General Structures Flashcards

Question

The strength of concrete is determined by what factors?

Answer 1

1 - **The Water-Cement Ratio**: The most critical factor in obtaining the proper curing and maximum strength of the concrete. Too much water causes excessive shrinkage and can allow the ingredients in the mix to segregate. Too little water (although less water produces very strong concrete) will make the mix less workable and will prohibit the concrete from bonding with the aggregate. The amount and type of cement is also important. 2 - **Admixture**: Material other than water, cement or aggregate, added to the concrete to change its properties such as accelerators, retarders and air-entertaining agents. A - _Fine Aggregate_: Coarse sand will produce grainy surfaces and can cause difficulty in finishing. Excessively fine sand will require additional water resulting in weakness and shrinkage. B - _Coarse Aggregate_: This represents the bulk of the mix. The properties of the aggregate; size, shape, material and quantity must all be considered for optimum strength, weight and fire resistance.

Answer 2

1 - **Beam Shape**: Most beams are rectangular in section, adjustments can be made for other shapes such as round. 2 - **Beam Size**: Beams larger than 12" nominal depth have reduced value of F_b 3 - **Grade of Wood**: Varies depending on quality. The better the Grade, the higher the F_b. A No.1 grade of a certain species is better than a No.3. 4 - **Duration of Load**: Dependent upon the amount of time the member is subjected to a load. 5 - **Beam Stability**: Adjustments are made for beams prone to compression buckling. 6 - **Moisture Conditions**: Kiln dried grade (15% MC) has higher F_b than air-dried grade (19% MC). Note: Design loads for visually graded lumber are based on species of wood, size, use and direction of loading.

Answer 3

**C - Steel** ## Footnote Although shear stress occurs in all beams, wood and concrete beams are most susceptible. Wood, in particular, is susceptible especially to horizontal shear. Beams that are heavily loaded and have a short span are often governed by shear. The main concern in the design of a steel beam is deflection and sometimes bending, when the compression flange is not adequately braced. Unless a large concentrated load is placed close to the support or it is a short span, steel is seldom governed by shear.

Answer 4

**They are nearly structural opposites**. ## Footnote Although their loads are resisted by horizontal and vertical reactions, an arch is in compression and a cable structure is in tension. The arch has thrust acting outward and the cable has thrust acting inward. The arch tends to spread out at the base and the cable pulls in at its supports. The rise of the arch is like the sag of the cable which forms a catenary curve. Thrust decreases as the height of the arch increases and also decreases as the sag of the cable increases. A cable (flexible) will change in shape as the loads change whereas an arch (rigid) cannot.

Answer 5

1 - **Steel Strength**: If the strength of the steel reinforcing is increased, the allowable stress is also increased which requires the need for greater bond stress or embedment lengths. 2 - **Concrete Strength**: If the specified compressive strength of the concrete is increased, the potential for bond strength development is also increased. 3 - **Reinforcing Bar Size**: An increase in rebar size will increase the ability of the bar to resist tension forces. 4 - **Encasement**: If the concrete mass around the reinforcing bar is limited, due to spatial requirements, the capability of the mass to develop bonding force may also be limited. 5 - **Rebar Location**: The concrete near the bottom of the pour cures more slowly, has a higher density and is of better quality due to the weight of the concrete above it. The capability of bond development is affected by this difference in quality.

Answer 6

With regard to reinforced concrete design, it is the length of embedment of reinforcing bar necessary to develop the required design strength of the reinforcing at points of maximum stress. One method of achieving the anchorage is to bend the end of the bar into a hook. When construction details limit the ability to extend the reinforcing bars sufficiently, a standard hook may be used to achieve adequate anchorage. The deformations on the surface of the rebar enhance bonding to the concrete and improve anchorage.

Answer 7

_PROs:_ 1 - **Faster Construction Time**: No on-site tying of reinforcing, no assembly and removal of formwork, no curing time. 2 - **Economy**: Elimination of formwork construction allows for a significant reduction of on-site labor costs. 3 - **Quality Control**: Factory produced components are more consistent in quality, and uniformity of strength, density, color, finishes, etc. 4 - **Standard Elements**: Pre-designed and engineered components can reduce the time necessary for the design development phase. _CONs:_ 1 - **Transportation**: Often sizable, heavy and bulky, these members may pose significant difficulty in transportation to the site. Larger members will require a larger capacity crane. 2 - **Cost**: Manufacturing, transportation and handling costs may be comparable to the savings gained in the reduction of on-site construction time. 3 - **Connections and Details**: These members lack the continuity, stability and resistance to lateral forces found in cast-in-place concrete construction and may impede the integration of other building materials. 4 - **Form**: Sculptural possibilities are limited in comparison to those achieved in cast-in-place concrete.

Answer 8

**B - Durability** Durability is the capacity of the plies to withstand different exposure conditions and remain bonded together. This capacity is a function of the adhesive used in the construction of the panel. Two main 'types' Type 1 - **Exterior**: Made with waterproof adhesive. Must withstand full weather exposure. Type 2 - **Interior**: Made with water-resistant adhesive. Must resist occasional wetting. --------------------------------- _Grade_ refers to the visual quality of the face veneer. _Core Construction_ refers to the material used for the inner plies. _Wood Species_ is a way of classifying plywood by category (construction or decorative), and hardwood or softwood.

Answer 9

An impervious fabric membrane supported by interior air pressure greater than the natural atmospheric pressure. Generally a tension structure that _may occasionally be subjected to compression or bending_. Commonly used as a roof enclosure for warehouses, athletic fields, etc. It can simply be inflated like a balloon using fans. When the necessary tension in the membrane exceeds its strength, it must be reinforced. This is usually done by wrapping the membrane in a grid of steel cable. The resulting tension in the cables gives the building it's form. In addition, another type of pneumatic structure consists of a hollow ribbed element that gains rigidity through its own internal pressure, independent of the internal pressure of the space, and may be used as a wall, a roof or both. Inherently flexible, these structures are dependent on the pressure differential between interior and exterior air to support even their own weight. Because of their exceptionally light weight, consideration of wind forces is critical since it may cause the structure to flutter. Uneven wind pressure (windward/leeward) may cause non-uniform loads which are detrimental to the structure. Unlike traditional compression foundations, _pneumatic structures need tension foundation_ to tie them down.

Answer 10

A convex or upward curvature built into a beam, truss or slab intended to counteract deflection when subjected to an anticipated load. Especially when the dead load represents the majority of the load. Common in pre-stressed concrete, it is intended to result in a nearly level or flat member after all service loads are applied. In light frame wood construction, this curvature is not intentional, however, the member is installed with the curve up to limit deflection.

Answer 11

Typically, for all truss members to be in pure axial compression and tension, loads are placed at the panel points (truss joints). Loads placed between panel points will cause bending moments in the loaded member and should be avoided. Members that are loaded in-between panel points will end up larger in area since they will be subjected to combined (axial & bending) stresses. Regarding open web steel joist (K, LH, DLH), these units are designed on the basis of a uniformly loaded top chord. Beams or girders must be placed under concentrated loads since open web steel joists are designed for uniform loads only.

Answer 12

_Steel_: No effect to consider _Timber_: Immediate deflection is computed with the standard formulas similar to those found in AISC manual. A long term factor of up to 2 is then applied. _Concrete_: A special computation for moment of inertia (I) is required. The modulus of elasticity (E) is an estimate but otherwise immediate deflection is computed normally as per AISC manual. Long-term factors are considered a percentage of the load always in place. Creep is the term associated with the long term deflection of concrete members.

Answer 13

1 - **Reinforced Concrete** 2 - **Reinforced Masonry** 3 - **Structural Steel** 4 - **Plywood** or other sheet material on studs. Nailing patterns are an important factor in this construction. This type of wall is designed to resist lateral forces, such as wind or earthquake loads, in its own plane. This wall can also be designed as a braced frame or a bent, which resists loads through the development of axial stresses.

Answer 14

The computations look the same for each, however, the location of the reinforcing steel is different. Reinforcing steel is located at the tension face of the beam. For positive moment (all simply supported beams) the steel is located toward the bottom of the beam. For negative moment (overhangs and cantilevers) the steel is located toward the top of the beam.

Answer 15

In this structural system, two-way concrete slabs are site-cast on top of each other on the ground slab, and then raised into place using special hydraulic jacks. The slabs are then attached to the columns. _PROs_ This method requires little supported formwork. This can result in a lower construction time and lower labor cost. In addition, this method offers ease of concrete placement. _CONs_ Satisfactory slab-column connections can be difficult to achieve and are not moment-resisting (pinned). This can be a problem when the system is subjected to lateral loads. Building plan must be highly repetitive, otherwise this method is not economical.

Answer 16

**D - Has deflected and water will not drain.** ## Footnote This phenomenon is a result of excess deflection of the roof structure and causes water to collect. It usually occurs in a roof that is too flat (less than 1/4" per foot). Water is unable to reach the drains and continues to collect as deflection increases. In extreme situations, it will lead to accelerated roof failure, particularly in long-span roofs. Note that water weighs 62.4 lbs. per cubic foot (PCF).

Answer 17

1 - **Flat Plate**: The simplest system. It is used for light loads and short spans, because it can't accommodate large shear loads near the columns. 2 - **Flat Slab**: Similar to the flat plate, but has thicker drop panels near the columns to handle shear loads. Best used in rectilinear buildings when columns are in line and equally spaced. 3 - **Waffle Slab**: Used for heavier loads and longer spans up to 40'.

Answer 18

To determine if there is enough cross-sectional area in a beam to prevent shear failure. This typically occurs at the supports where the value of shear is highest. A significant location on the shear diagram is that of zero shear, with it being has the greatest potential for failure in bending. The location on a beam where the shear value(s) equal zero coincides with the point(s) of maximum moment.

Answer 19

3' x 175psf = **525 pounds per linear foot.** ## Footnote Multiply the joist spacing by the uniform load being supported (lbs./s.f.) to determine the load per linear foot. PROCEDURE: Go to the table with the joist span (24') and the load per linear foot (525 lbs. per linear foot) and select the lightest member that will work from the steel joist load table.

Answer 20

**D - Buckling** ## Footnote A column is a vertical structural compression member that transmits axial loads through its longitudinal axis. The chance of buckling is the main concern in column design. The chance of buckling increases as the unbraced length and the slenderness ratio (SR) increase. The degree of buckling is determined by the stiffness of the member. The larger the slenderness ratio, the greater the chance of buckling and the less compressive stress the column can withstand.

Answer 21

**Bearing Capacity** ## Footnote Bearing Capacity limits the load-carrying capacity of a short wood column and is a greater concern than buckling. Bearing in compression parallel to the grain is dependent upon the mass of the material and its stress limit in compression. In a short column, the ratio of length-to-width is relatively small, which permits a larger allowable stress and reduces the possibility of column failure due to buckling.

Answer 22

**A - Web** ## Footnote In wide flange shapes and I-beams, although most of the material lies in the flanges, it is the area of the web that resists shear. The deeper and thicker the web of a steel beam is, the greater its shear capacity. The material in the flanges is ignored with regard to shear. In contrast, the flanges are primarily responsible for any bending moment. The wider the flanges and the further apart they are, the greater the moment-carrying capacity of a steel beam (greater section modulus).

Answer 23

This is the process of _rebuilding, stabilizing or strengthening the foundation of an existing building_. Three possible solutions are to enlarge the existing foundation, insert a new, deeper foundation under the existing one or treat the soil directly to increase bearing capacity. This may be necessary for several reasons. Typically it is due to the excavation necessary for the construction of an adjacent building with a deeper foundation. A number of potential problems can occur at closely spaced footings set at different levels. Excavation for the new building my disturb the soil around the existing foundation and must be done in a way that will not cause the existing foundation to settle or fail. Any settlement of the lower footing may cause additional settlement of the upper footing. Additionally, if the upper footing is carrying a significanrt load, the pressure in the soil below it may cause additional lateral load on the lower footing.

Answer 24

The moment of any section of a simply supported or continuous beam is equal to the sum of the areas under the Shear Diagram to the left of the section. **The location of zero shear is the point of maximum moment**. Remember, in shear diagrams, vertical increments represent force (lbs.) and horizontal increments represent span (feet).

Answer 25

Beams that rest on more than two supports (**continuous beams**) or beams that are fixed or partially fixed at one or more supports to resisit rotation, (**fixed-end beams**) and **rigid frames** for example. The bending moment cannot be determined by using only the equations of static equilibrium, Additional information is required regarding the elasticity of the material and the size of the members (E and I).

Answer 26

**1 - Unfinished A-307** They have the lowest load capacity due to their limited shear strength. Some movement occurs in the development of full resistance. Nuts are tightened enough to ensure a snug fit. Used for minor connections. **2 - High-Strength A-325 or A-490** Nuts are tightened sufficiently to create a tension force that causes a high degree of frictional reistance between the parts. High-Strength designations are: F: The limiting resistance is friction between the joined materials. N: FUnction in bearing and shear. Threads are not excluded (included) from shear planes. X: Similar to N, but threads are excluded from shear planes.

Answer 27

**Resultant** The Resultant of two or more forces acting on a body is a single force that has the same cumulative effect on the body as all the forces it replaces. The resultant is a vectorial summation of two or more forces. Since the Resultant is a vector force, it follows that it is defined by a Point of Application (P.O.A.), a Magnitude, a Line of Action (L.O.A.) and a sense (arrowhead). **Equilibrant** The equiplibrant has the same Point of Application as the Resultant. It also has the same (or parallel) LIne of Action, and Magnitude. However, the Equilibrant has a sense (arrowhead) opposite to that of the Resultant. The Equilibrant brings the Resultant to equlibrium

Answer 28

Although buildings are not supposed to move, their motion, however small, as a response to an applied force is a major consideration in structural design. 1 - _Translation_: This type of motion occurs as a simple linear displacement, where the path of a moving point is a straight line (either up/down; or left/right). 2 - _Rotation_: This type of motion occurs if the path of a moving point can be measured angularly as it revolves around a fixed reference point in a radial manner.

Answer 29

**B - Below the fiber saturation point.** ## Footnote Green lumber has not been seasoned and has a high moisture content. Water is present in the cavities between the cells and is called free water. Water is also present in the cell fibers and is called bound or absorbed water. As wood dries, the free water evaporates first. The wood fibers are still fully saturated and this is called the fiber saturation point (in most species it is about 30% moisture content). _Only when bound water begins to evaporate will the wood shrink and increase in strength._ As the wood dries, the moisture content decreases and the strength increases. It is usually unnecessary to season framing lumber below a moisture content of 11%-13%, because at this point the wood will continue to shrink or swell in order to maintain equilibrium with the moisture content of the air. Small dimensional changes in a single member are usually not a concern. However, the total chnage of several members such as in a wood floor may cause gaps or buckling.

Answer 30

The most common method used to calculate the strength of a concrete member. It is calculated by using a factorred design load called the uiltimate load. Code establishes the required strength (U) to be not less than: U = 1.4D + 1.7L Where D = Dead load and L = LIve Load. The new loads include the safety factor to be used in sizing and reinforcing the concrete member. This is a different design method than that currently used on wood or most steel member calculations. Wood and steel members may be designed using the Allowable Stress Design (ASD) method. The trend today is to use the Load-Resistant Factor Design (LRFD) for designing steel members and a similar method for wood members.

Answer 31

**Internal Moment = External Moment (moment Diagram)** ## Footnote Internal bending stress in the fibers of the beam resist external bending moment caused by the forces acting on the beam. Internal resisting moment is greater with an increase in depth. The issue of depth is critical in both span and load determination. The cross-sectional shape, area and materal of the beam act together to resist internal moment. Deflection is the strain that results from bending and needs to be limited to prevent excessive sagging. The allowable deflection is specified by model codes as a fraction of the span (1/180, 1/240, 1/360, etc.)

Answer 32

A system of compression and tension members that act together like a beam and can span very long distances. Composed of straight members, their intersections usually form a number of triangulations. Chord froces increase toward the center of the span as web forces decrease toward the center of the span.

Answer 33

1 - **Fixed**: Does not allow for rotation at the base. This condition can cause a high bending moment at the mid span. 2 - **Hinged**: More common than fixed, this flexible condition allows for rotation at its base. This rotation counters the stresses caused by live load, thermal expansion or uneven foundation settlement. Low bending moment at the supports, high bending moment at the apex. Note: The addition of a third hinged connection at the apex will reduce the bednign moment at mid span and makes the structure statically determinate.

Answer 34

**B - 0.001166 inches per inch** 0. 35 inches is refered to as the rod's total elongation. Since the original length of the rod was 25' (300"), the Unit of Elongation, or elongation per unit of length is 0. 35/300 = 0.001166 inches/inch.

Answer 35

**2 - Parallel to the grain** ## Footnote The main concern for shear in a wood beam is the longitudinal or horizontal splitting parallel to the grain (horizontal shear). This typically occurs at the ends of a simply supported wood beam. Resistance to shear perpendicular to the grain (vertical shear) is relatively high in a wood beam.

Answer 36

Shop-fabricated trusses of various standard sizes and details, usually used to support roof and floor decks. Top and bottom chords are made of cold-formed steel or double angles. Web members consist of steel rods or formed steel angles sandwiched between the teh angles of the top and bottom chords. These members are light-weight and can span long distances. Since long span members are deep and slender they are susceptible to buckling, especially during construction. They depend on secondary framing members called bridging (cross-bracing) for full stability. Open-web steel joists are categorized as follows: K - Standard LH - Long Span DLH - Deep Long Span Details and load capacities are standardized by the Steel Joist Institute.

Answer 37

**B - It supports the axial load of a beam.** ## Footnote A girder is a large primary horizontal supporting member of steel, reinforced concrete or timber. While it does support the load of a beam, _these are transverse (normal or perpendicular) loads, not axial loads_. Columns or bearing walls are common supports for girders. A girder supports a beam, beams support joists, decks or slabs. A _Beam_ is a horizontal structural member that carries transverse loads such as those from a joist, rafter, or purlin. It spans between girders. A _Joist_ is one of a series of relatively light-weight prallel members that span between larger beams, girders or bearing walls. They are horizontal members used to support floor and ceiling loads. For wood members, typical spacing is 12", 16" or 24" intervals, on center. A _Rafter_ is similar to a joist, however, it is installed at an incline and is used to support roof loads, whereas joists support floor or ceiling loads. A _Purlin_ is a horizontal member that spans across the slope of a pitched roof, parallel to the ridge beam and the header. Purlins are used to transfer roof loads from the decking to the rafters.

Answer 38

The primary objective of any building code is **to protect the life and safety of a buildng's occupants**. Other objectives include: 1 - To protect against structural failure in the event of a fire or earthquake 2 - To ensure that building components are assembled correctly. 3 - To ensure that the building materials are appropriate to a building's function. Accessibility, structural, mechanical, electrical, plumbing, and other codes have more specific requirements in addition to health, safety and welfare. Some Structural concerns expressed in any building code include: 1 - Load determination 2 - Allowable stresses in structural members 3 - Formulas for designing members of various materials. 4 - Construction requirements.

Answer 39

**C - A long-term shortening under a sustained compression load.** ## Footnote This is the deformation of a member subjected to a high level of compression stress (load) over a long period of time. This deformation, or shortening, lessens over time and the amount of movement varies enormously depending upon the material and the load. It will cause columns to shorten, and an increase in beam deflection. Note that a portion of a beam's cross-section is always in compression.

Answer 40

This is a type of foundation that distributes the building load over a large area of soil to ensure that the bearing capacity (F_p) of the soil is not exceeded. 1 - **Wall Footing**: placed under a continuous foundation wall and used to support a bearing wall. 2 - **Isolated Pad Footing**: supports a single column. 3 - **Combined Footing**: supports to or more columns 4 - **Cantilever Footing**: distributes the column load to each footing to equalize soil pressure 5 - **Mat Footing**: one large footing used for low soil bearing capacity or heavy loads.

Answer 41

_Shallow Foundation_ Mat: A type of spread footing, acts like one large footing that distributes the load under the entire building. Raft: Where the foundation is similar to a amt but the excavated soil is roughly the same weight as the building causing the building to essentially float. Both are two-way slabs and can be stiffened by shear walls above the foundation. _Deep Foundation_ Used in conjunction with a amt foundation are piles, where long stilts are driven into unstable soil and transmit the building load theough skin friction (perimeter surface area) or to a more stable layer of soil, that provides end bearing for the piles. Piers, or caissons, where a hole is excavated and filled with concrete are also used to support mat and raft foundations. The lower end may be belled to achieve the necessary bearing area.

Answer 42

This is the measure of the linear expansion of a material as it is heated (a). It has a unit of inches per degree Fahrenheit. ## Footnote Values for typical materials are: Lead: a = 0.0000159 Aluminum: a = 0.0000128 Copper: a = 0.0000093 A-36 Steel: a = 0.0000065 Concrete: a = 0.0000055 Timber (Fir): a = 0.00032 (perpendicular to the grain) a = 0.0000021 (parallel to grain) The similarity in the coefficients of concrete and steel make them complimentary materials. This is why steel reinforced concrete members or building components are exceptionally functional

Answer 43

**C - A lack of redundancy** ## Footnote Although failure of these members usually occurs in the connections it is most likely due to a lack of redundancy. Since there are realtively few members working together, a failure in one member may result in a progressive failure of the remaining structure. In these structures, deflection, as well as lateral loads are expected and included in the design of the members. The reliability of each of the members is crucial, therefore, it may be adviseable to design additional connections at the calculated points of failure, above and beyond the minimum required by building codes.

Answer 44

**Subsidence** A widespread sinking of the ground surface or settlement of the soil mass. Some causes may include highly compressive organic material or loose fill, seismic events or the addition or removal of oil, gas or water which creates compressive void spaces. **Liquification** A sudden loss of shear resistance in the soil. It is the transformation of otherwise stable granular soil from a solid material to a material with liquid characteristics, such as sand. It can be the result of earthquakes or other vibrations in the earth's surface.

Answer 45

The most common type of steelfor structural applications. The designation ASTM means that the steel has been manufactured according to the requireemnts set by the American Society for Testing and Materials, and A36 is the specification number. **Yield Point**: Fy = 36 ksi (specified minimum) **Specified Ultimate Tensile Stress**: 58 ksi. According to AISC: **Maximum allowable Stress**: F_v in shear = 14.5 ksi (Margin of safety is between 14.5 and 36 ksi) **Maximum allowable Stress**: F_b for bending = 24 ksi and depends on the adequacy of the lateral support of the compression flange. **Modulus of Elasticity**: (E) = 29,000 ksi Note: E refers to stiffness, not strength.

Answer 46

The tendency of a force to create a rotation about a point. It is equal to the magnitude of the force multiplied by the perpendicular distance from the line of action (moment arm) to the point (center of mass). For a simple beam with a distributed load: M = (WL)/8 or M = (wl²)/8 For a simple beam with a single concentrated load in teh center of the beam: M = (PL)/4 _Maximum Moment_ occurs where the shear diagram crosses zero. moments are expressed in foot-pounds, inch-pounds, kip-feet, or kip-inches. The amount of maximum moment (from the loading diagram) along with the allowable bending stress of the material, determine the required Section Modulus of the member necessary to resist bending moments.

Answer 47

**Greater than 12" in depth.** Larger wood members often have undetected, internal flaws that reduce their load carrying capacity. As a result, the allowable stress in bending (F_b) must be decreased for larger members and is multiplied by a size factor (C_f) which accounts for beam depth. F_b x C_f Where: C_f = (12/d)^1/9 and d = beam depth in inches C_f is a fraction which is often close to 1 and is thus a slight reduction of F_b.

Answer 48

Examine the terms in the deflection equation for a uniform load _/\_ = (5wl⁴)/(384I) = (5WL³)/(384EI) 1 - Decreasing the uniform load coefficient (w) by 25% will decrease _/\_ from 1" to .75" 2 - Decreasing the span (L) will significantly decrease _/\_ 3 - Increasing the Modulus of Elasticity (E) will decrease _/\_ 4 - Increasing the moment of Inertia (I) will decrease _/\_

Answer 49

**B - 0.1591"** ## Footnote Since the change in Length = _/\_L = (PL) / (AE) P = 600 kips A = 31.2 in² E = 29,000 ksi L = 20' or 240" Then: _/\_L = (600 x 20 x 12) / (31.2 x 29,000) = 0.1591"

Answer 50

**Concrete Design Strength (f_c') is the maximum compressive strength of cured concrete.** ## Footnote Commonly designed to be 3,000 psi for general use. Strengths of 2,000 psi may be appropriate for some applications whereas 9,000 psi may be required for ground floor columns in tall buildings. Higher strengths (5-7,000 psi) are also required for pre-stressed and tilt-up concrete since they will be picked up by a crane and transported. Slump tests can be performed during placement to determine the consistency of the mix, too wet or too dry, usually a result of the water/cement ratio. However, strength tests called cylinder tests (cylindrical mold test), are performed on cured samples cast at the time of the pour. The cylinders, usually 6" in diameter by 12" high, are subjected to compression tests after a period of 7 days and again after the cured concrete has reached its Maximum Compressive Strength, in 28 days. Core Cylinder tests are performed by drilling a cylinder from cured concrete and subjecting the sample to compression to determine if it meets or exceeds the specified f_c'.

Answer 51

1 - The geometric properties of the **Area** are analyzed and are independent of the material. The area of the section (and its material) determines its ability to resist shear. 2 - Every section has a **Section Modulus** (S_x). The section mudulus (in³) of the section (and its material) determines its ability to resist bending moments. 3 - Every section has a **Moment of Intertia** (I_x). The moment of inertia (in⁴) of the section (and its material) determines its ability to resist deflection. Both section modulus and moment of inertia have reference to a neutral axis passing through the centroid of the section. When the section is used along its strong axis I_x and S_x are used, and when used on its weak axis I_y and S_y are used. 4 - Every section has a **Radius of Gyration** (r). The radius of gyration (inches) is used mostly to determine the slenderness of a steel column. 5 - Every section has a **Centroid**, which is defined by the intersection of all neutral axes for the section (x-x, y-y, z-z).

Answer 52

**C - 0.3042"** ## Footnote The change in length (_/\_l) is equal to the change in temperature (_/\_T) multiplied by the coefficient of thermal expansion (a). Since the coefficient of thermal expansion of steel is 65' x 10^-7 in/in per degree Fahrenheit then: _/\_l = _/\_T x : x a _/\_l = 60 x (65 x 12) x 0.0000065 _/\_l = 0.3042"

Answer 53

Maximum moment and maximum shear must be known or calculated. Grade and species of timber will determine the allowable Bending Stress (F_b) and the allowable Shear Stress (F_v). 1 - Knowing the maximum moment about the x-axis and the allowable Bending Stress (F_b), compute: Section Moduls S_x = (M_MAX)/F_b. This will satisfy bending moment requirements. 2 - Section Modulus for rectangular sections: Sx = (bd²)/6 3 - Knowing the maximum shear (V_MAX) and allowable Shear Stress (F_v) compute: Area = (3V_MAX)/(2F_v). 4 - Area for rectangular section A = b x d 5 - Select a member that has a larger area than found in 4, above and a larger section modulus than found in 2, above. Note: Adjustments are to be expected since timber is supplied in standard sizes.

Answer 54

A - **Simple Beam**: Ressts on a support at each end. The beam ends are free to rotate. B - **Cantilevered Beam**: Projects from its support and is fixed (restrained) at that support. C - **Overhanging Beam**: End porjects past the support. D - **Continuous Beam**: Supported by more than two supports. E - **Restrained Beam**: Has one or both ends fixed and restrained against rotation.

Answer 55

1 - The span and support condition of the structure being designed and the uses proposed. 2 - The loads that will be applied (uniform, concentrated, variable). 3 - The weight of the beam itself. This must be estimated and adjustments must be made if estimate is shown to eb wrong. This is most important in concrete beams but should be checked in other materials. The span, loads, beam weight and support conditions will determines the resulting maximum shear and bending moment to which the beam will be subjected. 4 - The material to be used will determine the allowable stresses F_b and F_v and the _Modulus of Elasticity_ (E). For steel beams the grade of steel must be known. For timber beams, the species (pine, fir, etc.) must be known, in addition to the grade (No.1 , No. 2, etc.). For reinforced concrete members, the strength of the concrete (f_c') and strength of the rebar (f_y) must be known. 5 - The degree of lateral support for the compression side of teh beam (critical for steel beams).

Answer 56

**C - Near the tension face** ## Footnote This is the face that elongates under a load. In this case, the load of teh retained earth.

Answer 57

Tributary Area for column B-2 = 30' x 25' = **750 ft²** The load on the column = A x (load/ft²) = 750 x (100 + 80) = 135,000 lbs. or 135 kips

Answer 58

Point A represents the point of **Maximum Bending Moment** and **Zero Shear**, which occurs directly under the concentrated load. Note that the right reaction is larger than the left since the concentrated load is applied to the right of midspan. The shear diagram under thge uniform load is sloping straight line and the moment diagram is a parabola. There is a sudden drop in shear under the concentrated load in the shear diagram.

Answer 59

**R = F_v x A** ## Footnote Where: R = Resistance to shear failure F_v = Allowable shear stress A = Area of bolt cross section In a simple connection between two steal bars, the force (in this case tension) is transferred from one area to the other through the connector (bolt). Assume the bolt is loosely tightened and acts as a pin connection. While each plate is subjected to tension, the bolt itself is subjected to single shear. The amount of tension that each plate can handle depends on its srength (F_y) and dimensions (width and thickness). The amount of shear that the bolt can handle depends on its strength (A-325 or A-490), diameter and whether the threads are included in the plane of shear (type N) or excluded (Type X).

Answer 60

**Wall Foundation** ## Footnote A oad distributing, or spanning element, used in foundation systems. Often used to span unacceptable soil, it combines the functions of a foundation wall and a wall footing into a single structural element. May be self supporting or bear on piles.

Answer 61

**A - Hoops** **B - Meridians** The meridian lines in a dome act as individual arches transferring loads through compression to the ground. This compression is countered by the hoops that are in compression in the upper portion and in tension toward the ground. One of the most efficient structural systems if it is not too flat (high rise-to-span ratio).

Answer 62

**82.156K (or 82,156 lbs)** Refer to steps 1-4 of this chart for help in calculating the allowable axial load.

Answer 63

A - Rotation Fixed, Translation Fixed (strongest type). K = 0.5 B - Rotation Free (pinned), Translation fixed. K = 1.0 C - Rotation Fixed, Translation Free. K = 1.0 D = Rotation Free (pinned), Translation Free. K = 2.0 The end conditions in steel are either fixed or pinned. The end condition (K) of a column affects its load carrying capcity. K is the variable that deals with end conditions concerning the slenderness ratio SR = Kl/r

Answer 64

A covering over an area that behaves like a series of adjacent Roman arches. Commonly constructed of masonry, the arches create a continuous structure, supported at its longitudinal edges. Like an arch, a vault needs lateral restraint at the spring line.

Answer 65

**A - 9,091 psi** ## Footnote Since the radius of gyration of one rod is: r = d/2 = 0.75/2 = 0.375" Then the area of each rod is A = πr² = (3.142)(0.375)² = 0.44 in² Its share of the load is 8,000lbs/2 = 4,000lbs. Using f = P/A then 4000/0.44 = 9,091 psi on each rod. If these rods are A-36 steel, then the allowable tensile stress is F_T = 22 ksi which is greater than 9.09 ksi and the rods are far being stressed to their limit.

Answer 66

**D - Its circular plan allows for direct radial access from the ticket counters to the gates.** Designed by Eero Saarinen, it was built near Washington D.C. in 1964. Although the plan was organized around the circulation, the building has a rectilinear footprint.

Answer 67

A framework for forming domed or arched surfaces, typically roof structures. It consists of two sets or parallel arches that intersect to form a grid in plan. _Advantages include_: Repetition of like elements and joint details, the ability to span great distances, can be made of wood, steel or concrete. The italian engineer Pier Luigi Nervi is crdited with using this system extensively foir teh construction of airplane hangars

Answer 68

**Frei Otto** ## Footnote Completed in 1972, it is composed of a series of steel nets stretched between steel masts and anchored to the ground by steel cables. The roof structure is a high-tech PVC-coated polyester fabric tent that has inspired the use of tensile structures in archtectural design.

Answer 69

Total area of the built up section: 16.8 + 2(8 x 1) = 32.8 in² I_X = I_X of the W shape plus I_X of the cover plate plus the transfer of I_X of the cover plate to the new X-axis. The distance from teh X-Axis of teh W16 to eth X-Axis of teh cover plate is 16.43/2 + 1/2 = 8.715" Therefore: I_X = 758 + 2((1/12 x 8x 1³) + 8(8.715)²) I_X = 758 in⁴ + 1216.55 in⁴ = **1974.55 in⁴**

Answer 70

**4'-0" x 20'-0" = 80 ft²** ## Footnote This area is equal to the beam span multiplied by beam spacing. It is the area of effective load that a structural member must carry.

Answer 71

The horizontal force exerted by an arch that causes it to spread outward. This force is proportional to the load and the span, and inversly proportional to the rise. To calculate, use the formula: F = M / D Where: M = Moment of the Arch D = the rise of the arch. Can be resisted by supporting the arch with an adjoining arch, or by using steel tie-rods, a sufficient foundation or a shear wall.

Answer 72

**Maximum Moment = Pab/l at the load** **Reactions:** **L = (b/l) x P** **R = (a/l) x P** Since b \> a, R will be smaller than L (load is closer to L than to R). It is important to think of L and R as being fractions of teh 25k load. L larger than R since teh 25k load is closer to L. So the 25k load breaks down into teh foolowing reactions: L = (16/20) x 25k = 20k R = (4/20) x 25k = 5k L = 20k and R = 5k

Answer 73

A type of two-way concrete slab normally used along with long spans and heavy loads. Two perpendicular sets of ribs typically reinforced and cast in place using reusable square forms. The waffles around a column are filled solid to avoid two-way punching shear. To accomodate rectangular spaces ribs may be deeper in one direction or contain additional reinforcing. Often left exposed, this structural system tends to convey a sense of mass and rhythm.

Answer 74

A series of rigid frames that intersect each other in a grid pattern and are rigidly connected at their intersection points. This system works as a unit, similar to a two-way reinforced concrete slab. Space frames are used to span large, column free areas. They are statically indeterminate. A space frame is more efficient when it overhangs one bay in each direction.

Answer 75

1 - The sum of all vertical forces equal zero 2 - The sum of all horizontal forces equal zero 3 - The sum of all moments or the moment of all forces equal zero. If these equations can be solved, the beam is staically determinate. Note: The branch of mechanics that focuses on forces and equilibrium or bodies that are held motionless by forces acting upon them is known as statics. Where the resultant of all forces acting on a body os zero.

Answer 76

A - **Penetration**: The depth of a weld from the surface of the base metal to the point at which the two metals cease to fuse. B - **Reinforcement**: Additional material beyond the dimension of the throat. C - **Throat**: In a fillet weld, it is the ditance from teh intersecting surfaces of the base metal perpendicular to the hypotenuse of an inscribed isosceles right triangle. D - **Size**: The length of the leg of the inscribed isosceles right triangle. THis is the specified diemnsion, for example 3/16" E - **Root** F - **Toe**

Answer 77

A member that rests on three or more supports and is statically indeterminate. Maximum bending takes place over the first interior support. Negative moment occurs over the supports. Positive moment occurs between the supports.

Answer 78

A - **Compression**: Shortens B - **Neutral Axis**: Remains the same length C - **Tension**: Elongates The length of molecular bonds will change in deflection. The molecules resist this change in length and create an internal force which resists an external force. A deeper beam will have more molecules to create greater resistance to loads. Increasing the depth of a beam is an effective solution to accomodate larger loads or longer spans. The compression increases from zero at the Neutral Axis to a maximum (C) at the top of the beam. Likewise, Tension increases from zero at the N.A. to a maximum (T) at the bottom of teh beam. The couple that is formed between Compression at the top and Tension at the bottom, separated by a couple arm equal to the depth of the beam, is what resists external moments.

Answer 79

**A measure of the stiffness of a material, and its resistance to deformation. It is a ratio of stress to strain (deformation) and is unique to each material.** This stress/strain ratio was identified by Thomas Young (1773-1829) and was first published in 1807. He was also responsible, in part, for the decipherment of Egyptioan hieroglyphics. E is sometimes called **Young's Modulus**.

Answer 80

**D - Thin Concrete Roof Elements** ## Footnote Born in 1910, Candela's roof structures incorporated cleverly 'folded' cast-in-place concrete plates in paraboloid , dome and saddle shapes. These folds increase the stiffness of the structure and act like trusses or beams.

Answer 81

**Three-hinged Arch** ## Footnote An arch with hinges at each support and a third hing at the apex. Due to the third hinge, the structure is statically determinate, that is, the reactions can be found by using the equations of equilibrium. This type of arch allows for the thermal expansion and contradiction and uneven settling without bending derormation of the two arch members. A two-hinged arch, like continuous and fixed-end beams are statically indeterminate and require complex calculations to find the reactions. Both types of arches must resist outward thrust at the abutments. A simple method of resisting this thrust is by using a steel tie-rod. similarly, domes must resist thrust and this can be achieved by the use of a tension ring at the dome base. This may be seen at the base of the Pantheon dome, where a chain was added to prevent the dome spreading.

Answer 82

**S = M/F_b** ## Footnote Used in beam design to determine the required property for the beam cross-section. S is equal to the maximum moment (M) divided by the maximum allowable bending stress (F_b). A measure of the bending capacity of the shape, not a rating of the material. Steel beams with a higher percentage of material in the flanges will have a larger value of S and will be able to resist greater bending. It is equal to the moment of inertia divided by the distance from the center of gravity to the extreme fiber. For rectangular sections, S_x = I_X/c = bd²/c Section modulus is measured in in³.

Answer 83

A Stress (y) / Strain (x) graph which is a graphic representation of a material's ability to resist force and resulting deformation. This graph is different for different materials. ## Footnote a - **Elaatic Limit**: The greatest unit stress that a material can resist without permanent deforamtion when the stres sis removed. Described by mathematician, architect and physicist Robert Hooke. Hooke's Law of Physics states that stress is directly proportional to strain and remains constant up to the elastic limit. The slope (rise/run = stress/strain) of this diagram in the elastic limit is called the Modulus of Elasticity (E) and is a measure of a material's resistance to deformation. Twice the force will produce twice the deformation and when the force (up to a material's Elastic Limit) is removed, the material will return to original size and shape. b - **Yield Point**: A point beyond the elastic limit, up to which a material deforms with only a small increase in force and after which a material will continue to deform with no increase in force. The deformation is permanent. c - **Ultimate Strength**: This is the maximum unit stress of a material that occurs just at or before failure. d - **Rupture**: Material fails.

Answer 84

To determine the immediate deflection of a steel or timber beam, use teh standard formulas found in teh American Institute of Steel Construction (AISC) handbook, all of which take the form of: _/\_ (deflection) = (coeffcient)(load) x (span³)/EI The coefficient will vary according to the load type and pattern. The load will be either, the total uniform load on the beam, or one (possibly one of several equal) concentrated loads. The coefficient indicates that a concentrated load on a span creates greater deflection than a uniform load of the same magnitude. The coefficient also indicates that a cantilever will deflect more than a simple beam for the same loads. Span is the dominant factor in determining deflection. The stronger the material (E), the less the deflection. The deeper the member d and I, the less the deflection.

Answer 85

shortening = Compression elongation = Tension _/\_L = PL_o/AE Where: _/\_L = The change in Length of the member (inches) P = The axial load (lbs. or kips) L_o = The original length of the member A = The cross-sectional Area of the member (in²) E = The Modulus of Elasticity (psi or ksi, or similar units to P). The resulting unit strain is: _/\_L_o/L The final length is: L⁺/_- _/\_L_o (elongation or shortening)

Answer 86

**B - Distance from the first fiber of steel reinforcing to the first fiber of concrete** ## Footnote In reinforced concrete design this is the minimum required distance from the edge of the reinforcing steel to the outer surface of the concrete component. Governed by code, the general requirements are: Concrete cast directly against soil = 3" Concrete exposed to weather or soil: #6 bars or larger = 2" #5 bars or smaller = 1.5" Concrete slabs and walls with no exposure .75" to 1.5" Concrete beams and columns with no exposure 1.5" Concrete can achieve any amount of fire resistance by increasing this dimension. - The area of contact between concrete and soil may be a factor of the soil bearing capacity (F_p). - The distance from centroid to the compressive force is known as the effective (structural) depth (d). - The length of embedment is the length of steel necessary to develop a required internal strength (l_c).

Answer 87

A section property **r = sqrt(I/A)** A formula used for the design of slender compression members (columns, truss members, bracing members, etc.) Used mainly in steel design for Kl/r ratios (slenderness ratio) which are used to determine the allowable axial compression stress of a steel column. A property of the Area (A) and the Moment of Inertia (I) of the cross section of the member. Since typical sections (other than square or round) will have a different moment of inertia about the x and y axes, it follows that such sections will have different radii of gyration about each axis. The smaller radius of gyration will govern the slenderness of a steel column. The units for the radius of gyration are inches.

Answer 88

**B - 11.31 ksi** When a member is restrained at both ends, any change in temperature will cause internal stress. The formula is: (P/A) = _/\_T x a x E where: _/\_T = 60^oF a = 0.0000065 in/in/^oF E = 29,000 ksi P/A = 60 x 0.0000065 x 29,000 = **11.31 ksi** Fixing both ends of a member will create a very large build-up of forces and moments (the equivalent of increased loading). This will result in increased stresses at the supports as the member tries to stretch, as a result of the increase in temperature. This could lead to failure unless the connection is detailed to allow for that movement. Connection should be pinned if temperature differentials are anticipated.

Answer 89

1 - Find the _Slenderness Ratio_ (SR): With formula for axial compression, F_a is a function of F_y and Kl/r Determine the end condition: either fixed (K=0.65) or pinned (K=1). Determine the unbraced length of teh column (l) Determine the least radius of gyration (r). See steel chart and selct the least radius, either r_xx or r_yy. Solve for F_a using table C-36 in the AISC manual. 2 - Find the _Allowable Load_ (P): P = F_a x A Where F_a = Slenderness Ratio (step 1) Determine A = Area of section (from steel chart). Solve for P If dealing with both x-x and y-y: Compute SR with each r and use the highest SR (the highest SR will correspond to the smaller of r_xx or r_yy) for F_a in the steel chart.

Answer 90

**n = E_s / E_c **or **n = E_x / E_m** ## Footnote A ratio of the moduli of elasticity of different materials, steel (E_s) and concrete (E_c) or Masonry (E_m). Typically used to determine the properties, such as bending strength, of cross section made of two materials. For example, reinforced concrete and masonry or composite beams of W-type steel sections with concrete slab top flange. This is also used for flitch beams that combine wood and steel members.

Answer 91

In a compressive member, this ratio relates the unbraced length to the cross section. For steel, the SR is equal to: Kl/r where: K = stiffness of the end condition l = the largest unbraced length r = the least radius of gyration (related to the stiuffness of steel). By using the SR, we can determine the F_a, the maximum allowable axial compressive stress. A higher SR will reduce F_a and will reduce the amount of stress and therefore the axial load that the column can withstand. For timber columns, the cross-sectional dimension is used, the ratio is l/d. where l = unbraced length and d = lease cross-sectional dimension.

Answer 92

**D - Unit Strain and Unit Deformation** Both terms are synonymous and describes the effect on a member as a result of stress. The deformation _/\_L is defined as the change in length of a member. The Unit Deformation is the ratio of the change in length of a member with respect to its original length. Similarly, the unit strain is defined as the change in length per original length. s = (_/\_L) / L_o Where: s = Strain _/\_L = Dimensional change (elongation or shortening) L_o = Original Length For example, a 10' steel rod is subjected to a tension force. The rod is now 0.18" longer. 0.18" / 120" or 0.0015 in/in. ------------------------- Stress is the the internal resistance of a material as a result of an external force and is given by F = P/A Deflection is the resulting displacement (strain) of a beam under a load.

Answer 93

**M = f_b x S_x** ## Footnote The formula for the resisting moment (M), that considers the section modulus (S), size and shape of a beam in cross section, and the allowable stress of the beam (f_b), wood or steel for example. This formula is used in the design of a homogeneous beams (beams that are made of a single material). M is equal to or less than the bending moment. The section modulus and the resisting moment are directly proportional.

Answer 94

**B - the beam now has a greater load carrying capacity.** ## Footnote Due to an increase in the effective depth and therefore also in section modulus in the new beam, it is now able to support a greater load , moment, and span a greater distance. Also with the increase in effective depth, the beam will have a greater moment of inertia and will this result in less deflection. The resistance to shear at the supports remains relatively unchanged, since the height and area of the beam at the supports appears to be unchanged.

Answer 95

Using the formula M=wl²/8 or WL/8 Where: w = 500 lbs/ft. W= (500 lbs/ft.) x 12' = 6000lbs. Therefore: M = (6000 x 12)/8 = **9000 ft.-lbs.**

Answer 96

**Diagram D** Also called the a semicircular or round arch, the Romans were first to introduce the shape of a half circle. This offered convenient construction properties since each unit can be the same shape. They were commonly used to span modest distances and could be used to create vaulted space of 100'. ------------------ A is Parabolic arch, similar to a catenary arch. B is a three-centered arch or basket arch. C is a four-centered arch or Ogee arch.

Answer 97

**Panel Bracing or Staggered Truss** ## Footnote A vertically-oriented lateral load resistive system incorporated into the structure when additional wind or seismic bracing is required. It is achieved by stiffening alternate bays at alternate floors with panels or diagonal framing members. Considered a structural irregularity by SEAOC, Structural Engineers Association of California, because of its "unusual or novel structural features."

Answer 98

**A - Framing Anchor** **B- Truss or Joist Hanger** **C- Gang Nail Plate Truss Connector (toothed plate)** Used in light frame wood construction, these metal plate connectors are typically fastened using short nails meant for 2" wood members. They serve to tie the members together and improve resistance to lateral and uplift loads. All of the connectors shown are considered pins or hinges and do not restrain the joint against rotation.

Answer 99

A - **Stretcher** (3 core): unit most common for unreinforced construction. B - **Double stretcher**: Most common for reinforced construction. Large cores with vertical steel bar and grout create internalize informed concrete columns. C - **Low Web Bond-Beam**: Unit provides space for horizontal reinforcing bar and grout. Used to tie wall together at critical points or create a bond beam. D - **Channel Bond Beam or Linte**l: Unit provides space for horizontal reinforcing bar. Used to create lintels or over openings. E - **Control Joint**: used for making expansion or vertical shear type control joints in construction. Sealant is used between interlocking units. F - **End**: Used where the end of the unit will be exposed. G - **Corner**: half block used at ends of walls to create a running bond pattern.

Answer 100

**Diagram C** ## Footnote The continuity of the beam stiffens the loaded span by restraining the rotation that would normally occur at the ends of a simply supported beam. The beam behavior of the loaded span is similar to that of a fixed-end beam. The load causes bending and shear in the unloaded spans and is considered to be carried by the entire beam, not just the span in which the beam is loaded. Compared to a simply supported beam with the same load and span, the continuous beam is subjected to less bending moment and therefore, less deflection.

Answer 101

**Diagram D** ## Footnote Since the beam ends are fixed and rotation is prevented, bending Stess will result. The point of inflection (POI) occurs at the mid span and the two beam halves react as though they are cantilevered and carrying loads at their tips.

Answer 102

**Continuous**: Extreme fiber stresses change from compression on the top and tension on the bottom (positive bending), to tension on the top and compression on the bottom (negative bending). Bending stress is transferred from one bay to the next. **Simply-supported**: The beam ends are hinged, the beam can adapt by rotating around the hinge. No additional stress is imposed on the beams and they remain straight, though tilted.

Answer 103

Common in steel frame construction. It serves as permanent formwork and tensile reinforcement for concrete floor and roof decks. Made from galvanized sheet steel, the corrugations increase the stiffness of the deck by forming concrete into one-way ribs. The spanning distance is largely a function of the gage of the steel and the depth and frequency of the corrugations. It can be used instead of temporary plank flooring during construction, and is usually puddle-welded to the beams. Metal shear studs (Nelson studs) are sometimes welded to the top of the steel beams and protrude into the concrete. This integrates and strengthens the structure by developing shear resistance between the beam and the concrete. Shear studs improve the rigidity of the connection between the diaphragm and the supporting steel frame. In order to increase fire protection, the exposed metal deck is often covered by fire-resistive panels or sprayed-on material.

Answer 104

**A foundation connection for a hinged-arch.** A - **Glued-laminated beam or arch.** B - **Concrete abutment** C - **Steel shoe plate** D - **Bridge pin or bridge hinge** E - **Anchor bolt** The hinged connection allows the arch to rotate independently of the concrete foundation under applied loads or temperature changes. Hinged arches are relatively flexible structures and tend not to develop much bending stress. If stress is to be avoided all together a hinge may be added, usually at the apex of the arch, and each of the members is free to move without deformation. This is called a three-hinged arch.

Answer 105

1- a bolted beam-to-column, shear connection. It is a shear-only connection because the beam flanges are not rigidly connected to the column. Note the gap shown between the bottom flange of the beam and the column. 2- a welded beam-to-column, moment (rigid) connection. The beam flange welds transmit full flange strength to the column. The shear tab, welded to the column, and bolted to the beam web supports the beam until it is welded and offers permanent shear resistance. Electric Arc Welding is generally used in steel construction with the fillet weld being the most common type of weld. A moment connection is one that can stabilize a frame against lateral forces without the use of diagonal bracing or shear walls.

Answer 106

In a simply-supported beam with a uniformly distributed load, there are three possible, distinct tendencies to fail. 1 - **vertical shear**: critical at the support locations for short beams with small cross-sectional areas that are heavily loaded (especially if large load is concentrated near support). 2 - **Bending**: Occurs at the point of maximum moment or at the mid-span of the a uniformly loaded beam. Usually occurs with larger spans that are uniformly loaded. 3 - **horizontal shear**: This type of failure happens mostly in wood members where layers (growth rings) may slip past each other in a horizontal direction. Wood and concrete are more prone to failure by shear stress than steel members. Shear stress is critical in the design of members with a small cross-sectional area, and those that have short spa sand carry heavy concentrated loads.

Answer 107

Framing members must sometimes be notched, drilled, or cut to accomodate mechanical, electrical or plumbing equipment. However, it should be done in a way that will limit the impact on the member's structural integrity. Notching should _not_ be done in the center 1/3 of the beam span and should have a depth _\<_ 1/6 of the beam depth. Holes should _not_ exceed 1/3 of the beam depth and should not be located within 2" of the top or bottom (as close as possible to the Neutral Axis). Holes should be located away from the beam end supports to minimize the exposure to shear and away from the midspan to minimize the exposure to large bending moments.

Answer 108

1 - **Two-way or Punching Shear**: the column tends to punch through the footing. 2 - **One-way flexural shear or diagonal tension**: Considered ordinary beam shear, the two-way reinforcing in the tension face of the footing resists bending. Normally, if shear is a problem the footing is made thicker. The footing generates stress in the soil mass directly under the footing where, for calculation purposes, the soil bearing capacity is considered to be a uniformly distributed constant reaction. This is the most common type of shallow foundation and is called an isolated pad footing, a type of spread footing. It is poured directly on the soil and the transfer of stress from the footing to the soil is through direct bearing pressure.

Answer 109

1 - **Face Nailing**: The strongest nailing method 2 - **End Nailing**: The weakest method. A nail is driven through one piece of wood into the end grainof the adjoining piece. WOrks best for holding members in alignment until additional framing or sheathing can strewngthen the connection. 3 - **Toe Nailing**: About 2/3 as strong as face nailing. Used when access is not available to end nailing. Note: Blind nailing is a method of toe-nailing where the nail head is to be concealed by the adjacent piece of wood.

Answer 110

This type of spread footing is used to support two or more, possibly several, closely spaced columns. Ideally, the footing will have such area and weight that the centroid of the footing will coincide with teh centroid of the coulmn loads plus the footing itself. HOwever, this is not necessary as long as the resulting eccentricity does not create an overstress in teh bearing soil. In addition, the column spacing and allowable soild bearing capacity will control the footing size.

Answer 111

**Cover Plates** ## Footnote These steel plates are welded to a steel wide flange beam to increase the load carrying capacity of the member. They are usually added to both the top and bottom flanges. Can be used to slightly minimize section depth where space is limited.

Answer 112

A - **Diagonal web member** B - **Top chord** C - **Panel point (truss joint)** D - **Bottom chord** E - **Vertical web member** The top and bottom chords are analogous to beam flanges and are thus primarily responsible for bending stresses. Diagonals and verticals are analogous to the beam web, and are thus primarily responsible for shear stresses.

Answer 113

At point 'A' the beam tends to lift off the support. The design of the connection must be sufficient to hold beam end 'A' on the support and foundation. Summing moments about point 'B' will show that reaction 'A' needs to be downward to provide a counter-clockwise moment to balance the clockwise moment created by force 'P' about point 'B'. Thge downward reaction implies that it is a tie-down that must resist uplift at 'A'.

Answer 114

Assuming the three spans are equal in the two beams: _Beam 1_: The continuous beam will develop a given set of moments and is statically indeterminate. _Beam 2_: The location of the internal hinges will affect the moments in this beam and may result in a lower design moment. It is statically determinate. In this beam, the middle span is simply supported on the two overhanging end beams. The reaction from the middle span is a concentrated load onto the end bay and thus may generate more moment than the uniform load could generate on beam 1.

Answer 115

The taller building will have a tendency to cause the snow to drift or accumulate on the shorter building when the two buildings are adjacent. Codes require the structure of the shorter building to account for this phenomenon. This loading tendency lessens as the spacing between the buildings gets larger and is eliminated as a code requrieemnt at about 20'. Note: the right reaction in the sketch below is greater than the left due to additional snow accumulatiuon.

Answer 116

For a simply supported, uniformly loaded beam the maximum moment at mid-span = wl²/8 or WL/8. Maximum shear at each end = wl/2 = WL/2 which is equal to the reaction (R) at each end. Where: w = load per foot of span. W = total uniform load l = length of the span Remember that the maximum shear is in lbs/kips and maximum moment is in lbs-ft. or kip-ft.

Answer 117

The ability of a beam to resist bending forces. It is determined by a material's resistance to internal tension and compression forces. It is a critical factor in the design of any load bearing horizintal structural member that is loaded perpendicular (Normal) to its axis. The drawing shows compression of the top fibers of the beam, and tension of the bottom fibers (with failure). This is positive flexure.

Answer 118

1 - **C - Catenary** Cable 1 has a uniformly distributed load _along_ its length. This type of load configuration would result from weight of the cable itself. This naturally draped shape forms a catenary and is evident in electrical transmission lines, for example. 2 - **B - Parabolic** Cable 2 is uniformly loaded with respect to a horizontal projection, similar to that found in a suspension bridge deck. Note that this parabola is the shape of the moment curve for a uniformly loaded beam. Both curves have similar configuration and a sag ratio of about 1/3 span, where sag is the vertical distance between the lowest point in the cable and the supports. In both curves the supports resist downward force as well as inward thrust. A cable is the structural opposite of an arch. The cable is always in tension while the arch is in compression. Neither system can handle much shear or bednding moment. While these forces will change in magnitude, the sag can be adjusted without changing the load by simply moving the supports closer together or further apart. The steeper the cable (or arch), the larger the vertical component of the reaction. The shallower the cable, the larger the horizontal component of the reaction.

Answer 119

**C - 271.13 in³** Moment is equal to the moment due to the evenly distributed load plus the moment due to the two concentrated loads For evenly distrubted load: M = WL/8 or wl²/8 For the concentrated loads: M = P x a where P = load and a = distance of load from teh supports. (wl²/8) + (P x a) Therefore: (2 x 27²/8)+(40x9) = 545.25 kip-ft The allowable bending stress (F_b) is equal to 24 ksi for members with full lateral support. So the minimum section modulus capable of carrying the moment is **S = M_Max/F_b = (542.5 kip-ft. x 12)/24 ksi = 271.13 in³**

Answer 120

**B - The throat** ## Footnote afc = right isosceles triangle ab is perpendicular to fc, therefore: abc is also a right isosceles triangle. ab (throat) = bc According to the Pythagorian Theorem (ab)² + (bc)² = D² Since ab = bc, then 2(ab)² = D2 or (ab)² = D²/2 Throat = ab = D/(sqrt 2) = .707D

Answer 121

1 - The force in BC = CP = GK = HJ = 0 The method of joints allows us to examine the equilibrium of each joint independently. For each joint ΣF_x = 0 and ΣF_y = 0

Answer 122

**Stirrups** ## Footnote These bent steel reinforcing bars, usually a #3 or #4 bar, are used in reinforced concrete construction. They are open or closed and are placed transversely in beams. They are placed at specified intervals throughout the member, most closely near columns or where diagonal tension is highest. They serve several purposes: 1 - They resist _Shear_ where the stresses are greater than the capability of the concrete. 2 - They resist T_orsional (twisting) moments_ when present. Whenever possible, torsion will be designed out of the structure. 3 - The restrain any compression reinforcing from buckling. 4 - Although their true purpose is structural, they help hold the longitudinal reinforcing in place and maintain proper spacing during concrete placement. Their usage is covered in ACI 318, the usual governing code.

Answer 123

**C - On an axis of symmetry, when one exists** ## Footnote Any axis of symmetry will always pass through the centroid. The other answers could be correct, for example A - W-Shape steel sections B - Rectabgles D - Channels and angles

Answer 124

Eccentricity (e) is the perpendicular distance from the neutral axis of a steel column to the _Line of Action_ (L.O.A.) of the applied load or reaction. The eccentricity can be about the x-x or y-y axis of the column. B1 creates moment about the y-y axis B2 creates moment about the x-x axis Thus, the steel column has to carry combined loading (axial and bending).

Answer 125

In a 3/16" fillet weld, the throat dimension is: (3/16) x 0.707 Since the length of the two welds combined is 4" x 2 = 8" then: 3/16 x 0.707 x 8 = 1.605 in² The stress in the throat: Stress = Load/Area = (10,000)/1.0605in² = 9,429.5 psi

Answer 126

A bolted connection between two structural members that transfers force from one member to the other through frictional stresses between the contact surfaces of the members. No bearing action occurs because the parts are assumed not to slip.

Answer 127

**B - an increase in beam efficiency** ## Footnote Shifting the supports toward the center of the span creates two overhanging ends that balance the load between the supports. This reduces the bending moment and the deflection at midspan and reverses the curvature of the beam near the supports.

Answer 128

For simple solid wood columns, the SR is equal to L/d, where L= the unbraced length (laterally supported) and d is the column's least sectional dimension. The SR is used to determine the maximum allowable unit stress for the column (F_c') for compression parallel to the grain, and is the most critical consideration in column design. The taller the unbraced length (L), the greater the slenderness ratio, the less F_c'. Similarly, the smaller the least dimension (d), the greater the slenderness ratio and less F_c'.

Answer 129

**C - Wall Footing** ## Footnote Generally classified as a shallow bearing foundation. It supports vertical loads and consists of a continuous strip of concrete, typically reinforced longitudinally, and transversally, rectangular in cross-section and placed under a foundation wall. Its continuous nature allows it to distribute the building load over a broad area and span over isolated areas of weaker soil. A keyed joint is commonly used to strengthen the connection between the foundation wall and the footing.

Answer 130

Loads located at a distance (d) from either support that are equal or less than the depth (d) of the beam.

Answer 131

**C - L= 40k, R = 32k** Resolve the 30k load at point C into vertical and horizontal components. C_v = 3/5 x 30k = 18k C_h = 4/5 x 30k = 24k The pin/hinge connection has two reactions, H and L and the roller has one reaction, R. Thus: Without further calculation, we know that L should be larger than R since 30k at point B is greater than 18k at point C. So the only possible answer choice is C. If calculation is necessary, isolate asymmetrical loads from symmentrical loads as shown. The 30k load at point B exceeds the 18k load at point C by 12k, otherwise the load is symmetrical. L = L1 + L2 = 30k + 10k = 40k R = R1 + R2 = 30k + 2k = 32k

Answer 132

**B - Beam Depth** ## Footnote An increase in beam depth will reduce deflection.

Answer 133

A - Keystone B - Voussoir C - Springers Though each wedge-shaped unit in an arch or vault os known as a voussoir, the keystone and springer are further defoned. _Keystone_: The center stone or masonry unit at the apex of an arch. Often decorated, embelished or exaggerated in size, no true arching action occurs until this unit is in place. _Springer_: The lower-most voussoir, located where teh vertical support ends and the curve of the arch begins. During construction, a temporary formwork called _centering_ is used to support the vault or arch until the structure is self-supporting.

Answer 134

**C - 168.75 in³** ## Footnote Moment M = WL/8 or wl²/8 3(30)²/8 = 337.5 kip-ft. If full lateral support of the compression flange is provided, then for A-36 steel. F_b = 24 ksi. To find the minimum _Section Modulus_, S_x = M_Max/F_b = 337.5/24 x 12 = **168.75in³**

Answer 135

**Dimension D** The distance from teh compressive face of the beam to teh centroid of teh steel reinforcing. This is the dimension used in beam design calculations. ----------------------- Dimension A is called cover, and is goverened by building codes . It ranges from 3/4" (slabs) to 3" (foundations) depending on the type of member and its exposure to soil or weather. A higher fire rating can be achieved by increasing cover. Dimension B is the width of the concrete. Dimension T is the location of the Tension Stress Dimension C is the location of the Neutral Axis. Dimension H is the overall depth of the beam. This is the dimension, in addition to the woidth (B), that is used to calculate the weight of a concrete member. In a continuous beam, the top face chnages from compression to tension over the supports. This requires reinforcing at the top of the beam as well.

Answer 136

Since the load is closer to the left end of the beam, the Left reaction is greater than the Right. L = (P x b)/l x 60kips = **42.86 kips** R = (P x a)/l x 60kips = **17.14 kips** _Maximum moment:_ (P x a x b)/l = (60 x 8 x 10)/28 = 342.86 kip-ft. Since the compression flange has full lateral support: F_b = 24ksi F_b = M/S ⇒ S = M/F_b = (342.86 x 12)/24 = **171.43 in3**

Answer 137

A simple beam with a concentrated load at the center of the span. P = the concentrated load L = the length of teh span P/2 =- each reaction L/2 = Shear (v) passes through zero at center span where Bending Moment (M) is at its maximum. To solve for maximum bending moment at mid-span, calculate the sum of the areas in the shear diagram to the left of the mid-span

Answer 138

Since M = PL/4 M = (9 x 12)/4 = 108/4 = 27 kip-ft.

Answer 139

Refers essentially to the connections in a structural framework. These connections are capable of resisting any significant rotation of one member about the other. They include moment connections or moment-resisting connections that transfer bending between the joined members. These frames resist both lateral and vertical loads and are statically indeterminate structures. If the frame exists in a single-plane two-dimensional column-beam construction, it is called a bent. Commonly a welded connection. A rigid frame is a misnomer, since this construction generally produces a relatively flexible internal resistive system compared to the stiffness of a shear wall or a braced frame for example.

Answer 140

The phenomenon where both the axial force of tension or compression, and bending moment occur simultaneously at the same point in a cross-section of a structural member. This condition occurs with eccentrically loaded columns and may lead to P-Delta type failures.

Answer 141

The symbol is used with a horizontal reference line connected to an arrowhead line that points to the location of the weld. The symbol is placed above the reference line if the weld is on the side of the member away from the arrow, and below the line if the weld is on the same side. If it is to be welded on both sides, the symbol is repeated above and below the line. The example indicates a 3/16" near-side fillet weld 2" long and 5" on center to be done in the field.

Answer 142

Calculate MAximum Shear (determines minimum Area), and calculate Maximum Bending MOment (determines minimum Section Modulus). W = wL = 100lbs/ft x 16 ft = 1600 lbs. V_max = W/2 = 1600/2 = 800 lbs. M_max = WL/8 = wl²/8 = (100 x 16²)/8 = 3,200 lbs-ft. Shear F_v = 3V_max/2A ⇒ A_{min =}3V_max/2F_v = (3 x 800)/(2 x 95) = **17.63 in²** Bending F_b = M_max/S ⇒ S_min = M_max/F_b = (3200 x 12)/1500 = **25.6 in³**

Answer 143

**D - Vierendeel** ## Footnote It incorporates no triangulation or diagonal web members but relies solely on the shear and bending resistance of its components and the stiffness of its joints. The joints are fixed and act like a rigid frame in resisting loads. It requires more material than a triangulated member with the same span-to-depth ratio. It is often used for bridges and as deep support beams when clear openings are required in the bays. Under the same load, a shorter truss depth would require larger member sizes and would experience larger member deformations.

Answer 144

**A Needle Beam** ## Footnote It is a temporary cross-beam used to support an existing wall or floor during repair or underpinning work. Needle beams are typically made of timber or steel. One end of the beam is supported at grade. The other end of the beam is supported by a jack. This process is called needling.

Answer 145

The point of inflection or point of contraflexure. It indicates the points of change in curvature of the deflected beam. The points of zero moment or zero bending stress, where the moment diagram changes sign from positive to negative or vice-versa and passes through zero. These points are particulalry significant for reinforced concrete beams since they indicate where the reinforcing needs to be reversed. By definition, positive moment is when the top of a beam is in compression and the bottom is in tension. This happens between the supports, and the rebar will be placed on the tension side (bottom). The negative moment at the supports signifies that the rebar needs to be placed at the top (at the supports the tension is at the top of the beam).

General Structures Flashcards

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