Wood Beams

Question 1

What type of stress is produced in a wood beam where it rests on the support?

Answer 1

Compression perpendicular to graining (bearing)

Question 2

Glue laminated lumber (glulam)

Answer 2

structural engineered wood product comprising a number of layers of dimensioned lumber bonded together with durable, moisture-resistant structural adhesive

Question 3

Where does the maximum shearing stress occur in a rectangular beam section?

Answer 3

At neutral axis

Question 4

How does the increase in the slope of grain along a wood member’s length affect its strength?

Answer 4

An increase in slope decreases strength.

Question 5

Because of the effect of grain, wood beams have lower allowable stresses in ______ ?

Answer 5

shear (longitudinal)

Question 6

In visually graded lumber, what is the meaning of species?

Answer 6

“Species” actually refers to a group of wood species having similar mechanical
properties.

Question 7

Give some examples of factors which affect visual wood grade.

Answer 7

Splits, knots, grain, and warping

Question 8

Machine Evaluated Lumber (MEL) uses x-ray technology t measure what?

Answer 8

wood density

Question 9

Machine Stress Rated (MSR) lumber uses pressure rollers to measure what?

Answer 9

E modulus, by measuring load and deflection

Question 10

What is a typical application for MSR lumber?

Answer 10

Glulam timbers.

Question 11

What is the difference in meaning of a 2x8 vs. an 8x2?

Answer 11

orientation - 2x8 would be edgewise and 8x2 is flatwise orientation.

Question 12

What is the smallest timber size?

Answer 12

5x5

Question 13

Post & Timber and Beam & Stringer are size designations of what wood categories?

Answer 13

timbers

Question 14

What are the three size categories of sawn lumber?

Answer 14

boards, dimensioned lumber, and timbers

Question 15

What is the meaning of S4S?

Answer 15

surfaced four sides, finished or dressed lumber

Question 16

Which of these wood defects is NOT due to curing: split, check, shake?

Answer 16

shake

Question 17

Moisture content in a living tree can be as high as what % of the dry wood by weight?

Answer 17

200%

Question 18

At what point of drying will wood begin to shrink

Answer 18

when the moisture content becomes less that the fiber saturation point

Question 19

What’s the oldest “engineered wood product” used in the US?

Answer 19

plywood

Question 20

list examples of engineered wood products

Answer 20

plywood, glulam, oriented strand board (OSB) I-Joist, Laminated veneer lumber, parallel strand lumber (PSL)

Question 21

Is determining the moment capacity of a beam a design procedure or analysis?

Answer 21

analysis

Question 22

In finding the moment capacity of a beam, what is the actual stress set equal to?

Answer 22

the actual stress is set equal to the factored allowable stress

Question 23

which of the following grading stamps represents the strongest wood ( 10:2, 11:1, 12:3)

Answer 23

B. 11:1

Question 24

Which procedure is more likely to require iteration (cycling through the problem again) - analysis or design?

Answer 24

design

Question 25

after a bending member has been chosen to resist shear and moment based on strength, what serviceability criteria should be checked?

Answer 25

deflection and end bearing

Question 26

what is a reasonable range for the aspect ratio b/d of a wood member?

Answer 26

1:3 to about 1:7. This is roughly a 2x6 to a 2x14

Question 27

what factor limits the practical use of beams beyond an aspect ratio of 1:7?

Answer 27

lateral stability or lateral torsional buckling - they flip out sideways

Question 28

in beam design, what value is generally used to select a section size

Answer 28

the section modulus Sx

Question 29

under what conditions is shear likely to control beam design

Answer 29

shear is more critical in short spans with heavy loads

Question 30

how does shear being critical impact the section dimension

Answer 30

the width b should generally be increased, which gives a thicker section with more area and lower aspect ratio in the range of 1:1 to 1:2

Question 31

what code is used in the united states for the design of wood structures

Answer 31

the national design specification for wood construction (NDS)

Question 32

what design method is used in the NDS for the analysis and design of wood

Answer 32

both allowable stress design (ASD) and load and resistance factor design (LRFD) are covered in the NDS

Question 33

what is the main difference between ASD and LRFD

Answer 33

the main difference is how the factor(s) of safety are applied

Question 34

In determining the allowable bending stress values with the NDS, what type of factors are used?

Answer 34

In the NDS tabulated values are multiplied by “adjustment factors” which adjust the stress for a particular application. ``` Cd - load duration factor Cm - moisture factor Cl - Beam stability factor Cf - size factor Cfu - Flat use factor Cr - repetitive member factor Ci - Incising Factor ```

Question 35

Based on values used for the load duration factor, will wood carry a higher stress for short term loads (eg wind loads) or long term loads (eg occupancy loads)

Answer 35

wood can carry a higher level of load for a short duration

Question 36

What are the 3 methods of grading wood members?

Answer 36

visual grading, machine evaluated lumber, machine stress rated

Question 37

For what cross sectional shape is 1.5V/A = VQ/lb?

Answer 37

1.5V/A applies to rectangular sections

Question 38

For dimensioned lumber sections, are the actual dimensions larger or smaller than the nominal dimensions

Answer 38

the actual dimensions are always a little smaller due to shrinkage and surfacing

Question 39

in the design of structural members we generally refer to the three S's as design criteria. what are the 3 criteria for design

Answer 39

strength, stability, and serviceability

Question 40

what adjustment factor accounts for stability in wood beams

Answer 40

Cl stability factor

Question 41

which is more critical in the design of wood beams: horizontal shear or vertical shear

Answer 41

horizontal shear (parallel to the grain)

Question 42

what consideration can be made regarding shear loads in the vicinity of a support

Answer 42

Loads may be neglected within the d distance of a support

Question 43

In a safe beam analyzed using ASD, which will be larger - the allowable bending stress F'b or the actual bending stress fb

Answer 43

the allowable stress F'b is greater

Question 44

for which orientation are beams inherently more stable, edgewise or flatwise

Answer 44

flatwise

Question 45

which axis designation is used for the stronger (stiffer) axis

Answer 45

x-x

Question 46

for a typical wood beam, what approach is used in design regarding shear and moment

Answer 46

one typically designs for moment and checks the shear is safe

Question 47

which of the following width to depth ratios represents a well proportioned cross section for a laminated timber beam

Answer 47

1:4

Question 48

true or false: bearing stress should taken into account in the design of wood beams

Answer 48

true

Question 49

list defects that weaken the strength of wood

Answer 49

checks, splits, and shakes

Question 50

the formula used for shear stress in wood sections is 1.5V/A. this is ______.

Answer 50

exactly equal to VQ/lb

Question 51

when calculating maximum flexure stress with fb = M*c/I in rectangular wood beams, what value is used for c

Answer 51

c = d/2

Question 52

How are the basic tabulated stress values for wood listed

Answer 52

tabulated stress values in NDS are listed by species, grade, and size classification

Question 53

What are checks?

Answer 53

Checks are separation of the wood occurring across or through the rings of growth, usually as a result of seasoning

Question 54

What are splits?

Answer 54

Splits are separation of the wood through due to the tearing apart of wood cells

Question 55

What are shakes?

Answer 55

Shakes are lengthwise separation of the wood along the grain, usually occurring between or through the rings of annual growth. it is a naturally occurring defect