Flashcards in Truss Systems Deck (107):
What distance can modern trusses span without the installation of intermediate load-bearing partition walls?
18 m (60 ft)
How do increased spans affect the interior design of a building?
Greater freedom in design and allow the inclusion of large living spaces without the need for beams, columns or load-bearing partition walls.
What is a trussed rafter?
A pre-assembled unit built to form the shape of a desired roof style.
What is the maximum allowable moisture content for lumber used in the manufacture of trusses at the time of construction?
In the last 50 years, what advance in the design/construction of trusses has had the greatest impact on the development of the truss?
The development of metal connector plates.
What are the two most common sizes of material used in the construction of wood.
How are truss connector plates attached to the truss members?
Pressed or rolled into place by means of large hydraulic presses that exert well in excess of 1000 lb per square inch. Most presses exceed 60 TONs in capacity.
Which truss design is prone to horizontal deflection as well as to vertical deflection?
A roof with a single slope (shed roof) is built using which type of truss?
What are the names of the 2 styles of flat roof trusses?
1. Pratt Design
2. Warren Design
When would you use a combination truss?
A combination truss is used when you want a roof with two different exterior slopes and two different interior slopes.
Why should connector plates not be installed with a hammer?
They do not develop the resistances (solid anchorage) for which they are designed. These resistances can only be achieved when the plates are pressed into the lumber.
What is a girder truss?
A girder truss is comprised of two, three or more trusses laminated together to form a single truss capable of transferring much greater loads than a single truss.
Where can the instructions for the proper lamination of a girder truss be found?
They should be sent out with the shipment and will be included with the truss documentation.
Loads can be divided into two groups. What are they?
Live and Dead
Which type of load is a layer of roofing plywood attached to the trusses?
A dead load
Which type of load is a pile of roofing plywood placed on the roof trusses prior to installation?
A live load
What is a point load?
A concentration of loading in one area of a building or roof. An air conditioning unit would be a point load on a roof.
What are two basic forces that continually act on a truss?
Tension and compression
On what does the compressive strength of a member depend?
It's unsupported length. The longer the unsupported length the less resistance it has to a compressive load.
What is the purpose of the lateral bracing that is installed on the various members of a truss?
It serves to reduce the unsupported length of the members, which increases the compressive strength of those members.
Against what type of forces is wood the weakest?
Against tension forces that are perpendicular to the grain.
Why has the 1/2" maximum bearing gap rule been established for the installation of certain types of trusses?
Research has shown that following this rule helps reduce truss failure due to horizontal shear.
How can the amount of deflection in a particular member be decreased without decreasing the unsupported length of the member?
By decreasing the depth of the member. For example, if the floor in a building deflects too much when a person walks across it, a deeper truss remedies this problem.
Which truss will deflect the most given the same span and loading: a higher pitched truss or a lower pitched truss?
The lower truss will deflect the most.
What can be done to help eliminate the movement in the walls of a building when the truss being installed is prone to horizontal deflection?
Special truss hangers that allow the truss to deflect horizontal and independently of the wall can be installed when trusses prone to deflection can be used.
When is the best time to inspect for missing or damaged trusses?
While they are being in-loaded on-site. Any damage or missing items should be noted on the receiving documents. If this is done at the time of delivery, it is likely that the manufacture will take responsibility.
What type of info is included in the document package with the truss delivery?
Truss design drawings, truss placement and layout drawings, detail drawings for laminating girders, applying "T" braces and any other assembly or installation details. Also hanger installation instructions as well as the WWTAs recommendations for handling, erecting and bracing wood trusses.
When trusses are delivered to the site, they are usually banded together. Is it good practice to use the banding for lifting the pile of trusses off of the truck?
No. They keep the trusses in an orderly pile and is likely to fail if used to lift the bundle of trusses
Why must care be taken when lifting a truss that is laying flat on the ground?
They are very weak laterally and bending the truss while it is laying flat can easily cause damage to the truss at the point of the connector plate.
What is the maximum amount of lateral deflection that can be applied to a truss before it is likely to start causing enough stress at the connector plates to begin weakening the joints?
Lateral deflection greater than 150 mm (6") over 6 m (20') is considered excessive and can cause weakening at the joints.
What is the best position in which to keep a truss while it is being lifted and moved?
A vertical position
When should you consider getting a crane or picker in to hoist your trusses into place rather than lifting the trusses by hand?
When the size and situation make it difficult to place the trusses without placing undue stress on the joints and connectors.
For what size of truss is a single pickup point suitable hoisting?
Trusses twenty feet and under in length.
When lifting a truss over 30' in length it is recommended that two lifting lines be used. What is the max recommended angle between the two lines and why is the recommendation made?
60 degrees in order to minimize the possibility of the truss buckling during the lift from pressure created by the lifting lines.
What type of of equipment is recommended when lifting trusses ranging in length from 30' to 60'?
A spreader bar and slings should be used.
A strong back should be used when lifting large trusses over 60' in length. Where should the strong back be positioned in relation to the truss?
At or slightly above the mid-height point of the truss, to prevent overturning of the truss while it is being lifted.
What are two advantages in using a crane for lifting multiple trusses onto a roof?
Faster and safer
What are 4 types of braces that should be installed when performing multiple truss lifts with a crane?
1. Lateral bottom chord bracing
2. Lateral top chord bracing
3. Diagonal top chord bracing
4. Diagonal centre web bracing
Why should care be taken to limit the amount of extension of braces and lifting beams beyond the ends of the trusses on a multiple truss lift?
So that the braces and/or lifting beams do not interfere with trusses already installed on the roof.
When can you find info regarding appropriate pickup points on a truss when performing a crane lift with multiple trusses?
Why is it critical to ensure that the first truss is solidly anchored and properly braces before setting the subsequent trusses?
Because all subsequent trusses are anchored to it and if it topples, the whole set of trusses will fall like dominoes.
What is the function of top chord bracing?
Transfers loads, keeps the trusses plumb and maintains proper spacing between trusses until sheathing or purlins are installed.
When placing lateral top chord braces, how should they be positioned in relation to the vertical braces?
Directly in line with with the vertical braces to allow forces to be transferred directly to the ground or floor rather than through the final hoed and then down.
What is the max distance recommended between runs of bottom chord bracing?
Where is diagonal bracing require on the bottom chord?
At either end of a building and repeated at a max of 6 m (20') between diagonal braces.
What are the 3 main functions of permanent bracing In a trusses roof system?
1. To decrease the unsupported length of compressive members.
2. To increase the stability of the truss system.
3. To assist the truss system with load transfer.
What are the 3 main types of permanent bracing?
Top chord bracing
Bottom chord bracing
Bracing of the web member plane
How is lateral movement prevented if purlins are installed as opposed to continuous sheathing?
Diagonal bracing should be installed on the underside of the top chords.
Where will permanent web bracing be specified?
On the truss drawings.
Trusses are also available for floor joists and their use is becoming common practice in many areas. T or F
What are all the components for a typical truss?
Bottom chord, tension web, ridge, compression web and top chord.
Metal connector plates replaced wood gussets in the 1950s. T or F
What are the three most common thicknesses for connector plates?
20, 18 and 16 gauge
How much pressure is required to fully press a connector plate into spruce lumber?
1000 lb/sq inch
How much pressure is required to press a connector plate into 3x6 lumber?
8200 kg of force
What are the grades of lumber used in truss fabrication?
Select structural, no. 1, no. 2, machine stress rated (msr) and machine evaluated (Mel)
Scissor trusses are used for buildings with vaulted ceilings. T or F
Which truss is most prone to horizontal and vertical deflection due to their design?
Are special anchors required to anchor the ends of these trusses to the wall to prevent damage to the structure or finishes. T or F
Which truss requires the use of special hangers that allow for horizontal movement, but restrict any vertical moment?
Flat trusses top and bottom chords can be oriented either on flat or on edge. T or F
Where are flat top trusses usually used?
What is a jack truss?
Short trusses used to frame the corners of hip roofs.
When a roof design requires the installation of valley, the trusses upon which the valley trusses will be installed must be _____.
Sheathed. This anchors the tops of the trusses that support the valley trusses and also provides a better working environment. Provision for airflow in the attic space should be considered and an opening should be made at some point in the sheathing to prevent the creation of a dead air space in the roof.
When the dead load to be supported is to be increased, a truss designer must assess the truss's ability to carry the additional load. T or F
The total load from the carrier member is transferred to the girder through the nails and only the nails. T or F
All face nails must be used when attaching hangers in girders. Double shear nailing through the truss into the carrying member is required. T or F
The strap of a hangar must be bent over the header a min of ___ ". A minimum of four top and four face (double shear) nails are to be used.
To prevent failure due to horizontal shear the Truss Plate Institute established a ___ maximum bearing gap rule. This is the gap between the bearing surface (top plate of wall) and the webbing on the truss.
12.5 mm (1/2")
The ability to resist deflection is known as stiffness. T or F
The higher the pitch of the truss, the stiffer the trus and thus the higher pitched truss will deflect less than the lower pitch truss under the same load. T or F
As a ruler of thumb, when installing trusses, there should normally be a web or multiple webs above any bearing wall. T or F
T. The web or webs help transfer loads either away from the bottom or through the bottom chord to the wall. If there are no webs the truss may deflect improperly and possible failure of the bottom could occur.
Trusses are designed with a slight camber so the bottom cord will be flat when the roof dead loads are applied. T or F
Nails should be left slightly loose on the special hangers for scissor trusses. T or F
T. The loose nails allow the hanger to slide on the nail. These hangers allow the truss to slide in and out independent of the wall as the truss deflects.
To calculate the metric values of truss intolerance simply move the decimal place of the truss height two places.
T or F
T. Eg. 1750mm becomes 17.5 mm
The installation tolerances for trusses are an 8th per foot. T or F.
T. Eg. 12" = 1/8"
24" = 1/4th
Trusses that are damaged must have the damage assessed by the truss designer and necessary repairs made before they are installed. T or F
It is normal to have trusses banded together when they are delivered on site. T or F
Do not lift single trusses with spans greater than ___ by the peak.
Trusses that are stored for longer than two weeks must be covered to protect them from elements. T or F
The maximum a truss should bend is 6 inches over 20'. T or F
Smaller trusses, usually 9 m (30') or under, can be lifted into place by hand. How is this done?
Placing ends of truss roughly in position on the wall while the truss is upside down. Then, using a pole or board, the truss is rotated up into position.
For trusses 6m (20') or less, a single person positioned at the centre of the truss can rotate the truss into position without over stressing the connector plates. T or F
It is important to ensure that the truss bearing walls are properly braced against lateral movement prior to walking on the walls or setting trusses in place. T or F
Soft slings should be used for attachment to the truss opposed to cables and/or chains. T or F
T. Cables and chains can cause damage to the members.
When lifting ting trusses, the sling should not be attached to web members. T or F
For spans of ___ or less, a single point may be used to hoist a truss.
6 m (20')
For spans up to ___, use two pick points located so that the distance between them is approximately one-half the length of the truss. The angle between the two cables should be 60 degrees or less.
9 m (30')
A spreader bar and short cable slings should be used to lift trusses of spans between ___ and ____. The slings should be slightly toes-in to keep the truss from folding in half.
9m (30') and 18m (60')
Braces should be applied to the bottom chord on both sides at a spacing of no more than ___.
3 m (10')
When lifting long span trusses or multiple trusses the lifting beams should typically be ___.
Placing the first truss up on the roof of a building requires that you have bracing in place prior to hoisting it up into place. T or f
Temporary bracing on the TOP CHORD plane must have lateral brace within ___ of ridge. And ___ oc max.
3 m oc
What type of truss would be used for a sloped or vaulted ceiling in a residence?
Which truss will allow for a greater thickness of insulation at the roof and exterior wall intersection?
Raised hill truss
All trusses deflect. T or F
Trusses are specifically designed to support which type of loads?
What must be used when lifting large span trusses into place with a crane?
Spreader bar and slings.
What is the max oc spacing for temporary lateral bracing at bottom chord locations?
3.0 m (10')
Temporary bracing keeps the trusses plumb. T or F
A jack truss is used in which type of roof?
In a hip roof
What is a mono truss?
A truss used for single slopes.
What is the weakest strength property of wood?
Tension perpendicular to the grain.
Sizes for lifting beams are decided by the truss designer. T or F
Max distance between runs of bottom chord lateral bracing should be not more than ___.