Frequently Asked Technical Questions
Girders are trusses specially designed to carry extra loads that are a result of the structural framing members they support. Sometimes a single ply girder truss is insufficient to carry the entire load, so the truss designer designs a multiple-ply girder. This is where identical trusses are built and fastened together to act as one unit to support the load. ANSI/TPI 1 states that girder trusses up to three plies thick can be fastened together with nails. Girders over three plies must be pre-drilled and bolted rather than nailed. The maximum number of plies is five for girders that are supporting the extra load from only one side. If the loads are tying into both sides, the maximum number of plies is six. No matter what type of fastener is used, the fastening schedule will appear on the girder truss design drawing. Some connector companies have recently introduced another option for girder truss fastening: a high strength wood screw that does not require pre-drilling. Truss design software vendors have reported that they are working on adding this option to their programs. We do not assemble our multi-ply girder trusses in the plant; the framers are expected to do this in the field. Are there any requirements that would suggest assembling them on the production floor is necessary? I would like to change our company policy but need help backing up my opinion, aside from a series of unhappy framers.
I am a building designer on a salt storage building project. I would like to use metal plate connected wood trusses in the design. What should I specify for the metal connector plates?
Has there been any research or studies on long-term deflection of metal plate connected wood trusses? How much deflection is allowed?
We have designed a custom home that needs a good solid floor, with low vibration perceptibility. The system we have specified consists of 2 in. of gypcrete over 1 and 1/8 in. thick plywood or OSB. The floor trusses are 20 in. deep at 16 in. O.C., spanning 31 ft. 6 in. The preliminary truss design shows a double top and bottom chord with a total load deflection of 1.30 in. (l/290) and a live load deflection of 0.65 in. (l/581). The basement ceiling will be 1/2 in. gypboard nailed to the bottom of the trusses. We are looking for a way to analyze the system to determine the vibration characteristics. We are familiar with analysis methods for steel joists and beam systems. Are there any programs for wood truss floor systems?
Some building designers believe that gable end webs need to be L-braced to 90% of the web length, which sounds fine. However, they spec that scissors gable end webs need to be braced to 100% of the web length. That means that in the field they are notching the braces to fit around top and bottom chords – you can imagine how difficult that can be. Is that necessary or even the intent of the web L-bracing?
Is there a chart available on the allowable amount of deflection on floor trusses?
Are there charts illustrating allowable HVAC duct sizes that can fit between truss chords for various truss depths?
Do you have sound rating information for floor truss assemblies (i.e., STC and IIC ratings)? We are looking specifically for ratings for 12 in. floor trusses, with 1 and 1/2 in. concrete topping and without the topping. We are looking to meet the minimum code rating of STC 45 and IIC 45.
What is the recommended standard spacing for open-web wood joists? I have seen them placed 24 in. O.C. Is this acceptable for a customer who will have an exercise room with 500 pounds of free weights? I understand there are problems with bouncy floors with 24 in. spacing. Is this true?