Deflection Of Long Span Scissors Trusses
Question:
I was wondering if you have any information or anecdotes relating to the deflection performance of long span scissors trusses? We recently sold a job with 70 ft. long scissors trusses. The customer is in the process of installing the trusses and doesn't like the deflections he is seeing. To make matters worse, the scissors trusses are framing into a valley set (which we provided) that is erected on 35 ft. Howe trusses. The Howe trusses are exhibiting very little deflection. Any ideas?
Answer:
Since the trusses are currently being installed, first check the installation bracing. With 70 ft. scissors trusses, the code (IBC 2303.4.1.3) requires that the contractor enlist the services of a registered professional engineer to design the temporary bracing system. This requirement applies anytime trusses span 60 ft. or more. If the trusses are buckling due to inadequate bracing, this will certainly have an adverse effect on their deflection performance.
If the trusses are sheathed and plumb, the contractor should check to see how much lateral movement is occurring at the top of the bearing wall. The vertical loads applied to the scissors truss create a horizontal thrust at the bearing. This thrust may push out the wall if the truss is not allowed to slip or the wall is not stiff enough to offer resistance. This horizontal movement translates into a vertical deflection, which is most visible at the ridgeline. If the outside wall is a long clear run with no perpendicular walls to take out load and eliminate the movement, both the horizontal and vertical deflection could be noticeable.
Vertical deflection criteria for a Roof Truss supporting drywall is l/180 for total load and l/240 for live load, from section 7.6.1 of the Truss Plate Institute's ANSI/TPI 1 and IBC 1604.3. For a 35 ft. truss that would allow 2 5/16” total load deflection and 1 ¾” live load deflection. On the 70 ft. truss, the limit is 4 5/8” total load deflection and 3 1/2” live load deflection, unless the building designer specifies more restrictive criteria. The building designer is responsible for designing the supporting structure and truss-to-wall connection accordingly.
Horizontal deflection (section 7.6.3 of the Truss Plate Institute's ANSI/TPI 1) is limited to 1 and 1/4 in. due to total load and 3/4 in. due to live load.
Theoretically, if the top of the wall is restrained laterally, and the trusses are rigidly fixed to it (preventing slip), the vertical deflection due to horizontal movement will be minimal. The trusses will need to be re-designed as pinned-pinned and checked for stress changes due to the horizontal thrust. If this can be accomplished (much easier said than done) the deflection performance will be enhanced. For additional information on designing connections for horizontal deflection, see this article by Charles C. Hover, Jr., P.E. at Alpine Engineering.
This example helps illustrate the importance of having the building designer specify deflection limits in contract documents. Addressing this issue beforehand would certainly help reduce the chance of future problems. Also, if the building designer had specified the handling, erection and permanent bracing requirements in the contract documents, this situation may have been avoided.