Location Of The Metal Connector Plates
Question:
I was working on a job that had trusses that span more than 78 ft. with a 9/12 pitch. We had the #5 truss being installed when the #4 truss gave way. The trusses that were being installed are a two part system. The bottom set is what you would call hip trusses. The trusses contained 2x6 bottom chords, 2x8 top chords and rafters, and 2x4 webs. The roof sheathing was to be 3/4 in. plywood, 15 lb. felt paper and roof shingles. The ceiling was to be 5/8 in. drywall. The #4 truss sheared about 1 in. off the wall. A knot was in that location on the bottom chord and both bottom chord and rafter were splintered apart into strands of wood. Also, the roof rafter on the bottom side (the edge that touches the top edge of the bottom chord) contains wane. Would this truss be considered ineffective? If not, why? How does one choose the size and location of these plates?
Answer:
One cannot determine by sight whether a truss is ineffective. One would have to look at the original QC output to determine how many effective teeth are needed in each member. This output is available from the original truss design software.
The fact that wane was present on the top chord does not necessarily mean that the joint is ineffective. One chooses the size and location of the truss plates by first determining the load that the truss needs to distribute. Once the load is determined by the building designer and given to the truss designer, the truss designer can begin to size the plates and the lumber species according to the design load. The location of the truss plate is based on the design load and conditions. Teeth placed in knots, bark, pitch pockets, holes, and joint gaps are considered ineffective.
Gaps between wood members cannot exceed the tolerances listed in section 3.7.6 of the Truss Plate Institute's ANSI/TPI 1. These maximum gaps range from 1/16 in. to 1/4 in. depending on where the joints are located (i.e., heel, splice, compression, tension).
According to ANSI/TPI 1, splits in any wood member caused by metal connector plate teeth or the manufacturing process shall not exceed those permitted in the grade and species of the lumber used and specified in the Truss design drawing.
One question that might be asked: How was the temporary bracing designed for these 78 ft. trusses? Was an engineer experienced in large truss erection involved? The industry recommendations for handling, installing and bracing trusses specify that the recommendations are for trusses that span up to 60 ft. When dealing with trusses with spans larger than 60 ft., complex temporary bracing is required and a professional engineer should be contacted.