The Advertiser - June 2017 | Page #30
Compliance with ANSI/TPI 1–2014 requires maintaining member to member gaps at less than 1/8 inch. An exception would be for floor truss chord splices where the limit is 1/16 inch.
While building trusses, the code calls for tight joints. In a roof truss where member to member contact exists, the joint calculation for design assumes one member is hard against the other members. In a compression situation, the force will compress the area consuming the gap. When this happens, it creates a change which effectively reduces the member length and changes the resultant geometry. Often this will create additional deflection that was not considered in the original design.
Most readers are aware of the issues with member to member gap, but as a refresher it might be important to have a review. Recently, an evaluation was made of a fabricator via a video conference. In the review, the following truss was inspected. Figure 1 shows the actual photo [See PDF or View in Full Issue]. The truss was passed, and it was communicated to the manager that the truss looked great. Subsequently, a field audit was made the next day on-site. The trusses were re-inspected (Figure 2) [See PDF or View in Full Issue]. Do you see the problem?
While it might not have been apparent to the person looking at the photograph, the truss pitch transition joint needed a more thorough investigation. That on-site investigation led to the following findings.
The trusses had member to member gaps that exceeded 1/8”. Can you see the problem now? At another joint, the same issues are a little easier to identify from a fresh vantage point. In both cases, the trusses (while they were being fabricated) had modifications to the design that are not allowed per the ANSI/TPI 1 Standard. In both cases, truss builders inserted a wooden wedge at the joint to close the member to member gap. Their intention was admirable. It was great that the workers realized gap was a problem! The solution, however, was wrong. Per ANSI/TPI 1 the proper method to facilitate this gap would have been to:
- Recut the material if the material was mis-cut.
- Re-adjust the jigging if the profile was out of dimension.
- Shim the joint per ANSI/TPI 1 section 3.7.6.3:
3.7.6.3 Correction Procedure.
Correction procedures for joints with gaps exceeding these tolerances shall require shimming, unless otherwise specified by a Truss Designer. Shims shall be of galvanized metal, or alternatives approved by a Truss Designer, to obtain firm bearing between members. Metal shims shall be at least ¾ in. (19 mm) wide and long enough to bend over at least 1 in. (25 mm) along the member being shimmed. The metal shim shall be fixed in position with a deformed–shank (i.e., ring- or screw-shank) 6d nail (0.120 in. (3 mm) diameter and 2 in. (5 mm) long), or other fastener capable of resisting withdrawal, to prevent loss or accidental removal (see Figure 3.7-5).
Using this procedure, the shim should be a galvanized metal or alternatives approved by a Truss Designer. (The truss designer in this situation was never consulted.) The shim would never have been adequate due to its fragility and compressibility. Due to its size, the overall lumber values are zero.
Should the workers have resorted to shimming? This is an important consideration. Shimming should only be the last recourse. When re-cutting or re-jigging is not reasonable, this may be the only option. Remember, your customer is paying for “store bought” trusses, not cobbled together, “we will get it right eventually” trusses. Yes, it’s true that some clients might not even notice or realize the issue, but for those who do, you will be losing a potential future client forever.
An ANSI/TPI 1 3rd Party Quality Assurance Authorized Agent covering the Southeastern United States, Glenn Traylor is an independent consultant with almost four decades of experience in the structural building components industry. Glenn serves as a trainer-evaluator-auditor covering sales, design, PM, QA, customer service, and production elements of the truss industry. He also provides project management specifically pertaining to structural building components, including on-site inspections and ANSI/TPI 1 compliance assessments. Glenn provides new plant and retrofit designs, equipment evaluations, ROI, capacity analysis, and CPM analysis.