Compression or Tension Web Configuration—Which is Better?

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Issue #11240 - July 2019 | Page #32
By Glenn Traylor

On a recent Quality Assurance Audit at a large facility, we witnessed a floor truss builder carefully spacing out his floor truss webs in the allotted space available with precision and care. He was very meticulous. I asked him why he was doing that and he explained that his in-house inspector did not like any gaps in the webs. I showed him how he could adjust for this gap by tapping the webs away from the chase, widening the chase thus removing any gap. He then explained that, when he took that action, the truss salesmen would complain that the webs didn’t align. I commended the builder for his care and concern, but the situation revealed the need to discuss more issues during our review.

When we sat down with the production manager, shift supervisor, and several other key individuals to discuss in depth the issues and problems we found, the following issues were in our discussion.

  1. Lumber dimensions will always vary slightly. It’s the nature of wood.
  2. Chord thickness change impacts the web location related to the length.
  3. It’s not reasonable to adjust standard web lengths to suit variations in chord thickness.
  4. Webs are cut with reasonable accuracy but the precision is not sufficient to assume all webs are equal.
  5. Hiding member to member gaps under the connection plate does not resolve the gap issue—it merely hides and obscures the problem.
  6. The Warren design of the System 42 truss accommodates the variability of the floor truss web lengths and the chord thickness variations as long as the chase width is flexible (in other words, that the chase can float).
  7. Gaps in a Warren configuration where the webs are designed in compression (see blue in diagram) [See PDF or View in Full Issue] can cause additional deflection in the floor truss as the truss is put into service.
  8. If the Warren configuration is designed in tension (see orange in diagram) [See PDF or View in Full Issue], this minor gap 0” to the maximum gap 1/8” will not add to deflection, because the connector plate will be designed to transfer the tension load.
  9. Holding chase sizes creates problems when sometimes the request is not justifiable.
  10. Tension web configuration can require a larger connection plate.
  11. Compression web configuration can result in a smaller plate.

Our conclusion: spreading out gaps is never a desirable action. Webs should be pushed until tight towards the ends of the floor trusses rather than spacing out gaps. Chases should be allowed to float to accommodate the natural variations of lumber. If the design requires the fabricator to hold a particular chase size, agree to a minimum size but allow that size to increase.

When using a compression web configuration, it will be important to eliminate member to member gaps in order to help eliminate deflection issues. Tension web configuration is better suited where some gap exists.

 

Glenn Traylor is an independent consultant with almost four decades of experience in the structural building components industry. While he is a TPI 3rd Party In-Plant Quality Assurance Authorized Agent covering the Southeastern United States and performs 3rd party safety auditor services, these articles represent his personal views, knowledge, and experience. 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.

Glenn Traylor

Author: Glenn Traylor

Structural Building Components Industry Consultant

You're reading an article from the July 2019 issue.

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