Do You Understand Deflection and Deflection Differential?

Do you understand deflection and deflection differential — and how your truss systems and, more importantly, your clients’ perceptions will be affected? Deflection has been around for an exceptionally long time and always needs to be considered and addressed. To do so, however, we need some understanding as to why deflection causes a problem, and we need to understand that deflection is always going to be a metric involved in any design.

Probably the most common after-delivery problem a component fabricator encounters is a customer complaint that is either directly or indirectly related to deflection. Most often, this is related to floors, but other instances occur too, in particular with attic trusses. It’s very important to keep in mind this adage: “The customer’s perception is the primary measure of performance.” However, this perspective often does not consider the actual conditions within the floor system. Frequently, complaints may arise that are not based on comprehensive or accurate information.

Deflection Facts to Consider

1. Deflection cannot be prevented.
2. Deflection is not always caused by the component itself but is a factor in the overall system.
3. Deflection is not a problem if the design can accommodate for that deflection — see my article from July 2024, “What is Camber and Why is It Important?”
4. Deflection issues can be mitigated by adjusting the truss configuration — see my July 2019 article, “Compression or Tension Web Configuration–Which is Better?”
5. Utilizing camber, deflection can be reduced. This needs to be incorporated into the actual assembly of the component.
6. The longer the member, the more the deflection is going to be (see figure). [For images, See PDF or View in Full Issue.]
7. Deflection tolerance is dependent on the type and class of the structure.
8. Deflection issues contribute to “Truss Partition Separation” and this phenomenon is often attributed (sometimes incorrectly) to problems with deflection — see “The Enduring Problem of Truss Partition Separation” by Frank Woeste, P.E. from November 2020.
9. Deflection issues can be a result of less effective drywall installation techniques that ignore the need for movement.
10. Deflection issues can sometimes be a result of improper loading calculations — see “Errors in Substituting Dead Load for Live Load in Wood Design” by Scott Coffman, P.E., October 2018.
11. Deflection issues can also be caused by improper use of the structure. Scott Coffman’s article includes circumstances of cabinets stuffed full of heavy materials and situations in student housing. As a student at NCSU, I recall having a king size waterbed in the dorm. (It was the 1970s and not uncommon back then, so no judgement, please.) Hopefully, building designers of college dorms expect some idiosyncratic arrangements.
12. Per Scott Coffman’s article, localized deflection of floor systems is the primary serviceability complaint of homeownership.
13. Flooring elements are going to behave differently based on variations of dead loads.
14. Creep should be considered during design, as it refers to the long-term effect of dead loads that can become more pronounced as the building ages.
15. Gaps in fabrication can contribute to deflection issues — see my article “Why is the Gaps Limitation So Important to Truss Fabrication?” from August 2023.
16. Even when in compliance with ANSI/TPI 1, gaps can lead to long-term creep issues.
17. Bad installation habits (often attributed to lack of understanding) will reduce the positive impact that bracing, strongbacks, and sheathing nailing schedules provide to ensure the component acts as a system — see my June 2023 article, “Are You Providing Enough Information to Avert Failures?”
18. A floor truss can have deflection variations within the component. For example, the chase can present more flex than other parts of the truss — see last month’s article by MiTek, “Deflection Across the Chase in a Floor Truss.”
    • Floor performance is greatly impacted by the variation of lengths of floor elements and the materials they use. The figure on the first page illustrates how stepping from the longer green element to the shorter blue element and then to the rigid wall support is going to result in a different user experience for each step because of what is under the floor. Mitigating these situations depends greatly on the incorporation of strongbacks and internal blocking and bracing. The installation of premium sheathing materials is a potential recommendation based on the customer’s needs and expectations.
    • Camber should be introduced and incorporated in the design to counter the impact of permanent loads and live loads.
    • Custom kitchen islands with granite, tall partition walls, and added loads not considered during component design must be addressed.
    • Careful manufacturing procedures can mitigate deflection problems.
    • Consider including an additional top chord element stacking longer panel areas in components that should have tighter restrictions.
    • Promote the adoption of SBCA’s BCSI usage and tagging procedures to facilitate instruction for erection contractors.

The Bottom Line

Don’t roll over when you are confronted with complaints regarding deflection. Take this opportunity to help your client understand the nature of the product and their responsibilities of installation. But, even before you submit a proposal, consider all of the ways your team can minimize the negative effects of this very vulnerable issue.

An ANSI/TPI 1 3rd Party Quality Assurance Authorized Agent covering the Southeastern United States, Glenn Traylor is an independent consultant with over 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, expert witness and ANSI/TPI 1 compliance assessments. Glenn provides new plant and retrofit designs, equipment evaluations, ROI, capacity analysis, and CPM analysis.