TRIFORCE® Open Joist Installation: Use Your “Floor System Logic”

Out in the field, I occasionally run into situations that could have been avoided if workers had a clearer idea of fundamental floor system logic.

If you have the basic principles of loads in your head it helps you steer clear of mistakes that result in annoyingly bouncy floors, complaints, repairs and even tear outs and reworks. So let’s talk about loads:

Bearing loads

A floor system, basically consisting of horizontal members (joists) covered by wood sheets has 3 kinds of bearing loads:

  • Live loads, which are temporary and will vary depending on the buildings function (office, home or industrial).
  • Dead loads, which are the permanent weight of the building materials (structure, MEP equipment, ceiling finish etc.).
  • Special loads, which are also permanent and part of the design, but are concentrated in specific areas of the floor system. Additional blocking or reinforcement will usually be found in these areas.

Joists (any joist) bear these loads through the compression of the top chord and tension in the bottom chord. TRIFORCE® open joist design accommodates this load with a top and a bottom chord specifically designed for this purpose. This is why it is important to always check indications/stamps on the joists to make sure they go in right side up.

Compromised load-bearing

Problems arise when the joist’s load-bearing capabilities are compromised.  Here is an example:

Triforce_Floor_system_logic

Compression pressing down on the top chord has nothing to stop it at the location of the hole. This gives the floor more opportunity for movement. It also puts more stress on the bottom chord as well.

Furthermore, this is below a bathroom, which will likely have ceramic tile flooring and receive daily traffic. See the prospects of cracked tiles and possible plumbing fixture leakage and complaints?

Repair detail

Here is our repair detail for such a problem.

Triforce_repair_detail_for_a_cut_top_chord

Strongbacks

Here is some more floor system logic. One of the least expensive and effective ways to reinforce a floor’s overall resistance to load and reduce floor vibration is the application of strongbacks. They provide uniform load sharing through continuous ties across the floor system. Here is an example of how not to install them.

Triforce_Angled Strongback_mistake

Proper strongback installation

What’s wrong with this picture? Think again about compression and tension. You can see that this wood beam can’t provide stiffness or support lying on its side. Also, there’s no connection to the top and bottom chords. It has to be, to help counter compression and tension across the floor system. Here’s how we recommended fastening the strongback to the joists.

Triforce_proper_strongback_install

The beam’s vertical position gives it strength and through the use of the brace attached to the top and bottom chord, it is properly countering tension and compression. This reduces floor deflection and vibration, which not only helps increase perceived comfort, but also reduces floor system wear and tear over the long term. Keep in mind that it is best to fasten the strongback at center span.

See our installation guide

If you’d like to brush up on your floor system logic, I recommend you download the Triforce installation guide. Let us know if you have any questions.

See you out there on the site.

Triforce Spec Guide

3 Responses

  1. Tom

    The adding of strong backs to support and reduce vibration in longer open truss is well worth the little extra time involved in using the method you illustrate. I did it the other way and my ceiling drywall finished tape joints cracked the length of the factory beveled edge. I’m planning to make the repairs soon and your input will have saved me some potential grief! Many thanks.

    Please keep up the good work.
    Tom

    • Eric

      Thanks very much Tom. Glad it could save you some work!
      Eric

  2. Just a reminder that strongbacks need to be inserted as the trusses are being placed as you will most likely not have the room to slide it in later.

    Also thanks Eric for all your excellent help on my project. I can already feel that my floor diaphragm is solid and I have not even attached the strongbacks yet.

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