Reinforcing wood joists with steel strapping - reprise

12 years ago

In the thread on this forum "Reinforceing floor ..." started by "Galroc,"


respondent "brickeyee" provided considerable information on the approach of adding steel strapping to the underneath of the joist. The steel strapping approach argument that the steel will resist stretching is unarguable, but joist stiffness depends on whether composite action succeeds, or not.

I was enthused about this approach for a problem with my own house until I came across a paper by the Corps of Engineers titled "Evaluation of Expedient Techniques for Strengthening Floor Joist Systems in Residential Dwellings." (Available from NTIS as ADA013987.)

In this paper, experiments with various reinforced joist sections were performed, directed toward strengthening floors to resist nuclear blast effects by testing reinforcement approaches to failure. That isn't my goal, but some aspects of the steel strapping results caught my attention.

Three of the 15 test samples were steel strapped underneath. The steel straps were 1.5-inch by 1/8-inch, and were nailed to the joists with 16p nails 6-inches on center. The straps extended the entire length of the 16-foot joists, which were supported so that there was 14 feet of span.

The moment of inertia of the three 2 x 10 southern yellow pine joists used for a test section was calculated to be 296.8 inches^4. That for the steel reinforced joists was calculated to be 512.5 inches^4. However, while the measured moment of inertia for the unreinforced joists was 300.4 inches^4, very close to the calculated value, that for the steel strap reinforced joists was only 324.4 inches^4, a very modest increase for a lot of work. The inferred conclusion was that the performace was well below that expected from composite action. This conclusion is supported by the deflection vs. load curves that were published.

I am unable to see any obvious material difference between the experimental configuration and the description in answer to Galroc, although nail hole diameter in the strapping wasn't specified and may be key. So, has anyone on this forum tried the steel strap method and observed, even qualitatively, a significant reduction in sag or an apparent significant increase in floor stiffness?


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