I am both a truss designer and a firefighter. I find myself constantly defending trusses against the current teachings and layman’s understanding of wood truss performance in a fire.
1. Steel is a better performer than wood. WRONG! Steel can expand 9″ per 100′ length with direct flame impingement. A single steel member failure can be much more significant in causing a structural collapse than a single wood member failure!
2. A plate without nails through it is weaker than a plated and nailed joint. WRONG! We know that some manufacturers use nails to tack on plates in a pit manufacturing set up. Some jig table with rollers don’t use any nails. I would venture a guess that a nail-thru-plate joint is actually slightly weaker as there would be less tooth penetration at the nail and less wood density (very minimal less but not stronger.)
Improvements to fire rating in the wood truss design:
If there is a builder/customer/architect/designer who wants to make a better fire performance wood truss design, here are a few tips that work:
1. Use two layers of drywall as this will increase the time before the ceiling is penetrated.
2. Add a second bottom chord under the structural one and plate. This will double the thickness of the bottom chord and allow the first chord to be burned through without affecting structural integrity of the truss.
3. Use a fire retardant treatment on the trusses. To keep cost down, just use it on the bottom chords as an option.
4. All girder trusses fall over interior walls so truss to truss connections are protected by the interior wall.
5. Don’t use girders in the design at all.
6. Engineer the trusses at 12″ on center but set at 24″ on center to allow for structural over-design which will allow more flame impingement before failure.
We need to be diligent in spreading the word and dispelling untruths about wood trusses and fires!