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Optimal floor truss depths

Optimal floor truss depths

Optimal floor truss depths

(OP)
For standard residential and office loading (40 and 50 psf respectively) I have always been told that 1" depth per foot of span is a good economical depth for discussion purposes. Obviously they can be shallower than that but it might start to cost more per truss. I've got some MiTek tables that validate this answer to a certain degree.

My question is, does this rough rule of thumb apply to 100 psf loading as well? If not, is there some sort of rough rule any of you go by?

RE: Optimal floor truss depths

my first rule of thumb: deeper is cheaper.
and I would expect economical depth/spacing/span solution for 40/50psf floor system to be different from a 100psf floor system

RE: Optimal floor truss depths

(OP)
That was my thought as well. I didn't think it would need think be twice the depth for twice the loading but something like 25% deeper would be about right.

Thanks guys.

RE: Optimal floor truss depths

in my mind, truss depth follows same principle as bd^2/6 and bh^3/12. a little increase in d gives a lot more increase in moment capacity and deflection capacity

RE: Optimal floor truss depths

For residential floors I like to see a minimum ratio of about 18 to one. i.e. a maximum 18' span for a floor member that's 1' deep. Any more than that, and you get into problems with vibration/bounce.

The "deeper is cheaper" idea has limited merit. Sometimes you can increase the depth of a floor truss and use lower lumber grades to get the same span.

But - Truss manufacturers tend to view deeper trusses as more valuable. So for instance they may price a 16" deep floor truss higher than a 14" deep one even though it costs them about the same to build each one.

I've fund that floor pricing varies a great deal from one manufacturer to the next, and also from one region to another. So it might pay to talk to a local manufacturer.

RE: Optimal floor truss depths

Truss chord axial stress varies linearly with depth. And truss stiffnes varies with the square of depth. I'll go with the assumption that stiffness will govern design. Otherwise, you would want to double the depth. So, for equal stiffness but double the load, you'd want to increase the truss depth by 41%.

The analysis above assumes that all one is concerned with is the cost of the trusses. When considering the cost of the buildang as a whole, the calculus changes (building height etc). Then, it usually pays to make your 100psf trusses work a little harder than your 50 PSF trusses. You get into MSR, double chord, and reduced spacings. When I worked as truss designer, this was how it would typically play out for buildings where the 100 PSF occupancy only occurred in local areas. We wouldn't up the depth at all. If the majority of the building will be 100 PSF that's another story of course.

One nice thing about 100 PSF spaces is that they never see the load and, as a result, you never get call backs about those areas if they're properly designed. I'd be comfortable designing most 100 PSF spaces for deflection at 50 PSF loading.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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