The live load shall be taken as follows.
Attic truss bottom chord live load.
Trusses w storage on the lower chord most trusses will handle 10 psf live load anywhere in the attic non concurrent with other live loads i e.
The live loads a truss is required to bear are determined by local building codes.
Lately many have also been designing for an alternate load of 20 psf in the rectangle mentioned in the previous post.
20 4 10 means that the truss was designed for 20 psf top chord live load tcll 4 psf top chord dead load tcdl 0 psf bottom chord live load bcll and 10 psf bottom chord dead load bcdl.
Every truss design drawing must specify the loads that have been accounted for in the design.
If the bottom chord of a roof truss functions as a floor joist such as in an attic room it carries a live load that also varies by use but a typical live floor load for a residential space is about 40 pounds per square foot.
The truss shown below is known as an attic truss or a room in attic truss.
If the number next to bcdl bottom chord dead load is less than five psf pounds per square foot then the trusses and the building are not designed to support a ceiling.
Analysis of an attic truss requires multiple load cases with multiple duration factors.
In some cases a live load can be applied to a bottom chord either for storage purposes or if there will be movement within the truss such is the case in an attic truss.
In other words the truss design drawing should tell you what the truss was designed for.
An example of this is a vaulted ceiling application framed by scissors trusses.
This type of truss carries a combination of roof l r or s and floor l loading in addition to the standard top and bottom chord dead loads.
Live loads on roof trusses.
The remaining portions of the joists or truss bottom chords shall be designed for a uniformly distributed concurrent live load of not less than 10 lb ft2 note that in footnote b the uninhabitable attics without storage have a 10 psf non concurrent live load while this same load in footnote g i e.
A the top chord and bottom chord dead load.
In floor truss applications 2x4 or 2x3 material is typically used with the wider portion of the board resting on the bearing or what is known as a 4x2 configuration.
A when access is provided 1500 n m 2 b when access is not provided 750 n m 2 except for maintenance ii sloping roof with slope greater than 10 degrees.
On the truss drawing will be a section which outlines all of the live and dead loads which the trusses are designed to support.
Tcdl truss 1 65 psf 6 6 plf 2 3 3 psf 2 linoleum 1 00 psf per 1 concrete 12 0 psf 8 psf inch 1 5x8 ply 2 40 psf 0 4 1 8 total 17 05 psf tcdl 17 psf bcdl truss 1 65 psf 6 6 plf 2 3 3 psf 2 gypsum 2 00 psf 0 55 1 8.
I for sloping or flat roof with slopes up to and including 10 degrees.
