Roof Truss Bottom Chord Live Load

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.

Roof truss bottom chord live load.

Assume that the truss is spaced at 2 foot centers and has double 4x2 top and bottom chords. Purlins are beams of light sections spanning between trusses carrying dead load of roof live load and wind load. The area of the attic where storage loads are not applied is listed as a concurrent 10 psf live load. Flat the most economical flat truss for a roof is provided when the depth of the truss in inches is approximately equal to 7 of the span in inches.

2319 17 2 1 3 roof trusses shall be designed for a minimum live load of 30 psf 1436 pa a minimum dead load of 15 psf 718 pa on the top chord and a minimum dead load of 10 psf 479 pa on the bottom chord. In other words the truss design drawing should tell you what the truss was designed for. Most economical when the difference in slope between the top and bottom chords is at least 3 12 or the bottom chord pitch is no more than half the top chord pitch. 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.

The international building codes have a provision to add 20 psf of live load onto the bottom chord of trusses where a box 42 tall and 24 in width would fit between the top and bottom chords of prefabricated roof trusses in areas where a truss web would not interfere. 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. Assume a 1 0 pounds per square foot ceiling load for a c ducts plumbing and wiring. 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.

Generally the purlins are so spaced that they are supported over the top chord joints of the truss. 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. 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. And wind loads per chapter 16 of this code.