Roof Truss Bottom Chord Live Load

Purlins transmit these loads to the trusses.

Roof truss bottom chord live load.

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. 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 ceiling attached to the bottom chord is inch drywall and there is 6 inches of fiberglass insulation above the ceiling use 0 045 psf for insulation.

Assume that the truss is spaced at 2 foot centers and has double 4x2 top and bottom chords. 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. 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. 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.

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. 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. 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. 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.

Generally the purlins are so spaced that they are supported over the top chord joints of the truss. Assume a 1 0 pounds per square foot ceiling load for a c ducts plumbing and wiring. On the truss drawing will be a section which outlines all of the live and dead loads which the trusses are designed to support. And wind loads per chapter 16 of this code.