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January 2025 #17306 Page #112
Using the Roof Live Load Reduction By MiTek Staff
onsider a roof with a live load of 20 psf (pounds per square foot). This load is intended to account for construction loading (workers, materials, and equipment placed on the roof during construction or maintenance). If the roof has a large surface area, like a post-frame building, is it really likely that the entire roof area would be loaded with 20 psf of load at the same time? If we could design with a smaller roof live load, we could build more efficient and cost-effective trusses.
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Because a large roof area will likely never experience the full design load across its entire area, the roof live load may be reduced using the ASCE live load reduction. ASCE 7, Minimum Design Loads for Buildings and Structures, is a standard published by the American Society of Civil Engineers. This document, representing a consensus of engineering judgment, provides the minimum load requirements that Truss Designers, Truss Engineers, and Building Designers must use when designing truss components. One of the provisions of ASCE 7 is the uniform roof live load reduction, which allows a Truss Designer to use a smaller roof live load when designing a roof truss with a large tributary area. Tributary area is the area that a particular truss (or other structural component) is responsible for supporting. A common roof truss, spaced at 2 ft. on-center, with a 40 ft. span, has a tributary area of 2 * 40 = 80 ft2, while a 7 ft. jack truss has a tributary area of only 14 ft2. A 60 ft. pole barn truss, spaced at 6 ft. on-center, has a tributary area of 360 ft2.
Calculating the ASCE Roof Live Load Reduction The formula for finding the reduced roof live load is fairly simple. The nominal roof live load, Lo, is multiplied by two reduction factors, R1 (based on the tributary area of the truss) and R2 (based on the roof slope). The result is Lr, the reduced roof live load. Lr = LoR1R2
where 12 ≤ Lr ≤ 20 psf
Lr = reduced roof live load per ft2 of horizontal projection supported by the truss. Lo = unreduced design roof live load per ft2 of horizontal projection supported by the truss. 1 for AT ≤ 200 ft2 R1 =
1.2 – 0.001 AT
for 200 ft2 < AT < 600 ft2
0.6 for AT ≥ 600 ft2 AT = tributary area in ft2 supported by the truss.
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