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Wednesday, 7 February 2018

DEAD LOADS and LIVE LOADS

Dead loads are due to the weight of all materials that constitute a structural member. This also includes the weight of fixed equipment that are built into the structure, such as piping, ducts, air conditioning, and heating equipment. The specific or unit weights of materials are available from different sources. Dead loads are, however, expressed in terms of uniform loads on a unit area (e.g., pounds per square foot). The weights are converted to dead loads taking into account the tributary area of a member. For example, a beam section weighting 4.5 lb/ft. when spaced 16 in. (1.33 ft.) on center will have a uniform dead load of 4.5/1.33 = 3.38 psf. If the same beam section is spaced 18 in. (1.5 ft.) on center, the uniform dead load will be 4.5/1.5 = 3.5 psf. The spacing of a beam section may not be known to begin with, as this might be an objective of the design. Moreover, the estimation of dead load of a member requires knowledge as to what items and materials constitute that member. For example, a wood roof comprises roof covering, sheathing, framing, insulation, and ceiling. It is expeditious to assume a reasonable dead load for a structural member, only to be revised when found grossly out of order. The dead load of a building of light frame construction is about 10 lb/ft.2 for a flooring or roofing system without plastered ceilings and 20 lb/ft.2 with plastered ceilings. For a concrete flooring system, each 1 in. thick slab has a uniform load of about 12 psf; this is 36 psf for a 3 in. slab. To this, at least 10 psf should be added for the supporting system. Dead loads are gravity forces that act vertically downward. On a sloped roof, the dead load acts over the entire inclined length of the member.




Example 2.1 The framing of a roof consists of the following: asphalt shingles (2 psf), 0.75 in. plywood (2.5 psf), 2 × 8 framing at 12 in. on center (2.5 psf), fiberglass 0.5 in. insulation (1 psf), and plastered ceiling (10 psf). Determine the roof dead load. Make provisions for reroofing (3 psf).

SOLUTION-

LIVE LOADS

Live loads also act vertically down like dead loads but are distinct from the latter as they are not an integral part of the structural element. Roof live loads, Lr, are associated with maintenance of a roof by workers, equipment, and material. They are treated separately from the other types of live loads, L, that are imposed by the use and occupancy of the structure. ASCE 7-10 specifies the minimum uniformly distributed load or the concentrated load that should be used as a live load for an intended purpose. Both the roof live load and the floor live load are subjected to a reduction when they act on a large tributary area since it is less likely that the entire large area will be loaded to the same magnitude of high unit load. This reduction is not allowed when an added measure of safety is desired for important structures. FLOOR LIVE LOADS The floor live load is estimated by the equation

                                                    L k = L0
where L0 is basic design live load (see the section “Basic Design Live Load, L0”). k is area reduction factor (see the section “Effective Area Reduction Factor”). Basic Design Live Load, L0 ASCE 7-10 provides a comprehensive table for basic design loads arranged by occupancy and use of a structure. This has been consolidated under important categories in Table 2.1. To generalize, the basic design live loads are as follows: Above-the-ceiling storage areas: 20 psf; one- or two-family sleeping areas: 30 psf; normal use rooms: 40 psf; special use rooms (office, operating, reading, and fixed sheet arena): 50–60 psf; public assembly places: 100 psf; lobbies, corridors, platforms, and stadium*: 100 psf for first floor and 80 psf for other floors; light industrial uses: 125 psf; and heavy industrial uses: 250 psf. Effective Area Reduction Factor Members that have more than 400 ft.2 of influence area are subject to a reduction in basic design live loads. The influence area is defined as the tributary area, AT, multiplied by an element factor, KLL, as listed in

Table 2.2. The following cases are excluded from the live load reduction:
1. Heavy live loads that exceed 100 psf
 2. Passenger car garages
3. Public assembly areas Except the aforementioned three items, for all other cases the reduction factor is given by 
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