LOAD ANALYSISDESIGN OF A STRUCTURE A lightweight concrete fl

LOAD ANALYSIS/DESIGN OF A STRUCTURE A lightweight concrete floor system (shown below) is to be supported by four wide- flange steel beams spaced 12 feet apart that rest on masonry walls along the 44 foot length. The floor carries a live load of 60 Ib/sf and the slab depth is 5\". Assume that reinforced concrete weighs 120 #/ft and the beams are simply supported on the above mentioned masonry walls. Design the lightest weight steel beams required to support this structure for moment capacity assuming: a. the beams are braced at the mid-span only b. the beams are brace at the third points of the span c. the beams are continuously braced along their top flange. Disregard any torsional loading conditions and don\'t forget the weight of the beams!) 54 ft 44 ft Concrete deck Steel Beams Section A-A CMU Walls Section B-B

Solution

Thickness of slab = 5\"

unit weight of lightweight concrete = 120 lb/ft³

area load due to self weight of slab = (5/12)*120=50psf

live load = 60 psf

tribitary width of beam = 12 ft

uniform load on beam due to dead load of slab = 50*12/1000=0.6 kip/ft

uniform load on beam due to live load = 60*12/1000=0.72 kip/ft

Load on beam due to load combination 1.2D+1.6L = (1.2*0.6)+(1.6*0.72)=1.872 kip/ft

span of beam = 54 ft

maximum moment in beam =moment at midspan = 1.872*54²/8 = 682.34 kip-ft

a) unbraced length of beam = 547/2=27 ft

the lightest beam that has moment capacity greater than 682.34 kip-ft for unbraced length of 27 ft is W33x118

b)unbraced length of beam =54/3=18 ft

the lightest beam that has moment capacity greater than 682.34 kip-ft for unbraced length of 18 ft is W30x90

c) continuously braced top flange

the lightest beam that has plastic moment capapcity greater than 682.34 kip-ft is W24x76