The walk way structure shown in Fig. 4 allows commuters to cross from the parkin
ID: 2312157 • Letter: T
Question
The walk way structure shown in Fig. 4 allows commuters to cross from the parking structure to the office building across the street. The walkway is a 72 long truss that has 6 spans (each span 12 ft). The height of the truss is 12 ft. The width of the bridge is 10 ft. The live load is 100 psf and the snow load is 40 psf. The weight of the concrete deck steel floor beams, portions of the diagonals and the vertical members and additional dead loads and utilities is estimated at 90 psf. The top chords and bottom chords vertical members and diagonals will be HSS-shapes. The snow load will be applied at top joints H through N. The self-weight of the top braces and top chords and portions of the verticals and diagonals will be estimated at 1.5 kips/per joint applied at the top joints. The weight of the glass panels is 7 psf (the weight of the glass panels is distributed equally between top and bottom joints). Required: 1. Determine the maximum factored load, Pu in member CDExplanation / Answer
The load in member CD can be determined considering the truss as a beam subjected to the loads mentioned. The bending moment at the midspan due to those loads can be calcultaed and then that bending moment divided by the truss height gives the axial tension in CD
Length of truss=72 ft
height of truss=12 ft
width of bridge=10 ft
trbutary width of each truss=10/2=5 ft
live load=100 psf
live load on each truss=100*5=500 plf=0.5 kip/ft
bending moment at midspan due to live load=0.5*722/8=324 kip-ft
tension in CD due to live load = 324/12=27 kips
snow load on each truss = 40*5=200 plf=0.2 kip/ft
bending moment at midspan due to snow load=0.2*722/8=129.6 kip-ft
tension in CD due to snow load = 129.6/12=10.8 kips
dead load on each truss from weight of concrete deck,steel floor beams,etc = 90*5=450 plf=0.45 kip/ft
bending moment at midspan due to dead load = 0.45*722/8=291.6 kip-ft
tension in CD due to dead load= 291.6/12=24.3 kips
Load on truss due to weight of glass panel = 7*12=84 plf=0.084 kip/ft
bending moment at midspan due to weight of panel = 0.084*722/8=54.432 kip-ft
tension in CD due to panel weight = 54.432/12=4.536 kips
load on truss due to self weight of top braces,top chords,etc = 1.5 kip/jt
Bending moment at midspan due to self weight of top braces,top chords,etc = 1.5*12 + 1.5*24+ 1.5*72/4 = 81 kip-ft
tension in Cd due to self weight of top braces,top chords,etc = 81/12=6.75 kips
Therefore, load in CD due to dead load = DL=24.3+4.536+6.75=35.586 kips
Load in CD due to live load = LL=27 kips
Load in CD due to snow load = SL=10.8 kips
Load combination per ASCE 7-10 is 1.2D+1.6L+0.5S
Factored load in CD = (1.2*35.586)+(1.6*27)+(0.5*10.8)=91.3 kips
Pu = 91.3 kips
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