Using LRFD, design the cross-section of an I-shaped plate girder shown in Figure 4.12a to support a factored moment Mu of 4600 kip ft (6240 kN m); dead weight of the girder is included. The girder is a 60-ft (18.3-m) long simply supported girder. It is laterally supported at every 20 ft (6.1 m) interval. Use A36 steel.

Proportion of the girder web: Ordinarily, the overall depth to span ratio d/L of a building girder is in the range 1 to 1,. So, let us try h = 70 in.

Also, because h/tw of a plate girder is normally in the range 5.70^/(E/Fyf) to 11.7^/(E/Fyf), using E = 29,000 ksi and Fyf = 36 ksi, let us try tw = 1- in.

Proportion of the girder flanges: For a preliminary design, the required area of the flange can be determined using the flange area method

So, let bf = 20 in. and tf = 11 in. giving Af = 22.5 in.2 Determine the design flexural strength f bMn of the girder: Calculate Ix:

= [8,932 + (21.88)(0)2 ]+ 2[2.37 +(22.5)(35.56)2 ] = 65,840 in.4

Calculate Sxt, Sx,


Calculate rT: Refer to Figure 4.12b,

(1.125)(20)3/12 +(11.667)(5/16)3/12 22.5 +(1/6)(21.88)

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