Question

5.5-4 The beam shown in Figure P5.5-4 has continuous lateral support of both flanges. The uniform load is a service load consisting of 50% dead load and 50% live load. The dead load includes the weight of the beam. If A992 steel is used, is a W12 x 35 adequate? a. Use LRFD. 6.5 k/ft |6- — 18– 767

Please write very clearly.

Determine whether a W12x35 can work if A992 steel is used considering the conditions stated above.

Answer 1

Total load=6.5 kip/ft

Negative moment at support=6.5*6=39 kip-ft

Reaction at support=6.5*(6+18+6)/2=97.5 kips

Positive moment at midspan=97.5*9-(6.5*(6+9)*(6+9)/2)=146.25 kip-ft

Therefore, moment at midspan will govern the design

Moment at midspan due to Dead load=0.5*146.25=73.125 kip-ft

Moment at midspan due to live load=0.5*146.25=73.125 kip-ft

Per LRFD load combination, 1.2D+1.6L,

Design moment=1.2*73.125+1.6*73.125=204.75 kip-ft

Since both flanges are continuosly braced, beam will develop plastic moment capacity

Moment capacity of W12x35=192 kip-ft per Table 3-2 of AISC 360-10,

Since the capacity is less that demand, beam is inadequate