A simply supported steel beam carries a factored concentrated load of 25 kips at the center of its 36-ft span. The beam has lateral support at the center of the span and at both ends (Lb = 18 ft). Use the AISC design graphs and the LRFD design method to select the most economical beam section. Neglect the beam weight. Design for bending only. The steel is A992, Grade 50.