Question

NAME: PROBLEM #4 Select the most economical bar joist loaded with 80 psf DL and 100 psf LL. The joist spans 32 and has members adjacent at 3' on centers. Allowable live load deflection is /360.

Answer 1

SELECTION OF STEEL JOISTS -LRFD METHOD GIVEN DATA Span of joist Spacing of joist Dead Load Live Load Max. ive load deflection I32 ft 3 ft 80 psf 100 psf span360 CALCULATIONS Tributary width for each joist, W S 3 ft Total Dead Load 80 80 psf Therefore Uniformly distributed dead load DE80x 3 = 240 plf Uniformly distributed live load L100 x 3 300 plf Total factored uniformly distributed load 1.2 x 2401.6 x 300 768 plf Unfactored uniformly distributed live load L 300 plf SELECTION OF JOISTS From Vulcraft's steel joist catalog (LRFD load table), for the given span, select joists across the span such that the total factored uniformly distributed load and unfactored uniformly distributed live load capacities exceed the values calculated above, i.e., 768/300 Since the total uniformly distributed load capacity is below 825 plf, try K-Series joists The RED figures in the load table represent the uniform load in pounds per linear foot, which will produce an approximate joist deflection of L/360. For joist deflectionof L/240, multiply the RED figure by 360/240 Note: 775/337 823 393 778 / 407 772 / 438 Select whose capacities are approximate weight11.7 plf whose capacities are approximate weight11.7 plf whose capacities are approximate weight10.4 plf whose capacities are approximate weight 22K10 24K10 26K9 28K8 9.8 plf Select the lightest joist out of the above options and also which satisfies the check below, i.e

28K8 (self-weight1.7 pl823/393 CHECK Including self-weight, the total uniformly distributed dead load D-80.012)x 3 275 plf Therefore, total factored uniformly distributed load TL1.2D+1.61 810 plf823 Hence, it is O.K