Question

Steel trusses: Problem 9.10.A and 9.10.B *change span length to 28' for B. For both, find lightest and shallowest

Answer 1

Ans a) Let us first determine the load per ft on the joist ,

Live load = Joist spacing x Live load in psf = 4 ft x 25 psf = 100 lb/ft

Dead load = Joist spacing x Dead load in psf = 4 ft x 20 psf = 80 lb/ft

=> Total load = 100 + 80 = 180 lb/ft

Now, choose the possible joist section according to standard specificatiojn load tables for steel joist, 1986 , that will just carry these loads . Note that joist weight must be reduced from the total capacity of selected joist . Possible choices for the span 48 ft are shown below :   

Load condition/Joist	24 K 9	26 K 9	28 K 6	28 K 7
Total capacity (lb)	211	229	184	206
Self weight (lb/ft)	12	12.2	11.4	11.8
Net usable capacity (lb)	199	216.8	172.6	194.2
Load for L/360 deflection (lb)	101	119	105	152

So, according to above table lightest choice for joist will be be 28 K 7

The shallowest choice of joist is 24 K 9

Ans b) Let us first determine the load per ft on the joist ,

Live load = Joist spacing x Live load in psf = 5 ft x 30 psf = 150 lb/ft

Dead load = Joist spacing x Dead load in psf = 5 ft x 18 psf = 90 lb/ft

=> Total load = 150 + 90 = 240 lb/ft

Now, choose the possible joist section according to standard specificatiojn load tables for steel joist, 1986 , that will just carry these loads . Note that joist weight must be reduced from the total capacity of selected joist . Possible choices for the span 28 ft are shown below :    

Load condition / Joist	14 K 6	16 K 4	20 K 3
Total capacity (lb)	265	249	261
Self weight (lb/ft)	7.7	7.0	6.7
Net usable capacity (lb)	257.3	242	254.3
Load for L/360 deflection (lb)	124	138	189

So, according to above table lightest choice for joist will be be 16 K 4

The shallowest choice of joist is 14 K 6