Select an open-web steel floor joist for a span of 30'–0". Th e total design load is 150 psf, of which the live load is 100 psf, and the joists are to be spaced 2'–0" on-center.
What will be an ideal response?
Loading conditions
Live load = 100 psf
Total load = 150 psf
Calculations
For joist spacing of 2'–0" on-center:
Live load in pounds per lineal foot = 2' × 100 psf = 200 plf
Total load (excluding joist weight) in pounds per lineal foot = 2' × 150 psf = 300 plf
we first read down the far left column, Span (ft .), to 30 and then across to 302/219 . These are the first numbers that equal or exceed the known 300 plf total load and 200 plf live load. Following that column up to the top of the table, we find a 22K4 joist, which weighs 8.0 plf.
Adding the joist weight of 8.0 plf to the 300 plf yields a total load of 308 plf, which is more than the total allowable load of 302 plf listed on the table. So, we must move further along the 30 row to 331/262 corresponding to a 24K4 joist with a weight of 8.4 plf. Adding the joist weight of 8.4 plf to the 300 plf yields a total load of 308.4 plf, which is less than the total allowable load of 331 plf listed in the table.
Th e 200 plf live load previously calculated is also less than the 262 plf allowable live load for this joist. Thus, the actual deflection of the joist under live load will be even less than the allowable 1/360 of the span length for floor joists.
With a 24K4 joist selected, one more step is necessary to complete the design procedure. We must determine the number of rows of bridging required for the joist at a span of 30–0. reading down the Section Number column, we stop at #4 because that number corresponds to the last digit of the 24K4 joist. Reading across the table, we see in the 3 Rows column that the 24K4 joist with a span of 28 thru 38 requires three rows of bridging. Th us, the most economical floor joist for the loading condition described would be a 24K4 open-web steel joist with three rows of bridging.
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