The loads and span load tables for the k-series

minimum required bearing seat depths for if the joist was also to be installed at a slope,

a given slope. Stating these minimum bear- the bearing seat depth may have had to be

and manufacturing schedule. and bottom chord bridging aligning in the

When the loads for a top chord exten- same vertical plane, this configuration is

seat depth should be specified? A good rule regions with high uplift values. The engi-

of thumb, in most cases, is that the depth of neer of record should avoid dimensioning

plf load on the joist and extension, and a sometimes cost-effective, to state on the

1,000-lb concentrated load at the end of contract drawings: “Bridging as required

on a 5-ft, 6-in. extension is 375 plf for a economical. For example, if a lighter

type R12 extended end. Therefore, a 2½- joist requires erection stability bridging

appropriate top chord extension depth can weigh selecting the heavier section. The

make. For the example, the loads and span load tables for the K-Series, LH-Series,

If a special joist is being used, the structural engineer must determine if the load requirements for the joist can be ade-quately described in one gravity load case and one uplift load case. If yes, then the loads can all be pre-summed and speci-fied as total (or factored) loads. If no, then the loads should be broken out by load category, and load combinations must be specified. These loads can be noted in several ways:
➜ individual load diagram
➜ additional notes on the framing plan➜ load schedule
➜ separate loading plan
The end goal is for the joist design loads to be specified clearly and concisely. The difficulty is in knowing how to accom-plish this, and the biggest stumbling block is in trying to pre-sum loads for conditions that require consideration of multiple load cases with different load category sum-mations. Trying to mix a simple specifica-tion with a complex load condition almost always leads to confusion, loss of econ-omy, and the potential for an inadequately designed joist for the situation.

—Joseph J. Pote Director of Engineering – Research & Development, CMC Joist

four angle top chord extension. If a depth of 5 in. had been allowed, the manufacturer would have had the option of using two C5 channels to form the top chord extension. From this, one can see that increasing the extension depth simplifies the construction

MODERN STEEL CONSTRUCTION MARCH 2007