design of open web steel joist

Currently, open-web steel joists are still relatively small, parallel-chord trusses, but hot-rolled steel shapes usually make up the components of design of open web steel joist . (For a time, cold-formed steel shapes were preferred for chords to utilize higher working stresses available in cold-formed sections of ordinary carbon-steel grades. Unfavorable fabrication costs, however, led to a change to the hot-rolled steel chords.)

DESIGN OF OPEN WEB STEEL JOIST

Joists are suitable for direct support of floors and roofs of buildings, when designed according to SJI ‘‘Standard Specifications, Load Tables and Weight Tables for Steel Joists and Joist Girders,’’Steel Joist Institute, 3127 10th Ave., North Ext., Myrtle Beach, SC 29577. Moreover, since 1972, the American Institute of Steel Construction (AISC) has cooperated with SJI in producing an industry standard for steel joist design. However, exact forms of chords and webs, and their methods of manufacture, then as now, have continued to be in the provenance of SJI members. above Figure shows a number of proprietary design of open web steel joist.

joists are designed primarily for use under uniform distributed loading with substantially uniform spacing of joists, as depicted in below Fig. . They can carry concentrated loads, however, especially of loads are applied at joist panel points. Partitions running crosswise to joists usually can be considered as being distributed by the concrete floor slabs, thus avoiding local bending of joist top chords. Even so, joists must always be size-selected to resist the bending moments, shears, and reactions of all loads, uniform or otherwise. So joist loadings given in tables for uniform loading should be used with caution and modified when necessary.

One cardinal rule is that the clear span of a joist should never exceed 24 times its depth. Another rule is that deflections should not exceed 1⁄360 of the joist span for floors and roofs to which plaster ceilings are attached or 1⁄ 240 of the span for all other cases.

SJI publishes loading tables for K-series (short span). LH-series (long span), and DLH-series (deep long span) joist girders. The K-series joists are available in depths of 8 to 30 in and spans of 8 to 60 ft in 13 different chord weights to sustain uniform loads along the span as high as 550 lb/ft. LH-series joists are available in depths from 18 to 48 in and spans of 25 to 96 ft in six different chord weights capable of supporting total loads of 12,000 to 57,600 lb. DLH-series joists are available in depths of 52 to 72 in and spans from 89 to 144 ft in 17 different chord weights with total-load capacities of 26,700 to 80,200 lb. Load capacities in the foregoing were based on a maximum allowable tensile strength of 30 ksi, which calls for high-strength, low-alloy steel having a specified minimum yield strength of 50 ksi or cold-formed steel having the same yield strength.

Fire resistance ratings of 1, 11⁄2, 2, and 3 hours are possible using concrete floors above decks as thin as 2 in and as thick as 31⁄2 in with various types of ceiling protection systems. The Steel Joist Institute identifies such ceiling protection systems as exposed grids, concealed grids, gypsum board, cementitious, or sprayed fiber.


References:

  • BUILDING DESIGN AND CONSTRUCTION HANDBOOK for Frederick S. Merritt & Jonathan T. Ricketts
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