Including uniform dead load and live load

a more
Practicaljoist Specification

by bruce brothersen, P.e., and tim holtermann, s.e., P.e.

The revision to the 43rd Specification and COSP was pre-pared for inclusion by reference in the 2012 International Building Code (IBC). Due to the extent of the changes, 40 balloted revisions were included in the 43rd Specification. Some of these revisions include previously unpublished design checks and assumptions, better use of “mandatory” code language and reorganization of certain sections for better clarity. Most importantly, the changes are all aimed at making SJI products safer, easier to use and more eco-nomical. This article highlights just two of the changes in

MODERN STEEL CONSTRUCTION february 2011

of the chords, a concentrated load with the magnitude of the Add-Load is placed at the panel point closest to the cen-ter of the joist to maximize the design moment. For each web member, a concentrated load is placed throughout the length of the joist to maximize the design shear.

A Bend-Check Load is a vertical concentrated load used to design the joist chord for the additional bending stresses resulting from loads being applied between the panel points. The accompanying gravity load must already be included in the specified joist designation, uniform load, concentrated load or Add-Load. The Bend-Check Load is only used to add additional bending stress in the chord and does not con-tribute to the overall axial forces within the joist. Also, as part of the design requirements of this type of load, it must be specified whether the Bend-Check Load is to be applied at the top chord, bottom chord or both. An ideal use of this type of load is for cases when incidental loads, accounted for in the design loads, may induce additional bending stress due to their occurring at any location along the top or bottom

The use of an Add-Load and/or a Bend-Check Load will aid the specifying design professional in accounting for loads that are hard to locate and gives more flexibility to the joist products. As an example, there may be a design need to have a 500 lb load occur at any location on the top or bottom chords of the joist. This can be easily defined by specifying a 500 lb Add-Load and a 500 lb Bend-Check Load on the top

be used to define an Add-Load and/or Bend-Check load.

appropriate use of an Add-Load and Bend-Check Load. 1. D

2. D + L

8. 0.6D + 0.7E

Bruce F. Brothersen, P.E., is the engineering
corporate engineering manager for Canam Steel
Corporation and is based in Washington, Mo. He

Add-Load = definition per COSP
Bend-Check Load = definition per COSP
Values in this table shall be multiplied by the factors in ASCE7-05 section 2.4.1 by the joist designer.

Special Seismic Load Combinations
9. D + 0.7Em
10. D + 0.525Em + 0.75L + 0.75(Lr or S or R) 11. 0.6D + 0.7Em

For diagonal bridging, the bridging force, Pbr, is the hori-zontal component. The axial force in the diagonal bridging member is then a function of the angle of the diagonal mem-

bridging ber relative to the horizontal plane. The deeper the joist and

The bridging figure above demon-
strates the new requirement that appears
in new Table 2.7-3 in the COSP. When
the joist depth is 52 in. or greater, and the
joist spacing is less than 0.7 times the joist
depth, horizontal members are required
in addition to the diagonal members. For
example, 72DLH joists spaced less than 4
ft, 2 in. apart would require both diago-
nal bridging and horizontal members,
with the required horizontal member size
shown in Table 2.7-3. For 72DLH joists
spaced 4 ft, 2 in. or more, diagonal bridg-
ing with a slenderness ratio limit of 200 or
less is adequate.

The possibility of narrow spaces and
deep joist becomes more likely with a new
expansion of the DLH-series Load Tables
in the 43rd Edition. In the tables the maxi-
mum depth for a DLH joist increases from
72 in. to 120 in. and the maximum span
increases from 144 ft to 240 ft.