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General Description
Section 1
1-44
The designation for the G series Joist Girders is as shown in page
73 of the Steel Joist Institute publication. STAAD.Pro incorporates
the span length also in the name, as shown in the next figure.
Figure 1.33
Modeling the joist - Theoretical basis
Steel joists are prefabricated, welded steel trusses used at closely
spaced intervals to support floor or roof decking. Thus, from an
analysis standpoint, a joist is not a single member in the same
sense as beams and columns of portal frames that one is familiar
with. Instead, it is a truss assembly of members. In general,
individual manufacturers of the joists decide on the cross section
details of the members used for the top and bottom chords, and
webs of the joists. So, joist tables rarely contain any information
on the cross-section properties of the individual components of a
joist girder. The manufacturer’s responsibility is to guarantee that,
no matter what the cross section details of the members are, the
joist simply has to ensure that it provides the capacity
corresponding to its rating.
The absence of the section details makes it difficult to incorporate
the true truss configuration of the joist in the analysis model of the
overall structure. Any load or selfweight applied on the joist is
transferred to its end nodes through simply supported action as if
it were a truss. The joist makes no contribution to the stiffness of
the overall structure.