Vulcraft Steel Joists and Joist Girders Catalog - Sites at Lafayette

JOIST GIRDERS IN MOMENT RESISTANT FRAMES
When a Joist Girder is used as a component of a
moment resistive frame, both the design wind moment
and any continuity (usually live load) moment must be
specified for each end of each affected Joist Girder.
Provided this information, Vulcraft will design the Joist
Girder as a simply supported truss for full gravity
loading. The "fixed end" moments are then applied to
the Joist Girder. Using the appropriate combinations of
the gravity loads, the wind moments, and/or the
continuity moments, the critical member stresses are
identified and the Joist Girder members are sized
accordingly.
A one-third increase in allowable stresses is permitted
in all load combinations involving wind. (Vulcraft does
not design the Joist Girder for any dead load
moments unless specifically instructed to do so
on the structural drawings.) For this reason it is very
important that on the structural drawings the
specifying professional specify that all dead loads be
applied to the Joist Girders before the bottom chord
struts are welded to the stabilizer plates.
One of the most important considerations of using a
Joist Girder in a moment resistive frame is the
connection of the Joist Girder to the column. As with a
beam connection, special provisions must be made to
develop the required moment capacity. As can be
readily seen in Figure 1, the use of a standard Joist
Girder seat results in an eccentric moment due to the
depth of the seat. This moment must be resisted by
the weld group connecting the Joist Girder seat to the
cap plate of the column.
Vulcraft has conducted an extensive testing program
to determine the maximum capacity of this connection.
Table 1 gives the maximum capacity of a Joist Girder
with a six inch deep bearing seat sitting atop the
support. To achieve these strengths, both A 3 2 5
erection bolts must be installed and tightened "snug
tight" (ref. AISC for definition) in the Joist Girder seat.
In addition, the Joist Girder seats must be welded to
the support with a minimum of two 1/4" fillet welds 5
inches long. The bottom chord extensions of the Joist
Girder must be welded to the column stabilizer plate
as required.
Table 1
MAXIMUM A X I A LLOAD CAPA C I T Y
J O I S TG I R D E R
FOR 6" DEEP
W G H TPER FOOT J O I S TGIRDER SEAT S
0 - 30 8 K i p s
30.1 * Up 16 Kips
The capacities in Table 1 can be increased by onethird
for all load cases involving wind.
If the axial load due only to the wind moment does not
exceed the values in Table 1 (after increasing them by
one-third), a strap angle connecting the Joist Girders
together as shown in Figure 2 can be used to resist
the continuity moments, By tying the Joist Girder ends
t o g e t h e r, the Joist Girder-to-cap plate connection
need only resist the wind loads, the strap angles do
not transfer wind moments. The design of such a
strap angle to resist the continuity moments is the
responsibility of the specifying professional.
When the end moments on the Joist Girders are too
large for the seat to resist, it is necessary to utilize a
moment plate as shown in Details A-F. The use of this
simple moment plate virtually eliminates all
eccentricity problems.
By using the equations and Table 2 below, the
specifying professional can determine the minimum
Joist Girder top chord width for most Joist Girders. If
the end moments are very large, the Joist Girder loads
and/or spacings vary, or other special conditions exist,
a more exact analysis is required. Once the Joist
Girder top chord width is known, the specifying
professional can easily size the moment plate and its
weld requirements to complete the connection detail.
EQUATION 1 (ODD NO. OF JOIST SPACES)
. 028P
A=
D
(N 2 S - .67N + .67 - S)
EQUATION 2 (EVEN NO. OF JOIST SPACES)
. 028P
A = (N 2 S - .67N + .67)
D
Where:
P = Panel point load (kips)
N = No. of joist spaces
S = Joist spacing (ft.)
D = Joist Girder depth (in.)
Table 2*
Minimum Top Chord Width
A
0.95 - 1.19 6"
1.20 - 1.78 7"
1.79 - 2.48 8"
2.49 - 3.75 9"
3.76 - 4.76 11"
4.78 - 8.44 13"
Greater than 8.44
Consult Vulcraft
Please note that this chart is to be used only for
designing moment plates. It is not intended for
use as a general detailing aid.
*The bearing seat width may be larger than the top
chord width. Contact Vulcraft if seat width is needed
for determining column plate sizes.
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