Nominal moment capacity the composite joist

MATERIAL MATTERS

Understanding steel joists

covering the Load and Resistance Fac-

tor Design (LRFD), manufacture, and

sibilities of the design professional,

code of standard practice, weight

nies. The recently published CJ-Series

catalog contains standard specifications

design parameters checklist.

Definition of composite steel joists 50 ksi (345 MPa). Web sections are be required to follow the same design

systems. Members may consist of hot- or 5 ksi (34.5 MPa). CJ-Series joists provide a number of

rolled or cold-formed steel, including advantages, including the following:

nection between the joist top chord across the United States and Canada webs of the CJ-Series joists instead

and overlying concrete slab allows the during the last 40 years. With SJI’s of running beneath the joist bottom

or 3/4-inch- (9.5-, 12.7-, 15.9-, or multiple SJI member companies will L/30 permit the use of shallower

19-millimeter) diameter shear studs be able to provide composite steel joists joists for any given span.

sections is based on a yield strength of to manufacture CJ-Series joists will joist weight for any given joist depth.

28 Structural Engineer DECEMBER 2007 www.gostructural.com

Lighter weight joists translate into	composite joist bearing seats provide	a = Mn /(0.85 f ′cbede) ≤tc, in. (mm)
potentially lighter weight columns	negligible rotational restraint. There-	(Equation 1)
	fore, CJ-Series joists are modeled as	de = dj – ybc + hdeck + tc – a/2, in. (mm)
	pin-ended members when manufac-	de = dj – ybc + hdeck + tc – a/2, in. (mm)
	tured with the typical underslung bear-	(Equation 2)
	ing seats. CJ-Series joists are cambered	where,
joist system reduces construction	for 100 percent of the non-composite	where,
time. Wider joist spacing reduces	dead load. This is in contrast to com-	•a = depth of concrete compressive
the number of joists to be erected	posite wide-flange beams where con-
and, when required, the number to	nections at the ends of the beams may
	only permit approximately 85 percent
	of the camber to be removed under the	•dj = steel joist depth, in. (mm) •f ′c = specified minimum 28-day con- crete compressive strength, ksi (MPa)
	full non-composite dead load.
	full non-composite dead load.
The composite steel joist designa-
	The moment capacity of a compos-
depth, the letters “CJ” followed by	ite steel joist can be calculated using

	The distance between the centroid
load, and finally the factored uniform	of the tension bottom chord and the
	centroid of the concrete compressive
		the bottom of the bottom chord, in.
has a steel joist depth of 24 inches (610


kN/m), a factored composite live-load	side of the joist centerline, each of	top chord to the moment capacity of
	which shall be the lowest value of the
		first top chord end panel member is
of 240 plf (3.5 kN/m).	• one-eighth of the joist span, center-

	• one-half the distance to the center-	member.
All CJ-Series composite steel joists			(Equation 3)
			(Equation 3)

Figure 1 (left): Shear studs are welded through the steel deck to the top chord of the CJ-Series joist. Note that the shear studs are alternately installed on both joist top chord angles. Figure 2 (right): The CJ-Series composite steel joists permit passage of HVAC, plumbing, and electrical conduit through the open webs of the joist system. This makes it possible to reduce floor-to-floor heights.

MATERIAL MATTERS

Figure 4: The newly released First Edition Catalog
contains standard specifications for composite
steel joists, weight tables, bridging tables, and
code of standard practice.