Floor Framing With Steel Continued
Bridging, Blocking, and
The decking applied to the top flanges of the joists acts as
bracing to prevent the joists from rolling; spans of over 12
feet must also be braced at the bottom flange. A drywall
ceiling makes acceptable lower-flange bracing, but in a
basement, the usual approach is to run a strip of 1 1/2-inch
steel strapping at the center of the joist span for the entire
length of the floor section, securing it at each joist with one
screw (Figure 10). This is the equivalent of metal or wood
bridging in a wood joist. In addition, blocking is required
between joists at intervals of no more than 12 feet. The
blocking can be up to 2 inches narrower than the joists
themselves, and is fastened with clip angles.
10. Joists are braced with 1 1/2 inch steel
strapping at mid span, and spaced blocking fastened to
the joists with clip angles.
Dampening floor noise. A steel floor is
naturally stiffer than one framed with wood or I-joists, which
translates to a slightly harsher feel underfoot. Carpeting
takes care of most of this, but to provide some additional
cushioning, we also use a 1 1/2-inch-wide foam material that is
manufactured for use with I-joists (Integrity Floor Gasket,
Weyerhauser Advanced Building Components). It's a self-adhesive
material that's pressed onto the top flange of each joist
before it's set in place.
For decking, we use 3/4-inch Advantech. We tack the decking
in place with a few 1 1/4-inch knurled pin fasteners from a
model 500 ET & F pneumatic gun - the same gun and fasteners
we use for wall sheathing -- then screw them down with 1
1/4-inch self-drilling collated strip screws. The air fasteners
keep the decking from rising as the first few screws go in.
Plumbing, wiring, and HVAC. Steel joists have
punch outs for water lines, gas pipe, and wiring, so these are
not a problem to install. However, ductwork and PVC drain lines
do require some thought. More often than not, the home can be
designed to allow the ductwork and drain piping to run parallel
to the joists. Where this isn't possible, they're suspended
below the joists and enclosed in a soffit or dropped
Maximum Hole Dimensions and Spacing
Floor & Ceiling Joist Webs
(1) The dimension of the
hole measured across the depth of the
(2) The dimension of the hole measured
across the depth of the joist
(3) Holes shall not be located at
mid-span +/- 2 feet (610 mm).
(4) Circular holes shall have a
minimum diameter of 6 inches (127
(5) Spacing is the center-to-center
distance between holes.
(6) Edge distance is measured from the
edge of the hole to the edge of bearing
Courtesy of NAHB
11. The prescriptive method permits limited
holes in steel joists, but with careful planning, holes
can be reduced or eliminated.
In practice, this is rarely a problem, because you're
usually down in the basement, where the main trunk line from
the furnace has to drop below the joists no matter what. We try
to run the trunk line right next to the beam at mid-span, and
run the main waste line right next to it. It's easy to enclose
both in the same soffit if the basement is finished later.
If absolutely necessary, the prescriptive method makes
allowances for joist penetrations (Figure 11), but I have never
had a case where a plumber or HVAC contractor had to cut
through a joist.
framed more than 75 steel homes in the course of eight years as
operations manager of Generation Homes, in Valparaiso, Ind. He
is the president of Malak Properties, developers of the
SteelXpert estimating system for light-gauge steel