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visitormaddeningUrban and Civil

Nov 25, 2013 (4 years and 1 month ago)

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Hold the completed girder to the string with temporary 2X4 braces so
that it won't shift sideways when the floor joists are installed. Since the
beam pocket will be somewhat oversized, drive cedar shim shingles
between the girder and the concrete to a
nchor it sideways in its pocket
and to bring it flush to the top of the sill.* Even when not required by
local code, it's still a good idea to slip a steel bearing plate between the
masonry and the bottom of the girder to protect it from rot and to give
go
od bearing. The girder should have no less than four inches of bearing
in the pocket.


STEEL BEAMS

Steel beams are often used in residential construction where longer
spans than wood girders can safely support or where extra headroom is
called for. The siz
e and span lengths of steel beams are calculated in the
same way as for wood girders. Of the two common types of steel beams,
the "S" (standard) and the "W" (wide
-
flange), the W beam is most often
used in residential construction (see 6
-
14). It has a wide
flat flange,
which provides a better bearing surface for attaching wood framing
members than the narrow curved profile of the S beam.

The designation for a steel beam gives its height in inches, a letter for its
type, and its weight in pounds per linear fo
ot, e.g., 8W15.0 (refer to 6
-
14).

There are several ways to attach wood framing to steel beams (see 6
-
15).
Normally, the beam is set level with the top of the foundation wall
(about
Vs
inch higher if sill sealer is used), and the beam pocket is filled
with

grout under the beam. Steel beams should never bear on wood
posts; always use concrete
-
filled Lally columns. Bearing caps should be
welded to the top and bottom of the col
umns. The top cap is either
drilled and bolted to the beam flange or welded in plac
e. Holes drilled
through the flange at the bottom of the column allow it to be lag
-
bolted
into the concrete floor.

Use bolts, or a Ramset gun

a tool that uses blank cartridges to shoot
special hardened fasteners

to secure a 2 X 6 to the top flange of the
b
eam as a bearing pad for the floor joists (see 6
-
16).

If the joists must be flush with the bottom of a steel beam, they are
notched on their bottom edge to fit over the beam flange (see 6
-
17). A
second, deeper, notch at the top edge accommodates the top fl
ange of
the beam. The ends of the joists are toenailed into a two
-
by ledger ripped
to fit and bolted to the beam web.

*Some experts recommend setting wood girders 'A to Vs inch higher
than the sill to account for the difference in shrinkage between a 12
-
in
ch
-
wide plank and a two
-
inch
-
deep sill. While this may make sense
with unseasoned lumber, it would also make a hump in the subfloor over
the trimmer joist. Since the pressure
-
treated wood now required for
mudsills tends to shrink more than untreated wood,
the whole issue is
probably moot.


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In most cases, the floor joists will be deeper than the steel beam. A two
-
by ledger bolted to the web and bearing edgewise on the flange

or else
bolted flatwise to the flange itself

is used to position the joists at
the
desired height (see 6
-
18). The top edges of the joists are notched to fit
around the top flange of the beam, in any case. Nail a two
-
by joist tie
across the beam. Since the joists will eventually shrink and the steel
won't, leave a
3/8
-
inch space betwe
en the beam and the tie.

LAYOUT AND INSTALLATION OF FLOOR JOISTS

The position of the floor joists is laid out on the rim joist and the girder.
To prevent small measuring errors, use a 100
-
foot steel (or fibreglass)
tape instead of a pocket tape to mark all

the joists along the entire length
of the wall. Unlike a pocket tape, the hook of a long tape also stays put
as the tape is extended.

Back along the length of the rim joist (or slide back
wards while
straddling the girder) with one hand stretching the tap
e tightly, and, with
the other hand, draw a V point
ing to each spacing interval (see 6
-
19).
Then, with a com
bination square, draw a straight line through the point
of each V across the top of the girder and down the inside

face of the rim
joist. Place an

X at the side of these lines to show the position of the
floor joist. An X always marks where a joist, rafter, stud, or any other
framing unit will be nailed. Framing is normally spaced 16 or 24 inches
on center
(o.c.) so that 4X8 sheets of plywood and wa
llboard, or any
standard modular building material, will "break" over the middle of a
joist or stud, with a minimum of trimming or waste. Since the layout
lines actually corre
spond to edges and not centers, they must be offset
half the width of the stud o
r joist. For example, for two
-
by's at 16 inches

o.c., the first mark is at 15¼

inches, the next at 31
¼

inches, 47
¼

inches,
and so on. Each mark is "three
-
quarters under," because planed two
-
by
dimension lumber is 1
¼

inches thick. Full
-
dimension, rough
-
sawn

lumber would be marked an "inch under."

A single floor joist seldom spans the width of the build
ing. Most
framing plans require multiple sets of joists car
ried on one or more
girders or bearing partitions. While the ends of joists can be cut to butt
eac
h other over a girder, this is an unnecessarily time
-
consuming
method


especially since the butted joints must be stiffened against
deflection with overlapping "scabs" of short joist stock. It's a lot easier
to let the joists "run by" (overlap) each other
at the girder (see 6
-
20).