Figure 1-23.—Joists supported by steel beams.
figure 1-21. These can be left out if the line of panels
from the plywood subfloor straddles the butt joints.
Butted against a Girder
Butting joists against (rather than over) a girder
allows more headroom below the girder. When it is
necessary for the underside of the girder to be
flush with the joists to provide an unbroken ceiling
surface, the joists should be supported with joist
hangers (fig. 1-22).
Blocking between Joists
Another system of providing exterior support to
joists is to place solid blocking between the outside ends
of the joists. In this way, the ends of the joists have more
bearing on the outside walls.
Interior Support
Floor joists usually run across the full width of the
building. However, extremely long joists are expensive
Figure 1-24.—Joists supported on steel plates.
and difficult to handle. Therefore, two or more shorter
joists are usually used. The ends of these joists are
supported by lapping or butting them over a girder,
butting them against a girder, or lapping them over a
wall.
Supported by a Steel Beam
Wood joists are often supported by a steel beam
rather than a wood girder. The joists may rest on
top of the steel beam (fig. 1-23, view A), or they
may be butted (and notched to fit) against the sides
of the beam (view B). If the joists rest on top of a
steel beam, a plate is fastened to the beam and the
joists are toenailed into the plate. When joists are
notched to fit against the sides of the beam,
allowance must be made for joist shrinkage while
the steel beams remain the same size. For average
work with a 2- by 10-inch joist, an allowance of 3/8
inch above the top flange of the steel girder or beam
is usually sufficient.
Another method of attaching butted joists to a
steel girder is shown in figure 1-24. A 3/8-inch
space is shown above the beam to allow for
shrinkage. Notching the joists so they rest on the
lower flange of an S-beam is not recommended; the
flange surface does not provide sufficient bearing
surface. A wide plate may be bolted or welded to
the bottom of the S-beams to provide better
support. Wooden blocks may be placed at the
bottoms of the joists to help keep them in position.
Wide-flanged beams, however, do provide
sufficient support surface for this method of
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