Page 332 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch07.qxd 9/29/05 5:17 PM Page 7.16

FLOOR AND ROOF SYSTEMS

7.16 CHAPTER SEVEN

FIGURE 7.17 Open-web steel joist supports gypsum deck.

7.9 OPEN-WEB JOISTS

Although they are more frequently used for moderate- to long-span roof framing, open-web steel
joists (Fig. 7.17) are sometimes used for floor framing in multistory buildings, particularly for small
office buildings. Joists as floor members subjected to gravity loadings represent an efficient use of
material, particularly since net uplift loadings that are sometimes applicable for roof joist design are
not applicable for floor joist design. Also, the open webs of joists provide an effective means of routing
mechanical ductwork throughout the floor.
Joists can be designed to act compositely with the floor deck by adding shear connectors to the top
chord. In cases where increased future loadings are likely, such as file storage loading in office areas,
the web members and their connections can be oversized and additional shear connectors can be
provided in the original design at minimal additional cost. At the time when the increased loadings must
be accommodated, reinforcement plates need only be welded to the easily accessible bottom chord of
the joists, since the added shear connectors and increased web sizes have already been provided.

7.10 LIGHTWEIGHT STEEL FRAMING

Cold-formed steel structural members can provide an extremely lightweight floor framing system.
These members, usually C or Z shapes, are normally spaced on 24 in centers and can span up to
about 30 ft between supports. Because of their light weight, these members can be handled and
installed easily and quickly. Connections of cold-formed members are usually accomplished by
welding or by the use of self-drilling screws.
This type of floor-framing system is frequently used in conjunction with cold-formed steel load-
bearing wall studs for low-rise construction. Spans are usually short to keep depth of floor system
small. This depth has a direct bearing on the overall height of structure to which costs of several
building components are proportional.
Space in residential buildings often is so arranged that beams and columns can be confined, hid-
den from view, within walls and partitions. Since parallel walls or partitions usually are spaced about
12 ft apart, joists that span between beams located in those dividers can be short-span.
In Fig. 7.18, the joists span in the short direction of the panel to obtain the least floor depth. They
are supported on beams of greater depth hidden from view in the walls. With moment connections to
the columns, these beams are designed to resist lateral forces on the building as well as vertical load-
ing. (Depth of the beams may be dictated by lateral-force design criteria.) As part of moment-resisting
frames, the beams usually are oriented to span parallel to the narrow dimension of the structure. In that
case, the joists are set parallel to the long axis of the building. When beam and joist spans are nearly
equal, framing costs generally will be lower if the joists are oriented to span between wind girders,

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