Page 322 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FLOOR AND ROOF SYSTEMS

7.6 CHAPTER SEVEN

FIGURE 7.6 Two-hour fire-rated floor systems, with cold-formed steel deck. (a) With lightweight concrete
fill; (b) with normal-weight concrete fill.

Diaphragm Action of Metal-Deck Systems. Concrete fill on metal deck readily serves as a rela-
tively stiff diaphragm that transfers lateral loads, such as wind and seismic forces, at each floor level
through in-plane shear to the lateral load-resisting elements of the structure, such as shear walls and
braced frames. The resulting shear stresses can usually be accommodated by the combined strength
of the concrete fill and metal deck, without need for additional reinforcement. Attachment of the
metal deck to the steel framing, as well as attachment between adjacent deck units, must be suffi-
cient to transfer the resulting shear stresses (see “Attachment of Metal Deck to Framing” below).
Additional shear reinforcement may be required in floor decks with large openings, such as those
for stairs or shafts, with trench headers for electrical distribution, or with other shear discontinuities.
Also, floors in multistory buildings in which cumulative lateral loads are transferred from one lateral
load-resisting system to another (for example, from perimeter frames to interior shear walls), may
be subjected to unusually large shear stresses that require a diaphragm strength significantly greater
than that for a typical floor.

Attachment of Metal Deck to Framing. Metal deck can be attached to the steel framing with pud-
dle (arc spot) welds, screws, powder-driven fasteners, or when composite floor framing is used
(Art. 7.8), by means of shear connectors welded through the metal deck onto the top flange of the
framing member. These attachments provide lateral bracing for the steel framing and, when applic-
able, transfer shear stresses resulting from diaphragm action. The maximum spacing of attachments
to steel framing is generally 12 in.

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