Page 321 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch07.qxd 9/29/05 5:16 PM Page 7.5

FLOOR AND ROOF SYSTEMS

FLOOR AND ROOF SYSTEMS 7.5

resulting effect on floor levelness is objectionable, the top surface can be finished level, but this will
result in additional concrete being placed to compensate for the deflection. The added weight of this
additional concrete must be taken into account in design of the metal deck to ensure adequate strength.
The concrete fill, however, need only resist the stresses resulting from superimposed loadings.
Unshored metal-deck construction is the system most commonly used. The additional cost of the
deeper or thicker deck is generally much less than the cost of shoring. To increase the efficiency of
the unshored deck in supporting the weight of the unhardened concrete and construction live loads,
from both a strength and deflection standpoint, the deck is normally extended continuously over sup-
porting members for two or three spans, in lieu of single-span construction. However, for loadings
once the concrete is hardened, the composite slab is designed for the total load, including slab self-
weight, with the slab treated as a single span, unless negative-moment reinforcement is provided
over supports in accordance with conventional reinforced-concrete-slab design (disregarding the
metal deck as compressive reinforcement).
In cases where the metal deck is designed to span between supporting members without shoring,
but there are a few isolated locations with excessive spans, it may be more economical to provide
shoring in those isolated locations rather than increase the depth or thickness of the metal deck for
the entire floor. Although shoring is normally supported from the floor slab below, an alternative
approach is to support shoring from adjacent floor beams and girders, which would provide less
interference with ongoing construction operations on the floor slab below the metal deck.
Lightweight versus Normal-Weight Concrete. Either lightweight or normal-weight concrete can serve
the structural function of the concrete fill placed on the metal deck. Although there is a cost premium
associated with lightweight concrete, sometimes the savings in steel framing and foundation costs can
outweigh the premium. Also, lightweight concrete in sufficient thickness can provide the necessary fire
rating for the floor system and thus eliminate the need for additional slab fire protection (see “Fire
Protection” below). For exposed floor slabs, the potential for increased shrinkage and corresponding
cracking of lightweight concrete should be considered. The tradeoffs in use of lightweight concrete
versus normal-weight concrete plus fire protection should be evaluated on a project-by-project basis.

Fire Protection. Most applications of concrete fill on metal deck in buildings require that the floor-
deck assembly have a fire rating. For noncellular metal deck, the fire rating is usually obtained either
by providing sufficient concrete thickness above the metal deck or by applying spray-on fire protec-
tion to the underside of the metal deck. For cellular metal deck, which utilizes outlets that penetrate
the concrete fill, the fire rating is usually obtained by the latter method. As an alternative, a fire-rated
ceiling system can be installed below the cellular or noncellular deck.
When the required fire rating is obtained by concrete-fill thickness alone, lightweight concrete
requires a lesser thickness than normal-weight concrete for the same rating. For example, a 2-hour
1
1
rating can be obtained by using either 3 / 4 in of lightweight concrete or 4 / 2 in of normal-weight con-
crete above the metal deck. The latter option is not often used, since the additional thickness of heavier
concrete penalizes the steel tonnage (i.e., heavier beams, girders, and columns) and the foundations;
however, for situations involving high, nonreducible live loads, such as public assembly areas, the
improved composite action of the beams and girders with the thicker floor deck can result in little or
no penalty to the beam and girder weight.
If spray-on fire protection is used on the underside of the metal deck, the thickness of concrete
above the deck can be the minimum required to resist the applied floor loads. This minimum thick-
1
ness is usually 2 / 2 in, and the less expensive normal-weight concrete may be used instead of light-
weight concrete. Therefore, the two options that are frequently considered for a 2-hour-rated,
1
noncellular floor-deck system are 3 / 4-in lightweight concrete above the metal deck without spray-on
1
fire protection and 2 / 2-in normal-weight concrete above the metal deck with spray-on fire protection
(Fig. 7.6). Since the dead load of the floor deck for the two options is essentially the same, the steel
framing and foundations will also be the same. Thus, the comparison reduces to the cost of the more
expensive lightweight concrete versus the cost of the normal-weight concrete plus the spray-on fire
protection. Since the costs, and contractor preferences, vary with geographical location, the evalua-
tion must be made on an individual project basis. (See also Art. 4.12.)

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.