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  • Slab Loads




    Referencing manufacturer drawings and field observations, we are assuming the floor slab is 4” thick normal weight concrete with fibermesh, on .6C28 non-composite steel deck and bar joists 2’-0” o.c. This type of deck is normally used as a form for the pouring of a reinforced slab. After curing, the rein- forced slab is designed to carry all load. The Steel Deck Institute (SDI) specification states that randomly distributed fibrous admixtures shall not be substituted for steel reinforcement. (Sec. 3.2d)

    Due to the lack of welded wire mesh, or deformed bar reinforcing in this slab, the steel deck strength must be maintained long term. This requires that the underside of the deck be maintained with a protective coating. Approved paint, or galv coatings are cited by the SDI. (Sec. 2.2)

    Unreinforced concrete has a tensile strength of approximately 300 psi, Research shows this improves somewhat (to 400-500 psi) with fibermesh admixtures. However, we are unaware of any authority approving fibermesh to supercede the ACI 10.2.5 provision that tensile strength be zero for design. The primary effect of fibermesh in concrete mix is to prevent sudden bursting or shattering. It toughens the concrete, but in no way approaches the strength of continuous steel reinforcing.

    Tanks or other heavy equipment supported by the floor must be installed so as to not depend on the slab and deck bending strength to transfer load to the bar joists. This means that point loads, and line loads running parallel to the joists, must be avoided, or else some means of bridging the load to the joists be provided. (See sketches). In general, rigid equipment and flat bottomed tanks that are spread out over 2 or more joists will not depend upon the slab and deck to transfer load to the joists.

    The steel deck alone, spanning 2’-0” between joists, supports 152 psf uniform load at its design capacity. This is about ¾ of the original total design load. The deck will support a 95 lb. point load plus the concrete weight at its design capacity, or 150 lb. point load with no concrete weight. It is reasonable to assume that the fibermesh slab can support it’s own weight spanning the 2’-0”, and since it will in fact take some bending, a 150 lb. limit is probably somewhat low. A point load spreads out to an 8” dia. or larger circle on the steel deck. Therefore using 2 x 152 (psf) = 304 lb., provides a reasonable limit on point loads, beyond which steps should be taken to spread the load across a minimum of 2 joists. Note: point loads are assumed to be spaced at least 12” apart.

    Calculations utilize an allowable deck steel stress of 26 ksi.

    References:

    “Reinforced Concrete Design” Wang, Salmon, 1979

    “Designing with Steel Joists, Joist Girders, Steel Deck” Fisher, West, Van de Pas, 1991

    “Concrete Reinforcement with Recycled Fibers” Wang, Wu, Li, Journal of Civil Engineering Materials, Nov. 2000

    “SDI Specifications and Commentary” Steel Deck Institute, Vulcraft Steel and Floor Deck, 1996

    “Building Code and Commentary ACI 318-95” American Concrete Institute, 1995


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