Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.137
requirements. The tendons may be placed in the inclined plates or, more conveniently,
in small thickened edge beams. For cast-in-place, folded-plate construction,
double forming can usually be avoided if the slopes are less than 35 to 40�.
Since larger transverse bending moments develop in folded plates than in cylindrical
shells of about the same proportions, a minimum thickness less than 4 in
creates practical problems of placing the reinforcing steel. A number of area in the
plates will require three layers of reinforcing steel and, near the intersections of
plates, top and bottom bars for transverse bending will be required. Ratios of span
to total depth are similar to those for cylindrical shells, commonly ranging from 8
to 15. (See also F. S. Merritt, ‘‘Standard Handbook for Civil Engineers,’’ Sec. 8,
‘‘Concrete Design and Construction,’’ McGraw-Hill Publishing Company, New
York.)
9.93 SLABS ON GRADE
Slabs on ground are often used as floors in buildings. Special use requirements
often include heavy-duty floor finish (Art. 9.35) and live-load capacity for heavy
concentrated (wheel) load or uniform (storage) loads, or both.
Although slabs on grade seem to be simple structural elements, analysis is extremely
complicated. For design load requirements that are unusually heavy and
outside common experience, design aids are available. Occasionally, the design will
be controlled by wheel loads only, as for floors in hangars, but more frequently by
uniform warehouse loadings. (‘‘Design of Slabs on Grade,’’ ACI 360R; American
Concrete Institute; ‘‘Concrete Floors on Ground,’’ EB075D, Portland Cement Association;
‘‘Design of Floors on Ground for Warehouse Loadings,’’ Paul F. Rice,
ACI Journal, August 1957, paper No. 54-7.)
A full uniform load over an entire area causes no bending moment if the boundaries
of the area are simple construction joints. But actual loads in warehouse usage
leave unloaded aisles and often alternate panels unloaded. As a result, a common
failure of warehouse floors results from uplift of the slab off the subgrade, causing
negative moment (top) cracking. In lieu of a precise analysis taking into account
live-load magnitude, joint interval and detail, the concrete modulus of elasticity, the
soil modulus, and load patterns, a quick solution to avoid uplift is to provide a slab
sufficiently thick so that its weight is greater than one-fifth the live load. Such a
slab may be unreinforced, if properly jointed, or reinforced for temperature and
shrinkage stresses only. Alternatively, for very heavy loadings, an analysis and
design may be performed for the use of reinforcement, top and bottom, to control
uplift moments and cracking. (‘‘Design of Floors on Ground for Warehouse Loadings,’’
Paul F. Rice, ACI Journal, Aug., 1957, paper No. 53-7.)
Shrinkage and temperature change in slabs on ground can combine effects adversely
to create warping, uplift, and top crackling failures with no load. Closely
spaced joint intervals, alternate-panel casting sequence, and controlled curing will
avoid these failures. Somewhat longer joint spacings can be specified if reinforcement
with an area of about 0.002 times the gross section area of slab is provided
in perpendicular directions.
With such reinforcement, warping will usually be negligible if the slab is cast
in alternate lanes 12 to 14 ft wide, and provided with contraction joints at 20- to
30-ft spacings in the direction of casting. The joints may be tooled, formed by joint
filler inserts, or sawed. One-half the bars or wires crossing the contraction joints