Building Design and Construction Handbook - Merritt - Ventech!

9.142 SECTION NINE
depth. Hollow-core slabs may come with grouted shear keys to distribute loads to
adjacent units over a slab width as great as one-half the span.
Manufacturers should be consulted for load and span data on hollow-core slabs,
because camber and deflection often control the serviceability of such units, regardless
of strength.
(‘‘PCI Design Handbook,’’ Precast/Prestressed Concrete Institute.)
9.101 PRECAST RIBBED SLABS, FOLDED
PLATES, AND SHELLS
Curved shells and folded plates have a thickness that is small compared with their
other dimensions. Such structures depend on their geometrical configuration and
boundary conditions for strength.
Thickness. With closely spaced ribs or folds, a minimum thickness for plane
sections of 1 in is acceptable.
Reinforcement. Welded-wire fabric with a maximum spacing of 2 in may be used
for slab portions of thin-section members, and for wide, thin elements 3 in thick
or less. Reinforcement should be preassembled into cages, using a template, and
placed within a tolerance of �0 inor� 1 ⁄8 in from the nearest face. The minimum
clear distance between bars should not be less than 1 1 ⁄2 times the nominal maximum
size of the aggregate. For minimum concrete cover of reinforcement, see Art. 9.97.
Compressive Strength. Concrete for thin-section, precast-concrete members protected
from the weather and moisture and not in contact with the ground should
have a compressive strength of at least 4000 psi at 28 days. For elements in other
locations, a minimum of 5000 psi is recommended.
Analysis. Determination of axial stresses, moments, and shears in thin sections is
usually based on the assumption that the material is ideally elastic, homogeneous,
and isotropic.
Forms. Commonly used methods for the manufacture of thin-section, precastconcrete
members employ metal or plastic molds, which form the bottom of the
slab and the sides of the boundary members. Forms are usually removed pneumatically
or hydraulically by admitting air or water under pressure through the bottom
form.
(‘‘Architectural Precast Concrete,’’ Precast/Prestressed Concrete Institute.)
9.102 WALL PANELS
Precast-concrete wall panels include plain panels, decorative panels, natural stonefaced
panels, sandwich panels, solid panels, ribbed panels, tilt-up panels, loadbearing
and non-load-bearing panels, and thin-section panels. Prestressing, when
used with such panels, makes it possible to handle and erect large units and thin
sections without cracking.