Building Design and Construction Handbook - Merritt - Ventech!

9.144 SECTION NINE
TABLE 9.27 Maximum Joint Widths for Sealants
Type of Sealant
Maximum joint
width, in
Maximum
movement,
tension, and
compression, %
Butyl 3 ⁄4 �10
Acrylic 3 ⁄4 �15–25
One-part polyurethane 3 ⁄4 �20
Two-part polyurethane 3 ⁄4 �25
One-part polysulfide 3 ⁄4 �25
Two-part polysulfide 3 ⁄4 �25
(‘‘PCI Manual for Structural Design of Architectural Precast Concrete,’’ Precast/
Prestressed Concrete Institute.)
9.103 LIFT SLABS
Lift slabs are precast-concrete floor and roof panels that are cast on a base slab at
ground level, one on top of the other, with a bond-breaking membrane between
them. Steel collars are embedded in the slabs and fit loosely around the columns.
After the slabs have cured, they are lifted to their final position by a patented jack
system supported on the columns. The embedded steel collars then are welded to
the steel columns to hold the lift slabs in place. This method of construction eliminates
practically all formwork.
PRESTRESSED-CONCRETE CONSTRUCTION
Prestressed concrete is concrete in which internal stresses have been introduced
during fabrication to counteract the stresses produced by service loads. The prestress
compresses the tensile area of the concrete to eliminate or reduce the tensile stresses
caused by the loads.
9.104 BASIC PRINCIPLES OF
PRESTRESSED CONCRETE
In the application of prestress, the usual procedure is to tension high-strength-steel
elements, called tendons, and anchor them to the concrete, which resists the tendency
of the stretched steel to shorten after anchorage and is thus compressed. If
the tendons are tensioned before concrete has been placed, the prestressing is called
pretensioning. If the tendons are tensioned after the concrete has been placed the
prestressing is called posttensioning.
Prestress can prevent cracking by keeping tensile stresses small, or entirely
avoiding tension under service loads. The entire concrete cross-section behaves as