Precast Concrete Structures, Second Edition By Kim S Elliott

Precast Concrete Structures, Second Edition By Kim S Elliott
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What makes precast concrete different from other forms of concrete construction? Whether concrete is precast, that is statically reinforced or pretension (prestressed), is not always apparent. It is only when we consider the role concrete will play in developing structural characteristics that its precast nature becomes significant. The most obvious definition of precast concrete is that it is concrete which has been prepared for casting, cast and cured in a location which is not its final destination. The distance traveled from the casting site may only be a few meters, where on-site precasting methods are used to avoid expensive haulage  (or VAT in some countries), or maybe thousands of kilometers, in the case of high-value-added products where manufacturing and haulage costs are low. The grit basted architectural precast concrete in Figure 1.1 was manufactured 600 km from the site, whereas the precast concrete columns, beams, and walls traveled less than 60 m; wall panels have been stack-cast in layers between sheets of polythene adjacent to the final building.

What really distinguishes precast concrete from the cast in situ is its stress and strain response to external (load-induced) and internal (autogenous volumetric changes) effects. These are collectively known as ‘actions’ in the Eurocodes, and those mainly applicable to precast concrete structures are the ‘keynote’ code EC0 (BS EN 1990 2002), the loading or ‘actions’ code EC1 (BS EN 1991-1-1 2002) and the ‘concrete design’ code EC2 (BS EN 1992-1-1 2004).

A precast concrete element is, by definition, of a finite size and must, therefore, be joined to other elements to form a complete structure. A simple bearing ledge or corbel will suffice, as shown in Figure 1.3. But when thermal shrinkage or load-induced strains cause volumetric changes (and shortening or lengthening), the two precast elements try to move apart. Interface friction at the mating surface prevents movement, but in doing so creates a force F = μR which is capable of splitting both elements unless the section was suitably reinforced. Figure 1.5a shows an example of where frictional forces due to relative, unreinforced movement between precast slabs and beams caused spalling in the beam. In other cases, spurious positive bending moments due to the restraint of relative movement or end rotation have caused cracking in the soffit of slabs, or at a beam-to-column corbel connection.

Flexural rotations of the suspended element (the beam) reduce the mating length lb (bearing length), creating a stress concentration until local crushing at the top of the pillar (the column) occurs unless a bearing pad is used to prevent stress concentration. If the bearing is narrow, dispersal of stress from the interior to the exterior of the pillar causes lateral tensile strain, leading to bursting of the concrete at some distance below the bearing unless the section is suitably reinforced.

Content :

What is precast concrete?

Materials used in precast structures

Precast frame analysis

Precast concrete floors

Precast concrete beams

Precast concrete columns

Shear walls

Horizontal floor diaphragms

Joints and connections

Beam and column connections

Ties in precast concrete structures

Design exercise for 10-story precast skeletal frame


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