Mitchell's Structure & Fabric Part 2: Pt. 2 (Mitchell's Building Series) by J S Foster

Author:J S Foster [Foster, J S]
Language: eng
Format: azw3
ISBN: 9781317904274
Publisher: CRC Press
Published: 2013-11-18T16:00:00+00:00

Figure 4.27 Precast concrete structures

These illustrations show storey height frames with a pair of columns, but they can be constructed three or four columns wide and with only a top beam, provided the beam is substantial enough to withstand the hoisting stresses and the weight is within the capacity of the lifting crane. Two- or three-storey height frames can also be used. These multiple frames reduce construction time by the reduction in the number of joints to be made.

Column connections are illustrated in figure 4.27 A, B, C, details 1, 2, 3. They may also be made by means of high tensile steel bars passed through holes formed through the height of the columns. These are connected at the joints by screwed couplers and tightened by a torque-controlled spanner. Connections between beam ends are made by coupling plates or rebated ends and dowels. Rectangular frames may be formed of half-columns at each side and half-beams at head and foot. The half-columns are channel shaped so that when erected a void is formed between them which is filled with in situ concrete, which can be reinforced if necessary. The half-beams are bolted together.

Composite structure Reference is made above to the use of precast soffit elements as permanent shuttering to portions of in situ cast work required to form T- and L-beams at the junctions of precast beams and slabs. The use of reinforced precast concrete units as permanent shuttering, designed to act with in situ concrete to form a composite structure as illustrated in figure 4.27 E, is a means of obtaining the continuity and rigidity inherent in in situ cast work without the use of normal formwork. It also reduces the amount of precast work which factory overheads and transport costs tend to make more expensive than in situ work.

For economy the units should be shallow, but for ease of handling their thickness should not be less than one-fortieth of the length. In order to obtain units of reasonable length, therefore, the section should be of such a shape as to give stiffness to a thin member (see figure 4.27 E) or be stiffened in some other way. In the case of beams the pre-cast element should extend no higher than the soffit of the floor slab, so that in some instances the precast unit may be only a shallow strip carrying the tensile reinforcement similar to the prestressed element shown. To stiffen this during transport, stirrups and any top reinforcement for the beam may be introduced. Increased stiffness is given if diagonal bars are used to form the reinforcement into a lattice girder similar to the elements of this type used for floor slab construction shown in figure 5.2 G.

Satisfactory bond between the precast units and the in situ cast concrete is essential in order to transfer shear stress. A roughened surface on the precast unit is usually adequate, but this does assume good site supervision and workmanship in forming the junction between the two. A definite mechanical bond is preferable, achieved by means of projecting wire stirrups or castellations.

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