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3.3.1 Construction Details

An arrangement that makes more extensive use of precast concrete, and avoids the placing of cast-in- place concrete in the congested beam-column joint core regions, is shown in Figure 3.1(b). The success of this system depends on tighter than normal tolerances. The reinforced concrete columns can be either precast or cast-in-place to occupy the clear height between beams. The precast portions of the beams extend from near midspan to midspan, and hence include within the precast element over the columns the complex arrange- ment of joint core hoop reinforcement, which is fabri- cated in the precast factory. The precast portions of the beams are seated on the concrete column below with a suitable jointing material between and propped for construction stability.

Protruding longitudinal column bars from the rein- forced concrete column below pass through preformed vertical holes in the precast beam element and extend above the top surface of the element. The holes in the precast beam elements are preformed using corrugated

steel ducting and are grouted with the horizontal inter- face gap, as discussed in Section 3.6, after the column bars have been passed through.

Protruding bottom bars of the precast beam elements are lapped in the cast-in-place joint at midspan or, alternatively, they can be connected by welding to steel plates that are bolted together in the cast-in-place joint. A precast floor system is seated on top of the precast beam elements and spanning between them. Rein- forcement is then placed in the top of the beam and topping slab, and cast-in-place concrete is poured. One variation on this system is for the top steel of the beam to be cast within the precast beam section. This is particularly suitable for perimeter beams as no edge formwork for the topping is then required. Columns of the next storey are then positioned above the beams using grouted steel sleeves to connect the vertical bars if columns are precast, or using normal reinforced concrete details if columns are cast-in-place.

Figure 3.6(a) shows a 22-storey building under con- struction using this system. The structure consists of moment-resisting perimeter frames with interior frames carrying mainly gravity loading. Some construction details are shown in Figures 3.6(b), (c) and (d).

Figure 3.6(b): Precast concrete beam corner unit being lowered into place using temporary plastic

tubes as guides Figure 3.6(a): Structure of building frame incor-

porating precast concrete beam elements pass- ing through the columns

Figure 3.6(c): Column bars after being grouted in the joint core of a precast concrete beam unit

3.3.2 Some Design Aspects

General

An advantage of System 2 is that the beam-column joint core reinforcement can be incorporated in the

precast concrete beam element and the potential plastic hinge regions in the beam occur within the precast elements away from the vertical cold joints between precast elements and cast-in-place concrete. Also, it is possible to incorporate in precast concrete beam ele-

ments reinforcing details to relocate the potential plas- tic hinge regions away from the column faces if neces- sary (see Figure 3.7).

Splicing at Midspan of Beams

An aspect of the previous concrete design code [3.10] that caused problems in design was the requirement that, when the critical section of a potential plastic hinge region of a beam is located at a column face, no part of the splice of the longitudinal reinforcement was to occur within 2d of the column face, where d is the effective beam depth. To satisfy this requirement the clear span of the beam had to be at least 4d + l_s where l_s is the splice length. This code requirement made it difficult to use beams of relatively short span. However, as reported in Appendix B2, a number of tests on midspan joints have now been conducted [3.14, 3.15, 3.16] and the 1995 edition of the concrete design standard [3.12] modified this requirement by permitting the splice to commence at distance d from the column face. Hence conventional straight bar splices are now possible at midspan of short span beams.

Some details for midspan connections in beams which have been used are illustrated in Figure 3.8. The con- ventional straight bar lap of Figure 3.8(a) can be short- ened by using hooked laps as shown in Figure 3.8(b) and (c). The double hooked lap (see Figure 3.8(c)), involving the use of hooked “drop in” bars, is the most convenient hooked lap to construct. These details, in

some cases with slight modification, have all shown excellent performance in laboratory tests (see labora- tory tests [3.14, 3.15, 3.16] described in Appendix B2) and hence can be recommended as suitable for use. Diagonal reinforcement has been used where shear forces in the beams are high (see Figure 3.8(d)). The design and detailing of the welded connection details require extreme care. Significant vertical ties are re- quired between the bends in the diagonal reinforce- ment to resist the vertical component of the force in the diagonal bars. Also, it should be checked that bearing failure of the concrete cannot occur under the bends of the diagonal reinforcement (see the laboratory tests [3.15, 3.16] described in Appendix B2). The joints should be capacity designed and any eccentricities of plate and reinforcing bars be minimized and provided for by basketing reinforcement. Grinding back of the reinforcing bars to provide 45˚ double-V butt welds may be required to provide the necessary high standard welded connection.

Strength of Grouted Connections

Other aspects of System 2 frames which have been of concern are the performance of grouted column bars and horizontal joints between the columns and the precast beams, and the performance of column ties around the grout-filled ducts which are oversized to provide construction tolerances (see Figures 3.6(c) and 3.9). Laboratory testing in New Zealand [3.14, 3.15, 3.16], summarised in Appendix B2, has indicated that

(a) Reinforcement for conventional plastic hinge positions

A B

A B

(b) Two reinforced arrangements for relocated plastic hinges

Figure 3.8: Some details for midspan connections for beams which have been used in New Zealand column column cast-in-place joint column precast beam column precast beam

(a) Conventional Straight Bar Lap

cast-in-place joint column precast beam ≥ d ≥ _s ≥ d n ≥ 2d + ≥ d ≥ dh ≥ d d d d (b) Hooked lap

(c) Double hooked lap

column precast beam cast-in-place joint precast beam s ≥ d ≥ 2 _dh ≥ d column precast beam overstrength stub steel plates bolted together cast-in-place joint

diagonal bars welded to steel plate

column

precast beam

(d) Diagonal Beam Reinforcement

provided design and construction are adequate, these aspects are satisfactory. The performance of this sys- tem was shown to be similar to that of a conventional cast-in-place joint.

3.4 System 3 - Precast T or