Inversió pública i dotació d’infraestructures

4.3.1 General

In monolithic construction, precast elements are joined

continuous vertical reinforcement of vertical joints and adjacent to panel ends within full height metal ducts

cast-in-place joint concrete

prefinished hollow core floor planks.

n bars from top and bottom wall panels.

precast concrete wall panels

floor ties at the junction of each plank

Figure 4.1: Nearly monolithic precast wall construction horizontal joint of the SCT System, Yugoslavia (Reference 4.12)

horizontal hairpin bars. By altering the size and spacing of these bars the performance of the joint can vary from “jointed behaviour to “monolithic” behaviour

continuous vertical reinforcement adjacent to panel ends within full height metal ducts

precast concrete wall panels continuous vertical

reinforcement within the vertical joints

cast-in-place concrete joint

Figure 4.2: Nearly monolithic precast wall construction vertical joint of the SCT System, Yugoslavia (Reference 4.12)

by reinforced concrete connections possessing stiff- ness, strength, and ductility believed to be comparable to cast-in-place concrete. Monolithic precast wall sys- tems are often used as a cost-effective alternative to cast-in-place ductile cantilever shear walls or ductile coupled shear walls. Designers expect these to be capable of sustaining the significant inelastic deforma- tions required of a ductile structure.

a proportion of vertical bars are lap welded together cast-in-place concrete fill precast concrete floor planks precast concrete wall panels

Figure 4.3: Jointed precast wall construction example of horizontal joint for 2-storey houses in Japan (Reference 4.9)

cast-in-place concrete column

precast concrete wall panels

the joint tends to behave “Monolithically” if all horizontal reinforcement extends into the joint and is lap welded. If only a proportion of horizontal reinforcement extends into the joint then the joint will behave in a “Jointed” manner.

Figure 4.4: Jointed or monolithic precast wall construction example of vertical joint for 2-storey houses in Japan (Reference 4.9)

The satisfactory performance of well-detailed cast-in- place concrete walls has been established by exhaus- tive laboratory testing and observation of their behav- iour during earthquakes. If designers of precast con- crete wall systems detail the connections between the components to possess stiffness, strength and ductility comparable to cast-in-place concrete, then the com- pleted walls can be expected to perform satisfactorily.

Care is needed, however, in design and construction as indicated below.

4.3.2 Design of Monolithic Walls

In the absence of a New Zealand code of practice written specifically for precast concrete construction, designers usually design monolithic precast structural walls to the requirements of NZS 3101 [4.1].

continuous reinforcement at ends of panels only

cast-in-place concrete infill

precast concrete wall panels

Figure 4.5: Jointed precast wall construction horizontal joint of the “Rad” System, Yugoslavia (Reference 4.15)

precast concrete wall panels cast-in-place concrete infill

by varying the size and spacing of the hairpins the joint can behave in either a “Monolithic” or “Jointed” manner

Figure 4.6: Jointed or monolithic wall construction vertical joint of the “Rad” System, Yugoslavia (Reference 4.15)

Since NZS 3101 has been developed principally for cast-in-place concrete construction, there are some difficulties in applying this standard to precast con- crete wall designs. Care is needed when applying the dimensional limitation rules. In NZS 3101 it is as- sumed that structural walls have full flexural continu- ity about their minor axis at each level of lateral restraint (floor level). With precast concrete walls there may be a major discontinuity in the flexural

strength about their minor axis at each horizontal joint location. Designers are therefore encouraged to adopt a conservative approach to the ratio of the clear storey height to the wall thickness.

NZS 3101 requires two layers of longitudinal (vertical) reinforcement when structural walls are more than 200 mm thick. This can be easily achieved over the height of the wall panels, but having two layers of reinforce-

Note that the vertical continuity reinforcement occurs at the end of each wall panel only (USA example [4.10, 4.20])

grouted floor/wall joints

prefinished hollow core floor planks

bearing strip

grout placed within 1 hour of stressing 2 post tensioning Dywidag bars at each end of wall

panels precast concrete

wall panels grouting duct cast-in stressing head stressing plate stressing nut

paper or plastic dam floor ties at 1200 crs in each floor slab joint dry pack for 150 each side of stressing bar alternative location of coupler sleeve 600 mm unbonded in plastic sheath coupler sleeve

Figure 4.7: Jointed precast wall construction horizontal joint — platform type construction

vertical continuity reinforcement of ends of wall panels only (welded or bolted connections) cast-in-place infills at ends of panels precast concrete floor panels precast concrete wall panels

Figure 4.8: Jointed precast wall construction elevation of horizontal joint (Reference 4.8)

ment at horizontal panel joints is usually impractical. Furthermore, NZS 3101 has a restriction that the diam- eter of bars used in any part of a wall shall not exceed one-tenth of the thickness of that wall. This limitation can often be a major constraint when using central vertical lap bars at horizontal joints that lap with pairs of smaller bars placed either side of the central bar (see Figure 4.10, Section A-A).

NZS 3101 typically requires staggering of the splices of the principal vertical flexural tension reinforcement within potential plastic hinge zones. This is very difficult to achieve with practical details. Usually, laps in the cast-in-place concrete bandage joints and the cast-in-place end wall thickenings are staggered in level from those in the precast wall sections. Yielding of the bars splicing the precast wall sections together is

usually avoided by selecting a lap bar size and/or steel grade that is slightly stronger than the vertical bars that lap with it, i.e. one D24 (452 mm2_{) could lap with two}

D16 bars (402 mm2_{). This helps to minimise flexural}

cracking over the lapping region.

Research is therefore needed to check the sensitivity of monolithic precast concrete wall design and construc- tion to the difficulties which have been identified. 4.3.3 Vertical Joints

Vertical joints between wall panels are typically cast- in-place “bandage” type joints. Horizontal reinforcing from a precast panel projects into the joint zone and is lapped with horizontal reinforcing from an adjacent panel. The amount and spacing of the horizontal shear reinforcement is established using capacity design principles and concrete code requirements [4.1]. The widths of the cast-in-place vertical joints are deter- mined by concrete code requirements for lap lengths of the horizontal reinforcement. Typical details of mono- lithic vertical joints are shown in Figure 4.9.

A wide range of details are in common use in New Zealand. The design of the lap or splice regions should comply with the requirements of NZS 3101, or alterna- tively be laboratory tested. In many instances, the ends of monolithic precast walls have cast-in-place columns which improve the wall stability where flexural rein- forcement and transverse stirrups and ties are concen- trated.

4.3.4 Horizontal Joints

The vertical reinforcement in precast walls is usually lapped at horizontal joints. Proprietary grouted steel sleeve splices may be used for this purpose, or alterna- tively the lap can be formed by grouting a bar extending from one unit into a metal duct in the matching unit. The spacing of steel sleeve splices or metal ducts is usually at no more than 450 mm centres to comply with the maximum spacing provisions of NZS 3101. Some typical details of horizontal joints in monolithic con- struction are shown in Figure 4.10. Some steel sleeve splices have been cyclic load tested and comply with the concrete code requirements as high strength me- chanical connectors suitable for use in a plastic hinge zone, provided that the grout used complies with the sleeve manufacturer’s specification [4.7].

When corrugated metal preformed pipe ducts are used, starter bars that project into the ducts are usually designed for a full lap length as defined in NZS 3101. Generally central starter bars are lapped with pairs of smaller bars, one on each face of the precast concrete wall section or, alternatively, all of the main flexural reinforcement is lapped on the precast concrete wall

centreline and some additional basketing cover rein- forcement is provided.

The horizontal joint between precast concrete panels is usually scabbled or retarded and cement paste removed to provide appropriate surface roughness to avoid a sliding shear failure. The joint is then grouted using the method outlined in Chapter 6. When using steel sleeve splices, these are usually grouted individually. Foam plastic rings are used to seal the base of the steel sleeve splices to allow the separate grouting of the sleeves and the horizontal joint between the precast concrete panels.