MATERIAL Y METODOS

Voids are not normally provided near slab edges to ensure a robust and continuous edge detail.

5.3.4 Defl ection control

5.3.5 General considerations

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Prestressed hollowcore units are produced by an extrusion or slipform process with a typical width of 1200 mm, in lengths of up to 200 m. Each length is prestressed before casting. After curing, the units are sawn to the required length. Figure 5.17 shows a typical production layout.

It should be noted that where the only reinforcement in the units is the prestressing strands, as is common, it makes the support zone particularly vulnerable since this is where the maximum stresses due to bearing, shear and anchorage occur. The design should be in accordance with Eurocode 2.

5.4 Prestressed hollowcore

units

Figure 5.17

Typical hollowcore unit production.

Hollowcore units have lateral edges provided with a longitudinal profi le in order to make a shear key for transfer of vertical shear through joints between contiguous elements. For diaphragm action these joints are designed to resist horizontal shear.

Hollowcore units are often specifi ed from manufacturers’ tables rather than designed from fi rst principles. These tables are based on assumed loading, support and reinforcement details, and where the actual situation varies from that assumed in the tables, e.g. the existence of concentrated loads or different fi re rating, detailed calculations should be made to verify such units are appropriate.

BS EN 11683 _{describes the requirements and the basic performance criteria and specifi es}

minimum values where appropriate. It covers terminology, performance criteria, tolerances, relevant physical properties, special test methods and special aspects of transport and erection. Reference should also be made to Precast Prestressed Hollowcore Floors28_.

An example of the design of a hollowcore unit is given in Precast Eurocode 2: Worked

Examples29_.

Resistance at the end of the hollowcore unit relies on the interaction of shear and bond, therefore it is very important to understand the end prestressing conditions of hollowcore units. Figure 5.18 shows how the stress in the prestressing wires or strands and the moment of resistance, builds up from the end of a unit and further guidance is given in Eurocode 2, Cl. 8.10.2.2.

5.4.1 Anchorage of

prestressing tendons

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The transmission length, l_pt, for the prestressing wires or strands is that length required to transmit the full prestress, σ_p. l_pt is defi ned in Eurocode 2, Cl. 8.10.2.3 where σ_pt1 and l_pt1 are the values at ‘transfer’ and σ_pt2 and l_pt2 are the values after all losses (as shown in Figure 5.18). The ultimate design strength of the tendon requires further anchorage length. The slope of the line between σ_pt2 and σ_pd is less than that for the transmission length, l_pt2, because the tendon reduces in size as it is stressed. The reverse is true within the transmission length over which there is a wedging effect. One reason for assuming a linear build-up of stress is because any fl exural stress in this region will tend to reduce the section size and nullify the wedge effects.

5.4.2 Transmission length

Build-up of stress in prestressing wires or strands from end of unit.

The cracking length, l_cr, is the distance from the end of the unit to the point where the bottom fi bre stress resulting from all actions (bending, prestress and horizontal forces at the bearings) equals f_ctd. Figure 5.18 shows the components of actions and the net effect on the bottom fi bre stress. Note that if l_cr is less than l_pt2, the prestress is reduced.

Figure 5.19 indicates the results from the example given in the Precast Eurocode 2: Worked

Examples29_.

The following points are of particular note:

Consider all action effects to determine where the unit is likely to crack. Where dry or mortar bearings are used large horizontal forces may arise from

temperature and shrinkage effects.

In this example the horizontal force at the bearing may cause cracking close to the end of the unit, before l_cr is reached, see Figure 5.19(d).

If cracking does occur close to the support, the shear resistance is likely to be exceeded.

5.4.3 Cracking length

σ σ σ

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a) Stress due to flexure b) Stress due to prestress

c) Stress due to horizontal force at support

d) Net bottom fibre stress showing cracking length,lcr

Support CL of unit Bottom fibre stress C_L Support of unit Bottom fibre stress CL Support of unit Bottom fibre stress Bottom fibre stress 0 fctd lcr Tension Compression CL of unit Possible overstress

near end of unit

fb,m = Mx/Zb

fb,P = P/Ac + Pe/Zb

fb,H = H/Ac + Hyb/Zb

fb,Net = fb, M + fb,P + fb,H

Figure 5.19

Build-up of bottom fi bre stress in concrete from end of unit.

The total anchorage length, l_bpd, is the distance from the end of the unit to the point beyond which the full design resistance of the wires or strands can be obtained, as shown in Figure 5.18.

When the prestress is transferred from the anchor blocks to the hollowcore units, there is anchorage bond along the full length of the strand, apart from the transmission length at each end of the prestressing line. The concrete is then cut into the required lengths and at each end a further transmission length is introduced. Although expressions have been developed to determine the relationship between the end slip of the strands and the transmission length, it has been shown27 _{that, for hollowcore units that have been sawn,}

there is no simple relationship between transmission length and initial slip at these positions. This is discussed further in Section 6.6.

5.4.4 Total anchorage length

5.4.5 Tendon slip at ends of

units

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Figure 5.20 shows the three typical types of end failure that may occur. It should be noted that types a) and b) can interact, one reducing the resistance of the other.

Anchorage bond failure, see Figure 5.20a, may occur due to cracking close to the support which does not allow the full anchorage resistance to develop and strands start to slip. This causes the crack to grow until the unit fails. The most common cause of anchorage failure is when the end of the unit is subject to movement relative to its bearing. This may be the result of the effects of one or more of the following:

shrinkage

temperature changes humidity changes vertical loading.

It is important that the designer considers each of these possible effects. This is especially important for units with spans greater than 8 m. Reference should be made to

Movement, Restraint and Cracking in Concrete Structures26_.