6.4.1 General
6.4.1.1 The provisions of this subclause apply to flexural composite elements consisting of precast concrete units acting in conjunction with added concrete where provision has been made for the transfer of horizontal shear at the contact surface. The precast units may be of either reinforced or prestressed concrete. Analyse and design composite concrete structures and elements in accordance with clause 4 or clause 5, modified, where appropriate, in accordance with 6.4.3 and 6.4.4. Pay particular attention, in the design of both the components and the composite section, to the effect of the method of construction, on stresses and deflections, and to whether or not propping is to be used.
6.4.1.2 Base the relative stiffnesses of elements on the properties of the concrete, gross or transformed sections, as described in 3.4.3.1. If the concrete strength in the two components of a composite element differs by more than 10 MPa, make allowance for this when stiffness is being assessed.
6.4.1.3 Differential shrinkage of the added concrete and precast concrete units may require consideration in analysing composite elements for the serviceability limit states (see 6.4.3.4);
differential shrinkage need not be considered for the ultimate limit state.
6.4.1.4 When precast prestressed units, having pre-tensioned tendons, are designed as continuous elements and continuity is obtained with reinforced concrete cast in-situ over the supports, the compressive stresses due to prestress in the ends of the units may be ignored over the transmission length of the tendons when the strength of sections is being assessed.
6.4.2 Shear
6.4.2.1 Carry out the analysis of the resistance of composite sections to vertical shear due to design ultimate loads in accordance with 4.3.4 for reinforced concrete and 5.3.4 for prestressed concrete.
However, when in-situ concrete is placed between precast prestressed units and the composite concrete section is used in design, ensure that the principal tensile stress does not exceed 0,24 fcu anywhere in the prestressed units; calculate this stress by making due allowance for the construction sequence and by taking into account only 0,8 of the compressive stress due to prestress at the section under consideration.
6.4.2.2 Calculations for horizontal shear between the two components of a composite section are governed by the ultimate limit state. The methods given in 6.4.4.1 to 6.4.4.4 ensure that composite action does not break down for the serviceability limit states and that the design shear strength is adequate for the ultimate limit state.
6.4.3 Serviceability limit states 6.4.3.1 General
In addition to the provisions given in clause 4 and clause 5 concerning deflection and control of cracking, the design of composite construction will be affected by the provisions of the following subclauses.
6.4.3.2 Compression in the concrete in the case of prestressed precast units
For composite elements comprising prestressed precast units and in-situ concrete, the methods of
analysis may be as given in 5.3.3. However, the compressive stresses in the precast unit at the interface may be increased by not more than 50 % above the value given in table 29, provided that the ultimate failure of the composite element is due to excessive elongation of the steel.
6.4.3.3 Tension in the concrete in the case of prestressed precast units
When there is a danger of corrosion (e.g. if there is non-prestressed reinforcement in the in-situ
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concrete), the flexural tensile stress in the in-situ concrete should be limited by crack control
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measures, in accordance with 4.3.7. Amdt 1, Apr. 1994
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Table 41 - Deleted by amendment No. 1.
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Where continuity is obtained with reinforced concrete cast in-situ over the supports, the flexural tensile stresses and the hypothetical tensile stresses in the precast prestressed units at the supports should
be limited in accordance with 5.3.2.3. Amdt 1, Apr. 1994
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6.4.3.4 Differential shrinkage
6.4.3.4.1 The effects of differential shrinkage are not generally of great importance in simply supported elements. However, where there is an appreciable difference between the age and quality of the concrete in the components, differential shrinkage may lead to increased stresses in the composite section and these must be investigated. The effects of differential shrinkage are likely to be more severe when the precast component is of reinforced concrete or of prestressed concrete with an approximately triangular distribution of stress due to prestress. In particular, the tensile stresses due to differential shrinkage may require consideration in design, and the engineer should refer to specialist literature in deciding when these stresses may be significant.
6.4.3.4.2 In the calculation of the tensile stresses, a value will be required for the differential shrinkage coefficient (the difference in total free strain between the two components of the composite element), the magnitude of which will depend on many variables. For a structure in a normal environment, and in the absence of more exact data, assume a value of 100 x 10-6 for the differential shrinkage when calculating stresses in composite T-beams with an in-situ concrete flange.
6.4.3.5 Continuity in composite construction
6.4.3.5.1 When continuity is obtained in composite construction by providing reinforcement over the supports, give consideration to the secondary effects of differential shrinkage and creep on the moments in continuous beams and on the reactions at the supports. Take the hogging restraint moment Mcs at an internal support of a continuous composite beam and slab section due to differential shrinkage as
Mcs = εdiffEcfAcfacent ψ (19)
where
εdiff is the differential shrinkage strain;
Ecf is the modulus of elasticity of the flange concrete;
Acf is the area of effective concrete flange;
acent is the distance from the centroid of the concrete flange to the centroid of the composite section;
ψ is a reduction factor to allow for creep, taken as 0,37 (see 6.4.3.5.4).
6.4.3.5.2 The hogging restraint moment Mcs will be modified with time by creep due to self-weight load and creep due to any prestress in the precast units. The restraint moment due to prestress may be taken as the restraint moment that would have been set up if the composite section as a whole had been prestressed, multiplied by a reduction factor ψ1 taken as 0,92 (see also 6.4.3.5.4).
6.4.3.5.3 Use the information given in 6.4.3.4 for assessing a value for the differential shrinkage strain.
6.4.3.5.4 Equation (19) for calculating the restraint moments due to creep and differential shrinkage is based on an assumed value of 2,5 for the ratio βcc of total creep to elastic deformation. If the design conditions are such that this value is significantly low, then the engineer should calculate values for the reduction factors ψ and ψ1 from the following:
ψ (1 e βcc
) βcc
ψ1 (1 e βcc
)
where e is the base of Napierian logarithms.
6.4.4 Ultimate limit state
6.4.4.1 Horizontal shear force due to design ultimate loads
The interface of the precast and in-situ components occurs either in the tension zone or in the compression zone affecting the horizontal shear force due to design ultimate loads so that this shear force is either:
a) where the interface is in the compression zone: the compression from that part of the compression zone above the interface, calculated from the ultimate bending moment; or
b) where the interface is in the tension zone: the total compression (or tension) calculated from the ultimate bending moment.
6.4.4.2 Average horizontal design shear stress
The average horizontal design shear stress is calculated by dividing the design horizontal shear force (see 6.4.4.1) by the area obtained by multiplying the contact width by the beam length between the point of maximum positive or negative design moment and the point of zero moment.
The average horizontal design shear stress should then be distributed in proportion to the vertical design shear force diagram, to give the horizontal shear stress at any point along the length of the composite component. The horizontal design shear stress v so detained, should nowhere exceed the
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appropriate value in table 42. Amdt 2, Mar. 2000
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Ah = 1 000 bvh 0,87fy where
b is the contact width;
vh is the average horizontal design shear stress, as in 6.4.4.2; and fy is the characteristic strength of links.
6.4.4.5 Differential shrinkage between added concrete and precast units
Differential shrinkage between added concrete and precast units need not be considered for the ultimate limit state.
6.4.5 Thickness of structural topping
The recommended minimum thickness of structural topping is 40 mm nominal with a local minimum of 25 mm.