Section 5. Design and detailing: precast and composite construction
5.4 Composite concrete construction 1 General
This sub-clause is applicable where precast reinforced or precast prestressed concrete units combine with added in-situ concrete to resist flexure, provision for horizontal shear transfer having been made at the contact surface.
5.4.2 Analysis and design of composite concrete structures and members
The analysis and design of composite concrete structures and members should be in accordance with Section 3 or Section 4 (modified where appropriate by 5.4.6 and 5.4.7).
5.4.3 Effects of construction methods
The design of component parts as well as composite sections should take account of construction methods and whether props are used; stresses and deflections will both be affected.
5.4.4 Relative stiffness of members
The relative stiffness of members should be based on the concrete gross or transformed section; if the concrete in the two components of a composite member differs by more than 10 N/mm2 an appropriate
allowance should be made.
5.4.5 Assessment of strengths of sections of precast pre-tensioned units designed as continuous members
When such units are designed as continuous members 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 for the tendons.
5.4.6 Serviceability limit states 5.4.6.1 Serviceability
5.4.6.1.1 General
In addition the recommendations given in Section 3 and Section 4 concerned with deflection and control of cracking, for composite construction (except generally, with simply-supported members) differential shrinkage may be relevant (see 5.4.6.4.2 and 5.4.6.4.3). Horizontal shear is governed by the ultimate limit state. The methods given in 5.4.7.1, 5.4.7.2, 5.4.7.3 and 5.4.7.4 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.
5.4.6.1.2 Prestressed precast units
Where these are used, the methods of the analysis may be given in 4.3.4. However, the compressive stresses in the precast unit at the interface may be increased above the values given in 4.3.4.2 by not more than 50 % provided the failure of the composite member would be by excessive elongation of the steel.
5.4.6.2 Tension in in-situ concrete
5.4.6.2.1 Prestressed precast units in direct contact with in-situ concrete
When flexural tensile stresses are induced in the in-situ concrete by imposed service loading, the design tensile stresses in the in-situ concrete at the contact surface should be limited to the values given in Table 5.4. However, these values may be increased by 50 % provided the design tensile stress in the prestressed concrete unit is reduced by the same numerical amount.
Table 5.4 — Design flexural tensile stresses in in-situ concrete
5.4.6.2.2 Prestressed precast units not in direct contact with in-situ concrete
In this case the design flexural tensile stresses in the in-situ concrete should be limited by cracking considerations in accordance with 3.12.11.2.3, 3.12.11.2.4, 3.12.11.2.5, 3.12.11.2.6, 3.12.11.2.7,
3.12.11.2.8 and 3.12.11.2.9. If greater bar spacings are required, e.g. between groups of bars, the crack
widths should be checked by calculation (see 3.8 of BS 8110-2:1985).
5.4.6.3 Tension in prestressed precast units
Where continuity is obtained with reinforced concrete cast in-situ over the supports, the design flexural tensile stresses and the hypothetical tensile stresses in the prestressed precast units at the supports should normally be limited in accordance with 4.3.4.3.
5.4.6.4 Differential shrinkage 5.4.6.4.1 General
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 should 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; the engineer should refer to the specialist literature in deciding when these stresses may be significant (see also Section 7 of BS 8110-2:1985).
5.4.6.4.2 Calculation of tensile stresses
When calculating tensile stresses, it is necessary to know the differential shrinkage coefficient (the difference in total free strain between the two components of the composite member), the magnitude of which will depend on many variables.
5.4.6.4.3 Approximate value of differential shrinkage coefficient for building in a normal environment
In the absence of more exact data, a value of 100 × 10–6 may be assumed for the differential shrinkage in
computing T-beams with an in-situ concrete flange.
5.4.7 Ultimate limit state
5.4.7.1 Horizontal shear force due to design ultimate loads
At the interface of the precast and in-situ components the horizontal shear force due to design ultimate loads is either:
a) where the interface is in the tension zone: the total compression (or tension) calculated from the ultimate bending moment; or
b) 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.
5.4.7.2 Average horizontal design shear stress
The average horizontal design shear stress is calculated by dividing the design horizontal shear force (see 5.4.7.1) 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 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 member. The design shear stress vh, should be less than the appropriate value in Table 5.5.
Strength class of in-situ concrete Maximum tensile stress
N/mm2
C20/25 3.2
C25/30 3.6
C32/40 4.4
5.4.7.3 Nominal links
When provided, nominal links should be of cross-section at least 0.15 % of the contact area. Spacing should not be excessive. The spacing of links in T-beam ribs with composite flanges should not exceed four times the minimum thickness of the in-situ concrete nor 600 mm, whichever is the greater. Links should be adequately anchored on both sides of the interface.
5.4.7.4 Links in excess of minimum
Where the horizontal shear stress from 5.4.7.2 exceeds the value given in Table 5.5, all the horizontal shear force should be carried on reinforcement anchored either side of the interface.
The amount of steel required Ah (in mm2/m) should be calculated from the following equation:
5.4.7.5 Vertical shear 5.4.7.5.1 General
The design of composite members resisting vertical shear due to design ultimate loads should be carried out in accordance with 3.4.5 for reinforced concrete and with 4.3.8 for prestressed concrete.
For prestressed members, no shear calculations are necessary for the serviceability limit state.
5.4.7.5.2 In-situ concrete between precast prestressed units
In such cases, when the composite concrete section is used in design, the design principal tensile stress should not exceed 0.24Æfcu anywhere in the prestressed units; this stress should be calculated by making
due allowance for the construction sequence and by taking 0.8 times the compressive stress due to prestress at the section considered.
5.4.8 Differential shrinkage between added concrete and precast members
Differential shrinkage between added concrete and precast members need not be considered for the ultimate limit state.
5.4.9 Thickness of structural topping
The minimum thickness recommended is 40 mm nominal with a local minimum of 25 mm.
5.4.10 Workmanship
Workmanship is important in providing a good shear connection. In general, the topping should be well vibrated onto a surface that has been dampened but is without standing water (see 6.2.2 and 6.2.3).
equation 62
Ah 1 000bvh
0.87fy --- =
Table 5.5 — Design ultimate horizontal shear stresses at interface
Precast unit Surface type Strength class of in-situ concrete
C20/25 C25/30 C32/40 and
over
Without links As-cast or as-extruded 0.4 0.55 0.65 Brushed, screeded or rough-tamped 0.6 0.65 0.75 Washed to remove laitance or
treated with retarder and cleaned 0.7 0.75 0.80 With nominal links projecting
into in-situ concrete As-cast or as-extruded Brushed, screeded or rough-tamped 1.81.2 1.82.0 2.02.2 Washed to remove laitance or
treated with retarder and cleaned 2.1 2.2 2.5
NOTE 1 The description “as-cast” covers those cases where the concrete is placed and vibrated leaving a rough finish. The surface is rougher than would be required for finishes to be applied directly without a further finishing screed but not as rough as would be obtained if tamping, brushing or other artificial roughening had taken place.
NOTE 2 The description “as-extruded” covers those cases in which an open-textured surface is produced direct from an extruding machine.
NOTE 3 The description “brushed, screeded or rough-tamped” covers those cases where some form of deliberate surface roughening has taken place but not to the extent of exposing the aggregate.