Precast concrete construction

5.2.1 Framed structures and continuous beams

It is in general more difficult to provide full moment continuity in precast concrete construction than in in situ structures but, where this is to be the basis of design. then the procedures given in Sections 3 and 4 in the Code may be adopted. including the redistribution of moments. Redistribution may be particularly useful in reducing design moments at connections.

5.2.2 Slabs

5.2.2.1 Design of slabs

Again the basis for analysis and design of precast slabs should be that given in Section 3 or 4 as appropriate.

5.2.2.2 Concentrated loads on slabs without reinforced topping

This clause makes empirical recommendations on the width of a slab (perhaps made up 111

Handbook (0 BS8IIO:19&5

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of a number of precast units) which can be considered to be helping to resist concentrated loads, including line loads from partitions in the direction of the span. The type of partition will have a considerable influence on the way the load is distributed transversely across the slab: moreover, the type and width of the precast units and the connection betxveen them can have a considerable influence. A limited amount of testing has been carried out on a range of standard floor units and generally this has shown that the actual transverse distribution can be defined accurately by means of the load distribution or grillage analysis for bridge decks. which are in common use in this country. If. in a particular case, a more accurate assessment is required than is given by this clause.

references 5.1., 5.2 and 5.3 should-be consulted.

Many manufacturers will have results from load tests on their units in structures and I

these should be available for guidance.

5.2.2.3 Concentrated loads on slabs with reinforced topping The comments to 5.2.2.2 apply here also.

5.2.2.4 Slabs carrying concentrated loads

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5.2.3 Bearings for precast members

The definitions in 1.2.5 are important and have specific meanings when used in the following clauses.

5.2.3.1 General

This section comes from the work of a Committee of the Institution of Structural

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Engineers(S.4t. The clauses do not require that there is a definite check on the provision of overlap of reinforcement (a); it is clearly impossible in bearings on brickwork etc.

The use of the clauses will however give overlap where it is appropriate to be provided.

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5.2.3.2 Calculation of net bearing width of non-isolated members

When assessing the effect of potential rotation. the restraint and support of the supporting I

member should be considered when assessing its likely rotation.

5.2.3.3 Effective bearing length

Figure 5.4 is in error in that the vertical dimension in the lower part of the figure is shown as bearing width when it should show bearing length. see definition 1.2.5.5.

5.2.3.4 Design ultimate bearing stress

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The requirement to rely upon the weaker of the bearing surfaces is clearly important if the bearing length of the supported member is similar to the available length for bearing

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of the supporting member and vice-versa. Where for example one member is narrow with respect to the other, higher bearing stresses in the wider member. subject to test or the provisions of reinforcement to prevent bursting, based on clause 4.11 in Part 1.

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will be appropriate.Higher bearing stresses than 0.8f~ may be used when justified by tests. References 5.5^—5.9 provide data on bearing stresses.

5.2.3.5 iVet bearing width of isolated members

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5.2.3.6 Detailing for simple bearing

This refers fonvard to Clauses 5.2.3.7 and 5.2.4.

5.2.3.7 Allowances for effects of spalling at supports

Plastic load shedding packs are now available which reduce the effects of the problett3.

described in 5.2.3.7.4.

5.2.4 Allowance for construction inaccuracies

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112 5.2.5 Bearings transmitting compressive forces from above

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K4K ~4K4K4 •4~ 4K ..K. — K — — . .

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Pan]: Section S 5.2.6 Other f~rces at bearings

52.6.1 Horizontal forces at bearing

Particular attention should be placed on the detailing of both supporting and supported member. Continuity reinforcement must be anchored to both members in such a way as to avoid planes of weakness away from the support. It should be realised that the provision of tensile restraint will render both supporting and supported member prone to tension cracking. Reinforcement should be provided to minimise crack widths in this regard since it can be a serious cause of failureif proper provision to control and distribute cracks is not made.

It will often be sufficient, instead of providing a full sliding bearing (a), to provide a flexible bearing which allows sufficient capability to move laterally.

5.2.6.2 Rotation at bearing offlexural members

The use of suitable elastic bearing materials will do much to distribute and smooth out the bearing stresses.

5.2.7 Concrete corbels 5.2.7.1 General 5.2.7.2 Design

The essence of the design method recommended for a corbel is the assumption that it behaves as a simple strut-and-tie system, as indicated in Figure H5. 1 for loads appropriate to the ultimate limit state. So that it can function in this way, it is first necessary to eliminate the possibility of a shear failure and5.2.7suggests that the total depth of the corbel (h) be determined from shear considerations in accordance with 3.4.5.8. The corbel width(b) will normally be determined from practical considerations and the size of the bearing plate transmitting the ultimate load (Va) to the corbel should then be calculated by using a bearing stress not greater than 0.8f~, as suggested in 5.2.3.4, provided that it may be shown that the horizontal force at the bearing is low (less than 0.1 Va).

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A

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0.454~cos (3 0.9x 0.9xc

m tcl force diagram

The requirements of 5.2.7 for the proportioning of the corbel and the detailing of the reinforcement are illustrated in Figure HS.2. Of the three methods shown for anchoring the main tension steel (A~1) at the outer face of the corbel, that in diagram (a) is the most efficient technically. It also has some practical advantages in that the ratio a~/d is higher than for the other two methods where the requirements of 3.12.8.24 regarding the minimum radii of bends haye to be met for the main tension steel.

For higher a~/d ratios, design will be controlled principally by flexure at section A-A (Figure H5.l). Particular attention has to be paid to the occurrence of horizontal forces at the bearing, since these can considerably reduce the corbel strength; this problem is discussed and dealt with in reference 5.10.

5.2.7.2.1 Simplifying assumptions

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Figure HS.1: Design basis for corbels (5.2.7.2).

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Ha,tdbookto BS8I/O:]985

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II (A.,)

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Cal ~ —

...~..L... outside edge of bearing

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-I - tobe kept clear of bendin I main reinforcement (minimum

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DETAILING RULES

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~ma form of hot zontal loopsn re nforcement in the

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II . bars provided to anchor (cL... L~II II horizontal stirrups

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Figure ff5.2: Possible methods of anchoring main tension reinforcement in corbels.

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5.2.7.2.2 Reinforcement anchorage 3.2.7.2.3 Shear reinforcement

5.2.7.2.4 Resistance to applied horizontal force

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5.2.8 Continuous concrete nibs

Reference 5.11 has a considerable amount of information on the reinforcement of nibs.

The designer often has to consider the strength of continuous nibs supporting regularly

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or irregularly spaced discrete loads. e.g. from double tees. In this condition. a global case. supposing failure of the complete nib in bending and shear. as well as local bending

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and shear failure modes beneath the discrete loads, should be considered.

5.2.8.1 General

See above.

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5.2.8.2 Area of tension reinforcement

See above.

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5 ‘8 3 Position of tension reinforcement See above.

1 3 4 Design shear resistance

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5.2.8.5 Links in the member from which the nib projects

This Thang up steeV is important and should also be considered in any design where primary beams are loaded away from supports by secondary beams.

In document BS8110 structure use of concrete (Page 107-111)