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P(1) This chapter covers the common serviceability limit states. These are:—

Other limit states (such as vibration) may be of importance in particular structures but these are not covered in this code.

4.4.1 Limitation of stresses under serviceability conditions

4.4.1.1 Basic considerations

P(1) Excessive compressive stress in the concrete under the service load may promote the formation of longitudinal cracks and lead to micro-cracking in the concrete or higher than predicted levels of creep. If the proper functioning of a member is likely to be adversely affected by these, measures shall be taken to limit the stresses to an appropriate level.

(2) Longitudinal cracks may occur if the stress level under the rare combination of loads exceeds a critical value. Such cracking may lead to a reduction in durability. In the absence of other measures, such as an increase in cover to reinforcement in the compressive zone or confinement by transverse reinforcement, it may be appropriate to consider limiting the compressive stress to | 0.6 | fck in areas exposed to environments of exposure Class 3 or 4 (see Table 4.1).

(3) Creep may exceed the amount predicted using the methods given in 2.5.5 if the stress in the concrete under the quasi-permanent loads exceeds | 0.45 | fck. If creep is likely to significantly affect the functioning of the member considered, the stress should be limited to this value. For reinforced concrete flexural members, this check should be considered if the span/effective depth ratio exceeds 85 % of the value given in 4.4.3.2 for the case considered. A check may be necessary at transfer in prestressed elements.

(4) Durability requirements for prestressed concrete may impose other limits on the stresses in the concrete; for example, that the section should remain in compression (see 4.4.2.1).

(5) Stress under anchorages and bearings. No check will be necessary under serviceability conditions where the provisions of 2.5.3.7.4, and 5.4.6 or 5.4.8 have been complied with.

P(6) Stresses in the steel under serviceability conditions which could lead to inelastic deformation of the steel shall be avoided as this will lead to large, permanently open, cracks.

(7) This requirement will be met provided that, under the rare combination of loads (see 2.3.4) the tensile stress in ordinary reinforcement does not exceed | 0.81 fyk. Where the stress is due only to imposed deformations, a stress of | f^k will be acceptable. The stress in prestressing tendons should not exceed | 0.75 | fpk after allowance for losses.

4.4.1.2 Methods for checking stresses

P(1) In calculating the stress, account shall be taken of whether or not the section is expected to crack under service loads and also of the effects of creep and shrinkage. Other indirect actions which could influence the stress, such as temperature, may also need to be considered.

(2) The stress limitations given in 4.4.1.1 may generally be assumed to be satisfied without further calculations provided:

a) the design for the ultimate limit state has been carried out in accordance with 4.3.

b) the minimum reinforcement provisions of 4.4.2.2 are satisfied.

c) detailing is carried out in accordance with Chapter 5.

d) not more than 30 % of redistribution has been carried out in the analysis for the ultimate limit state. It should be noted that creep and shrinkage in partially prestressed members may lead to high stresses in both normal reinforcement and prestressing tendons which could lead to fatigue problems.

(3) Long term effects may be ignored except for situations where more than 50 % of the stress arises from quasi-permanent actions. In this situation, a modular ratio of 15 may be assumed.

(4) Stresses are checked employing section properties corresponding to either the uncracked or the fully cracked condition, whichever is appropriate.

(5) In general where the maximum tensile stress in the concrete calculated on the basis of an uncracked section under the rare combination of loads exceeds fctm (see Table 3.1), the cracked state should be assumed.

(6) Where an uncracked section is used, the whole of the concrete section is assumed to be active and both concrete and steel are assumed to be elastic in both tension and compression.

(7) Where a cracked section is used, the concrete is assumed to be elastic in compression but to be incapable of sustaining any tension. (In checking stresses in accordance with these rules, no allowance should be made for the stiffening effect of the concrete in tension after cracking).

(8) At least the minimum area of reinforcement given by 4.4.2.2 is required to satisfy the limitation on the stress in ordinary bonded reinforcement under the action of restrained imposed deformations.

4.4.2 Limit states of cracking

4.4.2.1 General considerations

P(1) Cracking shall be limited to a level that will not impair the proper functioning of the structure or cause its appearance to be unacceptable.

P(2) Cracking is almost inevitable in reinforced concrete structures subject to bending, shear, torsion or tension resulting from either direct loading or restraint of imposed deformations.

P(3) Cracks may also arise from other causes such as plastic shrinkage or expansive chemical reactions within the hardened concrete. Such cracks may be unacceptably large but their avoidance and control lie outside the scope of this chapter.

P(4) Alternatively, cracks may be permitted to form without any attempt to control their width or be avoided by measures, such as the provision of joints, which can accommodate the movement, provided they do not impair the functioning of the structure.

P(5) Appropriate limits, taking account of the proposed function and nature of the structure and the costs of limiting cracking, should be agreed with the client.

(6) in the absence of specific requirements (e.g. watertightness), it may be assumed that, for exposure classes 2—4, limitation of the maximum design crack width to about | 0.3 | mm under the quasi-permanent combination of loads (see 2.3.4) will generally be satisfactory for reinforced concrete members in buildings with respect to appearance and durability. For exposure class 1, crack width has no influence on durability and the limit could be relaxed if this was acceptable for other reasons.

(7) The durability of prestressed members may, for exposure classes 2—4, be more critically affected by cracking (see 4.1). In the absence of more detailed requirements, the limitations in Table 4.10 are considered to be generally satisfactory. The decompression limit requires that, under the frequent combination of loads, all parts of the tendons or duct lie at least 25 mm within concrete in compression.

(8) Special crack limitation measures may be necessary for members subjected to exposure class 5. The choice of appropriate measures will depend upon the nature of the aggressive chemical involved.

P(9) Limitation of cracks to acceptable widths is achieved by ensuring a) that, at all sections likely to be subjected to significant tension due to restraint of imposed deformations, whether or not the restraint is combined with direct loading, a minimum amount of bonded reinforcement is present sufficient to ensure that yield of the reinforcement will not occur until above the cracking load, and b) that bar spacings and diameters are limited in order to limit the crack width.

This also applies to parts of prestressed members where tension could develop in the concrete.

Table 4.10 — Criteria for prestressed members

Exposure class

Design crack width, wk, under the frequent load combination (mm)

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