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Flexural cracks on the sides of a beam start at the tension face and will extend, at most, up to the neutral axis. Crack widths will be greatest at the tension face and will reduce with distance from that face. In general, the cracks will be uniformly spaced along the most heavily loaded portion of the beam, i.e. near the mid-span in sagging or over the supports in hogging.
Flexural cracks on the soffit of a slab will run at right-angles to the span, again roughly uniformly spaced in the region of maximum moment. In beams and slabs in buildings that have been correctly designed, average crack widths should not exceed 0.3 mm. In bridges, the cracks should generally be narrower.
Guidance on the provision of reinforcement is given in Eurocode 2; Part 1-1 covers buildings, Part 2 covers bridges and Part 3 covers Liquid retaining and containing structures. These parts of Eurocode 2 have replaced BS 8110, BS 5400 Part 4 and BS 8007. Design to the appropriate codes should ensure that flexural cracks are limited to the required widths.
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