Lecture_21

# Lecture_21 - Chapter Five Design of Reinforced Concrete...

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190 Chapter Five _____________________________________________________________________ Design of Reinforced Concrete Beams The design procedure of reinforced concrete members consists of a series of interrelated steps and checks, which may generally be condensed into the following three basic stages: 1. Preliminary analysis and member sizing 2. Design and detailing of steel reinforcement 3. Serviceability calculations 5.1 Preliminary Analysis and Member Sizing The layout and size of members are often controlled by architectural details. The engineers must either check that the beam sizes are adequate to carry the loading, or alternatively, decide on sizes that are adequate. 5.1.1 Preliminary analysis and basic dimension requirements The preliminary analysis of beams generally only provides the maximum bending moments and shear forces in order to ascertain reasonable dimensions. Basic requirements of beam dimensions

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191 o overall depth ( h ) and effective depth ( d ): d = h – (50 ~ 60) mm o breadth ( b ): 1 1 ( ~ ) 3 2 b h = o cover to reinforcement: (generally minimum cover = 25 mm ) Fig. 5.1-1 Beam dimensions 5.1.2 Suitable dimensions for b and d Suitable dimensions for b and d can be decided by a few trial calculations. (1) For singly reinforced section (no compression reinforcement), 156 . 0 2 < cu f bd M For doubly reinforced section (with compression reinforcement if the area of bending steel is not to the excessive), cu cu f f bd M 10 156 . 0 2 < <
192 (2) In no case should v = V /( b v d ) exceed or 0.8 f cu or 5 N/mm 2 (Hong Kong code: 0.8 f cu or 7 N/mm 2 ) whichever is the lesser. To avoid congested shear reinforcement. The value of v should preferably be somewhat closer to half (or less) of the maximum allowed . (3) The span/effective depth ratio (for l 10 m) should be within the basic values given below: Cantilever: 7; Simply supported: 20; Continuous: 26 All calculations in design are generally based on the effective span: o For simply supported beams – distance c/c of the supports, or clear distance between the supports + d (effective depth); o

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