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Structural Concrete - Hassoun

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19.8 Design for Shear 769<br />

Figure 19.8 Distribution of shear forces along span. The middle diagram shows shear<br />

capacity of a simply supported prestressed concrete beam. The bottom diagram shows<br />

ACI analysis. (Stirrups are required for shaded areas.)<br />

The cracking moment can be determined from the following expression ACI Code, Section<br />

22.5.8.3.1:<br />

M cre = I y t<br />

(6λ √ f ′ c + f pe − f d ) (19.52)<br />

where<br />

I = moment of inertia of section resisting external factored loads<br />

y t = distance from centroidal axis of gross section neglecting reinforcement to extreme fiber in tension<br />

f pe = compressive strength at extreme fibers of concrete section due to effective prestress force after<br />

all losses<br />

f d = stress due to unfactored dead load at extreme fiber, where tensile stress is caused by external loads<br />

λ = modification factor for concrete<br />

The web-shear strength, V cw , is based on shear cracking in a beam that has not cracked by<br />

flexure. Such cracks develop near the supports of beams with narrow webs. The ACI Code, Section<br />

22.5.8.3, specifies that V cw be computed as follows:<br />

V cw =(3.5λ √ f ′ c + 0.3 f pc )b w d p + V p (19.53)

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