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compression chord

compression chord

Figure 6.24. Redistribution of forces in a shear-loaded web by strut rotation

Because of a smaller strut inclination, a larger number of stirrups is activated, and the shear capacity is increased. A result of the smaller strut inclination is, however, that the stresses in the concrete struts are larger, so that an appropriate upper limit to the shear capacity has to be defined. This method, with strut inclinations smaller than 45° may be assumed for design, and is known as the "variable strut inclination" method. This approach is not only attractive because of its agreement with the physical reality, but also because it is a simple equilibrium method, giving a transparent view of the flow of forces in the structure.

In ENV 1992-1-1, in chapter 4.3.2.4.4, the designer is allowed to choose the strut inclination between 0,4 < cot 0 < 2,5

which means that 0 may be chosen between 21,8° and 68,2°. The choice of the lowest value mostly leads to the most economic design. In this case the compression struts are supposed to rotate from an initial value of 45* to a lower value of about 22°. If the strut inclination is 0 and the (vertical) shear reinforcement yields, a shear force

Is transmitted, Fig. 6.25a. If the shear reinforcement yields, the truss can, by rotation of the compression struts to a lower inclination, activate more stirrups for the transmission of the shear force and, as such, extend the zone of failure.

Due to strut rotation, the stress in the concrete struts increases. Consequently, rotation can only continue until crushing of the concrete occurs. For an ultimate compression stress fc1 in the concrete struts, the corresponding shear force is (Figure 6.25b).

cot0 + tan0

where fc1 = vfc In those equations web width inner lever arm = 0,9d stirrup distance yield stress of stirrups inclination of concrete struts cross-sectional area of one stirrup effectiveness factor, taking account of the fact that the beam web, which is transversally in tension, is not as well suited to resist the inclined compression as cylinders used to determine fc.

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