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Beam Shear Behavior
- Reinforced masonry beams must be designed for shear as well as bending.
- Maximum shear forces generally occur near supports.
- Shear failure is actually a diagonal tension failure that is brittle in nature and should be avoided.
- To better understand diagonal tension consider the basic mechanics of a beam with no shear web reinforcing:
- Recall from Mohr's circle - an equivalent state of stress different than that shown above is obtained by rotating the differential element 45°.
- This rotated element yields principal tension and compressive stresses which are occuring simultaneously with the previous maximum fv state of VQ / Ib.
- To better understand diagonal tension consider the basic mechanics of a beam with no shear web reinforcing:
- In general this ft will exceed the inherent tensile strength of masonry, before fv exceeds masonry shear strength.
- When this happens, diagonal cracks, originating at the n.a. begin to occur and grow with increases in beam loading.
- The three general modes of shear failure in masonrybeams are:
- When you have diagonal cracking:
- The propagation of the tensile cracks is not resisted by the longitudinal flexural steel.
- Under slight increases of load, the diagonal cracks will spread into the compression zone, reducing the compression area to an ineffective amount; resulting in beam collapse.
- This potential for failure by diagonal tension is quantified in terms of shear stress:
- Compare fv to Fv
- fv = V / b j d (UBC '97 2107.2.17)
for flexural members without shear reinforcement (UBC '97 2107.2.8)
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