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.

    • 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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