Are there Code requirements, in addition to horizontal reinforcement, for masonry laid in stack bond?

ANSWER: 

Stack bond is one particular bond pattern that falls into the category of “not laid in running bond”, where running bond is defined as placing units in successive courses so that head joints are offset at least one-quarter the unit length.  In addition to the requirement for horizontal reinforcement (See the FAQ “Are there Code requirements for joint reinforcement in masonry laid in stack bond?"), several other Code requirements are imposed when the masonry is not laid in running bond.  These primarily affect the structural design, but also affect the amount of reinforcement, particularly in the higher seismic design categories.

  1. The bearing area under concentrated loads is limited by the location of head joints when the masonry is not laid in running bond.  (MSJC 2011 Section 1.9.5)  Distribution of the concentrated loads is not permitted to occur across head joints in masonry that is not laid in running bond.  (MSJC Section 1.9.7.2)
  2. When masonry that is not laid in running bond does not have bond beams spaced no more than four feet on center, the length of masonry that resists compression per reinforcing bar is limited to the length of one masonry unit. (MSJC 2011 Section 1.9.6.2)
  3. The allowable flexural tensile stress (allowable stress design) and modulus of rupture (strength design) of clay masonry and concrete masonry that is not laid in running bond is zero unless the masonry includes continuous grouted bond beams.  (MSJC 2011 Table 2.2.3.2 and Table 3.1.8.2)
  4. Allowable shear stress (allowable stress design) and nominal shear strength (strength design) is reduced when clay masonry and concrete masonry are not laid in running bond, are constructed of open end units, and are fully grouted.  Stress capacity is zero when that masonry is not fully grouted.  (MSJC 2011 Section 2.2.5.2 and Section 3.2.4)
  5. In special reinforced masonry shear walls, in which the masonry is not laid in running bond, the maximum permitted spacing of vertical and horizontal reinforcement is reduced from 48 inches to 24 inches while the additional maximum spacing limits of 1/3 the shear wall height and 1/3 the shear wall length still apply.  The minimum required area of horizontal reinforcement in special reinforced masonry shear walls is approximately doubled when the masonry is not laid in running bond.  When masonry in a special reinforced shear wall is not laid in running bond, it must be fully grouted and constructed of hollow open-end units or two wythes of solid units.  (MSJC 2011 Section 1.18.3.2.6)
  6. In Seismic Design Categories E and F, a greater amount of reinforcement is required in nonparticipating masonry elements that are not laid in running bond, as compared to those elements in Seismic Design Category D.  Those elements are also required to be fully grouted and constructed of hollow open-end units or two wythes of solid units. (MSJC 2011 Section 1.18.4.5.1)
  7. The Code contains additional requirement for Autoclaved Aerated Concrete masonry that is not laid in running bond.  (MSJC 2011 Chapter 8)

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