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Are steel shelf angles required to support masonry veneer with block backup?


The Building Code (MSJC 2011, which is referenced in IBC 2012) permits masonry veneer to be designed either by engineered methods or prescriptively.  According to the prescriptive provisions, veneer that is backed by wood framing is not required to have shelf angles because the height of such veneer above the support is limited to 30 ft, except that gables are permitted to be 38 ft. tall.  When the veneer is backed by cold-formed steel framing, the veneer must be supported at each story above 30 feet to the plate or 38 feet to the gable (MSJC 2011 Section    The veneer height limits for wood framed and steel framed backing are permitted to be waived if an engineering analysis is performed.  However, shelf angles are not required for structural support of prescriptively designed masonry veneer backed by concrete block, regardless of veneer height, provided that the veneer is supported on an appropriate foundation.

When the veneer wythe is clay masonry and the backup wythe is concrete masonry, the differential volume change movements (due to clay masonry expansion and concrete masonry shrinkage, as well as thermal movements, creep, etc) must be considered in the design, regardless of whether the engineered approach or the prescriptive approach is utilized.  The taller the wall, the more significant the differential movement.  Detailing at penetrations through the wall must be carefully designed and executed.  As a practical matter, the height of buildings with such walls should not exceed four stories because it becomes more difficult for detailing (at the top, at ties between wythes and at openings) to accommodate the magnitude of differential movement that result from taller structures.  Anticipated volume changes can be calculated in accordance with Section 1.8 of MSJC 2011.

When the veneer wythe is the same concrete masonry material as the backup wythe, but the cavity between the wythes is insulated, some differential movement will also occur between wythes, although less than with a veneer wythe of clay masonry.  The anticipated differential movement should be calculated so that appropriate details can be developed to accommodate it.

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