I recently read a technical paper that provides exhaustive data for engineered design and detailing of "relief" angles. However, the article does not discuss when – if ever – it is a good idea to incorporate relief angles into a structure, except to acknowledge in the conclusion that states: "Severe damage can result from the improper design, construction, and maintenance of shelf angle details."
Since maintenance, and to a large extent construction, are beyond the control of even the most conscientious engineer, I was wondering if you or any of your peers have written a paper with rational criteria for the incorporation of steel relief angles into masonry structures with, hopefully, some considerations for designing the veneer to resist wind loading at interruptions and discontinuities.
I have BIA technical note 28B (1980) which recommends: "In order to alleviate the many problems associated with the use of shelf or relief angles at each floor, it is suggested that the brick veneer may be designed to support its own dead weight on the foundation, unless heights (in excess of 100 feet) or number and location of openings in the veneer make it mandatory that walls be vertically supported by the structural frame."
We have been happily following this rule of thumb for many years by reinforcing the perimeter of veneer openings and providing lateral anchorage consistent with the UBC wind load requirements. Recently we discovered that our approach was considered "unorthodox" and "unconventional" in some engineering circles.
We were shocked to discover the aesthetic, cost, and detailing implications of trying to inject relief angles at every floor of a four-story institutional buildings which we had previously been designing successfully without a single relief angle. I reviewed more recent BIA documents and found that the language I quoted had been deleted.
Is something going on in academic and/or professional circles to squelch relief angle free veneers?
Other than regions of the country with high seismic risk, relief angles are typically provided to accommodate differential growth between the clay masonry and the structural frame of the building. The Brick Industry Association (BIA) in their Technical Notes on Brick Construction 18A states: "In low-rise masonry buildings (below three stories) and buildings with shear walls, it is not necessary to provide horizontal relief, but differential movement should be accounted for in the tie system, window details, and at the top of the wall. High-rise frame structures typically have horizontal expansion joints located at every floor level or every other floor level." Figures 10 and 11 in this technical note indicate that the minimum space beneath the shelf angles should be 1/8 inch.
BIA Technical Note 28B states: "Structures with a maximum veneer height of 30 feet (9.1 meters) from the foundation to the top of the wall and 38 feet (11.6 meters) from the foundation to the top of the gable can have their entire brick veneer supported directly on a foundation wall, footing, or noncombustible support without shelf angles. Structures with brick veneer above this height should have a shelf angle at each floor."
Horizontal expansion joints are generally provided in buildings four stories or more in height because it is difficult to accommodate differential movement at ties, windows, and the top of walls in tall buildings when horizontal expansion joints are not used. This differential movement between the brick masonry can be especially great when the structural frame is reinforced concrete because concrete frames shrink vertically due to creep and drying shrinkage over time.
I have investigated several buildings where differential vertical movement created significant problems. In one 14-story building, our firm measured more than 2 inches of differential movement at the top of the building, which lifted the shelf angles so that the anchor bolts bound up at the top of the wedge inset and the angle rotated upward. The magnitude of differential movement for unrestrained movement is calculated by using equation 1 in BIA Technical Note 18A.
mu = (ke + kf + kt (Delta)T) L where:
mu = total unrestrained movement of the brickwork, inches
ke = coefficient of moisture expansion, inch/inch (0.0005 inch/inch)
kf = coefficient of freezing expansion, inch/inch (0.0002 inch/inch)
kt = coefficient of thermal expansion, inch/inch/oF (0.000004 inch/inch/ oF)
(Delta)T = temperature change in brickwork from the time the walls were constructed, oF
L = length of wall, inches
Although this equation is used in the technical note to calculate horizontal growth, I recommend using it for determining expected vertical growth as well. The magnitude of expected structural frame movement should be added to this number to estimate the total differential movement. Using this procedure errs on the conservative side because it does not take into account the effect of mortar joints and long-term creep deflection that occurs in vertical growth versus horizontal.
However, based on my experiences investigating numerous failures in masonry walls resulting from moisture expansion, it is not unusual to have differential vertical movement in the magnitude of 1/10 inch per 10 feet of height, which is about 3/8 inch in a typical four-story building. If the windows and backup walls are attached to the building structure, which is often the case in nonbearing walls, any interface details must be designed to accommodate these movements without causing sealant failures or other problems.