Q: Should brick veneers over steel stud backings required by the International Building Code (IBC) be supported at each floor line?
There is some confusion in our office regarding this issue. If the floor heights are 20 ft, what is the difference between masonry veneer walls that are supported by each floor and walls supported every other floor in a building with 10-ft floor-to-floor heights? How tall can masonry veneer walls be built before shelf angles are required?
A: The permitted height of the anchored masonry veneer depends on the type of backing and the distance above the foundation. Chapter 14 – Exterior Walls of the IBC in Section 1405.5 – Anchored Masonry Veneer, states “Anchored masonry veneer shall comply with the provisions of Sections 1405.5, 1405.6, 1405.7, and 1405.8 and Sections 6.1 and 6.2 of ACI 530/ASCE 5/TMS 402.”
Section 220.127.116.11.1.3 of ACI 530-05/ ASCE 5-05/TMS 402-05 (2005 MSJC Code) requires that if anchored masonry veneers are taller than 30 ft at a horizontal plate and 38 ft at the peak of a gable, the weight of the veneer must be supported by non-combustible construction for each story above these height limits. However, there is no requirements for the maximum floor-to-floor height. Therefore, if the floor-to-floor heights are greater in some buildings than others, a greater height of masonry is permitted to be supported.
The requirement that the spacing of supports be on a per floor basis rather than providing a maximum height for each subsequent section of masonry veneer most likely is for practical reasons. It is very difficult to install shelf angles at intermediate levels between floors since additional structural members would have to be added to support these angles. For most buildings, floor-to-floor heights vary from 9 ft to about 12 ft. Therefore, relief angles would typically be required at roughly 10-ft intervals.
Shelf angles at each floor facilitate differential movement between the veneer and the structure of the building. Expansion joints are provided beneath each shelf angle to accommodate this movement.
I have encountered some problems with masonry veneers which – due to block expansion joints – acted as tall sections of veneer. Most of these problems were related to interfaces between the masonry veneer and windows, balconies, or other elements that are attached directly to the structure, or were attached to a structural backing that in turn was attached to the structure.
The movement between the masonry and the structure or element attached to the structure can result in cracking in the masonry or damage to the windows or other elements. When the brick veneer grows or the structural frame shrinks, the resulting differential movement can – in some cases – exceed that allowable displacement of the wall ties, causing them to fail.
If the expected movement can be accommodated in the interface details and wall ties, it would be possible to design a high-rise masonry veneer with spacing of supports that exceed 20 ft. Variations may be permitted by local code officials in these special cases if there are compelling reasons in the building design to limit the number of shelf angles and expansion joints.
Using plastic separators
Q: I have seen a plastic separator used between shelf angles and stainless steel or copper flashing to prevent corrosion. We have a repair project where a stainless steel flashing drip edge, along with rubberized asphalt flashing, is being used over existing steel support angles. The angles will be sandblasted and painted as part of the repairs.
Is it necessary to use a plastic sheet as a separator in this case?
A: Stainless steel can cause uncoated steel to corrode at a faster rate due to a process known as galvanic corrosion. Galvanic corrosion occurs whenever dissimilar metals come in contact with each other in the presence of an electrolyte (water with salts or other ions that allow a current to pass).
In an assembly with two dissimilar metals, one corrodes at a faster rate than it would by itself, and the other does not corrode or corrodes at a slower rate than it would by itself. The metal that corrodes faster is the “anode” and the one that corrodes slower is the “cathode.”
Methods of evaluating the potential for galvanic corrosion between two dissimilar metals are described in ASTM G82 – Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance. The methods in this standard guide use a Galvanic Series, which is a list of metals presented in the order of observed corrosion potential.
The risk of galvanic corrosion is greater when the metals in question are farther apart on the Galvanic Series. The metal closer to the “Active” end of this series becomes the anode and the one closer to the “Passive” or “Noble” end becomes the cathode.
There are two types of Galvanic Series listed in this guide. One list shows the relative positions of the metals, but does not give their corrosion potentials. When using this series, the farther apart these metals are on the list, the more likely galvanic corrosion will occur. However, since the corrosion potentials are not given, the magnitude of this difference is not known.
The second type of Galvanic Series provides the corrosion potential expressed in terms of volts measured in flowing sea water by a saturated calomel half cell reference electrode.
Norbert V. Krogstad is a consultant at Wiss, Janney, Elstner Associates Inc. Northbrook, Ill.