David Patterson, AIA, Deborah Slaton, and Kenneth Itle, AIA
Expansion bolts are commonly used to support new structural attachments in masonry or concrete. For example, expansion bolts are often installed into concrete spandrel beams to hold up new steel relieving angles, which, in turn, support building cladding such as brick or stone. Although relatively easy to install, expansion bolts must be placed into properly prepared holes drilled in the substrate—and torqued to the force specified by the manufacturer—to correctly engage the wedge portion of the bolt.
As part of a recent relieving angle replacement project on a three-story masonry building constructed in the early 20th century, three to four courses of existing brick masonry were removed to expose the original continuous relieving angles that were significantly corroded. These angles were removed and new galvanized relieving angles installed, attached with expansion bolts into the concrete spandrel beam at each floor. New through-wall flashing was provided, and the original brick veneer was reinstalled.
Within a few months of completion of the shelf angle work, widespread horizontal cracking was observed at bed joints between the bricks that were removed and reinstalled to access the shelf angle locations, and the adjacent original brick veneer. Continuous cracks of these types generally relate to masonry with poor bond and potentially inadequate support. Where exposed to view at the head of window openings, the new relieving angles appeared to be out of level and sloped downward toward the toe of the angle.
On removal of the brick masonry and portions of through-wall flashing at representative locations, the nuts on the expansion bolts were found to have backed out somewhat with many being able to be removed by hand. This lack of adequate anchorage resulted in rotation of the relieving angles and the resultant cracking of the masonry above.
In this application, the expansion bolts were subject not just to a vertical shear force, but also a tensile (pull-out) force created by the eccentric load on the outer half of the bottom leg of the relieving angle by the brick veneer. This condition was exacerbated by positioning the anchor near mid-height of the vertical leg of the angle rather than closer to the top edge, which would have reduced the tensile force and provided more edge distance between the anchor and bottom of the spandrel beam.
It is important to note where anchors are subject to both shear and tensile (pull-out) forces, the designer should be aware the manufacturer’s allowable load tables for shear or tension often do not address combined loading. For bolts subject to both forces, combined loading must be considered. In addition, burring the exposed bolt threads after the nut is sufficiently torqued (or other means of preventing the nut from backing out) is a good way to ensure the nut does not loosen overtime.
David S. Patterson, AIA, is an architect and senior principal with the Princeton, New Jersey, office of WJE, specializing in investigation and repair of the building envelope. He can be contacted at firstname.lastname@example.org.
Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at email@example.com.
Kenneth M. Itle, AIA, is an architect with the Northbrook, Illinois, office of WJE, specializing in investigation and preservation. He can be reached at firstname.lastname@example.org.