Question: Our firm is designing a multi-story steel frame office building with brick masonry insulated cavity walls. The shelf angles are designed with horizontal expansion joints beneath them at each floor line, roughly 10 feet on center. The windows are a relatively wide 12 feet and we would like to hang the window lintels from the floor above.
Can this plan create problems since the lintels will be attached to a different floor beam than the masonry on either side of the window?
Answer: Yes. Supporting lintels from the floor above can cause cracks to form in the masonry near the ends of the lintels. Masonry above and adjacent the window is supported on different beams. These beams deflect differently depending on loading and spans.
Moisture and thermal expansion of the masonry also can cause differential movement between the sections of masonry supported at these different levels. Differential movement resulting from either deflection or masonry expansion can result in cracking at the junction of these two masonry sections.
This potential problem is addressed by supporting the masonry from the same floor or by accommodating the differential movement. For example, vertical expansion joints can be added at both ends of the lintel to accommodate differential movement. Depending on the number of windows, this approach may result in many additional joints that affect the aesthetics and increase the maintenance costs.
Masonry above the windows can be supported on the same floor as the masonry on either side of the window by using steel or pre-cast lintels supported by the masonry at the jambs. These lintel beams must be designed to limit deflection to the length of the span divided by 600.