I am replacing the windows on an old building. All the steel lintels have sagged downward in the center, some as much as 1 inch. Were these lintels built this way? Or did loads from the masonry cause them to sag over time?
The deflection of the lintels is most likely caused by corrosion. Corrosion products of steel are more than 10 times larger than the steel itself. This exerts tremendous pressure between the lintel and the masonry above, causing the lintel to deflect downward. Since the lintels in your building have deflected as much as an inch, the steel angles could be damaged substantially. I recommend making openings in the masonry to determine the condition of these lintels. If there is substantial loss of section, the lintels should be replaced. If not, the steel can be sandblasted and coated with an epoxy surface treatment. New masonry through-wall flashings should be installed to protect the lintels.