I am making repairs to a building that was constructed in the early 1950s. The masonry walls have diagonal cracks extending up from the ends of the steel lintels over the windows. These cracks occur on both sides of the lintels and are worse at the top floor of the building.
Do you have any ideas as to what is causing these cracks?
Diagonal cracks that extend up from the ends of the lintel are often caused by corrosion of the steel lintel. The corrosion product of steel will occupy 10 to 20 times as much space as the steel itself. This expansion generates tremendous pressure when confined and is capable of bending the steel angles, breaking apart the brick masonry, or lifting the brick.
Corrosion that builds up on the top surface of the steel lintel lifts the masonry. Diagonal cracks form at the ends of the lintel because this area is the weakest plane. The cracks are more likely to occur when there are only a few feet of masonry above the head of the windows because there is less weight to resist the expansion pressure generated by the corroding steel.
Where there is greater confining pressure, such as at lower levels on a building that does not contain horizontal expansion joints, the pressure generated by the corroding steel causes the portion of the angle above the window to deflect downward and the brick at the jambs to crush.
To repair this problem, it is often necessary to replace the lintel with a new one. Typically three to four courses of the exterior brick masonry wythe will need to be removed in order to perform this repair.
Temporary support must be provided for the masonry immediately above the angle. This support is often obtained by using steel rods at close spacing that are drilled into the backup to serve as shear pins or by using a shore to support the masonry from a lower floor or the ground. You should work with a structural engineer to develop an effective method of shoring for your particular set of conditions.
I recommend replacing the lintels with new, galvanized steel lintels; however, painted mild steel lintels may be used. If a proper corrosion resistant paint coating is applied, the finished appearance of the lintel can be used to match the window system. A portion of the lintel is often exposed above the window and will match the surrounding materials better if painted. Galvanized angles can also be painted; however, special surface preparation is required to develop proper paint adhesion.
After the lintel is replaced, flashing must be installed above it to protect the angle from water and to prevent water leakage problems in the wall system below. The flashing should extend beyond the ends of the lintel and should have upturned ends—called end dams—to prevent water that reaches this flashing from flowing off the ends and remaining within the wall.