Brick Lintel Trouble
A builder recently asked if I could help them with a brick cracking problem at the lintel
supports at their double garage doors. The brick in question is full-sized brick veneer
with stick-framed wall behind.
Here is a sketch of the issue (the deflection shown is at an exaggerated scale.)
The problem did not occur at window, door, or single garage door lintels, only at the 16’
double garage door lintel. This builder uses L5x3-1/2x5/16 steel angles for all lintels.
The cracking problem is likely due to deflection (sag) of the lintel. When the lintel sags,
its ends rotate upward, distressing the brick and mortar there.
I found it strange that only one size lintel was used regardless of the span. Recall from
my previous writings that bending stresses and deflection go up exponentially with
increased span. Meaning long span lintels should be beefier than short span lintels. And if
we want to be green, we’ll use only what is required by code and no more.
Back to our 16’ lintel, what size angle should be used? The answer surprised me.
Here is how to analyze a veneer brick lintel.
The load comes from a 45-degree triangular area, which can be simplified to a uniform
load 2/3 the height of the triangle. This assumes there is adequate brick above the
triangle, 8” to 16” minimum, to provide resistance to arching thrust. With this assumption
only the weight of the brick in the triangle will act on the lintel; the weight of the brick
outside the triangle is supported by the piers on each side via arching action. If there isn’t
enough brick over the triangle to achieve arching, you must include the entire weight of
the brick above the lintel.
The maximum allowed deflection for any steel lintel, per the Brick Industry Association
Technical Notes 31B, 5/87, is L/600. For our 16’ opening that’s 0.32”.
Using the Custom Beam feature in ProBeam™, I calculated the midspan deflection of our
L5x3.5x5/16 at 2.07”, a whopping 6.5 times the maximum allowed! Also this lintel fails
in bending stress by 38%.
[Technical Note. ProBeam™ and most similar software assumes the vertical leg
(compression “flange”) of the angle is continuously braced against sideways buckling.
With our lintel, the only practical way to accomplish this would be to connect the top of
the vertical leg to the framed wall behind using masonry ties or similar. If this is not
done, the lintel becomes unstable and can support very little load. I don’t know if the
subject builder installs any sort of lateral bracing but if not he definitely should.]
So the L5x3.5x5/16 is grossly undersized.
I tried an L6x3.5x5/16 and found the deflection is 1.25”, still way too much.
Even an L7x4x3/8 has 0.66” deflection, much better than what we started with, but still
two times more than the brick industry recommends.
You can see that there is probably no reasonable angle that’ll calc in this application. So
what to do?
One option would be to weld vertical rebar studs to the horizontal flange of the angle
which are then grouted in hollow cores of brick or are mortared between bricks. This
converts the angle into a composite beam: the tension component of bending resisted by
the angle, the compression component by the bricks above. In this case the original
L5x3.5x5/16 would probably be fine. In order for this to work correctly, the angle would
have to be temporarily supported at third or quarter points until all the brick above was
placed and the mortar cured.
Another option, and the one I prefer, would be to split the single, 16’ wide door opening
into two, 9-foot wide openings. I.E. build a small wall pier between two 9’ doors. This
reduces the load (load triangle is only 4.5’ tall vs. 8’ tall) as well as shortens the span.
Now the L5x3.5x5/16 easily meets the L/600 criteria (0.18” max allowed deflection) with
a sag of only 0.12”.
If neither of the above options are appealing, the builder should at least use a lintel larger
than L5x3.5x5/16, which while it might not completely solve the cracking problem,
would be better and safer than what they’re using now.
Tim Garrison is an award-winning author, public speaker, and professional engineer. He
welcomes correspondence via his blog at BuildersEngineer.com