Bearing length of masonry lintels 1

[saaveze]

Hi Everyone,

I have been told that the minimum recommended bearing length for lintels is 150 mm (approx. 6 inches). For lintels located at the top of CMU walls with relatively large live and dead loads (and no reduction for triangle area above lintel), it seems to me there would be an advantage to increase bearing length in order to distribute the load over more area. What disadvantages are there (other than cost) for increasing the bearing length?

Thanks!

 

[KootK]

Are you sure that you're not thinking of loose steel angle lintels with respect to the 150 mm recommendation? 200 mm seems like a more reasonable CMU lintel minimum.

You'll need to provide enough bearing length to make your CMU lintels satisfy code specified bearing stress limits at the support reactions. You'll also need to ensure that detailing requirements at the supports are satisfied. Other than that, I can see no benefit to providing additional bearing length.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

What disadvantages are there (other than cost) for increasing the bearing length?

Yes, there is a disadvantage. When the lintel's bearing length is "short", the lintel is a good approximation of a simply supported beam. The load on the masonry is compression only and can be calculated with reasonable accuracy.

As the lintel's bearing length becomes "longer", the masonry's support of the lintel is increasingly fixed. With fixed supports the lintel does deflect less, but the supports have to dissipate the negative moment at the ends of the span. The forces on the masonry are no longer simple to calculate and will tend to include not only compression but also tension. Tension in masonry means cracks.

Bottom Line: Within acceptable limits, minimize lintel bearing length, otherwise there will likely be more cracking at the supports.

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[saaveze]

KootK and SlideRuleEra I appreciate your feedback.
KootK - 200 mm (approx. 8") does make more sense since it would cover half a standard 16" block and allow for the cells below to be grouted much easier.
SlideRuleEra - Do you know of a reference for masonry lintel design which includes checking for bearing stress? All the references I have go over design of the lintel itself but leave out checking the support conditions. I have the reactions at each support. Is it as simple as taking the reaction and dividing it over the support area and comparing that to f'm?

Do either of you know how analyze a support condition for a lintel / bond beam combination?

Thank you

 

[KootK]

Bearing checks are just that simple. I'm not sure what a combined lintel/bond beam is. Can you describe it in more detail?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[concretemasonry]

The combination "combined lintel/bond beam" that I have seen consists of a 2, 3 or 4 course of CMUs over a large garage door type opening on a tall wall. It is usually a conventional bond beam in the top course that is basically a continuation of the normal top course in the remainder of the other walls. The bottom course of the "combined lintel/bond beam" is a conventional solid bottom bond beam with reinforcement. The opening is supported by posts for several days or longer if the walls will be carrying floor or roof loads early.

Usually, the vertical joint above the bottom of the lintel is distance that is a masonry module (usually 8", since 6" is not a practical masonry module) away from the side of the opening for appearance and constructability. It may be 16" away if there is a "belt and suspenders" amount of vertical steel on each side of the opening.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Ron]

A precast lintel should match the masonry coursing....so 200 mm would be reasonable.

 

[SlideRuleEra]

saaveze - Take a look at publications by the National Concrete Masonry Association:

http://www.ncma-br.org/topics.asp?p=503

and the Brick Industry Association:

http://www.gobrick.com/Technical-Notes

In particular, see "Structural Steel Lintels", Bearing, on page 5:

http://www.gobrick.com/Portals/25/docs/Technical Notes/TN31B.pdf

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