STRENGTH DESIGN OF REINFORCED LINTEL BEAM - LATERAL WIND LOADS

[oengineer]

I am working on the design of a CMU Lintel using strength design. I am using example 11.4-1 in the Masonry Design Guide. In the example, they checked the lintel for gravity DL + LL, but they do not mention anything regarding lateral Wind Load design for the lintel.

I thought if I just rotate the lintel/beam around on its weak axis and follow the same procedure as shown in the example I should be okay. I am having issues with the Mn > 1.3Mcr check & the required reinforcing. My required reinforcing is not a lot, but when I enter the amount of reinforcing required for the lintel, then my Mn > 1.3Mcr check is not satisfied.

Are there any good examples for lateral load lintel design available? Comments/suggestions are appreciated.

 

[oengineer]

Also, I am using the TMS 402 & the Masonry Design Guide for my deflection design of the lateral load. If any one has an example of lateral deflection due to wind load it would be appreciated.

 

[masonrygeek]

Typically lintels aren't checked for out-of-plane loading unless there is no wall surrounding the lintel. It is the entire wall which is checked for wind load that also happens to include the lintel. In many cases, the wall spans from floor to floor so the lintel reinforcing isn't considered, just the vertical rebar. As you get into shear walls, openings affect the loads a lot more and a more sophisticated analysis is required.

 

[Aesur]

@masonrygeek, why would you not check the out of plane loading on the lintel? The lintel supports loading from the wall above and the window in the opening below. I typically distribute the wind loading to the lintel based on the tributary area the lintel carries. The vertical reinforcing does nothing for out of plane in the lintel, only the wall being supported by the lintel. The lintel is supported for out of plane bending and deflection using horizontal bars. Most programs I have seen account for out of plane design, including EnerCalc.

@oengineer, try designing the lintel as a wall with out-of-plane loading/bending spanning between the jambs. Then the jambs would be designed as a wall/column with axial load and an out of plane point load from the lintel.

 

[KootK]

It is my experience that header are not checked for lateral load in any of these materials:

1) masonry
2) wood
3) cold formed steel.

I can offer no rational explanation for why that is.

Mn > 1.3Mcr check is not satisfied.

I wouldn't sweat this. Lateral loads are transient and short lived so the logic of providing ample warning to the soon to be crushed doesn't really apply in my opinion.



HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[BridgeSmith]

The AASHTO spec has a provision for concrete that if Mn is not greater than 1.2Mcr, it has to be greater than 1.33Mu. Isn't there something similar for masonry?

 

[WARose]

I've checked it a few times. It rarely changes anything. Typically when I've done it.....it was because the stresses in the other direction were borderline (i.e. I was doing it by working stress).

 

[oengineer]

@everyone

Thank you all for your help. I have completed my design of the CMU lintel for gravity & lateral wind loads.

 

[lexeng18]

I ran into this very question a few weeks ago and never came across a good answer. Everything I found pretty much didn’t address it at all or approached it from the same perspective that Koot did, which I can understand but doesn’t satisfy me intellectually.

Say you have reinforced masonry wall with windows. The aspect ratio of the windows is such that the mullions will be designed to span head to sill. How do you justify not addressing out of plane bending?

I would love to be pointed in the direction of a technical reference that explains this in more detail, or even just what industry practice is.

 

[jimstructures]

oengineer;

Please, if you are willing, share your solution so others can see your methods and assumptions.

Jim

 

[oengineer]

@jimstructures

I used example 11.4-1 in the Masonry Design Guide to create a spreadsheet. In my analysis, I rotated the beam on its weak axis and used 0.6D + 0.6W for the service load deflection check and & 0.6D + 1.0W for the strength check. I did 2 different calculations: 1.) DL+LL based on example 11.4-1 2.) WL based on factored & service load deflection check with rotated member.

 

[masonrygeek]

There are a couple of good articles by David Biggs in Structure magazine that cover openings in masonry walls here:

https://www.structuremag.org/?p=4474

and here:

https://www.structuremag.org/?p=3834.

While it doesn't address this issue specifically it shows a procedure for collecting the loads around an opening. How you treat the masonry lintel will depend on where you place movement joints, whether you are designing a shear wall, and the size of the opening.