CMU Deflection Limits Extents

[LPPE]

Suppose I have a beam directly supporting CMU. Deflection limit = L/600. What about the girder that supports this beam? Not directly loaded with CMU, but still required to meet L/600? What if this girder frames in to another girder? Does this girder, now twice removed from direct CMU loading, need to meet L/600?

 

[haynewp]

No, it's just the beam that is directly supporting the CMU that is required to meet a certain deflection criteria for the CMU.

 

[samdamon]

Respectfully disagree with you, Haynewp. Agree that for a single span condition l/600 is very appropriate for new construction. However, if the load path through supports for the masonry is more complex, and depending on the extents and geometry of the masonry being supported, both local (for a single member) and macro (for all members that are helping carry the load) limits on deflection may need to be investigated. In the end, to keep movements in the still-plastic masonry tolerable it might be necessary to use stricter deflection limits for a portion of the support system. My two cents. Regards.

 

[haynewp]

I think it would be the differential deflection at each end of the support beam that would cause a problem and not an overall movement. If each end of the support beam deflects a certain amount, unless the deflection is different on each end, then how does this affect the wall?

I think you are right, it does need to be looked at, but to keep a girder to L/600 because it supports another girder (at L/600) that supports a beam (also restricted to L/600), because the one beam has CMU on it? Seems like overkill to me.

 

[samdamon]

Good point about differential movement of support points.

Perhaps I am generally being conservative. However, if the total of combined deflections of the entire support structure exceed the l/600 guideline, that could be a potential issue. LPPE's situation is a little unusual in that he has a greater number of members sharing the load from the masonry (relatively rare condition, I think).

 

[UcfSE]

I think LPPE is speaking of a hypothetical situation. Personally I would not count rigid-body deformation in with the deflection of the beam to find whether it meets L/600. Basically I would hold only the beam directly supporting the cmu to the L/600 value unless there are special detailing considerations to tell me otherwise. If there is an adjacent wall for instance I would not want a great deal of deflection relative to the two walls. I would also try to put some sort of a practical limit on the total deflection.