behavior of steel frame with infill CMU 1

[Robbiee]

When a steel frame has infill CMU built tight and anchored to the columns but isolated from the beam above for deflection, the block resists lateral loads by the diagonal compression strut action. Correct? or it needs to be also connected to the beam?
Your input is appreciated. Thanks

 

[Lion06]

It definitely doesn't need to be connected to the beam above to take lateral loads if it is built tight to the columns. That being said, it makes it a lot more difficult to determing how much of the lateral load is going to the CMU infill walls in that case.

 

[MiketheEngineer]

And is the CMU re-inforced and how did you connect it to the colums. Just butting up might not help that much because there may be gaps.

 

[RareBugTX]

This topic is interesting and before I comment on your post I want to say that IMO this building structural system rationality is not widely understood and a lot of empirical assumptions in analysis, design and construction are the norm.

The way I see it is that yes, there must be a continuous attachment both at the top and bottom of the wall to attain the CMU compression strut masonry action, and the tables and formulas by TMS, ACI and ASCE 530 document reflect those assumptions. If there were no continuous attachment, then the wall shear capacity for in plane forces would not apply and I think the loading would resemble more of a column (pier) with uniformly loaded at its jambs (column flanges), in other words, the wall would deform as a column does.

I have been looking for an example where the compatibility betwwen the steel frame drift and the cmu wall in plate deformation is addressed.

Best regards


Rarebug

 

[civilperson]

CMU without cells filled and reinforced has a large percentage of void space. The mortar joints are much weaker than the faces and webs. Without continuous bond beams or totally filled cells, no significant strength may be added to the column for lateral deflection.

 

[Robbiee]

Thanks all. CMU is connected to columns by z-shaped flat bars welded to columns/or hooked to clip angles welded to columns. Block is reinforced.
My understanding is this:
compression strut will develop only if the lateral force applied on CMU is smaller than:
1-shear capacity of block/joint
2- bearing capacity of an effective width of block.
The lateral force could be calculated by assuming supports at the locations of the Z-ties. The reactions at the supports are the lateral forces. Now for deflection calculation, a compression diagonal brace with equivalent axial stiffness is assumed considering cracked section.

 

[csd72]

This exact topic has been covered previously and I provided some good references.

Use the google search at the top of the page to find the previous thread.

 

[Robbiee]

csd72:
The link to the references you provided is not working. Can you please check if it is correct? Many thanks

 

[csd72]

Yes sorry that link is now redundant, try page 7 on the following:

http://www.eng.upm.edu.my/hrc/pc/handouts/precast frame - 6 stability.pdf