CMU Height Guidlines for Exterior Walls Bearing at Grade 1

[gregeckel]

Are there practical or code based height restrictions on exterior CMU walls which are tied laterally at each floor but gravity loaded to the foundation?

Obviously I need to account for floor deflections and thermal movements.

Code or design guide references would be helpful.

Thank you,
Greg

 

[dik]

Catch the recent thread...

thread507-310458

Dik

 

[gregeckel]

Good info there but that thread is for a wall spanning a large single story. Also in that case the wall is load bearing. My question relates to a wall which is tied laterally at each floor and only supports its self weight, and cladding.

-Greg

 

[dik]

If it's an exterior wall, and is spanning from floor to floor, I'd use this as a pretty safe guide to wall thickness and height... might change a little, but, not much...

Dik

 

[dik]

If cladding... then why not use steel studs... likely a lot cheaper..

Dik

 

[DST148]

gregeckel - As mentioned in the last thread there are no limitations on h / t for engineered masonry. Here are the limits using Empirical Design of Masonry - Ref ACI 530 -05:

Construction Maximum l/t or h/t
Bearing Walls
Solid units or fully grouted 20
All other 18
Nonbearing walls
Exterior 18
Interior 36

t is the nominal thickness of wall.
The code defines wall carrying vertical loads greater than 200 pounds/ft in addition to its own weight as loadbearing.
Many times the thickness of the exterior nonbearing cmu wall used as a back-up is governed by deflection requirements of the finish / cladding /veneer attached to the wall.

 

[dik]

Thanks DST... rescued again... I don't have access to my codes, here, but think that even walls (concrete and masonry) designed as slender have limiting h/t ratios... maybe not with ACI... will check, just out of curiosity...

Dik

 

[gregeckel]

Sorry, I should clarify. In the case I'm considering the floor to floor heights are small (approx 12'). The design of the CMU for out of plane forces is not the primary issue.

The question is about if relieving the CMU is necessary. For example with brick veneer we limit the maximum unrelieved height.

I.E. Can the wall be built 25, 50, 100 feet tall off the foundation wall without relief?

Thanks,
Greg

 

[DST148]

gregeckel - What is relieving the CMU? Did you mean relieving angles for brick veneer?
dik - Ref ACI 318-05: Commentary R10.8 - Design dimensions for compression members - With the 1971 code, minimum sizes for compression members were eliminated......... Commentary R10.11.5 - For slender compression members, an upper limit of 100 is imposed on the slenderness ratio if designed by the moment magnified method. No similar limit is imposed if design is carried out according to section 10.10.1 - forces obtained using second order analysis considering nonlinearity.
ACI allows concrete walls to be designed either using provisions of Chapter 10 or Chapter 14. Thicknesses of walls designed using provisions of Chapter 14 shall not be less than 1/25 the supported height or length, nor less than 4". For slender walls provisions of Chapter 10 would apply.

 

[dik]

Thanks...

Dik

 

[concretemasonry]

DST -
The relieving for brick veneer is a practical/detail situation since brick has a long term expansion, while concrete and concrete masonry have a long term shrinkage/creep and steel is just normally creep due to the magnitude of the permanent loads. It is because compatibility of materials to allow openings (doors, windows) to match the exterior.

Dick



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

 

[msquared48]

You guys are sick, talking about CMU walls relieving themselves. You really ought to be ashamed.

Mike McCann
MMC Engineering

 

[LonnieP]

I may not be up-to-date on this one, but isn't Ht/150 the service load deflection limit for slender wall design?

LonnieP

 

[charliealphabravo]

I have used publications by the metal building industry to address these kinds of issues. They have these issues when using masonry wainscot walls on metal building since the buildings are generally much more flexible than the masonry. Most of the "relieving" is done with specially designed embed plates and anchor rods that are attached to bond beams or filled cells at top and bottom of the wall and then to the steel frame.