h/t Limitations for CMU Walls

[dakota99]

I am working under the 2002 masonry code using allowable stress method. I am looking for limitations on height to thickness ratios for hollow concrete masonry with load bearing walls. I have always designed cmu walls with an h/t> 30 as slender walls. h being the height of unbraced wall and t is the thickness of block i.e. t=7.625 for 8"cmu block. Are there any other restrictions on h/t for walls? also does anybody ever use the equivalent solid thickness as t?

 

[concretemasonry]

Have you looked at ACI 530? The commentaries (code and specifications) provide some valuable insight into the thought behind the codes and specifications. This is the basis for the IBC and much of the IRC.

for higher walls, you should consider partially reinforced instaed of hollow masonry or going to 12" for extremely high wall sections.

Equivalemt thickness is not really a viable measure. the excess mass in the old higher % solid units and complete grouting can be anti-productive in some locations.

Dick

 

[DaveAtkins]

As far as I know, there is no limit on h/t if you are doing an engineered design (perhaps with empirical design there is--I don't know).

The h/t ratio will affect Fa, because h/r is in the formula.

DaveAtkins

 

[dakota99]

thanks for your input. I found a table in the ACI 530 that gives h/t ratios for emperical design but these are pretty limiting.

 

[concretemasonry]

The tables for empirical design are obvious and they are based on dimension of the elements and a long history of performance.

They are conservative and take little real engineering.

When you start into the code requirements, you go to the other sections of ACI 530 beyond the General Design Requirements to the Allowable Stress Design, Strength Design of Masonry and Prestressed Masonry, you are dealing with real Engineered masonry, but you should not try to work with a combination of the well accepted methods.

The Empirical Design follows and is meant as a proven guide for basic structure meeting the requirements and generally uses an assumption of minimal strength matherials or materials lumped into broad categories. This is intended for very basic structures built using common material, requireing little testing and inspection - garden variety buildings.

If you want to design high walls or 22 story load bearing buildings, you need to use one of the other design methods such as Allowable Stree Design or Strength Design of Masonry when you can be more speific about the materials, testing, inspection to make the engineering a value to the owner.