ASD Masonry Wall Design for out of plane bending- Not in ACI 530-05?

[rharting]

Has anyone looked at ACI 530-05 Chapters 2 (allowable stress of masonry) and 3 (strength design of masonry) and noticed the differences between them?

I'm specifically looking at section 3.3.5, Wall design for out of plane bending and i've found that there is no similar section in ASD. After looking further, i noticed that chapters 2 and 3 are very different in the type of design they specify which seems odd considering they are simply different methodologies.

It may be my ignorance with masonry, but i don't understand why they don't either provide something like AISC (where each design check is provided for both ASD and LRFD/strength design) or like ACI 318 (just eliminating ASD all together).

Thanks for any comments.

 

[JAE]

In the ASD section I think they just provide you with allowable stresses for flexure, axial, etc. It's up to you to put them together to check the stresses.

 

[concretemasonry]

You also have to rely on a knowledge of masonry elements and how they relate to standards and codes that are usually the basis for ACI 530. The NCMA (ncma.org) has a great series of technical notes (over 100) that provide some insight into the various codes applications.

Masonry elements should be treated as as unit and not individual materials. The basis of most standards are past testing of elements and correlation to the material properties.

Because so much of masonry is based on history and the properties of available materials, much of the older sections are based on the very conservative history and do not reflect the current knowledge. - Sort of like trying design a building with 3000 psi concrete when 5000 psi is possible. I have seen 8500 psi CMUs when the current minimum standard is 1900 psi, so it is not surprising that a consensus standard is a little behind the times in some areas of application.

Dick



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

 

[SteelPE]

These threads are a little off topic but they might be of some interest to you.

http://www.eng-tips.com/viewthread.cfm?qid=252451&page=2

http://www.eng-tips.com/viewthread.cfm?qid=252182&page=2

 

[slomobile]

The difference is the varied basic assumptions made between Working Stress and Strength Design. Working stress assumptions include: plane sections remain plane, materials remain elastic etc. Strength design does not limit itself to these assumptions.

The section (MSJC 3.3.5) you speak of is known as 'Slender Wall Design' and is derived specifically to account for second order P-delta effects of the deformed wall under combined axial and lateral loading. The derivation of the included equations is based in Strength Design assumptions and not Working Stress. Therefore there is no corresponding ASD section as you noticed. This is why there are limits to slenderness of a wall that can be designed with ASD methodology (h/t=30), anything above this limit, and the P-delta effects can typically be major players so second order effects must be considered. This is not possible with the previously mentioned basic ASD assumptions.