(Un)Cracked Masonry Section

[EIT2]

When designing a working stress method, concrete masonry shear wall, are there differing design procedures and/or formulae, based on the determination of whether the section is cracked or uncracked?

To explain further, if one should have the book, Design of Reinforced Masonry Structures, First Edition, I am comparing two different design examples 4.9.2 and 4.9.3. They appear to adopt two different procedures after the cracked or uncracked determination.

What is the difference between cracked and uncracked concrete masonry sections?

Any comments? Thank you!

 

[ishvaaag]

The more important thing when doing calculations one is not familiar with is to try to get the gist of the question. One way may be as you are doing asking to those usually practice the method.

Other important thing is to be consistent. This is usually made by following some standard procedure, ideally that of a sound and clear apllicable code, but if not, a sound and clear technical text.

Now respect to your question, for unreinforced masonry, or for reinforced to levels where cracks have not appeared some use the whole section, following some consistent procedure.

When cracked some of the usual assumptions of reinforced concrete appear, such that some tensile force at the reinforcement is balanced bya a simple compressive block, such a triangular one (which was typical for reinforced concrete in the classical working stress method, and some still support for masonry designs) or a rectangular stress block.

 

[ERV]

EIT2,

In masonry design a cracked section is one that has exceeded the allowable stress (Fb). An uncracked section is within the allowable stress (Fb).

In order to determine the reinforcement requirements for masonry shear walls, a check for uncracked sections is performed, which is the allowable stress. If OK than the section is uncracked, if not, than the section is cracked and recalculated.

The other equation may be in regards to incipient cracking. In which case they are offering a formula for an alternative calculation. This calculation may include the use of more steel, particularly in the end cells.

Sorry I do not have a copy of this book.

 

[dutchie]

This is a question I have worked on a bit lately having just started to do engineered masonry design as opposed to empirical masonry design

The cracked section comes into play when calculating moments in reinforced masonry. As a section is stressed beyond it's tensile capacity, cracks develop. This reduces the effective moment of inertia of the section. The canadian code requires us to calculate an effective moment of inertia using formulas that include the moment of inertia of the uncracked section and of the cracked section.

Basically, you are trying to pinpoint the location of the neutral axis of the section once cracks develop. the neutral axis will shift from the middle of the wall in an uncracked section to a location closer to the edge of the wall as the section cracks under load. It may be useful to visualize the section as a T-beam once cracked.

That is what I have come up with anyway. any other input on this one would be apprecieated.

 

[ERV]

EIT2,

Try this link for more info:

http://www.ncma.org/etek/TEKs/T14-7AF.cfm?Referer=blocklite.com