Shear capacity equation for 100 year old brick piers

[carlosqw]

Hi everyone,

Other than performing testing on mortar strength, I will like to predict the shear capacity of an unreinforced brick pier of a building constructed about 100 yrs ago. I have the UBC shear capacity eq. for reinforced mansonry walls and will like to use it, by just setting the shear capacity of reinforcement equal to zero. Is this OK?, or is there any other eq. that only calculates shear strength of unreinforced masonry walls?

 

[JAE]

I'm not sure you can use the current equations for shear on 100 year old brick. Primarily for the reason that the testing and research that went into setting these shear values was probably based upon today's more rigid mortar vs. the older high-lime content flexible mortars.

 

[dik]

The shear capacity of the brick pier will be largely dependent on the quality of bond between the brick and mortar and the load applied. You can likely check with a parks department that is involved in restoration and I have a *.pdf file on historic mortars as well as one that I've prepared for historic brickwork. Service load shear capacity will likely be in the order of about 20-40 psi for the soft mortars typically used (if in good condition). Testing shows that within reason, the soft mortar strength is independent of the mortar and is primarily govened by the joint thickness. This is kinda reasonable where a thin joint will transfer shear better and the values are low enough that the scatter of data clouds results.

dik

 

[Taro]

Default lower bound shear values for unreinforced brick are given in FEMA 356 (available at

http://www.degenkolb.com/0_0_Misc/0_1_FEMADocuments/fema356/ps-fema356.html).

These default values should not be used for critical facilities and must be further reduced by a "knowledge factor" of 0.75.

 

[bkgd]

you definetly do not want to use current code equations. FEMA 356 will give you several different equations to check - rocking, bed joint sliding shear, ect. you also need to realize that FEMA 356 is a deformation based code meaning that your loads are higher as compared to current code since you are trying to determine actual deformations in elements as compared to forces in elements. Basically the loads will be about 4 to 6 times higher than a UBC/ASCE 7-02 approach, depends on your rehabliation objective and analyis procedure linear static, linear dyanmic, pushover or non linear dynamic.

Just make sure you use FEMA 356 in its entirety, great document but somewhat complicated and cumbersome but you really get the most out of the exisitng structure.

I have used it for many upgrades of all types, you will also need to make sure your jursidiction will approve its use, Portland, Oregon allows its use with an approval.

regards,

bp

 

[youngstructural]

This is an excellent reference of which I became aware while working with the Canadian Fed. Gov. as a bridge engineer (a VERY junior one mind you!)...

You want "Design and Construction of Stone Columns", 1985, FHWA-RD-83-027, PB85-215416/AS... For some reason listed as a Ground Improvement...

http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

The engineers with PWGSC are some of the best bridge engineers in the world when it comes to existing bridges. Our National Capital is a monolithic stone architecture museum, unlike the american capital which always strikes me as somehow cold and polished, but absolutely stunning (as, of course, it was intended). If you're looking for some expertise in the area, I would suggest sending them an e-mail. They're very friendly and usually more than willing to help, government department or not. They will not consult for external organizations, so expect to have their help limited to suggestions and tips by telephone and e-mail.

I've done a few of these analyses myself over the years, just remember that the quality of the pointing makes a big difference. With stone that old, it almost certainly is not cementious grout. Make sure you match your grout for any repairs! If your grout is more stiff/stronger than the original grout, or even worse stronger than the stone themselves, you can be in for big trouble.

Good luck! Let us know how it turns out...

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[youngstructural]

My apologies gentlemen: There is a good reason that resource is listed under Ground Improvement, it's not the document I was thinking of, and actually does relate to stone columns used to improve soft clays, etc. I will endeavour to find the right reference, which is essentially a run-down of how to consider the mass as a measure of ensuring a mechanical friction developing between the stones (and hence providing shear capacity).

If I can put my finger on it, I'll post the link...

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...