Raising the height of a freestanding brick masonry wall 1

[PSEPK]

I have to analyze an existing 9 in. thick burnt clay brick masonry wall and to propose measures to be adopted for raising its height from 6 ft to 12 ft above ground level. Existing wall has RC columns at 10 ft on centers. wall has been constructed in 100 ft lengths with 1 inch expansion joint between consecutive lengths. For wind analysis as per ASCE 7-05, with 100 mph wind in exposure category C, I need your comments on the following:
a. What should be the value of B in Fig 6-20 for this case, 10 ft (equal to column spacing), or equal to length between expansion joints?
b. How the stability of the raised wall can be ensured? Foundation of brick masonry wall consists of 13.5 inch brick masonry supported on 6 inch thick PCC. RC columns are supported on 6" thick RC foundation, 2' - 3" long (normal to the wall length). Column foundation has been placed 1'-10" below ground level.
c. Are there some recommended ratios and spacings of brick masonry buttresses for supporting freestanding walls? Where can I find them?
d. Can one refer to some example or article dealing with similar situation, or design of brick masonry walls with RC columns.

Thanks.

 

[JStephen]

Is tearing the old one out and building a new one a viable option?

 

[PSEPK]

JStephen,

It is definitely one of the options, but might not be the first choice.

 

[CELinOttawa]

Agree with JStephen. From foundations to unforeseen site conditions, this is bad news. Remove and replace. Sometimes clients need to be told 'no'. Your current wall height is well protected from wind loading by ground effect, the higher you go the less true this is. Unless you're going to build in new columns with new foundations between, and farting around trying to save a few bricks at the (silly) expense of labour, this is not going to go your way.

I have, however, had no choice but to do this once before. We extended the height of garden walls around a heritage property. We put columns to each side of the wall and built atop. Terrible, gastly looking And inefficient job. Tearing down and building new would have been cheaper and easier.

 

[Hokie93]

Have you verified that Exposure C is the correct Exposure Category? Sometimes we take "C" as the default when, in reality, Exposure B might be appropriate. As you know, there is a pretty good reduction in wind pressure going from Exposure C to Exposure B. As for Figure 6-20 of ASCE 7-05, I would use 100 feet for dimension 'B'.

"Masonry Design and Detailing" by Christine Beall provides good information and rules of thumb for the design of brick screen walls.

 

[PSEPK]

Thanks for your comments.

@CEL: The client has already used masonry buttresses to support some other walls in the past, and is not happy with that arrangement. Accordingly, this time some other solution is sought.

@Hokie93: Wall is located mostly in Exposure C area. I'll check the Beall's book too.

 

[oldrunner]

You also want to consider any uphill wind condition (Kz factor).

 

[PSEPK]

oldrunner,

The site is in a plain area, as such there is no uphill wind effect.

 

[PSEPK]

And, now the closing remarks.

I was asked to suggest 2-3 options to my boss, for handling the stated problem . Two options were obvious.
Firstly, removing the existing low-rise wall and constructing the new one of a proper design.
Secondly, using the masonry buttresses to support the raised height walls (already in place in some locations, and working fine.)

I was in fact looking for some other simple, practicable, reliable, time- and cost- effective solution. It however turned out that there was none.

I have submitted my opinion to my boss, for recommending the first option mentioned above (i.e., new construction) to the client . Alternately, client may go for masonry buttresses, if he likes to.

Thanks to all of you for spending your time to comment on the matter and providing valuable professional advice.

 

[dicksewerrat]

Is there room on the upwind side of this wall to build a covered walkway with more brickwork columns to help hold the actual 12 ft. wall? I look back to 'Flying Buttresses' from the late Middle Ages.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[PSEPK]

Yes. I myself had thought of the same once, but the idea was not floated, as the client was not interested in protruding type of wall supports.

Anyway, thanks for the idea, dicksewerrat.

 

[dhengr]

PSEPK:
That’s a pretty tall order (pun intended). When you are going to double the height of the wall from 6' to 12' and then object to almost any means of buttressing the existing wall, I’m not sure it can be done. Then, you have an existing footing/found. condition, not well explained/detailed which was certainly not designed for this significant increase in the lateral loading, and how to make that measure up to the new loads? If you could do something to make the footings capable enough to support the cantilever, and if you had or could gain access to mid-wall cavities you might consider post-tensioning the entire wall height down to the footing at regular intervals. Alternatively, you might try to make the wall span the 10' btwn. conc. columns. Then rework the footings at the columns to take the lateral loads. Then, add width to the columns perpendicular to the wall plane, making them 3-4 times wider at the base, on the new footings. Slope these new sides to be narrower at the top, and reinforce them as a canti. with sloped tension and compression faces. These new column portions might be cast right over/around the existing columns and provide the same masonry keying feature as exists now. This reinf’g. could be post-tensioned also, with the fixed ends in the footings. These would be kinda like a buttress but more closely mimic the existing column condition and spacing.

 

[PSEPK]

"Alternatively, you might try to make ..."

Wow, what a solution!, dh. you have got the point (although, conveyed a bit late. In fact, due to urgency at my office, I have already given my recommendations more than a week ago). This is a very good idea, indeed. Rather, i should say, it is the kind of solution the client might have been looking for. Requires strengthening the columns only and additional footings, with minimum width of protrusions visible. Has the big advantage of keeping the masonry wall as it is (saving a considerable amounts of construction material as well as time, otherwise required for reconstruction).

It is very likely that I might not be able to work on this option right now, because of time and other constraints at my office. I however appreciate your valuable views on my posted problem and suggesting a solution which is not only structurally very sound and practicable, but is architecturally attractive as well. Thanks a lot.