Masonry parapet 8' extension

[RareBugTX]

Hello all:

Working on a Strip Mall (150' x 35') project where cantilevered parapet is being extended eight feet. 20 psf wind controls. Roof structure is open web bar joists @ 5'. Upon performing structural analysis it is found a 0.9 klf at the roof/wall connection. Largest moment appears there. Mason wants to only grout and reinforce only 8" cmu ABOVE the existing roof/wall connection. My clear concern is that we have an moment of same magnitude (+- 0.9 klf)just below of roof and wall location. Question is about constructability and ease of reinforcing/grout placement. Existing wall below is 4" brick outer face, 4"unreinforced cmu. collar is partially grouted. I am proposing to break the inner face shell at rebar locations and run my rebar down 36" (#5 @ 24", more than enough per calc) at the vertical reinforcing locations. Does this sounds like a lot to you guys?, Other option is to completely demolish wall and start from ground.

 

[concretemasonry]

Sounds like a "make pretty" redo to modernize a maal that was built withou any plans for modifying.

To increase the resistance of the new parapet, you must increase fixity of the currently projecting parapet. Apparently any bracing of the additional height is probably out of the question.

You probably no choice but to use the modified existing structure. If you want to install vertical reinforcement the prudent thing to do is open up a section of the wall at several locations and see what is there. You option to use the 4" block to develop the lateral resistance of the new parapet could be the best rout.

Seeing what the old parapet and connections are is necessary. Usually 4" block are very difficult to grout. Very often, 4" block have a "top" (as laid) that is almost closed and have 2 cores. The other common type would have 2 cores and a relatively open core from top to bottom. Unfortunately trying to meet the code requirements for normal grouting can be difficult whee you try to put rebars in the small space and still have room for the grout to develop the continuity and bond. A 11" slump grout with very small aggregate might be possible, but difficult to assure.

Dick

If the new extension as much less and not 8' high, the mason might have some credibility. There is little flexural strength in 3-5/8" thick masonry wall.

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

 

[msquared48]

I do not think that 4" CMU can take #5 bars with grout. Moreover, I do not think it will work in flexure.

Looks to me like you need to take down the existing wall and construct one that will work.

Mike McCann, PE, SE (WA)

 

[JAE]

I would agree. Tear the wall down and start over.
To begin with - 20 psf pressure is not correct on a parapet. That should be closer to 35 to 40 psf total (see ASCE 7 for parapet component wind pressures.)

You stated an 8 ft. extension of the parapet. You didn't say what the existing parapet height is.

For a wall + parapet, the bending moment extends quite a distance down the "backspan" below the roof. Any reinforcement would have to extend past the backspan inflection point and be developed properly.

Since this is an existing structure, how well do you know about the current roof-to-wall connection? Adding 8 feet of cantilever would be adding about 300 plf of load there.

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[JedClampett]

I'm a little worried about the statement that 20 psf wind controls. Parapets have huge wind loads, with windward on one side, leeward on the other, all multiplied by a very large coefficient. Three times typical wall loads are not uncommon. You're going to have difficulty designing any 8'-0" cantilever, much less the bottom connection.
If you're governed by any code that references ASCE 7-05 or later, take a look at the section for wind loads on parapets.

 

[JAE]

Jed - I was probably unconservative suggesting 35 to 40 psf. I think we've seen 40 to 50 psf for 90 mph zones.

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[JedClampett]

We must of pushed submit at almost exactly the same time. I wasn't commenting on your post, just the 20 psf in the OP.

 

[JAE]

Jed - No I understood that - just responding to your "Three times typical wall loads are not uncommon". I previously didn't want to get carried away with overestimating the parapet pressures and just wanted to reinforce afterwards that your estimate was probably closer to reality.

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[XR250]

I would also be concerned about the MWFRS

 

[JAE]

XR250 - that is a good point - adding 8 ft. of parapet would definitely add to the overall building lateral wind and possibly compromise the roof diaphragm and whatever type of lateral system is there.

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