Plain Load Bearing Masonry 2

[Soldierson]

I am involved in a renovation of a 1-story 1950's era warehouse built with plain load bearing masonry walls (no steel) supporting a simple bar joist roof system. The masonry is a closed cell hollow block. i.e. you can't grout it. Any references on how to reinforce the masonry for wind loads without disturbing the original building look and architecture? I am proposing to remove the existing brick veneer and apply a reinforced cement stucco. Are there any other ways? Also in analysis of the existing structure, I am using an allowable compressive strength of 100 psi (gross area) and allowable shear stress of 22 psi based on BIA tech references. Sound reasonable?

 

[dik]

Can you use W8's on the inside?

Dik

 

[concretemasonry]

Sounds like you may have a typical 16'-20' high wall warehouse built out of 12" hollow block. - It was probably built according to code at the time.

Does it have pilasters at 18 or 20' o.c. or any type of lateral bracing?

Since you are removing the brick, the BIA suggestions are not relavent. Removing the brick will obviously change the appearance. Brick veneer usually does not contribute to the strength of a wall, so the structural effects do not exist.

Your problem will not be with compressive strength or shear strength.

I suggest looking at ACI 530-02. There are provisions for several different types of design/analysis methods including Empirical design (which may have been used at that time), Allowable Stress Design, and Strength Design. You can even look at Prestressed Masonry with segmented steel for a retrofit if you can get access.

 

[Soldierson]

This building has no pilasters on the inside. The bar joists are on 8' centers and span 36' which is my concern. The owner is trying to keep the interior clean and avoid adding pilasters, beam columns, or other 'unsightly bracing". The building is about 14' tall and uses a 6" hollow block.

Dear concretemasonry:

I did review the ACI 530-02. Are you telling me that this code applies to plain load bearing masonry? I propose replacing the brick after retrofitting the load bearing wall.

Thanks for your input. It is very helpful.

 

[jike]

Is there evidence of distress in the wall due to wind load?

It sounds like it was built by emperical methods which I would guess doesn't check out by engineering analysis. If it checks by emperical methods would you really have to do anything?

Please refer to ACI 530-99, Table 5.5.1 and notes 1 and 2 below. 12x14/(4+6) = 16.8 < 18. What kind of wall ties do you have or do you have bonded headers?

 

[concretemasonry]

Gerald - The Empirical Design Provisions of ACI 530-02 can apply to loadbearing unreinforced masonry.

What you have may have been satisfactory for the code when the structure was built. Once you get into changes that effect the structural portion of the building, the existing codes will govern. Replacing the brick, if it is bonded or tied, is not a practical alternate.

Determine if you have unbonded brick veneer or bonded brick veneer (5.7 of the ACI 530-02). The hollow 6" is a very rare construction, although it is used with high strength units on 20+ story load bearing buildings elsewhere. Once you remove the brick veneer that may be bonded and structural, the current codes will apply. It may be prudent to look closer at the wall construction details and the possibilities of partial grouting of the walls.

What part of the world is this project in? American engineers have not been very progressive when it comes to high performance of masonry.

 

[Soldierson]

Jike,
There is no evidence of wind distress. The owner is cutting new openings in the wall so I must analyze the new masonry columns created by the cuts. I may have to do more destructive testing to determine if this is bonded brick and headers. Thanks for your input.

concretemasonry,
again, thanks for your insight. This project is in Dallas, Texas.

 

[jike]

Bonded brick headers can generally be seen from the outside.

How large are the openings?

 

[Soldierson]

Yes, you're right. These openings are supported with steel lintels, not headers. There are five openings 3' to 9' wide, new and proposed, in a 57' long wall. The remaining six panels are 4' to 7' wide for a combined length of 29' to support the 57' long wall in wind shear and vertical load. Code wind loading is 90mph.

 

[dik]

At the time, it was common for cavity walls to be the equivalent thickness of the sum of the wythes (nominal dimension) and the unsupported height being 18 x the thickness...

if 8+4=12 * 18 = 18'...

Dik

 

[Soldierson]

OK, I understand, The maximum unsupported ht is 18' which this building meets, but I am analizing for combined wind shear and vertical load with new openings in the structure. See last reply above. Thanks for your interest.

 

[jike]

The header that you mention is different then what I am talking about.

The headers that I am refering to is the brick turned 90 degrees to fit into the CMU in lieu of wire ties.

 

[msquared48]

Any chance of vertically post-tensioning (unbonded) the wall from the roof?

Mike McCann
MMC Engineering

 

[Soldierson]

Jike,
No, there are no header (row lock) brick. I am not sure of the attachment. Like I said earlier, this may require some disassemby to figure out the exact construction details.

mssquared,
Yeah, I am now considering alternatives. Post tensioning could work, but where the heck do I hide the strands? This is an exposed visible wall, architecturally. I cant put them inside the closed cell hollow block.

 

[msquared48]

Why not? - you could drill from the top down and anchor the strand in the foundation - right thru the voids in the wall. Probably will have to underpin the foundation where the PT breaks thru the bottom of the footing to install a plate to weld it off to, and tension from the roof.

Mike McCann
MMC Engineering

 

[Soldierson]

OK, I am going to put that in my back pocket to consider as I proceed with the analysis. Thanks for your input.

 

[csd72]

Keep the cavity, remove a few bricks/blocks and place a plate across the cavity with a nut welded to the underside.

Then put a threaded A36 rod dow into the cavity threaded into the nut and tied to the roof members. This will bear against the underside of the hole you just created.

The dead load of the wall should be sufficient to resist the uplift if you spread it at a 60 degree cone..

Typical practice in Australia.

 

[Soldierson]

This is the third good responce I have received. Thanks for your help.

 

[concretemasonry]

While in Phoenix. I saw homes built using unbonded post-tensioned 6" block walls.

They used threaded segments of rods and a nut and washer conbination that was calibrated for the proper torque/tension. This was new construction and I think the base rod sections were embedded in the foundation.

 

[csd72]

Where you have lintels, you may get away with just rodding down to the lintel if there is sufficient weight over. If not then you will need to provide rods either side of the openings.

For wider openings you could rod down to the lintels and then tie the ends of the lintels down with a rod chemically anchored into the footing at the end of the cavity (may need a skewed anchor).