Multi-wythe brick walls

[SteelPE]

I have an existing structure that a client would like to do some renovations to that require a change in occupancy and a virtual gut of the existing building. The age of the building is unknown but there was an addition that was constructed to the building in the 50’s. The building, as it appears to me right now, is unreinforced multi-wythe clay brick bearing walls with poured in place concrete slabs.

The project requires us to remove us to remove a wall between the existing structure and the addition. I am a bit worried about removing the wall as I suspect that it is part of the LFRS for the building. If the wall is removed, we will need to check the shear resistance of the remaining walls to make sure they are capable of resisting the lateral forces generated into the building.

Does anyone have any guidance as to what values can be used on the existing unreinforced mulit-wythe bearing walls (shear and bearing) for a building of this age?

 

[dhengr]

SteelPE:
There are methods for testing that brick insitu and there are labs which can do that testing. These various tests will get you the design values you need for evaluating the structure; gross wall shear and bearing, bond, mortar and brick strengths, etc. I can’t see it from here, but I’d want to see the existing plans or bldg. for its current condition, as this relates to the new layout and usage. I would generally discourage the wholesale removal of major interior masonry walls. You will find that they are needed for the stability that the bldg. has shown over the years. They provided a significant part of the total lateral stability for the bldg. and supported the perpendicular exterior walls ever 16-20 feet, or so. They were also bearing walls for floor and roof systems, which were allowed/made to span that far, at that time. When you start removing these interior masonry walls, everything has to be completely redesigned and rebuilt/reinforced. As it is, you might still have to reevaluate the lateral stability of the bldg. for these significant changes. We can usually rationalize cutting some fairly large openings in those walls, but it is really tough when you want to completely remove them. Obviously, one part of the party wall was once the exterior wall of the original bldg., and how was that treated as relates to any gravity or lateral loads from the newer addition. These details really must be evaluated for their affect. Then, to make the project work, and save a good portion of the essential walls, the architectural layout, etc. must be finessed to allow the structure to be made to work. Architects/clients have to be made to understand that wholesale gutting and saving just the pretty exterior brick walls is not an easy structural problem when confronted with today’s codes. That’s no pat answer, right?

 

[bookowski]

You can have in situ flat jack testing to provide you with shear and compression values. In my area it's relatively expensive though so it's a tough sell. The shear test is also at least a little iffy when I've seen it because the dead load is still in place, so really you get a test value that will tell you X shear capacity under Z dead load - use that as you will for other conditions.

Not sure what size/type of building you are looking at but most likely it did not have an explicitly designed lateral system. So the existing walls may not work for code loads even before you make your opening. A lot of times I see that there is so much wall that shear stresses are low but it's not easy to resolve uplift (tension) in the masonry, especially on front and rear walls which are typically not load bearing other than self weight and broken up into piers with windows.

Usually a change of occupancy would trigger a full upgrade but depends on your local code.

 

[SteelPE]

dhengr/bookowski,

I agree with most of what you are saying. We are making a site visit to the project this week. The jurisdiction I am in requires the use of the IEBC. I had a question posted about appendix A1 of the IEBC a few weeks ago and nobody seemed to be able to answer my question (which was related to this same project).

http://www.eng-tips.com/viewthread.cfm?qid=376448

I found, after I made the post, that provision which requires the use of IEBC Appendix A1 has been eliminated at the recommendations of the engineering community due to the cost implications associated with it. Appendix A1 does give some guidance for tests which we may decide to follow. Taking core samples in accordance with A106.3.3.2 seems like it wouldn't be too difficult.

I don't think the building requires a "complete overhaul" as this isn't necessarily required in the IEBC.... although we are required to evaluate the seismic strength of the existing building using R=1.25 (jurisdiction requirements).... but the jurisdiction also only requires us to use 50% of the seismic load in this instance (basically equating R=2.5)

I find that when I post questions about the IEBC they never get answered. I don't think the engineering community is as versed in this code as they are with many other codes.

 

[bookowski]

Didn't get what you were looking for.

I have not had samples taken to a lab but have had in-situ testing. For most of the conditions I've seen I think it would be difficult to remove and not disturb the mortar joints to get it to a lab. You could test the brick masonry itself but that would be of limited value without mortar info.

For the in-situ tests (at least the ones I've had done) the test report provides you with recommended design values for shear strength, f'm, and Em.

 

[XR250]

I usually end up adding a steel frame to take the place of the loss of stability rather than testing

 

[SteelPE]

Excel,

That is usually what I would do too, but I'm not sure I can get the architect to move his floor plan (I haven't necessarily tried yet). I may have to do it in this instance as I will probably end up with a diaphragm problem too.

I do work with lots of fabricators and I have a few example projects prepared by other engineers where they do virtually nothing to reinforce the existing building. I'm just wondering how these other engineers (who are much bigger and better than I am) do these projects.

 

[XR250]

They don't consider the stability - like most of my competitors..

 

[SteelPE]

These guys are not competitors. These are engineers from top notch companies.

 

[XR250]

Bigger, but maybe not better…

 

[bookowski]

Why don't you just recommend the in-situ testing? Just had it done recently, it was $12k to do two locations and provide report for shear, compression, and stiffness.

 

[XR250]

In the buildings I usually work on, the mortar has almost reverted to its constituent elements. I doubt anything positive will come out of the tests.
A moment frame could likely be had for less and gives some belts and suspenders.

 

[bookowski]

I think the OP was saying that he couldn't do a steel frame.

I often do steel frames as well but the cost is much more than testing in my neck of the woods, not even close. As soon as you introduce the frame you need some form of a footing/concrete for anchorage, you need to detail the diaphragm to deliver the forces, it also should be stiff or it doesn't make much sense with the compatibility of what is left of the masonry. There can also be an issue that you are unloading the P from the basement wall, which is usually garbage anyway, and it becomes hard to justify the existing wall resisting soil pressure with the lower axial. Once I go steel frame I'm usually in for new basement walls plus frame plus diaphragm detailing.

On a test that I just had done they had a measured shear strength of 248psi (to failure). The test reports refer you to asce 41-13 for making further use of the test data.

 

[Gumpmaster]

Check out ASCE 41. It will deal with the capacity of un-reinforced multi-wythe walls as they relate to seismic loads. I believe there are conservative capacities where you don't have to test the materials, but they leave you the option of testing to obtain higer values. Someone tell me if that's not the case. It's been a while since I've used ASCE 41.

There's a new version, ASCE 41-13, which is apparently much improved.