Alterations to Existing Shear Wall - IEBC

[bookowski]

I have an existing r/c shearwall building from the late 1950's. The client is proposing a new door opening through an existing shear wall. The wall is a straight wall segment (no returns, piers etc) that is about 28' long. The proposed door would be a 3' rough opening centered along the wall length. The building is wind controlled (low seismic region).

My first impression was that this should be ok as the location and size of the door would have little impact on moment capacity (ignore shear for now). However, after checking the wall I am finding that the demand/capacity ratio exceeds 1.0 even before the opening - this is based on current code wind load. The building would have been designed for lower wind pressures.

The way that I am reading the International Existing Building Code it states that as long as I don't increase the D/C ratio by more than 10% I am ok, is this the correct interpretation? What if I am already exceeding capacity under modern loads - say I am at D/C = 1.05, am I allowed to take it up to 1.15? What if I am able to reinforce such that my D/C ratio is not changed, or is even improved but the D/C still exceeds 1.0? This is in the NY area the code provides little guidance on requirements for this situation.

 

[SteelPE]

For some reason I don't really like the IEBC and not many people appear to use it on these boards because I have asked questions before that went unanswered.

What method in the IEBC are you using? Perscriptive, work area or performance? I have always taken the loads as they currently sit today. That is the demand capacity ratios are done using the loads provided in the code today. Technically it doesn't matter if the wall is 10000% overstressed as it currently sits as long as the demand capacity ratio is less than 10% you do not need to update. That doesn't mean you can't update if you don't have to (and if it was 10000% overstressed then I might want to).

I look at it this way. I live in an area where snow loads are greater than the roof live loads. The people who write the code love changing the snow loads for my area. If I have a building where I am adding a suspended ceiling that wasn't in the original building and since the building was constructed the code people decided to change the roof snow load from 30psf to 38.5psf now the roof has a demand capacity ratio of 1.18 (assuming a 15psf dead load and if it was efficiently designed). Does that mean my client who wants to add a 2 psf suspended ceiling needs to reinforce his entire roof because someone decided to change the snow load???????? No. The demand capacity ratio for this roof is only be 1.037 which is less than 5% therefore it's OK. Once you go over these limits then you need to start digging further in the code to see what needs to be done.

How large is this building where you have a 26' long reinforced concrete wall that is overstressed by wind loads?

 

[bookowski]

Thanks.

You mean that as long as the change in D/C ratio isn't over 10% correct (you said as long as D/C is < 10%).

The building is a little over 20 stories tall, and there isn't a lot of wall. At the time it was built the wind requirements were not as demanding so it would have been sufficient. Under current loads it's a bit over capacity, around 10%-15% depending on a few assumptions. The opening doesn't change that too much and I can reinforce to essentially bring it back to about original condition.

 

[SteelPE]

I have always look at being able to take 10% of the length of a shear wall away. I'm not sure if this is correct or not. I have never really gotten to worried with regards to door openings... but then again I have never worked on a structure that is over 5 stories tall.

Once you go over the limit then you need to start worrying about what you are going to need to do and whether or not you need to comply with the new provisions of the code. In my area there are a ton of URM walls..... and for some reason I always get seismic to control so that causes some huge problems.

 

[JAE]

bookowski,
Just a thought but after 60 + years perhaps the f'c is a bit higher than what you are using? Might help a bit on the Vc side of things.

I presume the rebar fy is 40 ksi for that time period.

 

[bookowski]

Yeah, I was thinking that as well. The original drawings specify 3.5ksi, if it has come up higher it would help with the Vc and also the with P-M diagram. Steel is 40, you are correct.

 

[Michel60]

Good morning bookowski,

I believe that the condition is referenced (at least in the CBC) in Section 3404.1 that the altered structure would be "...no less complying with the provisions of this code than the existing building or structure was prior to the alteration." You'd still get the < 10% increase by 3404.4 but not be "required" to upgrade the system as a whole due to the alteration. Of course if in your review you found a significant issue it should be brought to light, but for something in the 5% range it would seem to be splitting hairs for a nearly 60 year old building. And once you start pulling on that thread the end can be hard to find.

I think you have to look at it in the light of if you were doing nothing to the building it would be acceptable for it to be continued to be used in perpetuity, over stress or no overstress based upon current code forces. The procedure is some what arbitrary in terms of this force level. It is intended to provide a relatively simple point of reference to evaluate the change for a relatively small alteration, not to be taken necessarily as an absolute performance requirement for the element.

regards,
Michel