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IBC Chapter 34 Existing Bldgs Lateral Intent

IBC Chapter 34 Existing Bldgs Lateral Intent

IBC Chapter 34 Existing Bldgs Lateral Intent

(OP)
IBC (09) 3403.4 Existing Structural Elements Carrying Lateral Load:...... ... "Exception: Any existing lateral load-carrying structural element whose demand-capacity ratio with the addition considered is no more than 10 percent greater than its demand-capacity ratio with the addition ignored shall be permitted to remain unaltered."...

Is the following the intent/in compliance with the above:
Very simplified example:
- Existing building 5 stories, built 1900, no defined lateral system. Analysis shows existing capacity of urm walls = 50k base shear
- Existing wind demand = 100k base shear
- Existing D/C ratio = 2.0 (2x overstressed)

- New addition, add 2 stories onto the ex 5. New/addl wind force/V = 40k
- I provide a lateral system (say new elevator/stair cores) with capacity = 40k

One way to look at it: existing D/C unchanged, new takes new load and old takes old load. Still 2x overstressed.
Other way to look at it: total demand = 100k + 40k = 140k, total capacity = 50k + 40k. Total D/C = 140/90 = 1.56 (better than existing)

Long story short - can the code be interpreted that I only need to provide a system for the increased load?

RE: IBC Chapter 34 Existing Bldgs Lateral Intent

No. You need to design the new lateral system for the load it will actually see while not increasing the demand to capacity of any element in the existing lateral system more than 10%. If the new system is stiffer than the existing, it will take loads from the lower 5 stories. Conversely, if the new is more flexible than the existing, the new construction will dump loads into the existing which will probably exceed the 10%. A more detailed analysis is required. This is how you might proceed.

1. Analyze the existing building with the current code loads to determine the loads to the existing lateral elements.
2a. Analyze the entire building with the new construction and current code loads.
2b. Verify the loads to the existing lateral elements has not increased 10% (if the load to an element from the first analysis is 100 kips then it needs to be less or equal to than 110 kips during the second analysis)
2c. Verify the new lateral system meets current code requirements.

This might satisfy the code requirements, but it may not be good engineering practice. I would strongly discourage building two stories on top of a building when it is already excessively overstressed with current code loads. This is my opinion and I would like to hear what others think.

Also, chapter 34 only allows a 5% increase for gravity loads. I doubt you can add 2 stories and meet this requirement

RE: IBC Chapter 34 Existing Bldgs Lateral Intent

I agree that you need to ensure that the existing walls will not absorb (much) new additional load. The IEBC equates the allowable 10% increase directly to story shear. Seems like it would be difficult to add a 7 story high core with a high enough relative stiffness to absorb just the new loads. Unless the new floors and core were somehow laterally isolated from existing.

RE: IBC Chapter 34 Existing Bldgs Lateral Intent

(OP)
I was not talking about a specific example (although 2 on 5 is nothing special in my area, larger is definitely done). My questions was more general, if you increase the loading (say just wind to keep it simple) is there a threshold where you are required to upgrade all elements or do you just have to 'do no harm' and not make the overstress higher on any existing elements.

It wouldn't be that hard to come up with an example. Modify my example - flexible diaphragm wood framing, say in the Y direction the building currently has a front and rear urm wall and an urm core around stairs. We add 2 more floors and at the same time upgrade the stair and add a new concrete core around that stair (or reinf m walls). The 2 new floors have a smaller footprint and the diaphragms cantilever off the core, i.e. all new 2 story lateral load goes into new core. So old walls take same load (assuming flexible diaphgrams load dist by trib). New core takes all load from new 2 stories + whatever old load was in that core, we design it to work. The front and rear walls are still overstressed by 2.0.

I am not inventing the problem, this comes up all the time and I've seen this justified where others say that you have to upgrade everything. I don't think it's clear in the code one way or other. (Obviously better to upgrade everything where possible).

RE: IBC Chapter 34 Existing Bldgs Lateral Intent

I think wannabeSE's response is very good - you have to take into consideration the relative stiffnesses of the new configuration and compare the NEW load path demand with the original condition.

bookowski, your second post suggests adding the two floors but detailing the addition in such a way to avoid any lateral forces getting into the original unreinforced masonry walls. If you actually could do this (it seems doubtful) then yes, by not adding more than 10% to the lateral loading of the original, the code suggests that is acceptable to leave the original walls alone.

But I also agree with wannabeSE on this as well:

Quote:

This might satisfy the code requirements, but it may not be good engineering practice.

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RE: IBC Chapter 34 Existing Bldgs Lateral Intent

(OP)

Quote (JAE)

I think wannabeSE's response is very good - you have to take into consideration the relative stiffnesses of the new configuration and compare the NEW load path demand with the original condition.

I assumed at this point that we don't need to preface our questions with stating that we'll perform structural analysis to determine the demands.... yes the analysis would need to consider relative stiffness as all of our analyses do. Please feel free in the future to assume this for all of my posts (and presumably everyone else who posts here and performs engineering).

The essence of the question is whether or not the code, in letter or spirit, intends for you to upgrade all elements at some threshold or is satisfied if you do not increase the demand to capacity ratio on existing elements.

I do not agree that this approach is wholesale poor engineering practice, in fact I would argue that the reverse may be true.

RE: IBC Chapter 34 Existing Bldgs Lateral Intent

bookowski,
Sorry if I mis-interpreted your question. Your statement here:

Quote:

One way to look at it: existing D/C unchanged, new takes new load and old takes old load. Still 2x overstressed.
Other way to look at it: total demand = 100k + 40k = 140k, total capacity = 50k + 40k. Total D/C = 140/90 = 1.56 (better than existing)

Long story short - can the code be interpreted that I only need to provide a system for the increased load?

is comparing two approaches to calculating the actual demand ratio change. I was responding to that specific question.

I didn't see in your first post anything about what to do if one or the other provision was exceeded, which was apparently the focus of your second post.

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RE: IBC Chapter 34 Existing Bldgs Lateral Intent

And to answer your second post with my humble opinion - if you do exceed the 10% limit, then yes, the entire building lateral force resisting system must be altered meet the current code by upgrading the existing materials/assembly, providing a different load path, or developing a different stiffness arrangement to keep the demand ratio below 10%.

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