International Existing Building Code

[penpe]

I'm seeking your opinions about whether an exception for lateral loads in the IEBC (International Existing Building Code) can be applied to structures that are NOT buildings. I have just purchased access to this document, so I'm just beginning to look for answers. This question pertains to an industrial 140 foot tall heater structure, (half of that height is exhaust stack), whose foundation footprint is 36' x 20'. The client wishes to increase the height of the convection section, which increases the overall height. Wind shear is increased about 9%.

The exception to 2015 IEBC section 1103.3 says "the lateral force-resisting system of EXISTING BUILDINGS to which additions are made shall comply with sections 1103.3.1, 3.2, 3.3. Exception #2: Any existing lateral load-carrying structural element whose demand-capacity ratio with the addition considered is not more than 10 percent greater than its demand-capacity ratio with the addition ignored shall be permitted to remain unaltered."

My analysis concludes that the addition causes the allowable soil bearing capacity to be exceeded by 6%. Would it be appropriate to include in the report of my findings that the foundation is adequate because of the IEBC exception?

 

[1503-44]

I think you already know my answer. According to your description, the IEBC allows such increase to apply only to the lateral force resisting system only. IMO that means typical cross bracing. Furthermore it apparently defines that system as a lateral load carrying structural element. Again, typical cross bracing. That's not soil.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[TLHS]

That clause doesn't apply to this situation, that being said a 6% overage in bearing pressure wouldn't generally be a problem unless you've got unusual soil conditions where settlement or foundation movement aren't the primary design issues. Can you accept a bit more foundation movement?

If this is outside of your experience, though, throw some fees at a geotech engineer to have a quick look.

 

[KootK]

I disagree with the other guys and feel that the IEBC may be of some use to you in this. Here's how I see it:

1) Obviously, you need to do your due diligence in ensuring that there isn't some other, more applicable standard floating around that would supercede the IEBC. An "International Existing Stack Code" or whatever. No doubt you've already done this.

2) The spirit of the IEBC provision is simply to acknowledge and quantify an engineer's prerogative to be reasonable in not directing clients to undertake major remediation works in response to trivial increases in demand. In my opinion, it is your prerogative to employ such reasoning with or without the IEBC. Referencing the IEBC simply provides a precedent for this logic within our profession, even if it's targeted at a different class of structure. In your report, I would just state all of this plainly and frankly, acknowledging that the IEBC is not targeted at industrial structures but sets a precedent none the less in your professional opinion.

3) The foundation to soil interface is what "braces" your structure from flopping over or sliding under lateral load. Conceptually, I feel that makes the interface as much a part of the "lateral system" as any superstructure element.

 

[penpe]

Thank you for the responses.

1503, I see your point that usually this would be applied to structural elements such as concrete, wood, and steel, but I think a GeoTech guy would argue that the soil qualifies as a structural element too - one that is part of the system that carries the lateral loads.

TLHS, we were asked to investigate the impacts of increasing the dead load and the height. We find that the soil pressure exceeds the allowable. I feel that our report needs to convey that finding, and let them make the call regarding what they want to do about it. I do think it's probably okay, and would like to say that IEBC backs me up, but is it a "building"? Probably not. We do intend to recommend a new soils study, as the soils report from 1967 is convoluted, confusing, and generally outdated.

KootK: I agree that IEBC provides a precedent, and could be mentioned as such, even if not specifically applicable. I think that since it's less than 10%, and it's been fine for over 50 years is a pretty strong argument for not modifying the foundation.

It's so much easier to make sure the design is adequate when it's new!

Thanks again.

 

[1503-44]

Absolutely the soil is a structural element, but it has nothing to do with lateral structural elements the building code is talking about, so you absolutely cannot link that reasoning.

If your geotech agrees with soil bearing increase, that's entirely a different animal; go for it. But forget using the building code reasoning line to do that.

Anything else and you're potentially opening a can of worms that you could see on your plate for years.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[KootK]

...but it has nothing to do with lateral structural elements the building code is talking about, so you absolutely cannot link that reasoning.

How would you know which structural elements the building code is talking about? The relevant clause that I'm familiar with is shown below and it does not mention any particular structural elements. So, in my mind, all of the elements of the lateral system air fair game. And that makes sense to me logically since, after all, what could possibly be the difference between a foundation and a bolt in a drag strut somewhere when the method is just a %difference thing? That's a pretty blunt instrument.

Or is there a commentary section someplace that does specify certain structural elements?

 

[canwesteng]

I'm personally more comfortable with going 10% over bearing capacity than structural members, and I think most engineers who work in industrial brownfield would be the same. Depends on the site, but in general bearing pressure relates to an amount of settlement expected and not a shear failure of the soil. Are you really concerned about 10% more settlement than the stack has already experienced? And why should these sizeable wind loads be applied against a settlement criteria in the first place?

 

[JLNJ]

Check to see that your jurisdiction adopts the IEBC. Not all which adopt the IBC also adopt the IEBC.

That said, I would not worry about 6% over for soil bearing, regardless of the adopted code. Allowable soil bearing numbers are not exact to 4 significant digits.

 

[EZBuilding]

How could you increase the lateral demands of a lateral system without increasing the lateral demands on the foundation?