Modifications to Existing Wall 1

[DCBII]

An important client has a 100' long wastewater retaining structure that isn't ready for an earthquake. The vertical bars are too small, the wall doesn't have enough shear strength, and the strip footing can't handle the overturning load. He can't afford to do any retrofits. It's not in the scope of work for this project. He just needs a 12" strip of the strip footing removed to make his process work better. Other than the potential for leaking some wastewater nobody is directly in harm's way.

A strict reading of the IEBC says if I reduce the seismic capacity of a lateral element by more than 10%, I need to bring that element up to code. Cutting out a 12" width from the footing leaves nowhere sound for that 12" wall strip to dump its lateral load. There wasn't a viable load path to begin with. There are some expensive ways to handle it, but it's not in the budget. If I then say "Not feasible", a competitor will say "Yes" and become his new favorite client. Is anybody (beside myself) really any worse off? Probably not. The weakened portion may be the first to go, but judgement tells me this whole thing is likely to fail anyway.

How do you handle this? Is there a way to push the risk onto the owner when he's bent on a certain way of doing things?

 

[MotorCity]

By "wastewater retaining structure", I assume you mean a tank? If that's the case, the strip footing for the wall does very little other than to provide a flat working surface to construct the wall and prevent leakage. The tank resists the load by hoop tension, not by acting as a traditional retaining wall such that it can "overturn". Think of it this way. Say you were to construct a round, concrete enclosure (think cylinder) with no strip footing. Now suppose you were to fill the middle of the cylinder with soil. Think the cylinder would hold back the soil? I do. See.....no strip footing required. Obviously, my analogy is based on how "most" tanks work with the understanding that you could have some sort of odd arrangement or construction.

 

[DCBII]

MotorCity. This is a rectangular tank. I can find a solution, just not the one the client wants without getting into this ethical dilema.

 

[wannabeSE]

Are you proposing to remove 1' (12") from a 100' long continuous footing? If yes, it probably isn't a big deal. But, we don't know all of the details.

 

[DETstru]

DCBII, can you post a sketch? Are you removing a 1'x100' strip of concrete?

 

[DCBII]

I'm not asking you guys to analyze the wall. I've already done that part. It's no good - whether we cut out part of the footing or not. I'm asking if there's a way to make the owner take ownership of the risk, or if I tell him "tough luck - can't do it", and then watch our competitors run off with our client because the competitors are willing to take on the risk.

 

[kipfoot]

The short answer is "no."

If the owner goes against your recommendation, then they take the risk. If they find another engineer who is willing to approve an unsafe condition, then that engineer takes the risk. Ethics prohibit one engineer from supplanting another, but generally any client might someday "run off" with another engineer.

 

[JStructsteel]

If the risk is a failure of the wall, and you said it can be done just to save the client, and the wall fails, who will the client come after?

If you say it cant be done, and someone else does, and if fails, who do you think he will come after?

A client is someone that pays us to take his or her risk. . sounds like you need a new client if cant understand it wont work but chooses someone else anyway.

 

[DCBII]

I'm looking for a risk-based approach, similar to ASCE 41, that allows the owner to determine the performance level they are comfortable with (Life Saftey, Collapse Prevention, etc.).

After a few days of mulling through it, I think I have a solution. I think I'm looking at the wrong seismic event. As I get reading into the IEBC Section 301.1.4.2 (and ASCE 41 Section C2.2.1), it looks like I can use reduced forces for existing buildings. If I'm reading it correctly, existing buildings don't require the same seismic return period to be considered as that considered for new buildings. This wall may be able to handle the smaller forces. This structure was built in the 80's. It wasn't designed for the modern earthquake forces, but it was designed for something.

That approach seems to me to balance the seismic risks with the retrofit costs in a manner that doesn't shut down every existing building project we work on. Otherwise any structure built more than 20 years ago would require a major retrofit with even minor alterations.

 

[SlideRuleEra]

DCBII - IMHO, you don't have a solution, you have an excuse... and a very feeble one at that. Kipfoot and Jrisebo have answered your question about "tricking" the Owner into taking the risk. Frankly, as a former "sophisticated" Owner's employee, I would fire your firm based on that intent.

Anyway, back to facts. ASCE 41 concerns buildings.
As you know (but I just want to mention) building loads are statistical probabilities. Design load combinations acknowledge that.
Life safety allows a limited amount of failure.

A fluid containing wall is different. The calculated (hydrostatic) load is real (not a statistical probability) and is ready to "act" on a 24/7 basis.
Given the first opportunity (say, a seismic event), the fluid will "swing into motion" to ensure total failure... not just a partial (life safety equivalent) failure.

Of course do what you think best, just wanted to put another viewpoint into words.

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[nackra]

I am not sure where your project is, but have the environmental costs (fines) been considered as part of the analysis that reviewed the retrofit costs?

 

[DCBII]

SlideRuleEra:
1. Full disclosure to the owner is certainly part of this, as it would be if I were using ASCE 41. Codes for existing buildings are risk based. The intent of ASCE 41 is to let the owner choose the risk. So it goes with FEMA P-58 as well. IEBC doesn't require me to tell the owner anything, but I think it's fair to do so anyway.
2. "Building" codes have not been written with tanks in mind, but in many cases they are the best source of guidance available for a subject. The exact provisions may not apply, but a cantilevered tank wall will behave much like a retaining wall would under lateral earth loading. Building codes can provide guidance. There is no standard for retrofitting tanks. The underlying principles for buildings can be applied with judgement however.
3. Hydrostatic loads are somewhat predictable, but I'm concerned with hydrodynamic loads, which are highly unpredictable. Even so, the head on this tank varies from one hour to the next. ACI 350 seems to recognize that by giving me a 1.2 factor on the hydrostatic load when designing in combination with seismic loads. What are the chances the max head will occur at the same time as a 2500 year or even 500 year quake?
5. This thing is fine for hydrostatic loading. It's hydrostatic + hydrodynamic that causes the problem.
6. Do you think it's rational to scrap a structure built in the 80's because it can't handle an event with a 2% chance of exceedance in 50 years? Especially when there may only be 20-30 years of life left in it? Everything built prior to the 2000 IBC was designed for a 10% chance in 50 years. Think about how many structures you're condemning. This provision in IEBC, effectively reducing the return period, seems like a rational move to prevent the country's entire infrastructure from being condemned, while still weeding out the grossly under-designed structures. If it still fails, then I'm stuck telling the owner that.
7. This is not aimed at you, but in my experience, it is the firms and engineers that throw their hands in the air and say "it can't be done" that get fired from projects. Sometimes it can't be done, but a good SE knows where the exceptions are in the code, and where they do and do not apply. I get sick of seeing "structural" engineers applying costly seismic provisions where they do not apply. I've found that the term "ductility" is often code for "I have no idea what's going on here, so I'm going to triple the rebar, the welding, the inspections, and the cost." And we wonder why our country can't afford to repair its infrastructure...

 

[SlideRuleEra]

DCBII - Aah... now I understand what you are doing. We have often looked at modifying or repurposing our outdated or damaged industrial structures. Just have gone about the analysis in a different way.

Instead of trying to justify that the structure is code acceptable, look at the problem "backwards". That is, determine the best that can be done within budget.

Based on what can done, determine (in your case) the maximum seismic event the structure can withstand. With this information, your firm and the Owner can make an understandable, informed decision if that seismic event is an acceptable risk, then whether to proceed or not.

Believe me, I don't make this stuff up. We had a particularly difficult project that defeated two qualified consulting firms. For the third try, retained Thornton Tomasetti. It took Mr. Richard Tomasetti himself to resolve our problem using a method similar to the above.

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