I Hate Pricing Episode #476 - Determining Wind Loads on a Replacement Membrane for a Domed Stadium 2

[KootK]

So I have a client with an interesting project. They're going to replace the roofing on an entire, domed stadium. Like an NHL stadium, non-retractable. I'm going to help by working out the wind loads on the new membrane. I'd like recommendations on fee considering:

- Even done with great care, I could see being done with this in 12 HRS or less. So... $1,500?

- If the new roofing flies off during a Hurricane, I could see getting sued for ONE. BILLION. DOLLARS! So... $4,500?

I'm looking into the construction budget and the scale of my client's fee. We'll see how that goes. I like these guys so I certainly don't want to be perceived as gouging. But, then, I also don't want to be foolish from a business perspective as is, unfortunately, often the case.

What would you do?

 

[JStephen]

On the billion dollar end of it- that's where you call up the wind-tunnel guys and let them earn their money for a change.
Did the original cover blow off? Any reason to change what it was designed for?
And is there supporting structure that limits the loads that can be applied anyway?

 

[JAE]

If the dome is irregular enough, a wind tunnel study might be more appropriate. You would then change from being an engineer diving into ASCE 7 (or appropriate code) and turning instead to being an engineer coordinating with the folks in Guelph, Ontario, or Colorado or wherever and then interpreting the tests and writing a report summarizing (and perhaps simplifying) the results for the use by the roofers.

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[EZBuilding]

I agree that a wind tunnel appears to be the best recommendation. I would be surprised if the stadium did not have one performed to begin with during design. If a wind tunnel is found to be cost-prohibitive for the project I would make sure you have a limit of liability equal to your fee; 1500 or 4500 is peanuts compared to what you could be on the hook for otherwise.

 

[KootK]

On the billion dollar end of it- that's where you call up the wind-tunnel guys and let them earn their money for a change.

Yeah, I was wondering about that too. You certainly would have wind tunnel for new build. Do you feel it's appropriate for this too? To be honest, I'm not 100% clear yet on how I'd determine uplift on the membrane as separate from uplift on the roof deck as a whole (my normal thing). I'm just assuming that I can figure that out, probably with some desperation posting 'round here.

Did the original cover blow off? Any reason to change what it was designed for?

Nope. Not sure that I can find out what it was designed for though. In my entire time on this earth, I can't say that I've ever seen a "net pressure on membrane" calc. I would certainly like to see one now.

And is there supporting structure that limits the loads that can be applied anyway?

I'm not sure where you're going with this. You're thinking that the re-roofing would change the net uplift on the roof deck as a whole?

 

[WinelandV]

Are you JUST determining wind loads? There's no other expectation for, say, connections, or the actual framing, etc.? What are the odds of scope creep? I assume that you've verified that you're not in a special wind region (or whatever the Canadian equivalent is)?

 

[KootK]

Are you JUST determining wind loads?

Yup, just the wind loads. I'm also sort of behind my client in terms of liability. Everything goes out on their letterhead and under their PM's stamp. That said, it's my understanding that doesn't really shield me from liability in meaningful way. As a licensed engineer, I'm only able to proffer one kind of advice: professional engineering advice.

I assume that you've verified that you're not in a special wind region (or whatever the Canadian equivalent is)?

No, not sure if there is a Canadian equivalent but I'll look into it.

 

[WinelandV]

Good to know that it's just the wind loads.

If the new roofing flies off during a Hurricane, I could see getting sued for ONE. BILLION. DOLLARS! So... $4,500?

And this is really where the rub is. I don't suppose you could get them to sign off on a hold harmless for any liability above your insurance limits? I'm mostly joking (unless they sign one!), but there does need to be some discussion about the risk involved. Now, from behind my keyboard, I THINK that your liability should be limited because you are only providing applied loads. Presumeably someone else will be taking those loads and doing... something with them. So, at the end of the day, you could apply a factor of ignorance and go home and sleep at night.

*** The thinking behind the liability being limited is that if it blows off, I think it will be difficult to figure out what the root cause of the failure was: poor construction, inadequate connection design, exceedance of the capabilities of the membrane system in general (that is, wind loads caused the membrane to rip), possible debris from a wind storm striking the roof, and finally, under-estimated wind load. Good luck sorting through all that. ***

 

[CANPRO]

What are the expectations for deliverables? Do they want a nice drawing showing the varying wind load according to the different zones on the roof, or just a quick email saying your worst case wind pressure is xx psf? Also, who is going to make/verify assumptions regarding the internal pressure category, exposure category, etc... Is the existing geometric info available and clear? You could waste a lot of time just trying to chase down info to verify these assumptions.

I'd be nervous about the potential liability as well - obviously if you're short on the wind calc that will cause problems, but being overly conservative could cause problem on this scale as well - I don't know all the details but say you overestimate the windload by 10% and that puts the roof supplier into the next thickness or strength of fabric and now the project is millions over budget. I don't know if that is a realistic concern, just throwing out potential problems for you to sweat over.

I do my own drafting and I can be a little particular about the drawings, so if they wanted a clean drawing showing the varying pressure across the roof, I'd be into this for 12+ hours for sure by the time its all said and done.

 

[JLNJ]

Does FM (Factory Mutual) play a role? They might have some prescriptive requirements which would get you out of having to calculate anything.

 

[Agent666]

Unless your code has rules for large domed roofs, I'd say your approach should involve a wind consultant. Usually they will demonstrate either a better understanding/accounting of the local effects and/or a lowered wind pressure due to their testing (whether its physical testing in a wind tunnel, or some sort of computational assessment). If you are outside the applicability of your local standard then this is the only option as I understand it.

Often undertaking this type of assessment can yield significant savings for contractor/client if there is a lesser loading requirement over a large portion of the roof.

Approach two is to simply replace like for like, unless you are undertaking this work due to some original defect that needs to be better understood. This is more like normal maintenance replacement.

 

[TehMightyEngineer]

I suspect one big ticket item is determining dynamic effects (vortex shedding, buffeting, etc.) of the roof membrane. Not sure if this is in your scope but it would seem that such dynamic forces would cause localized pressures that exceed the normal components and cladding loads normally calculated. I suppose also that this roof isn't a perfect dome (ribs in the fabric, guy wires, parapets, signs, flags, etc.) and thus might be too far off from what went into deriving the dome C&C loads.

The other aspect that might push you to wind tunnels is the effect of adjacent structures. Is there anything nearby that's large enough to cause channeling or buffeting effects?

I think in the end your best bet is to look at the value side rather than the cost side. The client is also looking at many zeros of liability but also many zeros of cost. Something tells me they'd want to not only skinny down the design as much as possible but also at the same time ensure they're 100% covered.

Maybe one way to price it is quote your price of ~$2,000 for engineering and list another line item of $XXX for wind tunnel testing or computer modeling ($XXX = cost of wind tunnel or computer modeling sub-consultant marked up 30% for your work coordinating). Tell them that such testing may be required and that the value it adds in potential reduced cost of the roof will be evaluated over the course of the design. If it is required and/or saves them cost down the road to do the wind tunnel testing, then you perform it. Ideally this makes you look both inexpensive and high-end at the same time.

Plus you get to play with wind tunnels and scale models! Hard to advertise about that banner project KootK, LLC worked on when all you have to show for it is 2 pages of calcs.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[MIStructE_IRE]

For a project like this I wouldn’t be comfortable without doing the wind tunnel test.

 

[CrabbyT]

ASCE 7-10 recommends wind tunnel testing for anything greater than ONE. BILLION. FEET. 200' in diameter, check out the commentary on page 552.

I don't know how competitive the stadium engineering market is, but I think you might be selling yourself short at $1500. In my opinion, $4500 - $6000 sounds more reasonable, plus whatever it would cost for subcontracting some wind tunnel people. Seems like it would be easy to get muddled down in the details. Do they have an air handling unit up there? Fans?? Lights??? Hot dog fume vent???? Also, is the dome pressurized? Are power outages a concern?

Definitely sounds like an interesting project.

 

[Ron]

There is not now, nor has there ever been a relationship between fees charged and liability for engineers. A $500 job can result in $5 million in liability.

On the other hand, what you are doing has downstream implication so you should be paid for such specialty work. It isn't just a few hours of playing with factors and coefficients from ASCE 7 or others....it is the application of reasoned engineering judgment in considering internal and external pressures. As discussed in other threads, positive or negative interior pressure can have a huge impact on an exposed flexible membrane.

Not sure what your local fee structure is but $125 per hour is low in my opinion. Should be at least $200.

 

[jayrod12]

I'd agree with Ron that the hourly rate is low, but not as low as he thinks. Over to the east in the polar vortex area we charge 150.

 

[SteelPE]

I can't say that I would feel comfortable with what you are doing (wind loads on a membrane) and I would probably pass on the project but I know the pain of pricing such items.

Recently I was approached with a project with similar pricing issues..... it was 370,000 square foot building single story warehouse. Pretty simple in terms of design which would only take few days to complete (think 1,500 30K12's everywhere). However when it came time to write the proposal I did it based upon a % of construction costs not based upon the required work (a low percentage at that). I din't think my insurance company would appreciate a low fee on such a large project. I then explained to my client the thought process behind the proposal and if they think I am out of line with the proposal please let me know.

 

[KootK]

Thanks all for your valuable input.

I think that there's been some confusion with regard to my use of the word "membrane". This is't a fabric structure or anything like that. In this context membrane is just the moisture barrier element of the roofing system. As such, I've have little concern for any of the fancy dynamic behavior stuff.

 

[TehMightyEngineer]

I for one misunderstood; so you're saying you are simply coming up with a loading for the waterproofing material? Is it like an adhesive PVC or EPDM membrane that adheres to the skin of the structure of the stadium room?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[KootK]

Exactly right TME. Were I doing the wind load on flappy tension membrane, I'd probably thinking something to the tune of $15K.