Wind vs Seismic Control, whats appropriate? 5

[xkcstructural]

Situation: an engineer is analyzing a structure and first pass-through determines the Wind exposure category as B, then does the seismic evaluation and finds that seismic controls, but "decides" that they do not want seismic to control and changes the wind exposure to C and then say voila wind controls and I do not need to meet the seismic requirement. I've also experienced it on the flip side where I put a structure in wind exposure category C and its an exisitng structure and meets exposure category C but they say that wind probability will likely never happen and brining the structure up to meet Exposure Cat c is too much for the contractor so treat it as B.

How does this make other engineers feel? For me it raises a red flag about that person's judgment and engineering understanding. I have never had to deal with this kind of environment and I am wondering if my gut telling me that, that behavior is not engineering is in check or am I out of line. Is this behavior okay if they are the EOR?

Lately, I've just been having a bunch of ethical dilemmas and Im the only one concerned about this behavior. Any advice if it is a red flag on how to approach it? I get told I stick to code too much and the probability of that wind event happening is not likely. It feels dismissive and belittling. But if it is the EOR's call then maybe Im asking for no reason and I can have a better understanding of whats acceptable.

 

[Just Some Nerd]

If this engineer (multiple of them?) was a dating profile, it would be entirely made up of red flags. In this one post we have:

- Someone who will willingly increase the wind category, thus loads and subsequent costs, just so wind can govern for some unknown reason
- Someone who will decrease the wind category to appease a contractor, rather than value engineering the building to meet the loads or giving the contractor the correct answer of "no"
- Someone who probably ignores seismic detailing requirements (if applicable) because they go straight to wind loading alone
- Someone who decides to come up with their own return periods/intervals for events instead of those they are required to follow, because they feel like it suits them better (I don't actually know about other regions, but things like the choice of 1-in-X wind/earthquake events are not up to interpretation and determined by the building class/usage as set out in the design codes & construction codes)

As for how to approach? I don't know if there's a solution that doesn't involve finding another company to work at. I had to do that once, it wasn't worth the loss of sanity to stay worrying about every unethical decision made.

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Why yes, I do in fact have no idea what I'm talking about

 

[JAE]

Agree with JSN's list totally.

And I might add - there's no such thing as wind controlling all of a building vs. seismic, or visa versa.

You have to always check both. There are seismic provisions, such as Omega factors on connections, that might control in that particular connection design vs. wind, even though the wind load on the overall building is larger than the static seismic demand on the overall building.

 

[SWComposites]

Xkc - what is your role / relationship in this situation?

 

[xkcstructural]

Right now I'm on the on boarding process and leaning how this company designs and how they run their projects with the goal to work independently on my own projects and be a project manager, EOR. I have my PE so I'm fully aware of my responsibility and my ethical responsibility. Anytime I bring it up, I get made out to be like I don't know the industry very well and it's something I'll learn. But Everytime I return home I feel gas lit.

The projects range from residential to commercial. And since I'm new to residential it makes me uncertain but when I see it being done on commercial projects I am pretty dumbfounded.

 

[human909]

I have my PE so I'm fully aware of my responsibility and my ethical responsibility. Anytime I bring it up, I get made out to be like I don't know the industry very well and it's something I'll learn. But Everytime I return home I feel gas lit.

With this experience it sounds like you are asking a question you already know the answer to and are just looking for reassurance. (And there isn't anything wrong with that!)

It sounds like a toxic company management combined with engineering of questionable ethics. Just Some Nerd summarized the problems and solutions quite well. Get the hell out of there before it drives you mad or are pushed into ethically questionable territory.


As an aside changing company culture is HARD even if the company hierarchy is open to it. I've managed to change my company culture regarding our approach to design, it wasn't always easy but at least the company was mostly supportive of change. It doesn't sound like you have that option.

 

[Jinal Doshi]

Say the structure is located in a low seismic region, where you are under SDC B / C, then ASCE allows you to design some structures with low ductility, like R=3 systems and you don’t have to worry about AISC 341. I can understand some reasoning behind that.

But, if this is happening in high seismic region and if the building is relying on high ductility, then I would say whoever gave you the advice is probably not thinking correctly and does not understand the philosophy of structural design. Not only it is a bad advice, but it is also being ignorant and either not designing structures properly or he doesn’t care about added tonnage of material.

Wherever you have high seismic design category, then regardless of the wind being the governing load or not, you have to meet all the seismic detailing requirements. Period. And with some capacity based principles, if you over design yielding elements, then all the protected zones will need to get equally bigger to remain essentially elastic. Adding cost and carbon to the building construction. If what he said were true, then none of the tall buildings on the western coast of US would have ever been detailed for seismic.

 

[quasiblu]

Where's written that you do not meet the seismic requirements when wind governs, and how much should it govern? And should it govern on wind assuming which R value?

 

[phamENG]

Does not sounds like a good place to be. Just Some Nerd summed it up nicely and JAE filled in the only missing piece I could think of. They're playing fast and loose with minimum reliability requirements and gambling that the wind/seismic loads will never come.

I will say - they are not alone in this mindset. It's the way I was taught at my first firm. They even had a note under loading on the drawings sets that indicated which one "controlled." It wasn't until I dug in and really started understanding what each one was that I realized they were wrong.

 

[Eng16080]

I don't know how you could ever come to the conclusion that one type of loading will always "control" over another without fully designing the structure for both.

Even after calculating the wind and seismic loads, it's not always obvious which controls. As an example, take a wood shear wall with a wind shear force acting at the top of the wall which is 20 percent higher than the associated seismic force. The wind force is higher, so wind must control, right? Not necessarily! In designing the shear wall per NDS requirements, the nominal unit shear capacities used to resist wind are about 40 percent higher than the nominal unit shear capacities used to resist seismic. So, in this example, even though the wind force is 20 percent higher, seismic still controls the design of the shear wall.

 

[xkcstructural]

Thanks for everyone's response.

I find this happening alot in the projects Jinal is talking about where it's seismic region B and just by switching the wind exposure to C you can "decide" wind "controls" but those call seem out of convenience vs good judgement. I don't think this engineer has an understanding of what to do in seismic category C/D and stays away from those project if he can it seems. One of the engineers he's trained tried pulling the same behavior on a structure with an R value of 3 1/4 and 3 1/2 and I said to design this you need to take over strength in to account and consider your connections and it seemed to be foreign concept.

I've never had to question another engineers judgement, etichally. I guess I've been lucky so far as far as previous mentors and coworkers maintaining integrity.

 

[KootK]

How does this make other engineers feel?

Shitty. It feels shitty. That said, I feel that this thread needs a dissenting opinion. So I'm going to attempt to provide it.

1) The "seismic always matters" thing really is about ductile detailing rather than force transfer. As such, for regions and projects where low ductility seismic systems are used, I do see some merit in the "wind governs" approach. That said, I concede that I find that a lot of the engineers who make that argument wind up lacking the nuance of understanding that I feel is necessary for them to be qualified to make the call.

2) I have had the privilege of being taught the art of business development by some of the best in the business. Truly. Consistently, I hear them express views that go something like this:

You can do profitable work in any space. The trick is to recognize what clients are willing to pay for engineering in that space and then give them a product tailored to suit.

This is, of course, the opposite of what we are taught in engineering ethics classes etc. Still, everything that I've experienced in the real world of engineering practice supports the views that I've seen expressed by my business development mentors. I see this as the truth of engineering practice.

3) Recognize that the group of people that you are speaking with here is a group self selected to be more uptight about technical things than the average. Me, more than most. Welcome to your personal echo chamber. In most jurisdictions, the metric for acceptable engineering practice is whether or not your practices are consistent with those of the other engineers working in your area. In this sense, one could make the argument that your employer's apparent success in their market is, in and of itself, evidence of the technical sufficiency of their practice. Yes, yes... there's always someone at the bottom of the range and perhaps that is your employer.

 

[phamENG]

Just to throw this out there in relation to KootK's point 1:

Say you have a steel frame detailed with R=3 (steel systems not detailed, etc. etc.). Overstrength for that system is 3. So if your wind load is 3 times your seismic load, it's fair to say that nothing is going to be in an E*Ω > W condition. But if you're in a position where fiddling with the wind exposure is enough to tip the scale one way or the other, something is going to need overstrength on the seismic load and it will be the larger lateral load. Maybe not the entire building, but certain pieces will.

 

[Aesur]

I would caution you to make sure you fully understand what they are doing and saying as they could be stating this incorrectly to you. I say this because it is common in low seismic zones for wind to control lateral design over seismic, and in doing so many seismic provisions are not required, ie over strength etc (there are exceptions for certain materials). It is true you need to investigate both forces to make this decision, however this is done quite easily. If you are in a high seismic zone, then this is definitely an incorrect practice. You say "to avoid seismic requirements" this does lead me to think you are not talking SDC A or B and are in C or higher, in which case you are correct in your assumption. I would also caution against wind B versus C, many times projects are actually C, it's an annoying little thing to calculate/prove, there was another post on here a few months back about this.

 

[xkcstructural]

Kootk and phamg, good points

And I wanted a check outside of what I think and you've given me something to think about another perspective as far as the business side and the general consensus as far as building in this area.

And Aesur, I think if the decision was more objective as far as changing wind category because it is necessary cause that is what it actually is vs tipping it so that they don't need to consider seismic requirements and retrofitting because it's too much. Projects range from all SDC, 60% in Category B.

Yeah I've dealt with wind exposure C vs B working on mountain tops and any clear path to your site can put it in the C group but that's not the thinking that's driving the change. There are many factors to consider when making this change but I think "saying I can change this to c and wind controls and I don't need to worry about the moment frames being welded for seismic ...etc is I think dangerous. And the inconsistency in these calls when it's very clearly C and they say its B to reduce the demand cause the contractor wasn't anticipating that "and the structure probably won't see that wind"

 

[phamENG]

I've told this anecdote before on here, but I'll say it again because I don't have anything better:

I'm on the coast, not the mountains (not right on the coast, several miles inland in a swamp). My home office is in a half story inside the gable roof. When we moved in, we were in the middle of a dense forest. In the past 4 years, we've had about 150 acres around us on three sides cleared. I can hear and feel the difference when the wind blows between the two conditions, and I don't mean the wind itself. I can hear the rafters creaking more, the roof sheathing flexing, the nails popping. I can feel my floor shifting in big storms. Is it design level? No. I'm not dumb enough to stick around for that. But there is a tangible difference between the two, and it can be caused by something as simple as somebody clearing their land and selling the timber. Now, I go with C unless it's in an established neighborhood/developed area or the trees I'm counting on are on protected land that will prevent deforestation. Just something to think about when considering using B.

Frankly, where this sort of behavior is common, it's common because it 'works.' It may not be the most ethical or represent the highest commitment to integrity, but it works from an economic standpoint. Will the house ever see it? Probably not. The odds are really good that it won't. So they discount it to get more work and life goes on. Until it doesn't. Someday the 'big one' will hit there...may not be as big as Northridge, but it'll be bad. It may happen tonight, it may happen in 200 years. Who knows? When it does, somebody will have to answer for the buildings they didn't design properly. The odds of it being the people alive now aren't high, and for some that's enough confidence to gamble with. The chances of finding a highly successful structural engineering firm in an area with little to no environmental risks to the building (powerful earthquakes, catastrophic hurricanes, apocalyptic snow loads) that is ALSO devoted to doing everything 'just right' is pretty slim. Sad but true fact, I'm afraid. The market will force a good many to places they'd rather not go when they have a chance to consider things from an idealistic standpoint.

 

[AaronMcD]

Often times here I don't even check wind, seismic will be many times higher by inspection without so much as thinking about calculating it. And since wind doesn't have the special detailing requirements seismic does, that makes it easy. Just get enough info to put wind criteria on the general notes, and no plan checker has ever asked for a wind calc.

Sometimes I do have to check wind, just in case in SF, say for a 3 story long skinny house, check wind on the large wall.

The image is from a big project on a mountain. So super high wind loads despite being a single story. Flexible diaphragm, check what governs for each grid line, lateral system, and for sheathing (different NDS capacities for wind and seismic), components (basic stuff), overstrength, or collectors (1.25 for seismic, wood frame flexible diaphragm).

http://www.hellodwell.design

 

[Eng16080]

Frankly, where this sort of behavior is common, it's common because it 'works.' It may not be the most ethical or represent the highest commitment to integrity, but it works from an economic standpoint. Will the house ever see it? Probably not. The odds are really good that it won't.

That's the dilemma as engineers. Constructing a house that 'works' can most likely be accomplished without us.

 

[canwesteng]

I'm with Kootk, in that for SDC A/B, if you understand the detailing requirements, if any, and don't introduce any irregularities, this could be a valid approach. That being said, it doesn't sound like it's being properly thought out, and reducing wind exposure on a whim so that the design is lighter is pretty concerning. Ultimately, we need to make safe designs, and sometimes that means overdesigning things, if that's all the engineering the client is willing to pay for.

 

[human909]

I was going to comment earlier. I certainly regularly use the phrase seismic or wind governs. But I thought it would be nitpicky.

I don't know how you could ever come to the conclusion that one type of loading will always "control" over another without fully designing the structure for both.

For some structures it really is obvious.
-I design slender industrial steel towers, they are completely wind governed I don't bother with adding seismic loads.
-I design structures indoors with industrial equipment that aren't subjected to direct wind pressures. I adopt throw generous seismic loads at it and provide lateral restraint accordingly. Though to be honest half the reason for my lateral restrain here is sensible serviceability so II don't end up with a wobbly platform.
-For the multi storey industrial structures I design, I never know whether seismic or wind will govern. Sometimes some members are seismically governed and others are wind. I apply both loads appropriately and the design the structure accordingly.

I've never had to question another engineers judgement, etichally. I guess I've been lucky so far as far as previous mentors and coworkers maintaining integrity.

I do it all the time. I enter building and immediate start picking up on design issues, many of the times there are no doubt ethic issues behind that too. Too many people simple just don't care. And too many people are just not competent.