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Checking calculations
19

Checking calculations

Checking calculations

(OP)
I'm writing this thread in regards to what was said in this old thread here:
http://www.eng-tips.com/viewthread.cfm?qid=321928

It leaves me feeling uneasy to say the least. I also feel compelled to ask the following question:

- In the interest of meeting the deadlines AND feeling comfortable, why not be generously conservative in most of your designs?

- Many admit to finding errors often in their own work and in others. It stands to reason that not all errors get discovered. This would lead me to want to be conservative in my design.

- Another thought, given the old triangle (cost-quality-time), if deadlines are very rushed (low time) and the structure simply needs to be satisfactory (low quality/less refined), then being conservative (higher cost) appears to be the way to solve this puzzle.

Thoughts?

---

Personally, I have 8 years of design experience, and spend a lot of time reading books and teaching myself as much as I can. I usually buy a few books a year to refine my skills and knowledge. Honestly, I don't see too many peers doing that. I'm just looking to stimulate some discussion.

RE: Checking calculations

stiman86,

This is why the factor of safety sometimes is called the factor of ignorance.

--
JHG

RE: Checking calculations

Overdesigning to compensate for your expected but unfound design errors is wrong and costly. Don't make errors. Check your work.

www.PeirceEngineering.com

RE: Checking calculations

Errors are common. More common than we'd like to admit. I've made my share.

Big firms have some kind of checking policy (usually). One man shops? Rarely, unless it's big enough to require a peer review.

I found an error today from a vendor who was reporting a load of P. It looked to me like he forgot that his columns were loaded from two sides. When I asked, he told me, "Thanks, they should have been two-sided." Plus some additional equipment loads. The total was 2.4P. I'll probably add 15% to that and use 2.8P.


RE: Checking calculations

so what PEinc says is what most everyone will say on the record...and in a perfect world yes...but overdesigning is what happens with unrealistic deadlines...is it the goal? No, but do you have time to sharpen the pencil on everything? Sometimes no. And sometimes, sharpening the pencil costs the client more if they really need the building more quickly. Especially industrial clients where production losses can be huge.

For me this boils down to judgment and value systems...I'm not saying go crazy with overdesign, but if you use the same size columns to avoid having to design every single one to the nth degree, I say go for it. Repetition also has a way to pay for itself...and overdesign leads to future flexibility.

As for errors, every engineer makes mistakes, and that should be expected. Otherwise you expect perfection, which is not realistic. Building in conservative design to accommodate for errors isn't a PC way to put it, but it does make one tend to want to lean that way when they design things. And, frankly, I believe an argument can be made for being conservative for this reason. For one thing, the cost to "fix" things later would be significant versus minor cost increases in material up front...

I have about 9 years experience now and I used to sweat this a lot more than I do now. The reason? I worked for another firm and the things I was worried about in terms of design weren't even considered by some engineers. When I asked certain questions, people didn't know what I was talking about. What you'll find on this board is engineers that care enough to ask. A lot of engineer don't care, it's a paycheck. So being on this board and asking questions means you care enough that you're less likely to make a big mistake.

I'm a pragmatic realist and I call it like I see it. I've also been accused of being cynical. I don't like the negative connotation with that. The truth and cynicism are often confused, at least as I see it.

RE: Checking calculations

What if your error causes you to be off by a factor of three?

RE: Checking calculations

My understanding is that there is a field of study related to the complexity of systems and as the system’s complexity increases at some point a threshold is crossed where there is virtually guaranteed to be a failure. I hope the code writers bear that in mind.

I did a quick internet search on the topic and this was one of the articles that popped up:

http://web.mit.edu/2.75/resources/random/How%20Com...

I thought point #14 was particularly relevant:

14) Change introduces new forms of failure.
The low rate of overt accidents in reliable systems may encourage changes, especially the use of new technology, to decrease the number of low consequence but high frequency failures. These changes maybe [sic] actually create opportunities for new, low frequency but high consequence failures. When new technologies are used to eliminate well understood system failures or to gain high precision performance they often introduce new pathways to large scale, catastrophic failures. Not uncommonly, these new, rare catastrophes have even greater impact than those eliminated by the new technology. These new forms of failure are difficult to see before the fact; attention is paid mostly to the putative beneficial characteristics of the changes. Because these new, high consequence accidents occur at a low rate, multiple system changes may occur before an accident, making it hard to see the contribution of technology to the failure.

RE: Checking calculations

@271828, if you’re off by a factor of 3 and can’t see it without running any numbers, you are most likely designing something you ought not be designing. Or, if it’s newer to you, something you should be having peer-reviewed.

Case and point is a question i had on this board earlier this week on an embed plate. The initial design was off by a factor of 4ish, but I knew i needed to study the issue more, so I came to this board after doing my own homework and came up with a better solution.

RE: Checking calculations

I agree with a lot of what njlutzwe said. Too many different design cases makes for false economy. There are savings in repetition and simplification. However, I have heard many engineers say that they overdesign because they cannot be sure that the contractor won't make mistakes. I disagree. That's an inspection problem. The designer usually isn't responsible for inspection. Design it correctly and let someone else worry about building it correctly. You can't design conservatively enough to prevent stupidity in the field.

www.PeirceEngineering.com

RE: Checking calculations

And to PEinc’s point, if it’s too conservative and the contractor knows it they are less willing to take you seriously and they may change something somewhere that is critical because they think you way over-designed EVERYTHING...

A good engineer once told me that I don’t have to design everything to a 1.0 unity ratio, but i needed to have a REASON for why it’s sized the way it is. As drawoh said, this is where the “factor of ignorance” can come in handy...

Also, imagine a situation where you have a 1,000,000 piece of equipment supported on a platform. Let’s say bare minimum with 1.0 unity all the way around, the platform will cost $15,000 to build. Or for $20,000, you can build a platform that will be stout as all get out. So for 0.5% cost of the project we greatly exceed the MINIMUM which is the code. This is a value judgment. I don’t think there is a wrong answer, but i lean towards the $20,000 job and move on.

RE: Checking calculations

(OP)

Quote (PEinc)

Overdesigning to compensate for your expected but unfound design errors is wrong and costly. Don't make errors. Check your work.

Let me give you a scenario so that you can understand what I'm talking about.

Say that, in reading a textbook I bought on steel design, I find out that under certain seismic system, a moment frame structure must have columns of a certain size because of X reason that isn't in the code, but is for some other reason based on tested assemblies. Had I not read this book, I wouldn't know this, and other engineers around me don't know about this. This is what I'm really talking about, not just checking your wl^2/8 calcs and column run-downs.

Another example is, lots of engineers I've met don't consider that a simply-supported beam on one side of a column, causes a moment in the column ( V x e = M).

Quote (njlutzwe)

Building in conservative design to accommodate for errors isn't a PC way to put it, but it does make one tend to want to lean that way when they design things. And, frankly, I believe an argument can be made for being conservative for this reason. For one thing, the cost to "fix" things later would be significant versus minor cost increases in material up front...

I agree with this. I think I should employ this way of thinking more often.

Quote (njlutzwe)

the things I was worried about in terms of design weren't even considered by some engineers.

This is what I'm talking about. This stresses me out! Your typical older engineer who stopped learning and still uses his codes from 20 years ago will just give you the "here, I can design this moment frame building no problem, by hand. See how fast I did that." These types don't even consider that there might be stuff they are missing from continuing education. I just gave one example above, but there are countless things like this that come up the more you read.

@Archie264
Interesting read. Thank you for contributing that.

Quote (PEinc)

overdesign because they cannot be sure that the contractor won't make mistakes. I disagree. That's an inspection problem. The designer usually isn't responsible for inspection. Design it correctly and let someone else worry about building it correctly. You can't design conservatively enough to prevent stupidity in the field.

All I'm going to say is, it's often more complicated than you make it seem. You can't simply "let someone else worry about building it correctly" because it's always your problem. You also need to be comfortable enough to sign that letter of conformance at the end of a project. So you absolutely want to have it built right. Some contractor's make zero effort.

Quote (njlutzwe)

Also, imagine a situation where you have a 1,000,000 piece of equipment supported on a platform. Let’s say bare minimum with 1.0 unity all the way around, the platform will cost $15,000 to build. Or for $20,000, you can build a platform that will be stout as all get out. So for 0.5% cost of the project we greatly exceed the MINIMUM which is the code. This is a value judgment. I don’t think there is a wrong answer, but i lean towards the $20,000 job and move on.

Excellent example. I believe this is the right mindset to have for most projects. At least for these fast-paced projects (which is all of them theses days!).

RE: Checking calculations

(OP)

Quote (Archie264)

My understanding is that there is a field of study related to the complexity of systems and as the system’s complexity increases at some point a threshold is crossed where there is virtually guaranteed to be a failure. I hope the code writers bear that in mind.

On this point, I attended a concrete workshop recently, held my cement.ca regarding the upcoming updates to the code for the new NBCC/OBC and I was not impressed. These PhD types who perform the research and write the codes are making thing more and more complicated. There was a portion of the session dedicated to breaking down certain code clauses so that normal people can understand them! I'm not kidding. At one point the presenter had to bring in his colleague to help him clarify something.

It's just plain obvious that if the people writing these codes can barely make sense of them, how is your typical engineer supposed to? How are the people who don't attend these workshops supposed to? And shouldn't we be making things simple enough that the bottom 50% of designers can implement them correctly?

RE: Checking calculations

3
stiman86,

I disagree heavily with the examples you picked.

I agree that there are many mistakes made in design, but your examples don't seem to actually touch on that. Your examples relate to assumptions, models and maybe approximations rather than errors. I can tell you that a hand designed moment frame is likely to be as safe as something done via a complicated finite element analysis as long as the limitations of the analysis are understood. Additionally, treating a simply supported beam on a column as a pure axial load isn't going to fail the structure and is generally going to be negligible. It's acceptability will depend on connection design, detailing, your bracing scheme and other similar things. When Alexander Newman and Charles Carter, two very good engineers, are pretty handwave-y on the topic, I'm going to be careful calling any reasonable set of assumptions wrong if applied appropriately ( https://www.aisc.org/globalassets/modern-steel/ste... ). The code you're using may have explicit directions on the topic, but not all do and there's more than one way to skin a cat.

There are mistakes and then there are disagreements in methodology. There are a wide variety of choices of models that will give different answers and still be right within an acceptable margin.

More detailed analysis is not always the right path forward. It can easily result in more confusion and errors than using straightforward analysis appropriately. You're significantly more likely to lose the forest for the trees when you have added complexity. There are definitely applications where you want to utilize every tool in the box but you don't want to overcomplicate things if it's not necessary. Being overly precise can also give a false sense of security. Failures aren't in the few percentage points you generally get from refined analysis methodology. They're not even in the 20% error you might get from screwing up the math on one item. They're in an inability to recognize load paths, properly define loads or account for local and global stability issues. Alternately, they're in combined errors or complete fuckups that you don't catch because you don't check that your inputs and outputs actually line up and make sense. This is significantly mitigated if you keep your analysis straightforward or verify with a straightforward analysis.

This is the opinion of someone in the earlier portion of his career who is very much in favour of keeping up with new technique and continuing education.

RE: Checking calculations

stiman86, it is the design engineer's responsibility to properly design (not excessively overdesign) a structure. It is the contractor's and inspector's responsibility to see that the structure is built properly. If the designer is also responsible for inspection and/or certifying the construction, then it is the designer's responsibility to first design it properly AND secondly to inspect it properly. These should be two separate responsibilities. The owner is paying a designer, a contractor, and an inspector. It isn't the designer's responsibility to overdesign a structure in case the contractor and inspector improperly perform their work. I'm sure that the owner expects each to do his work properly and would not be happy to know that the designer is inflating the owner's project cost in order to compensate for potential, shoddy construction and inspection by others.

www.PeirceEngineering.com

RE: Checking calculations

Some quick thoughts on this subject:
1) A really good, experienced engineer working on a type of project that he has done many times before, can probably come up with a reasonable design without doing much in the way of calculations. Is this wrong? Maybe, maybe not. If the structure does calc out when the calculations are run and it is not excessively conservative, then this is just an example of engineering experience paying dividends to the person who hired that engineer.

2) Now, a less experienced engineer or one working on a type of project he is not as familiar with would not have as good of a feel for what the final design would be. In that case, this is not a good idea and more thorough calculations have to be run. This takes time. You can reduce that time a bit by making conservative simplifying assumptions and such. But, this time reduction can have a major cost to the owner. For me, that's what the OP's question is about. And, there just isn't one solution for all projects.

3) Ultimately, it's the owner's decision. Does the reduction in project completion time justify the increase in construction cost associated with a conservative design. I've seen fire re-build projects for industrial facilities where the lost revenue associated with two weeks of lost operation exceeded the total construction cost of the project. In that case, conservative and fast is much, much better for the owner. In fact, we built a "temporary" facility really, really fast just to stop the losses. But, knowing they would replace the majority of the equipment which was salvaged from another site with new equipment manufactured specifically for that sites specifications. So, in essence it was better / cheaper for them to build the facility twice in a year that it would have been to wait for everything to be economically designed.

RE: Checking calculations

(OP)
TLHS, I appreciate your input. You do bring up a good point that about disagreements in methodology. The article you posted in which Newman tackles the question of unbalanced load on HSS column is interesting. I have a textbook that says you should consider the moment. I agree with you that it comes does to judgement and that there is more than one way to skin the cat.

PEinc, no one is saying we should excessively overdesign. In general, I don’t feel like I’m arguing to do the things that you’re arguing against doing.

JoshPlum, good points.

RE: Checking calculations

(OP)
Twice in the last few months I was retained to re-design projects that had been completed by another engineer. The owners had a sense that it was over designed, and the engineers stuck to their design. I was able to save the owner enough construction and material cost to more than offset the fees they were paying to redesign.

All that to say, some people do excessively over design, and I don't consider myself one of those people.

RE: Checking calculations

stiman86,
Would you mind sharing with us what type structures you found to be "over designed", and in simple terms how you arrived at that decision?

RE: Checking calculations

(OP)
The first was a pre-eng building that is to be constructed over an existing ice pad. The first engineer required boring underneath to tie each opposing footing together to take care of the outward thrust that resulted from the frames due to snow loads. By designing a suitable (albeit larger) footing that could resist the sliding, uplift and overturning, I eliminated the need to bore under the ice pad.

TLHS will be happy to know that I made use of Newman's textbook on light metal building foundation design.

The second one was a sign foundation where the contractor had done some funny things to the reinforcement and anchor rods. The original engineer wanted them to rip it all out and start over. When I looked at it, the sign base had about 3 times as much overturning resistance than needed. So I didn't feel it necessary to rip everything out and was able to make it work with a couple small alterations.

RE: Checking calculations

Honestly, as I read some posts on this thread, I read a holier than thou attitude. Mistakes happen. If you think you don’t make mistakes, then honestly, all i can say is wow. I don’t believe for a second that stilman is trying to say he intentionally overdesigns things. Please re-read his OP

Also, any good structural engineer knows that it is an art as much, or more so than a science. There are so many layers of assumptions it’s hard to even list them all. Choosing to land on the conservative side of that science is what I think Stilman is trying to say.

Lastly, people talking about inspection and design responsibility...please...a lawyer will pick that apart faster than you can blink. And even if the argument they have is ridiculous, your insurance provider will be scared anyway.

RE: Checking calculations

I wouldn't call those examples of "over design". Your solutions may be more logical and economical than those proposed by the other engineers, but they represent different constructability issues, not necessarily extravagance.

RE: Checking calculations

Could also be different serviceability limits. One engineer may be fine with code minimum while another may want double that. I run into this all the time in residential house design.

RE: Checking calculations

2
The original posting said, "In the interest of meeting the deadlines AND feeling comfortable, why not be generously conservative in most of your designs?" What's the difference between generously conservative and excessively overdesigned? Codes specify safety factors, load factors, and resistance factors. Does an owner really want to pay for an engineer to arbitrarily overdesign just because a contractor may not build it right? I don't think so. Yes, mistakes do happen; some contractors are bad; some inspectors are incompetent. Engineers are often sued even though theirs designed are correct and proper, and there will always be some forensics engineer claiming that the design was incorrect. However, in my opinion, that is no reason to excessively or generously overdesign as a preventive measure. I also am not saying that engineers should design every single individual member. Members can be grouped into several different design cases based an the engineer's judgement. I also have not accused anyone here of excessively overdesigning, so let's not go there.

www.PeirceEngineering.com

RE: Checking calculations

njlutzwe wrote:

Quote:

@271828, if you’re off by a factor of 3 and can’t see it without running any numbers, you are most likely designing something you ought not be designing. Or, if it’s newer to you, something you should be having peer-reviewed.
Forget "3." What if he's bumping up designs by 10% but his calculations are off by 40%? He probably wouldn't realize that error, and he has almost eaten the factor of safety.

This reminds me of balancing a checkbook and arbitrarily subtracting $50 from the check register just in case I've forgotten something. What if I forgot something that costs $80? I need better practices than that, or else my checkbook will be in disarray.

An arbitrary bump-up is a horrendous practice.

RE: Checking calculations

Quote (PEinc)

However, in my opinion, that is no reason to excessively or generously overdesign as a preventive measure


I typically over-design metal studs member and connection strengths as it has been my experience that about only about 50% of what I show on the plans gets done correctly (I may be exaggerating). There have been countless times that over-design has saved the contractor's butt when re-work would have been required. Another reason I may over-design some areas is to simply keep the number of details to a minimum

RE: Checking calculations

Quote (271828)

An arbitrary bump-up is a horrendous practice.

Isn't this what we do when apply any factor of safety?

I remember talking to a professor about it once and he said "NO! Safety factors are for substandard materials, poor workmanship, and extreme overload, NOT design errors!". I always found that a bit arrogant. I'd wager that in practice safety factors have saved many an engineer's backside.

RE: Checking calculations

Not for design errors, rather the 'imponderables' mentioned in the West Gate collapse royal commission report. If in doubt, make it stout.

Important things can be overdesigned but I'd recommend only doing that if not over-expensive. Eg a beam that is repeated 100 times would be refined but a one-off brace that stabilises a large part of the structure could have some fat.

Also think about what failures really occur. To me, that's connections, buckling, fatigue etc (brittle failures mostly). Something like a slab with substantial reserve capacity through redistribution can be on code limit comfortably.

RE: Checking calculations

Tomfh wrote:

Quote:

Isn't this what we do when apply any factor of safety?
I'd say no. In modern specifications, load and resistance factors are established using rational methods. They're not arbitrary. The methods are laid-out in the literature, in commentaries, etc. They're not established by one guy guessing in his office.

RE: Checking calculations

2
2 cents from a non-SE mechanical engineer who designs all manner of things (but not buildings):

Quote (njlutzwe)

Honestly, as I read some posts on this thread, I read a holier than thou attitude. Mistakes happen.

First.. I echo this sentiment. We all make mistakes, and it's ridiculous to assume that you could work over an entire career, spend 40+ years performing detailed engineering and calculation, on what is more than likely to be hundreds of unique jobs and many thousand unique calculations by the time you are finished, without releasing a single mistake into the world.

It just isn't possible, even in the age of computerized almost-everything. Maybe even less possible than it used to be, since the computerization of engineering has allowed the code to balloon and become more complicated.

With that said. My work very frequently involves looking at existing designs and adapting them, evaluating their capabilities, etc. If, after doing my first pass of analysis, I feel that the structure or widget being evaluated is massively over-designed, my first instinct is ALWAYS to ask the following question:

What did the designer whose work I am evaluating know that I do not? What load, code phrase, environmental condition, material restriction, etc am I missing?

Just because your gut tells you that something is oversize does not mean that the guy who did the first design is an idiot.

In my opinion, assuming that you can subtract from someone else's work without at least considering what you are missing is a dangerous way to operate.

Just my two cents, gents.

RE: Checking calculations

For the most part, errors which could lead to failure are fairly obvious. Sizing a beam as a W18X35 when it should have been a W21X44 will not result in a collapse--sizing a beam as a 2X12 when it should have been a W21X44 will result in a collapse.

In the end, a structural engineer can only do his/her best, then get the work reviewed by a colleague (if possible).

DaveAtkins

RE: Checking calculations

Years ago I saw an interview with Joe Paterno where he said that he thought the way to reduce injuries in football was to take the facemask off their helmets…then people wouldn’t lead with their face and would return to tacking in lieu of hitting.

In that vein I’ve come to wonder if we wouldn’t be better off if we not only chucked not only the computers but the calculators as well and just stuck with slide rules.

Neither will happen, of course, but a fella can dream…

RE: Checking calculations

njlutzwe wrote:

Quote:

Honestly, as I read some posts on this thread, I read a holier than thou attitude. Mistakes happen. If you think you don’t make mistakes, then honestly, all i can say is wow. I don’t believe for a second that stilman is trying to say he intentionally overdesigns things. Please re-read his OP

Also, any good structural engineer knows that it is an art as much, or more so than a science. There are so many layers of assumptions it’s hard to even list them all. Choosing to land on the conservative side of that science is what I think Stilman is trying to say.

That's not how I understand the OP, who typed the following.

Quote:

In the interest of meeting the deadlines AND feeling comfortable, why not be generously conservative in most of your designs?

Quote:

- Another thought, given the old triangle (cost-quality-time), if deadlines are very rushed (low time) and the structure simply needs to be satisfactory (low quality/less refined), then being conservative (higher cost) appears to be the way to solve this puzzle.
Paraphrase: I don't have adequate time to be reasonably sure that my calculations are correct. I can fix this by bumping up my design by an arbitrary amount. (Even though I don't know the magnitude of the errors I think I might've made.)

Everybody makes mistakes. It's this manner of dealing with potential mistakes that is objectionable.

Example reasonable strategies: peer reviews, comparing program output to established tables, and visually inspect the sizes and see if they look right based on your experience.

RE: Checking calculations

Archie264,

I have seen claims that rugby has more injuries than American football. This may reflect that statistically, an injury occurs when the coach opens a first aid kit. Injuries in American football may be more serious because the people hitting you are wearing padding. This is a poor analogy for structural analysis and checking.

I am a mechanical designer, not a structural engineer. I have argued on other forums that the design checker must catch errors that cost more than the design checking process. My background is that I have prepared packages of ten to twenty drawings for one-off prototypes. Two or three parts may require re-work due to errors, which usually is way less expensive than a day or two of design checking and the subsequent squabbling. If, due to size or quantity, the mistakes will cost thousands of dollars, and/or kill people, checking is justified and/or necessary.

In civil/structural engineering, what is the cost of correcting non-fatal mistakes?

As a non-PEng, I have submitted structural calculations for safety purposes, clearly noting my qualifications. I felt way better when someone at the other end, went through my stuff and critiqued it.

--
JHG

RE: Checking calculations

@271828:

You and I disagree, which is no big deal, it's rare two engineers agree on anything. I read his OP as someone coming to the realization of what goes on in the consulting world with unrealistic deadlines. I read the OP to mean, you can't get into the weeds with everything, so why not be conservative with some of those things and move on, because if you sharpen your pencil too much, you may make an error.

As Structural engineers, we all live in a glass house...so I would recommend we don't start throwing rocks...

RE: Checking calculations

Quote (njlutzwe)

so I would recommend we don't start throwing rocks...

Especially if the designed safety factor for projectile penetration of the wall diaphragms of said structure is unknown.

RE: Checking calculations

To my way of thinking- if there was a consensus among engineers in general that calcs ought to be arbitrarily bumped up just to be conservative, we'd simply see that written into all the codes and standards. The fact that it isn't done that way implies that there is no such agreement.

Although, it's worth noting that some specific fields do use higher safety factors. ASME vessels, lifting and rigging equipment, for example.

One effect not really noted above is that on one-off type items, it is easy to spend $50 worth of time to save $30 worth of material. So on something like that, a quicker but more conservative approach may actually be more economical.

Along with this, if you realize early on that there may be minor adjustments in the calculations prior to the job being finished, a little bit of conservatism there can save a lot of changes later on.

"I have seen claims that rugby has more injuries than American football."- My older brother played rugby in college and then tried out for the football team. His observation was that American football was actually rougher on the players. In rugby, you had no pads, but only occasionally hit someone. In football, you were hitting someone every single play. I don't know if his perception matches statistical injury rates or not.

RE: Checking calculations

2

Quote:

I have seen claims that rugby has more injuries than American football. This may reflect that statistically, an injury occurs when the coach opens a first aid kit. Injuries in American football may be more serious because the people hitting you are wearing padding. This is a poor analogy for structural analysis and checking.

drawoh,

If that's correct then it would seem to be consistent with the item I posted earlier taken from the article about systems' complexity. That is,

14) Change introduces new forms of failure.
The low rate of overt accidents in reliable systems may encourage changes, especially the use of new technology, to decrease the number of low consequence but high frequency failures. These changes maybe [sic] actually create opportunities for new, low frequency but high consequence failures.


To wit: unpadded rugby players suffer more frequent but presumably less catastrophic injuries than padded football players. So while padded football players might not get as many bloody noses as rugby players, it seems once per season (or so) a professional football player might get paralyzed from a broken neck. To say nothing of the amount of concussions occurring inside those helmets.

I used that analogy to make the point, perhaps unsuccessfully, that with each refinement of the code to correct high-frequency-low-consequence failures there comes the opportunity to introduce the possibility of low-frequency-high-consequence failures. Especially since the new engineer has to learn it all. Mixing metaphors, the seasoned engineer only has to keep up with this years changes and, much like a frog being slowly boiled, doesn't realize just how hot the water is. At some point expecting the new engineer/frog to jump in the water and think it's fine -- or be able to check calculations against the entirety of the code without missing something catastrophic -- might be expecting a bit much.

RE: Checking calculations

Quote (271828)

I'd say no. In modern specifications, load and resistance factors are established using rational methods. They're not arbitrary.

What I meant was the safety factors, whilst in theory being there to protect against this that and whatever, in reality catch design stuff ups too.

Any safety factor is simply a layer of fat. If an engineer decides he wants a slightly thicker layer of fat, that's his choice as far as I'm concerned.

RE: Checking calculations

Quote (Archie264)

...

I used that analogy to make the point, perhaps unsuccessfully, that with each refinement of the code to correct high-frequency-low-consequence failures there comes the opportunity to introduce the possibility of low-frequency-high-consequence failures.

...

Do building codes prevent high frequency, low consequence accidents, and cause low frequency, catastrophic accidents?

--
JHG

RE: Checking calculations

>>>Do building codes prevent high frequency, low consequence accidents, and cause low frequency, catastrophic accidents?<<<

Who knows? The ending sentence of that point I posted from that article discusses this possibility:

These new forms of failure are difficult to see before the fact; attention is paid mostly to the putative beneficial characteristics of the changes. Because these new, high consequence accidents occur at a low rate, multiple system changes may occur before an accident, making it hard to see the contribution of technology to the failure.

RE: Checking calculations

We engineers are truly an arrogant lot!
Yes, mistakes happen and no, we can't (pre-)fix them with a safety factor (although, after a sleepless night, those extras came in handy - also note: sleepless because I checked and found a mistake). My strategy, when up against a schedule, is to simplify and envelope the design. As an abbreviated example, if I have a distributed load, find the total load and apply it as a point load at center span in order to size a beam. An enveloping strategy like this is not a mistake, often meets the client's requirements, easy to check and good in a quick pinch.

Because we all have to sell our services, its important to manage the clients perceptions. The obvious downside to enveloping like this is the potential for a poor reputation as an over-designer. However, most of the projects I deal with today are happy with an aggressive schedule over material costs.

RE: Checking calculations

Quote (TEGUCI)

Yes, mistakes happen and no, we can't (pre-)fix them with a safety factor

I'm still not sure why not.

Whenever a safety factor catches someone's underdesign (as opposed to catching substandard material) has it not fixed it?

And the bigger the safety factor, the bigger the issue that can be caught.

RE: Checking calculations

"And the bigger the safety factor, the bigger the issue that can be caught." The bigger the safety factor, the bigger the project's cost. Clients who hire design engineers expect the engineer to know how to properly and economically design a structure according to code and project requirements. Pumping up a safety factor, without a really good and understood reason, in order to cover expected mistakes doesn't seem right to me. If a designer is expecting to make mistakes, maybe more training or schooling is needed. If you expect the contractor to build it wrong, you have the wrong contractor. If you expect the inspector to do a poor job, you have the wrong inspector. Design it right; build it right; inspect it right. I know it's not always that simple, but try?

www.PeirceEngineering.com

RE: Checking calculations

(OP)

Quote (PEinc)

If you expect the contractor to build it wrong, you have the wrong contractor
Tell me, how many projects have you been hired for your engineering services where you've also been tasked with personally choosing the contractor of your liking. How many developers/businesses/institutions call you up and ask you which contractor they should hire?

Quote (PEinc)

If you expect the inspector to do a poor job, you have the wrong inspector.
You've used the term "inspector" several times in your replies. In Canada (Ontario), our professional organization strictly avoids using that term because it has legal implications that are far beyond what is expected as the engineer of record. Using the terminology used by the professional organization I belong to (PEO), which is also the terminology used by contract lawyers who teach seminars to engineers on providing professional services, we as the engineer of record perform periodic reviews of the construction work to ensure that it is in general conformity to the plans and specifications.

My point being that we don't get to see everything the contractor does. We have to make the drawings clear and easy to understand and build. Having 30 different column types that are designed just right, and most economically, is a construction nightmare, likely to lead to mistakes by the contractor. Lawsuits happen all the time, and even if it's 100% designed right, built wrong, you end up having to share some of the costs/liabilities.

--

A lot of people in this thread have shared examples of situations where they use some judgement in being on the conservative side. I appreciate everyone's input and I have gained some valuable take-aways. Thanks everyone.

Edit: spelling

RE: Checking calculations

I'm shocked by the arrogance in this thread.

If safety factors aren't explicitly designed to cover possible mistakes (by either the engineer or contractor or possibly both) then I would love to hear why people think we use them for everything.

RE: Checking calculations

Quote (jgKRI)

If safety factors aren't explicitly designed to cover possible mistakes (by either the engineer or contractor or possibly both) then I would love to hear why people think we use them for everything.

Doug Jenkins (a regular here on Eng-Tips) wrote a paper on limit state design principles. To quote:

"Statistical analyses used to calibrate code load and resistance factors do not include significant sources of risk, such as failures in the design and construction process, or unforseen events after completion."

RE: Checking calculations

Quote (jgKRI)

Statistical analyses used to calibrate code load and resistance factors do not include significant sources of risk, such as failures in the design and construction process

That's a worry, because if it's true it means we are completely ignoring the risk of design and construction deficiencies, preferring instead to rely on a fantasy of making zero errors.

RE: Checking calculations

Worked on the interference analysis of a USN man-rated mini-submarine a few years ago.
Budgets were tight, the boat was overweight, the spaces and clearances were "negative" = Everything hit everything when you tried to replace it or remove for maintenance or even to install the internal comuters, radios, wires, fans, crypto gear, HVAC and atmosphere controls, etc.
So, in the middle of everything else, we had to put in four man-rated submarine pressure hull hatches.

Turns out the only "acceptable" hatch was from the deep-diving submarines because they were certified for underwater explostions because they had been tested post-WWII on diesel submarines, so they were apporved for the newer nuclear submarines so they were approved for our little shallow -water submarine. Get the logic?

Turns out the fabrication dwgs were from 1932. Had never been updated from the "made a wood form and cast it in the foundry" era.
Were the four hatches too big, too thick, too heavy in some places? Ansolutely.
Where were they over-built, and where was the steel "just enough" and where was it "never failed yet" ... but was still undersized? Don't know. Can't tell.
But we could not get a "new lightweight hatch" designed and approved, even one that would have been designed in 3D with CAD/CAM certs and a finite element pressure check.

Typical of that project. Yes, it failed. Too expensive, too heavy, not enough power to do the intended job.

You've GOT to do your best design, and use a second-check to verify YOUR assumptions and YOUR approximations. Else the project fails because it becomes too expensive to be built, to be run, to operate as needed. Or, like the nuclear power porgrams, they get managed to death without adding the ultimately needed levels of safety and reliability.

Because you CANNOT truly know what your design will actually face.

RE: Checking calculations

2
jgKRI wrote:

Quote:

I'm shocked by the arrogance in this thread.

If safety factors aren't explicitly designed to cover possible mistakes (by either the engineer or contractor or possibly both) then I would love to hear why people think we use them for everything.
I am not familiar with other standards, but in the AISC Specification, the resistance factors do not account for human errors in design.

From Page 81 of Galambos 1981 AISC Engineering Journal article, "Load and Resistance Factor Design," which is the basis of modern LRFD in steel: "The reliability is to be interpreted as 'notional,' i.e., it is a comparative concept. It should not be confused with actual structural failures, which are the result of errors and omissions. Only the national statistical variation of the parameters is included, and, as in other traditional specifications, human errors must be guarded against by other control measures."

Part 2 of the AISC Manual spells out what sources of variability are included. Design errors are not in there.

Here's how LRFD phi factors are determined:
1. Test as many specimens as possible to get measured failure loads. Dig more out of the literature if possible.
2. Select a predictive model. Predict the failure load for each specimen.
3. Compute the ratio of measured-to-predicted load for each specimen.
4. Use reliability calculations, such as in one of the SSRC Guide appendix, to determine the phi factor.

Nowhere in that process is a factor for human error, other than in the development of the predictive model. Errors in the predictive model are deficiencies in the best current human understanding of the failure mode. These errors are quantified in the process listed above. They are not arbitrary. These factors are not intended to help with potential errors -- of totally unknown magnitude -- that designers might make. How would one include those types of errors in a reliability calculation?

I am sorry, but using an arbitrary bump-up is an awful practice. Errors most certainly will occur. They must be dealt with some other way.

RE: Checking calculations

In my mind the terms 'load factor' or 'resistance factor' or phi from LFRD are not interchangeable with the term 'safety factor'.

Quote (Tomfh)

That's a worry, because if it's true it means we are completely ignoring the risk of design and construction deficiencies, preferring instead to rely on a fantasy of making zero errors.

I couldn't agree more.

RE: Checking calculations


Quote ( )

"Statistical analyses used to calibrate code load and resistance factors do not include significant sources of risk, such as failures in the design and construction process, or unforseen events after completion."

Yes, but the structure doesn't know the difference between being overloaded because of a code limit load and being undersized for a more realistic maximum load. So in the words of a wise sage "I'd rather be lucky than good."

Quote (271828)

I am sorry, but using an arbitrary bump-up is an awful practice. Errors most certainly will occur. They must be dealt with some other way.

What specific way do you deal with them?

RE: Checking calculations

3
The derivation of the phi factor is aligned with what 271828 lists above -
I might add that this includes, within the reliability methods, all the individual parameters that contribute to the capacity of the structural member.

For a concrete beam, those parameters would include concrete strength, rebar yield, depth to rebar, width of beam, and the equations used to determine strength.

Each of these parameters has a "natural" variability in itself and the phi factor is developed based on their variabilities plus calibration with an expected probability of failure.
The "natural" variability of the parameters, including the predictive equations, creates the phi and none of this involves egregious errors.

However, when combined with load factors, you have a margin of safety now built in that does in fact cover some errors as long as the errors are not excessive.

In some cases, we engineers find ourselves telling owners and contractors "the structure doesn't work under code" while they are scratching their heads looking at a structure that has stood for several decades. Been there many times.

So I would suggest that small errors are many times unintentionally covered by the load and phi factors while larger errors are not intended to, and can not be, covered by the level of built in safety.

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RE: Checking calculations

I agree with stiman86's comments about inspection. However, it is common and usually code-required that companies be hired to do on-site inspections or special inspections. These special inspectors are supposed to be hired by the owner or owner's rep. It is the special inspector's specific job to make sure construction is done correctly, keep records of their inspections, and to notify the proper authorities or entities if there are construction problems. When I said "you picked the wrong" engineer, contractor, or inspector; I was referring to whoever does that hiring, not necessarily and not usually the design engineer. By the way, I have many, many times been asked to recommend contractors to my clients. I always recommend more than one contractor so that my client has a choice from several contractors that I know are experienced and capable of performing the specific work required.

EDIT: P.S., in New York City, the design engineer of record has to sign a form that he or she approves the chosen special inspector.
www.PeirceEngineering.com

RE: Checking calculations

njlutzwe wrote:

Quote:

What specific way do you deal with them?
Depending on the situation, one or more of:

1. Knowing from experience approximately what I should get in most cases. Bird's eye view of the sizes on the plan.
2. Check my primary calculation method with standard tables, textbook examples, etc.
3. If a result looks weird, break down the problem into a simplified version which should give a similar result.
4. Peer reviews.

We all make mistakes. Our work is too complicated to pretend otherwise. I just disagree with the particular method described in the OP. I am sorry for being too forceful in this thread, though. Maybe I need a load factor.

RE: Checking calculations

@271828

I agree with your methods of checking on things. I often do quick dirty calculations and am surprised by how close they are to a complex RISA-3D analysis complete with semi-rigid diaphragm load distribution.

Regardless of the exact interpretation of the OP, i think there is more common ground than uncommon...

Another thing to consider is the fee/schedules of typical design shops. The adage “you get what you pay for” is so true. And, i think that if clients were willing to pay more and give more reasonable design schedules, engineers could save them time and money by spending more time designing with a sharp pencil. For example, connections with the UDL. That is typical, but we could fine tune that, especially for short beams, but we often don’t.

Now, with that said, I think that if the fee was $10,000 or $100,000, with established practices, you would probably get about the same product. In other words, if clients were willing to pay more, I even wonder if we as engineers would give them a better product, or if we would revert to the current design methodology we follow.

Ultimately, what i’m Getting at is that every engineer could sharpen the pencil more than we do. And the level that we do is based on judgment, experience, and ability to sleep at night (Both structurally and ethically). So it’s something to hold in tension. Folks like 271828 make us better engineers by encouraging us to sharpen the pencil. Meanwhile, Stiman and company make us more efficient. I think there is a time and place for both, and the tension is healthy.

Just my two cents...

RE: Checking calculations

Quote (JAE)

In some cases, we engineers find ourselves telling owners and contractors "the structure doesn't work under code" while they are scratching their heads looking at a structure that has stood for several decades. Been there many times.

Here's one I sometimes wonder about. Let's say we design a steel beam to support a 200mm thick concrete deck. We factor the dead load by 1.2, and reduce the steel beam's capacity by the relevant phi factor. Assume the strength ratio comes out to be 0.98, the beam is working hard, but has not exceeded its capacity. The contractor pours the slab, and actually pours it 220mm thick. Our original load factor of 1.2 covers this 10% variance in dead load - the load factor has done its job.

But say a new engineer comes back and measures and checks the structure from scratch - the beam doesn't work anymore - because they're now factoring the actual slab thickness, which is larger than the slab thickness in the original design.


RE: Checking calculations

Sorry for joining late.
I vote not to overdesign, instead:

1)Search hard to see if there exist already standardized stuff: a)Some place somewhere somebody solved a similar problem, executed it and catalogued their work, b)German engineers steel use standard tables the way engineers did in the fifties and sixtees where competent designs/methods were sought for and tabulated. Modern software killed this great tool.

2)Sharing designs/methods : I asked a friend for design of a huge spiral staircase which he knows worked, I modified it to make it more economical and remove errors, built it and passed my design back to him. Yes sharing works. Modern I-am-the-best approach kills sharing.

What do you think?

ijr

RE: Checking calculations

Most serious failures do not occur because the member is not quite strong enough, most occur because of something fundamentally wrong with the structural scheme or with the way it has been installed.

Conservatism will not negate this only checking of the design will negate this.

Even with preliminary schemes we do what we call a 'sanity check' i.e. we get one of our experienced colleagues to run their eyes over the job and see if it all looks right to them. This can take as little as 5 minutes for small jobs.

I have checked work from graduates to those top of their field and almost ready to retire. I have picked up errors in all levels although the later end errors are generally a lot more subtle.

RE: Checking calculations

IMHO you have to consider the life cycle of the structure. I work in power and mining industry and almost all structures in the field have extensive section loss from corrosion just due to the conditions. I have also seen several very questionable repairs. To compensate I favor fairly conservative design just knowing how things will be treated during their expected life.

As a second point I have done several projects where I need to go back and check an existing structure for additional loading. It gets very expensive for the client when they need to add 10% more weight but the original structure was designed to close to IR=1 that now they need to additional steel work.

RE: Checking calculations

Ideem,

I agree. If you design to 100% then you’re up against the limit from the get go.

If economy is the number 1 concern then by all means go for full efficiency, however it’s not always as simple as that.

Contractor pour the concrete 10% too thick? FAILS a fresh analysis

Client wants to put slightly more load on their building? DOESNT WORK

Steel tensile area corrodes by 10%? DOESNT WORK, structure non compliant.

So leaving a bit of headroom isn’t necessarily bad practice in my opinion.



RE: Checking calculations

Regarding the two previous posts, I like to have the client involved in/making over-design decisions. Eg be honest with them about the limitations of paint/galvanising in preventing corrosion and have them agree to a documented corrosion allowance. Ask them if they're sure they'll never use a structure for any other purpose. A lot of the time they say lowest initial cost is their prime directive and I can live with a lean design in that case.

Regarding concrete poured a bit thick: a decent code will allow a reduced dead load factor for loads based on survey of an existing structure. If only they were all so decent...

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