Eng. practice of allowing 5% overstress 33

[radair]

It seems to be standard structural engineering practice to allow up to 5 percent overstress in structural design. It's been this way since I graduated college in 1980 and I've seen the practice commonly used in the tower analysis field for the last 15 years.

I've been asked my opinion by a government agency as to why this is a safe and acceptable engineering practice, including citing any relevant structural codes. They are not questioning my work but are asking me for a signed & sealed letter of opinion. It seems to me that this would be a better question for their state engineering board of licensure.

Can any of you help?

 

[youngstructural]

Okay, I really don't see anymore value to this post and will probably stop reading the thread...

It has been interesting, and I do believe the majority agree that 1.05 is nothing to stress about, while NOT being something anyone shoots for...

I honestly think we've exhausted the issue. See you all in the next thread.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[JaredS]

MechEng,

I never claimed the code was a hassle, it isn't, the code is probably the most valuable tool a structural engineer owns. It is just that the code is so limited. The code cannot possibly cover all the situations that an engineer is bound to encounter, however, because of broad generalizations made by the code, it covers a great many of cases. Because these conditions are so generalized though it makes the code in particular very conservative.

Furthermore, most structures, not bridges, have great degree of redundancy. These structures, should they have one element fail, will simply redistribute the load. Also, this "failure", unless the engineer responsible for designing the structure exhibits gross negligence, is not the kind of failure that results in building collapse. In fact one specific code reasons that because of redistribution effects that members undergoing torsion do not need to be designed to resist the applied torsion and that they only need to be designed to control cracking.

 

[BRGENG]

I think MechEng last post says it best. While the majority will agree that the built in factor of safeties would not cause a failure on a structural member that is designed 5 percent over stressed.

However if one of your buildings did have a failure and they see a calculation that the allowable load on a column in 100 kips and the actual load on the column is 105 kips. Guess what, you probably just lost your case, your license and a bunch of money.

When doing a code review for one of our clients we will kick back every design we see that has a 5 percent overstressed member. Unless you can show me in the Code where it is allowed. Besides if you are going to bend this rule what else are you bending?

Apsix and frv, I can only assume your experience is lacking or you have been mislead at some point. Long ago I remember the old timers using the 5 percent factor and wonder what the hell are they thinking.

Unfortunately 1/2 of engineering is avoiding litigation. Breaking the code is not a good start in doing that.

Your arguments that if everyone else is doing it then it is standard practice is weak and negligent. Per the hundreds of review we do each year many engineers don’t use the new requirements for splice lengths in masonry or development lengths for anchor bolts. You would be amazed at how many don’t even look at seismic loads and just put “ by inspection wind controls”. Even worse is the use of the ASD 9th edition when it has now been removed from the Codes in most cases. People still use old version on Enercalc that are based on the 1997 UBC code and not IBC.

While the building will probably still stand if you don’t design per code you are wrong. It is that simple. As I said before, if you think the 5 percent rule is okay go inspect PEMB after a big snow or ice storm.

 

[apsix]

BRENG
Are you refering to "It does not lead to over-stress of the member"?
It depends on your definition of over-stress. I am equating over-stress with failure, eg. going into yield. Obviously that doesn't happen when you design to 1.05 instead of 1.0.
The point I was trying to make is that it's a term which can mislead people and lead to statements like "Designed to fail".

I thank you for your concern regarding my experience/competence, but assure you that you have no need to worry.

PEMB; a different kettle of fish entirely, not to be confused with a robust structure.

 

[frv]

BRGENG-

I, too, am getting bored of this thread.

It seems obvious that you skipped over large parts of this discussion.

But I can say, beyond a shadow of a doubt, that if any building fails, it is not because there is a 5% "over stress" in any one member. There is some huge mistake somewhere else.

As others and I have repeated ad nauseum, as engineers we are not simple number crunchers. We are allowed to use the knowledge knowledge and experience we've gained to make INFORMED judgment calls.

Misled? EVERY ONE of my (engineering) professors alluded to this in one way or another. And before you go off discrediting them, don't; trust me, you won't win that battle.

Anyway, I'll join youngstuctural in abandoning this thread.

 

[oneintheeye]

i see the code as a deign aid/guide/safe method to acheive structural safety. It says itself compliance to code does not allieviate any legal responsibilty if the structure fails. Stating 'its to code' is not an entire defence, especially as the code cannot cover all types of structure etc. constructed today. There are also numerous statments (i cant be bothered to look for the actual quote) that state unless can be proved otherwise take ...... i.e. if you can prove by first principles that the structure is ok then you can deviate from code on certain issues, e.g. proving stresses etc.
I would suggest any engineer worth calling himself one can recognise an indivudual member that in theory is 'ovestressed' by 5 % can be, if you feel inclined, be proved from first pronciples to work as part of a structural frame.

 

[oneintheeye]

oh and I would suggest by your post brgeng you sum up what people who are 'checking' engineers are deemed to be like to those actually doing designs. Crucual yes, but frequently code junkies who insead of looking at practicalities and 'engineering' a solution are overly concerned with ticking every minor code clause as they are the only ones who have time to do it.
Dont take offence, its what your post implies. Not what your like i'm sure!

 

[csd72]

herewegothen,

r.e: "ticking every minor code clause as they are the only ones who have time to do it. "

To be fair, engineering firms should quote with enough in the budget to check full compliance with the code. This is a failing of the engineering industry, not the checking engineer. It is amazing how much better the checks and measures are in accounting than they are in structural engineering. It does annoy me too though, when I have a pedantic checker.

As per the 5%, what if the original designer allows 5% and then the engineer for the renovations also allows 5% (as the code implies they can), the building is now taking more than 10% additional load above what the code says it should - are you going to say this is negligible?

 

[swearingen]

That's kinda goofy, csd72. If the original engineer went to 105%, the equally competent engineer doing the renovations would see that it's at 105% and wouldn't go further. It appears that many folks on this thread, however, would then stop the project, kick the contractor off the site, put red tape around it and scream "STAND BACK - IT'S OVERSTRESSED - SHE'S GONNA BLOW!!"



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[csd72]

If it was a concrete building with no calculations supplied, how would you know?

 

[HgTX]

If it's a concrete building with no calculations supplied, you reanalyze, no?

Hg

Eng-Tips policies: faq731-376

 

[miecz]

If it's a concrete building, or any type of framing, and calculations are not supplied (a very common scenario), I would simply ensure that the new loading does not add to (or subtract from)the existing loading by more than 5%, and I'm done. A reanalysis is not common, or expected. If that building happened to be understrength by 5%, something I wouldn't expect, well, now it's under by 10%.

 

[DaveAtkins]

And 10% overstress won't cause failure either.

Look at it this way--failure is not caused by using a W18X35 beam where you should have used a W21X44. It is caused by using a 2X12 where you should have used a W21X44.

Design to Code, yes--but use engineering judgment as well. How many of you, if you find a mistake in your work after something is built (a mistake that results in, say, a 10% overstress), will call the Owner, tell him his structure is in danger, pay for repairs, etc.? No--you sharpen your pencil, try to justify what you have done, and use your engineering judgment to determine if there is really a problem.

DaveAtkins

 

[oneintheeye]

miecz,

if i was given a building I would reanalyze it whatever, how do you know the original engineer didnt make an error? Fo instance the bridge collapse over there. You in theory would have told them they could increse load by 5% on an already incorrectly detailed connection. The japanese shopping mall collapse (forget the name), increased loading combined with poor construction led to collapse. I cant believe anyone would advise a structure is ok just by inspecting the load and 'assuming' it was originally correct. You would definetly end up in court in it failed justifying. I find it kind of shocking actually.

 

[miecz]

herewegothen,

Here's how I see it. Say there are two buildings standing near each other, and an unusually strong hurricane is approaching. One building has been standing for 40 years, and the other was completed yesterday. If you had to bet which one would be standing after the event, which would you chose? I'd go with the old building, as it has proven itself over time.

I believe this reasoning applies to a 5% overload. We know something about a building that has stood for 40 years that we don't know about a building under construction. It has proved itself time and again under load, the only true test. The probability that a 5% additional load is going to fail an old building is less than for a new building.

 

[oneintheeye]

unless thats the first ever hurricane of that force it has encountered.
If it is underdesigned from the start a further increase may cause problems. You wouldn't know that it was underdesigned unless you check. I would check personally.

 

[strguy11]

For steel buildings, I have no problem with a 3 to 5% "overstress" for this reason. The typical "design" Fy values are specified as a minimum value of either 50 or 36 ksi, depending on the steel specified. It is this 50 or 36 ksi we design to, however if we have a 5% overstress on a member, this would theoretically require a 52.5 ksi steel. My experience has shown that the mill certification reports for the steel are typically higher than this, which is why I dont have a problem using the 5% overstress on a couple of members, but I would never design the entire building this way.

For concrete it would be similar, since test reports show that the in-place 28-day strenghts are higher than what is normally specified. I would think that other materials would be similar.

Based on this engineering judgement, along with the embedded code safety factors, I feel comfortable with this design practice, and feel that the I steel meet the intent of the code.

 

[frv]

just when I thought I was out, they pull me back in!


strguy11-

Although I agree with you, higher compressive strength doesn't do much for flexural capacity. I think here you rely more on conservative assumptions about loading and the fact that you can get a lot of moment redistribution than on the fact that you get higher strength concrete.

 

[strguy11]

FRV-

True, but with concrete, you typically get about 3500 psi, even when specifying 3000, which represents a 16% increase. Looking at the design equations for concrete beams, (A = T/.85 * F'c) The higher F'c would actually give you a smaller Area of concrete tequired to resist the compressive force from bending. This smaller block, would actually increase he effective depth to rebar. The rebar usually has a greater Fy than 60 also. (More like 62 or 63 based on mill certs) Therefore a higher F'c would help in bending. With all of these factors, I feel comfortable since we are only talking about 5% anyway.

 

[Lion06]

Higher f'c does do a little for flexural capacity (though admittedly, very little), but it does add ductility. The reduced compression block depth means greater strain in the steel at ultimate loads.
It will also help with shear strengths, which can be an issue in concrete (and typically is not in steel).