Overstress in existing building evaluation 3

[boileralum]

My firm's current steel design standard is to allow up to a 1.03 overstress on new designs. We are reviewing the design of an existing building, no new codes or loads but we are finding some isolated overstresses on the order of 1.10 or less but greater than the 1.03 that we usually allow. Mainly looks to be resulting from compression flange local buckling on OMF Rigid Frame columns and rafters. The building is being checked under ASD. Question is would anyone feel uncomfortable allowing a 1.10 overstress for this situation when considering the safety factor that you still have in place? This is a pretty straight-forward building, not high occupancy, no cranes, no second floors.

 

[hokie66]

I wouldn't feel uncomfortable, provided I fully informed the building owner of my findings and the meanings and implications thereof.

 

[strucguy]

Try evaluating the design using LRFD principles. It may be helpful, but I seriously doubt if it makes any difference. Also, take a second look at the loads and masses being used in your analysis. I have seen drawings that used 10 psf for MEP, and that too as a live load. That's very conservative.

 

[paddingtongreen]

Same as hokie.

I question your 1.03 for new work. Does your contract with the client allow judgment or does it say you will comply with codes? it is unlikely to be a problem, but if you do have a problem somewhere, the bean counters will be all over that 1.03, not because it caused the problem but because it lowers your credibility.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dcarr82775]

For a new building there is no excuse to use 1.03, it is 1.00.

For an existing structure I'll use something around 1.05 as an upper bound.

 

[JedClampett]

In cases like this, you need to evaluate the whole issue. For instance, is the overstress due to overly conservative live loads? Or is it due to a very unlikely combination of events, like a maximum earthquake in the worst direction possible? If the steel mill certs were considered (this might not work for buckling) would that help? Have you sharpened your pencil in every way possible?
Especially for an existing building, I tend to come out on the side of accepting it. Don't forget, you're really not overstressing the member; you're reducing the factor of safety. Document your reasoning in your report to the client.

 

[rscassar]

Was the original engineering design performed by your firm? I would not feel comfortable giving the building owner advice that their structure is compliant with the building codes if you cannot justify yourself that the structural design is within the strength provisions. It is a question of ethics and I personally would tell the owner that their building is non-compliant.

If you are finding that the building is 10% overstressed, I would have another engineer in your firm independently review the structure and if they also establish a 10% overstress, I would tell the building owner. It is a duty of care to the community.

 

[woodman88]

“the bean counters will be all over that 1.03”
How many engineers here feel we can justify more than two significant figures for the loads (live and dead) being applied to a building? I was taught that if one of the input numbers were significant to only two figures than the answer could only be significant to two figures. So that 1.03 would need to the rounded to the nearest two significant figures of 1.0. Would any of you require the number to be rounded up to 1.1? Or do some (or most) feel that the loads are to three significant figures and that the answer should be too?

Garth Dreger PE
AZ Phoenix area

 

[SteelPE]

I was told at a recent seminar that “steel does what we tell it to do”. This was at a connection design seminar and had to do with the load paths through the connection.

There was an interesting story that went along with the seminar in regards to an existing structure the speaker was asked to apply additional loads to. The speaker said that according to his calculations, the structure was overstressed and that no additional load could be applied and that the building in fact needed to be reinforced. However, the building had been standing for a long time without incident. The job never proceeded forward.

After a few years, the speaker came across a paper that described an old hand method to designing structures (this structure was quite old). Applied to the overstressed building previously and the structure worked.

Depending on how old your structure is, there may be more to the system than running it through computer program.

 

[mtu1972]

woodman88 - Well said.

I hear this all the time from my compatriots who go ballistic if anything over 1.00 is accepted (or whenever members turn red on the computer screen). I learned very early to be conservative on my load assumptions. Then knowing that when I get something that checks out just slightly over 1.0, I can still feel comfortable with it.

Your argument about significant figures is very pertinant. Collectively we seem to have lost some of the common sense thinking that was needed "when life and Codes were so much simpler".

gjc

 

[ishvaaag]

It must also be said that if you find 3% overstress by ordinary analysis, I have found that through more precise FEM 3D analysis use to show NO overstress. This is not however a general rule, but a trend within the structures I have checked this way, mainly Reinforced concrete.

When you model the actual structure the threedimensional restraints within the material come to work, and maximum stresses (at least the ones we use to worry about) come somewhat down.

 

[Doublest]

It sounds more like a legel problem than an engineering problem. I wouldn't feel uncomfortable for 3% overstress from engineering sense, but I will feel uncomfortable for this 3% overstress in court.

 

[paddingtongreen]

Woodman and followers, There was a time when some clients hired other firms of engineers, accountants and lawyers to go over engineer's designs to check for due diligence. Try to explain to a lawyer or an accountant about the inexactitude of what we do. I just don't think the 1.03 should be written down as a general allowance, I let it go in the calculations with notes such as "Conservative Loads" alongside.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ishvaaag]

I precisely was pointing that very likely the tools to prove in court that no 3% overstress exist are standing.

 

[rharting]

Typically for existing buildings i would use 7% as long as i used conservatice loads and/or the probability of the beam seeing all of the design loads was extremely low. I would certainly let the owner know the results (7% overstress is small compared to factor of safety, likelyhood of the members actually seeing the exact loading conditions, etc)

I couldn't agree more with paddingtongreen. There is absolutely no excuse for allowing 3% overstress in new construction. The cost to increase your capacity by 3% should be rather small, but the cost to fix a problem or add new loads would be extremely large.

 

[topsisu]

You may want to review IBC chapter 34(2006 - Section 3403 or 2009 - Section 3404) to see if the 5% load increase limit can be utilized or if the original live loads can be used to justify the design.

 

[woodman88]

"There is absolutely no excuse for allowing 3% overstress in new construction."
So do you have an allowable overstress figure? Such as 1.02, 1.005, etc?
What if a typical connection or member in the building is at two or three locations 1.005 overstressed? If you would not allow this, have you checked out each program you use to be sure it meets this requirement? Because some of the programs I have checked would give a 1.00 answer for a 1.005 calculation. Others will show the 1.00 as overstress.
I personally like to keep to a maximum of .90 stressed for connections and members. But will allow a 1.049 (my personal limit) to avoid going to a odd size, spacing or connection.

Garth Dreger PE
AZ Phoenix area

 

[sandman21]

Funny how some people don't apply there logic the other way, if 1.03 is fine, how about 1.031 or 1.049 or 1.1? I don't overstress any design, I will fine tune the loading but if it does not work, it gets a retrofit. I have seen too many buildings with missing rebar, wrong grade bolts, poor or incomplete welds, to try and justify using values close to 1.0 let alone above 1.0.

 

[JStephen]

I remember a LONG thread a while back on the 1.03 issue.

Where I first ran across this in the past, the people there had some old textbook that said that in soil analysis, 5% more or less didn't matter, so the design procedure there was to design every foundation for 105% of the bearing recommended by the geotech report.

As to the original post, I would also wonder if it wasn't a matter of how the analysis was done rather than just having an overstress.

 

[ishvaaag]

I neither overstress anything ever by design. But I use the available tools at hand to proof that such is the case, be it for a technical or reasonable economical or constructive advantage, or defend a rightful personal position.

In this view, the "old texts" are still the new texts on this: we use standard procedures to generate our structural design, but that doesn't mean that such procedures, stated wherever they be, represent in the most accurate form the actual behaviour, or the solicitations for the case; there maybe more accurate solutions elsewhere, or simply, the methods themselves maybe conventional or notional but never corresponding to accuracies in the quantificable measures in the order of a 3%, or even 10%. Try to predict with such accuracy the deflections in concrete, or believe the axial forces you are determining (commonly) for your columns are (or will be) the actual ones: such won't be the case.

And this is not to advocate for imprecision, or lazyness, but a necessary statement on that the art and science of designing structures has much ahead to progress.