existing structure overload!

[oneintheeye]

A bit of possibly a hypothetical question here. We have been doing more and more assessments of existing structures for not what I would say is change of use exactly but increased loading. New loading pattern etc. A couple have been very close to overload. Some of the new loading conditions have already been applied before the check. My question is this;

1) Say the new load (already placed) shows that the reinforcement in the slab in theory fails, but the slab is standing. The allowable design stress is less than calculated. But the structure is standing. The calculated material stresses are less than the manufactured 'target' for the time which (with design safety factors) account for the fact that the slab is ok. In the (somes) client s eyes I would look crazy for insisting remedial works are undertaken. How is the best way to handle this?

 

[desertfox]

Hi herewegothen

I am not a structural engineer however I know exactly what you mean, I have seen this sort of thing in various mechanical components from time to time.
For instance when you design a pressure vessel you follow a particular code but lets say the stresses are over what the code allows but still within the yield stress or failure critera of the material specification, then you have to make a call on whether your going to conform to the code or not if you have that option.
If you have to adhere to the code then the design as to change and I would say to the customer or client for the vessel to be complient we have to change this and this etc otherwise we can't meet the stresses in the code or specification. Sometimes for certain things you can apply for a non conformance and again I would put to the customer the option of complying or obtaining a non conformance so it least its recorded.
Always at the back of my mind is that the safety factors used in code's etc are usually derived from experience over the years for things that cannot be calculated or are unforseen circumstances that the component might have to experience at some point without failing, so were possible I always try to stay within the code of practice, that way you cannot be out on a limb.

desertfox

 

[ishvaaag]

One thing is that the things stand and another that they have the required safety as per current law statement. The owner needs then must be informed on that reinforcement is necessary according to the law and otherwise, in more than putting to risk the lives of anyone there and his own products and means, is anytime exposed to a closure of the facility since not complying what mandatory.

 

[Ron]

herewegothen...you have an obligation to point out the deficiencies, the potential impact, the building code implications, and the reason the building is still standing (variables in design and construction working together...they could just as easily be divergent, creating structural failure...in short, LUCK). Be clear with your client that you are further obligated to recommend the remediation to comply with code and accepted practice. They can decide what to do from there.

Do you have a further obligation to inform code officials? Perhaps, but that can depend on a lot of factors. If there is a potential for danger to the public, absolutely. I would suggest that if you show more than about a 10 percent overstress, you make some hard recommendations and consider informing local code officials. In some codes, there is a provision that states if analysis shows a deficiency, you are obligated to load test the structure.

I would suggest that you pose this same hypothetical situation to your state board and ask their opinion (from their legal counsel).

 

[frv]

Out of curiosity, how much are the slabs failing by? Have you been overly conservative in your design assumptions?

I ask because code minimum live load tends to be on the conservative side to begin with. Code live load accounts for accidental impact loading (a few people doing jumping jacks, or a couch falling when moving it, for example), among other things.

If you are only slightly over-stressed, it's probably OK. There was a very lengthy post some time back on the issue of whether a small overstress was acceptable. Those of us who took place in that discussion were pretty convinced of our own point of view, so I don't know whether that would be helpful.

 

[SlideRuleEra]

...I would look crazy for insisting remedial works are undertaken.

IMHO, the key is not to INSIST that the work be performed. Hopefully, the Client hired you be cause they value your opinion. Tell the Client the truth about the situation. If they don't accept your recommendation, suggest that they have a qualified third party (another Engineer) review the situation.

We (an Owner) had to do this on several occasions over the years and the results could be surprising. Sometimes the original Engineer was right, other times wrong. More often than not, a QUALIFIED third party would find a compromise solution (perhaps less expensive remedial work or a slight alteration in what the Owner wanted to do that minimized the problem).

Better than going to Building Officials with what may be incomplete or inaccurate information (possibly leading to legal action against you).

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[weab]

I agree with frv. A few percentage points over just means that you have a slightly reduced safety factor. I might not accept it for a new design, but maybe for existing.

You say "The calculated material stresses are less than the manufactured 'target' for the time which (with design safety factors) account for the fact that the slab is ok." I don't quite understand; to me, this seems to conflict with your previous statement. I understand you to say that stresses exceed allowables.

You asked how do you explain it to an owner. It's all about safety factors and the straw that broke the camel's back. Explain that while it is still standing, tell him that it probably hasn't endured its design wind or seismic event. Explain that the original design engineer knew that some owners will put more load than expected to "save" owners such as himself. Or tell him that the reduced safety factor is such that he'd better have workers that weigh 100 lb. If you put a bunch of 300lb guys in the area, tell him that you'd personally evacuate the structure. Yeah, maybe this is a bit over dramatic, but I think it makes the point.

Owners don't understand safety factors and don't understand that a few cracks or some deflection can indicate imminent failure. If the owner refuses to fix the problem, explain in a letter that you can't do additional work on the structure, and copy the world.

 

[jike]

Before going to the Owner, you must answer the question how accurate is your analysis? Can you refine your analysis and perhaps justify not doing anything? For instance 3D analysis versus 2D analysis. Slabs often span in 2 directions even though we assume only one direction.

 

[msquared48]

Also, since this is an older slab, the f'c may be higher than the original manufacturer's specs, allowing for a somewhat greater capacity than the original design.

Have you done any testing of the current f'c?

Mike McCann
MMC Engineering

 

[csd72]

herewegothen,

If you are in the uk, then the structural refurbishment code (or something like that as its been a long time) goes on about accepting the capacity of a structure if it has already been used for its intended use for a period of time. You will have to look this one up yourself.

We rely on load testing for bridges so why not for this type of situation. Keep in mind that concrete generally increases in strength over time even after the 28 day strength has been achieved.

That said, for concrete I would be a little bit wary about the fact that the gross concrete strength of an uncracked beam/slab could exceed the working load. This would mean that the first time it was overloaded to cracking it could suddenly lose this strength and collapse.

If it has already cracked then this issue would not necessarily apply.

 

[oneintheeye]

hi thanks for all your replies I will try and clear up points.

1) The design code used in a lot of these cases was allowable stress (i.e. not factored load) therefore the stresses are greater than allowable but less than the yield stress.
2) Wind in some of these cases would not be an issue, these are very large buidlings and I am looking at internal slab areas. Seismic does not apply in UK.
3) Due to point 2 is the fact that the load is applied a load test?

Another question, if I send a recommendation to the client but hear nothing back what should be done? In theory the client could have appointed another engineer to check, design and could carry out remedials without me ever knowing. Does my respnsibility end with the report?

 

[desertfox]

Hi herewegothen

What exactly are you contracted to do? if just to do a study of the structures and report back how can your responsibility at that point not end.
You can send a report and reccomedation to the client with a proviso:- if I do not hear from you within 30 days... 2weeks whatever then ......

desertfox

 

[oneintheeye]

generally we would offer in our fee for assessment of the structure for the new conditions (or it may be a conbined inspection of condition and assessment). Design of remedial works is excluded. Therefore my report would say, structure fails either do not change use for new loads or remedial action which will be on the lines of ......
Therefore does that remove any liability on my behalf if they do not carry out either? I don't see how I could demand proof.

 

[desertfox]

Hi herewegothen

If you tell them in the report that the existing structure fails if its put to some other use and they ignore it how then can you be responsible.
I would just tell them exactly how it is, if you feel you can change the design you could offer at some fee a redesign or modification to existing structure to do what they wish, at the end of the day its upto your client.

dsertfox

 

[jike]

I recommend that you discuss your findings with the client face to face or over the phone before issuing the final report. Ask if there is any additional things that the client would like you to address. This type of discussion and follow up can be used as a great marketing tool for future work! Don't miss this opportunity.

 

[paddingtongreen]

herewegothen, I am disturbed by your loose use of the word "fails". It may be overstressed but not fail; don't say, "fail to meet code requirements" say "higher sresses than allowed by code". Of course, if you find rebar stressed up to the yield, then say "may be close to failure". It is the difference between unnecessary and necessary alarm by using "fail".

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

 

[SlideRuleEra]

I agree with paddingtongreen's comments on the use of the word "fail". This is an important distinction and is easily misunderstood.

About your followup question if the Client ignores your report. IMHO, that should be handled on a case-by-case basis; jike's suggestion for direct contact with the Client (rather than just written reports and letters) is the way to go. I'll add to that to be sure to distribute timely minutes of these meetings/phone calls to all who participated.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oneintheeye]

i probably wasn't clear. My use of the word 'fail' was for discussion here and between us I assumed the word fail would be taken as 'design fail' so aologies for that. Of course I have direct contact with the client, there is no problem with any of our relationships with clients as far as I know. The report would follow an initail discussion M query was more to do with, is the structure load tested if it is standing under a load already imposed (there may be some error in the assumed load on the conservative side). What I am getting at is to a non engineer saying that the structure needs remedial work when he can see it standing there may seem a bit strange.

 

[rwpe]

When dealing with existing building, we generally see a range of ages from 20 years old to 300 years old. My first comment would be to verify (if possible) if the structure was initially designed correctly. If so, then imo, it becomes a judgement call on your part.

There are so many Factors of Safety that go into everything we design as Structural Engineers, that a degree of overstress in a structure would not necessarily present a threat to the life-safety issue. If the slabs you are reviewing are in good shape, have little to no creep, etc., then a FoS of 1.25 vs. 1.5 may be acceptable.

When we design, we don't necessarily take into account all the little things that could contribute to a structural element. For example, wood joists. We normally do not include the contribution of the sheathing to the strength of the wood joist. Would it be incorrect to do so? Assuming that the connection between the two elements is good and solid, then no--it would not be incorrect.

What it boils down to is are you comfortable with standing behind your stamp if the structure do not quite meet all the code requirements based on conventional analysis?

 

[csd72]

I think paddingtongreen makes a very good point.

There was a case in Australia about 10 years back where a young engineer used the words 'emminent shear failure' in an overreaction to some cracks in a slab. The whole office of personnel was moved out and was too scared to move back in after things were cleared up.

Be clear and concise but also remember that we engineers tend to throw around a lot of words that would make most laymen panic.