Design Life of a Steel Structure 2

[VAStrEngr]

What is the standard for determining the design life of an exposed steel framed structure?

Although the observed structure in question has what appears to be a suitable covering of paint, it is unknown what the condition of the steel beneath the paint is. To me, a good paint job doesn't necessarily indicate structural integrity.

The structure was built 60 years ago and is an open framed canopy in upstate New York.

 

[kelowna]

Is it me, the posting not being clear or are you a bit confused?

The design life on a steel structure is not a 'best before date', it is just an statistical number given by the appropriate design code or client! It is used to calculate things like fatigue, environmental loads (wind, earthquake,...) and corrosion protection.

It does not mean that if the design load of a structure is 75 years, it will be OK for 75 years and fall down the first of January of the 76th year. It does not mean either than you can assess an existing structure and assign 'residual design life'.

Not unlike a house, if properly maintained, a structure can be sound and solid well passed its 'design life' and for ever. If not maintained, it can collapse well before it.

If you are assessing an existing structure, you should assess its physical condition, its capacity and the loads present. That will tell you if it is sound or what deficiencies it has.

In your case, good paint condition is a good sign, it means good maintenance, no corrosion and therefore maybe section loss is not a problem. I would take some measurements to confirm no significant section loss, look at the connections and strip the paint on heavily loaded areas or fatigue prone areas to look for fatigue cracks. I would look at weld cracking (if it has any welds) I would also look for local issues like local corrosion, buckling or signs of any problems.

 

[VAStrEngr]

Thanks for the response. Perhaps I was a bit unclear but your answer was very adequate.

I was not intending to assign a shelf life or "must replace by" date...I simply didn't know if there was any figures for when a structure has served its lifespan and continual maintenance or upkeep financially outweighs the benefits of new construction. I'm sure it can be calcualted but perhaps there might be a good rule of thumb that would save a more thorough analysis.

Due to the location, it has also seen its share of harsh winters and freeze-thaw cycles. I didn't know if there was anyone who could speak to the defiency of older structures subjected to repeated internal temperature changes and any loss of strength associated with this fatigue loading.

I didn't mention in the first post, but the facility is considered an essential facility so a failure could have far reaching consequences of life and safety...one reason I wanted to thoroughly consider all possibilities.

 

[SlideRuleEra]

There are many significant older steel structures that are still in continuous use and yet have a long expected lifetime ahead. One that comes to mind is New York's Empire State Building (pushing 80 years old).

With proper maintenance and conservative loading another way to look at your question is to consider that major STEEL structures have only been in use for about 125 years... that is appears to be not long enough to accurately determine an answer.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[paddingtongreen]

In this century, I designed changes to an "EL" (elevated commuter railway) constructed at the beginning of the last century. The trusses, open to the weather, were in good shape except where in contact with other members or the concrete platform, places where capillary action would hold water.The reason they were in good shape is that they were coated with a lead based paint.

What they used to look like:

http://www.workshopoftheworld.com/west_phila/el.html

Some of the changes:

http://world.nycsubway.org/us/phila/market-frankford.html

Instead of having pay windows on each side, we put bridges in over the tracks, with stairs and elevators.

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

 

[kelowna]

I get it now.

Increased maintenance on older structures is a fact, and an economic analysis can be made comparing increased maintenance costs v.s. replacement cost, but that will be fairly unique for each structure. I think the main point is, again, assessing the structure, determining in which conditions it is, and seeing if it is adequate or not for the loading present using the current design code.

Past designs for steel could be very conservative and it might surprise you. Besides, even new structures have to be maintained!

Michael, beautiful job!

 

[Ron]

VAStrEng...In order to assess and older structure, a detailed inspection should be made. As others have noted, the designs were usually conservative, but depending on maitenance, the structure might or might not be in good condition. Further, while there is usually not a requirement to bring existing structures up to current code, if it has such life safety implications and is a critical structure or a part of a critical structure, then analysis to current code should be considered and offered.

Paint coatings hide a lot, but also reveal a lot. Past corrosion with some loss of section can be readily seen by the highlighted texture through the paint. If the paint is peeling or flaking, there's a possibility of exfoliation from corrosion.

Life spans of well-designed and well-built concrete and steel structures are generally indefinite, depending on maintenance and maintainability.

Historical performance of a structure is a key indicator of it's expected useful life. If it has performed well for 60 years, and continues to be reasonably maintained, it will likely be serviceable until obsolete.

I would approach the evaluation in this manner:

1. Do a detailed field inspection of the structure. Not the lawyer-preferred "observation" but a true inspection. Check member thicknesses, section loss, bolt conditions, weld conditions, and particularly note areas of deterioration that might correlate with other features (lack of drainage, contact with dissimilar materials, etc.). Look for subtle deformation or rotation, or connection movement. Look at past layers of paint to see if movement has occurred since the painting...this can give a movement history if they know when the painting occurred.

2. Do a detailed drawing of the present condition.

3. Offer to analyze to existing code. What out for the double edge sword of this one. What do you do if it doesn't meet code? What do they do if it doesn't meet code? If it is a critical structure, they'd be foolish not to bring it to existing code, just because they would have documentation showing that deficiency. That document then becomes a liability to them if they don't act on it.

4. Stay away from offering finite estimates of life, since they probably won't be accurate. Give an "opinion of remaining useful life". Remember that "estimates" carry a higher duty of accuracy than do "opinions".

Good luck.

 

[VAStrEngr]

Great information, thank you.

Unfortunately, I am facing the same challenges with this project as with other Structural projects that come through my company. I don't work for a "structural" company and am often times very misunderstood as to what is needed for our portion of the work. For instance, a large component of this job relates to the project mentioned above...yet they didn't budget for a structural to visit the site, instead they had a stormwater guy go and do the inspection from which I am supposed to base my reccomendations. While he is a very capable stormwater engineer, bringing back wide angle pictures and taking approximate measurements of one of the columns isn't quite the information that a structural guy would have instinctively known to get. Sometimes it can get very frustrating...and maybe dangerous!

I've been looking for a good opportunity to ruffle some feathers over the lack of appreciation for what it takes to do structural work and this might be the chance to do it. How can I help to evaluate the options for a replacement structure when I don't have the proper information in hand?!

Back on topic, I sincerely apprecaite everyones feedback...I know this was a somewhat vague topic but your advice is greatly appreciated.

 

[csd72]

VAStrEngr,

Over this side of the pond there is a real emphasis on the designer actually visiting the site and not relying on second hand information.

Dont accept it. It is managements fault that they did not involve the right people from the start and any financial losses resulting cannot be pinned on you. Just make sure they consult with you in future.

I know where you are coming from as I am the only person of Structural or Civil Engineering background in my company and I am still training them into consulting me about these things.

 

[kelowna]

Wouldn't it be like asking your doctor to assess your health by e-mail from a couple of pictures and the opinion of the neighbor that did visit you?

Crazy!

 

[csd72]

Kelowna, a star for you. That is how little some people respect our profession.

Anyway, a few links regarding the original topic:

http://eurocodes.jrc.ec.europa.eu/doc/background/EUR23252EN.pdf

http://www.discountpdh.com/course/maintwater/maintenance of waterfront structures Ch-7.pdf

http://www.scribd.com/doc/19534603/...on-and-Repair-of-Hydraulic-Steel-Structures-1

http://www.sustainablebridges.net/main.php/SB3.4.pdf?fileitem=9584839

http://infoscience.epfl.ch/record/116033/files/SIE_AN_Kühn.pdf

 

[kelowna]

And a star for you csd72 too. Great reference material. It is going directly to my library.