Fire Damaged Warped Frame Steel Beams 1

[TFreeman]

We are currently looking at a steel framed building which was subjected to a fire some 30 years ago. The current state of the building has several frames in the middle of the building severly warped and the frames misaligned by as much as 7 inches at the peak. The building is ~80ft by 200 ft with clear span frames at approx. 20 ft on center.

Question is how to determine current structural stability?
Also are there any relevant references for this problem?
Looking for some insight into how to address the warped section in calculations in order to determine the capacity of a frame.

The other question is how much warping is too much? The current condition is well outside mill tolerances for steel sections but we don't have any other reference to determine what rotations, etc. constitute a dangerous condition?

For the record this building is a process building at a gas processing facility. Contains several vessels, treaters, pumps, etc. The site people only recently brought the warped beams to our attention but asking around it seems as if everyone says it has always been that way.

My gut reaction says the frames are in bad shape and should be repaired. However, one needs something more substantial to convince those with money to solve the problem.

 

[Wopshistos]

Before anyone here suggest FEM again you can solve this one by simple statics and take resultants that are unwanted or extrordinary to a new bracing set. Since the building has been up this long I would assume the damage was only minor. If not, you should check the metallurgy - microscopically at first and then tensile tests to see that material properties are similar to those assumed in design.
Finally if your gut feeling is that the building is wrong and should be demolished, say so. The money isnt your problem only the responsibility for continued use.

 

[ishvaaag]

Well, wop is right that with proper bracing maybe the thing will stand, if ugly thing (distorted welds to weld and bolt against).

If the steel is amnable to straightening, it could be feasible, and maybe even worth if only for some members, or outright substitution. There are specialty firms doing straightenig jobs usually for bridge but I think if for more than a scarce number of repairs too dear and so substitution be preferable.

On the FEM side, P-Delta with initial imperfections should help you significantly to ascertain the strength at the factored level. It is simply a matter of properly include the initial status in the model, use short segments to through the P-Delta capture the in-member status, and then use either K=1 and strength of materials approach or K from non sway and chapter H of LRFD equations. Ensure in the process you use stiffnesses consistent with the final status at your factored loads' level.

Respect buckling for a given level of initial out of alignment imperfection, LRFD does not contemplate but the industrial standards, but the Eurocode formulation gives a limit stress for any initial out of alignment imperfection. Not even this will be required if you follow the process above and model your structure with segments short enough.

And again the observation on the actual status of the steel is relevant, for surely regularization loads of concrete will be intervening etc, so don't forfeit the chemical analysis and mechanical testing of what standing that you pan to reuse. You may finally subject your structure to tests of service level loads in the affected zones, what will get the owner satisfied but this is of no help at the design stage.

 

[TFreeman]

Just to further the discussion:

The frames are welded, tapered "I" shape with 6" wide flanges and varying depth.

In the worst locations the flanges are rotated as much as 15 degrees, as well the web is bent/warped to 10 to 15 degrees from vertical.

The misalignment is as much as 3 inches from edge of top flange to edge of bottom flange.
With deformations such as this repair or reinforcement seems very difficult if not impractical.

 

[Ron]

TFreeman...my colleagues touched on the major issue but it bears repeating....
If you have this much deformation, the temperature during the fire has compromised the metallurgy and has yielded the steel in a plastic state. Sometimes this is OK, but more often it changes the mechanical properties to such a state as to be non-predictable for analytical purposes. This state alone would preclude the use of the system as a frame must be interactive through all points.

While it is true that the frame might have been standing and performing for quite some time, it is also likely that it has never experienced its design conditions. The result is as if we neglected code requirements and decided that only those conditions to which the building would be subjected on an average day would be necessary for design. NOT SO!

While the structure could be repaired, the cost of investigation of properties to an adequate extent, coupled with the difficulty of affecting appropriate repairs, would likely make replacement attractive.

 

[TFreeman]

Thanks Ron,

You have mirrored my thoughts exactly.

I have little experience with repair by heat straightening, could one of you please let me know what type of costs are inlvolved in the process? Ball park figures, cost per foot, cost per pound kind of numbers?

I would like to be able to at least speak to it if the idea is brought up.

Also, as I asked before, is there a reference available for fire or mechanically damaged steel assesment and repair? Much of the information I have found and been told seems to rely heavily on the inspectors experiences and rather broad catagorizations of damage.

Again, thank you all for your opinions. It always helps to hear your peers echo your thoughts.

 

[aurelio]

According with my studies and reading is impossible warp 7 inches in fire condition without loss their physics and chemistry property of the steel, besides the structural capacity of the building at itself change to worse. The steel is very friendly and god are gradiose but is very dangerous live or work in there. I don`t if the building are in heigh wind or earthquake zone but the best are remove and supersede each element in warp condition and retrofit other after a good study.

 

[Ron]

TFreeman...heat straightening is at least as much art as science. There are a few people in the country who are very good at it, mostly in the sense of cambering large beams or girders.

To add heat straightening to the already compromised metallurgy is just one more unpredictable variable to deal with. As an example, if we have new steel, predictable metallurgy and controlled conditions, we can camber or heat straighten a deformed section. In your case you have none of these control variables, thus the result of heat straightening would likely be unacceptable.

 

[rkchalla]

This is from memory.
API-579, a publication by the American Petroleum Institute has a chapter that deals with evaluation of fire damage. However, please note that the publication broadly covers fitness for service issues of pressure vessels and tanks and not structures per se. It might give you some pointers about metallurgical issues.

 

[oldrunner]

API-579 will cost you $579 + !