Beam Deflection? 2

[caleb]

Hi everybody,

I have a problem, and a possible solution that I would like to hear your comments on as far as its validity.

Problem:

An existing structural steel beam(say beam 1) has been loaded by the addition of some new structural steel in order to support the weight of some substantial machinery. The new loading on beam 1 is fairly complex in nature and of course the steel drawings have dissapeared. So my problem becomes how can I determine the loading on this beam and the applicable stresses present so I may determine if even more weight (thats right my client wants more) can safely be added to beam 1. I do know how I can step through the process with FBD's and calculate the loads, deflections, stresses ect.. in this manner, however the environment is extremely hostile and again the loading is very complex. So an Idea I am considering and would appreciate your comments on is as follows:

Solution?:

Using a laser alignment tool used for aligning couplings, shafts and also for determining straightness of plates I would like to measure the existing deflection of this beam. I can easily start at a point of zero deflection, say the beams connection to its column and procede outwards toward the beams center. Now I realize this method is going to require some substantial assumptions and this is where my concerns lie. But what I would like to do is assume that at the very least this beam was made to AISC published values regarding mill tolerances of Camber. Then I would assume that the beam was placed into the structure "crown up" and all measured deflections would be added (down being positive) to the maximum allowable beam camber listed by AISC. So the measured deflection + The maximum allowable beam camber would be the value used in my calculations regarding beam streses. After obtaining this information I'm confident in my ability to perform the calculations remaining. I hope everybody understands what I'm stabbing at here, and that you might be able to offer some insight. I sure would appreciate any comments regarding this problem, and would be happy to keep you posted if your intersted on how the job turns out.

Many thanks,

Caleb J. Bell

 

[DaveVikingPE]

What kind of "hostile" environment? A refinery? In the ocean? Another planet with a poison (to earthlings) atmosphere? A volcano? Super-corrosive? A nuculear plant? Etc.

What is so complex about the loadings?

Can you get close enough to the beam/structural system and make your own as-builts? Or can you "shoot" elevations from a safe distance and make your as-builts from survey points?

If you can get close enough to use the laser alignment tool, you should be able to measure the beam and draw a pretty accurate cross-section. And maybe take a piece of the material for analysis, no?

Let's put it this way: if the beam was designed along the lines of the AISC ASD, then you shouldn't have such a hard time. ...and you're going to need the beam's cross section to back-calculate to a bending stress anyway. Sound like what you're doing is interesting, though.

 

[Watermelon]

Wouldn't do it. Too many variables and unknowns. You have no guarantee the beam was installed crown up. (I remember once while walking the steel on a 14 story hi-rise, I discovered a beam that was installed upside down, and I am not talking about camber.) I am reminded of the old adage: When I "ASSUME", I make an ASS out of U and ME.

While it MIGHT be possible to reverse-design a beam based on deflection calculations, that is not what we are supposed to do.

I'd identify the beam section, calculate the load it, add some extra weight so that I can sleep well, and then add reinforcing as required. The owner is probably hoping to get away without having to spend any money, and is probably half-expecting to do some kind of modification.

 

[dougantholz]

Caleb,
If you are going thru that much expense just to measure the beam, I think you can economically justify just modifying the beam for the additional load. I would say that the beam works now, and that you should modify the beam so your modifications carry all of the addtional stresses, so there is no net change in the existing beam stress. And as a bonus, you'll definately be able to sleep at night.

 

[caleb]

Thanks everybody for your advice, you gave me the answer that I expected. As a side note its not beyond reason for me to calculate the loadings on that beam, my supervisor and myself were trying to come up with a quicker and less calculation extensive method for analyzing beams as we are both doing structural analysis almost exclusively now.