182' tall 4-legged tower & 175' span 9' dia tube - built 1965 - evaluating safety

[ARS97]

I've been given the task of evaluating a conveyor tube gallery (9' dia., 175' span, 1/4" shell, approx. 182' in the air) that spans over a major highway in WV. The state is requiring the customer (coal mine facility) to have a PE evaluate the safety of the tube and it's support tower (182' high, 4 legged tower). These structures were designed & constructed in 1965 (48 years old), and time has taken it's toll.....heavy corrosion, the welded intermediate connectors of the double-angle bracing are popped apart, damaged members from various incidents over the years, etc.

There's a lot of issues to deal with here.

1) Codes and specifications have changed quite a bit since 1965 (just a tad). Even if this structure was checked in a "new" condition, my results will likely look much different than what the original design had. It has stood the test of time so far, but if you're evaluating safety, simply stating that it hasn't fallen down SO FAR isn't a good reason to say it will remain safe. If the newer codes and specifications are the engineering community's best estimate to real structural behavior and load application, then I believe that the evaluation needs to utilize those sources (such as ASCE 7-10, AISC 360-10, etc.)

2) A few years back, the customer contracted a PE who performed this same inspection. To be blunt, this PE, who is in his early 70's, has a reputation to stamp anything for the right price. His report included an overly simplified 10 page long design calculation that supposedly encompassed the structural analysis of the structures I mentioned above. Many of his methods are UNDER-conservative and seem more like a preliminary feasibility study rather than a robust analysis. His conclusion was that the structure was safe, with a few small exceptions for which he included reinforcement provisions (which the customer did not implement). He didn't even check anything with the 182' tall tower, which is shocking.

3) In my opinion, due to the use and location, this structure should be classified as a Risk 4 structure (ASCE 7-10). For one, it spans over a major highway (WV Rt 2) and would likely result in loss of life if a collapse was to happen. Also, this is the sole conveyor system for this company's coal production, so a collapse would result in substantial financial impact due to significant down time. I might consider a Risk 3 category, but no lower.

I guess my general question is this - how would you approach this situation? My report needs to satisfy ME.......I can't worry about what some other PE did or didn't do in the last report. However, in regards to evaluating older structures.....does my reasoning listed above make sense?

 

[JedClampett]

I think you know the answeer. Research the codes in place at the time of design (so you know where you stand), note the differences and evaluate for all current codes. If it's not safe, it needs to be fixed. It might have to be taken out of service or put in reduced service.
Saying that, I think the problems might not be all that grim. It's not like dead and live load and wind were not known about in the 1960's. They might of used smaller values or maybe for something this massive, the design was conservative.
One last thing. The owner is likely to have some idea that bad news is coming. They know it's a 50 year old structure and was designed a long time ago.

 

[SlideRuleEra]

I appreciate your problem and will keep my comments general:

Keep in mind that industrial structures often, and most likely were not required to meet building codes. This does not mean that good engineering practice and standards were not followed (AISC for steel design comes to mind). Bottom line, keep and open mind on whether design details comply with building code requirements (either 1965 or now).

When evaluating steel members base your calculations of existing dimensions of "good" (uncorroded) steel, not on the nominal size of members. This may require getting a testing lab to make detailed measurements at critical location. This comment also applies to the material properties of the steel (perhaps A36).

Take lots of detailed photographs and make use of them in your report, along with a non-technical explanation of what they show.

Get the company's opinion of the importance of the structure to them before you make the final decision of the risk category. As much as possible keep the company involved in this type of input to your report. This will give you an idea of how they "like" the direction your report is headed. There is no point in you giving them a report that is ignored nor is there a reason for you to make unnecessary assumptions. IMHO, better to walk away from this job if the company won't "listen".

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[paddingtongreen]

If your PE oath is similar to Pennsylvania's you must not comment on the other PE's work unless he has been so informed by the client. Better to ignore his work entirely. Are you legally required to work to the latest codes or is the original code grandfathered in? Does your client know this. If the original code is grandfathered in, does your agreement allow you to use the modern code if more conservative? Since no work was done on the previous recommendation consider that the client may be politically connected.

Negatives, I know, but they are based on a lifetime of work. All businesses need work but make sure that the work you get is the work you need.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[ARS97]

JedClampett - I have a preliminary analysis completed. The results are pretty grim actually. There's a few main reasons though:
1) I generally like to consider ice load on trussed towers. I feel it's appropriate due to the nature of the structure and actually can be a significant factor in design of those types of structures. I'm fairly confident that the original design did not include this load (see #2).
2) In order for some of the members within the tower frame (mainly the diagonal bracing) to be anywhere near code compliant, tension-only assumptions must be made. I have no problem with that usually, but for ice-sensitive structures, I don't like it. Tension-only doesn't allow for the effect of bending to be accounted for.
3) P-delta effects.....the oriignal design was likely a first-order analysis. Something of this size has some decent P-delta effects.

SlideRuleEra - the pointers regarding documentation of the field measurements and observations are noted. We'll likely get the steel tested as well. It's likely that the tube is CORTEN (50 ksi) and the structural steel is A36. As far as the expectations of the report - they know it's coming. We have a good relationship with this customer and they trust us to do what we feel is right. Now whether they follow the recommndations or not is another story.......

paddingtongreen - I'm licensed in PA, MD & WV (PA is my original license). I know the problem with commenting on another PE's work. I wasn't going to bring up to the customer, but I'm sure that the customer will compare the report that they received a few years back with the report that I'll be providing. It's bound to come up, but the bottom line is that my report is what I feel comfortable with. In regards to the codes....I'm not sure. I don't think the customer cares honestly.....there's no specifications. We're dealing with a customer who we do a lot of work for and they just trust us to do a good job. They're looking to us for what is the best method. I don't really like the idea of using outdated codes though.....there's a reason why they've changed over the years (besides the superficial reasons).

 

[paddingtongreen]

Andy, I don't want to highjack your thread but although code updates were needed, a high proportion of the designs we did then are still working. We had a built in redundancy because we could only used primary load paths.

No reply to this required, back on track.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin