Coal Belt Conveyor Truss Deflecton

[mmettler]

I am working with a coal mine that has been in operation for approximately 4.5 years. The conveyor that originates near the mine portal runs horizonatally along the ground and then extends up to a stacking tube structure at a fairly steep incline. The conveyor was constructed of steel box trusses and steel bents. The top chords are channel sections while the remaining members are rolled angle sections of varying size. The last truss section near the top of the stacking tube spans approximately 130' horzontally with a 34' rise. The mine's engineer was able to observe visible vertical deflection of the truss immediately after the conveyor was commissioned for service. Surveyors have measured the deflection at 0.54' (~6.5") at mid-span which equates to an approximate deflection ratio of span/241. I am not familiar with servicibility limits for structures of this type (industrial/material handling)and am concerned about advising the operator appropriately. I plan on contacting the original supplier who I understand designed, erected and commissioned the conveyor in the first place. I have suspicions with regard to why the truss is deflecting but need more information to confirm my suspicions. It shall be noted this is the longest span in the series of trusses that make up the entire conveyor. It was also observed the truss in question was fabricated with larger sections than the others that have spans that are less than 100 feet.

 

[paddingtongreen]

What is your question?

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

 

[mmettler]

paddingtongreen:

My main questions are what standards and guidelines exist for servicibility (deflection) limits for trusses supporting material conveyors and is span/241 an unreasonable amount of in-service deflection? It shall be noted the deflection observed appears to be permanent.

 

[hokie66]

I think you answered your own question. Deflection is a serviceability issue. Does this amount of deflection affect the serviceability, i.e. the operation of the conveyor, in any way? If not, there is no serviceability issue. As to strength, if there is a question about that, you would have to do the analysis.

 

[mmettler]

hokie66:

Understood and for last couple of years the conveyor operates as intended. The deflection observed is bothering the mine's engineer and he is looking for peace of mind as well as any advice regarding repair or replacement. I was curios if there is a standard or guideline for deflection of these types of structures similar to the IBC's deflection limits for building structures. Thank you all for the responses thus far.

 

[miningman]

Maybe the Mine Engineer should focus on what weight loading his production personnel are imposing on the structure and compare that with the design criteria. Speaking as an experienced production guy , we are notorious for exceeding design limits. Production is the name of the game, and if the designer failed to incorporate adequate factors of safety, then that doesn't say much for the mine's representative who dealt with the design engineer. Then again , maybe the installers didn't quite tighten up everything the way they should have. Maybe they did but vibration etc has loosened things up. What I am really trying to say is someone needs to get out of their office and get up onto the structure... which undoubtedly is dirty, filthy, and probably way up in the air (poor access, poorly maintained perhaps) and do a good visual examination., rather than reviewing codes that may or may not be applicable.

 

[paddingtongreen]

This conveyor is very steep, it must be some kind of bucket. We were given 14^o as the maximum slope. The deflection makes it a little steeper and the top and less steep at the bottom. If is not overstressed and if the coal does not run back down the hill, enjoy the day.

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]

When I first read the post, I thought it said 0.5"......much different outlook since it's 0.5'!

That seems excessive.......and that condition is likely primarily dead load only with perhaps a portion of the material load. With full design loads, the deflection will be even worse. Even if the truss isn't cambered that seems like alot.

Is it possible that there's too much "play" in the bolt holes of the splices?

Perhaps this a truss that supports a vertical takeup weight for the conveyor? If so, a large concentrated load will be present.)

My experience tells me this - a conveyor truss deflecting that much under those conditions (far from worst-case design loads) is likely underdesigned. Check into it further. I typically use ASCE 7 for the applicable snow, ice, wind and seismic loads. AISC for the steel code check.

A word of caution - design efforts for conveyor trusses vary drastically depending on the engineer. I spent the better part of 1 yr investigating the numerous design shortcomings of a rather well known design/build company in the industry. After various structural failures, the owner decided to get a second opinion. Cheap designs are great for everyone involved....until something falls down.

 

[ARS97]

I wanted to add - typically these kind of trusses have a span-to-depth ratio of somewhere in the range of 10 to 20. I typically stay around 15, which gives reasonable deflection amounts when the members are sized appropriately.

 

[mmettler]

andysines

Thank you for your responses. I am still trying sort out when exactly the reported truss deflection was measured, mainly were they conveying coal on the belt at the time of the survey. When I reviewed the conveyor last Thursday, the truss deflection was visible with no coal being conveyed (the long wall miner was shut down that morning). Furthe, the mines survey crews were not available to measure the truss during my visit.

Your hypothesis on member rotation within the bolted splices is worth looking in to.

The belt tensioning assembly and belt drive are located below this truss roughly at mid-length of the conveyor; thus, the weight itself is not imparting a direct load onto the truss in question but belt tension is certainly imparting an axial load into the truss since it is at the end and on an appreciable incline. Besides the reported vertical deflection, it appears the truss is twisting slighty because of the torsional loads from the catwalk and pedestrians.

I am not certain the original designer took into account the debris (year round) and ice load that occurs on the truss during the winter months. Reportedly, the ice build-up will resemble rime ice because of the prevailing winds and coal particles that are contained within the ice. As you would imagine the operator is not real diligent in removing the debris and ice regularly. Part of this is the lack of access. There is one catwalk on approximately 3 below the top chord on one side of the truss for access along the conveyor and to the stacking tube. It was difficult for me to even gather enough measurements to complete an analysis at this time. I will be attempting to retrieve original design and shop fabrication details from the design-builder but I am not confident they will cooperate for numerous reasons including the fact they were dismissed from the project prior to finishing their work. But we shall see and I have a suspicion this section of the conveyor is not adequate. It shall be noted the other truss spans are significantly shorter with a one of them still having visible camber. I am leaning towards recommending the replacement of the defelcted truss during their next scheduled long wall move which can take up to a month to complete.

 

[ARS97]

If there are belt tension forces acting on the truss, it could be a significant cause to the problem. Typically, the only belt tension forces that box trusses are subjected to are small to medium gravity take-ups (vertical load) and small truss-mounted drives (usually in-line with truss). Large gravity take-ups and drives on the other hand, are usually supported by braced towers.

You say the truss appears to be twisted from past catwalk usage? Usually the loads on the catwalk don't reach the assumed design level. If the truss is twisted without any catwalk LL being applied, that sounds pretty sketchy.

My gut feeling is you have a poorly designed "cheap & dirty" truss design on your hands. It's not uncommon.....and don't be afraid to let the customer know that you get what you pay for. Some of these "design" companies get away with flat-out negligence until something finally goes wrong. Unfortunately, in the coal industry, design reputation is temporary....the almighty dollar usually determines the final decision.

Out of curiosity.....the original designer.....did their company name begin with a "T"?

 

[Sawbux]

My primary concern would be stability of the compression chord. You noted that it is composed of channel members. Are they adequately braced for lateral buckling? Is there a horizontal truss or just the frames of the conveyors idlers attached?

A possible fix would be to put some harped cables and tighten to remove deflection. But check to make sure top chords can take the additional axial load or anchor the cables to something other than the truss.

 

[mmettler]

There is a horizontal truss for both the top and bottom chords. Whether they are adequate would require an analysis.

 

[hokie66]

While I was not alarmed by the deflection, some of the other things would ring the bell...design-build, termination of contract, drawings not available, twisting of gallery. In addition, I would imagine that corrosion is/will be an issue.