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Energy transfered on impact by a crumbling objectHelpful Member! 

Biss73 (Mechanical)
23 Feb 12 11:15
Hi,
I'm working on the demolition of a stack and we plan on cutting the stack in sections and removing those one at a time with a crane. The problem I have is that inside this stack there is refractory material and we need to block the bottom of the section with a plate to avoid having chunk of refractory falling on people's head.
I'm the one sizing this plate and I'm having a hard time figuring out how.  I did the energy conservation equations with potential energy and spring energy (E=mgh=0,5*k*x^2) but I think it is way too conservative.  This approach does not take into account the fact that the piece will crumble on impact and not all the energy will be transfered on the plate.

Does anyone have any insight?

Thanks for your collaboration,
Biss
MintJulep (Mechanical)
23 Feb 12 11:43
If it doesn't all go to the plate, then where do you think it goes?
rb1957 (Aerospace)
23 Feb 12 11:58
crumbling would extend the time of the impact, by making a series of smaller impacts.

not sure how you could analyze th eplate ... how stiff is the plate ? (infinitely = rigid impact)  i think it's a very complex calc ... how large a piece of liner ? does the impact energy cause it to fracture (as you surmise) ? how long does the impact last ? (the time to convert the KE into an impact force) how does the plate respond ? (deflects, absorbs strain energy, ...)

1) put in the thickest plate you think you can; or
2) clear out the base of the stack when you're removing segments.
3) test a plate ... rig it up like it's in the stack, drop a piece of liner on it, see what happens !?
MiketheEngineer (Structural)
23 Feb 12 13:22
This has been discusses MANY times on these forums.  Search them.

Basically it boils down to how fast/slow the collision takes place which defines your de-acceleration/time.  Then it is straight forward.

But if it were so simple why do Ford, Chevy, BMW, etc wrecks so many cars a year to figure collision problems.  BECAUSE it is unknown!!

Only testing will completely satisfy your questions!
Wrenchbender (Mechanical)
23 Feb 12 14:39
Not knowing any details about stack demolition, i can only offer the following for a mitigating approach:
a. Put up barricades so no one's head is in the hazard zone?
b. Pre-demonish the bricks in a controlled manner in each section before lift-off?
c. Install airbags or a network or cribbing to temporarily hold bricks in place?

hydtools (Mechanical)
23 Feb 12 15:31
What would OSHA do?
People directly below a demolition?
Can you imagine the noise underneath the plate?
Put a layer of sand on top of the plate to absorb the impact of the falling pieces.

Ted

Biss73 (Mechanical)
24 Feb 12 7:52
Thank you all for your input.

Do not worry the area will be secured during the demolition.  It is the other equipments surrounding the stack we want to protect.

Biss
dvd (Mechanical)
24 Feb 12 23:23
As I read it the stack is being disassembled in sections and the refractory could fall out of the section being lowered.  

I am curious what the stack diameter is?  

Also, how tall is the stack?

How tall will the sections of stack be cut?

I assume that the stack cannot be tilted over and lowered to the ground like it may have been erected?

Is the refractory stuck on weld pins and only some of it will fall?

Do you think that all of the refractory will fall at once?

If it did all fall at once, is the refractory volume physically able to move inside of the stack?

It seems to me that if all of the refractory fell the energy would not be delivered instantaneously.  The problem is convincing yourself and the plant owner that your assumptions are correct.  

The simple thing might be to ask your insurance carrier how to do this, since they will be paying off if there is damage.
IRstuff (Aerospace)
25 Feb 12 0:27
I don't understand why you'd have people milling about under the shadow of a demo project.  Seems to be highly unsafe.  

Even if there were no chance of refractories falling, the shaft section itself could be structurally weakened and break apart hanging from the crane.  How would you attach this plate to the section without weakening it?  You either have to drill holes for fasteners or hang the plate with hooks on the top rim.  All the excess weight is bad for a material that is used primarily in compression and has little tensile strength.  You'd almost need two plates, one on the bottom and one on the top, and the plates would be held together to keep the stack section in compression.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

cloa (Petroleum)
26 Feb 12 18:05
Posters please note this OP statement

"Do not worry the area will be secured during the demolition.  It is the other equipments surrounding the stack we want to protect."

Despite the original misleading description, no more general OHSA worries.
:)
rb1957 (Aerospace)
26 Feb 12 22:09
"The simple thing might be to ask your insurance carrier how to do this" ... would they answer this (without a consulting fee) ?  don't most insurance c/- either disappear when it comes to parting with their hard-earned fees and squirm like a turkish wrestler to avoid paying ?  why would they accept the responsibility for the demo ??

some intersting posts about how will you get the plate in place without seriously weakening the structure ...
Biss73 (Mechanical)
27 Feb 12 20:15
Hi,
Here are some answers to your questions:
__

dvd:
Also, how tall is the stack? diameter of 4' by 200ft height

How tall will the sections of stack be cut? Sections of 50, 60 and 90'

I assume that the stack cannot be tilted over and lowered to the ground like it may have been erected? Yep, pretty crowded around it(by equipment, not workers)

Is the refractory stuck on weld pins and only some of it will fall? No, wire mesh in refractory attached to shell with nelson stud

Do you think that all of the refractory will fall at once? No

If it did all fall at once, is the refractory volume physically able to move inside of the stack? No
__

rb1957:
how will you get the plate in place without seriously weakening the structure?  1- Support with a crane the top of the stack 2-Cut the bottom of the top section 3- Lift the top section 4- Insert the plate where the cut was made 5- Lower the section on the plate 6- Weld 7- Lift and put down somewhere else
__
IRStuff:

The shell of the stack is in carbon steel with 3in refractory inside.  We inspected the shell for the remaining thicknesses and made our calculations with those. We checked for compression, tension and shear in the steel and everything is under 0,9 times the yield with an impact factor of 2.

I hope I answered your questions and thank you all for your collaboration,
Biss
Helpful Member!  dhengr (Structural)
1 Mar 12 23:31
Biss73:
Why not a 3' - 10" dia. work platform hung from the top of the stack and able to be lowered about 50'?  You may want to weld a stiffening and lifting ring around the outside top of the stack, and to be able to work from an outside platform at that elevation too.  How big is the clean-out and maintenance door at the bottom of the stack, large enough to remove small rubble?  Working from the inside, with a small pneumatic chipping hammer, break the refractory into brick size pieces which fall down a chute to the base of the stack, from which point they can be removed.  The work platform has a railing system, it is a chord of the circle from 10 to 2 o'clock, and the chute is under that area, which is the working area from about 11 to 1 o'clock.  The platform has a grating floor from 2 to 10 o'clock for the worker to stand on and to allow ventilation to flow vertically.  The debris chutes can be rented and some of them have a means which causes heavy masonry debris, etc. to tumble down short inclines, back and forth, rather than falling the full height.  This whole arrangement can rotate on a circular sub-frame which is attached to the support cables hooked off at the top.  This sub-frame has 3 or 4 spring loaded wheels that ride against the inside of the stack below the platform, and also prevent it from rotating when the work level is rotated.  

When you get down to 50', maybe only 20', having removed the refractory (brick?), bolt your work platform off to steel stack, and unhook and lower the cables.  Cut the stack from the inside and lift it off with a much lighter crane.  Weld a new top stiffener ring in place and start over again.  The crane has to be able to pick from above the 200' elevation, it has to have sufficient boom to do that in any case.  So, cutting lighter, shorter, lengths might mean a lighter, less expensive, crane, which probably has to be on site for the duration of the project in any case.  You work out the details.
 
MiketheEngineer (Structural)
2 Mar 12 10:36
OSHA says either completely protect the area - how you do that is your problem - OR cordon off the area so nobody can possible get hit.  I like the latter idea.

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