Author Topic: Help needed in load design  (Read 1281 times)

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1114
  • breaking knots is fun
Help needed in load design
« on: March 12, 2018, 12:53:43 pm »
I am looking at designing a gate to a cave and would like some advice on strength.  I am considering using 3mm thick steel sheet with 10mm dia holes on a 15mm triangular pitch which gives 40% voidage.  The question I have is what is the maximum distance I can have between flat bar supports to permit the sheet to withstand a pseudo point load of 200kg?  (That is a stag stepping on the near horizontal gate; fortunately I don’t have to worry about cattle.)  And, of course, what would be the minimum dimension of the flat bar to provide that support?

Many thanks in anticipation

Online PeteHall

  • Global Moderator
  • forum star
  • *****
  • Posts: 501
Re: Help needed in load design
« Reply #1 on: March 12, 2018, 01:16:15 pm »
Do you have any other dimensions Bob?

A sketch of what you are proposing would help.
The distance between stupidity and genius is measured only by success.

Offline Wayland Smith

  • stalker
  • ***
  • Posts: 258
Re: Help needed in load design
« Reply #2 on: March 12, 2018, 01:20:31 pm »
Victorian engineering.
Design it so it looks right, then add 50%  ;D

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1114
  • breaking knots is fun
Re: Help needed in load design
« Reply #3 on: March 12, 2018, 02:21:40 pm »
The location is Rana in Assynt but I don't have an image of the entrance.  In simple terms it is an off vertical triangular opening which would be covered by an isosceles triangle of some 2m side.  Two of the edges would be supported on angle iron but I am unsure about the third as it would lie on the soil rather than rock.  In my mind I had simplified the problem to a horizontal square supported on its 4 edges and placing the load at the centre point so the question transforms to how large is the square?

Offline Ed

  • player
  • **
  • Posts: 91
Re: Help needed in load design
« Reply #4 on: March 12, 2018, 03:18:06 pm »
need fine mesh / gaps to keep the pesky midges out of the entrance pitch of Rana

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1114
  • breaking knots is fun
Re: Help needed in load design
« Reply #5 on: March 12, 2018, 05:21:02 pm »
Sorry our spec only wants to keep out deer and sheep.  We do intend to allow bats in.

Offline AR

  • Black shadow
  • junky
  • ****
  • Posts: 950
  • PDMHS, ATAC, ANHMS
Re: Help needed in load design
« Reply #6 on: March 12, 2018, 09:02:58 pm »
Do you need to use flat bar for the plate supports or could you use angle? That would significantly increase the strength without upping the weight; if I was building a shaft lid to withstand "half a cow" of weight I'd be looking to use angle iron to support the plate.
Dirty old mines need love too....

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1114
  • breaking knots is fun
Re: Help needed in load design
« Reply #7 on: March 12, 2018, 09:49:04 pm »
Do you need to use flat bar for the plate supports or could you use angle?
Fair point though I was hoping a calculation would show I only needed flat bar of a few mm thick.  It all has to be carried about 3km up a valley.

Offline Maj

  • junky
  • ****
  • Posts: 766
  • MNRC, ATLAS, CPC.
Re: Help needed in load design
« Reply #8 on: March 13, 2018, 04:33:07 pm »
   The question I have is what is the maximum distance I can have between flat bar supports to permit the sheet to withstand a pseudo point load of 200kg? -  That is a stag stepping on the near horizontal gate; 

Asian or European ?



Your King - Aka Maj. 
Confucius say "War does not determine who is right, war determine who is left."

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1114
  • breaking knots is fun
Re: Help needed in load design
« Reply #9 on: March 13, 2018, 05:56:34 pm »
Asian or European ?
I am not aware of an influx of Asian stags into the UK.  But I see that an auction for a hunt for one raised $19,000!  That would be a boost for the region.

Offline marysboy

  • obsessive maniac
  • ***
  • Posts: 308
Re: Help needed in load design
« Reply #10 on: March 15, 2018, 01:28:57 pm »
Do you need to use flat bar for the plate supports or could you use angle?
Fair point though I was hoping a calculation would show I only needed flat bar of a few mm thick.  It all has to be carried about 3km up a valley.
The 3mm plate alone will be a heavy carry!

Plate area 1.73 m2
Volume 0.0052m3
Density 7850kg/m3
Weight 41kg
Less 40% voids = 25kg.

And that's without it being assembled with flat / L / T / square bars, hinges etc. Plus its a really awkward size for carrying. And it'll rust. And it'll be difficult to open from inside (or outside?)

Lots of issues here. Suggest collect some photos of the location, and let us know why you picked the voided plate over (say) a grille. Is 200kg the static weight of the stag (and one foot only?). Must the gate be disassembled to fit in a car or carried up the hill. How would it be reassembled on site. How will the gate be held in place, and how will it be opened? Instead of gating the entrance, can it be fenced?

From your description the situation could be reduced to a simple described as a simple beam problem - there are many calculators online that can help here, and people on the forum will no doubt help too. But i think as others have hinted above there are many unknowns and to safeguard against that it would be prudent to add a large factor of safety.

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1114
  • breaking knots is fun
Re: Help needed in load design
« Reply #11 on: March 15, 2018, 02:18:22 pm »
You are correct in identifying many possible issues.  However we think (hope) we have a handle on those so far identified bar the question I asked.  Whilst a grill would be simpler, there is a concern that unless we go to very small spacing, animals could get trapped.  We also are unsure about whether we should be trying to stop drafting, given the dig opened up a new draft route.  200kg is the cited weight for a stag and my simplifying assumption is it’s weight would all come down through one foot as an allowance for the energy of it moving.  My thought was the plate would probably take a three person carry and other trips for other gear which would be put together on site using nuts and bolts. (There is no current plan to lock the gate.)  Detail on the door are yet to be thought about.  Staggs are well known for jumping fences so a gate seems better. 

So my pressing question is how much force is required to dent a plate?  I don't see how the problem can be simplified down to a beam set up.  The essence of the problem is how much force can be put onto a thin sheet supported around its four edges assuming a rectangular support structure to flex it but not dent it.  That fundamentally is a plate bending problem.  (And please don’t confuse it with a related problem of sheet metal bending.)  I am unclear how one can simplify from that to a beam where the support is at two ends.  Plate bending seems to be only covered by costly commercial software which is why I was hoping someone might just have access to such programs and be able to use it.


Offline marysboy

  • obsessive maniac
  • ***
  • Posts: 308
Re: Help needed in load design
« Reply #12 on: March 15, 2018, 04:23:25 pm »
Okay I see now your question is more about the issue of the hoof deforming your plate locally, between supports. (I originally took your question to relate to the strength of the sheet/gate as a whole as i assumed the under plate supports would be very close).

Plates can be modelled as very thin, broad beams if you assume they span in one direction so this is still do-able. (e.g. beam of length 200, width 200, depth 3). Bending moment beam theory would allow you to find the maximum distance between supports if you calculate / decide a maximum allowable moment (based on tensile strength with safety factors?) From the tone of your post i guess you're happy doing beam calcs. You would have to make a decision on the effect of voiding though it sounds like you're not sure it will be included anway.

It is a long time since i did any plate analysis so i can't offer more on that subject, but i would be interested to see how you solve the problem.

Offline marysboy

  • obsessive maniac
  • ***
  • Posts: 308
Re: Help needed in load design
« Reply #13 on: March 15, 2018, 04:29:39 pm »
Found some pics online that might help

http://www.darkanddeep.co.uk/caving_scotland.asp









Offline mikem

  • forum hero
  • *****
  • Posts: 2099
  • Mendip Caving Group
Re: Help needed in load design
« Reply #14 on: March 15, 2018, 04:44:01 pm »
Asian or European ?
I am not aware of an influx of Asian stags into the UK.  But I see that an auction for a hunt for one raised $19,000!  That would be a boost for the region.

What about Sika (although they are smaller than Reds; or is Maj after cave nesting Swallows)?  :tease:
https://www.bds.org.uk/index.php/documents/deer-species/8-sika-deer-poster/file

Mike
« Last Edit: March 15, 2018, 04:58:51 pm by mikem »

Offline mikem

  • forum hero
  • *****
  • Posts: 2099
  • Mendip Caving Group
Re: Help needed in load design
« Reply #15 on: March 15, 2018, 05:09:20 pm »
A possibly more useful photo of the entrance & current gate:



& this might help (or it might not):
http://www.onlinestructuraldesign.com/calcs/Fixed_beam_metric/Fixed_beam_metric.htm

Mike
« Last Edit: March 15, 2018, 05:35:53 pm by mikem »

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1114
  • breaking knots is fun
Re: Help needed in load design
« Reply #16 on: March 15, 2018, 09:23:09 pm »
many thanks to Marysboy who has PM’d me with some calculations.  Whilst they suggest I can’t use 3mm sheet with holes, I do at least now have a firm handle on looking at the problem.  I have also found a piece of coding which claims to calculate plate bending but that is being difficult to run.

Mikem's image also shows one of the major problems.  The edge of the shaft is currently held back by scafold bar and mesh.  Memory is dim but I fear that edge does not become solid rock for some considerable depth.

Offline The Old Ruminator

  • forum hero
  • *****
  • Posts: 2838
Re: Help needed in load design
« Reply #17 on: March 16, 2018, 06:23:54 pm »
In a horizontal setting surely the answer is to have a raised lid. The farmer was happy with this one.



A bit of shrinkage since construction.



There are often cows in the field so this is ideal.

Online Mark Wright

  • forum star
  • ****
  • Posts: 657
  • NCC, BBPC
Re: Help needed in load design
« Reply #18 on: March 16, 2018, 06:50:28 pm »
I particularly like the way it blends beautifully into the landscape.

Mark

Offline mikem

  • forum hero
  • *****
  • Posts: 2099
  • Mendip Caving Group
Re: Help needed in load design
« Reply #19 on: March 16, 2018, 08:26:56 pm »
Mikem's image also shows one of the major problems.  The edge of the shaft is currently held back by scafold bar and mesh.  Memory is dim but I fear that edge does not become solid rock for some considerable depth.
It's a photo from SWCC.

Mike

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1114
  • breaking knots is fun
Re: Help needed in load design
« Reply #20 on: March 16, 2018, 10:42:43 pm »
I think I may have got a handle on predicting what thickness is required to provide a suitably thick steel cover.  But it is now almost impossible to progress further without an on-site inspection to sort out the site characteristics before making progress with the design.  I hope to organise that during the Mendip Migration.