structural support for a raised platform
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Posted by: smd79 ® 07/29/2004, 08:55:23 Author Profile Mail author Edit

Hi, I have designed a safety walkway platform and have a question regarding the integrity of the column support I need. The platform is 13' high and supported from the ground. The platform itself is 30"x30" so these support legs aren't really far apart, and I am concerned about the platform tipping over and/or buckling. What can i do to prevent these failures from occuring? I have set the 4 legs to have a 2"x2" tubular cross section, is this appropriate. How does one go about determining the leg support requirements? How does adding diagonal cross beams, and horizontal beams address these issues? Any input or advice is greatly appreciated Thank you Shawn Din

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Re: structural support for a raised platform
Re: structural support for a raised platform -- smd79	Post Reply	Top of thread	Forum

Posted by: smd79 ® 08/04/2004, 10:06:50 Author Profile Mail author Edit

Hi and thanks everyone, yeah I will be adding some horizontal beams for structural support. I have generated a table with all the critical load/stress values for the different 2x2 leg thicknesses (0.065" - 0.25"), and it ranges from 10.4 to 12.4 MPa. Now this is for each column, therefore im assuming with 4 columns the structure is even stronger. Furthermore, I calculated the stress on a column if I applied a 1000kg load (which is 10 times what i expect to actually be applied). Now as long as the critical stress I calculated is greater than the applied stress I should be safe, correct? The legs would be stainless steel, and used a value of 200 GPa for the modulus of elasticity. here are the values for a 2x2 - 0.25" thick square tube    critical load = 11700 N    critical stress = 10.4 MPa    applied stress of 1000kg = 8.7 MPa do these numbers make sense? Any input is very much appreciated. Thank you once again Shawn. Modified by smd79 at Wed, Aug 04, 2004, 10:15:44

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Re: structural support for a raised platform
Re: Re: structural support for a raised platform -- smd79	Post Reply	Top of thread	Forum

Posted by: angelos ® 08/05/2004, 16:43:17 Author Profile Mail author Edit

It is sure that your structure will not break down due to overstress . You shouldn't even think to make a walkway platform with applied stress  in the  same rank  with critical stress. (i  mean that applied stress must be far far smaller than critical , 0-5 % i suggest ). Your platform has to  be stable. This will only occur if you put rigidly the columns in a proper base. And the horizontal beams will not help at all. Imagine this-- You will have a strong box [12' x 30'' x 30''] but  nothing will prevent this box to overturn. Finally you have to calculate the stess that will occur due to the bend .-- the lenght of the columns is bigger than the critical so the columns are more likely to break down due to bend than overstress.

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Re: structural support for a raised platform
Re: structural support for a raised platform -- smd79	Post Reply	Top of thread	Forum

Posted by: angelos ® 08/02/2004, 12:24:32 Author Profile Mail author Edit

If the legs are rigidly supported from the ground (ex. concrete base,1.5' depth ) then your platform is very  safe and stable. If not I believe that  the only reasonable solution to your problem, is to construct a proper base (made of concrete). If you try to add diagonial or horizontial beams you will only make the whole structure more expensive .

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Re: structural support for a raised platform
Re: structural support for a raised platform -- smd79	Post Reply	Top of thread	Forum

Posted by: acroduster1 ® 07/30/2004, 15:30:00 Author Profile Mail author Edit

If you use X bracing, no larger size columns or special base plate detailing would be needed.  They're not so good standing on their own, but even if you just brace them to the middle (one brace point half way up), they'd be good for around 24,000 lbs of axial load EACH (assuming they are 1/4" wall thickness and made of 36ksi steel).  Bracing them only in the dead center would look funny, though, so I recommend going with 4 "bays" of bracing, breaking the column into 4 equal parts of the same length.  By the way, in this application, you don't need X bracing.  You can use single braces going one way diagonally then horizontally across to the next one forming a zig-zag shape.  If they're designed correctly you'll save material and fit up costs. Acro

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Re: structural support for a raised platform
Re: structural support for a raised platform -- smd79	Post Reply	Top of thread	Forum

Posted by: Cragyon ® 07/29/2004, 10:24:13 Author Profile Mail author Edit

Well, to calculate the "buckling load" on the vertical support visit the following page /column_buckling/column_ideal.htm . In general, it is a lot easier to simply over design the structure.  I would add a couple of  "X" cross braces between all of the legs as well as double the size of the vertical supports.  You can simply weld another  2" x 2" tubular cross section parallel and onto the current supporting legs. Better be safe than sorry.

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Re: structural support for a raised platform
Re: Re: structural support for a raised platform -- Cragyon	Post Reply	Top of thread	Forum

Posted by: brewnog ® 07/29/2004, 11:47:43 Author Profile Mail author Edit

Over-engineering is definitely the way forward, for the minimal extra expense involved. Make sure the bottoms of the columns are fixed very rigidly to the floor somehow, (fabricating a proper base might help) as you may then be able to model these as rigid (rather than pin-jointed) supports, and be even more confident of your calculations!

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