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Which of these girder desighns would probably be the strongest?


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#1 SNIPE

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Posted 14 April 2012 - 02:14 AM

Hi,

I am busy designing a 1m cubed box that is made of lego girders, it does not have walls, floor or a roof yet.
I always thought that the X or 'triangle' shape girder pattern was strongest, (like this) - Posted Image

However in lego you cannot bind the bars of one cross to the centre, each bar would have to be behind one anothe.
The first design I created could support 112 ibs aprox and it was the one at the front in the below image.

Here are the 4 designs , I think the two at the front of the image are the most likely but I only need one type of girder so far.
Posted Image

LDD File here:
http://www.brickshel...mfr/girders.lxf

Edited by SNIPE, 14 April 2012 - 01:34 PM.
Removed oversized pics


#2 Tragic Banjo

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Posted 14 April 2012 - 06:21 PM

The one in front is most likely going to give you the best ratio of load to number of bricks used. Since structural members carry the bulk of their loads along the top (compression) and bottom (tension), you'll want to focus most of the strength there and just connect them as efficiently as possible. While the cross-bracing works well for steel girders, plastic bricks are a whole different animal - my gut tells me you'll want to play to their strengths by only using 90-degree connections rather than trying to mimic the angles of a steel truss. Finally, I'd also suggest bulking up the connections in the bottom member, since stacks of bricks are going to be weaker in tension than in compression.

(And this is mostly theoretical speculation, of course - my structural engineering classes taught us how to work with wood, steel, and reinforced concrete, but for some reason they neglected ABS.  :tongue: )
Posted Image

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#3 SNIPE

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Posted 14 April 2012 - 07:15 PM

Thanks for the reply,
I was wondering about the different types of tention too and as you pointed out, I don't want to use loads of bricks just for each support in the girder, here are some strength test videos I carried out , they is a different design for each video, yes it is an odd way to test them but they were incredibly strong for lego, it felt kind of 'weird' standing on a hollow lego box (the part I was standing on was only 2 studs thick).

The designs have mostly 2x4 bricks and some 2x8 bricks too.

Which dimensions do you think the girder should be?, the current ones are regtangular and are 10x12 bricks (8x10 diameter hole as the wall is 2 studs thick all the way round)

#4 Tragic Banjo

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Posted 15 April 2012 - 05:35 AM

If the current dimensions work, you might as well stick with them, though you could probably play around with the spacing of the vertical supports depending on how you want the load distributed. Again, I'm just projecting what I know about steel construction so I don't know how well it translates to LEGO, but try making the vertical members more closely spaced towards the ends to get a more efficient distribution.

Edited by Tragic Banjo, 15 April 2012 - 05:35 AM.

Posted Image

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