MadSci Network: Engineering
Query:

Re: What structural design(triangle,square,etc.) can bare the most weight?

Date: Tue Jan 1 15:07:37 2002
Posted By: Chas. Hague, Staff, Bridge Design Department, Alfred Benesch & Co, Consulting Engineers
Area of science: Engineering
ID: 1009137085.Eg
Message:

Amy

I'm pleased to find a young woman interested in Engineering.

First:  Every structure is made up of members, which support loads in two 
basic ways.  In tension  (by being pulled at) or by compression (being 
pushed on).  Tension is what happens in a rope.  The stress in
the material is simply the weight divided by the area of the rope.  Exceed 
the stress and the rope breaks.

Compression is more complex.  The load that a compression member, such as 
a post or column, can support is governed by several factors.  The longer 
a column is, the more flexible it will be, and the less it
can support.  Increasing a column’s cross-section -- Making it thicker --
gives it more stiffness, and increases the load it can carry.  
 
The members making up a structure are assembled into frames.  Here’s an 
experiment for you.  Cut strips from the cardboard back of a pad of 
paper.  Make 2 strips four inches long, two five inches, and one six
inches.  Punch holes in the strips a half inch from each end.  Now use 
brads to pin the four inch and five inch strips together into a 
rectangular frame.  Lay the frame flat on a desk, and push gently down on 
the middle of the longer strip. Notice that although the frame will stay 
rectangular if the force is applied at right angles to the strip, it will 
fold up as soon as any sideways force is applied.  

Now insert the longest strip diagonally between the corners.  The frame is 
now a pair of triangles with the two long sides adjacent.  But it has 
become more rigid.  

That's why almost all structures have triangles in their design.  
Buildings have diagonal bracing in some floors to resist the wind.  Truss 
bridges are a series of triangles that transfer the loads from mid-span to
the ends.  A geodesic dome is a ball made up of triangles connected at the 
corners.  Wind turbine towers are triangles made up of three or four legs 
in a pyramid shape.  

A triangle is the only shape that is naturally rigid.  Of course it's hard 
to live inside a triangle -- spent any time in an attic or a tent?  It's 
not too efficient with space.  Also, a pyramid-shaped frame will have to
have longer members to enclose the same space as a box.  Because they're 
longer, the compression members in the pyramid would have to have a larger 
cross-section to support the same weight than shorter, vertical columns.

 When an engineer designs a building, bridge, or whatnot, she's trying to 
place the structural elements in the optimum location where they can carry 
the loads.    So, while a frame made of triangles will bear loads
most efficiently, most structures are constructed in a pattern of columns 
and beams (with diagonal cross bracing) to make the best use of space.


Here's a building using triangles:  http://www.greatbuildings.com/cgi-">http://www.greatbuildings.com/cgi-">http://www.greatbuildings.com/cgi-
bin/gbi.cgi/John_Hancock_Center.html/cid_john_hancock_002.gbi

And here's more than you probably wanted to know about bridges: http://www.brantacan.co.uk/trus
s.htm

And a page about geodesic domes: http://www.insit
e.com.br/rodrigo/bucky/geodome.html

And here's a bridge that uses triangles in a unique way.  My family drove 
under it last Sunday: http://www.aisc.org
/msc/0007_05_oldplankroad.pdf






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