Joel OF

Tension VS Compression bracing

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What factors influence your decision making whether to use tension or compression braces in items such as gates and sign brackets?

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Steel is somewhat poor in compression and great in tension, which is why rebar works so well inside concrete.  The concrete has great compressive properties but isn't good at stretching.  Combine the two and you get an outstanding structural material.

So as a general statement, I would always design steel in tension if possible.  An example might be an L-shaped sign bracket hung on a wall.  A support bar that forms a closed triangle would therefore hold more load if the hypotenuse was on the upper side.

However, in decorative blacksmithing, we should also consider the fasteners and other elements as potential failure points.  A large gate collared together might fail of its own weight only because the fastening method was not sufficient.  That's essentially what occurred on the Titanic - the rivets failed and started the path to destruction.

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Tension requires very little engineering, compression requires a great deal of engineering. Tension can be carried through a simple shape such as a round or flat bar, compression requires structural sections (box angle chanel,pipe, etc.) except in the case of significant oversize and short span. 

Tension is easier. 

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Function of the finished item is paramount. and the design of the component parts and their layout should reflect this

To Quote HW 

 An example might be an L-shaped sign bracket hung on a wall.  A support bar that forms a closed triangle would therefore hold more load if the hypotenuse was on the upper side.

However, in decorative blacksmithing, we should also consider the fasteners and other elements as potential failure points. 

A bracket like this may hold more load, however there would be more leverage on the upper mounting point which could prove a potential safety problem, 

Edited by John B
Expand original statement

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Engineering is indead a factor, but it realy comes down to weight. A tension brace is going to be much smaller, lighter and less visualy appearan t were as a compresian brace is going to be large, heavy and a apera t desighn eliment. Think of the diferance between a bicycle wheel and a wagon wheel. The spokes on the former hold the axle up by hanging from a thin wire wile the later holds the axle up on a thick piller of wood. A 26" bicycle wheel weighs much less, and is rated at 300# than a 26"wagon wheel. Tho the latter may be more visualy apealing.

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Just to add on a bit.  Consider how a blade of grass is pulled perfectly straight by tension.  In compression, the blade will surely bow - but where?  That being said, we need compression assemblies.  It wasn't until I was in a structural course in college that it occurred to me that a tree trunk in cross section has compression and tension. 

Diaphragms add shear planes add nuance to how these things come together.  It's remarkable how strong the right assembly of flimsy fabric on matchstick lumber can be.

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So don't think so much about compression on steel as bad.   Steel is strong in compression otherwise hammers would not work very well.   Ehh?

Think more about buckling.   Steel is fine in compression as long as it does not buckle.   Once it begins to bow now we are in to bending which is buckling.  

The shape of steel and how the load is applied have a very great deal to do with how well it will handle compression.    A round bar will begin to buckle under compression more easily than the same amount of metal shaped into an I beam shape or even a pipe or square tube.  

In engineering, which I are one, it is all about buckling.    Diameter and the distance from the centroid of the material and the direction of force applied make all the difference.  

Cross braces that prevent buckling are a big help!  That is a huge hint into compression and the prevention of buckling!   I am sure people have real life examples of avoiding buckling with cross braces that can be applied here.   Might be a balancing act but...   My two cents...

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I generally get along better with folks that aren't engineers.  

Engineers can be:

  • Too snooty sometimes.  
  • Other times not very practical.  
  • Sometimes have no experience using a tool or actually doing stuff other than calling on the phone for people to do stuff for them that they complain about not being correct.  
  • Want to talk about safety and call things out without ever actually having done what they wanna throw the safety card at!

Anyway...   I have a healthy disrespect for my profession.   

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