A backbone?
You mean something like this?: http://www.odt.co.nz/your-town/dunedin/14440/moko-symbol-inner-strength
I think that the Chinese symbol for serenity is a good idea. I am getting it on my wrist next month.
there is brute strength and there is spiritual strength. I would say the latter is non-violence in Buddhism, and therefore all buddhist symbols represent strength. Further than that you would have to look into Hindu, or Vedic, symbolism I'm afraid.
Using a rose to represent something in the poets inner world would be an example of symbolism.
As a symbol, a screw would represent technological sophistication.
You think probable to the chemical symbol of an element.
poridge poridge
In the Celtic tradition it would depend on the individual you asked. Here are some tattoo or craft ideas a Celtic individual might consider when asking oneself this question. 1.) Zoomorphic figures are often used to symbolize an animal which has given them knowledge or strength. 2.) Coats of arms also show strength... being involved in a Clan. 3.) One can use Ogham letters to spell out the word strength. Even draw knotwork around it or whatever you want. 4.) There is a symbol (with symbol meaning) for "inner strength" displayed at celtic-tattoo.net
What symbol would represent a chlorine ion that has ionized to have a stable electron configuration?
Rectangles
A symbol to represent happiness would be bluebirds (happy birds...),flowers,a beautiful scene, smiley face, rainbow...........Anything that makes you happy.....
I think you are looking for a symbol not a word. That symbol would be the bald eagle.