help:what is role of rigidity in the structure?

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The thickness of material is too large to decrease yield strength,but increase the rigidity of  material .so we like use thick material in weld structure. i want to why do it ? what is role of rigidity in the structure?
Reply:From an engineering perspective, both strength and rigidity are typically important.   If the structure is not strong enough, it may break or permanently bend in service.  If the structure is not rigid enough, people may perceive it as unsafe.A footbridge which is not rigid enough may be perceived as unsafe and people will be be unwilling to use it.   A skyscraper which is not rigid enough may sway too much in the wind, causing motion sickness.A flagpole, on the other hand, may not have a rigidity requirement; it may be acceptable if it bends and sways in the wind.  Aircraft wings are designed to flex when the plane hits a downdraft or updraft.A machine lathe is an example of a machine where if it is not rigid enough, it will not work well.  This is one reason that lathes are generally heavy.It all depends on the application; most applications have requirements for both strength and rigidity.Interestingly, steel is a very good material for rigidity, even if weight is important.   Surprisingly, titanium is not much more rigid per pound than steel.  Aluminum is used where weight is important, but rigidity is not.Richard Originally Posted by wqcThe thickness of material is too large to decrease yield strength,but increase the rigidity of  material .so we like use thick material in weld structure. i want to why do it ? what is role of rigidity in the structure?