25-foot span -- what materials to use

huangyhg

25-foot span -- what materials to use
after posting this question with too many details previously, i will try the basics.
i am looking for a material (angle iron, box steel, or similar) to span a 20-foot length without sagging. it will only need to support its own weight + 15% or so. weight is an issue so lighter is better.
great site, by the way.
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try an aluminum association i-beam or an aluminum i-beam made from two formed channels back-to-back.  where is the load?  what is is supporting?  what is under it?
i suggest a piece of cotton thread.
the question is meaningless, any real system that is non-active will sag.
what is the real load?
cheers
greg locock
rule of the thumb for acceptable deflection is l/240 and to get this, the beam depth must be about l/20 to l/24, where l is the span in inches. the member must be symetric about the vertical axis or the   
say is your post 25 ft span or 20 ?. my reply is for 25' span.
you will always get deflection, it is only a matter of degree.  you best bet is to have a beam arched to an amount of the expected deflection.