25-foot span -- what materials to use

[matt13421342]

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.

 

[IFRs]

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?

 

[GregLocock]

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

 

[lilliput1]

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 member will twist when subjected to loading. A light gauge steel joist about 14" high should be about what is required for 1 1/4 " deflection. Now I am a mechanical engineer and I learned this from another mechanical engineer. Suprisingly not many structural engineer seem to be aware of this. I am curious to know if other structural engineers can refute or validate the above rule of thumb.

 

[lilliput1]

Say is your post 25 ft span or 20 ?. My reply is for 25' span.

 

[chicopee]

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.