Steel strength measurements

Steel is so much stiffer than wood that when you make a sandwich of the two, the steel winds up taking most of the load, the wood just flexes out of the way. You can simplify your calculations by just assuming that the steel channel is taking all the load.

A second effect of the stiffness is that in wood rafters the limiting factor is usully the stiffness so you calculate that first. (Check for strength obviously but do it last) For steel, start with the strenght calc.

Net: the steel probably needs to be quite a bit bigger than you might expect. You might be better off sistering on some LVL wood beams that have been ripped to width. With the price of steel these days, the LVL has gotten a lot more attractive. This would allow you to take advantage of the strength in the wood rafter that is already there.

There are several decent strength calculators > i am strengthening some roof rafters with steel channel cupping the original

Flexing properties? I guess you want to know about what maximum deflection you will get for a particular section under a particular load. Here's a site with links to the section properties for common beams:

There's a lot more to it though. Beam deflection is afffected by the way the load is distributed, how the beam is supported, and by the direction of the load with respect to the cross section.

How critical is your need? The criticality of your need will determine how detailed an answer you need.

Hello, r.p.mcmurphy! You wrote on Sat, 6 Nov 2004 08:40:15 -0000:

rm> i am strengthening some roof rafters with steel channel cupping the rm> original rafters to increase resistance to flexing and allow removal of rm> purlins. is there anywhere on the net with measurements of various rm> flexing properties of channel etc?

rm> steve

With best regards, ahlbebuck. E-mail: snipped-for-privacy@saftour.co.za

On Sat, 6 Nov 2004 08:40:15 -0000, "r.p.mcmurphy" vaguely proposed a theory ......and in reply I say!:

remove ns from my header address to reply via email

Two programmes

beam.exe and beamby22.exe (beamboy) are very useful. B ut you will still need to work out a few things for yourself.

Also, when you say "cupping", if you mean by placing the channel under the beam, with flanges vertical, then all bets are off.

beam ------ || || || || | ------ | -------- channel

???

***************************************************** Dogs are better than people.

People are better than dogs for only one purpose. And then it's only half of ofthe people. And _then_ most of them are only ordinary anyway. And then they have a headache.........