Information on column design unity equation

[frostrobn]

I would like to know the significance of AISC ASD equation H1-3 (unity equation for compression members) and the preceding equations H1-1 and H1-2. When are these equations used and what is the significance of one over the other? I have not been able to get a real good understanding of these equations. I have read the ASD and it's commentary. I'm just looking for another interpretation. Any information anyone could share would be helpful.

Thank you.

 

[Neilmo]

Hello Frostrobn,

Suppose you have a member which is subject to both compression and bending, then the unity equation goes like this - you can work in `stresses` or `forces`, it doesn't matter.

(Actual compression / Allowable compression + Actual bending / Allowable bending) <= 1.0

I am not familiar with the standards you are quoting, but they should be similar to the above. Some times other factors are introduced, such as 0.5 x Actual bending, etc. etc.

The equation I have given is pretty general and usually gives a rather conservative approach to the use of the unity factor. Other factors are introduced for a more realistic approach and to give a greater economy of materials used, usually at the expense of more calculating power required.

If your standard gives more than one equation for the unity check, you can use whichever one suits you best, ie. the quick approach, or the more exact approach. If you have time, check them all out and then pick the one which gives the greatest economy of material. I usually just plum for the quick, conservative check as being simple and easy to use. How accurate do you want to be anyway!

Hope this helps.

Regards,
Neilmo