notional loads direct analysis method 1

[ELSARWAN]

dear experts,
I want to ask about notional loads
first in direct analysis method according to AISC360-05 or AISC360-10 I have to used notional loads which equal 0.002 from gravity loads and apply it for each story level ,but there is condition in code where the ratio of maximum second-order drift to
maximum first-order drift (both determined for LRFD load combinations or 1.6
times ASD load combinations, equal to or less than (1.7 in aisc 360-10) &(1.5 in aisc 360-05)

my question is when I add the notional load to asd combination I must multiply the combinations with 1.6 or not
thx in advance

 

[JoshPlumSE]

Yes, your notional loads would also be increased. The idea here is to amplify the second order effects (i.e. P-Delta) so that it is on the level with what you would get if you were using LRFD load combinations.

Just don't do it TWICE. Meaning, you do the following 1.6*(0.002*(DL+LL)), not 1.6*(0.002*1.6*(DL+LL)).

 

[ELSARWAN]

Joshplum, thanks a lot for your helpful reply
so what I will do in ASD combination with notional load will be as follow
(DL+LL+1.6*(0.002*(DL+LL))...is this ok or not?

also i have another question
i always use ASD in my design but i don't know which will be more economic in our design ASD or LRFD ?
thanks in advance.

 

[JoshPlumSE]

Actually, you have to factor up BOTH the gravity load and the lateral loads. So, it would be something like the following:

Gravity load = 1.6*(DL+LL)
Lateral / Notional Load = 1.6*0.002*(DL+LL)

Then your total results can get divided by 1.6.


Note:
If you're using an analysis program then check to see whether this is automatically accounted for. I can speak for others, but in RISA (the analysis company I work for), if you request a P-Delta analysis and your steel code check is set to AISC 13th or 14th editions, then the program will automatically handle the 1.6 factors internally. I would imagine that most of the other commercially successful programs would do basically the same thing.

 

[ELSARWAN]

I really appreciate your help
thanks a lot joshplum