LRFD Load combinations with positive and negative numbers

[Tim_B]

I have number of fixed end moments on a roof truss. (due to dead, live, snow and wind) Some of the wind moments are negative. Do I just combine the negative numbers straight into the load combination with the positive moments for example from Eq 16-6 in IBC 2015...

0.9D + 1.0(-W) + 1.6H

Is that correct? It makes sense in my head. I can expect positive moment due to dead load (or anything else) to counter-act the negative moment due to wind. (And obviously H is zero in my example)

 

[klaus.]

Easy answer...you have to combine whatever is necessary
If you know that a certain combination will not give max forces the you can omit this combination
But generally you need to consider whatever gives the extreme forces
Positiv or negativ is sometimes difficult to see in a bigger structural model


best regards
Klaus

 

[Tim_B]

So the answer is "yes" and these load combination will not likely govern if the result is small (in absolute value) due to the negative number...?

 

[klaus.]

well if you know that they do not govern the of course you not need them.....
there is no general answer




best regards
Klaus

 

[MotorCity]

If you have reversible loads (such as wind and seismic) you need to apply them with a different sign in each load combination to get the net effect