Hi everyone,
When we combine sustained load case and occasional load case(like PSV thrust force) the combination method recommended by Caesar is scalar.Scalar combination of stress gives a very conservative value which will never occur in actual piping.
To get a true value of stress at point algebraic combination is required.
So can anyone give me reference form ASME 31.3 which states that combination has to be scalar or algebraic.

read this

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Main=8497&Number=38534#Post38534

Regards,

Thanks dan,
well scalar combination gives results which will never occur in piping as the scalar combination simply adds stress as numbers.
Take a simple case , when we add a spring to a system we simply add a force to system F=Kx or a constant force F (in case of constant sprinf)to reduce stress.What Caesar does is combines this spring force algebrically and gives the resultant stress and forces.So in this case if i am right it is algebraic combination.
Why cant the same be applied when we add a psv thrust force,which simply is a force.
I hope dan you got my point.
Regards

The Occasional loads are primary loads and are treated as primary loads.

Stresses created by occasional loads are primary stresses.

How you combine these?

As you saw, the code require that the combination method is scalar.

Why?

I'm afraid that I will not give you a satisfactory answer, but I will try.

I was searching for this answer and the only reference that I found was that the occasional effects are treated similar to the AISC practice.


Regards,

Scalar is implied by the piping code. B31.3 (302.3.6(a)) states the sum of longitudinal stresses ... due to sustained loads ... and of the stresses produced by occasional loads ... may be as much as 1.33 [Sh]. Other Codes with equations show the stress magnitudes summed, not stress vectors.
Why? I figure many occasional loads have a "rebound" component so if you say the pipe moves 25 mm in +X, it might just as easily be 25 mm in -X a moment later. Just my assumption. To be conservative I figure those occasional loads are + and -.
Of course, CAESAR II will allow you to do different if you wish. That's between you and the owner.