Hello Friends.

Background:-
ASME B31.3(2012 & 2014) editions have explicitly defined a equation for calculating stresses due to sustained loads-equation 23a of clause 320.2.
In this equation a term "Sa=stress due to sustained longitudinal force due to pressure and weight" is defined.

Query:-
My understanding is that the force due to weight(Mass X gravity) will impact only the vertical legs of beam analysis software like Caesar II and horizontal legs will not face any force due to weight.
Some US codes consider only pressure force(which is simply longitudinal stress due to pressure) and ignore weight while calculation of Sa. Weight is only considered while calculating Sb(Primary bending).
Is my understanding correct?

Thanks

Horizontal lines can see an axial, weight load. It may not be much but the system response in different directions (X,Y,Z) are coupled through the stiffness matrix.
Yes, before 320 provided an equation for stress due to sustained loads, most engineers would calculate a stress due to moments alone and simply add on PD/4t for the (axial) pressure stress. CAESAR II provided switches to adjust that stress formulation.

Hello Dave.

Sorry, I can't understand your reply. Can you provide a simple clarification of an example?
Do you mean to say that we have to consider the shear force due to weight on a horizontal line at the nodal points when we build up the stiffness matrix?

Thanks

Neeraj,

You said force due to weight will impact only vertical legs. I said horizontal legs can see axial load due to weight as well. Here's an example:

Because the pipe to the right of the support in the center wants to rotate around the same support, it's exerting a force onto the support on the right. Because it exerts a force on the support on the right, it has to exert force onto the anchor point, as well. If there's friction on the center support, the horizontal load is split between the anchor and center support.

Now that as per equation 23a (Section 320.2) clearly states that we need to take F/A (Axial stress - equation 23d) into consideration then what is the need for switch for F/A? Is not it a requirement now of the code?

That F/A switch in the configuration file is available for all piping codes and intended to allow the user to adjust the stress equation where Codes do not provide them. Now that B31.3 has an explicit equation for sustained stress, this switch no longer functions for B31.3 sustained stress.