It appears that the default is still yes however I'm noticing significant changes in restraint loads with pressure changes when using this option. Should this option be marked NO?

What are we talking about here?

Yes "pressure stiffening" can change the restraint loads. Why? Because pressure stiffening of elbows changes not only the SIF, but also the flexibility factor, used in the generation of the element stiffness matrix. You change the stiffness, you change the load.

hi all/Mr richrad,
can u plz tell me what is the meaning of pressure stiffening in CAESAR II. practically what is the difference between normal case and pressure stiffening. can tell me why stiffness increases in case of pressure stiffening.

See the note in Appendix D of either B31.1 or B31.3.

Hello Raju,

Bends and elbows gain additional flexibility as they deflect because the pipe cross section ovalizes with this deflection. When this happens the pipe cross section will have a "strong" axis and a "weak" axis. The "weakening" of the axis that resists the deflection (due to ovalization) results in increased flexibility (this is why the flexibility factor for bends and elbows is greater than that of straight pipe). Internal pressure tends to keep the pipe cross section round and because this pressure retards the ovalization it thereby retards the increase of flexibility. The greater the internal pressure, the stiffer the bend or elbow will be (i.e., the more it will resist deflection).

Regards, John.

Mul211,

Please note the following points:

1) The SIF and Flexibility factors gets changed due to effect of pressure. This is as outlined in B31.1 & B31.3.The effect is more pronounced on Large Diameter, then walled pipes.I will give one case study: In a particular analysis ( not using liberal stress), the expansion stress ratio ( computed vs allowable) was getting reduced, when the pressure was increased. This was due to the stiffening of the elbows ( and hence lower SIF).

2) Bourdon effect : This is similar to displacement induced loading, only thing, it is pressure loading which results in the displacement. Needless to say that if unrestrained such a loading ( strain) will not result in stress.

3) Stress stiffening ( in CAESAE II terminology) is the simulation of the effect which can physically be envisioned as : Imagine a bar stretched by axial force. Intuition tells us that this will increase the lateral stiffness also. If this thing is mathematically quantified, by writing the equilibrium expressions, we get a [P]=[K][D]effect but the [K] is a function of geometry only and not material properties.Hence this is also known as "geometric stiffening". The other name for tis phenomenon is P-Delta effect.

Regards

Anindya