Hi,
This post is regarding the change in axial stress allowbles when we change the piping pressure which directly changes hoop stresses.
As per ISO 14692- there are separate allowables for hoop and axial stress.The allowbles hoop and axial stress determine the failure envelope for GRE piping.This is in accordance with ISO 14692.
My question is why do allowable axial stresses change with the pressure(hoop stress.) if we have fixed axial stress allowbles defined.
i have observed only axial allowbles change with change in pressure.With increase in pressure axial allowbles also get increased till value reaches the hoop stress allowbles.Once hoop stress allowbles are shown in static out put no more change in allowbles occurs.
Can some one explain why it is so.

Your allowable axial stress has two major components - pressure and bending. Also, the allowed axial stress is also a function of the hoop stress - as hoop stress changes, the total allowed axial stress changes. CAESAR II reduces the specified/calculated allowable axial stress by the axial pressure stress and lists the remaining stress as the allowable axial stress for bending.

Hi,
When we define the failure envelope as per ISO-14692 , we specify three points on the graph.
1.al(0:1) Long Term Axial Stress at 0:1 Stress Ratio-(hoop stress is 0 at this point)

2.al(2:1) Long Term Axial Stress at 2:1 Stress Ratio-(This is a natural condition when a pressurized pipe is enclosed at both ends)

3.hl(2:1) Long Term Hoop Stress at 2:1 Stress Ratio

The region bound by these points determines the failure envelope as per my understaning.
I agree that axial stress is a funtion of hoop stress.If hoop stress increases axial stress also increses, but why should the allowble axial stress change with hoop stress if it is already defined by the region bounded by three points.Axial allowable stress and hoop allowable stress are already defined by the points on graph.How can the graph change with change in hoop stress.
As hoop stress is directly proportional to axial stress.If i use your statement that axail allowbles change with hoop stress , then i will have lower allowables for low pressure and higher allowables for higher pressure.How can GRE material axial stresses allowbles change with pressure.
I hope u got my point .
Regards

In the CAESAR II flexibility analysis for ISO 14692, the axial stress we check is the longitudinal stress due to bending (and non-pressure axial loads). The envelope you mention has two axial components - pressure and this bending. Yes, axial stress due to pressure is proportional to hoop stress. The allowable stress we show though is the evelope limit MINUS the axial stress due to pressure. That's why the axial allowable stress changes as hoop stress changes.

Hi Dave,

Just want to clarify about the axial allowable stress of FRP
is the envelope limit MINUS the axial stress due to pressure? Because in Equation D.14 of Annex D ISO 14692 the axial pressure stress is part of the induced effective axial stress rather than the allowable axial stress..Caesar II 2013 is now using this Equation D.14 as what I've manually checked. Or Maybe this is from previous Caesar version..

Sorry if my interpretation is incorrect.

Regards,

You have it right, in CAESAR II, the ISO14692 limit for axial stress is the Code's limit less the axial stress due to pressure.

Thanks Dave....

My query is a bit out of the subject but it is as follows:

1) Do we need to perform seismic analysis for underground piping??
American life line guideline suggests the approach for the checking but is it applicable for GRE lines also??