The FRP manufacturer provided the following allowables:

1500 psi for constant loads
3000 psi for occasional loads

I'm not familiar with the code compliance cases per BS 7159, but it seems that all load cases are compliance cases, with the exception of the Expansion cases.

So, how do I set up my load cases to account for the 3000 psi allowable? The SUS+WIND case is comparing the stresses to the 1500 psi allowable. Is that correct? Why isn't this considered to be an "occasional" case? B31.3 uses the typical 1.33*allowable, but this isn't right for FRP. (B31.3 isn't really appropriate for orthotropic materials anyway.)

Below is an example of how my load cases are set up. I've set up T2, P2 as the same design pressure and temperature (T1=T2, P1=P2), but the first case has a 1500 psi allowable and the second case has a 3000 psi allowable.

L1: W+T1+P1 (OPE)
L2: W+T2+P2 (OPE)
L3: W+P1 (SUS)
L4: W+P2 (SUS)
L5: W+T1+P1+WIN1 (OPE)
L6: W+T1+P1+WIN2 (OPE) {I have two wind directions}
L7: W+T2+P2+WIN1 (OPE)
L8: W+T2+P2+WIN2 (OPE)
L9: L5-L1 (OCC) (suppressed in output; construction case only)
L10: L6-L1 (OCC) (suppressed; construction case)
L11: L7-L2 (OCC) (suppressed; construction case)
L12: L8-L2 (OCC) (suppressed; construction case)
L13: L1-L3 (EXP)
L14: L2-L4 (EXP)
L15: L3+L9 (OCC)
L16: L3+L10 (OCC)
L17: L4+L11 (OCC)
L18: L4+L12 (OCC)

And I have other load cases for maximum restraint loads, too.

L1, L2: Operating cases
L3, L4: Sustained cases
L5: Operating (@T1, P1) + Wind (+X dir)
L6: Operating (@T1, P1) + Wind (+Z dir)
L7: Operating (@T2, P2) + Wind (+X dir) (should be same as L5, except has a different allowable)
L8: Operating (@T2, P2) + Wind (+Z dir) (should be same as L6, except has a different allowable)
L9 thru L12: These are construction cases only for use in the cases to follow.
L13, L14: Expansion cases
L15 thru L18: These are the SUS + WIND cases for each temp/press and wind direction.

I know a couple of these cases can be suppressed in the output. But what am I doing wrong? Why does the code compliance report still show the allowable in cases L15 - L18 as 1500 psi, not 3000? Is this because it's considered to be a sustained load and not an occasional case?

Please help!

Thanks.

BS 7159 doesn't permit an allowable stress increase for occasional loads, therefore CAESAR II doesn't implement an increase for OCC load cases under this code. In fact, BS 7159's allowable is defined not by stress, but rather strain (entered as SH = allowable strain times Eaxial). So we just verify that for no load case representing an actual stress state (i.e., OPE, SUS, or OCC, but not EXP, which actually represents a range between two real states) does the strain exceed the allowable.

It is likely that your FRP has a different strength in the hoop direction vs. the longitudinal direction -- often 2 to 1 (for hoop stress verification CAESAR II uses Eh/Ea * SH). So typically you might get an allowable stress of 1500 psi in the longitudinal direction and 3000 psi in the hoop direction -- are you sure that the vendor isn't giving you those allowables as opposed to SUS/OCC?

Hi Tom, thanks for responding.

So are you saying that even if I enter the allowable stress as 3,000 psi for load cases T3/P3 and T4/P4, that it will still use the allowable I entered for T1/P1 and T2/P2 (1500 psi)? Shouldn't it use 3,000 psi since that's what I entered as the allowable??

Actually, in this case our strength is the same in the axial and hoop directions according to the manufacturer. It is very clearly stated as such:

Laminate tensile strength (axial) = 15,000 psi
Laminate tensile strength (hoop) = 15,000 psi
Laminate compressive strength = 20,000 psi

"The allowable stresses for constant loads are 1/10 of laminate strength. Allowable stresses for occasional loads are 1/5 of laminate strength. Allowable stresses for loads produced by external pressure are 1/5 of laminate strength. Stresses produced by seismic, wind, or snow loading, in conjunction with any appropriate working stresses shall be less than 1/5 of laminate strength."

So I have the SUS+Wind case "exceeding" the allowable. I think the allowable for that load case should be 3,000 psi, but CAESAR II is comparing the SUS+Wind stresses to 1,500 psi. If the stress is, say, 1,507 psi, it fails. But I don't think it should since SUS+Wind could be considered as an occasional stress case. So the allowable (according to the manufacturer) should be 3,000 psi, and a stress of 1,507 should pass with no problem.

I was trying to get the program to use 3,000 psi for the SUS+Wind case. Is this possible?

I hope all that made sense!

Thanks.

You are trying to get BS 7159 to do something that it isn't supposed to do (give an increase for occasional stresses), so it is not very easy, but I think we may be able to trick CAESAR II into doing what you want.

The way CAESAR II works with BS 7159 is this: if you define a temperature case in an OPErating load case, CAESAR II will use the corresponding allowable (so a load case with T1 or T2 will use an allowable of 1500 psi while a load case which includes T3 or T4 would use 3000 psi). We normally have no idea which allowable to use with SUStained or OCCasional load cases, since they usually include only primary loads (such as weight or wind) which may occur at any operating temperature, so we conservatively use the smallest allowable. (Note that P1 has no inherent tie to T1, nor P2 to T2, etc. as pressures and temperatures can be mixed and matched as one wishes, so the presence of a P3 or P4 does not indicate that the load case takes place when the system is operating at T3 or T4 respectively.) So a strict W+WIND (OCC) case would use the smallest allowable, 1500 psi.

If you want to trick CAESAR II into using a 3000 psi allowable, you need to define an OPErating case that uses a temperature T3 which corresponds to a 3000 psi allowable, but that doesn't really add any stress to the system. So you should define a T3 which is very close to ambient, such as 71 degrees (but not too close to ambient, or CAESAR II will realize that it is a non-load and will wash it out of the load cases). Then assign an allowable of 3000 psi to SH3, define your occasional case as W+P+WIND+T3 and make it an OPE type. It should then use 3000 psi as you are hoping for.

Just keep in mind that this isn't a genuine BS 7159 analysis...