The B31.1 2020 edition (and later) replaces Appendix D and requires direct use of B31J, which provides flexibility factors on tees and branch connections in addition to the stress intensification factors (SIFs). Because Appendix D required the flexibility factors on tees to be 1.0, similar to the flexibility factors of pipe, B31J’s flexibility factors on tees have a large impact on the flexibility of the entire system and produces more realistic results, including the results for displacements and restraint loads.
Pressure Stiffening
Pressure stiffening is defined by default in the code. You can exclude pressure stiffening on bends in the analysis by selecting No for Use Pressure Stiffening on Bends in the Configuration Editor.
Flanged End Modifications
Modifications resulting from flanged ends are permitted in the code when the bend is not a widely spaced miter. CAESAR II does not verify the B31.1 criteria "B" length for closely spaced miters.
In the 2020 edition, branch connections can also be stiffened using flanges in accordance with B31J.
F/A Axial Stresses
B31.1-2018
B31.1 does not add F/A into the stress calculation by default. F/A and the pressure stresses are added to the tensile or compressive bending stress to generate the largest longitudinal stress component. You can include the axial force terms into the code stress by selecting Yes for Add F/A in Stresses in the Configuration Editor.
The F/A forces are structural forces developed in the pipe and are independent of
the pressure PD/4t forces.
B31.1-2020
In addition to the use of B31J, the 2020 edition changes the stress equations, using the directional SIFs and bending moments method instead of the combined/resultant moment method. The code also adds intensified axial and torsional stresses to all stress calculations. The longitudinal stress due to pressure is not intensified. This differs from B31.3, which intensifies the longitudinal pressure stress and the axial stress.
B31.1 includes F/A in the stress calculation by default. F/A and pressure stresses are added to the tensile or compressive bending stress to generate the largest longitudinal stress component.
B31.1 always includes F/A into the code stresses. The software ignores the value selected for Add F/A in Stresses in the Configuration Editor.
Occasional Load Stress
B31.1-2018
The code combines stresses from the sustained (SUS) loads and from occasional (OCC) loads.
SUS loads are loads such as primary force-driven loads acting on the pipe including weight and pressure, cold as-installed hanger loads, and force sets arising due to pressure. OCC loads are loads such as wind and earthquake loads.
B31.1-2020
Instead of traditional methods of combining stresses, the code requires using the combined SUS and OCC forces and moments for the OCC stress equation.
Reduced Branch Stress Intensification Factors (SIFs)
B31.1-2018
In 1980, B31.1 added a reduced branch SIF equation to Appendix D. This equation came from ASME Section III. However, B31.1 continued to use the effective section modulus calculation for the branch. ASME Section III rules clearly stated that the branch section modulus should be used with the new SIF. B31.1 continued to use the effective section modulus, producing unnecessarily high calculated stresses. This error was corrected in the 1989 version of B31.1. Prior to CAESAR II version 3.0, you had two options:
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Use the pre-1980 version of the B31.1 SIF rules
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Use the very conservative post-1980 B31.1 SIF rules
These options exist today with a correction to the section modulus problem. If you need to run the software without the section modulus correction, then select False for B31.1 Reduced Z Fix in the Configuration Editor.
B31.1-2020
B31.1 requires direct use of B31J. The branch section modulus, not the effective section modulus should be used.
Reduced Intersection Branch
B31.1-2018
Reduced intersection branch SIFs produce conservative results and were not intended for reinforced or welding tees. To disable the reduced branch fitting calculations for reinforced or welded tees, select True for No RFT/WLT in Reduced Fitting SIFs in the Configuration Editor. This produces less conservative results that can be applicable in some cases.
B31.1-2020
Reduced intersection branch settings are not relevant. The software ignores No RFT/WLT in Reduced Fitting SIFs in the Configuration Editor. The actual branch section modulus is used to calculate bending stresses at the branch for both reducing and non-reducing intersections.
Stress Allowables
Paragraph 102.4.3 indicates that weld joint efficiency factors are already included in the allowable stress values given in mandatory Appendix A. The allowables from Appendix A can be divided by the joint efficiency factor to determine the basic material allowable.
Use the equations below to calculate the stress allowables.
Expansion Allowable = f [ (1.25/Eff)(Sc+Sh) - SL ]
Noncyclic Displacement Stress Range Allowable = 3Sc (when you define Load Cycles as 1.0)
Sustained Allowable = Wc(Sh/Eff)
Occasional Allowable = Sh/Eff * (Occ)
Where:
f = Cyclic Reduction Factor.
Eff = Longitudinal Weld Joint Efficiency.
Sc = Cold Allowable Stress.
Sh = Hot Allowable Stress.
When calculating EXP allowables, the code limits Sc and Sh to 20 ksi.
SL = Sustained Stress.
Occ = Occasional Load Factor. The default is 1.15.
The code allows an Occ of 1.2 or 1.15 based on the duration of occasional loading.
You can control the value for each load case in the Static Analysis - Load Case Editor Dialog.
Wc = Weld Strength Reduction Factor.
Non-corroded Stress
By default, the code applies nominal (non-corroded) section modulus properties for stress calculations.
Set All Cases Corroded to True in the Configuration Editor to use the corroded pipe wall thickness for all stress calculations.
Stress Intensification Factors (SIFs) For Intersections
B31.1-2018
The code does not differentiate between in-plane and out-of-plane SIFs and uses a single value.
B31.1-2020
The code uses B31J for intersection SIFs and flexibilities. In-plane, out-of-plane, and torsional SIFs and flexibilities are separate and unique. For reduced branch intersections, the section modulus of the matching pipe is used to calculate bending stresses at the branch for both reducing and non-reducing intersections.
If you do not manually define SIFs, then the axial SIF is 1.0 for bends and equals the calculated out-of-plane SIF for other components under secondary load cases such as EXP. The axial SIF is 1.0 for all components under primary load cases such as SUS and OCC.
Reducer Default Values
B31.1-2018
The default flexibility factor value is 1.0. The SIF value is the maximum of 2.0 or:
0.5 + 0.01a(D2/t2)1/2
Where:
a = Reducer Cone Angle in Degrees = arctan[ (D1-D2) / (2*length of the sloped portion of the reducer*0.6) ]
D1 = Diameter of the Large End
t1 = Thickness of the Large End
D2 = Diameter of the Small End
t2 = Thickness of the Small End
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a is the slope of the concentric reducer transition in degrees. If unspecified, CAESAR II calculates a using 60 percent of the entered reducer length.
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a cannot exceed 60° and the larger of D1/t1 and D2/t2 cannot exceed 100. If the value exceeds these limits, the software displays a warning and then performs the calculations.
B31.1-2020
The B31J default SIF values are calculated and unique. The default flexibility factor value is 1.0.
The code states that the maximum SIF cannot be larger than 2.0, but user-defined SIFs can use a larger value when calculating stresses.