B31.1 - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

Calculate pressure stiffening using B31.1

Pressure stiffening is defined by default in the code. You can exclude pressure stiffening on bends in the analysis by including the Use Pressure Stiffening=No option in the setup file.

Flanged end modifications using B31.1

Modifications resulting from flanged ends are permitted in the code providing the bend is not a widely spaced miter. CAESAR II does not verify the B31.1 criteria "B" length for closely spaced miters.

B31.1 does not by default add F/A into the stress calculation. F/A and the pressure stresses are added to the bending stress, whether the tensile or compressive component of bending, to produce the largest longitudinal stress component. This is true for all codes where the addition of axial and pressure terms are concerned. You can include the axial force terms into the code stress by inserting the Add F/A In Stress=Yes option in the setup file.

The F/A forces are structural forces developed in the pipe independent of the pressure PD/4t forces.

Calculate reduced branch stress intensification factors (SIFs) using B31.1

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. The ASME Section III rules clearly stated that the branch section modulus, not the effective section modulus should be used with the new SIF. B31.1 continued use of the effective section modulus produced 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:

  • Use the pre-1980 version of the B31.1 SIF rules

  • Use the very conservative post-1980 B31.1 SIF rules

These options also exist in version 3.0 and later except that the section modulus problem is corrected. If you need to run version 3.0 and later without the section modulus correction, then include the B31.1 Reduced Z Fix=No option in the setup file.

Calculate reduced intersection branch using B31.1

Reduced intersection branch SIFs were not intended for reinforced or welding tees. Conservative results are produced, but the original researchers did not intend for SIFs to be used for these fittings. You can disable the reduced branch fitting calculations for reinforced or welded tees by including the No Reduced SIF for RFT and WLT option in the setup file. This produces less conservative results, but can in some cases be justified.

B31.1 102.3.2 (c) says to divide the allowable stresses coming from the stress tables in Appendix A by the applicable weld joint factors listed in Paragraph 102.4.3.

Calculate the B31.1 stress allowables

Use the equations below to calculate the stress allowables.

Expansion Allowable = f [ (1.25/Eff)(Sc+Sh) - Sl ]

Sustained Allowable = 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

Sl = Sustained Stress

Occ = Occasional Load Factor Default is 1.15

Calculate stress intensification factors (SIFs) for intersections using B31.1

Inplane and outplane SIFs for intersections are the same.

B31.1 reducer default values

The default flexibility factor value is 1.0. Use the following equation to determine the SIF value: maximum of 2.0 or 0.5 + .01*Alpha* SQRT(D2/t2).

Where:

D1- Diameter of the Large End

t1- Thickness of the Large End

D2 - Diameter of the Small End

t2 - Thickness of the Small End

Alpha - the Reducer Cone Angle in Degrees.

Where:

Alpha = atan[ (D1-D2) / (2*length of the sloped portion of the reducer*0.6) ]

Alpha is the slope of the (concentric) reducer transition in degrees. If unspecified, CAESAR II calculates alpha using 60 percent of the entered reducer length.

Alpha cannot exceed 60° and the larger of D1/t1 and D2/t2 cannot exceed 100.