Restrained Pipe (as defined in Section 833.1):
For Straight Pipe:
Both SL and SC < 0.9ST (OPE)
Both SL, and SC < 0.9ST (SUS)
SL < 0.9ST and Sc < ST (OCC)
and
* The Stress Report displays the calculation causing the highest percent of allowable along with its specific allowable.
For All Other Components
SL < 0.9ST (OPE, SUS, OCC)
Unrestrained Pipe (as defined in Section 833.1):
SL < 0.75ST (SUS, OCC)
SE < f[1.25(SC + SH) – SL] (EXP)
Where:
SL = SP + SX + SB
SP = 0.3SHoop (for restrained pipe); 0.5SHoop (for unrestrained pipe)
SX = R/A
SB = MB/Z (for straight pipe/bends with SIF = 1.0); MR/Z (for other components)
SC = Max (|SHoop – SL|, sqrt[SL2 – SLSHoop + SHoop2])
MR = sqrt[(0.75iiMi)2 + (0.75ioMo)2 + Mt2]
SE = ME/Z
ME = sqrt[(0.75iiMi)2 + (0.75ioMo)2 + Mt2]
S = Specified Minimum Yield Stress
T = Temperature Derating Factor
SH = 0.33SUT
SC = 0.33SU
SU = Specified Minimum Ultimate Tensile Stress
CAESAR II Determines
You can also let the software determine the appropriate stress. The software selects a stress equation based on the calculated axial load in the piping system. The software applies the fully restrained stress calculation when the axial pipe lo ad is within 2.5% of the limiting load or otherwise applies the unrestrained stress calculations. The limiting load is the load to fully constrain the pipe axially against the linear superposition of thermal expansion and pressure elongation (when the Bourdon pressure effect is selected in the Configuration Editor or in Special Execution Parameters). The restrained/unrestrained status can change from element to element.
The software uses the following loads:
-
Local axial force
-
Thermal force: E*a*AREA1
where AREA1 = p(Ro2-Ri2)
-
Bourdon force: (1-2*s)*P*AREA2
where AREA2 = pRi2
and determines the stress equation based on:
-
Limiting Force = Thermal Force + Bourdon Force
-
Delta = Limiting Force + Local Axial Force
-
When |Delta| / |Limiting Force| £ 0.025, then use the fully restrained stress calculations.
-
Otherwise, use the unrestrained stress calculations.
This option is most suitable with the Underground Pipe Modeler or when modeling soil/pipe interaction using restraints.
B31.8 distinguishes between restrained and unrestrained piping for the purposes of stress computations. To implement B31.8 you must define which sections of the piping system are restrained, as per Code Section 833.1. In general, restrained piping is piping in which the soil or supports prevent axial displacement of flexure at bends. Conversely, unrestrained piping is piping that is free to displace axially or flex at bends. For more information, see Section 833.1. Processing a B31.8 model through the Underground Pipe Modeler designates the buried sections as restrained.
For restrained pipe, B31.8 specifies that the operating case stresses should include the thermal axial stress component, a constant stress due to linear thermal expansion, but exclude thermal bending stresses from the SB component. Because CAESAR II cannot go back and segregate internal thermal forces and moments from those of other loads, the thermal axial stresses are calculated and included as part of SX (as opposed to added as a constant), and thermal bending stresses are conservatively included in SB.
Bending stress SB is defined differently for straight pipe or "large-radius" bends than it is for other components. CAESAR II resolves the ambiguity of exactly what constitutes a "large-radius" bend by considering any bend having an SIF of 1.0 as being a "large-radius" bend.
Occasional load default values
The occasional load default value for B31.8 is 1.111 (1/0.9) and is only applied to the allowable for SC combined stress calculated only in straight pipes. The allowable in this case is ST as opposed to 0.9ST. There is no provision for increasing or decreasing this allowable.
In the case of occasional stresses in straight pipes, there are potentially two stresses (SL and SC) to be compared against two different allowable limits. CAESAR II only prints the one that provides the greater ratio of calculated stress versus allowable stress. You can visually determine which stress prints by examining the magnitude of the allowable.
Calculate pressure stiffening using B31.8
Pressure stiffening is included by default in the code. You can exclude pressure stiffening on bends in the analysis by setting the Use Pressure Stiffening switch in the setup file.
Modifications to the flexibility factor and Stress Intensification Factor (SIF) using B31.8
Modifications to the flexibility factor and SIF of bends resulting from flanged ends are permitted by the code.
Calculate socket welds using B31.8
B31.8 makes no distinction between socket welds with undercut and socket welds without undercut. Unless you are specifying a fillet weld leg length, use a default SIF value of 2.1 for all B31.8 socket welds.
Using reducers with B31.8
Use of reducers is subject to the following limitations:
-
Alpha the reducer cone angle is limited to 60°
-
The larger of D1/SQRT(t1) and D2/SQRT(t2) cannot exceed 100 where D1/t1 and D2/t2 are the diameters and thicknesses of the large and small ends, respectively.