Notes on Occasional Load Cases - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

Several piping codes require that you add the stresses from occasional loads (such as wind or earthquake) to the sustained stresses (due to weight, pressure, and other constant loads) before comparing them to their allowables. You can recreate this combination in CAESAR II using the following load cases:

CASE #

1

W+P+H

(SUS):

Sustained stresses

2

WIND

(OCC):

Wind load set

3

U1

(OCC):

Uniform g load set for earthquake

4

L1+L2

(OCC):

Code stresses for wind

5

L1+L3

(OCC):

Code stresses for earthquake*

* Scalar Summation Method required

If you must model nonlinear effects in the system, the load case combinations are not so straight forward. Friction, one-direction restraints, and double-acting restraints with gaps are the nonlinear items which complicate modeling. For this example, we will use wind loading on a long vertical run of pipe with a guide. Assume there is a 1-inch gap between the pipe and guide. Under normal operation, the pipe moves ¾-inch towards the stop leaving a gap of 1-¾-inch on either side of the pipe and a ¼-inch gap on the other side. If you analyze the wind loads alone, the pipe is allowed to move 1-inch from its center point in the guide to the guide stop. Because occasional loads are usually analyzed with the system in operation, the pipe may be limited to a ¼-inch motion as the gap is closed in one direction, and 1-¾-inch if the gap is closed in the opposite direction. With nonlinear effects modeled in the system, the occasional deflections (and stresses) are influenced by the operating position of the piping.

The following list of CAESAR II load cases takes this point into consideration. The load cases displayed below are only for wind acting in one direction, that is, +X. Depending on the system, the most critical loads could occur in any direction +/-X, +/-Z, or skewed in XZ. The intention of the load case construction is to find the effect of the occasional load on the piping system in the operating condition. The stress due to the moment change from the operating to the operating plus wind case is added to the stress from the sustained case.

CASE #

1

W+T1+P1

(OPE):

Operation analysis

2

W+P1

(SUS):

Sustained stresses

3

W+T1+P1+WIND1

(OPE):

Operating analysis with wind

4

L1-L2

(EXP):

Expansion stresses (Algebraic summation)

5

L3-L1

(OCC):

Net deflection of wind (Algebraic summation)

6

L2+L5

(OCC):

Code stresses for wind (Scalar summation)

Case 5 computes the isolated wind effect on the piping system in the operating condition. Case 6 adds the stresses from Case 5 to the sustained stresses from Case 2.