# PIPESTRESS FAQ

##### 1) Sylvester error

To resolve the modal extraction convergence problem, see documentation for PP cards.

The modal extraction routine implemented in PIPESTRESS starts with a random initial vector and iteratively converges to the next eigen direction. The Sylvester error is usually due to an unlucky initial vector.

If this error occurs for a higher mode, it is usually sufficient to simply restart the modal extraction. However, when the error occurs extracting the first mode, this method cannot be used. Instead, you can use a PP card with keyword "SEED". This parameter is used to select the initial vector.

FREQ RF=200 RP=0 FR=400.0 MP=400.0 MX=9999 TI=/MODAL/
0001     PP    ITMAX   999
0001    PP    KILL
0001    PP    SEED   .66666

##### 2) Supports at start of branch

Supports at start of branch at a JUNC point must be defined before JUNC card.

##### 3) Difference between MATH EX=1 and MATH EX=2

When material table cards are used to define operating conditions, there are two options in PIPESTRESS for defining thermal expansion. These options are "linear thermal expansion" and "mean coefficienf of thermal expansion".

The EX field of the MATH card defines which option will be used (EX=1 is obsolete and should not be used).

The EX field of the MATD card defines the value of thermal expansion corresponding to the MATD card temperature.

##### 4) Kludge for operating pressure

Question: "I need to use operating pressure and transient pressure of 10 000 psi. PepS allows only 6 000 and 5 500 respectively. What should I do ?"

If the operating pressure for the structural analysis is defined using a OPER card, you can enter a pressure of 10 000 psi by adding the kludge field KL=1 to the card.

There is no way to "kludge" the transient pressure. We can suggest two work-arounds using the fact that these pressures are only used to calculate the inside film coefficient for thermal transient analysis :

1. Use the maximum pressure allowed by PIPESTRESS, 5 000 psi, provided that the temperature and pressure for this transient are in the same region of the steam table as would be 10 000 psi for the same temperature. In this case, the difference in the calculated film coefficient will probably be small and will have little effect on the solution

2. Manually calculate the average film coefficient and enter this value for the transient by replacing the TRAN card by a FLUD card.

##### 5) Restart - role of NF16

Question: "I am getting an error in my PIPESTRESS file using a merged restart file as an input to my final analysis. It says : NF16 for this problem = 1645 ////////// NF16 for restart file = 1394"

The parameter NF16 is the number of records-per-load case on the PIPESTRESS restart file. If a calculation using an input restart file does not have the same value of NF16 for the restart file and for the calculation itself, this error occurs.

On reason this error occurs is that the restart file model and the calculation model do not contain the same number of generated mass points.

If the calculation involves a modal extraction for which the FREQ card uses the field MP (Automatic Mass Modeling) whereas the restarted calculation : did not involve a FREQ card ; or involved a FREQ card with a different value of MP, then this error would occur.

PIPESTRESS can only restart problems where the calculation captured from the restart file and the actual condition are "topologically identical". A necessary condition for this to be true is that the values of NF16 are equal.

##### 6) TH (thickness factor) for modeling

Question: "Can I input valve stiffness using VALV card ?"

See the TH field defined for the VALV card.

##### 7) Missing mass vs Left-Out-Force (LOF)

"Seismic analysis will include the effects of missing mass by performing rigid response load case analysis. The results of the missing mass approach using the rigid response load case analysis are combined with the low frequency modes by SRSS method."

PIPESTRESS does not use the "missing mass" method to approximate the contribution to the total system response due to the higher frequency (uncalculated) modes.

Instead, PIPESTRESS uses the mathematically rigorous "left-out-force" method. To use the "left-out-force" method correctly it is necessary to :

1) FREQ card : calculate all the non-rigid modes. The standard cut-off frequency for seismic loads is 33 Hz. In this case, you should define the FREQ card field "FR=33". This will instruct PIPESTRESS to calculate and use modes with natural frequencies up to 33 Hz. You should also use the FREQ card field "MP=33". This will instruct PIPESTRESS to model the mass so that the mass points are sufficiently close together to correctly represent modes up to 33 Hz. If PIPESTRESS finds mass points which are not close enough, it then generates additional mass points. Note that the cut-off frequency of 33Hz is characteristic of earthquake loads. If, for example, the load is due to aircraft impact (a required load in Germany), the FREQ card fields should be "FR=50" and "MP=50".

2) RCAS card : typically, floor response analysis is defined in PIPESTRESS by a RCAS card. The method used for modal superposition is defined by the field "SU". Probably the most commonly used value is "SU=1". The "LO" field controls the method used by PIPESTRESS to approximate the contribution of the higher frequency modes. The RCAS field "EV" defines the "event number" which refers to the floor response spectrum as defined in PIPESTRESS by "SPEC"cards.

3) SPEC card : see User's Manual

4) Theory : using this method you should obtain a highly accurate envelope for the "primary" response. See discussion on "Dynamic analysis" in the User's Manual.

##### 8) Modeling GAPS

This point concerns non-linear supports. PIPESTRESS is basically a linear program. However, for static loads, it is possible to model simple bi-linear restraints.

Bi-linear restraints can be modeled in PIPESTRESS as follows :

* The load case is modeled by an NCAS card

* The bi-linear supports are modeled by NRST cards. However, for any static-equivalent dynamic case (LCAS with TY=1, 2 or 6) or any dynamic case (RCAS, DCAS, GCAS or RACC case) the restraints defined by NRST cards will act linearly.

1) Gaps : a gap may be modeled for a deadweight case by two NRST cards in the plane perpendicular to the pipe axis.

2) Constant supports with limit stops : a constant support with a limit stop may be modeled for weight analysis by two support cards, a constant support spring hanger (CSUP) card and an NRST card

3) Snubbers : snubbers only act in dynamic cases. Since bi-linear restraints act linearly for dynamic cases, snubbers with limit stops or gaps cannot be analyzed by PIPESTRESS.

##### 9) Bends and reducers : SIFS

For the ASME code, a bend has two sets of stress indices : one set based on the end conditions and one set based on the "body of the fitting". For each end point of each bend PIPESTRESS evaluates two stress values : one based on the end conditions and one based on the body of the fitting. The higher of the two is reported by PIPESTRESS. If the "body" indices are used, they are reported with "f" suffix. See in file .pri.

##### 10) Generalized Response Analysis (GRA) vs TRUE for time history analysis

There are two methods in PIPESTRESS to construct envelope solutions for time history loads. The "true time history" method is selected by using the LINEs option of the GCAS card. See page 2-82 of the PIPESTRESS User's manual. If the LINEs option is not used, PIPESTRESS calculates the envelope solution by the Generalized Response Analysis (GRA) method.

The PIPESTRESS envelope which is generated in the "pth" file is always the GRA solution.

The disadvantage of the GRA method is that, in most cases, it is much more conservative than the "true" method. The advantage of the GRA method is that it is much faster and uses less memory than the "true" method. For small and middle-sized problems, the "true" method is better. But for very large time history problems, the GRA method can be very userful. This is a complex subject which requires some study.

##### 11) Modeling Pipe and non-Pipe elements in PIPESTRESS

Stresses are calculated for elements whose stress intensification factors and flexibility intensification factors are defined in the ASME code. These elements are : straight elements (TANG), bend elements (BEND / BRAD), tees (two TANG + BRAN card), reducer elements (CRED and ERED cards), miter bend elements (modelled as combinations of bend and straight elements).

Other elements are used to model non-pipe elements contained in the structure. These can be valves, socket weld fittings, rigid elements, vessels, etc... The mechanical properties of these non-pipe elements are modelled using cards such as RIGD, BEAM, SPRF or SPRS, MTXF or MTXS, etc... For example, in some cases, it may be useful to model the mechanical properties of non-pipe elements using the mechanical properties of pipe elements. In this case, the STRU card is used.

All the non-pipe elements cards have in common that PIPESTRESS does not calculate stresses for these elements because the rules for calculating the stresses are not contained in the ASME code.

CROS and MATL cards are not required for BEAM cards.

Although the STRU card is used to model non-pipe elements because it uses the mechanical properties of pipe elements, it is required to define the cross-sectional properties (CROS card).

The circular cross-section is used for the STRU card (see preceding paragraph). However, the CROS card is not used for BEAM cards. The type of modeling used for BEAM cards is fully explained in the PIPESTRESS Theory Manual.

##### 12) Special modeling (relaxation with SPRS)

Question: "In one of the model, I want support valve actuators (2 valves on separate pipe parallel to each other) by a strut. If I use RIGD card, it provides rigidity in all directions. As on a support drawing, I am connecting 2 actuators by a strut that means only in direction of the strut. Only in one direction I want both valves to move together."

Use SPRS with appropriate stiffness AX, AY, AZ, BX, BY, BZ. For example:

SPRS PT=b DX=0.002 AX=1. AY=1. AZ=1.E9 BX=1. BY=1. BZ=1.

##### 13) THIST envelope - difference with PIPESTRESS envelope

Utility program THIST tries to reconstruct the PIPESTRESS envelope. However, the reconstructed PIPESTRESS envelope will only be equal to the original PIPESTRESS envelope if the options selected for the THIST execution are identical to the options used for the PIPESTRESS execution. The PIPESTRESS envelope calculated by THIST is only used for validating THIST.