Piping Seismic Analysis

PASS/START-PROF performs seismic analysis according to these design codes:

Seismic analysis considers responses from:

The software then calculates combined responses from all seismic and static load components.

Inertial Forces Analysis

PASS/START-PROF uses these methods for seismic inertial force analysis:

Equivalent static method calculates structural responses using equivalent static loads applied in different seismic directions. This method offers simplicity and accounts for structural nonlinearities like unilateral restraint engagement, which response spectrum analysis cannot handle. However, it doesn't consider higher vibration modes. To compensate, safety factors are applied, and seismic acceleration is set to maximum spectral values.

Response spectrum analysis provides more accurate results than the equivalent static method, typically yielding lower pipe responses due to frequency-dependent acceleration assessment. This method uses acceleration spectra to calculate modal seismic loads for each natural frequency. Modal responses are combined using specialized formulas to determine design values.

Time history analysis delivers the most accurate results, accounting for physical, geometric, and structural nonlinearities. This resource-intensive method is reserved for critical pipelines. It requires input acceleration records (accelerograms) and models pipe movement during seismic events, capturing maximum response values over time.

PASS/START-PROF implements the equivalent static method with conservative margins. Inertial forces are calculated as:

1.3 - static method correction factor accounting for inaccuracies versus response spectrum analysis (higher mode effects)

- pipe element mass (concentrated nodal or distributed)

- (Alpha1 for GB50011-2001) maximum spectral acceleration per selected design code (SNiP II-7-81*, SP 14.13330.2018, NP-031-01, GB50011-2001)

- vertical seismic coefficient from PASS/START-PROF input data, typically 0.65-0.75

For pipes on structures (buildings, ramps), seismic accelerations multiply by factor kh. Horizontal direction:

Vertical direction:

Determine factors according to applicable codes.

Eight load cases analyze inertial force responses:

Analysis accounts for structural nonlinearities (unidirectional restraints, gap supports), pendulum effects, and friction.

View results in output tables:

Maximum responses for each direction equal the difference between operating static loads (L1) and operating + inertial loads (L3-L8):

Access results in output tables:

For SRSS(X,Y,Z) combination, displacements, support loads, and expansion joint deformations calculate as:

L12: L1 + (max(L9,L10)2 + Xmax_SAM2 + max(L11,L12)2 + Ymax_SAM2 + max(L13,L14)2 + Zmax_SAM2)0.5

L13: L1 - (max(L9,L10)2 + Xmax_SAM2 + max(L11,L12)2 + Ymax_SAM2 + max(L13,L14)2 + Zmax_SAM2)0.5

Xmax_SAM, Ymax_SAM, Zmax_SAM - seismic anchor movement responses (see below)

Stresses calculate as:

L12: L2 + (max(L9,L10)2 + Xmax_SAM2 + max(L11,L12)2 + Ymax_SAM2 + max(L13,L14)2 + Zmax_SAM2)0.5

SUS - sustained load case

Note: Stresses calculate separately for tensile and compressive zones, using the absolute maximum value.

For MAX(X,Y,Z) combination, displacements, support loads, and expansion joint deformations calculate as:

L12: L1 + max(L9, L10, L11, L12, L13, L14, Xmax_SAM, Ymax_SAM, Zmax_SAM)

L13: L1 + min(L9, L10, L11, L12, L13, L14, Xmax_SAM, Ymax_SAM, Zmax_SAM)

Stresses calculate as:

L12: L2 + max(L9, L10, L11, L12, L13, L14, Xmax_SAM, Ymax_SAM, Zmax_SAM)

Maximum absolute responses calculate as:

L14: max(abs(L12), abs(L13))

Seismic stress analysis checks two zones: compressive and tensile areas. This is critical for codes considering axial force and bending moment interactions.

Tensile and compressive zone stresses calculate separately, with the highest value used for final assessment.

 

Seismic Anchor Movement Analysis (SAM)

Piping systems often connect to multiple building attachment points or heavy equipment that move independently during seismic events. In addition to inertial forces, these relative displacements create significant static loads on the pipe.

To analyze these effects, specify seismic support displacement values and directions. Supports belong to "phase groups" - for example, all supports on one building floor belong to phase group 1, while supports on another floor belong to phase group 2. Each group moves independently, with unlimited groups permitted.

Obtain phase group displacement values from structural seismic analysis using specialized software.

Seismic anchor movements represent maximum possible X, Y, and Z displacements occurring at different times.

For each phase group, input three maximum displacement values along XYZ axes. Six load cases analyze each phase group:

Second phase group:

Analysis accounts for structural nonlinearities (unidirectional restraints, gap supports), pendulum effects, and friction.

Maximum responses for each direction equal the difference between operating static loads (L1) and operating + displacement loads:

Phase group 1:

Phase group 2:

Final responses calculate as SRSS combinations across all phase groups: