How to Make your Process Pipework Earthquake–Proof

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Methods of Seismic Analysis

The earthquake characteristics are described by ground acceleration, the time of shaking and its response spectrum. The ‘design earthquake’ is generally defined by either of the magnitude of the largest earthquake that probably could occur or the magnitude corresponding to large return period e.g. 500 years, 1000 years, etc. The return period can be calculated as follows:

T = -t / ln (1 - p)

Where, p is the probability of exceedance in t years, generally 10 per cent in 50 years and T is the return period, in years.

Example 1: t = 100 years, p = 10, then T = -100 / ln (1 - 0.1). Therefore, T = 949 years ~ 1000 years

Earthquake Analyses – Effects on Pipework

The effect of an earthquake on a pipe can be analyzed by the following methods:

• ● Quasi-static or static equivalent method

• ● Response spectrum method (RSM)

• ● Time history analysis method

Quasi–Static or Static Equivalent Method (SEM)

SEM, which has been followed by most of the codes, assumes that the mass distribution throughout the height of the structure is uniform, with the centroid of mass and rigidity coinciding at each level. In most of the cases, this assumption holds well except for the structures of highly irregular shapes. In this method, a horizontal force in proportion to the ratio of seismic acceleration to gravitational acceleration is applied, as base shear, to get horizontal forces acting on the system.

Response Spectrum Method

It is a plot of maximum steady state response (displacement, velocity or acceleration) of a series of oscillators with different natural frequencies to a given shock. In this analysis the maximum response developed by a system to the transient load is analyzed. Response spectrum can be modified based on damping. The acceleration is in inverse proportion to the damping. RSM is the most common method in dynamic analysis of earthquake.

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