Seismic load

An earthquake moves the ground and with it every structure a pipe is attached to. Two different loadings result. The first is inertia: the mass of the line – pipe, contents, insulation, valves – resists the movement and loads pipe and supports with forces across and along the axis.

The second is overlooked more often: if a line is attached to two structures or at different levels, its supports do not move alike. This relative movement is imposed on the line like a thermal displacement – and at transitions between buildings it can be the larger of the two effects. It is commonly referred to as seismic anchor movement.

How severe an earthquake has to be assumed is not determined by the piping code but by the building code for the site – in Europe, Eurocode 8 with the relevant national annex. Seismic zone, ground conditions and the importance of the plant give the ground acceleration.

A line on an upper floor, however, does not experience the ground motion but that of the structure, which can amplify it. For demanding verifications the structural engineer therefore provides response spectra for the level at which the line is supported.

The simplest method applies the seismic action as an equivalent static load: the mass of the line is multiplied by an acceleration and applied as a horizontal or vertical load. The response spectrum method is more accurate; it takes the natural frequencies of the line into account and determines the maximum response for each mode. Time history analysis with a complete acceleration record is reserved for special cases.

Which method is required is set by the code and the project specification.

Earthquakes are rare and short. Piping codes therefore assess the inertia forces as an occasional load and, together with pressure and weight, permit a higher stress than for sustained loads alone; ASME B31.3, for example, 1.33 times the allowable stress at operating temperature. Wind and earthquake need not be considered as acting concurrently.

Besides stresses in the pipe, the loads on supports, structure and equipment nozzles are checked – in practice that is frequently where the bottleneck lies.

Seismic safety comes from supports that limit lateral and axial movement without blocking thermal expansion. Where the two conflict, snubbers help: they allow slow movement and lock against fast movement.