Pipeline Safety in Seismic Zones and Gimbal Expansion Joints

The point where a pipeline breaks during an earthquake is usually not the weakest part of the pipe, but where the line crosses between two different structural elements. Two blocks of a building oscillate at different frequencies; the rigid pipe spanning between them cannot absorb the difference and separates at a weld or flange.

Why are expansion joint crossings critical?

A building expansion joint is a deliberate separation that allows the structure to move. What is correct for the structure is a point of strain for the services crossing it. During an earthquake the two sides of the joint displace relative to one another both horizontally and vertically — the movement is triaxial, not uniaxial.

An axial expansion joint is inadequate here; it accommodates movement only along its own axis. The lateral and angular components load the bellows in a way it was not designed for.

The gimbal solution

Gimbal expansion joints can articulate in any direction thanks to their double-bellows, articulated construction. They have two important properties:

• They accommodate triaxial movement. The gimbal arrangement allows the two sides of the joint to move independently.
• They absorb the axial thrust within the unit. Because the thrust from fluid pressure is taken by the gimbal frame, it is not transferred to fixed supports — no extra load is placed on a structural element that is already stressed during the earthquake.

Why fire lines are treated separately

The system most needed after an earthquake is the fire-fighting line, and it is also among the systems most likely to have been damaged at exactly that moment. If no seismic provision has been made where sprinkler and hydrant lines cross building joints, the line cannot hold pressure in a post-earthquake fire.

For that reason, expansion joint crossings on fire lines are evaluated with a different priority from those in comfort installations: the aim here is not only to protect equipment but to keep the system operational after the earthquake.

Checklist

1. At which points does the line cross a building expansion joint? Are these marked on the drawings?
2. In which directions and how much relative displacement is expected?
3. Does the selected component accommodate triaxial movement, or only axial?
4. Where does the axial thrust land — on the support, or on the component itself?
5. Has the fire line been evaluated separately?

Summary

Seismic risk is not distributed evenly along the line; it concentrates at particular crossing points. Identifying those points and fitting the right flexible element is both more effective and more economical than trying to strengthen the entire line.