Support reaction

Every support that prevents a movement of the piping has to provide a force to do so. This force is the support reaction. At an anchor there are moments as well; at an equipment nozzle the same quantity is called the nozzle load.

A piping system with several supports is statically indeterminate. The distribution of loads therefore follows not from equilibrium alone but from the stiffness of the whole system. Moving or removing a support, or changing its type, alters the reactions at all the others – including distant ones.

Support reactions are not a by-product of the stress analysis but the interface to other disciplines.

A support does not carry the same load in every condition. In the cold condition, weight acts. In operation the line expands: some supports are loaded more heavily, others lift off and carry nothing. During the hydrostatic test a gas line is filled with water and many times heavier. Wind, earthquake and pressure surge cause short-term, mostly horizontal forces.

Design is based on the most unfavourable combination that can actually occur at the same time. For this purpose the reactions are listed per load case and direction – a single maximum value without the load case is of no use to the structural engineer.

When a line slides over a support during thermal expansion, a friction force arises: vertical load times coefficient of friction, opposing the direction of movement. For steel on steel a coefficient of around 0.3 is commonly used, for PTFE sliding pairs considerably lower values; the project specification governs.

The friction force acts horizontally on the steelwork. Along a long line the friction forces of all supports add up and load the next anchor – and with it the nozzles that lie beyond.

The force of the pipe on the support and the force of the support on the pipe are equal and opposite. Which of the two a table contains has to be stated above it – as does the coordinate system. A reversed sign is the most frequent error at handover.

The term should not be confused with the reaction force of a discharging safety valve. That force results from the momentum of the escaping fluid and is an external load on the piping, not a result at a support.