A building moves, and the riser has to move with it
A tall building is never perfectly still. Wind pushes the tower sideways and it drifts and comes back, and an earthquake shakes it much harder. The gas riser is fixed to the building at its supports, so every movement of the structure is forced onto the pipe.
Why height makes it worse
The drift of a tower grows with height, so the top floors move the most. A riser that stretches from the base to the top has to cope with the difference between the movement at the bottom and the movement at the top, and it has to do so at every floor in between.
What the structural designer must supply
The PNGRB guidelines for multi-occupancy buildings ask for the sway of the building in the plus and minus X and plus and minus Z directions, and a table of the displacement at each floor level that the pipe designer can use and cross-check. For earthquakes, the pipe analysis uses the seismic class and the response spectrum from the building design. The builder is responsible for these data, and the pipe analysis is only as good as the numbers it receives.
How the analysis uses the data
The sway displacements are applied to the supports in the model, one direction at a time, and the pipe stresses and support loads are calculated. Seismic effect is checked as its own case. The results show whether the riser can follow the building without being overstressed, and where guides, loops or other flexibility are needed.
Common problems
Sway data arrive late or not at all. The data are for a different version of the building. The table gives movement at the roof but not at each floor. Any of these can make the analysis unreliable, so ask for the data early and check that they match the current design.
Design for the building you have
A riser that is fixed too rigidly takes the building movement as stress. A riser that is guided sensibly lets the building move and keeps the pipe within safe limits. That difference is what the analysis is there to find.
If you have a tower in planning, send us the height and the structural data and we will outline the analysis. See our PNG riser stress analysis service.