The rule of thumb
Steel lengthens by about 12 millionths of its length for every degree Celsius it warms. The change in length is the length multiplied by that factor and by the change in temperature.
The design temperatures
The PNGRB guidelines for multi-occupancy buildings use an installation temperature of 20 °C and a design temperature of 65 °C. That is a swing of 45 °C.
Worked example
For 60 m of riser, the growth is about 60 × 12 millionths × 45, which is about 32 mm. For 120 m it is about 65 mm. For 250 m it is about 135 mm, and for 292 m it is about 158 mm. These figures are for a pipe free to grow.
What it means in a building
A riser is held at many points along the structure, so it cannot grow freely. If nothing allows for the movement, the growth turns into force on the pipe, the joints and the supports. The building also moves, so the real movement is a combination of the pipe and the structure.
Why the answer is not so simple
The result depends on the real temperature range, how the pipe is held, the sway and seismic movement of the tower, and the loops or guides that are provided. A simple growth figure is a starting point. The stress analysis uses all of these together.
What to do with the number
Use it to see why a tall riser needs flexibility, then have the riser analysed for the actual tower. The PNGRB guidelines require a stress analysis for buildings taller than 40 metres.
To have a riser checked for the real movements of your tower, send us the height and the structural data. See our PNG riser stress analysis service.