Height does not decide the design
It is tempting to reuse a riser design: same height, same area, same pipe size, same expansion loop. It does not work. Height is only one of the inputs. Two towers of the same height, on neighbouring plots, can need different pipe, different loops and different supports.
The structure is different
A tower stiffened by a large core and shear walls drifts differently from a framed tower or one with a different layout. The stiffness and mass of the building set how far it sways in wind and how it responds to an earthquake, and that movement is what the riser is forced to follow.
Wind is different
IS 875 (Part 3) sets wind loads from the basic wind speed, the terrain, the height and the shape of the building. Orientation, nearby towers and the building's shape change the wind on each tower, so two neighbours do not see the same wind.
Earthquake response is different
IS 1893 (Part 1) bases the design on the seismic zone, the soil at the site, and the natural period and mass of the building. These are different for every structure, so the movement at each floor is different.
The shaft and the gas demand are different
The position of the riser shaft in the plan matters, because a shaft far from the core moves differently from one beside it. Floor heights, podium levels, setbacks and transfer floors change the spacing of supports and the route of the pipe. The number of flats served by each riser sets the flow, and with it the pipe size and the pressure drop.
Pipe size and loop size are results, not look-up values
Pipe size comes from the gas demand and the allowed pressure drop, and wall thickness and fittings are checked against stress. An expansion loop is sized from the thermal growth and the building movement that has to be absorbed, so its size depends on the tower's own sway and earthquake data. A loop that is too small leaves the pipe overstressed. One that is too large wastes shaft space and adds cost and pressure loss. Only the analysis shows which is right.
What the codes say
The PNGRB guidelines for gas supplies to multi-occupancy residential buildings (2020) require a stress analysis for buildings taller than 40 metres. It uses the building's own sway in the plus and minus X and Z directions, a floor-by-floor sway table and the seismic data, and checks five load combinations. The builder is responsible for the building data. The PNGRB T4S regulations (2008) make ASME B31.8 the base design code for the network and require thermal expansion and stresses in above-ground steel piping to be evaluated at 65 °C. ASME B31.8 covers expansion and flexibility, longitudinal stress, and supports and anchorage for exposed piping (paragraphs 832 to 834 in the 2022 edition), with flexibility and stress intensification factors for fittings in Appendix E. IS 875 (Part 3) and IS 1893 (Part 1) give the wind and earthquake basis for the building, and the pipe analysis depends on the movements calculated from them for that particular tower.
When a design is reused
A riser copied from another tower is a guess. If the data match by chance, nothing is gained. If they do not, the pipe may be overstressed, the supports overloaded, or the loops too large to fit the shaft. The problem appears after installation, when it is expensive to correct.
Even twin towers need a check
Twin towers in one complex can still differ in shaft position, floor heights, the number of flats per riser and the structural data issued at the time. Building designs are revised during construction, so the analysis must use the current version of the data.
What a tower-specific analysis needs
The riser layout, the building height and floor levels, the sway and seismic data from the structural designer, the gas demand and pressure, and the shaft details. With these we run the analysis for that tower and give the pipe sizes, the loops, the supports and the report.
ERGNA Engineering is a registered vendor of Mahanagar Gas Limited for design, engineering and stress analysis of pipe natural gas networks in high-rise buildings. Send us the details of your tower and we will outline the scope. See our PNG riser stress analysis service.