Drainage: tall buildings
Drain of thought
15 May 2017
Proper design, specification and installation is essential for drainage in tall buildings, writes Daniel Oliver
From design to construction, tall buildings are some of the most complex projects faced by construction professionals. Although the effects of getting things wrong may not be immediate, without the proper expertise in place throughout then drainage in tall buildings may eventually fail.
Using the end-to-end expertise of a drainage manufacturer and installer is the best way to prevent any future complications. From design to product training, bespoke fabrication and installation, an holistic approach is needed, considering all potential issues, from complicated systems and high numbers of toilets to dealing with rainfall.
If drainage systems are to function properly, it is vital to assemble and install products correctly. Training on the benefits and specifications of a company’s entire portfolio allows the right products and systems to be installed on any given project. A “one size fits all” mentality does not work for drainage, particularly when systems are complicated.
Training on the specifications of a company’s portfolio allows the right products and systems to be installed – a “one size fits all” mentality does not work
Many construction professionals underestimate the complexities of designing and installing drainage in a building, with tall buildings presenting additional challenges. Working with a single manufacturer from the design to the installation of products can often obviate these issues.
Successful drainage systems require pressure to work successfully. For every 70.1cm of building elevation, 6.9kPa of air pressure is sacrificed, and a drainage system can be compromised as a building’s height increases. The pressure must be maintained throughout the entire system from roof to sublevel, and this presents a major challenge.
When water travels through a drainage system, the space left behind in the pipe must be replaced with air; otherwise negative pressure will build up. This in turn will lead to poor performance, with the potential for blown trap seals, siphoning and consequent drainage failure.
Figure 1: Millbrook Tower in Southampton used Polypipe's drainage services
The hydraulic jump associated with this change in pressure puts considerable strain on pipe joints and can lead to coupling failure, which must be counteracted at design stage by specifying restraining joints.
Air pressure valves
Tradition dictates that to prevent these rapid changes in pressure, a secondary ventilation stack must be installed. However, this can take up valuable commercial space and requires double the materials and staff to install.
More modern methods, including the installation of air pressure attenuation valves, are a proven alternative. The valves absorb the effect of positive back pressure, which can blast water out of traps, by slowing the speed of the airflow from a potential 320m/s to 12m/s.
The way water travels throughout drainage systems also needs to be considered, particularly in tall buildings. When water moves through a vertical pipe, it travels down the sides, creating a vortex with an empty space through the middle. While this presents no major issues when a pipe is vertical, any horizontal branch pipes – which are inevitable in a drainage system – will be compromised.
Water can travel at speeds of almost 5m/s down a vertical pipe, but this should be reduced to around 2m/s when it reaches an offset pipe. Failure to do so can result in air compression in the system and its eventual failure.
Failed drainage can cause the release of unpleasant odours, the backflow of foul water and even the spread of disease or potential building damage
This complication is worsened by the installation of a large number of toilets in a building, as in high-rise apartment blocks and hotels for instance. Again, the intelligent use of attenuation valves in the system can rectify this before it becomes an issue.
As mentioned, if the movement of air in the drainage system is not properly managed, there can be serious consequences. A failed drainage system in a building can cause the release of unpleasant odours, backflow of foul water, air contamination and even the spread of infectious disease or potential building damage.
As is often the case, prevention is better than cure, and by working with knowledgeable manufacturers from the outset of a project, not only can issues be avoided but additional benefits can be realised.
Because just a small amount of water separates a drainage system from the inside of a bathroom, if a trap blows then the sudden surge of pressure may result in contents of the soil and waste pipe spouting from toilets and sinks. In densely occupied buildings such as high-rise apartments and hotels – with an increase in toilets and therefore outlets for blown traps – the consequences would be exacerbated.
Partnering with manufacturers also enables a project to take predicted amounts of rainfall into consideration. While most rainfall events in the UK comprise less than 5mm of precipitation and can be controlled adequately by traditional systems, the increasing frequency of extreme weather means that any flood risk must be taken into account.
Severe rainfall and consequent flooding may compromise the integrity of a building’s above- and below-ground drainage, damaging the property, roads, cars and the surrounding area. However, by working with specialist manufacturers these issues can be avoided, for example with the installation of green and living roofs, which are now being incorporated into local authorities’ strategic flood risk assessment plans.
The most effective way to counteract any potential complications is to factor all possibilities in at the design stage of the project. Working with experts in air and water movement in tall buildings will ensure that the selected systems are appropriate for the project, as well as reducing the time and staff required for installation.
Each project will have its own complications and consequent solutions. When fitting complicated drainage in a building, incorporating a manufacturing partner from the consultation stage to design, manufacture and installation ensures that experts are on hand through to project completion.
With each additional storey of a building come additional complexities and challenges, and the correct product needs to be specified accordingly. If a building will be used primarily at night and the comfort of residents is the key consideration, as in residential towers, student accommodation and hotels, then acoustic – that is, more extensively soundproofed – soil and waste drainage may be deemed necessary.
With triple-layer pipes developed from co-polymer polypropylene and acoustically engineered fittings, the transmission of airborne noise through the building structure is limited. Office blocks, however, may be more suited to a system made from high-density polyethylene with strong resistance to abrasion, chemicals and temperature.
Once the system design has been finalised, working with a manufacturer that offers a fabrication service means that drainage stacks and products can be made specifically to the particular standards required on each project.
Figure 2: Using a fabrication service means products are provided and installed to the exact specification
Simplifying on-site connections, air-testing drainage stacks in advance and reducing on-site waste means installation time is reduced, drainage systems are more reliable and the product meets the project’s exact requirements.
With the construction industry shedding 343,000 jobs since the financial crisis in 2007 and an estimated 400,000 staff to be lost to retirement in the next decade, architects and surveyors are continually looking at ways to reduce labour time and demand for human resources. Using off-site construction is one of the easiest ways to do this, and is particularly effective in the manufacture and installation of drainage.
Even expertly designed and manufactured systems will fail if they are incorrectly installed, though. Training installers on the technical requirements for each product’s use, as well as in best practice for installation, will prevent such failure. Training can be conducted in dedicated seminars or on site, and will help ensure the integrity of the installation.
As technology advances, manufacturers are continually improving the methods by which their products are installed. Welded jointing can ensure that the weld area is as strong as the host material, reducing the risk of leaks, but if installers are not trained in this method then complications will occur, so proper training is needed.
The incorrect installation and maintenance of drainage can damage the infrastructure of the building, with dire logistical and financial consequences. While specifying and installing drainage in a building can be complicated, working with experts from design to completion will ensure that the system is not only functional but saves resources, reduces demand for staff, limits costs and meets all deadlines, making it a sound investment.
Daniel Oliver is a product manager at Polypipe Terrain
- Images © Polypipe Terrain
- This feature is taken from the RICS Building control journal (April/May 2017)