Riser design key to tall building safety

By Nick Atkinson, Director at Ambar Kelly

Our cities’ skylines are rising. Yet with the construction of multi-storey buildings, risk becomes a greater issue, one area of which being the riser zone. As with many aspects of building design, if risk is not considered at the design stage, the implications increase – and all too often so does cost – making it perilous for those constructing and indeed occupying these structures.

The riser zone is a vital feature of every tall building which interfaces many construction disciplines: architecture, structural engineering, fire, drylined walls, mechanical and electrical (M&E), main contractors and frame contractors. As a result of its cross-disciplinarily, the question ‘who is responsible for the ‘hole” can be rightly asked.

Chain of responsibility

As well as providing an artery for a myriad of vital utilities such as water, ductwork and electrics, without sufficient forethought, a riser is fraught with danger. Falls from height, fire, flood; the risk percentage rises when designers relinquish responsibility and pass the ownership of the ‘hole’ on to the contractor.

The chain of events usually runs something like this: an architect determines the location of the riser – which starts life as a hole in the structural floor – before a structural engineer calculates the strength of the infrastructure surrounding it. An M&E consultant will decide on the service to fill the ‘hole’, which is left in the overall care of nobody in particular, it seems.

According to ‘General principles of prevention’ outlined in ‘CDM 2015’ Managing health and safety in construction, dutyholders, including architects, are instructed to avoid risk where possible. However, there are loopholes in this guidance. The misconception is the riser is a risk issue not a feature of design. Although a riser is part of an architect’s principles of prevention, it is not being included in the design. It is left to the main contractor to manage the risk which means they look at the hole not the whole.

Where GRP falls short

It leads to the conclusion that designers are failing to meet their legislative obligations by creating a govern-less holes within a building. By planning walls, doors and floors around a riser, designers are effectively creating a shaft… or chimney.

Now, if the main contractor decides to use Glass Reinforced Plastic (GRP) grating as the maintenance flooring and installs it then this adds to a building’s smoke and fire loading in both its under construction and final state. GRP grating is generally seen as a safety device to prevent people falling through holes, but how effective is it?

The stakeholders all think that because the perimeter of the shaft is built to contain a fire for 2 hours this mitigates the risk. This is a false presumption as no risers are truly sealed and the fire doors that are employed have an air gap around until the intumescent strips expands through heat. This expansion can take time and before that activation through heat the air gap around the doors allows air to be drawn into the shaft which is a chimney. In a multi-storey building these air gaps can be numerous, feeding air to the fire, creating great heat and generating a self-serving air provision to sustain the fire.

There are three basic resins that are used to bind the woven glass fibre strands to form GRP: Orthophthalic (Ortho) resin; Isophthalic (Iso) resin, and Phenolic resin. Iso resin GRP grating is said to meet BS 476-7, the regulation relating to the measure of the surface spread of flame, but only in flat sheet form, tested vertically. One cannot test a grating

However, when installed in a riser’s chimney-style environment, Iso GRP grating is found to be extremely flammable. This makes the BS 476-7 classification an unsuitable test and is as much good as a “chocolate fire guard” in determining if a product is suitable for its use within a riser zone. Ortho and Iso GRP are prohibited on offshore oil platforms and the London Underground for fire and smoke reasons, yet it is readily used in high-rise construction. Surely, if we are paying more than lip service to the risk of fire shafts, then either Phenolic GRP (identifiable, as it is red/brown in colour) or solid steel maintenance platforms should be used.

Prioritise riser design

Therefore, as there are issues with using a GRP solution, it seems sensible to turn to an alternative system.

Riser safety ought to be of major concern for the building sector, particularly as HSE figures show falls remain the biggest cause of fatal injury in Britain’s workplaces. For example, 2017/2018 data reveals 26% of workplace fatalities – about 37 per year – resulted from a fall from height.

Clearly there is demand for a solution which assures complete riser zone security. Systems such as RiserSafe® offer a bespoke, prefabricated, fully-cast solution for issues relating to riser voids. Meeting individual project demands, these systems’ protective properties are designed to endure a building’s lifecycle.

Incorporating M&E services into its design, acting as barriers to smoke and flame these solutions negate the need for temporary handrails on site and double-up as a storage area to reduce labour at height.

With this approach, riser-related issues are addressed at the design stage. It requires designers being proactive, rather than reactive. The location of services, horizontal fire compartmentation and riser firewalls should be designed before projects go out to tender, helping to drive efficiencies on site with decisions being made long before work has started. A riser zone accounts for 0.1/0.2% of a total build’s cost, which ought to add weight to the argument for its design falling to the architects will give cost certainty.

By designing the riser zone, multiple benefits can be reaped and many risks can be eliminated. If efficient measures are employed to fill in the riser void, then frankly there is no risk of workers falling from height. If the riser zone is a core part of a building, then how long will it be before it becomes a core factor in building design? With some joined-up thinking and a holistic approach taken by architects, the question of ‘who’s responsible for the riser?’ will go some way to be being answered.