The features of safe stair design
Stair Safety: BWF Technical Manager Kevin Underwood summarises the features of good stair design
A timber staircase is an opportunity to show off design flair and innovation, but the desire to create a centre-piece for the home should not surmount consideration given to the safety of the stair. Stairs present a risk of injury through slips, trips and falls and while a home owner may believe that this type of accident would not happen to them the statistics belie this view. In their report Can the home ever be safe? the Royal Society for the Prevention of Accidents (RoSPA) states that every year there are over 300,000 visits to Accident and Emergency following falls on the stairs.
Most serious falls occur in descent a person falls backwards and slips down the stairs, injuring themselves on the nosings of the steps (they pass) or when they hit the floor or other objects at the bottom of the flight. Occasionally, more serious injuries occur when the user falls forward rather than backwards. In ascent, the injuries tend to be less severe as the fall is most usually forward onto the steps above; but again, should the user fall the other way (backwards rather than forwards) much more serious injuries can occur. Carpets can reduce the severity of an injury while hard, sharp edges are likely to cause more serious injuries.
Accidents can be reduced by attending to those characteristics of a staircase that affect a user’s ability to use the stair safely. The geometry of the stair, the rise, the going and the position and shape of the treads can affect the ability of the user to safely place their feet and avoid tripping or slipping on the stair. But if a user does begin to fall, a suitable handrail can allow them to steady themselves and at all times adequate guarding can prevent users falling to the side of the stairs.
Under-step and over-step
Many accidents are caused by an under-step or over-step in both ascent and descent. In ascent, an under-step is where the foot is not placed fully on the next step up and an over-step is when the toes catch the riser of the next step up or catch under the nosing. In descent, an over-step is where the foot lands beyond the tread of the stair and an under-step is where the heel catches the nosing or riser of the step above.
Consistent rise and going
A key cause of under-stepping or over-stepping is when there is inconsistency in the rise and going of a stair. When a person starts to ascend or descend a stair they subconsciously determine the appropriate movements they need to make with their legs and feet based on the first one or two steps they take. This process, known as ‘proprioceptive feedback’, makes the user vulnerable to variations in the rise and going of a stair as they expect to step up or down by the same amount throughout the flight. Variations between steps are unexpected and can lead to the user tripping or slipping. It is, therefore, important to allow only small variations between steps of around 1% of the design dimensions.
See video below on the steps to 36 Street Street New York Subway Stairs
When a user places their foot on a step the best support is provided when they can place most, if not all, of their foot on the tread. But when the going falls below 250mm this is not always possible and in order to get adequate support, users begin to turn their feet to the side at each step. 95% of domestic stairs fall into this range, and walking with turned feet has become the expected method of walking on a stair. However, the risk of slipping on nosings is increased when the amount of the foot that overhangs the tread increases; so the user that doesn’t turn their feet is in danger of slipping.
Where stairs are designed to have an open rise the treads should overlap and there should never be horizontal gaps between treads when the stair is viewed on plan. In order to protect children who may use the stair the vertical gaps between treads should be small enough to prevent a 100mm diameter sphere from passing through.
Fewer than expected accidents occur on stairs with winders (stairs change direction without a landing), even though winders are often used in narrow, steeper stairs when space is at a premium. It is possible that users take less care when walking on straight flights and are more cautious on winder flights. Well-designed winders providing an adequate going on the walking line and at least 50mm of tread at the newel can be safe to use. However, winders that cannot provide adequate support at each step can lead to users dropping over 600mm after a slight slip near the newel.
Users of stairs quite often don’t use the handrails provided and may feel they are unnecessary. Where there is adequate going for the user to place their foot comfortably and the rise is not too high, users tend to feel confident and do not use the handrail to steady themselves or to pull themselves up the stair. However, handrails are an essential safety item. When a loss of balance occurs users are able to catch hold of a suitable handrail and stabilise themselves within a fraction of a second. It is, therefore, essential that the handrail is positioned so that the hand can reach it quickly and, as a fall can occur on any part of the stair, the handrail needs to be within easy reach throughout the whole flight.
To adequately protect users from falling from the side of the stair any fall of more than 600m should be protected by guarding. This should be strong enough not to give way when weight is applied to it and it should be tall enough to stop people falling over the top of it in normal use. To protect children who may use the stairs, any gaps in the guarding should be small enough to prevent a 100mm diameter sphere from passing through.
The surface finish or covering to a tread can affect the likelihood of slipping and worn coverings can cause a trip. Where the going of the stair is 300mm users tend not to be affected too much by the slipperiness of the surface but on stairs with treads less than 300mm consideration should be given to a measure of slip resistance at the nosing. This is where first contact is made in descent and a slip resistant surface can help turn a potential slip into a slight loss of balance which can be corrected by gripping the handrail.
The characteristics described above are just representative of the issues affecting the safe use of stairs. There is a wealth of research into how people use and perceive stairs and into what makes stairs both safe and unsafe to use. So while the statistics show that there are a lot of injuries attributed to trips, slips and falls on stairs in the home, stairs should not be considered as inherently unsafe, as the risks associated with stairs can be mitigated through good design.
Article by Kevin Underwood, Technical Manager of the British Woodworking Federation
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