Barrier Heights, guard rails and hand rails

As I seem to have difficulty explaining the difference between handrails and guardrails and when each is needed, I have added an XY chart to my balustrade design spreadsheet so that I can illustrate the different situations.

Handrails and guardrails are not the same thing. Whilst it is common for industrial guardrails to use a top rail for both guardrail and handrail, the two functions are different and the rails have different characteristics.

Handrails

A hand rail has to be at a height that it is comfortable to hold, this generally cannot be achieved for both adults and children by the same rail. Schools are the primary situation where two handrails may be required: one low level handrail for children and a higher level handrail for adults. In other situations only the one handrail suitable for adults is required, in such situations vertical infill rails may be a suitable compromise for children. Not all locations require handrails. The function of an handrail is to assist with mobility and provide steadiness. Handrails are typically required on stairs, ramps, and in corridors of hospitals and aged care facilities, they may also be required on narrow footbridges and walkways. A handrail needs to be strong enough that it does not crush in the hand, it also needs to be large enough that it can be properly gripped. There are no clear structural design requirements for handrails, though AS1428.1 Design for Access and Mobility, does provide a 1.1kN design requirement for grabrails. A handrail needs to be stiff enough that it does not flex too much between supports: if it flexes too much then it does not steady a person’s movements. In a crowd loaded situation a handrail may be broken and become a hazard. However, given the situations in which a handrail are required, no crowd should be able to develop behind a handrail.

Guardrails

The function of  barrier is to obstruct and control human traffic. The simplest barrier is a rope or a single rail. Such structure has to be at a minimum height such that a person will not topple over the barrier. However if the barrier is too high, then a person may fall under the barrier. The old version (1992) of AS1657 set the heights for barriers between 900mm and 1100mm, the new version (2013) sets the height greater than 900mm. The Building Code of Australia (BCA) sets the minimum height for a barrier has 1000mm. Given that the 95th percentile height for the centre of gravity (COG) for a standing human is a little over 1000mm, a minimum height of 1100mm seems preferable.

The COG is approximately around waist height, or at the level of the belly button. If a rail is placed at this height then a small force is required, at shoulder height, to dislodge a persons footing and potentially topple them over the barrier. As the barrier height increases a much larger force is required to topple a person.

A rail needs to be of such a shape that when a person is pushed against such rail they do not experience injury, consequently a rail suitable as a handrail may not be suitable as a guardrail even if it has adequate strength. The code for the use of glass in buildings (AS1288) requires the use of load bearing handrails for infill barriers irrespective of their height. It would be better to refer to the rail as a guardrail, as its function is to prevent the primary load from being applied to the infill. If the rail is installed at the minimum height for a handrail, then it will not function as a guardrail, a person could topple over the handrail and then they would be bearing against the glass panel behind. The glass panel would then no longer be acting as infill but as the primary load bearing element. As a primary load bearing element the glass panel then needs to be designed for the AS1170 top edge loading requirements.  For tall panels this may not be sensible, as the panel maybe higher than the reach of the load source. For example a crowd is unlikely to have a reach above shoulder height (approx. 1500mm).

Top Rails and Top Edges

A rail type barrier of minimum height can have a single top rail which functions as both guardrail and handrail. If the barrier is made higher, then such top rail cannot function as a handrail and separate handrail is required. If the barrier is made significantly higher than minimum, then the top rail cannot function as a guardrail and is unlikely to experience the top edge loadings defined in AS1170. A full height glass panel for example has a top edge at ceiling level, applying the AS1170  top edge load to the top edge of such glass panel does not load the panel, it loads the ceiling plane and is clearly inappropriate. However simply assuming that such top edge load is a line load to be applied at some suitable height on the panel is also inappropriate: as a crowd pushing against the panel is likely to apply a force over a large area, somewhere between the height of the knees and the shoulders, with the greater load being between the waist and the shoulders. Therefore some judgement needs to be made on applying the AS1170 top loads.

Examples

Minimum Height Guardrailing, with knee rail and kickplate
Minimum Height Guard railing, with knee rail and kickplate

 

Barrier 1100mm high, with additional rails between knee rail and top rail to reduce spacing of rails.
Barrier 1100mm high, with additional rails between knee rail and top rail to reduce spacing of rails.

 

Tall horizontal rail barrier.
Tall horizontal rail barrier.

It should be noted that horizontal rails do not off themselves make a climbing hazard. The rails are a climbing hazard if people can climb over barrier and then fall or jump to the ground below. An enclosed foot bridge, with mesh barrier,  would only permit people to climb inside the cage and fall to the level of the walkway inside the cage. The main requirement would be to stop people accessing the barrier from the outside, that is getting on top of the bridge enclosure and walking along its top.

Barrier with infill panels, top rail and guardrail
Barrier with infill panels, top rail and guardrail

 

Panel type barrier
Panel type barrier

 

Tall Panel Type Barrier, with Guardrail
Tall Panel Type Barrier, with Guardrail

 

Tall Panel Type Barrier with guardrail, and handrail
Tall Panel Type Barrier with guardrail, and handrail

 

Tall Barrier with Top Rail
Tall Barrier with Top Rail

Here the top rail is too high to function as a guardrail, and the panels cannot be considered as merely infill, the panels need to be designed for the top edge load: but such load does not make sense. The AS1170 top edge loads need converting into some equivalent surface loads for the design and/or testing of the panels.

Barrier with Posts Set below Floor Level
Barrier with Posts Set below Floor Level

Base moments for the posts are based on the length of the columns, not on the height of the barrier.

Barrier with Posts Set Above Floor Level
Barrier with Posts Set Above Floor Level

If barrier posts are set on top off concrete upstands then those upstands need to be adequate to support the base moment and shear imposed by the barrier loads.

Loading Zones for Barriers

The following diagrams show equivalent surface  pressures, for different height barriers. The surface pressures are based on an assumption that the top edge loads are not applied at a height greater than 1100mm. The base moment therefore becomes a constant, then a surface pressure is calculated based on the load width from 1100mm to the top of the barrier. It is assumed that the surface pressure would be converted into an equivalent line load applied at the mid height of the loading zone, to determine base moments for the barrier. The surface pressure is assumed to act in combination with infill loads, and loads for walls of light weight construction: so base moments for higher barriers will increase.

Loading zone one: Below waist level
Loading zone one: Below waist level

Low barriers, those below 1100mm, should be designed for the AS1170 top edge loads: top edge loads and infill loads are taken independently.

Loading zone 2: above waist level below shoulder level
Loading zone 2: above waist level below shoulder level

Here a high surface pressure results as a consequence of a small load width, it may or may not be of benefit to the design and assessment of the panel.

Loading Zone 3: Tall barrier, higher than shoulder height
Loading Zone 3: Tall barrier, higher than shoulder height

 


Related Posts:

  1. TechNote#300 Barriers (Conversion of top edge loads into surface loads)

Notes:

  1. I typically prefer the spelling handrail and guardrail to hand rail and guard rail. In general I also prefer compound words without hyphens.
  2. I know we now have the National Construction Code (NCC), however the NCC is no more the building code of Australia (BCA), than a car is a wheel, or a wheel is a car. The BCA is part of the NCC, it is not the NCC, and I will continue to refer to it has the BCA.

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