Platforms and Towers Overview

Mobile Access Towers

Mobile access towers (MAT’s), commonly referred to as “alloy” towers or even “ali” towers, have evolved from specialist equipment developed in the USA during World War II for use in aviation maintenance and as quick erect radio masts for communicating with advancing troops. Since those early days the product has gradually evolved to become an essential tool of the building and maintenance industry world wide and particularly in the British Isles. Prior to the widespread use of aluminium in mobile access towers the building and maintenance industry relied on towers that were made from painted or galvanised steel sections. This made them extremely slow and heavy to build and even more difficult to move and dismantle.

Originally formed using aluminium alloy tube extruded in imperial 2 inch diameter, sections were joined using aluminium alloy castings and crimped or welded joints. MAT’s are now available made using a wide range of materials. These materials come in various tube thicknesses and diameters and components are fabricated using a variety of jointing methods and the resulting towers are capable of being configured into a wide range of widths, lengths and heights,

Aluminium alloy is still the most common material used in making towers but now both extruded and seam welded tube is used with overall tube diameters ranging from 44m to 50.8m and with wall thicknesses ranging typically from 1.5mm to 3.0mm. The original tube dimension of 2” (50mm) overall with a 1.5mm wall remains the most common size used. Advances in welding technology have enabled sections to be joined together using direct tube to tube (butt) welding as well as using traditional castings which can be welded or crimped together. More recently GRP tube has become available offering greater insulation properties and this tube is usually joined using nylon castings and epoxy resin glues. Nylon and plastic is increasingly being used in standard aluminium towers for components such as spigots and toe board clips as the cost of raw aluminium continues to rise.

Although there are any number of tower shapes and sizes on the market, in general terms mobile access towers are available in lengths ranging from 1.5m to 3.0m (1.8m and 2.4m being the most common) and in width from 0.6m to 1.5m (with 0.8m usually known as single width or s/w and 1.4m usually known as double width or d/w being the most common). There are some towers that are 2.0m wide (triple width) and even larger platform areas are frequently formed by linking two or more towers together.

After length and width, the next main variable in MAT’s is the means used to get to and from the platform. The four most popular methods are via a vertical ladder welded into the frames at one end of the tower, by a series of clip in ladders usually angled at about 70°, by the horizontal rungs of the frame where these have a non slip surface and are at the same spacing as ladder rungs (230-300mm) or by a stair ladder complete with banister rails

Mobile access towers have been built to heights far exceeding 30m but the current European Standard for MAT’s; EN1004 sets a maximum height for standard towers at 12m indoors and 8m outdoors. This difference reflects the effect that the weather, and particularly the wind, can have on towers used outdoors. In general terms however the average height of towers sold or hired has decreased over the past 20 years reducing from a typical height of 6-8m to a typical height currently of 2-4m. Care needs to be taken when considering the height of a tower as some suppliers quote the height of the tower as being to the height of the top platform, others take it as the height of the top guardrail while others take it to be the height that a typical user can comfortably reach. The variation between these three methods can be as much as 2.0m

The load that a tower can take varies from brand to brand and is affected by factors such as the thickness of tube used and the rigidity of the frame joint. Minimum loading criteria are specified in EN1004. In general terms however a typical tower will take a total load of somewhere between 750kg and 950kg with individual platforms limited to 1.5 or 2kn/m² which equates to something like 200kg+ on a typical 1.8m single platform. Some heavy duty towers can take total loads in excess of 2,500 kg.

Although this article has concentrated on the typical and traditional specification of Mobile Access Towers the introduction of the Working at Heights Regulations in 2005 has lead to the development of a whole range of small tower related products that come under the heading of low level access equipment. These are items that are made from the same materials as MATs but usually have small platform areas at heights below the 2.5m level which is the minimum height for products certified under EN1004-2004. This includes such items as Room Scaffolds, Podiums, Pulpits and Step units. Most of these fold for ease of transport.

Mobile access towers require regular inspection and maintenance although maintenance is usually fairly undemanding. A typical inspection by a user before use of the tower would call for visual inspection before use of all welds, all rivets and a check that the castors rotate freely and that the brake mechanism works properly. The components should also be free from contamination such as concrete or mastic.

An item with cracked welds, loose or missing rivets or dents that have creased the aluminium tube should be taken out of use and sent away for repair. The repair should ideally be carried out by the original manufacturer or by a repair company approved by the manufacturer. This ensures that the original warranties and performance criteria still apply.

Although fundamentally straightforward to build; all manufacturers recommend that towers are built by at least two competent persons and these days the Prefabricated Access Suppliers and Manufacturers Association (P.A.S.M.A.) training course is seen as the industry standard. This is a one day course available from a national network of training centres designed for operators, supervisors and managers which cover safe assembly, use, inspection, repositioning and dismantling of towers. PASMA can be contacted via their web site at www.pasma.co.uk

As with any other product that allows users to work at height a risk assessment needs to be carried out before a tower is used to ensure that it is suitable for the job in hand. Mobile access towers offer collective fall protection, ease of access and egress, they are easy to move and, provided they are built correctly, safety for operatives during the assembly and dismantling process. Two methods are currently recommended by PASMA and the HSE for assembling, altering and dismantling mobile access towers which take account of the need to prevent falls during these processes. These are by the use of advanced guardrails or by using the 3T (Through The Trap) assembly method. Details of both of these methods can be found on the PASMA and HSE web sites and in HSE Construction Information Sheet No. 10 (Revision 4)

There are a number of optional extras and non standard configurations that can be used with or added to MAT’s many of these are illustrated in BS1139 part 6 and can also be seen on manufacturer’s web sites. PASMA offer advanced training course for many of these such as cantilever towers, linked towers, chimney scaffolds and walk through units.

Author: Paul Pritchard