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façade access technology
Cento Engineering Company Limited
INTRODUCTION TO SPECIFYING A BUILDING MAINTENANCE UNIT (BMU)
Building roof space is at a premium, with demands for air handling units, lift overruns, satellite
dishes, and even future tenant space. Finding suitable space for a BMU and its track system on
the roof is an important part of the building design. Before you design a roof layout with
appropriate space for the BMU, you must first understand the effects the features you require
have on its design in terms of space required and loads supported by the structure. Please note
this document uses layman’s terms, not industry specific terminology.
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The key items that influence BMU static weight and wheel loads
Maximum safe working load within the cradle.
Maximum Jib outreach.
Requirement for telescopic jib function.
Requirement for luffing or self elevating jib function.
Counterweight radius restrictions.
Track gauge (Distance between centre line of front and rear track).
BMU hoisting height / Building height (Rope length).
Cradle design, size and weight.
Where a glass replacement facility is included, establish glass weight, surface area and
replacement procedure.
Wind speed (Non-guided platform, up to 40m high 14m/s. Guided platform, over 40m 20m/s)
as indicated in BS EN 1808:1999 page 18.
BMU Manufacturer - Not all BMUs are designed the same way. Manntech BMUs generally
impose lower loads on the structure when compared with other manufacturers.
BMU outreach and jib design
Once the available location on the roof is chosen, the cradle drop positions must be determined.
The two most important cradle drop positions are those furthest away from the BMU and closest
to the BMU. When specifying the outreach, the goal is always to keep the jib length to a minimum
to reduce wheel loads, cost and BMU mass. Large outreach BMUs sometimes are required to
provide access close to the BMU roof car (the part of the BMU which runs directly on the track or
runway). When the outreach is too large for a fixed length jib to also position the cradle close to
the roof car, a telescopic jib, or hinged jib design is required. Telescopic or hinged jibs are often
also used to reduce the BMU length when parked in garage or storage compound areas.
Standard Systems
Compact Crane Systems
Crane Type Systems
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Auxiliary hoisting capacity
The requirement for auxiliary hoisting capacity, for glass/cladding panel replacement or lifting
small plant items is often overlooked. This additional function can have a large effect on the BMU
design, increasing size, weight, wheel loads and cost. The weight of the item to be lifted must be
accurate. The load, which can weigh up to 1000Kg (the same as a small car) will be suspended
at the end of the jib causing high leverage forces and in turn will increase wheel loads on the front
track. To comply with current regulations, the wheels on the rear track should never lift up from
the roof. Therefore a very large counterweight is required to stabilise the BMU and its load; this
can amount to Tens of Tonnes (several small cars).
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Cradle
There are various types of cradle to choose from, but the choice will be based on any areas of
facade requiring access which are set back from the main facade line, for example beneath
balconies or brise soleil. Please refer to the drawing on the next page for more information.
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Track System
The track will be either Galvanised Universal Beam or Galvanised Universal Column structural
sections. The size of the track section is dependent on the specification of the BMU, and can
typically range from UB178 x 102 (19kg per Metre) to UC305 x 305 (137kg per Metre).
We describe the working zone of the BMU as the clear passageway. This is the minimum clear
space required for the BMU to traverse along the length of the track without colliding with any part
of the building or roof mounted plant. The clear passageway will dictate the track gauge (distance
between the centreline of the front and rear track). As a rule of thumb you will require a minimum
track gauge of 1500mm for a BMU with an outreach up to 10 Metres, 2000mm track gauge
between 10 and 20 Metres outreach, 2500mm track gauge between 20 and 25 Metres outreach
and 3000mm track gauge for an outreach between 25 Metres and 30 Metres.
In addition to the track gauge you need to allow clearance for the wheel units which run on the
track. Allow 400mm between the outside track and the inside of the parapet and 400mm from the
inside track to the plant screen. This gives you the absolute minimum clear passageway
dimension. For example, a 1500 gauge track will require a clear passageway of 1500mm +
400mm + 400mm = 2300mm.
More clearance is required at bends in the track, because the BMU rear wheels will form a chord
across the inside of the bend, causing the roof car to move away from the outside of the corner.
You will also need to consider whether the counterweight on the jib can slew clear of plant
screens or parapet details to the front or rear of the BMU. If it can, this helps keep the minimum
passageway dimension reduced. The minimum passageway condition can be reduced by
including a self elevating jib and counterweight feature which allows the jib to be parked lowered
towards the roof and raised when in use for operation above roof level obstructions.
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Track support options
Cast in stainless steel U-bolts – The traditional option. Cento can provide the stainless steel Ubolts to be tied into the structural steelwork and cast into concrete upstands by other contractors.
Galvanised steel posts – Steel upstands fixed back to the reinforced concrete or steelwork
structure. These can be through bolted or welded.
Freely laid rail (FLR) – Galvanised steel upstands, including links between the front and rear
track. There are no penetrations of the roof finishes when using the FLR system, therefore it is
quick and easy to install. It is designed to sit directly on top of the roof insulation and roof finishes.
In addition to the three options above, it is also possible to use a trackless BMU, which runs on a
reinforced concrete runway. The BMU is guided by a small angle guiderail, or by the inside of a
structural parapet when traversing.
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Further design support options
N.B.: Cento Engineering Company Ltd are not facade access consultants.
Centos core business is to supply, install, test, commission and maintain facade access
equipment. This information sheet is offered to assist you in the process of specifying
permanent facade access equipment.
Whilst we would like to offer design assistance at no cost, we are a specialist contractor and
recover our costs through manufacture and installation of permanent success systems. We
can offer limited design support without obligation. However, there are various specialist
facade access consultants such as Arup, D2E International, Hilson Moran, Hoare Lea Lift
Engineering, Lerch Bates, Reef Associates Ltd, and WSP Group, all of which can be found on
the internet.
Cento Engineering Company Ltd, 6 Baddow Park, Great Baddow, Chelmsford, Essex. CM2 7SY
Telephone: 01245 477708.Facsimile:01245 477748.E-mail: [email protected]

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