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EP_19.jpg March/April 2005
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BEARING THE LOAD
Keith Rainford looks at the role of cranes in off-site construction.

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CT_indexHP.jpg The adoption of off-site construction techniques is gathering pace; after all it is unrealistic to expect three million new homes to be built brick-by-brick. Walls, floors and even complete fully fitted rooms are now factory built in timber, steel or concrete. When setting up such off-site facilities it is essential to consider the need for safe, efficient and reliable handling of these large components.

It was the Venetians, not Henry Ford, who invented the first production line. Needing hundreds of ships to service their empire, they decided that a standardised design would cut costs, have lower maintenance and operational advantages. Their answer was to build a canal and float each hull through a series of points where key equipment was installed. This was an elegant solution, but not available to everyone.

Production lines are perfect for products that are required in high volumes and are of a fairly regular and usually light-weight design. Where products are non-standard, bulky, irregular and heavy – as with most building elements – a more flexible answer is required. As engineers know, overhead cranes are the ideal solution.

Overhead travelling cranes (OTCs) occupy no factory floor space. They provide access to most of the building volume including the complete production area from floor to hoist level. They can cope with a range of loads including large, irregular and heavy elements. They are based on proven technology and crane manufacturers have vast experience in problem solving from a diverse range of industries. A properly engineered crane system will provide 20 to 30 years service. Moreover, cranes can be designed to address the many unique requirements of manufacturers.

It is important to match the crane to its intended use. This means more than providing a crane to handle the maximum safe working load (SWL). By examining the production process in detail the crane engineer can optimise the structure, mechanisms and handling devices to ensure safety, reliability and efficiency.

Important operational questions to consider are:
• The maximum and minimum load the crane will handle
• he overall dimensions of the load
• The number of crane movements per day
• The proportion of loads that are at, or close to, maximum SWL

Critical environmental considerations include:
• The span of the bays where the crane is installed
• The height of the bay and available envelope for crane installation
• The load carrying capacity of the existing structure
Based on these and other relevant criteria a crane specification will be developed for optimum performance


An example of a building component supplier using cranes to boost output is floor supplier Milbank. Business is booming for this building systems supplier, which prompted the development of new facility to cope with demand for pre-stressed and reinforced concrete flooring units.

Inside the factory, Milbank has installed two overhead cranes of 16 tonnes safe working load. Each crane spans 34 metres and is fitted with twin eight tonnes hoists. Installed in the yard, is a 20 tonnes SWL crane, spanning 30 metres and fitted with twin ten tonne hoists. Using twin hoists together ensures safe handling of larger/heavier loads while the flexibility to use a single hoist allows smaller loads to be moved efficiently.

All the cranes are of double girder construction for maximum strength and rigidity. Load stability, productivity and safety are important issues so a sophisticated computer based load sway control system regulates all crane movements to ensure stable high-speed load transfer.

Carefully Matched
Crane specifications are carefully matched to the manufacturing process. Currently the factory produces around 1000 square metres of hollow core floor per day. This is due to double in the next expansion phase when throughput will rise to around 600 tonnes of concrete flooring per day. All crane mechanisms are therefore heavy duty reflecting the large number of heavy loads and high number of crane movements expected.

The cranes are employed in double hoist mode to transfer 50 metre slabs from the casting shop to a conveyor that then moves them to a cutting area. A homing system ensures that the cranes deliver the concrete sections to precisely the same location on the conveyor. Cranes are used in single hoist mode to move moulds and other process equipment. Control is by radio remote devices allowing the operators to position themselves perfectly for greatest safety and visibility.

This kind of crane technology can be found in modular building too. Several modular builders in the UK also use overhead cranes. The flexibility of OTCs enables them to be used to handle materials and building elements of different shapes. They are used in the fabrication of floors, walls and roofs, to assemble these elements together to create modules and in the final transportation of completed modules.

For greatest flexibility, unit manufacturers commonly use two cranes with twin hoists. Small items such as pack of plywood or pallet of other materials can be moved using a single hoist. Some manufacturers use vacuum devices in place of a hook to handle sheet material. Building sides can be moved from fabrication into assembly by using cranes in twin hoist mode while whole modules are moved using two twin hoist cranes operated in tandem to give heavy lift capability with maximum load stability.

Modern cranes provide a robust handling system for many off-site manufacturing environments. They enable the twin demands of safety and productivity to be met without compromise. Different hoist, control and accessory configurations can be used to meet special handling needs. Moreover, systems can be future-proofed to allow for expansion or process changes.

Keith Rainford is the sales director at Street Crane Company. For further information, telephone 01298 812456 E-Mail: admin@streetcrane.co.uk or visit www.streetcrane.co.uk

CASE STUDY
When Heiton Steels, the largest Irish owned steel stockholder, sought to speed material throughput in its stockyard, the company looked to a solution from Street Crane. The answer was a unique Goliath design (a Goliath is a crane that stands on legs and runs on rails) that maximises efficient use of storage space and allows separate loading and unloading lanes. The single girder crane is of a cantilever construction with the main beam offset from the support frames and the hoist offset from the horizontal beam to keep loads clear of the support legs. The beam projects beyond the track edge on both sides, permitting unimpeded loading and unloading of trucks on either side.

The design enables the whole of the area between the tracks to be used for storage because access roadways within the tracks are not required. The crane is used to handle universal steel beams and shares the stockyard with an overhead travelling crane. As the paths for these cranes cross, multi-level safety systems are installed. A sophisticated anti-collision system recognises when either crane enters the critical area where their paths cross and prevents the other crane entering this zone.

Radio control of crane movements allows the operator to select a safe position with good visibility from which to control operations. Floodlights improve visibility in the darker months. Audible alarms warn of the cranes advance while emergency stops allow the operators to bring the cranes to a quick and safe standstill.

To assist productivity, the new crane has travel speeds of 80 metres per minute in long travel, 30 metres per minute in cross travel and a lift speed of eight metres per minute. All movements are moderated by frequency inverter for smooth acceleration and deceleration and to assist load stability.  CT-E

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