Supporting structures and connections

Virtually all lifting appliances need a supporting structure. Some, such as overhead travelling cranes and slewing jibs, effectively incorporate their own. For others, an appropriate solution must be found. At its simplest, this may be nothing more than a suspension point on a building structure. Overall there is a wide range of options available to the buyer, each with its advantages and disadvantages.

As far as both safety and efficiency are concerned, the choice of support structure is as critical as the lifting appliance itself. Indeed, given that it often involves the use of elements not specifically designed for lifting, it is perhaps even more important that care is taken during the specification and selection process. As with any other item of lifting equipment, reaching the right decision invariably begins with an analysis of the lifting operation(s) involved. In particular, this should cover whether the load has to travel and, if so, is this along a fixed path or over a wider area? Is a temporary or permanent structure required? How rigid does it need to be? This last point is particularly important for operations where precision raising or lowering is required, as a light flexible structure could make the job difficult.

There are many operations where only a nominal amount of sideways movement is required, for example in a maintenance operation when a pump is lifted off its base and landed to one side. Provided there is sufficient height and a fixed lifting point, a little manual force may be thought sufficient to swing the load clear. Unfortunately there are several potential pitfalls. The lifting machine must be capable of working at the angle required, the operatives involved must work in unison and position themselves to avoid being trapped or crushed. Also, particularly with temporary free standing structures, pulling sideways can cause the structure to topple catastrophically.

So what are the options? For fixed-position lifting operations, it is often possible to use a suspension point that is part of the building structure. However, a check by a structural engineer or similarly qualified person is essential. It is important not to underestimate the load on the structure and include the weight of all the lifting equipment as well as that of the load and a dynamic allowance. Where manually operated lifting equipment is employed the dynamic allowance is 10%, increasing to 25% for powered appliances. The British standard BS 449 Use of structural steel in buildings gives guidance on this subject. Equally important are the method of connecting the lifting equipment to the structure and the line or lines of pull which will be exerted. When sheave blocks are used, the load is increased by the value of the hoisting effect. Approved lifting points should be clearly identified, marked with the appropriate safe working load (SWL) and the connection subject to periodic thorough examination.

Tripods and shearlegs

Tripods and shearlegs provide an alternative where a temporary suspension point is required and costs need to be kept to a minimum. They generally offer heights up to six metres and capacities ranging from 500kg to five tonnes. As with all temporary structures, it is important that the floor can take the imposed loads and particular care should be taken to avoid unseen weaknesses such as underground pipes, drains and cables. Floor spreaders may be required. Also there must be means of restraint to prevent the legs from splaying. There are no British, European or ISO standards for tripods and shearlegs, so the manufacturer’s guidance must be followed closely. In use, the load should never be pushed or pulled sideways as it may cause the tripod or shearlegs to topple.

Runways

In practice, many lifting operations do involve travelling the load. For repeated lifting and movement along a fixed path, runways are the most common solution. Comprising sections of standard rolled steel or special purpose track, lifting appliances such as hand chain blocks and electric hoists can be suspended by means of travelling trolleys. Runways may form part of the building structure itself, be built into building members, or be suspended from self-supporting structures. They can provide extremely sophisticated handling solutions, using switches, turntables and bends to allow movement of loads along numerous different routes.

Where the runway has its own independent structure, the designer should take into account all the appropriate factors. As with the fixed lifting point already referred to, if a runway is to be supported by the building, a structural engineer will need to be consulted to confirm the safety of any proposed design. In this case the loads imposed on the support points will also include a proportion of the weight of the runway and fixings.

In the UK, runways made from rolled steel sections are normally designed, tested and certified in accordance with the requirements of BS 2853 Design and testing of steel overhead runway beams, marked with the appropriate SWL and subject to periodic thorough examination. Given the different dynamic allowances, the Lifting Equipment Engineers Association (LEEA) recommends that the SWL marking should include the word ‘manual’ or ‘power’ and, to avoid any confusion, the marked capacity of lifting appliances should not exceed that of the runway.

Runways made from special purpose track should be installed in accordance with the manufacturer’s instructions and subsequently subject to periodic thorough examination.

Mobile gantries

Mobile gantries provide an alternative to the permanent runway, and are best suited to occasional applications requiring only limited travel of the load. Usually they are mounted on wheels or castors. However, mobile gantries are generally unsuitable for movement under load; they must first be positioned over the load, and aligned with the direction of travel required. Then the load can be raised, moved across the gantry and lowered. There are no specific British, European or ISO standards, but mobile gantries are essentially a mobile runway structure. One point to be wary of is the stability of the gantry. In use, the load should never be pushed or pulled perpendicular to the runway as it may cause the gantry to topple. Also, when moving the gantry, a small defect or obstruction on the floor can be equally perilous. As with tripods, the suitability and strength of the floor must be considered and the floor must be level.

Mobile gantry types include simple ‘goalpost’ designs, ‘A’ frames, adjustable height, foldaway and self-erecting variants and a choice of light and heavy duty versions. Demountable gantries are another option, effectively providing a semi-permanent structure that can be dismantled for occasional relocation. When deciding on the dimensions required, consider carefully the room available and the movement required. All mobile gantries require bracing between the track and the supporting frames. Buyers can choose between external bracing, which provides maximum internal clearance, and internal bracing which may overcome problems with external obstructions, but at the cost of limiting travel across the runway. Box braced, rigid or spliced braced designs may eliminate the need for compromise in this respect.

The selection of castors or wheels will largely depend on the floor surface. The use of brakes, wheel locks and castor rotational locks should always be considered where inadvertent movement may occur. In terms of SWL markings, the same guidance offered for runways applies.

Overhead travelling cranes

For lifting and movement in all planes across large areas of floor, the overhead travelling crane is the obvious choice. The relatively high capital cost means they are generally suitable only for regular lifting requirements. That said, overhead travelling cranes range from small moving beams fitted with hand chain blocks to massive bridge structures with integral crab units. Most run on elevated gantry rails, although semi-goliath and goliath cranes use one or two floor-mounted rails respectively.

Slewing jib cranes

A more economic alternative for ‘all planes’ lifting within the arc of a circle is the slewing jib crane. They can be mounted on building columns or walls, or built into their own self-supporting columns. In the case of the first two options, the suitability of the building must be checked by a structural engineer and prepared foundations will usually be required for free-standing columns. Again the design should be performed by a suitably qualified person.

In terms of the design of the jib itself, overbraced designs provide the greatest travel and coverage, but at the expense of headroom. Underbracing maximises the available lifting height, but reduces travel along the jib. Small, light loads can be slewed easily by hand, but the further a load is pushed from the vertical, the greater the risk that the jib will snatch and cause the load to swing dangerously. Power slewing is an option. The risk of collision must also be taken into account, and slewing or travel stops employed as necessary. BS 7333 Slewing jib cranes is the relevant British Standard and it includes a classification system to match the jib to the duty required.

Connections

Whatever type of support structure is chosen, the means of connecting the lifting appliance must obviously be compatible with it and transmit the load to the structure in the right way. A structure capable of carrying a particular load can still fail if wrongly connected. Where the load will travel along a track the usual connection is a trolley. It is essential that the trolley fits the size and profile of the track and that track has effective end stops which engage with the trolley to prevent it running off the end.

Where a fixed lifting point is required, it is either integral with the structure or by means of a beam clamp. There are no British, European or ISO standards for such clamps but, in broad terms, adjustable and non-adjustable versions are available, with the former suitable for use with a range of beam sizes. Clamps must engage with the beam in such a manner that the beam flange is not subject to local overload. Most clamps are designed so that the line of force must be at right angles to the flange but there are also designs suitable for ‘angled’ applications. The SWL of the clamp should allow for the weight of the lifting appliance and any other tackle which will be suspended from it. Where pulley blocks are used, the hoisting effect mentioned earlier must also be taken into account.

The LEEA firmly believes that all such trolleys, clamps and other connections should be subject to the same requirements of certification, testing and periodic thorough examination.