2.5.1 Selecting the Type of Crane

If the crane is to span the whole width of the building, the most likely choice will be a top running bridge crane and the supporting structure can be designed as an integral part of the building structure.

If the crane is to span only part of the width of the building, there are four options. First an under-slung bridge crane running on tracks suspended from the building. This option keeps the floor area clear of extra supporting structures. Second a semi-portal bridge crane. This option is only suitable if the crane span is to reach one side of the building. Consideration must be given to the possible hazards arising from the crane leg and the floor level track. The third option is a portal bridge crane and again consideration must be given to the possible hazards arising from the crane legs and the floor level tracks. Finally, there is the option of a top running gantry crane with one or both sides supported on a free-standing supporting structure. This has the disadvantage of a permanent supporting structure obstructing part of the floor but may be suitable if only a short travel run is required.

Whichever option is chosen, it is essential that it is decided early in the building design process so that the required capacity, span, travel and height of lift are provided for. The loadings imposed by the crane on the building, supporting structure and ground must be determined and allowed for.

It is essential that the building designer is familiar with designing structures for use with electric bridge and gantry cranes. The design must incorporate the facility to accurately adjust the line, level and span of the supporting structure rails or crane tracks. Many buildings will be subject to movement over time and this will make it necessary to re-align the supporting structure or tracks. The type of building structure is also important. Many modern buildings are of portal frame design. These can flex considerably under the influence of wind, snow and other imposed loads such as internal services, water tanks, etc. This flexing can result in an unacceptable variation of span. It is possible to introduce ties to limit the effect, but this should preferably be done at the design stage.

Consideration must also be given to access for delivery and erection of the crane taking account of whether the crane will be delivered fully assembled or not. The headroom required for a mobile crane to install it may require scheduling the installation for before the roof is completely covered. Similarly, access will be required for load testing the crane and supporting structure. This will involve travelling the crane under load down the length of the supporting structure. It should therefore be scheduled before any plant, machinery or other obstacles are installed in the building.

Clearly much will depend upon the design of the building. The options for the type of crane are essentially the same as for a new building but if the building was not designed to accommodate electric bridge and gantry cranes, it is highly unlikely that it will withstand the loads imposed without considerable strengthening or the installation of an independent supporting structure on new foundations. Also, the available headroom may not accommodate a crane of the required capacity spanning the full width of the building. These factors may limit your options.

The main options are a top running bridge crane and a semi-portal bridge crane. The decision mainly depends upon whether the building will support crane tracks on that side and the length of travel required. For the length of travel, it is a balance of cost between the more expensive semi-portal crane and the cheaper ground level track. Consideration must be given to the possible hazards arising from the crane leg and the ground level track.

The two main options for an outdoor crane are a top-running bridge crane with an independent support structure or a portal bridge crane. The choice depends on the required travel length, practical constraints and balancing the higher cost of the portal crane with the cheaper ground-level track. Hazards from crane legs and tracks must also be considered.