Austin Williams’ NBS Shortcut looks at the stresses and strains put on cranes
Cranes are delicately balanced pieces of engineering - literally. The tower crane comprises a long lifting jib and a counterweight jib which balances the lifting jib both when it is ‘at rest’ and when it is working.
The ‘derrick’, is named after 17th-century hangman Godfrey Derrick, who lent his moniker to this type of crane because of its similarity to the gallows. ‘Craning your neck’, perhaps.
There are three main classes of crane: mobile, tower and derrick. Mobile cranes, as the name suggests, are suitable in conditions where movement is required around the site. Tower cranes are suitable for confined sites and manoeuvre relatively light loads to considerable height and reach. Derrick cranes are suitable for heavy loads at long reach.
As well as the three generic types of cranes, there are a range of different crane configurations. These include:
• A-frame horizontal cranes. These are also called saddle jib cranes and have an extended tower from which tie bars or cables connect to the lifting and counterweight jibs to give added support. The hook is suspended from a trolley that runs along the length of the lifting jib;
• flat-top horizontal cranes (as shown in the main drawing). Saddle jib cranes are similar to the A-frame type, but without the extended towers; and
• luffing jib cranes. These tend to be permanently angled jibs with cable supports connected
to a smaller counter jib. Often, the pulley is mounted at the end of the main jib and the radius is altered by altering the height of the jib. These tend to have a lower capacity and a lower tower height than saddle jib cranes, but require more power.
A variety of other types and configurations are available and early discussions between the contractor and crane supplier are essential to determine the exact requirements. The crane supplier will assess constraints and potential problems to determine the best and safest crane for the job. The designer/specifier should keep out of decisions on site cranes - they are ‘contractor’s risk’ items. This Shortcut explores some of the principles relating to commonly used tower cranes.
As well as the long horizontal lifting jib (or boom) and the counter jib, tower cranes feature cables and a hook supported on
a trolley that can be moved along the main jib by the operator. In most cases, the jib turns either by ‘slewing’ gear located at the junction of the mast and the jib, or at the base, in which case the entire crane rotates. This turning movement, as well as the variable height of the hook is driven by motors on the counter jib.
The action of lifting loads alters the balance of the crane, exerting a ‘moment of a force’ at a rotation point which could cause the crane to topple. The ‘moment’ is the engineering calculation of force multiplied by the perpendicular distance between the force and the turning point. There tend to be two principle moments acting on the crane:
• overturning (destabilising) moment. This is the moment due to the lifting jib, load and wind forces tending to cause the crane to topple. This increases with the extra load being lifted and the distance of this additional load from the tilting fulcrum; and
• resisting (stabilising) moment. This is the moment of the dead weight of the crane minus the lifting jib plus the dead weight
of the foundation and any counterweight or force in tension piles about the tilting fulcrum, i.e. that which resists overturning.
Should the overturning moment exceed the resisting moment, the crane will fall over. As well as the different types, configurations and weights to be lifted, each exerting a different pull on the jib, the crane has to deal with:
• the torque transferring a turning force through the mast;
• the stability and slenderness ratio of the structure; and
• climatic loads - such as wind, ice and snow - which exert considerable additional forces on the crane.
It is important to bear in mind that wind pressure is the square of the wind speed (i.e. if the wind speed doubles, the wind pressure could increase by a factor of four). The crane manual will list the maximum safe wind speed at which a crane can operate, but
this may need to be modified to take account of abnormal gusting caused by surrounding buildings, large area loads, and lifting loads from sheltered into exposed areas.
As a rule of thumb, wind speeds at a height of 100m are twice those at ground level (notwithstanding site-specific factors). Given the potential hazards involved, clients and project managers must accept that the operator makes the decision on when to take the crane out of service.
Before selecting a tower crane, the contractor and supplier must ensure that the ground conditions are suitable and that the site logistics are appropriate for its use. If the site location is too cramped to house sufficient counterweights, cruciform grillage, ballast, etc., then additional support may be gained by tying the crane structure to the permanent building frame using specially designed struts that:
• reduce the tendency of the crane to overturn;
• reduce stresses induced in the crane tower by wind loading; and
• help deal with the rotational moments transferred from the slewing unit (the slewing torque), from balance wind loads and swinging loads
The forces from the struts have to be taken into account in the design of the supporting structure. When the crane tower is located within a building it can be propped in both directions and the torsion effects are due to slewing of the crane, swinging loads, and out of balance wind loads. When the crane is supported from one face of the building, and tends to tilt in a direction parallel to the face of the building, the necessary prop action acts eccentrically to the building structure, and this may also produce tensional effects.
The Health and Safety Executive’s ‘The Lifting Operations and Lifting Equipment Regulations 1998’ (LOLER) stipulate that a minimum area of 600mm around the base of the crane must be fenced off. Allowances must be made for the safe access (and construction and removal) of the crane. The ground conditions must be suitably firm and level to ensure that the loads can be accommodated safely. Even so, the crane’s footings will inevitably have to be taken below ground level or concrete pads provided for ballasted systems.
When the cranes are not being used they need to be left in the configuration recommended by the manufacturer. Some jibs are designed to ‘weathervane’ i.e. rotate on the slewing ring in order to present a minimal surface to the wind. Where these jibs are restrained from slewing for particular reasons (e.g. being restrained from rotating over a busy road), then the additional loads must be taken into account in the crane’s design. Similarly, increasing the surface area of the crane exposed to the prevailing winds by attaching advertising signage across the jib must be taken into account in the design.
Computer technology has increased safety on building sites, not least with the improvements in load-moment indicators. Where the moment reaches around 90 per cent of the crane rating, an audible and visual warning will alert the operator. Some cranes have a safety feature which can be set to indicate when the moment reaches 105 per cent, at which point the moment indicator will automatically trip an alarm and stop the lift. Instantaneous feedback is essential where the load is at or near the maximum, especially since wind gusts can blow the load beyond the safe distance from the radius (as can centripetal forces when the crane is operated too quickly).
Because of these and other advances over recent years, accidents are a rare phenomenon. The Health and Safety Executive has recently carried out a survey on the likelihood that a person will require rescue from a height, and deduced that there will be one major non-fatal accident requiring rescue from a crane for every 1.25 million hours of use (that is one every five thousand years). However, it still seems sensible to implement safety-training procedures, maintain a good ‘housekeeping’ regime to prevent trip and slip hazards, and to provide rescue equipment (commonly stretchers and a rescue kit for those using fall-arrest harnesses and lanyards).