A report published in Feb by the UK Health & Safety Executive stopped just short of calling for a total ban on assembling tower cranes using external climbing frames. However, it warned that ‘the climbing of tower cranes seems to be a high hazard operation’, adding ‘there may be potential for severe accidents during both the assembly and use of external climbing frames.’
The report, headed Discussion paper on the safe use of ‘external’ climbing frames on tower cranes, lists in detail the risks that these jacking systems present, making quite clear the safety organisation’s concerns (see below). The document does not attempt to make any firm recommendations but aims to ask questions of the crane manufacturers and alert the whole crane industry to the risks being faced.
‘There are serious risks that designers, manufacturers and users may not know,’ said Andrew East, HSE principal special inspector for the construction sector.
The HSE’s report is the result of a lengthy investigation prompted by a fatal accident at Canary Wharf in London on 21 May 2000.
Three crane erectors died when the top section of a Wolff Hydro 32BF luffer collapsed while being jumped.
In January the HSE presented the coroner with a preliminary report of its investigation of that accident. The report was not made public and the HSE emphasises that the discussion paper is completely separate from its accident findings. Although the coroner has yet to rule on the cause of death or establish liability, the HSE decided to release a paper because ‘we can’t delay discussion,’ said HSE chief inspector of construction Kevin Myers.
Nearly three years have passed since the Canary Wharf accident, but the HSE has still not reached any formal conclusions about it, owing to the difficulty of the investigation, Myers said. ‘It has not been a straightforward investigation. We have not been able to reconcile technical investigation with the evidence accumulated from witnesses, ‘ he said. ‘It is more technical than many investigations.’ If the HSE investigation finds fault, prosecution could follow. It may be, though, that no firm conclusions are ever reached in that inquiry.
For the consultation paper on external climbing frames in general, HSE also explored other accidents brought to its attention, including one in San Francisco, USA in 1989, and more recent accidents in South Melbourne, Australia, and in unspecified cities in Italy and Korea in 2001.
Although lifting frames are widely used, no official standards exist for their construction, use or maintenance. The discussion document is intended to ‘contribute to the development of future technical standards’, said the HSE, and ‘lead to improvements in the safety of climbing operations’.
The document outlines how climbing frames generally work, lists safety risks during operation, lists the issues involved in designing, building and operating safe lifting platforms and gives a list of relevant standards.
‘The whole top of the crane is disconnected from the mast that normally supports it and the load is transferred through the hydraulic cylinder to reaction points on the mast (climbing lugs),’ the document says. ‘The hydraulic cylinder assembly may have to support a dead weight of as much as 100t, depending on the crane in use. The climbing frame as a whole has to cope with the significant static and dynamic forces involved with climbing.’
Climbing frames attach to the top of a mast in progress and support the top of the crane – jib, counterjib and slew ring. Hydraulic cylinders inside them push up the top of the crane to create an opening big enough to fit in another mast section. The safety of the entire operation relies on workers making many exact connections – including mating mast sections on top of mast sections, hydraulic cylinders against lifting lugs and temporary support beams that hold up the lifting frame while the cylinders are being moved. These exact connections must be made under a shifting centre of gravity, from the back, on the hydraulic cylinders, when lifting, to the centre of the mast after a new section has been bolted in. The connections also must be made at height, possibly in wind that can affect the crane’s balance.
The HSE report suggests that designers of frames should take account of all the forces operating on the climbing frame, including wind loading, and perhaps build in better monitoring devices. It also suggests that crane manufacturers build in new slewing locks and other mechanical or electrical controls. It also suggests that suppliers train and certify erectors. It suggests that the crane operating company should perform a risk assessment as part of site preparation work and inspect the frame every six months.
East said that while he did not feel that lifting frames were being used dangerously or that they should be banned, he said that publishing the discussion paper was a way of ‘raising our concerns, our unease’.
He added: ‘A lot of this to us appears to rely on individual judgements.’
The discussion document does not apply to internal climbing frames inside tower cranes installed in building cores, which rely on a different lifting method.
Later in the year, the British Standards Institute will publish a revision of the safe use of cranes, BS 7121, p2, East said .
The full discussion paper is available online at www.hse.gov.uk/construction/index.htm. The HSE is seeking comments from the industry, for which the deadline is 16 May.
POSSIBLE RISKS …as described in Annex 2 of the HSE discussion paper Risks during assembly/dismantling of the climbing frame Information arising from investigations in Europe indicates that serious accidents have occurred because:
• a part-erected climbing frame has been disconnected from the lifting appliance before fully bolting it up and connecting it to the mast head or ensuring it was adequately supported by the mast e.g. on support shoes
• a platform (or other ancillary parts of the climbing frame) has become detached when not properly secured in place.
Erectors may also be at risk of falling from walkways or ladders during the assembly or dismantling process. People at ground level may be at risk from falling parts or tools.
Loss of support for the climbing frame The correct location of the hydraulic cylinder assembly on the climbing lugs or other reaction point on the mast will normally be critical. The mass of the crane top will have to be supported at this point and reaction forces transferred to the mast during the climb. Similarly, any support beam or shoe used to hold the top of the crane temporarily between strokes of a multi-stage cylinder should be deployed in the correct position.
If it is possible for the cylinder assembly or support beam/shoe to be misplaced (or accidentally displaced) during a climb the consequences might be serious. Displacement of a cylinder assembly from the climbing lugs could occur for example if the hydraulic ram is mistakenly retracted at a point in the climb when the top of the crane is being temporarily supported by the new mast section. That may occur unintentionally if the new mast section is misaligned when it is lowered onto the top of the mast.
Risks from wind loading Climbing of tower cranes is usually required where tall structures are under construction. Wind speed and direction can vary significantly in time and position around the construction site and neighbouring structures. Manufacturers or suppliers will normally specify a maximum wind speed for climbing. If the wind speed is substantially greater than the specified maximum at critical points in the climbing cycle the wind loading on the jib could give rise to an overturning moment sufficient to cause failure. Anticipation is the key. In some circumstances, strong gusts of wind in the middle of a climb might be give rise to substantial loading on the climbing frame unless the erection team can take rapid action.
Alignment and slewing of crane jib There may be a risk of structural overloading and failure if:
• the jib of the crane is not correctly aligned with the front face of the mast before climbing begins; or
• the crane is slewed at any stage during the actual climbing operation, either inadvertently (e.g. by wind loading) or deliberately.
Information available from the Occupational Safety & Health Administration report into the 1998 San Francisco accident suggests that the immediate cause of that accident was probably the failure of the mast as a result of the crane slewing while a new mast section was being fitted. It was postulated that the erection team had attempted to slew the crane to make it easier to push a new mast section into the opening in the climbing frame. It cannot be stated with any certainty why the crane was slewed as all the erection team were killed in the incident.
‘Balancing’ the crane Climbing instructions provided by different manufacturers suggest that when a crane is in true ‘balance’ the guide rollers or wheels should exert no significant force on the mast. However, it appears that normally the ‘balance’ is only checked at the start of the climbing cycle (and then indirectly) by checking the alignment of the mast head and mast. The fixings holding these two sections together are removed and the two sections are parted by a few centimetres. There seems to be the potential for significant variation in setting this balance point. The erection team may not keep to the supplier’s recommended balancing radius for a variety of reasons including:
• the existence of temporary wind effects
• the mast not being vertical
• the balance weight specified by the supplier not being available or not in use because of space restriction on site
• variations in judgement about the correct alignment of the two sections.
If the ‘balancing’ operation is carried out when wind is blowing, the crane radius may have to be adjusted to counteract the wind loading on the jib. Any subsequent change of direction in the wind during the climb may affect the ‘balance’ of the crane. Obviously, the effect on ‘balance’ would be most pronounced when the change of direction approaches 180°. The greater the wind speed the greater the change in wind loading on the crane jib and the potential out-of-balance forces acting on the climbing frame and mast. If the wind is blowing strongly, the out-of-balance forces may give rise to substantial loads on the mast or climbing frame. These forces might be sufficient to cause an overturn.
Structural failure The San Francisco incident also showed that a crane mast might be subject to overload by reaction forces from the climbing rollers or wheels under certain conditions.
Misalignment of the new mast section There is potential for problems when trying to move the new mast section into the climbing frame and fitting it to the mast head and mast. If the erectors have difficulty aligning the new mast section various measures may be adopted to get the section to fit, which might affect the crane balance. If the new mast section is misaligned when presented to the top of the mast and corrective action is not taken, the mast head (and the top of the crane above it) could be left resting on top of the mast without proper engagement. In that case, there would probably be no load on the hydraulic cylinder. This could be a highly dangerous situation. The unloaded cylinder assembly could become unseated from the climbing lugs and the mast head would no longer be restrained. Also, the balance point of the crane could then shift from the line of the hydraulic cylinder to the centre of the mast. The backwards overturning moment might then increase significantly. The use of smaller diameter temporary pins to secure the new mast section to the mast head may exacerbate this problem.
Hydraulic hammer Normally a check valve will be fitted into the port supplying the ram side of the double acting cylinder to prevent hydraulic oil escaping form the cylinder and the climbing frame it supports failing in the event of a pipe burst. However, the check valve can cause problems with erratic oil flow leading to unexpected and sometimes uncontrolled movements of the load. To lower the climbing frame the check valve has to be opened. The check valve will only open and let oil out of the ram side of the cylinder when oil pressure is applied to the other (annulus) side of the cylinder. If, due to the weight of the climbing frame, the oil on the ram side is forced out quicker than the oil can flow in on the annulus side, the check valve will snap shut. Once the valve is shut it stops flow out of the ram side and allows pressure to build up again on the annulus side and the cycle is repeated. The result could be violent bouncing of the climbing frame as it judders down, subjecting the climbing system to dynamic loading.
Fit of climbing frame If the tolerance of the fit between the climbing frame and mast sections is not well controlled during manufacture or the frame is not well maintained there might be a risk, in extremis, that the guide rollers or wheels can become displaced from the mast, with potentially serious consequences.
Lifting operations There would appear to be risks associated with lifting the climbing frame, during which the jib of a luffing crane will be at a very steep angle. In addition, there are conventional risks associated with lifting loads such as the climbing frame or new mast section.
Risk of falls of people or materials Erectors can be at risk of falling when uncoupling lifting tackle from the climbing frame and new tower sections. Also, the erection team use external working platforms, walkways or ladders built around the climbing frame to prepare for a climb. If platforms are not provided with robust edge protection (e.g. guard rails and mid-rails) and ladders are not in protected positions the erectors may be at risk of falling. There will also be a risk to those at ground level from items such as pins, bolts, hammers and other tools accidentally dropped by the erection team.
Write down what you do and asses the risks The discussion paper recommends that crane operating companies perform a risk assessment which, combined with the manufacturer’s instructions, would be used to create a statement of work procedures ‘for the safe transportation, assembly, erection, use and dismantling of the climbing frame’. The discussion paper suggests that such a document could include:
• transportation of the climbing frame to and from the site and its offloading/loading
• safe assembly of the frame around the crane mast to avoid collapse or dislodging any part
• positioning new mast sections on the ground to avoid or minimise the need to slew the crane between climbs
• positive inspection of the assembled parts of the frame by a competent person to check on safety critical features such as the tightness of bolts, the correct location of the guide rollers or wheels and the absence of damage
• establishing a suitable exclusion zone to protect others
• arrangements for obtaining site-specific weather forecasts at relevant heights and for measuring real-time wind speed and direction
• responsibilities for monitoring other safety critical parameters including crane jib alignment, actual jib radius and permitted free height of the crane
• arrangements to prevent deliberate or inadvertent slewing during climbing e.g. by ensuring that any interlock for the power supply is in sound condition, the slew brake is correctly engaged and any positive scotch or other device to prevent inadvertent slewing is in place
• checks on the positive engagement of the hydraulic ram assembly on the mast (pins and lugs or support shoes) and the means of preventing disengagement during climbing
• checks to ensure that moveable parts have been secured before a climb, e.g. any temporary support beam or a trolley which allows the hydraulic ram assembly to be moved clear of the mast during setting up
• arrangements for providing a suitable balance weight (where indicated in the climbing instructions)
• procedure for balancing giving permitted tolerances from the balancing radius specified in the operating instructions and other parameters such as wind speed • temporary fixing of mast sections and permitted arrangements for overcoming fit problems to move new mast sections into position
• contingency arrangements for recovering from emergencies during climbing and leaving the crane and climbing frame in a safe position
• safe dismantling of the frame.