There was a time when the great iron and steel spans of buildings such as railway terminals and exhibition centres had to be winched up piece by piece and bolted in place by men with no fear of heights – or regard for their own safety.
These days modern machinery and know-how make the job a lot easier, or at least that is the theory.
On a hot and windy afternoon, we are on a site near Abu Dhabi airport, where the new VIP terminal is being built. The main room in this complex is a large, rambling area, complemented by a domed tower in one corner.
The entire roof structure though, has been built in situ, but on the floor. The plan for today is to raise this vast structure from the ground and into position by using a sequence of massive jacks.
However, there is a saying about the best laid plans. On arrival, workers are still scurrying about, seemingly fitting air conditioning ducts to the structure, which currently is sitting on temporary I-beam piers, about a metre off the deck.
This seems to be a sensible plan – after all, why would they attempt to do this when the beams are several storeys high? In fact, this should have been done some time ago.
Posters have been advising of the time of the roof lift for ages, and in any case, the project has a tight schedule – the whole basic structure is supposed to be completed just three weeks after our visit.
But, two hours after the lift was supposed to begin, the ducting still isn’t ready. Still, this gave us the chance to find out a bit more about the project, as well as the lift itself. The VIP terminal is part of the ‘Presidential Flight’ unit of Abu Dhabi airport.
Developed by Aldar on behalf of the government, the project has Merco and Six Construction working in a joint venture as the lead contracting team.
STRANDS
However, due to the large amount of steel, and the frame that needed lifting, Swiss-based heavy lift firm VSL has taken part in meetings since the project’s original conception.
At somewhere below 400 tonnes, the steel section is quite heavy – you certainly wouldn’t want it dropping on your foot for example – but there have been heavier lifts, which begs the question, why not use a pair of large cranes instead of strand jacking?
David Gratteau from VSL explained, “The time schedule is quite short and assembling on the ground is much faster than building in situ. The quality is higher too – you won’t need to correct all of the time.”
In fact, the original weight was to have been about 15% heavier, but value engineers got the weight down, which makes little difference from a jacking point of view, but makes a big deal of difference to the ground loading.
The jacking procedure sounds complex, but it is actually relatively straightforward. Eight large hydraulic jacks stand on temporary gantries with the large metal I-beams braced underneath in a triangle shape.
The jacks then pull the large, pre-tensioned ‘strands’, or cables up, which are attached to the frame with big cramps.
Of course, it isn’t quite that simple. The jacks have to be synchronised exactly, or the whole structure is likely to be pulled up at an angle, and corrections have to be made for variances in temperature (as the steel will contract) as well as wind speed and so on.
There are various ways of measuring the height of the steels, but this site uses half a dozen Sick lasers, which are in turn all rigged to a computer housed in a mobile office in a nearby shipping container.
Looking after the computer is a man named Saddan Bin Mohammed Said, who had previously been working on VSLs projects in Singapore. He explained: “The lasers are useful for heights up to 300 metres, we went over 200 [on a project] in Singapore”
ROOF
Today’s lift will be just over 16 meters, so they should cope well. Other parts of the system include a computer, which displays a window that looks similar to a graphic equaliser on a stereo – in that a line of levels shoot up or down whenever a part of the structure isn’t at the correct height.
“This is the ‘Brow’ system.” Saddan clarified. “It is telling me the height. This gauge tells me what percentage of the load is being lifted and what capacity is available.”
These levels can be controlled or corrected with a pair of purpose-made instruments that feature a bewildering array of buttons, lights and keys.
“These are limit switches. As soon as a tolerance is reached, they light up and you press them to lock the lift,” explained Saddan, as he demonstrated by pressing one of the flashing buttons, and indicating the corresponding ‘bar’ on the computer.
“You can link these to any amount of control desks and with just one button the whole configuration will open,” he concludes.
However, although the systems are on, and the levels suddenly show readings when a worker presumably bangs into the roof frame, there is no real use for it yet, as although it is over an hour after the lift is supposed to start, and there is still no word as to when operations might begin.
It is a frustrating time, as the lift is best done in the afternoon, while being completed before it gets dark. Grattaux explains; “It is possible to lift in the dark, but it is much better done during the hours of daylight in case you miss something.”
The time ticks on, allowing us a chance to talk to some of the riggers who are also waiting.
There is also a chance to check out some of the machinery on site. There’s a big JCB 540 Loadall telehandler hanging about, as well as a bunch of rented cranes – all Tadano rough-terrain 20-tonne units as far as we could see. There are also the usual suspects – a large generator set of unknown make and rating idles away alongside the hydraulic compressor for the jacks.
Elsewhere, several small Belle cement mixers wait patiently for their next batch.
SCAFFOLD
Clambering back up to the scaffold to the roof, we find the VSL employees adjusting the equipment and waiting for the nod. “The jacks have a very long service life, over twenty years, as they are sent back to Switzerland regularly for complete overhaul” Gratteau explained.
Six o’clock comes and goes, and the shadows are getting longer, but at five past, just as we thought it was going to be too late to do it today the call goes out to start the jacks.
There is a flurry of activity, as men rush hither and thither trying to get the ground clear of boxes, clamps and other building detritus.
Site foreman Saeed Barsha barks commands at his charges while they clear out, and after a couple of minutes he is left standing next to of the I-beams on to which the frame sits.
The compressor’s engine note raises, as the flow to the pumps is turned on, and sure enough the entire structure raises by an inch and Saeed moves the I beams away. The whole structure is now entirely supported by the jacks.
People now screw reflectors for the lasers into the bottom of the lowest point of the structure, and with another surge of power from the compressor, the structure slowly lifts up.
Standing with us at the base of the unit are most of the workers who have just been shooed out of the main work site, most of whom seem excited by the lift, which is inching higher and higher.
The sun is starting to set, but it looks like the structure might just be in place by nightfall.
DASHED
These hopes are dashed though. At precisely 6.30 the workers who had been looking on in awe at the lift shuffled off to find their bus, as did just about everybody else who were needed to supervise the process, leaving a fairly angry team from the heavy lift company, as well as site manager Hossam Ibr Hamed making a furious round of phone calls, but there is little he can do.
Without the staff to supervise the mighty section just has to be locked in place where it is, ready to be lifted another day.
Strand Jacking
This lifting technique was invented in Europe in the 1970s based on principles learnt from post-tensioning concrete. Today they are used all over the world where precise lifting is required. The heavy items are lifted into position with computer-controlled hydraulic jacks.
Because the multiple jacks can be moved in unison and with great precision, heavy structures can be assembled at ground level (with increased safety and reduced cost) and then lifted into position, rather than having to be built in the air.
Traditional cranes and other lifting methods cannot provide this level of precision.In recent years, computer control has added to the versatility and safety of strand jack technology.
In theory, any number of strand jacks can be used simultaneously to achieve unlimited lifting capacity, with computer-controls to keep the motion of all jacks synchronised.
In practice, the maximum number of jacks that can currently be used simultaneously and kept under existing computer control systems is 80.
Note: The lift was successfully completed at the start of the following day’s shift and work on the project continues.
