Reaching for the sky8 October 2018
Modern cities are almost defined by high-rise skyscrapers clad in glass or in pre-cast panels. Julian Champkin looks at the lifting challenges presented by their construction.
One fundamental design has come to dominate high building design across the world: the concrete or steel core and flooring with non-load-bearing façades of glass or of precast panels. The facades are frequently known as curtain walling.
Installing these has been a task for tower cranes; nowdays, very frequently in Europe and increasingly in America, it is instead becoming a task for cranes at the other end of the size and cost spectrum, the mini-cranes.
“A mini gives a tool to a crew of men in a way that lets them operate independently of whatever else is happening at the construction site,” says Tony Inman, president of Maeda Mini-Cranes USA. “It means they do not have to wait for the tower crane to be available from other tasks; it is becoming common now for subcontractors to have their own minicranes so that they can do the job with their own tools.”
“In Europe, where buildings are closer together, the mini-lifter is preferred,” says Barry Wood, vicepresident, marketing and commercial development at Woods Powrgrip, manufacturers of vacuum lifters for cranes. “In the US, the big crane on the side of the building is the default. It depends on the contractor, though, and the equipment that he has; both systems work well. Which is used, will generally depend on what the contractor has available.
Certainly mini-cranes are becoming more and more popular.” Whichever crane type is used, for glass panels the below-thehook device is very frequently a vacuum lifting system. The smooth surface of glass, and the generally non-problematic weight of a glazing section, make vacuum lifters the ideal solution for lifting windows or glass curtain walling.
Vacuum cups can be used to lift façade panels also. “They can install facades,” says Wood, “but it depends on how you define a facade. Vacuum cups are able to lift more and more. But the problem, as you imagine, is that suction cups do require a smooth surface to grip, and this is one of the factors limiting the weights they can lift.
“Vacuum cups can lift curved glass panes also,” he says. That requires a frame that can position the suction pads correctly. “The installer has to know the radii, and how far the weight extends inside or outside the curve; operatives should ask the size, weight, chord, and diameter and analyse those for safety. So curves are not a problem, but you do need to be more careful.”
For installing glazing or light-weight panels, two techniques are common. A mini-crane can be lifted, generally by tower crane, to the top, or to an intermediate floor, of the building; from there it can lower panels for the floors immediately below it, where they are secured in place.
GGR have been pioneers of the mini-crane in Europe, and this was the technique, using one of their spider mini-cranes, that was used to glaze the upper stories of the iconic new Shard building in London. Glazing lower down was installed by tower crane. This combination of methods is not uncommon.
Alternatively the glazing can be installed from within the building, using a glazing robot—essentially a mini-crane on wheels or tracks with a telescopic or a knuckle boom that has an articulated vacuum lifter rigidly attached. The robot is small enough to fit inside the space to be walled. The boom with the glass panel attached rotates the panel to allow it to pass through its destined opening, holds it out over the floor edge, outside the building, then rotate it back to the correct orientation and pulls it back against the floor and ceiling edges for fixing.
“Glazing is one of the most common application for minicranes,” says Andy Crane, sales manager of Kranlyft UK, master distributors for Maeda. Maeda manufactures mini-cranes with capacities from 995kg to 8t.
He runs through glazing methods in more detail. “There are different ways of lifting glass. A vacuum lifter on the end of a hook is most common, but it does limit you to where you can place the load. It has to be within the hook fall.
“You can use a searcher hook to get round that problem. It is basically a fixed hook on the end of the boom, which the vacuum lifter is attached to. It holds the glass, and you can place it in awkward positions: it allows tight access where there is limited headroom, up near the ceiling for example or under an overhang. Using the searcher hook also eliminates swing of the rope and makes lifting more controllable and more precise.”
A typical application would be a shopping mall or a low-rise building where the crane is working inside the building on the same level as the glazing installation. In such applications—particularly when replacing a damaged pane in a building which is in use—various power options are available, including electric from mains cable or rechargeable battery.
“The MC285 is our most popular model,” says Crane. “It has 2820kg maximum lift capacity, and 8.7m lift height. It has three power options, diesel, diesel-electric and petrol, which also has the option of an LPG conversion. The dieselonly model weighs 1900kg, which means that it is light enough to go inside a construction lift. That makes it very useful.
“It has the option of an auxiliary winch. If you put the crane on the top of a building it will let you lower a load of up to 900kg a distance of 100m, which is a lot. For glass, that is perfect. There is massive scope for small machine on such tasks, and they are far cheaper than a tower crane doing almost the same job.
“Another option is a glass manipulator,” he says. “It is a vacuum lifter with its own articulated arm. It can fit straight onto most Maeda models and transforms the crane into a glazing robot.
“So there are some good options for glazing. In June this year we signed a dealership, alongside Lift Ltd, with the German company Uplifter, who make among other things vacuum manipulators that can fit on the end of our mini-cranes. It is our first dealership that is purely for glass handling, so it is a sign if you like that glazing is booming.”
Uplifter have been making minicranes and vacuum units for more than 15 years. Sebastian Bayer is their product manager. “We were pioneers in the field,” he says; and adds happily that glazing is not likely to cease to be in demand.
“It is a rising market everywhere,” he says. “There is a huge boom in construction; I cannot see the end of it. When money becomes more expensive you will get a problem, but I cannot see that happening for at least two years.
Travel along the Thames in London and on each side of you are new high towers full of glass. It is the same in every other major city. They demand a lot of different kinds of glass handling systems.”
He is particularly proud of the company’s newest glazing robot, the Glassworker. “The first model was introduced at the end of 2016. It is almost 100% German-made. It is a complete mobile system.” The GW 625 is claimed as the strongest offroad glass-lifter of its class.
On four rubber tyres, with a knuckle-boom arm and six rearrangeable suction cups, it is controlled by an operator walking behind it or by wireless remote. It has a capacity of 625kg and a lifting height of 3.1m vertically, or 3.6m for overhead working: installing roof or ceiling panels are another application of vacuum lifters. Six of these machines have been working on what will be Germany’s tallest residential building, the Grand Tower in Frankfurt, 172m high with an entirely-glazed façade. “It has been a very interesting project for us,” he says, “which has been going on for 18 months. The Glassworker can drive around different parts of the tower, on different stories, and put the glass through from inside.” The window elements are tilted slightly to the horizontal, rotated 90°, due to their size and set down with the lower edge fitting with millimetre precision onto a pre-positioned profile on the floor. The window is then pulled back to vertical and the top edge secured.
Thus in Europe, the mini-crane is well established as a glazing method for tall buildings. It is less well established in the US. “The product is not so entrenched over here,” says Inman. “We lag maybe eight to ten years behind Europe. But the product is gaining recognition.”
Where the mini-crane is being used, European and North American preferences are different. Electric power has yet to become ubiquitous in the US. “Here propane is a major fuel source,” he says, “more common than in Europe. A propane machine still produces exhausts, but is not a diesel, so it is more acceptable in a semi-enclosed environment.”
Nor are mini-cranes limited to glazing. “Minicranes can do many kinds of cladding, if you have one of the bigger models up on the roof.
Those models can get pretty big and heavy, pushing the envelope of what is a mini-crane as what is not.
A bit of capacity can be required for those jobs.
“And capacities are not that small any more,” he says. “They can lift higher and heavier; they are growing in size. Some are even becoming comparable to small RTs in capacity, but their manoeuvrability gives the advantage where there are access issues.
“For glazing, vacuum attachments are the usual method; for installing cladding or panels, greater weight and often rough surface textures require hooks and rigging. Aluminium frames with glass ready installed are becoming common. And full pre-cast concrete panels with glazing pre-installed have been common for a while.
They go by the name of unitised panels. These become more of a crane lift task, and for these rigging and hooks are preferred.” It has been claimed that using unitised panels can save two-thirds of installation time. “But actually installation of glass and facades is a very small part of the whole time of a construction,” says Wood. “The big benefit of vacuum lifting is that no hook or modification of the panel is needed, which means that workers can gain access to all the edges.
It is good to get rid of the stage of drilling holes for anchors. But there is no significant change in the necessities of the process of lifting and fixing.”
And he describes a fundamental limit to the power of vacuum lifters when used on heavy panels: “It is worth pointing out that lifting strength of vacuum pads cannot be increased indefinitely by increasing the strength of the vacuum; it is limited by the atmosphere. So the sky is not the limit; even if your vacuum was perfect there is an absolute and practical limit of one atmosphere of pressure, 76cm of mercury, pressing on the outside of the pad. This also has practical effects when working at locations high above sea level. We have to remember that our customers may be working at 5,000ft above sea level, where atmospheric pressure is considerably less. So we try not to push the vacuum limit too much. For heavier weights you add more pads rather than increase the suction in each pad.
“But concrete panels with glazing may be heavy, and beyond the limits of vacuum holding.”
Turning to pre-cast concrete panels, then, the trend is for larger panels, arriving on-site more completely prepared, often with glazing ready-installed. “With prefabrication it is easier to control tolerances and specifications in the factory,” says Wood.
“They arrive stacked horizontally on a semi-trailer. A panel can be only a certain width, so it can lie flat on the semi-trailer; as far as size goes, transportation, rather than lifting, is what sets the limits.
“Rotating the panel to vertical is no problem. If there is wind, that can cause problems. But transporters get creative, and use racks and so on to make rotations simpler and quicker.”
Such panels will have lifting points for hooks embedded in them. “Counterbalanced hooks can be used to get panels close to the building.
Many glass loads are not very heavy and the handlers can pull it in the few feet. For heavier loads you can angle the lift bar a little, so that the panel is close enough to the vertical but the lower end is reachable from the building. If it weighs around 100lb, the guys on the building can generally drag it in. But there comes a point when the panel is too heavy for man-handling. At that point an external counterbalance, a long beam with a moveable counterbalance weigh, becomes necessary to control the hang angle. It can also get in under overhangs or roof lines so that the hoist cable does not snag or interfere.”
Some advances in technique are common to both sides of the Atlantic. “These days, depending on the job, fi xing the panels to the building is generally done from inside the structure” says Inman. “A lot of personnel can be in the area working on it. It is much quicker and more effi cient that having workers in cradles hanging onto the outside.”
Some challenges are universal as well: “Getting the mini-crane up there, to the top of the building, can be done by tower crane, or landbased mobile crane, or construction elevator, depending on the size and circumstance,” he says. “Getting it down afterwards can be more of an issue. The tower crane may have been removed by that time.” Of course the issue stems only from the advantage that the mini-crane has already given: of allowing the expensive tower crane to depart the site before the very end of the job.
“Creativity is sometimes called for,” he says. “Sometimes it will be disassembled to get it into parts within the weight capacity for the elevator.”
Usually crane operators are concerned with raising loads. Getting loads down from the top of a building at least makes for an interesting challenge.
Case study: The Shard
Glazing by GGR mini-crane
Glazing the Shard, the iconic 95-storey 307m building beside the re-developed London Bridge Station, called for the highest-ever siting of a GGR UNIC mini crane and crane operator in the world.
Before the project began, GGR conducted tests in a mock up of the structure to practice lifting the UNIC 706 onto the pinnacle rooftop with millimetre precision.
For the actual project, the crane was dismantled and the boom removed. The main chassis was lifted onto the roof by a tower crane, with the outriggers open at the precise position required to set safely onto the roof. The outriggers were moved with millimetre precision before the crane was lowered onto four special support columns. The support columns were then welded to the structure. Next, the boom was lifted and fixed onto the crane.
The crane and operator then helped to install 11,000 angled glass panels on the upper floors of the building. There were also several smaller UNIC cranes working in between floor levels. During the final stages the smallest crane had to be dismantled inside the building and manually carried the down the stairs as there was no lift on that particular floor.