The choice between whether to use a bottom slewing self-erecting tower crane or a top slewing model that has to be assembled with an assist crane will often be an easy one. If you only need a crane on site for a week or two, and you don’t need too much reach or capacity, a small self-erector sounds like the tool for you.
If you are building a multi-storey building over the course of a couple of years, then a conventional top slewer will be required. But there is an awful lot of construction in between these two extremes where the decision may not always be so straightforward.
Potain is one of several manufacturers that offer a wide range of self-erectors and top slewers and the product lines overlap. Its largest self-erectors are bigger than its smallest top slewers, to ensure all customer needs can be met. Potain confirms that hook height and jobsite duration are the key factors. If a tower crane needs a mobile crane (anything from 40t to 100t capacity, typically) to put it together, then the erection costs need to be offset over the course of the job. Though there is no fixed rule, a cut-off time of six months might be borne in mind.
Potain confirms that a self-erector is more expensive to buy than an equivalent top slewer, but it costs next to nothing to erect. If you are going to be erecting and dismantling the top slewer many times, the total cost of ownership can soon overtake that of the self-erector.
It generally takes a day to put up a top slewer (Potain says up to eight hours for its MDT City range, and up to 12 hours for other models), whereas a self-erector can be ready for work within 15 minutes, so there is also the time consideration, as well as the money.
These calculations have been affected in recent years, however, by the progress that the manufacturers have made with simplifying their smaller top slewers – the city cranes. Potain is among those whose city cranes are designed for easy erection. So too is Comansa, stopped producing self-erectors some years ago to focus exclusively on its top-slewing flat-top cranes. Flat-top cranes are generally quicker to erect than conventional cat-head type cranes because they do not have pendant lines.
Comansa export manager Juan Mari Iturrarte says: “From my personal point of view, most of the original reasons for the existence of self-erectors, at least anything larger than 30m, would seem to have disappeared. In the 1960s through the ’80s, large self-erectors were ‘desirable’ because they were transported on the road in one bundle (plus ‘X’ trucks of counterweight). They could be erected and tested in an eight-hour day. Only two or three workers were needed to erect them.
“They did not have several thousand loose nuts, bolts and washers to lose between dismantling and erection. Many top-slewing, or ‘conventional’ tower cranes, of that era were frequently a complicated mass of components which could take weeks rather than days to assemble and erect. They also needed a skilled electrician just to wire it all up… so the big self-erectors made a good deal of sense.
“Since the small tower crane revolution started, eliminating torqued bolts and using long towers, water-proof electric plugs, and folding cross-bases with screw jacks, the decline of large self-erecting crane markets, with the possible exception of Germany, certainly began.”
Iturrarte says that that a team of two or three men can erect one of the new Linden-Comansa 500 Series flat-tops with a 50m jib in as little as four hours. And the crane can be transported to site on two or three standard 40ft trucks.
He adds that a top-slewer offers its owner more versatility. “Contractors are more-or-less obliged to take the work they can get it, so today they are building three floors, tomorrow ten or twelve. What is your self-erector doing then? Modern cities don’t have room for monstrous tail-swings at ground level, nor several extra trucks of ballast to give them a ‘reduced’ tail-swing, any more than they have room to maneuver 17m.-long machines. Can tomorrow’s builders and rental companies afford the cost premium, height and space limitations, and, in many cases, performance penalty of the big ‘self-erector’?”
Iturrarte says that traditional bottom-slewing markets, such as Germany, still demand large self-erectors. But elsewhere, including his native Spain, they are in declline. He says this is a direct result of the possibilities that manufacturers now offer their customers with top slewing city cranes. “It is a logical, market-led evolution,” he says.
Potain does not take such a dogmatic approach. “As both ranges are complementary, there is room for both types,” the company says. “The best crane can be a self-erector or a city crane. It is necessary to make a deeper analysis at each new job site.”
