Building bridges7 April 2011
The derrick continues to be an efficient bridge building tool, despite the perception that it is outmoded. The beauty of the derrick’s simple tripod structure is that it is lightweight and easy to operate, but it must compete with larger capacity, heavier, more specialised equipment. Cristina Brooks reports
Bridge building equipment trends are difficult to ascertain because each kind of equipment has its advantage, each contractor has their preferred method, and practices differ in the US and EU.
“It depends upon the size of the bridge, the site constraints, the materials, there are just so many factors that effect how you are building a bridge,” says Mike Wade, technical director at bridge building equipment manufacturer Dorman Long. The options get more complex as architects push design forward to create larger bridges. “There is no trend in equipment, it is more the way the equipment is used that makes it interesting,” he says.
Bridges made of steel or concrete segments can be built using any combination of derricks, mobile cranes, tower cranes, floating cranes, gantries, and nose recovery systems.
Thomas Eggert, operations leader at Enerpac bridge equipment manufacturers, clarifies. “It really comes down to the design of the bridge: Whether they have precast concrete segments or steel components. We tend to use hydraulic cylinders to push or pull the segment in or out,” hydraulic cylinders being generally either strand jacks or climbing jacks.
Mobile cranes remain the most popular option for steel bridges and smaller bridges. “Most small motorway bridges that you drive under every day will be put in by a crane, what proportion of bridges is that in the world, 85%? If you can get a crane in near enough to lift the bridge, that is the way to do it. But very often you just can’t do it, either because it its too big, or its over a railway track or a 4km wide river,“ says Wade. That is when contractors rely on launching systems and derricks.
F&M Mafco is the world’s largest derrick supplier. It is a family-owned business. Executive vice president Bob McKenna, Jr., says that the company has contracted derricks for use on many recent bridge projects. “For example, we used derricks on the Ting Kau Bridge in Hong Kong, the USA ship channel, Annersons Island in Vancouver. We just finished the Audubon Bridge in Louisiana. We’re working on the Port Mann Bridge project near Vancouver.”
Tim Fox, who works in international sales at F&M Mafco, says derricks can replace cranes and decrease logistics costs. “You could use a 500t crane or a tiny derrick. Cranes work by reducing the reach and the radius. For every yard you lose efficiency, and that is the principle of the derrick on the bridges. A crane needs a concrete counterweight, while a derrick uses the bridge as the counterweight.”
The great stability and substantial lifting power of derricks was demonstrated recently during building of the Colorado River Bridge, part of the Hoover Dam Bypass, when a cableway highline crane fell from the steep cliffs. F&M Mafco’s American S-70 Derrick, with a capacity of 446t, was adapted with an extended jib and platform to serve as a replacement until a new system could be developed.
Derricks working separately or in pairs can be used instead of floating cranes for lifting modular bridge segments of up to 200t, depending on what material they are made of, and may have the advantage of greater stability.
F&M Mafco has made a bid to use a derrick instead of a water based crane for work beginning on the Forth Bridge in Edinburgh. “Its hard to keep the segments stable on a water based crane, where the water is choppy and the boom moves 3m–4m. Also, if you did it from the water with a crane, it’s a higher cost, £50,000–£100,000 per week,” says Fox.
“If cost is a concern and speed is a concern, a derrick will win almost every time. On the Port Mann Bridge, they have derricks on one end of the bridge and a strand jack system on the other. We were millions cheaper,” says McKenna.
Similarly to a rail-mounted gantry, a derrick may be mounted on a movable sled for constructing different parts of the bridge. Two derricks may be set up to simultaneously work outwards from the tower in the centre of the water to construct bridges rapidly.
This capability gives the derrick an advantage over the gantry and jack, which requires reassembly for working in multiple directions.
Launching jack systems are more widely used than derricks in Europe. This is partially because of architectural differences. “In the US, the rooftops on power stations were originally built to take a derrick crane. Now they are built to use a tower crane and in the EU they are normally built to take tower cranes. Everyone in the EU has to come to America to hire derricks,” says Fox. Derricks are commonplace partially owing to their adaptability to the rooftops of high rise buildings in American cities.
UK-based Dorman Long provides strand jacking equipment to EU clients. It recently supplied DL-S185 strand jacks for a jacking system to Edimo Metallo S.p.A in Italy. The system was used to launch the 1,000t Ponte Verde steel arch bridge over a railway line in Padua.
Jacking systems are built to order and can involve different lifting equipment. They are used to construct cable stay bridges, suspension bridges and box girder bridges. Strand jacks can stretch wire rope across from pier to pier and pull bridge components across the gap. The jacks on one side let the wire rope out but simultaneously hold it back, while the jacks on the other side pull.
“Launched bridges are used over a railway because there’s often limited time you can work over a railway and launching is considered safer,” says Wade.
Alternatively, strand jacks can be attached to gantries that sit on the bridge and lift vertically to pick components up from underneath it. “Almost invariably, we use strand jacks mounted on some kind of gantry for lifting the deck sections. Basically strand jacks give very good power in a small package. Typically we might be using 300t strand jacks on a cable stay bridge,” says Wade.
Sometimes jacking systems work simultaneously with steelwork towers. “The tower system itself is just a structure. On the structure we put strand jacks or climbing jacks and those provide the motive power to lift the load. It can be a deck section, petrochemical vessel, it can be a roof structure, or an offshore application.” The simplicity of the strand jack system makes it good for offshore and export to Dormon Long’s largest market in China.
“We are not using more complex equipment because it is not necessary, and also because we can’t be taking very advanced technology into remote parts of the world. We have to use robust technology. It’s mainly to do with equipment control systems - things like sensors and load control systems. If we try to be clever with how much we move things in a hot, marine environment, that kind of equipment can mean less reliability. Simplicity means reliability. The benefit of a modular lifter over a crane is that the¬ mobilization cost is a lot lower. The operational cost is a lot lower. Also there’s far less machinery,” says Wade.
Comparing the strand jack to the derrick, he adds: “They each have their advantages.”
One of the advantages of derricks, says Wade, is that they can slew and luff, while strand jacks on gantries will only have limited transverse movement. “Derricks are generally used for lighter pieces you mount on the end of a bridge deck. You can pick up smaller units with a derrick, but what we can do is pick up pieces of 600–800t, and what we’d use is a gantry.”
The European and American bridge construction divide may not be a question of pure engineering theory, but of practice. “The biggest problem we have is that a lot of people don’t understand the operation of a derrick and it needs to be explained,” says McKenna, which is why F&M Mafco has a training program for derrick operators. Tim Fox observes. “One of the things I’ve noticed,” says Fox, “Is that in the US derrick bridge build is commonplace, and in the EU they seem to like the bridge launcher. Young engineers coming out of college use the launcher, but the derrick is very efficient. Its probably experience; if you used one before you’d probably use one again.” He adds that this perception has not hurt sales. “Its starting to feel like its coming back. We have a few more orders. It is more economical.”
The main reason that the derrick can be more economical is that it is rented and not sold like built-to-order jacks, which generally can’t have multiple applications. “One of the disadvantages is that they are pretty customized, although some companies have used adaptable launchers,” says Tom Eggert. It costs from $100,000–$200,000 for an Enerpac jacking system and rental is not common.
For strand jack buyers, “They end up with capital costs that they can get some recovery on, its normally quite long term. If you buy you end up with control over that cost if there’s a delay to the program,” says Wade. “For shorter projects our customers normally rent the equipment”.
The derrick’s small size and cost effectiveness make it popular for lifting on rooftops, particularly for installation of air conditioning units and building signage. F&M Mafco recently installed a sign for the Las Vegas Cosmopolitan hotel and Casino. Liebherr and Terex Comedil also have derrick packages for rooftop disassembly of tower cranes.
At Liebherr, Carlos Monteiro, project manager responsible for derricks, says that the Liebherr derrick package intended for dismantling tower cranes from rooftops is increasingly popular. “There is a growing demand for dismantling operations and this is fact. This is because high rises are an efficient use of space in cities. In our case we are looking to provide for this application.”
Timberland Equipment’s Lawrence Clark has also seen increasing use of the derrick package for tower deconstruction. “We have one derrick package locally. The market tends to be a little soft as the US market has been a little slow; the bulk is the Middle East and South America,” he says.
The future for derricks and their applications on larger bridges may depend on engineers and architects. However, by providing derrick and tower crane packages, manufacturers have ensured that derricks will remain a staple for rooftop lifts in cities growing upwards.