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05-04-2009, 09:26 PM
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 كيف تعمل الاوناش البرجية How Tower Cranes Work How Tower Cranes Work Inside this Article
All tower cranes consist of the same basic parts:
 On top of the slewing unit are three parts:
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05-04-2009, 09:29 PM
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| How Much Weight Can They Lift How Much Weight Can They Lift? A typical tower crane has the following specifications: Maximum unsupported height - 265 feet (80 meters) The crane can have a total height much greater than 265 feet if it is tied into the building as the building rises around the crane. Maximum reach - 230 feet (70 meters) Maximum lifting power - 19.8 tons (18 metric tons), 300 tonne-meters (metric ton = tonne) Counterweights - 20 tons (16.3 metric tons) The maximum load that the crane can lift is 18 metric tons (39,690 pounds), but the crane cannot lift that much weight if the load is positioned at the end of the jib. The closer the load is positioned to the mast, the more weight the crane can lift safely. The 300 tonne-meter rating tells you the relationship. For example, if the operator positions the load 30 meters (100 feet) from the mast, the crane can lift a maximum of 10.1 tonnes. The crane uses two limit switches to make sure that the operator does not overload the crane: The maximum load switch monitors the pull on the cable and makes sure that the load does not exceed 18 tonnes. The load moment switch makes sure that the operator does not exceed the tonne-meter rating of the crane as the load moves out on the jib. A cat head assembly in the slewing unit can measure the amount of collapse in the jib and sense when an overload condition occurs. Now, it would be a pretty big problem if one of these things fell over on a job site. Let's find out what keeps these massive structures standing upright. |
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05-04-2009, 09:30 PM
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| Why Don't They Fall Over? Why Don't They Fall Over? When you look at a tall tower crane, the whole thing seems outrageous -- why don't these structures fall over, especially since they have no support wires of any kind? The first element of the tower crane's stability is a large concrete pad that the construction company pours several weeks before the crane arrives. This pad typically measures 30 feet by 30 feet by 4 feet (10 x 10 x 1.3 meters) and weighs 400,000 pounds (182,000 kg) -- these are the pad measurements for the crane shown here. Large anchor bolts embedded deep into this pad support the base of the crane:  So these cranes are essentially bolted to the ground to ensure their stability. In the next section, you'll learn how tower cranes "grow." |
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05-04-2009, 09:31 PM
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| How Do They Grow How Do They Grow? Tower cranes arrive at the construction site on 10 to 12 tractor-trailer rigs. The crew uses a mobile crane to assemble the jib and the machinery section, and places these horizontal members on a 40-foot (12-m) mast that consists of two mast sections. The mobile crane then adds the counterweights. The mast rises from this firm foundation. The mast is a large, triangulated lattice structure, typically 10 feet (3.2 meters) square. The triangulated structure gives the mast the strength to remain upright.    To rise to its maximum height, the crane grows itself one mast section at a time! The crew uses a top climber or climbing frame that fits between the slewing unit and the top of the mast. Here's the process:
  Once the building is finished and it is time for the crane to come down, the process is reversed -- the crane disassembles its own mast and then smaller cranes disassemble the rest. |
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05-04-2009, 09:33 PM
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| Renting a Tower Crane Renting a Tower Crane Most construction companies rent their tower cranes from a company like Heede Southeast. Heede ships the crane to the site, assembles it and charges a monthly fee while the crane is on the site. The typical fee for installation and disassembly runs around $60,000. This price includes shipping the crane to the site, renting the mobile crane used to assemble the tower crane, the cost of the crew that handles the assembly, etc. A typical monthly fee for a 150-foot-tall tower crane is approximately $15,000, with an additional charge to rent the climbing frame and extra mast sections. For more information on tower cranes and related topics, check out the links on the next page. |
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