A Construction Crane Engineering Essay
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That is the one with a tall central shaft with a long boom and counterweights. Tower cranes are often built onsite and are even affixed directly to the structure of the building, especially in the construction of very tall buildings like skyscrapers. Take a drive downtown in any growing city and you'll see tower cranes affixed to the tops of twenty-story office buildings going up. The most common point of affixation for these cranes is the concrete elevator shaft in the center of most buildings, but other points can be used as well if necessary.
Cranes can be fixed in its position in many ways. One of them is to let a second crane, which is taller and lighter, to hoist the crane into position. Some cranes can also self assemble (e.g. Self-erecting cranes).
Buying cranes doesn't seem to be reliable, as construction cranes and tower cranes are very expensive to buy. The renting case seems to be the most suitable one in this issue. There are many companies out there, who make their profits by renting cranes out to construction contractors.
Truck mounted cranes vary widely in price, but you could expect to pay anywhere from $20,000 to $100,000 for a good one of this category, but when we talk about a big tower crane, you should expect a bigger range than that stated above.
Although the more common option is to rent a crane, some contractors and construction companies, who work in many large-scale projects, may find it better to buy one and use it from one construction site to another.
Moreover, tower cranes can be rent or bought online nowadays; as some web sites are specially designed for this sake.
To operate a tower crane effectively and efficiently, at least two crew members should help in this operation. At the very least, you need one to provide you signals during the lifting and someone in charge of the truck's parts.
it is important to know the capacity of your cranes. Tower cranes have built-in computers programs to help the operator in this. If the operator is lifting more weight than the capacity of the crane, the computer will alert him.
Do not forget the counterweights
Counterweights act as the support force of the machine during lifting. The counterweights' weight is calculated using a specific formula that requires you to know the weight of the object you are lifting, nevertheless, the radius of the crane you are lifting at.
8. Crane safety
Make sure the power is cut from any nearby power lines, and make sure the surrounding area is safe and clear of people, workers and obstacles before work. Also, be well prepared for dealing with urgent emergencies; if something does happen, make sure you know exactly what you should do.
SAFETY FACTORS FOR TOWER CRANES
Site-level safety management
Company-level safety management
Type of load
Employment source (operator)
Length of work shift (operator)
SOURCE: JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT, APRIL 2009, P. 316.
Tower cranes are considered a common fixture at major construction sites. They are used to lift steel, concrete, large tools like acetylene torches and generators, and a wide variety of other heavy building materials.
10. Parts of a Tower Crane
All tower cranes consist of the same basic parts, illustrated as follows:
The base is bolted to a large concrete pad that supports the crane.
The base connects to the mast (or tower), which gives the tower crane its vertical height.
the slewing unit (gear and motor) is fixed on the top of the mast; that allows the crane to rotate
Additional three parts are found on the top of the swelling unit:
The long horizontal jib (working arm), which is the portion of the crane that reacts against the load. A running trolley runs along the jib to move the load in and out from the crane's center
The short horizontal machinery arm, which contains the crane's motors and electronics as well as the large concrete counter weights:
The operator's cab (where the operator sits and control the crane)
The machinery arm contains the motor that lifts the load, in addition to the control electronics that drive it, and the cable drum, as shown in the following picture
While the motors that rotate the gears in the slewing unit are located above the unit's large gear
12. How Much Weight Can They Lift?
A typical tower crane has the following specifications
Maximum unsupported height - 265 feet (80 meters)
if it is tied into the building, the crane can have a total height much greater than 265 feet.
Maximum horizontal reach - 230 feet (70 meters)
Maximum lifting power - 19.8 tons (18 metric tons), (300 ton-meter)
Counterweights - 20 tons (16.3 metric tons)
It appears that the maximum load that the crane can lift is 18 metric tons (39,690 pounds), but it cannot lift that much if the load is at the far end of the jib. The closer the load to the mast, the more weight the crane can lift safely (due to the moment effects). The 300 ton-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 tons.
The crane uses two limit switches to keep the crane in the safe side:
The maximum-load switch; monitors the tension on the cable and makes sure that the load is less than 18 tons.
The load-moment switch; makes sure that the operator does not exceed the ton-meter rating of the crane. Sometimes a cat head assembly in the slewing unit can be used to measure the amount of collapse and depreciation in the jib and sense when an overload condition occurs.
13. 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. Largeanchor 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.
14. 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 orclimbing frame that fits between the slewing unit and the top of the mast. Here's the process:
The crew hangs a weight on the jib to balance the counterweight.
The crew detaches the slewing unit from the top of the mast. Large hydraulic rams in the top climber push the slewing unit up 20 feet (6 m).
The crane operator uses the crane to lift another 20-foot mast section into the gap opened by the climbing frame. Once bolted in place, the crane is 20 feet taller!
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.
15. 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.
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