TOWER CRANE

Tower cranes are a common fixture at any major construction site. They are pretty hard to miss - they often rise hundreds of feet into the air, and can reach out just as far. The construction crew uses the tower crane to lift steel, concrete, large tools like acetylene torches and generators, and a wide variety of other building materials.

Tower Crane Image Gallery

Tower cranes can rise 150 feet in the air and lift up to 19 tons. See more pictures of tower cranes.

When you look at one of these cranes, what it can do seems nearly impossible: Why doesn't it tip over? How can such a long boom lift so much weight? How is it able to grow taller as the building grows taller? If you have ever wondered about how tower cranes work, then this article is for you. In this article, you'll find out the answers to all of these questions and more!

Parts of a Tower Crane

All tower cranes consist of the same basic parts: 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

height. Attached to the top of the mast is the slewing unit -- the gear and motor --

that allows the crane to rotate:

On top of the slewing unit are three parts:

The long horizontal jib (or working arm), which is the portion of the crane that carries the load. A trolley runs along the jib to move the load in and out from the crane's center:

The shorter horizontal machinery arm, which contains the crane's motors and electronics as well as the large concrete counter weights:

The operator's cab:

The machinery arm contains the motor that lifts the load, along with the control electronics that drive it and the cable drum, as shown here:

The motors that drive the slewing unit are located above the unit's large gear:

Now let's find out how much weight this equipment can handle.

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.

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."

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:

1. The crew hangs a weight on the jib to balance the counterweight. 2. 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). 3. 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.

Fixed Base Tower Cranes

The fixed base-type tower crane has its tower mounted on a foundation block. By the use of another (mobile) crane, the tower can be erected to its full height at the beginning of the project or the crane can have the mechanical capability, usually hydraulically, to raise itself, allowing for the addition of structural sections to the tower.

Factors Affecting the Choice of Crane Location   

Tower Crane 1 Tower Crane 2 Two Cranes Crane Locations

1. Maximum coverage of work area 2. Maximum load to be hoisted 3. Strength and design of crane base 4. Presence of adjacent possible obstruction during swinging 5. Magnitude of wind load effect on the boom or load 6. Visibility of crane operator

Two fixed base tower cranes on concrete base were set up on-site; one with a working radius of 35m and the other of 65m (both with lifting capacity of 4 tonnes)

Selection Criteria for Tower Cranes

The choice and utilization of a tower crane requires considerable planning. It showed reach from its fixed position to all locations from which loads are to be lifted and placed. Therefore, when selecting a crane for a particular project, the engineer must ensure that the weight of the expected loads can be handled at their corresponding required radius. Tower cranes are selected for use based on factors such as:

1. Weight, dimensions, lift height and radius of the heaviest loads2. Frequency and duration of usage

3. Maximum free-standing height of the crane

4. Maximum braced height of the crane

5. Machine-climbing arrangement

6. Weight of crane supported by the structure

7. Available headroom which can be developed

8. Area which must be reached

9. Hoisting speed of the crane

10. Length of cable that the hoist drum can carry

11. Other normal procurement factors such as price, reputation and service back-up of manufacturer/ agent

Site Planning

Effective site planning is an important element for the successful completion of a construction project. Since the site of this project is located on steep slopes and in congested urban area, a number of constraints are imposed for the construction activities. The issues to be considered under 'Site Planning' include:

Site Constraints Site Layout Site Drainage

Site Services

1. Cast a concrete base for the basic mast of the crane with timber formwork 2. Assemble the telescopic cage section on top of basic mast 3. Assemble the top mast section just under the mast cabin on top of cage

section 4. Erect the section of top tower containing the mast cabin above top mast

section 5. Fix the counter-jib perpendicular to the vertical tower using the counter-jib tie 6. Hang the counterweights at the end of the counter-jib 7. Install the main jib with the main jib tie 8. Hang the jib trolley on the main jib

After erecting the whole tower crane, it was ready for "raising" to the required operating height. This was achieved by jacking up hydraulically the telescopic cage section and then bringing extra standard mast sections into the telescopic cage consecutively until sufficient height was reached.

Tower Crane Erection Process   

Basic Mast Cage Section Mast Section Top Tower

Counter Jib Counter Weights Main Jib Typical Section

Moving Jibs Connecting Jibs Securing Basic Mast

Placing Re-bar

Installing Lever Lifting Section Introducing Section

Subject: erection of construction cranes

Question:

How are the very tall construction cranes erected and taken down following completion of the high rise project?  Unfortunately I've never been present when that activity is taking place.  I'm particularly curious about how the horizontal swing arm is placed with the counter balance at the There are at least three different answers too your question.

1.)  Some of the smaller units are self erecting and self dismantling.  Essentially the boom (swing arm) folds like an accordian door with three hinge points.  The tower has a bottom hinge and folds over and then lays down.  This type of tower crane must be erected exterior of the building so all of this folding and hinging can occur.

2.)  Some are erected to final height at the beginning of the project, and they can either be built into the building or just outside of the building.  The boom is positioned high enough so that it will clear the top of the building when finished, and high enough so that it clears surronding buildings also.  The tower is erected a piece at a time (about 20' sections) until the proper height is reached.  A large mobile( on wheels) crane is used to hoist the pieces.  A pivot section is added on top of the tower, and then the boom and counter weight extenions.  Some times the boom has to be inserted in more than one section, adding counter balancing sections on the opposite side.  The boom is light enough and strong enough that some amount of moment is managed through the tower connections until the full boom and counterweights are all in place and the boom becomes balanced.  The process is just the opposite to remove it.    When the crane is erected in the building footprint and the building built around it; sometimes the crane to remove it must be really big; as it must reach up and over the shoulder of the building and still get above the standing tower high enough to lift the pieces off.

3.)  A variation on the theme is a jumping tower crane.  It usually starts with about about 100' of tower.  There is an apparatus in the tower section just below the pivot section that allows additional tower sections to be slid into a jacked up gap.  The tower crane then lifts its own tower section onto the apparatus and this new section is inserted.This procedure is repeated as the building rises.  This keeps the operator closer to the work so he or she can see it better.  This crane is dismantled the same as #2 above.  end.  

How do they dismantle tower cranes when they have been assembled within a structure?

A tower crane is usually assembled by a telescopic crane of smaller lifting capacity but greater height, and in the case of tower cranes that have risen while constructing very tall skyscrapers, a smaller crane will sometimes be lifted to the roof of the completed tower to dismantle the tower crane afterward. A self-assembling tower crane has been demonstrated, which lifts itself off the ground using jacks, allowing the next section of the tower to be inserted at ground level structure?