What Is A Tower Crane?

What Is A Tower Crane?

Meta Description: A tower crane is a crucial construction machine. Learn about its components, operations, and installation. Discover the significance of tower cranes in construction.

This article provides a comprehensive overview of tower cranes used widely at construction sites. Tower cranes are essential for lifting heavy materials and equipment efficiently. We will discuss the different types of tower cranes, their key components, and their operations.

The guide aims to explain how tower cranes work, their importance, and the installation and dismantling process. It also highlights factors to consider when selecting the right tower crane.

Key Takeaways:

• Tower cranes, including luffing jib and self-erecting cranes, are vital in the construction sector for lifting heavy materials.
• Construction companies often opt for tower crane rentals to handle heavy equipment on loading docks and transport trucks.
• Typical tower cranes consist of parts like the slewing unit, tower mast, and base, enabling them to lift a weight.
• Flat-top cranes are commonly used in construction, often installed on existing structures to aid construction workers.

Understanding the Vital Role of Tower Cranes in Construction

Tower cranes have become essential equipment for construction sites because they can lift and precisely position heavy loads over long distances with minimal ground space requirements.

Their versatility allows tower cranes to access entire job sites and support various construction activities from initial material delivery to completion.

A Brief History of Tower Crane Development

The first tower cranes used in construction were Derrick, named after inventor Thomas Derrick. These early lifting tools featured a horizontal boom hinged to a rotating base, a design that paved the way for later tower crane models.

In the early 20th century, gantry tower cranes were introduced but proved too heavy for most urban job sites. The modern tower crane design was conceived in 1949 by German engineer Hans Liebherr. It features a rotating tower and horizontal jib for improved mobility and efficiency. Liebherr’s innovative crane design inspired many subsequent tower crane prototypes.

By the 1970s, most manufacturers had switched from hydraulic to electric power systems, enhancing tower cranes’ functionality, performance, and energy efficiency.

Components and Mechanisms of Tower Cranes

Tower cranes have several key components that lift and precisely position heavy loads. A thorough understanding of these parts is essential for contractors utilizing tower cranes on construction projects.

Base and Support Structure

Its base and support structure are among the most essential components of a tower crane. This typically consists of a reinforced concrete foundation or pad that provides a sturdy, stable platform to anchor the entire crane assembly.

The base may be a steel grillage mounted on the building for tower cranes installed on existing structures. Proper engineering ensures the base can withstand substantial overturning and uplift forces from crane operations.

Tower Mast

The tower mast is the primary supporting element, rising vertically from the base. It must be sufficiently rigid to counterbalance heavy lifting loads. Tower masts commonly feature a square steel lattice configuration to achieve high strength-to-weight ratios.

Their height determines the crane’s operational reach. On self-erecting tower cranes, the mast can extend telescopically to climb the crane upward as the building rises.

Slewing Mechanism

The slewing unit is one of the critical mechanisms enabling a tower crane’s maneuverability. This consists of a significant circular bearing mounted to the top of the tower mast. It facilitates 360-degree rotation of the entire upper crane assembly, including the jib, counter jib, and operator’s cabin.

The slewing unit bearing allows smooth rotation via an electric motor or hydraulic system. It must withstand substantial overturning moments from load swings and shifts in wind conditions during crane operations.

Jib and Counterjib Arms

Projecting horizontally from the top of the rotating slewing unit are the jib and counterjib arms. The jib carries the trolley and hook block used for lifting loads. It can extend the crane’s reach up to its rated capacity.

The counter jib is located on the opposite end of the jib and helps balance the heavy jib and suspended loads. It also anchors counterweights necessary to counterbalance the jib and payload during lifting operations. Jibs and counter jibs commonly feature an open-web steel lattice construction for strength and low weight.

Trolley and Hook Block Assembly

One of the primary load-handling mechanisms is the trolley and hook block assembly. The trolley travels horizontally along the jib on rails, allowing the operator to position the hook block directly over the load. The hook block consists of pulley systems that enable the hook to be raised and lowered using wire ropes.

This raises and lowers suspended loads from the crane hook. For precise load positioning, most trolleys and hook blocks also include functions like powered trolley travel and computer-controlled load moment limiting devices.

Counterweight System

• Counterweights are necessary to counterbalance the heavy jib and payload during lifting operations.
• They are made of concrete, iron, or steel blocks and adjustable to match the crane’s lifting capacity.
• Counterweights are positioned on the opposite end of the jib from the hook block and suspended load.
• Tower crane operators can add or remove individual counterweight sections to achieve balance for different load configurations.
• Proper counterweighting is critical for safe and efficient lifting by preventing tipping or instability of the entire crane system under load.

Common Types of Tower Cranes

Tower cranes come in various designs to suit different lifting needs on construction sites. Understanding the primary types is important for contractors selecting the right crane.

Hammerhead Tower Cranes

One common variety is the hammerhead tower crane, recognizable by its vertical mast and horizontal jib, forming a “T” shape. This configuration positions the operator’s cabin at the end of the horizontal jib. Hammerhead tower cranes are versatile and can access work areas on multiple sides of the crane.

They are well-suited for urban construction sites where space is limited. The horizontal jib also allows for unobstructed lifting of steel beams and other long materials from all directions around the crane.

Luffing Jib Tower Cranes

Another common variety is the luffing jib tower crane. It features a diagonal jib that can “luff” or adjust its angle, extending and retracting to lift loads around obstacles. Luffing jib cranes can access tight spaces more quickly than other types.

They are well-suited for dense urban construction sites where space is constrained. The ability to angle the jib also allows for lifting loads from multiple directions around the crane.

Self-Erecting Tower Cranes

Self-erecting tower cranes are highly efficient on tall building projects since they can climb upward as construction progresses. The mast sections can be extended hydraulically to raise the entire crane.

An onboard climbing frame allows it to walk up the completed floors and anchor securely to the rising structure. This eliminates the need for a separate mobile crane to dismantle and re-erect the tower crane at each level. Self-erecting tower cranes provide continuous lifting coverage throughout the entire construction cycle of high-rise buildings.

Erecting and Installing Tower Cranes

Proper planning and execution are crucial for erecting and installing tower cranes on construction sites. Contractors must follow specific best practices.

Planning and Preparation

Thorough planning includes selecting the right crane type, consulting engineers for the foundation design, and obtaining necessary permits. The base/foundation and tower sections must be engineered to withstand significant weight loads.

A detailed lifting plan is also required to safely lift the crane components into place using auxiliary cranes or mobile cranes. Protection from overhead power lines must likewise be addressed.

Proper preparation of the laydown and assembly areas ensures efficient crane installation. Coordination with other trades prevents scheduling delays.

Assembly Process

Once the foundation is prepared, assembly begins with erecting the tower sections. Lifting each section precisely into place requires experienced riggers and signals from spotters. The mast, jib, and counterjib components are attached and anchored securely.

Assembly also involves installing the slewing ring, counterweight system, and hook block. Commissioning tests are performed before operations begin to ensure all crane functions meet safety standards.

Climbing Tower Cranes

• On tall building sites, climbing tower cranes facilitate vertical construction.
• They feature a climbing frame that locks onto completed floors, allowing the entire crane to hoist upwards.
• Hydraulic cylinders extend the mast sections telescopically during climbing.
• This process requires riggers to guide the sections and engage locking pins at each level.
• Climbing occurs in carefully planned stages to maintain the crane’s stability and center of gravity.
• Safety devices prevent accidental climbing until the area above is cleared for the crane to lift itself to the next level.

Operating Tower Cranes Safely and Efficiently

Once erected, tower cranes must be operated carefully according to safety protocols. This ensures efficient material handling without risks.

Operator’s Cabin and Controls

The operator’s cabin also called the operator cab, is positioned at the end of the jib or hammerhead. It contains seats, control panels, and instrumentation. The cabin provides the crane operator with a safe, stable platform to oversee lifting operations and view the surrounding work area.

Control panels allow operating functions like slewing, luffing, trolley travel, and load hoisting using joysticks or levers. Modern tower cranes also incorporate computerized load moment and radius indicators to prevent overloading and guide safe working radii.

Communication and Signaling

Clear communication is essential for coordinating lifting operations safely. Tower crane operators communicate with riggers, spotters, and other workers using two-way radios. Standard hand signals are also employed when verbal communication isn’t possible. Spotters relay signals from below to guide load positioning precisely.

Safety Mechanisms and Procedures

• Tower cranes incorporate load moment limiting devices and radius indicators to prevent overloading or exceeding the safe working zone.
• Regular inspections and maintenance per the manufacturer’s schedule ensure all safety-critical components function correctly.
• Regulations require comprehensive training and certification for crane operators. They must demonstrate proficiency in operating procedures, emergency response, and safety compliance.

Dismantling and Removing Tower Cranes

Once a construction project is complete, tower cranes must be carefully dismantled and removed. Proper planning and execution are crucial.

Disassembly Process

The disassembly process begins by removing the jib and counterjib sections. The trolley, hook block, and other lifting gear are also detached. Riggers then disassemble the mast in a controlled manner from the top down using tag lines. Each section is lowered securely to the ground using the crane’s lifting capabilities.

Using Auxiliary Cranes

An auxiliary crawler or mobile crane may be needed on some projects to lift the heavy tower crane components down. Coordination ensures the auxiliary crane has sufficient reach and capacity. Temporary ground stabilizers extend their outriggers.

Planning for Removal

Dismantling and site preparation require as much planning as the original installation. Contractors account for load limits, overhead obstructions, and transporting the crane off-site. An engineered lifting plan maintains safety, and permits are obtained for street closures during heavy load movement.

Considerations for Choosing the Right Tower Crane

Selecting the most suitable tower crane requires evaluating key site and project factors. Understanding these ensures efficient material handling.

Required Lifting Capacity

The tower crane’s maximum rated capacity is one of the most important considerations. Contractors must accurately determine the heaviest lift weights throughout construction, accounting for building materials like steel beams.

The capacity needed now and in future phases should be estimated. An under-sized crane will cause project delays from multiple lifts. Oversizing is cost-efficient. The crane’s lifting charts must satisfy all load requirements safely.

Site Conditions and Access

Another critical factor is evaluating the construction site itself. Contractors must assess space constraints and clearances and choose a tower crane with a jib and tail swing radius that avoids obstructions.

Site access for transporting and erecting large crane components is likewise important. Ground conditions are also reviewed to ensure the foundation design supports the crane loads adequately.

Project Timelines

Project timelines and phasing influence crane selection. Contractors estimate the duration of lifts in each phase to prevent crane overcapacity early on and undercapacity later on.

Self-erecting tower cranes may be preferable for tall buildings to facilitate vertical construction continuously without dismantling and re-erecting a crane at each level. Planning ensures the right crane capacity, type, and climbing ability (if needed) to service material handling efficiently throughout the schedule.