Steel buildings and bridges are generally erected with cranes, derricks, or specialized units. Mobile cranes include crawler cranes, rubber tired rough terrain cranes and truck cranes; stationary cranes include tower cranes and climbing cranes. Stiffleg derricks and guy derricks are generally considered stationary hoisting machines, but they may be mounted on mobile platforms. Guy derricks can be used where they are jumped from floor to floor. A high line is an example of a specialized unit. These various types of erection equipment used for steel construction are also used for precast and cast-in-place concrete construction. One of the most common machines for steel erection is the crawler crane (Fig. 2.3). Selfpropelled, such cranes are mounted on a mobile base having endless tracks or crawlers for propulsion. The base of the crane contains a turntable that allows 360 rotation. Crawlers come with booms up to 450 ft high and capacities up to 350 tons. Self-contained counterweights move the center of gravity of the loaded crane to the rear to increase the lift capacity of the crane. Crawler cranes can also be fitted with ring attachments to increase their capacity. Truck cranes (Fig. 2.4) are similar in many respects to crawler cranes. The principal difference is that truck cranes are mounted on rubber tires and are therefore much more mobile on hard surfaces. Truck cranes can be used with booms up to 350 ft long and have capacities up to 250 tons. Rough terrain cranes have hydraulic booms and are also highly mobile. Truck cranes and rough terrain cranes have outriggers to provide stability. A stiffleg derrick (Fig. 2.5) consists of a boom and a vertical mast rigidly supported by two legs. The two legs are capable of resisting either tensile or compressive forces, hence

the name stiffleg. Stiffleg derricks are extremely versatile in that they can be used in a permanent location as yard derricks or can be mounted on a wheel-equipped frame for use as a traveler in bridge erection. A stifleg derrick also can be mounted on a device known as a creeper and thereby lift itself vertically on a structure as it is being erected. Stiffleg derricks can range from small, 5-ton units to large, 250-ton units, with 80-ft masts and 180-ft booms. A guy derrick (Fig. 2.6) is commonly associated with the erection of tall multistory buildings. It consists of a boom and a vertical mast supported by wire-rope guys which are attached to the structure being erected. Although a guy derrick can be rotated 360, the rotation is handicapped by the presence of the guys. To clear the guys while swinging, the boom must be shorter than the mast and must be brought up against the mast. the guy derrick has the advantage of being able to climb vertically (jump) under its own power, such as illustrated for the construction of a building in Fig. 2.7. Guy derricks have been used up to 160 ft long and with capacities up to 250 tons. Tower cranes in various forms are used extensively for erection of buildings and bridges. Several manufacturers offer accessories for converting conventional truck or crawler cranes

into tower cranes. Such a tower crane (Fig. 2.8) is characterized by a vertical tower, which replaces the conventional boom, and a long boom at the top that can usually accommodate a jib as well. With the main load falls suspended from its end, the boom is raised or lowered to move the load toward or away from the tower. The cranes are counterweighted in the same manner as conventional truck or crawler cranes. Capacities of these tower cranes vary widely depending on the machine, tower height, and boom length and angle. Such cranes have been used with towers 250 ft high and booms 170 ft long. They can usually rotate 360.
Other types of tower cranes with different types of support are shown in Fig. 2.9a through c. The type selected will vary with the type of structure erected and erection conditions.
Each type of support shown may have either the kangaroo (topping lift) or the hammerhead (horizontal boom) configuration. Kangaroo and hammerhead type cranes often have moveable counterweights that move back as the load is boomed out to keep the crane balanced.
These cranes are sophisticated and expensive, but are often economical because they are usually fast and may be the only practical way to bring major building components to the floor they are needed. Crane time is a key asset on high-rise construction projects.
Jacking is another method used to lift major assemblies. Space frames that can be assembled on the ground, and suspended spans on bridges that can be assembled on shore, can be economically put together where there is access and then jacked into their final location.
Jacking operations require specialized equipment, detailing to provide for final connections, and analysis of the behavior of the structure during the jacking.


Comments

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.