TRAVELING CRANES BASIC AND TUTORIALS

A traveling tower crane is erected freestanding on a base frame and ballasted by the user to accommodate in service loads. When out of service, it may need to be parked and anchored down to prearranged storm ballasting blocks or guyed to resist storm winds. An installation might be on the ground or mounted on another structure such as a building roof.

The base travels on railroad-type rails set to a very wide gauge. At each corner of the base one or more wheels is provided; when more than one wheel is used, they are mounted in a bogie that will equalize the load on all wheels at any one corner (Figure 6.39).

Some crane manufacturers offer options on the number of wheels to be placed at each corner of any one crane model. As the number of wheels increases, the weight of track and the number of track supports need to decrease. This can have significant ramifications for installation cost, particularly if soil conditions are poor.

Crane rails can be supported in a number of ways, including wooden ties on stone ballast (in this case the term ballast is used to refer to the bed of material placed between the tie and the native soil or sand base), a continuous steel beam on wooden ties and stone ballast or on concrete footings, or a continuous concrete footing or concrete sleepers on stone ballast.

The best system is that which will support the crane properly at least cost; this will be a function of crane wheel loads, soil conditions, and availability and cost of the materials at the jobsite. The crane manufacturer provides the wheel-load data, but the installation designer must make the decisions from that point on.

The spacing of sleepers or ties can be determined from rail strength and the wheel loads. For multiple-wheel arrangements, some continuity can be taken into account, but we suggest that supports outside of the bogie should be taken as simple. Deflection should also be checked to avoid lifting the ties off their beds.

Track splices are designed to carry only shear loads, so that splices must be centered between close-spaced supports or placed directly over a support. The spacing must be set so that the two rail ends do not differ in elevation (as a result of deflection or any other cause) as the wheel passes over the splice. This will prevent horizontal impact forces from occurring, forces that can be quite significant given the inertia of the tall crane above.

Rails must be laid to comply with the tolerances given by the manufacturer or specified by code. There are strict limits to variations permitted in gauge, in elevation along the tracks and between the tracks, in straightness, and in slope.

Crane rails can be laid to curves but only if the bogies are designed to permit it. Centrifugal forces which develop as the crane travels a curve can have an important effect on stability. The manufacturer must supply data for minimum radius of curvature that will permit safe travel at the speed the crane is capable of attaining.

Curved track as well as slewing forces, wind, and rail misalignment induce lateral forces on the rails. Rail strength and anchorage must be sufficient to restrain these forces. Magnitudes, however, are not easily determined; the crane manufacturer’s recommendations should be sought and followed.

On poor soils, track differential settlements can be a problem as they may cause track elevations to deviate from permitted tolerances and endanger operations. It would be wise to monitor elevations at marked points. This will show whether settlement is ongoing or stabilizing.

With wooden ties, settlements can be corrected by jacking the rail and tie and resetting the stone ballast. For concrete supports it may be necessary to install steel shim plates with sufficient contact area to prevent the concrete from being crushed.

The parking area must be designed and constructed in advance of crane erection. It will consist, for most cranes, of an area with close support spacing for the rails that will be capable of resisting the storm-wind compressive wheel loads. In addition, there must be four buried ballast blocks to which the crane can be tied down by means of cast-in fittings.

In U.S. practice, the buried ballast together with the traveling ballast must be capable of counterbalancing 1.5 times the maximum overturning moment. At the ends of the tracks, trippers are set that will automatically cause the crane travel brakes to engage. At a distance somewhat beyond the crane stopping distance, end stops, or bumpers, are installed as a last means to prevent the crane from running off the rails.

EARTHQUAKE EFFECTS ON DERRICK AND CRANES BASIC AND TUTORIALS

Studies of earthquake effects on cranes are few, and code development in this area is in its infancy. Generally, permanent installations such as bridge cranes and port cranes can be subjected to seismic analysis using the same principles as those used for other fixed structures.

A decision to analyze a crane seismically should be based on the degree of risk as weighed against potential consequences of a loss. Risk may be assessed by study of earthquake maps. In areas of low or moderate earthquake risk, seismic study may be demanded only for the most-sensitive applications, such as nuclear work.

In adopting a philosophy for earthquake resistance, the crane analyst or designer might consider one or more of three risk mitigation levels, or limit states.

1. The earthquake design does not cause structural damage to the crane. All stresses remain in the elastic range. The crane should remain serviceable.


2. The design earthquake may result in some damage that could be readily repaired and the crane restored. Failure may occur in components that are not part of the main force-resisting system. Component failures cannot put workers or the public at risk, and significant collateral damage to surroundings is not permissible.

3. Controlled ductile yielding may result in the complete functional loss of the crane, which would be replaced, but the avoidance of a catastrophic failure leaves the public, workers, and surroundings protected.

A designer might choose to calibrate the design to only one of these states or, alternatively, consider associating each of them with a different magnitude earthquake event. As all three imply a high level of life protection, this decision would be based only on economic considerations.

Except for those few industrial cranes that are in near-constant use, probability favors the premise that the device will be unloaded if an earthquake should occur. Nearly all cranes used in both general industry and construction will have a substantial load on the hook only a small percentage of the calendar year.

With few exceptions, then, earthquakes might reasonably be evaluated only for out-of-service consideration. Though earthquakes are dynamic events, the simplest methods of seismic analysis make use of equivalent static loads.

These methodsare suited to areas of low or moderate seismicity or for structures that are relatively simple in their response to excitation. Other methods in the toolboxes of seismic engineers may be applied for more complex situations or where the level of risk warrants the investment.

A freestanding tower crane may respond well to moderate earthquakes because its long period of oscillation will not resonate with the higher-frequency ground motion. However, the crane could be at risk in a severe earthquake due to base shear or from vertical acceleration acting on the counterweights.

In some soils, liquefaction could pose a risk. On a tower crane that is secured to a building, tied to the outside or mounted within, the interaction with the building can lead to higher seismic loads compared to those expected for standard freestanding erections.

Generalized assessment of earthquake risks for a mobile crane can be difficult because a typical machine changes location frequently and its boom disposition changes constantly. Overall exposure should not be great. There could be vulnerability under certain conditions, however.

For example, ground acceleration might induce an unloaded machine with a short boom at a high angle to tip backward.


Perhaps the most studied seismic event with respect to cranes was the Kobe, Japan, earthquake of 1995. Documented failures included

• Total collapse due to foundation failures probably caused by liquifaction
• Overstress of towers from horizontal shear, with resulting diagonal and connection failures
• Leg tension failures due to overturning moment on pillar cranes and tower cranes
• A bridge crane girder lifting off its supports
• Overturning of a rail-mounted gantry crane

In 2002, two tower cranes toppled from the 60th floor of a steelframe building under construction in Taipei, Taiwan, during a moderately severe earthquake. The failures were not caused directly by the earthquake, but rather by the cranes oscillating in resonance with the building.

TOWER CRANES BASIC AND CIVIL ENGINEERING TUTORIALS

TOWER CRANES BASIC INFORMATION
What Are Tower Cranes?


Much like a mobile crane, a tower-mounted crane moves loads by executing three motions: the hook is raised and lowered by means of a winch and fall, carried in a circular path by the swing gear, and carried in a radial motion by either luffing the jib or rolling a trolley carriage on its underside. The simplest of tower cranes have only these motions, but more complicated arrangements include mechanisms that allow the base to roll on a track, the crane to change elevation by climbing, or the jib to articulate on a hinge point.

In the twentieth century, tower crane and mobile-crane industries developed independently on opposite shores of the Atlantic Ocean in response to needs and cultural proclivities of their respective lands. Simply put, Europe was the realm of tower cranes, North America the realm of mobile cranes, and the rest of the world a patchwork of the two.

The globalized marketplace and rapid diffusion of knowledge in the present century have eroded differences in construction practices worldwide so that the selection of a crane type now is more likely to be determined by its suitability to the work than by the country where the work is taking place. At least that is the broad picture; contractors in some localities cling longer to traditional practices than others.

Tower cranes are the lifting machines of choice worldwide for most mid- and high-rise building construction. They are used also on expansive sites where the broad hook sweep and the relative ease of coordinating multiple tower cranes is an advantage.

There are niche markets for these as well; cable stay and suspension bridges, offshore oil platforms, and power plants are some examples. In most of the world outside U.S., small tower cranes are used for modest-size residential and commercial projects.

Many of these rigs are self-erecting machines that are pulled to the site by truck. In North America, similar work would utilize a small telescopic crane.

Gradually, however, self-erecting tower cranes are penetrating the U.S. market. There is some doubt  whether these machines should be classified as tower cranes; though their operating motions fit thepattern, their deployment—setup time, operation, inspection, maintenance, and demobilization—is more like that of mobile cranes.

Freestanding hammerhead tower cranes range up to about 300 feet (91 m) in height; for luffing tower cranes the limitation is less. Though most tower cranes free-stand and remain at a fixed height, various self-climbing arrangements permit a tower crane to attach to a building under construction and rise with it.

With such supplemental means of support, a tower crane can ascend to any building height. Very high line speeds up to 1000 ft/min (5 m/s) available with some models yield good production rates even at extraordinary heights.

Some machines can operate in winds up to 45 mi/h (70 km/h), which is far above mobile-crane wind limits. Lift capability of tower cranes is gauged by a moment rating expressed as tonne-meters. The tonne-meter rating is obtained by multiplying rated capacity in metric tons by the working radius in meters.

This is done according to a method that averages a range of boom lengths and working radii. The smallest machines used for light construction have ratings of about 20 meter-tons and the very largest in production exceed this by a factor of about a hundred. Most used for heavy construction are in the range of 150 to 650 meter-tons.

The cost associated with installing and removing a tower crane is small for the self-erecting type but can be considerable for most others. At minimum, those costs would include trucking, hiring a rigging crew and an assist crane, construction of a foundation, electrical hookup, and the services of a trained technician. More complicated installations such as those which climb have considerable additional expenses.

The high costs of installation, as well as the considerable investment of time and planning, make all but the smallest tower cranes a tool for longer-term projects where these expenditures can be amortized.

Excluding self-erectors, tower crane designs follow the “erector set” concept; that is, they are composed of multiple components of common design. Components connected together by pins or bolts are often interchangeable among an array of models.

As a tower is a cantilever subjected to high-bending moments that shift and reverse, each tower leg must be designed to resist alternating compressive and tensile forces. The usual stress levels and the expected number of lifetime loading cycles are not exceptional; thus leg design does not pose a difficult fatigue problem for designers.

The connections between tower sections are another matter, being a perpetually nettlesome design and maintenance concern. Some designs use bolts and others use pins to join the legs of one section to the next. Bolts must be preloaded to carefully controlled levels.

Failure to establish and maintain the preload can result in bolt fatigue failure. The alternative pin-type connection requires tight fabrication tolerance; imperfect towers can be either too sloppy or difficult to fit together. They also must be carefully designed to avoid the pitfall of premature fatigue failure.

With the exception of a few diesel-powered machines with hydrostatic drives, tower cranes are powered by electricity and motors driving all the machinery are almost universally electric, too. The hoist motors on older machines have high- and low-speed ranges with stepped increments in each range.

More recent models have variable-frequency drive or other forms of continuously adjustable speed motors with friction or eddy-current brakes and creep speed. Automatic acceleration for all motions is typical on many cranes. Remote controls are sometimes offered.

A tower crane is said to be top-slewing—slewing being the preferred tower crane term for swing motion—if the swing circle is mounted near the tower top and it’s said to be bottom-slewing if the swing circle is near the. Bottom-slewing machines might have become largely obsolete had the concept not been revived by recent developments with self-erecting machines.

CRANE SUPPORTED (HANGING) LEADERS USED IN PILING BASICS AND TUTORIALS

CRANE SUPPORTED (HANGING) LEADERS USED IN PILING BASIC INFORMATION
What Are Crane Supported Leaders Used In Piling?


Although the complete piling rig with its base frame and leaders supported by a stayed mast provides the best means of ensuring stability and control of the alignment of the pile, there are many conditions which favour the use of leaders suspended from a standard crawler crane.

Rigs of this type have largely supplanted the frame-mounted leaders for driving long piles on land in the UK and USA. The usual practice is to link the leaders by the head of the crane jib and to control their verticality or backward or forward rake by means of adjustable stays near the foot of the leaders.

The latter bear on the ground through an enlarged foot which can be levelled by a screw jack. BSP International Foundations Ltd. TL series leaders (Figure 3.6) have heights of 19.0m and 21.9m and carry hammers of up to 3 tonne mass.

The 610mm and 835mm square section lattice leaders have a height to the cathead of 22.5 and 38m respectively, and can carry combined pile and hammer loads of 13 tonne and 21 tonne respectively.

Backward and forward rakes of up to 1:3 are possible depending on the stability of the crawler crane. There is a practical limit to the length of pile which can be driven by a given type of rig and this can sometimes cause problems when operating the rig in the conventional manner without the assistance of a separate crane to lift and pitch the pile.

The conventional method consists of first dragging the pile in a horizontal position close to the piling rig. The hammer is already attached to the leader and drawn up to the cathead. The pile is then lifted into the leaders using a line from the cathead and secured by toggle bolts.

The helmet, dolly and packing are then placed on the pile head and the assembly is drawn up to the underside of the hammer. The carriage of the piling rig is then slewed round to bring the pile over to the intended position and the stay and angle of the crane jib are adjusted to correct for vertically or to bring the pile to the intended rake.  The problem is concerned with the available height beneath the hammer when it is initially drawn up to the cathead.

Taking the example of leaders with a usable height of 20.5m in conjunction with a hammer with an overall length of 6.4m, after allowing a clearance of 1m between the lifting lug on the hammer to the cathead and about 0.4m for the pile helmet, the maximum length of pile which can be lifted into the leaders is about 12.7m.

A somewhat longer pile could be handled if the leaders were of a type which allows vertical adjustment. Occasionally it may be advantageous to use leaders independent of any base machine. Thus if only two or three piles are to be driven, say as test piles before the main contract, the leaders can be guyed to ground anchors and operated in conjunction with a separate petrol or diesel winch.

Guyed leaders are slow to erect and move, and they are thus not used where many piles are to be driven, except perhaps in the confines of a narrow trench bottom where a normal rig could not operate.