Contrastación de la Hipótesis

The planning of erection (lifting) devices and details for their attachment to the ele-ments must be completed. It is obvious that changes in methods or details of any phase of the construction may be difficult after actual commencement of that part of the work.

Lifting Devices for Handling Precast Concrete Units. A lifting device consists of two main parts: the anchorage element embedded in the precast unit and the attachment element, which is attached to the anchorage to fasten the lifting line to the component.

To provide adequate strength, the anchorage should bear against the reinforcement. A simple and common device is to embed several steel loops in the concrete, leaving the loop exposed for attachment of the crane hook.

Selection of proper anchors for lifting precast concrete products requires considera-tion of a number of factors including the type of load, type of lift, concrete shape and weight, concrete compressive strength at time of initial lift, number of lifting points and type of rig-ging to be used, direction of pull (cable or sling angle), reinforcement, ease of attachment to product, compliance with safety requirements and ease of use during final installation and cost.

The location of lifting devices in the components should be carefully considered, tak-ing full account of the special loadtak-ing that will be imposed on the concrete as a result of tilt-ing, lifttilt-ing, or moving the component, including an allowance for impact. For example, rais-ing a horizontally cast precast panel to a vertical position (e.g. columns cast on site) may in-duce stresses in the concrete that exceed any loading that may be imposed on the panel after it has been installed in a structure.

Selection of the correct lifting devices depends on a number of factors concerned with the type, weight, configuration, thickness, and strength of the precast component.

Selection of the lifting device and its location should be based on the manufacturer’s recommendation and an engineering analysis of the proposed installation. The locations and details of lifting and handling devices should be shown on the shop drawings.

Safe, effective, economical lifting and handling of precast concrete units is essential for producers and users of these products. Precast concrete products need to be lifted and han-dled a number times during production and installation, particularly when removing from the casting bed or form, transporting from the casting bed to storage or from storage to point of use, lifting into installed position or anchoring to a structural frame or other concrete members (where required).

Precast concrete products should not be moved until the concrete has gained sufficient compressive strength. In some cases, the form and the product are moved at the same time with the lifting hook connected directly to the form. Some forms are made to remain in place

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while inserts, loops, lifting eye bolts, hooks, plates, pulling irons and lift pins or slings are used to remove the product.

The method used in removing a casting from the mould and to position it in the struc-ture is influenced by the characteristics of the casting, temporary stresses developed, practical use of available equipment, and economy. In securing a physical attachment to the casting for lifting purposes, simple devices and inserts are frequently used. Lifting devices designed spe-cifically for precasting are available from manufacturers.

A spreader beam is used in transmitting vertical loads from the two surface inserts to a single point at the lifting hook. Larger elements may require three inserts to reduce the indi-vidual insert load or bending in the panel. A continuous cable through sheaves on the spreader and at the center insert may be used to equalize the load at all three inserts. The center insert may be moved higher or lower on the panel to avoid openings. Large or heavy panels are of-ten lifted with a four-point pickup with sheaves on the spreader for load equalization.

For direct vertical insert loads a double set of spreaders are used. To reduce bending in a four-point pickup, two inserts are placed near the panel top and two near the center with ca-bles run through sheaves on the spreader beam to rotate the panel to a vertical position.

Timber or steel strongbacks are often used to reinforce panels for bending stresses.

The panel may be lifted by a connection to the strongbacks or to other points on the panel.

a Device for lifting packages

b Composite slab lifting device with rolling blocks

c Lifting device for spatial elements

d Spreader beam for lifting

pre-cast beams in two points of pickup e Four-cable lifting device f Six cable slab lift-ing device with

roll-ing blocks

125 g Spreader beam for lifting precast

beams with two wire cables in two points of pickup

h Spreader beam for lifting precast roof slabs with four wire cables in

four points of pickup

i Staircase lifting de-vice with rolling

blocks Figure 11- 4 Typical lifting devices for precast concrete members

Source: Popa R., Teodorescu M., 1984; Suman R., 1989, Trelea A., 1997.

Cranes - a crane may be defined as a machine for lifting loads by means of a cable. The use of cranes has greatly increased in the construction industry due mainly to the need to raise the large and heavy prefabricated components.

Figure 11-5 Crane clearances

Caption: a. boom angle; b. maximum clearance height of cabin; c. maximum radius of tail swing; d.

center of rotation to boom foot pin; e. height from ground to boom foot pin; f. distance from center of boom point sheave to bottom of hook; g. clearance radius of boom; h. length of boom.

Source: Andres C., 1998.

Characteristics of Cranes - a crane consists primarily of a power unit mounted on a carrier with a hoist, a boom, and control cables for raising and lowering the load and boom.

The boom can be a welded steel lattice or a hydraulic boom made of 1 articulated base element and several telescopic sections, that are extended and retracted hydraulically (from 5 to 30 m) allowing the crane to be completely self-contained.

A jib, an extension to the end of the boom, is used for extending the height to which loads can be lifted; it can be added to a lattice boom or a hydraulic boom. A jib decreases the lifting capacity of the crane and should be used with caution. Two basic jib formats for this type of crane are available, namely the folding lattice jib and the telescopic jib.

Outriggers are hydraulic telescoping supports (4-6 suspension cylinders, individually controlled in both – horizontal and vertical – directions from the frame).

Crane Selection Criteria: The range of cranes available is very wide and therefore actual choice must be made on a basis of sound reasoning, overall economics and technical capabilities: of cranes under consideration, prevailing site conditions and the anticipated utili-zation of other erection equipment.

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The criteria by which to chose cranes is based on the following considerations:

a. Maximum hook height (H).

b. Extended / retracted boom length (f). The shallower the boom's angle, the less load it can lift. The longer the boom, the less load it can lift.

c. Clear radius of boom (R). Should the load be lifted at a grater radius, the angle of the boom be decreased, the load capacity of the crane is greatly decreased.

d. Lifting capacity (Q). To assure the cranes stability, it is necessary to respect the fol-lowing restriction: 0,75 lifting capacity of crane ≤ tipping load of crane

If not possible then the stability of the footing must be increased by leveling and com-pletely supporting them by their outriggers, which must be fully extended and positioned firmly on stable ground.

Depending on the circumstances under which a load is lifted, either of these can gov-ern the safe lifting load of the crane. Loading charts are provided with each crane and must be adhered to religiously. Load charts should never be exceeded.

e. Traveling clearance for crane (S = min. 500 mm) given by maximum radius of tail swing, width of chassis or length of outriggers.

Five basic types of cranes are commonly available for use on the construction site, they can be classified as: tower cranes, track-mounted cranes, lorry-mounted cranes, self-propelled cranes, and gantry cranes. There are several variations of each type, and each is available in a wide range of lifting capacities and boom lengths, thus providing the contractor with a generous selection of options.

Figure 11-6 Typical tower crane

Caption: 1. Outer tower; 2. Inner tower; 3. Jib; 4. Tail (counter jib); 5. Cab; 6. Load line; 7. Inner guy line; 8.

Outer guy line; 9. Fall back line; 10. Trolley line; 11.

Counterweight; 12. Tower base with central ballast coun-terweights; 13. Frame; 14. Drive motor (hoist gear).

Source: Andres C., 1998.

A tower crane comes in several basic variations (e.g. self-supporting static tower cranes, supported static tower cranes, traveling tower cranes and climbing cranes).

Advantages offered by using a tower crane:

- Tower cranes have one advantage over mobile cranes in that the operator's cab is placed at the top of the tower, providing the oper-ator an unobstructed view of the construction site.

- Tower cranes are usually set up at a location on the construction site from where all corners of the building can be reached, often in the elevator shaft of the building being constructed.

- High capacities range radii and heights (30 m and 100 m).

Limitations:

- Low lifting capacities 150 kN.

- At the completion of the building, they must be disassembled and lowered to the ground in relatively small sections by a mobile crane.

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The track-mounted cranes (crawler crane) come in a wide variety of designs and ca-pacities, generally with a 360° rotation or slewing circle, a low pivot, and jib.

Advantages:

- Mobility on the job site with load on hook.

- It can lift relatively heavy loads (10…900 kN) without the use of outrig-gers.

- Capacities ranges similar to the lorry mounted cranes, height capacities 60 m;

Limitations: slow speeds and large bulk, the crane cannot move from one site to another without some disassembly and the use of 10 tone trailers to transport it between sites.

Characteristics: The jib is of lattice con-struction with additional sections and fly;

jibs to obtain the various lengths and capaci-ties required.

Figure 11-7 Track-mounted crane

Caption: 1. Hook; 2. Jib; 3. Cab; 4. Boom sus-pension rope; 5. Hoist rope; 6. Pendant rope.

Source: Chudley R., 1999.

Lorry-mounted cranes come in a wide variety of designs and capacities, generally with a 360° rotation or slewing circle, a low pivot, and jib. To improve the mobility of the crane from one site to another lorry-mounted cranes have rubber tires and an additional cab were a driver drives the crane from site to site on public roads.

Characteristics: The crane is operated by a separate crane engine and controls. The capacity of lorry-mounted cranes ranges up to 2000 kN in the freestanding position but this can be increased by using the jack out-riggers. Their height capacity range to 100 m.

Mobile lorry cranes can travel between sites at speeds of up to 48 km/h, which makes them very mobile, but to be fully efficient they need a firm and level surface from which to operate.

Figure 11-8 Lorry mounted crane

Caption: 1. Hook; 2. Pendant ropes; 3. Hoist ropes; 4. Engine; 5. Jib. Source: Chudley R., 1999.

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Self Propelled Cranes (also re-ferred to as a crane truck) is a portable boom crane mounted on an industrial truck. They can be distinguished from other mobile cranes by the fact that the driver has only one cab position for both driving and operating the crane.

Characteristics: They are small capacity machines having a fixed boom or jib length, with small radii and low lifting ca-pacities 10 t.

They are extremely mobile but to be effi-cient they usually require a hard level sur-face from which to work. Road speeds ob-tained are in the region of 30 km/h.

Figure 11-9 Self propelled crane

Caption: 1. Hook; 2. Boom; 3. Cabin; 4. Chas-sis; 5. Outriggers; 6. Boom suspension ropes;

7. Hoist ropes; 8. Pendant ropes. Source:

Chudley R., 1999.

Gantry Cranes (also referred to as portal crane) is a rail-mounted crane consisting of a horizontal transverse beam that carries a combined driver's cab and hook supporting saddle.

This is a particularly safe form of crane as it requires no ballast, gives the driver an ex-cellent all-round view and allows the hook three-way movement of vertical, horizontal and transverse directions. Although limited in application this special form of mobile crane can be very usefully and economically on repetitive and partially prefabricated blocks of medium -rise dwellings.

Characteristics: They are small capacity machines having low lifting capacities 50 kN…320 kN and small lifting height 6…10 m.

Figure 11-10 Typical gantry crane