Lifting aids, attachment points
To move pre-fabricated structures, temporary lifting aids are necessary to absorb the additional stress between the lifting machine and the element. Their types and methods of use can be so specific that I can only provide a brief summary of their application here. Types of structures include rope slings and rigid lifting beams. Their application largely depends on the dimensions of the element to be moved, but they are also used in combination.
Lifting Aids
Lifting aids are temporary devices suitable for gripping, rotating elements, absorbing additional stresses during lifting, bracing elements, and anchoring them.
Lifting aids can be classified based on manufacturing and assembly technology:
Rope Slings:
Rope slings are the simplest tools. These can be made of hemp or steel. Hemp ropes are generally used for lighter elements, which are fastened to the elements by knotting or looping. Steel wire rope slings, due to their rigidity, are usually endless or have ends shaped for gripping. Endless ropes are used for loading simple structures. In such cases, care must be taken to ensure that the rope does not break sharply and does not damage the structure. They are attached to the crane hook by looping or hooking. Among steel wire rope slings, there are also multi-leg rope slings, which are designed with shackles at their ends. These structures, sensitive to stress, have four gripping points, where a balancing operation must be applied to ensure the ropes work together. The advantages of rope slings are their simple structure, quick manufacturability, and light structural weight.
Chain Slings:
These are used for loading operations. Their end designs are similar to those of wire rope slings, but they are more sensitive from a safety engineering perspective, as they are more susceptible to mechanical stresses occurring during assembly, so they can only be used for loading work.
Lifting Beams:
The disadvantage of the previous types is their large structural height. Lifting beams are used to overcome this. These are generally made from hot-rolled sections, and in larger designs, with a truss structure. Lifting beams are often supplemented with auxiliary beams for multi-purpose use. The disadvantage of lifting beams compared to rope slings is that they are structurally complex and have a high dead weight. The latter significantly burdens the lifting machines. Consequently, their size can only be increased to a certain extent, but this also incurs significant assembly costs.
Special Gripping and Lifting Aids:
Special auxiliary structures are also available for moving structures of various shapes. Examples include screw-on chain links, lifting shoes, and clamping jaw solutions. Their advantage is that no gripping point needs to be created on the structure to be lifted. There are two main groups: fork and scissor grippers. In addition to these, a so-called vacuum suction cup lifting beam can also be used for loading operations.
Lifting Beams Used for Lifting Columns:
For larger pillars, the requirement is to set the structure vertically, which can only be achieved with special lifting beams. For lifting, gripping higher than the upper end of the pillar is recommended, as this reduces bending stress when setting it vertically. For these, a rope branch usually attached to a short beam is used, with a pivot pin, clamping jaw, or frame placed at its end. A characteristic of these is that during lifting, one end of the pillar remains on the ground, so a different type of lifting beam is used for unloading from a vehicle.
Lifting Beams Used for Beam Lifts:
Lifting beams is simpler than lifting columns, as they remain in a horizontal position throughout manufacturing, transport, and installation. For these, rope slings or universal lifting beams are generally used, but if the beam cannot withstand two-point loading statically, then geared lifting beams combined with suspension ropes are used. For long-span girders, lifting beams are also made in a truss design. Using a rope sling for lifting is not recommended due to the high risk of buckling, but a solid lifting beam is not the best choice either due to its high dead weight. Girders with high webs cannot be gripped in their upper flange due to their thin structural thickness, so for these structures, a box-type grip is typically used.
https://www.youtube.com/watch?v=riDVFgc8VyILifting Beams for Floor and Stair Panels, Slab Structures:
For their lifting, fork grippers (generally for smaller structural elements), rope and lifting beams can be used. Among rope slings, the four-leg sling with a compensating mechanism is common. For longer structures, six or even eight gripping points may be required. With these lifting aids, rope compensation can be achieved with traditional structures or pulleys, making the rope sling suitable even for lifting inclined panels or stair elements.
Lifting Beams Usable for Wall Structures:
For lifting wall structures, lifting and loading slings are generally used. The latter cannot be used during assembly for safety reasons, as they hinder it. For loading blocks, the eccentric scissor gripper has proven effective, which is mounted with a pivot pin or a fork gripper. Wall panels are gripped in their upper part with hooked, carabiner, half-eye, and pivot pin designs, to which rope, beam, or universal slings are also attached for lifting.
https://www.youtube.com/watch?v=Q4BfhqDi-jM&embeds_referring_euri=https%3A%2F%2Farchphases.hu%2FAuxiliary Equipment for Lifting Other Structures:
These generally serve to provide temporary stability to the structure or assist in its placement by absorbing additional stress.
Gripping of Structural Elements
The movement required during the loading of structures can be done manually for smaller elements, but in most cases, we also use lifting machines. In such cases, an auxiliary device is attached between the lifting machine and the structural element. Gripping points must then be created on the elements. These must meet structural and technological requirements.
From a static point of view, a general requirement is that the structure should not suffer damage. For anti-tipping, gripping above the center of gravity is the most effective; otherwise, the elements must be secured against tipping. The risk of overturning is characteristic of lower, sturdier beams. This is not characteristic of taller beams gripped in the upper flange. Taller beams are sensitive to buckling. Flat-like elements are very sensitive to overturning due to their low structural height, so in such cases, multi-point gripping is recommended, and for irregular elements, three-point gripping. The most frequently used structural elements are right triangles, in which case four-point gripping is the best solution. The disadvantage of this is that initially only three gripping points work. The fourth only works after the structure has deformed.
From a technological point of view, a general requirement is that the gripping point ensures easy, quick, and safe attachment and detachment of the gripping structure. During manufacturing, the design of a simple structure with low material and labor requirements should be considered. It should not require skilled labor, and template manufacturing should allow for a simple design. The movement and storage of the element should not hinder or be sensitive to assembly and post-assembly work.
Gripping Points
The design of gripping points can be, depending on manufacturing and assembly technology: protruding lug, recessed lug, recessed lug with plastic insert, recessed threaded rod, recessed screw, DEHA-1, DEHA-2, passing tube, structural steel gripping, threaded concrete, lifting hole.
Summary:
The design of lifting aids and the gripping points of the element always depend on the mass of the given element, the crane's load capacity, and the shape of the structure to be lifted. However, if we diligently search the manufacturers' websites, we are sure to find the right one for us.
Sources:
BME Department of Building Construction and Management, Notes on Organization of Structure Assembly
YBL lecture notes