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This information sets out some of the basic considerations taken into account by the lifting design engineer.
Statements like those in AS3850, "Because of the mode in which failure can occur, it may be necessary to test complete systems and not calculate values obtained from a group of components that make up the system. The mode of failure of an individual component does not necessarily reflect the mode of failure of the system." But the standard does not continue to further the understanding required regarding test methods, the components that should be deemed as part of the system, the various modes of failure, and the interpretation of test results for each failure mode. And further in AS3850, "The strength limit state capacity shall be determined by a statistical analysis from the test results in accordance with Paragraph A4.5." and assuming the test data is taken from a statistically valid test method, the data is to be determined via statistical means to derive the Load resistance model, for the anchor. There are adequate load case coefficients available to estimate sling angle load amplification, suction from the casting bed, and various dynamic transportation load estimates. Load resistance factors of safety, FOS, set out in the Australian Code would typically denote a FOS of 5.0 for re-usable lifting equipment and an FOS of 2.5 for a lifting anchors.
The rigging arrangements can influence the applied anchor load, where statically indeterminate systems are not necessarily a design consideration, but can be used in practice. The determination of the loads through the rigging system must be a consideration whilst calculating the load resistant model, refer to the examples shown in Figure 3.
Even though years of experience accounts for a good gauge for the appropriate lifting anchor to be used, it should not be left to the reinforcement fabricators and precast factory personnel to select the lifting anchor. The design engineer should specifically account for the applied loads expected during the lifting, transport and placement (or re-usability requirements) of the element. Flexure, casting bed suction, load direction (axial 'tensile', angular 'sling', transverse 'shear') are also load considerations to be accounted for in the lifting design of the element. The anchor selection, together with additional reinforcement, and rigging arrangements is influenced by: - The dead weight of the element - The number of anchors in the element and the configuration of the anchor - Capacity of the anchor at the specific concrete compressive strengths at time of lift - The dynamic loads applied during lifting (suction to the casting bed, or crane dynamics) - The rigging configuration All of the above factors must be taken into consideration during the lifting design phase of the element. The weight of the element can be determined by the calculated volume, and using the specific gravity (normal weight reinforced concrete is approximately 24 kN/m3). Establishing the lifting anchor positions will influence the rigging arrangements used and therefore the static analysis of the rigging should be determined. Particular rigging configurations may be more suitable for particular job sites or lifting in place considerations, and the lifting design should denote the assumptions accordingly. For example, the statically determined systems, shown in Figure 3, where the determination of the loads is not always possible.
Dynamic loads considered in lifting design are accounted for in two stages; suction to the casting bed on the initial lift and then the dynamic loads induced from crane vibration. These crane impact loads must be accounted for during transportation in the yard and on-site, and the coefficient increases from an overhead gantry crane through to a crane moving over rough terrain. Consideration for the entire transportation loads must be taken into account during the lifting design. Anchor capacity, or load resistance, should be considered for tensile loads (axial), sling angle (angular) and shear loads (transverse). Consideration of different load combinations may result in wide variations required from the lifting insert. The load directions during production, transport and placement should be considered carefully. Depending on the planned load direction, either a different anchor may be included in the lifting design, alternatively, reinforcement may be included to reduce the possibility of element flexure crack damage. The configuration (size, position and quantity) of this reinforcement should be supplemented to the element reinforcement design to ensure for adequate capacity of the lifting design. Lifting design is influenced by the steel / concrete interaction of the specific anchor selected. Different load cases are considered by the lifting design engineer, such as anchor susceptibility to edge distance, placement sensitivity, and anchor capacity at the specific concrete strength at time of lift. For example, a footed pin head style anchor maybe more susceptible to edge distance than a hairpin style anchor. Or a splayed anchor does not have the same tensile/axial capacity with the equivalent anchor length (effective embedment is greater on a footed anchor than a splayed anchor of equivalent overall length, see figure 4).
Practical application must consider that the Load Resistance ≥ Applied Load
Applied load To determine the required anchor, the manufacturing plant handling and the site handling should be considered separately. Example: A thin walled rectangular section, 6.0 m long, 3.0 m wide and 150 mm thick is being considered to be edge lifted from a horizontal steel bed using an overhead gantry crane, and then lifted on-site using a tower crane. No panel rotation is being considered.
When selecting an anchor, consider the element formwork and the ease of placement and securing of the anchor prior and during the pouring of the concrete. For example, some of the anchors shown in figures 4-6, can be placed into thin wall elements as the anchor chair maintains the position relative to the element thickness. As the orientation of the void determines the lift position of the lifting clutch, the wire chair can be secured against the element reinforcement to maintain this orientation during the concrete pour and set. When an anchors load resistance must consider load reduction factors, this would imply that the particular selected anchor will form a different failure crack zone. For example, the anchors depicted in figure 5, a footed anchor has the tendency to overload the concrete cover in thin wall panels, hence is more susceptible to side blow-out failure than a hairpin style anchor, depicted in figure 8.
Lifting design if done correctly will consider many aspects which should be considered through the transportation load cycle of the concrete element. The considerations should cover the lifting system model and load resistance model. Using suitably qualified and experienced engineers is certainly recommended as the consequences of getting the lifting design incorrect can be fatal. Efficiencies can be gained from getting the lifting design correct, by optimizing the number of anchors, correct reinforcement detail of the element, the correct selection of the anchor type and the minimizing the complexities of the rigging configurations. [1]
Reinforced concrete, also called ferroconcrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having higher tensile strength or ductility. The reinforcement is usually, though not necessarily, steel bars (rebar) and is usually embedded passively in the concrete before the concrete sets. However, post-tensioning is also employed as a technique to reinforce the concrete. In terms of volume used annually, it is one of the most common engineering materials. In corrosion engineering terms, when designed correctly, the alkalinity of the concrete protects the steel rebar from corrosion.
A block and tackle or only tackle is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift heavy loads.
A crane is a machine used to move materials both vertically and horizontally, utilizing a system of a boom, hoist, wire ropes or chains, and sheaves for lifting and relocating heavy objects within the swing of its boom. The device uses one or more simple machines, such as the lever and pulley, to create mechanical advantage to do its work. Cranes are commonly employed in transportation for the loading and unloading of freight, in construction for the movement of materials, and in manufacturing for the assembling of heavy equipment.
A lifting bag is an item of diving equipment consisting of a robust and air-tight bag with straps, which is used to lift heavy objects underwater by means of the bag's buoyancy. The heavy object can either be moved horizontally underwater by the diver or sent unaccompanied to the surface.
In structural engineering, buckling is the sudden change in shape (deformation) of a structural component under load, such as the bowing of a column under compression or the wrinkling of a plate under shear. If a structure is subjected to a gradually increasing load, when the load reaches a critical level, a member may suddenly change shape and the structure and component is said to have buckled. Euler's critical load and Johnson's parabolic formula are used to determine the buckling stress of a column.
Prestressed concrete is a form of concrete used in construction. It is substantially "prestressed" (compressed) during production, in a manner that strengthens it against tensile forces which will exist when in service.
A shear wall is an element of a structurally engineered system that is designed to resist in-plane lateral forces, typically wind and seismic loads.
In geotechnical engineering, a caisson is a watertight retaining structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam, or for the repair of ships.
A concrete slab is a common structural element of modern buildings, consisting of a flat, horizontal surface made of cast concrete. Steel-reinforced slabs, typically between 100 and 500 mm thick, are most often used to construct floors and ceilings, while thinner mud slabs may be used for exterior paving.
Formwork is molds into which concrete or similar materials are either precast or cast-in-place. In the context of concrete construction, the falsework supports the shuttering molds. In specialty applications formwork may be permanently incorporated into the final structure, adding insulation or helping reinforce the finished structure.
In rock climbing, an anchor can be any device or method for attaching a climber, rope, or load to a climbing surface—typically rock, ice, steep dirt, or a building—either permanently or temporarily. The intention of an anchor is case-specific but is usually for fall protection, primarily fall arrest and fall restraint. Climbing anchors are also used for hoisting, holding static loads, or redirecting a rope.
Anchor bolts are used to connect structural and non-structural elements to concrete. The connection can be made by a variety of different components: anchor bolts, steel plates, or stiffeners. Anchor bolts transfer different types of load: tension forces and shear forces.
A deep foundation is a type of foundation that transfers building loads to the earth farther down from the surface than a shallow foundation does to a subsurface layer or a range of depths. A pile or piling is a vertical structural element of a deep foundation, driven or drilled deep into the ground at the building site.
A tieback is a structural element installed in soil or rock to transfer applied tensile load into the ground. Typically in the form of a horizontal wire or rod, or a helical anchor, a tieback is commonly used along with other retaining systems to provide additional stability to cantilevered retaining walls. With one end of the tieback secured to the wall, the other end is anchored to a stable structure, such as a concrete deadman which has been driven into the ground or anchored into earth with sufficient resistance. The tieback-deadman structure resists forces that would otherwise cause the wall to lean, as for example, when a seawall is pushed seaward by water trapped on the landward side after a heavy rain.
Cellular confinement systems (CCS)—also known as geocells—are widely used in construction for erosion control, soil stabilization on flat ground and steep slopes, channel protection, and structural reinforcement for load support and earth retention. Typical cellular confinement systems are geosynthetics made with ultrasonically welded high-density polyethylene (HDPE) strips or novel polymeric alloy (NPA)—and expanded on-site to form a honeycomb-like structure—and filled with sand, soil, rock, gravel or concrete.
Structural engineering depends upon a detailed knowledge of loads, physics and materials to understand and predict how structures support and resist self-weight and imposed loads. To apply the knowledge successfully structural engineers will need a detailed knowledge of mathematics and of relevant empirical and theoretical design codes. They will also need to know about the corrosion resistance of the materials and structures, especially when those structures are exposed to the external environment.
KOLOS is a wave-dissipating concrete block intended to protect coastal structures like seawalls and breakwaters from the ocean waves. These blocks were developed in India by Navayuga Engineering Company and were first adopted for the breakwaters of the Krishnapatnam Port along the East coast of India.
Rigging is both a noun, the equipment, and verb, the action of designing and installing the equipment, in the preparation to move objects. A team of riggers design and install the lifting or rolling equipment needed to raise, roll, slide or lift objects such as heavy machinery, structural components, building materials, or large-scale fixtures with a crane, hoist or block and tackle.
This glossary of structural engineering terms pertains specifically to structural engineering and its sub-disciplines. Please see glossary of engineering for a broad overview of the major concepts of engineering.
Offshore embedded anchors are anchors intended for offshore use that derive their holding capacity from the frictional, or bearing, resistance of the surrounding soil, as opposed to gravity anchors, which derive their holding capacity largely from their weight. As offshore developments move into deeper waters, gravity-based structures become less economical due to the large size needed and the consequent cost of transportation.
