Cranes are divided into two main groups- mobile and stationary.
Mobile cranes.
They can be moved easily around and/or between sites. They are three main types-wheeled cranes, crawler cranes and truck mounted cranes.
Wheeled cranes are mobile cranes mounted on wheeled chasses and have only one operator position from which the crane is controlled and vehicle is driven. Their boom is made up from standard sections and can be used with or without fly jab. These cranes can operate on hard ground with or without using the outriggers. For small loads they can operate without using the stabilizers and lifting up to 10 tones transferring the load into the ground through their big rubber tyre wheels. For larger loads outriggers are used and maximum lift is 30 tones. See (ceca14-7)
Crawler cranes can be universal power unit rigged as a crane or purpose designed track mounted crane with or without fly jib attachment. Their lifting capacities is up to 45tonnes.track mounted cranes can travel and carry out lifting operations on most sites without the need of special roads and hard stand provisions. See (chudley3 builder’s plant)
Compared to the wheeled cranes crawler cranes have the following advantages-
Can operate on poorer ground-lower bearing pressure on the tracks, operating costs are usually less.
The disadvantages are that they need a low loader for transportation between the sites and also they travel slower on the site.
Truck mounted cranes consist of strut or telescopic boom mounted on a specially adapted truck or lorry. They have two operating positions. The lorry is being driving from conventional front cab and the crane being controlling from a different location. The lifting capacity of these cranes can be increased by using outrigger stabilising jacks and the approach distance to the face of building decreased by using a fly jib. Lorry mounted telescopic cranes required a firm surface from which to operate and because of their short site preparation time they are ideally suited for short time period. Cranes with telescopic boom are available from 10 to 200tonnes lifting capacity. See (chudley3builder’s plant).
Stationary crane
These cranes can be defined as type of crane having a base which is fixed in position or has a limited amount of a horizontal movement. These are tower cranes. They are usually associated with high rise constructions. However, they are also useful for low rise work which is concentrated over a limited area, especially where mobile crane access is restricted.
The main advantage of the tower crane is that the boom, which is located on the top of the tower, is set at a height so that it is clear of all obstructions. It allows it to be placed very near to, or even within, the building under construction. For high rise buildings the tower cranes are usually the cheapest options.
They are static and rail mounted tower cranes.
Static tower crane is supported on a well prepared foundation. The mast is bolted to a steel cruciform base. Ballast usually consisting of concrete is placed on the base to provide a counter balance.
Another type is a climbing tower crane. This type is fixed within the structure to a climbing frame. As the height of the building increases, the tower crane is winched up floor by floor.
Rail mounted tower crane is on wheeled bogies which run on a truck placed on well compacted aggregate ballast. The crane can move along while lifting the load. The advantage of this type over the static one is that the crane can be used for construct both high and long buildings. It also can travel to the material storage area
Disadvantage is that the site must be level and have good ground.
2 Describe and illustrate the construction options of the structural superstructure of the multi storey car park
Framed structures can be built from structural steel, reinforced concrete or a combination of both.
Framed structures are used where there is need for large open areas uninterrupted by walls.
Structural steelwork is widely used for the construction of industrial plants, warehouses and for a range of multi storey buildings.
The steel framework can have floor slabs added which are made from either precast or in situ concrete.
Erection techniques vary but are basically divided into two main methods:
Erection of individual components, i. e. beams, columns, bracings etc and fabrication of sub assemblies at ground level with subsequent erection of the subassemblies.
It should be noted that the latter will result in lifting heavier units which consequently will require larger cranes.
Form work may be required if the steelwork has to be encased in concrete for fire proofing. See (CECA 18-8)
Another structural frame for construction of commercial and industrial multi storey buildings is commonly used in situ concrete.
Structural frames built from in situ concrete include columns, beams, walls, floor slabs. In some cases beams are eliminated and flat slab construction is used. This facilitates the use of simpler formwork and removes the need for beam reinforcement. Thus, the use of flat slabs increases the productivity.
In situ concrete construction requires the use of formwork and falsework.
Precast concrete can also be used in the construction of structural frames.
It is used for external structural elements which are part of the cladding. There two basic types of precast concrete structures;
Columns and beam elements erected in site-infill panels added latter and wall units which act as both structural elements and building envelope.
In both types there will also be precast flooring and staircases.
The joints between precast elements must be design it and constructed very carefully to ensure structural continuity
Precast concrete elements are heavy and, therefore, large cranes are required for the erections of the frame.
Although formwork is generally only required around joints where small quantities of in situ concrete are placed, temporary support may be required while lining up and levelling elements. These must stay in place until the jointing processes is complete. See (SECA18-7).
In this project I will choose steel frame work and precast concrete elements, based on information I have obtained from
I have concluded this from the following:-
Speed and predictability – Offsite prefabrication increases the speed, quality and safety of construction. The predictability of cost and programme are also improved. Time related savings when specifying a structural steelwork frame compared with reinforced concrete can be 2 – 3% of overall building costs.
Flexibility and adaptability – Long spanning structural steelwork systems create column free areas that offer greater flexibility of floor layout and enhance the lettability of the space. Light steel partition walls can be easily relocated, leading to adaptable buildings with the ability to meet future needs.
Sustainability – Steel is 100% recyclable without any loss of quality.Currently in the UK, when buildings using structural steelwork frames come to the end of their useful lives, 86% of the steel sections are recycled to create more steel products and 13% are reused in their existing form. The recycling rates for reinforcement bar are negligible by comparison.
Continuous development – The steel industry in the UK has a long history of development activity. Efficiency improvements in manufacturing and fabrication have been augmented by improved structural systems and knowledge development and dissemination in areas such as fire engineering, durability, vibration control and acoustic performance.
Economy is a great consideration on my choice of using precast concrete elements, as it reduces the requirements for formwork and access scaffolding; this saves cost through reduced resources and by shortening the programme. There is less reliance on wet trades, which can be delayed by unfavourable weather conditions.
There are also benefits in using precast elements for specific areas of the building such as stairs, where safe access is immediately available once installed.
Precast elements are designed by specialists with experience in ensuring that the structure can be erected quickly and efficiently. Off-site production provides a high quality product for the following reason - Precast elements are cast to close tolerances, and checked in the factory before delivery to site
Quality control systems, a consistent well trained workforce, and widespread use of self-compacting concrete ensure a high standard of workmanship High quality finishes are generally achieved through the use of robust, purpose made formwork and dedicated concrete mix designs in a factory environment
3 Describe the provision of temporary works to the reinforced concrete superstructure to the multi storey car park and the production of concrete for the scheme
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Concrete is a mixture of cement, fine aggregate-(sands, crushed rock, and pulverized fuel ash), coarse aggregate-(crushed rock, gravel), water and admixtures (retargets).
The four important properties of concrete are:
Workability in the fluid state
Durability
Strength
Ability to bond to and protect the steel reinforcement
To achieve these components are mixed together in various proportions to produce a material that has the required properties both:
1 in the fluid state (for transportation and placement)
2 in the solid state (to withstand the weather and applied loadings)
It will only be achieved if the concrete is correctly
Batched/mixed
Transported
Placed/compacted
Cured
Batching and mixing
The hopper is filled with the specified weight of which material, fine aggregate, coarse aggregate and cement in this particular order:
1coarse aggregate
2 cement
3 fine aggregate
These materials are mixed dry and then water is added.
A measured quantity of water is added and the concrete constituents thoroughly mixed again.
Transporting the concrete
Once the concrete mixture is ready, it is transported to the building site by a mixer truck.
Placing the concrete
Three main methods are used to place the concrete onto the formwork.
The mixer truck deposits it directly onto it (usually small foundation/slabs)
Pump (for large foundations, multistory frames/slabs)
Crane and skip (for multi storey)
Formwork and falsework
Formwork is a mould into which wet concrete is placed and compacted. It forms the concrete into the desired shape and size and controls its alignment and position.
Formwork includes the actual facing material that is in contact with the concrete, known as the form face or sheathing, as well as all the necessary framing and supporting structure.
Falsework is any temporary structure used to support the permanent structure during its erection until its self supporting. Falsework can therefore, by definition, be part of the formwork.
Materials used for formworks include timbers, metals, glass fibre, reinforced plastics, .and rubber.
Most formwork systems are made from variety of materials a common combination is steel framing with plywood sheathing used with aluminum or steel walers and soldiers.
The formwork and reinforcement onto which the concrete is to be placed should be well prepared.
The face of the formwork should have release agent applied to facilitate the removal of formwork after the concrete has hardened.
Any dirt and wood shavings etc must be removed from the form work.
Compaction
Compaction of the concrete reduces the amount of air voids .which has the following effects:
A reduction in the permeability of the concrete.
An improvement in the bonding to the reinforcement.
An increase in the compressive strength.
The compaction can be achieved in a different ways:
By hand-small jobs only
External vibration-fastened to formwork (for thin walls and narrow columns)
Internal vibration-pokers placed into the concrete (concrete greater than 150mm thick)
Surface vibration-vibrating tamps (thin slabs)
Curing
Concrete curing means controlling the rate and extent of moisture loss from concrete during the first twelve hours so that the concrete can achieve its potential strength and durability.
Once it is placed and compacted, the concrete must cured before it is finished to make sure that it doesn't dry too quickly. Concrete's strength is influenced by its moisture level during the hardening process: as the cement solidifies, the concrete shrinks. If site constraints prevent the concrete from contracting, tensile stresses will develop, weakening the concrete. To minimize this problem, concrete must be kept damp during the several days it requires to set and harden.
The most common methods to cure fresh concrete is through:
Covering the concrete with an impermeable membrane
Applying a suitable curing agent
Water curing
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