- Demolition
- Foundations
- Structural frame
- Floors
- Roof
- Schedule of finishes
- Description and position of the type of crane
- Detail drawings
Brief
Foundations
There are some factors influence the type of foundation and we should consider which type of foundation to be used in our building. The new central courtyard building is suitable for applying pad foundation. We decide to construct 9 pad foundations and the size of each is 1200 mm x 1200 mm x 700 mm. We as a group decided we should analysis some factors that can be affected to the choice of foundation before the construction. The factors are as following.
Ground conditions
According to the previous soil investigation report, we assume that the soil type and site condition of the existing central courtyard in MA block is likely to ordinary earth type.
Cost of constructions
It is economical to construct a pad foundation of columns of concrete supporting reinforced concrete ground beams, rather than excavating deep trenches and raising walls off strip foundations. The spacing of the columns is determined by the most economical construction.
Type of structure
Steel structural (skeleton) frame will be used in the new central courtyard building, MA block. The advantages of this structure are in the followings
- It can provide very long span
- Easy for erection
- It can provide a better integration of services
The equipment used for the construction of pad foundations are listed below:
Construction methods
Excavate pad foundation
Excavate the soil for pad foundation and ground beams by using excavator. In order to provide the working space for the labours, extra 600 mm of each side should be excavated. Moreover, a temporary spoil heaps for the excavated soil should be allowed to store near the excavated site, generally, the distance is not more than 10 m.
Support to excavation
Pad foundation will need additional temporary works, in addition to any for the support for the excavation while pouring concrete. It is necessary to using the timbering and strutting to prevent the soil collapse during the construction work for the foundation.
Placing blinding
In order to protect concrete from the soil during construction and provide a firm working base, placing approximate 100mm thick blinding layer of weak concrete over the bottom of the excavation.
Fix reinforcement and formwork
Placed and fixed 4 sides formwork to mould each pad foundation and 4 sides formwork to mould each column.
Fix reinforcement before pouring concrete. Provide at least 50mm thick concrete for concrete cover to protect reinforcement exposed.
Poured concrete to base
Prepared mixed concrete by using concrete mixer for pad foundation. Poured concrete to the mould formed by the formworks in the pad foundation. During pouring concrete using concrete vibrator to vibrate on the concrete for the pad foundation in order to prevent honey combs formed in the bottom of the pad. Furthermore, sprinkle water on the concrete surfaces to cool down concrete due to chemical reactions during concrete become hardens.
Remove all formwork and support material
After concrete harden, remove all formwork of pad foundation and support material of formwork and soil.
Backfill, compact and dispose of surplus
Backfill suitable soil or materials to the excavated pad, after remove all formworks and support materials. Then ram on earth around the back filling area by using rammer. Furthermore, the excess excavated soil should be disposed out of site.
Structural frame
The structural steel (skeleton) frame will be used in the new central courtyard building. The main elements for this frame consist of universal columns, universal beams, rib beams supporting for floor. Furthermore, there are angle cleat, cap plate, splice plate for connection two main elements by mainly using bolt and nuts or welding. Otherwise, in order to provide large internal space which are unobstructed by columns. The advantages of using structural steel (skeleton) frame
There are some factors that can be affected to the choice of structural frame before the construction. And the factors are shown in the following:
The availability of materials
Cost
Speed of erection
Size and nature of site
Fire resistance required
There are some advantages of using structural steel (skeleton) frame:
The erection of the steel is carried out by skilled labour, quickly and accurately within tolerances. Complete the structure at early stage, it permits the laying floors and roofs and the rapid sealing-in of the whole structure to permit other trades to follow on continuously and no interruption through adverse weather.
The plant and equipment used for the construction of in-situ reinforced concrete frame are as below
Construction method:
Ground columns and beams
After pad foundation is completed, the ground universal column can be erected. The universal column is hoisted into position over the concrete base and in the correct position and steel wedges are driven in between the plate and the concrete. The universal column weld to a steel base plate which fix on the pad with 4 holding down bolts. The gaps between base plate and pad foundation will be grouted by cement mortar. Universal beam will connect to universal column. The beam ends are bolted to the projecting flanges of the shelf angles. Angle side cleats are bolted to the flange of columns and webs to main beams and angle top cleats to the web of columns and flange of secondary beams . Generally, cantilever ground beam will connect existing wall by welding.
Upper floor and columns and beams:
The main beams will connect to the universal column same as connection in ground columns and beams. Since precast concrete T- beams will act as floor structure in this building. The connection of main beam and precast concrete T- beams will fully describe in the next section. Cantilever beams will connect existing wall by welding. Angles will be welded bottom of the beam and channels to provide a base landing for the precast concrete slabs.
There are main disadvantage of using steel structure. The steel member will begin to deform, twist and sag and no longer support its load, if there are fires and the temperature reached 550ºC. In order to reduce the amount of heat getting to the steel, a casing to structural steel members is needed. Precast ‘L’ and ‘U’ shape vermiculite gypsum casing will be fixed around column and beam section. The vermiculite and gypsum boards are screwed to steel straps fixed around the steel sections and the sheet metal faced casing by interlocking joints and screws or by screwing.
Floor
In the floor construction, precast concrete T beams will be used in the new central courtyard building. It can provide a long span floor, and also its depth is very acceptable. Using this floor, it is because it made in factory, then we can save much time and reduce the labour and formwork during the erection. It also can accommodate openings for services.
The plant and equipment used for the construction of floor are as below
The construction sequences for the construction of floors are as listed below
Ground floor
The ground floor slab will be used in-situ reinforced concrete slab and place the hardcore of 150 mm thick to the whole ground floor area to provide a horizontal surface at the appropriate level for the concrete slab and to form a dry working surface. Also, a thin layer of 25 mm thick sand blinding will be placed on the top of hardcore to achieve damp proof membrane. After that, reinforcement mesh will be covered to receive 100mm thick of concrete grade C25/20.
Upper floor
Precast concrete T beam will be applied on the upper floor and the size of the precast unit is 2400mm x 2400mm x 300mm thick. The tower crane will lift the precast double T beams and to be sitting on the beams and the ends of T beams fixed to steel beam with angle cleats. After all the precast T beams are laid, mesh reinforcement and a layer of 50 mm thick reinforced concrete topping will be cast on top of the floor units. The objective of laying the reinforced concrete is to joint all the floor units together. Moreover, post tensioned cables through holes formed at the bottom of the ribs to stiffen the structural frame in the building. Also, compression reinforcement should be added for resist the compressive stresses.
Roof:
The type of roof structure at the new proposed central courtyard building is a Pitched Trussed roofs. Furthermore, steel beams steel trusses will be used to form the roof. The considerations of choosing Pitched Trussed Roofs as follow:
- Using steel trusses, it will produce a stiffer component for handling in transport and erection and allows for the reversal of stress in members which may occur during these operations or through wind suction on the roof and which could cause buckling of the flat members. (2000 Foster J.S.)
- Steel structures are generally quicker to erect than those in in-situ concrete.
- The pitch is kept low to minimise volume, reduce weight and area to be covered.
The plant and equipment used for the construction of the roof are as below:
There are main tie, hanger, tee section rafter form a main part of the roof structure. Angle tie, angle struts act as a supporting member for main part of the roof structure. The main tie connects to UC stanchion by welding and bolting (see fig.). The tee section rafter connects to main tie by welding: another side connects to hanger and opposite rafter by welding and connecting plates in the middle and form a roof ridge. Besides, angle struts and angle ties connect to tee section rafter or each other by welding (see fig.). Angle purlin cleats will be welded to the tee section rafter to receive steel angle purlins.
Basically, there are two different levels for the existing roof, which are one at higher level and the other one at lower level. At the higher-level existing roof, the gutter will be remained. At the higher level of the new roof will be constructed slightly lower than the existing roof. New gutter will be placed underneath the existing gutter.
Rainwater down pipe will placed vertically at the edge of higher-level existing roof to allow the rainwater to flow out through the new gutter. For the lower level existing roof, it is the same as higher level. After complete steel roof structures, the roofing sheet will be laid on top of the purlins. Sheet metal covering should be used for this building. The strips of metal are jointed down the slope of the roof by means of a standing seam joint. The metal strips have to be secured to the roof surface at close spacing of the standing seams. Close spacing of the fixing cleats is necessary. Fixed cleats are fixed in the centre of the length of each strip and nailed to roof boarding through the felt underlay. One part of the expansion cleats are nailed to the roof, another part of the expansion cleats are folded in at the standing beam and moved inside the fixed piece.
Tower Crane
Since 1950 the crane has played a major part in the construction industry, as a standard piece of plant required for construction of medium to high-rise buildings. Cranes are available in many different forms with a horizontal jib carrying a saddle or a trolley, or alternatively with a luffing or derricking jib with a lift hook, as it’s extreme end. Horizontal jibs can bring the load closer to the tower whereas luffing jibs can be raised to clear obstruction, such as adjacent buildings, and advantage on confined sites. These basic types of tower cranes are available; self-supporting static tower cranes, supported static tower cranes, travelling tower cranes and climbing tower cranes.
Self-supporting static tower cranes
These cranes generally have a greater lifting capacity than other types of cranes. The mast of these self supporting tower cranes must be firmly anchored at the ground level to a concrete base withholding down bolts or alternatively to a special mast base section cast into a foundation. These are particularly suitable for confined sites and should be positioned in front or to one side of the proposed building with a jib or sufficient length go give overall coverage of the new structure. Generally these cranes have a static tower but types with a rotating or slewing tower and luffing jib are also available.
Supported static tower crane
These are similar in construction to self supporting tower cranes but are used for lifting to a height in excess of that possible with self supporting or travelling tower cranes. The tower or mast is fixed or tied to the structure using single or double steel stays to provide the required stability. This tying back will induce stresses in the supporting structure, which must therefore be of adequate strength. Supported tower cranes usually have horizontal jobs since the rotation of a luffing jib mast renders its as unsuitable for this application.
Travelling tower cranes
To obtain better site coverage with a tower crane a rail mounted or travelling crane could be used. The cranes travels on heavy wheeled bogies mounted on a wide gauge rail track with gradients not exceeding one in two hundred and curves not less than 11.000 radius depending on mast height. It is essential that the base for the railway track sleepers is accurately prepared, well drained, regularly inspected and maintained if the stability of the crane is to be insured. The motive power is electricity, the supply of which should be attached to a spring-loaded drum which will draw in the cable as the crane reverses to reduce the risk of the cable becoming cut or trapped by the wheeled bogies. Travelling cranes can be supplied with similar lifting capacities and jib arrangements as given for static cranes.
Climbing cranes
Design for tall buildings being located within and supported by the structure under construction. The mast, which extends down through several storeys requires only a small opening in each floor. Support is given at floor levels by special steels collars, frames and wedges. The raising of the static mast is carried out using a winch, which is an integral part of the system. Generally this form of crane requires a smaller horizontal of luffing jib to cover the construction area than a static or similar tower crane. The jib is made from small, easy to handle sections which are lower down the face of the building, when the crane is no longer required, by means of a special winch attached to one section of the crane. The winch is finally lowered to ground level by hand when the crane has been dismantled.
References
Advanced Construction Technology, 3rd edition, R. Chudley, 1997
Foundation Design & Construction, 6th Edition, M.J. Tomlinson, 1995
Project Management in Construction, 2nd Edition, A.Walker, 1989
Site Guide to Foundation Construction, W.S. Martin, 1996
CN2248-Construction Technology III