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Suitable selection of materials and labour
Insitu reinforcement concrete and structural steel are popular and dominates in the framing market of multi storey buildings. Through the above evaluation we found that the insitu reinforcement concrete frame's materials and labour is pretty much stable then steel and precast concrete frame. Therefore, we recommend using insitu reinforcement concrete frame approach in this development.
Cost is one of the crucial factors need to be considered in the selection of structural framing options and this costs of framed structures can be affected by the market condition. Through our experience and market analysis, we found that the insitu reinforcement concrete frame is much cost efficiency compared to the others type of structural frame. This is due to steel is particularly volatile and always influence by the exchange rates due to international competitions. Adversely, cement price is more stable and according to 'BCA (2007)', the statistic claims that the UK average cement price is stable over the last 10 years and it is raised below the inflation rate. Cement is one of the main components in the concrete mixture and thus cause the concrete price generally will be cheaper. Therefore it is wisely no to propose steel frame as its price fluctuate more frequently in comparing with concrete.
Speed of construction
In principle, the insitu reinforcement concrete frame has a disadvantage in term of construction speed it is relatively slow due to the time consumed for curing. However, lead time for steel frame actually is higher than insitu reinforcement concrete frame because one of the reasons is due to the steel frame need to pre-fabricate in factory and it is requires a number of fabrication processes.
Lead time can be defined as the actual time counted from putting in an order (by the builder) until to the actual construction one site of a particular element. According to the "lead time figure published in the Chartered Quantity Surveyor (1992), show that insitu reinforcement concrete frame's lead times is 2-8 weeks and it is less than the steel frame's lead times; with 8-14 weeks". Hence, the insitu reinforcement frame construction's "total construction time" would be shorter if compared to steel frame. Although steel frame construction has an advantage in term of faster structural erection time but in our opinion the shorter 'total construction time' produced by insitu reinforcement frame construction is more crucial. Therefore, the insitu reinforcement concrete frame construction is more suitable and is recommended in this project.
Ability to standardise
The insitu reinforcement concrete frame is more flexible and tolerant in any alteration during the construction process. Any subsequence alteration is straightforward and it would not much affect the following construction sequence, process, cost and importantly greater delay would not happen. But, both the steel and precast concrete frame has disadvantages in the ability to standardise. This is due to both are factory prefabricated products and later only deliver to site for installation. Therefore any subsequence alteration in steel frame or precast concrete frame construction either in design or construction sequence will cause an impact in the factory production line as well as to the subsequent following construction planning process.
The insitu reinforcement concrete frame has inherent fire resistance advantage compared to steel frame which fire resistance factor is not inbuilt. Therefore steel need additional fire protection work and this directly will involve additional construction time and cost.
Size and nature of site
As mentioned above both of the steel and precast concrete frame are prefabricated in the factory. Therefore, it has a disadvantage and limitation in producing huge structural frame during the prefabrication process and installation process at site. Similarly the delivery process of prefabricated huge frame will also tough and massive and not practical. But, this does not happen to the insitu reinforcement concrete frame where the huge structural elements can be adjusted and produced on site by constructing it in small part each time without affect by the factor of size and nature of site.
Finally we would like to propose that this 10 storey building to be constructed by using insitu reinforcement concrete frame construction due to the above mentioned advantages. In addition, this insitu reinforcement concrete frame is more useful in accomplishing 'green process 'compared to steel frame. Adversely, we also do not negligence on the disadvantages points of the insitu reinforcement concrete frame such as quality control, massive construction process and etc. However, this insitu reinforcement concrete's existent disadvantages factors can be reduced to an acceptable level by adopting a proper site management system and well planned construction process.
- Construction Technology 5, Heriot-Watt University
- Comparison Of Reinforced In-Situ Concrete And Structural Steel In Multi-Storey Building Framework Construction, RIAD QUADERY (ICE Membership Number: 64405090)
- BCA (2007
- Chartered Quantity Surveyor (1992)
Unit 1 (c)
In order to increase the building height to 30 storeys, the previous proposed structure need to consider and cope efficiently to the gravity loads and resists significant lateral loads or "sway" force cause by wind, while at the same time, not presenting excessive self weight loads on the foundation system. Therefore, some alteration in terms of frame will need to take into consideration for achieving the tall building design safety factor.
According to "Chew, Y L M., Construction Technology for Tall Buildings (2nd Edition), the amount of materials needed in a tall building to resist gravity loads is almost linear with its height, however the amount of material needed to withstand lateral forces increases with the square of the wind speed." The Figure 2.1 below is an illustration of the lateral forces imposed by wind increase exponentially with the building height.
A) Introduction of "Shear Truss - Shear wall Structure"
Shear walls normally is reflected to the vertical elements in the lateral force resisting system (LFRS). This shear wall is very famous apply in many structures. For conventional concrete frame system, shear wall is designed to function as a deep, thin vertical cantilevered beams members where it is robustly connected from the roof level onto the foundations level. While at the same time the insitu reinforcement concrete floor are designed in robust connection into the shear wall (vertical element) and performing the function as a horizontal diaphragms to transfer lateral loads to the vertical element and subsequently into the foundation. Please refer the below illustration of "Diaphragm of Shear wall (Figure B), 'Shear Wall Action (Figure C)', 'Diaphragm Action (Figure D)' and
For this 10 storey office building, it is design with a symmetrical floor layout and this layout makes it suitably to create few shear wall system. Moreover this shear wall design and its implementation will only involve some minor arrangement and structural design alteration, such as;
- alter the type of foundation,
- convert the existing wall element become vertical reinforcement concrete wall,
- change the floor system in order it must be robust enough and able to transfer the lateral force to the shear wall within the limit of design deflection.
- Similarly the design of the beam (underneath at open space area) must be rigidly tied into the supporting shear walls make sure the lateral load can efficiently for transfer to the foundation.
With the above alteration the whole shear wall will be formed (refer layout plan grid line A-B/3-6 & F-G/3-6) and this new created system not only carries vertical load but it floor system acting as a diaphragm to transfer the lateral loads to this shear wall and then transfer those load to the foundation efficiently. With this alternative design the building height will able to achieve 30 storeys and maximum to achieve 35 storeys height. Besides, through this approach the overall building concept will be remained unchanged.
B) Introduction of Frame Using Vertical Trusses, Belt Trusses and /or Outrigger Trusses
This concept is development from the earlier shear wall. The purpose of this design is to provide a vertical truss call 'Outrigger Trusses' where it is robustly connected to the perimeter column/exterior column with the purpose to form a much stiffer structure at external column and enables it to resist greater wind forces or other form of lateral load, as well as the resistant of gravity load. In addition the 'Belt Trusses' will be built and it is functioning in wrapping through the perimeter column at the same level as the outrigger trusses to further stiffen of the structure.
This design is suitable in apply to amend this building height into 10 storey. The Figure E and Figure F below shown two diagrams to illustrate the concept of this vertical trusses, belt trusses and outrigger trusses.
In order to apply this concept in this project, the perimeter building column (facade column) will be tied by using beam act as outrigger trusses and robustly connected to central core (also act as shear wall). Through this alternative frame solution the whole structure will act as a large system will enabling a structure approximately 25% stiffer than a original structure solely relying on a shear truss or shear wall system while without changing the floor plan arrangement. Figure F shown how the outrigger truss combine with shear wall to further enhance this whole building structure to resist lateral forces.
C) Introduction of Tube System and Bundle Tube System
The tube system has been the most efficient structural system used for tall building. This tube approach creates a 3 dimensional system (Refer Figure G). This system can be formed by using the conventional frame system where the external columns around the perimeter of the building are designed in much closer together. Subsequently, these columns around the corners of the four building facades are tied robustly with short beam and will form a continuity system around all four facades and effectively create a structure similar to a huge box section that cantilevers from the foundation to the top of the building. This structural system capable in resisting lateral forces in any direction as in principle a 'Box" section has inherent strength.
This tube system is appropriate to adopt in order to increase this building height to 30 storeys. From this building existing layout design (in term of shape) it is suitably to form a 'Tube shape'. Therefore, according to this tube system design criteria, the perimeter column (facade column) of this building will be placed much closer (Refer Figure H) and tied robustly by beam surrounded the four facades of the building and become more stiffened. This will enable the whole structure act as a whole system to resist the lateral force as well as functioning to transfer the gravity load to the foundation. The advantages of this tubes system is it is allow fewer interior columns, and so create more usable floor space.
In addition the above propose tube system can wisely interconnected or combine act as a whole with the existing tube frames, i.e. two number of lift core and the alternative proposed new shear walls (position at grid line A-B/3-6 & F-G/3-6), to create and perform another approach call 'bundle tube system'. Through this combination a stronger structure will be created and efficiently to resist the lateral forces and gravity loads. The Figure F illustrate the bundle tube system and through this integrate structural frame it is definitely will become more efficiently to resists lateral loads or "sway" force cause by wind.
As a conclusion, with the above solution of alternative frame design (i.e shear walls, vertical trusses, belt trusses and Outrigger Trusses, tube system). This building will be able to increase to 30 storey height with any one of the above single alternative frame design. However it is advisable to combine those approaches by looking at the advantages and permission of this existing symmetrical building layout with the objective to resists significant lateral loads or "sway" force (cause by wind), as well as cope efficiently with the gravity loads (vertical load). Moreover theses combination will not cause many changes in terms of the original design and the building concept.
- Chew, Y L M., Construction Technology for Tall Buildings (2nd Edition),
- Construction Technology 5, Heriot-Watt University
- Brick Industry Association, Technical Notes 24C-The Contemporary Bearing Wall, Introduction To Shear Wall Design, (Sept./Oct.1970)(Reissued May 1988) www.gobrick.com
- Tall Building Structures Analysis And Design, By Bryan Stafford Smith, Alex Coull)
Double skin facades is very popular apply in many European cities. This wall system is attractive due to its characteristic such as durability, ecology, greener technology, aesthetical viewing and etc. This double skin facade able to provide natural ventilation into a building space and also can reduce energy consumption. These double skin facades sometimes also referred to as a 'building in building'.
This wall system can be define as a traditional single facade doubled inside or outside by a second, essentially glazed facade. Each of these two facades is commonly called a skin. Each of the facade skin can be constructed by various different combinations of materials, commonly by two skin of glazed. However, it is also popular to apply an outer layer of glass used together with a solid inner skin. The area between the two skins can call 'ventilated cavity' or 'air space'. It is purposely in such designed for vertical air circulation purposes. The ventilated cavity between the two skins can be in various widths, normally range from as narrow as several centimetres to as wide as several metres (in order to form accessible cavities). The cavity width will influence the way that the facade is maintained. This air circulation space can be used in many different functions, but in the simplest analysis, the air will be drawn into the building by applying the circulation utilises stack effect, so that a natural ventilation effect will be created for the internal space of the building. However, this cavity space in some other design can be consists of fan supported or mechanically ventilated.
A) Natural Ventilation
The design of exterior part of the skin forming a protective shield for the building and through the circulation utilises stack effect the natural air will be drawn into the internal space of the building, so that a natural ventilation effect will be created. Therefore, the interior comfort would not be affected even though the windows maintain open throughout the whole day.
B) Promote Greener Technology
The natural ventilation for high-rise conditions relatively will reduce air-conditioning loads and thus will minimise the CO2 output produced during the building operational phase. According to the research, carried out by 'Franklin Andrews, Professor Michael Wigginton of the University of Plymouth and Battle McCarthy', who represent the United Kingdom Department of Environment, Transport and Regions has shown that double skin buildings when compared to advanced single skin building are capable to reduce 50% of CO2 emissions within the cold temperate climate prevalent in the United Kingdom.
C) Better acoustic insulation-improve noise protection
Besides improved the noise protection, this double skin facade are capable to gain an excellent acoustic insulation magnitude even though under the windows open condition. The magnitude of the acoustic insulation is equal to that obtained in classical glass facade with the windows closed condition.
D) Reducing heating energy requirement
The air stored in the cavity between the two skins would be heated by the sun rays especially in winter time. Thus improving both the heat-insulating functions of the facade and its thermal performance and subsequently will reducing the heating costs.
E) Reducing cooling energy requirement
Double skin facade can allow for nigh-time cooling of the interior building with make the window in open during night time and thereby lessening cooling loads of the building's HVAC system. Thus especially, apply during summer time the night cooling can cut down the building energy consumption particularly the costs of air-conditioning in the summer.
F) Exploiting solar power
Both energy consumption and costs are possible to reduce by utilize the sun's energy particularly with the incorporation of photovoltaic glass. By this method, the air stored inside the cavity will be heat by the solar rays and reduce the energy consumption.
G) Increased Natural Daylighting
The double skin facade will improves the access of natural light transmission into the building space and thus will produce a better indoor comfort and give positive effect to occupants health and as well as increase the productivity of office personnel. Physically with this increased natural daylighting will cause a significant reduce in the amount of electrical lighting required because the quality of light from natural daylight is more preferential to electrical lighting.
H) Fire Escape
For some of the widest cavities (normally width range from 600mm to maximum 2m), located between the two skins will be able to provide a fire escape during fire occur. Therefore, with this back up emergency escape will enable the fire brigade to save more life during there is fire to the building.
The above have indicated the benefit of double skin facade wall and make it gain a lot of popularity throughout the world.
- Construction Technology 5, Heriot-Watt University
- Franklin Andrews, Professor Michael Wigginton of the University of Plymouth and Battle McCarthy
Coursework Unit 6 (b)
The technical challenges that would need to overcome to produce a double skin facade for this building are as below;
1) Overheating challenges
The overheating problem may happen especially on warm day where hot air will collects and emerge at the top air space and this may cause the top floor offices suffer due to this overheating issue cause by this accumulation of hot air in the cavity. Therefore, technically approach to overcome this problem is to design the 'air space'. There are two type of air space,
- undivided air space, and
- divided air space.
The undivided air space will has advantage from the stack effect. On warm days hot air collects at the top of the air space and with the appropriate openings at the top of the cavity, thus will siphon out warm air and at the same time the replacement of cooler air is draw in from the outside.
The benefit of divided air space design is it can reduce over-heating particularly on upper floors. It is also can reduce noise, fire and smoke transmission within the division. Moreover, this floor-by-floor divisions add construction simplicity of a repeating unit and in turn can produce economic savings.
From the above, indicate that both air space using the natural physics principals (hot air rises) to draw air upward. We need to highlight that the second type; i.e. divided air space by floor is practically apply for fire protection and sound transmission purposes.
2) Maintenance Challenges
Although some of the building which is use the fully glazed double skin facade to achieve an aesthetical pleasing view but its maintenance is critical in terms of cleaning process. The air space need to be cleaned more frequently because this area is tend to emerge of dust particles which is circulates quickly during the ventilation process. From an research carried out by 'Terri Meyer, Associate Professor, school of Architecture, University of Waterloo' indicate that glazed double skin facade need to carried out full cleaning regularly from 2 to 4 times a year.
Therefore, the air space's design criteria need to consider the imparts of cleaning especially for the continuous cavity. Similarly, others barrier elements such as louvers placed within the cavity must be removable in order to facilitate access during cleaning process. Normally a device called bosun's chair platform which is similar to the window washing rig is used to access the interior space of cavity for cleaning purposes.
In some double skin facade design an 'open grates' will be put at floor by floor or at particular place act as the cleaner standing platform and without affected the airflow design.
For divided air space or cavity, normally the interior windows will function as the access panels for In some instances, where the cavity is more divided, the interior windows, whether operable maintenance purposes. Therefore the design of air space need a consideration to provide an adequate space for maintenance purposes besides the ventilation functionality. The interior clear dimension for air space is usually range from 600 to 900 mm.
2.2) Replacement of Deteriorate Mechanical Part
The high-tech mechanic which is incorporated for the functioning of double skin facade (particularly in ventilation process) tend to have a higher failure rate and repair cost. The same mechanics also necessitate higher replacement costs. For instances, the replacement of wiring after a certain number of years. Therefore, a preventive checking is need to carry out frequently in order to detect any mechanical problem in an earlier stage before its give a negative impact onto the ventilation system. Besides, a proper data record and operation manual on those relevant mechanic device also need to be keep properly. Through this data any cases of malfunction of mechanic device would be able to find the direct causes and the solution can be carry out as soon as possible as well as in finding the suitable spare part replacement within a shorter period of time.
3) Control of Natural Ventilation
For high-rise building with double skin facade, normally will found a problem on how to control and maintain its natural ventilation (here mean the quality of air) to its occupant. The area between the double skin facade in principle is not affected by high velocity wind because this area have been protected by the exterior skin. Therefore, this region typically will access by the inhabitants for natural ventilation and this will cause some unexpected impact of sound, smoke, noise or heat transfer over this zone either from one section, level or room to the proximity area. In order to eliminates these impact efficiently normally this 'buffer zone' will be propose in compartmentalize design and separate into regions with air supplied by grilles or vents at the individual zone or each level. Then with the use of vents or grilles allows for the control of the incoming air by reducing air velocity, as well as protecting from the rain and reducing the noise transmission from the exterior. Regular cleaning process also need to be carried out in order to make sure that the ventilated air is always in good quality such as out of dust particle. Hence, high-rise building is essentially need to plan and implement this control in order all its occupant will be provide with a natural ventilation.
4) Fire Regulation
Although the air space between the skins can be use for fire escape but the fire regulation might cause difficulties if no extra attention is provided. Therefore, for safety measured a proper indication of fire escape route have to provide and place at suitable location at each floor.
5) Reduce of Space
As mentioned earlier the width of the cavity can be formed from few centimetres until few meters. So, this will cause some reduce in the building usable space. Therefore, it is significant to find the optimum cavity width in order to gain an optimum office space in this building.
In some European country the double skin facade will face the climate issue such as humidity. For those double skin facade associate with the passive design strategies, (i.e. thermal mass) and radiant (hydronic) system , the condensation control will become an issue. Therefore, this critical factor need to be solved in order it would not give a negative comfort effect inside the building. This problem can be technically solved by adopting condensation control through the appropriated mechanical ventilated system, i.e. 'extract air system'. This system normally apply for location where the natural ventilation is not possible (due to it locations inherent with high noise, wind and fume). Through this system the fresh air will be supply by HVAC and it is precludes the natural ventilation. These systems tend not to reduce energy requirements as fresh air changes must be supplied mechanically. In addition, the occupants are advisable not to adjust the temperature even though belong to their individual spaces. However, the priority consideration will still be put on the potential use of natural ventilation. For instances, to come out a desirable hours of natural ventilation scheduled through the utilisation computerise control system can achieve this objective.
7) HVAC Technically Design
The HVAC will play a major role for the building where the natural ventilation is not suitable due to its negative climate conditions. Therefore, in cases the functionality of a HVAC system will become a significant point and it will give an impact onto the building ventilation system. So, in order to solve that problem, during the earlier design process need to fully integrates architectural and mechanical concerns is need to fully integrates during the earlier design process. By this earlier stage planning will able to achieve a smooth functioning of HVAC system. In economical point of view it will less costly and it is a wise planning because the cost saving figure is enough to compensate for the construction cost of the second facade.
8) Solar Heat Gain
This point come to the issue of excessive of the incoming solar radiation above the comfort level especially in the summer time. So, in order to maintain solar heat gain under a standard design level is by preventing the heat from initially entering the space. Particularly for a highly full glazed building, normally an external shading devices are the most efficient means of reducing solar heat gain. However, this external shading devices need to be cleaned frequently in order its can function effectively.
In the other approach is to use the special glazing such as 'spectally selective glazing' and where this glazing materials is able to respond differently to various wavelengths of solar energy or in other means is to permit visible light while rejecting unnecessary invisible infrared heat. An ideal spectrally selective glazing permits only the art of the sun's energy which is useful for daylighting.
Another type of glass called 'electrochomic glass' also able to improve the solar performance. The type of glass able to change its colour from clear to dark using electrical current. The electrical current can be activated in two ways, either by manually activated or by sensor reaction to the light intensity. In physic principle dark colour glass will reduce solar transmission into the building. Adversely when it is little sunlight, the glass will perform brightens in order to permit more suns ray into the building and minimized usage of artificial light.
Both of the above mentioned materials is very practically apply for the full glazed double skin high rise building. Besides, the application of horizontal blind can permit use of daylighting and at the same time still can achieve the exterior view and it is a more economic approaches.
9) DSF Self Loading Transfer.
This double skin facade (DSF) especially full glazed type, is unable to take its own load. Therefore, the dead load and imposed load of skins have to transferred to the adjacent structural wall and frames. Therefore, the design of structural wall and frames need to consider to carry the facade skins loading.
10) Installation Process Challenges
His installation process is difficult especially for full glazed double skins facade because it is tough to work with the increase of the building height. Also encounter near misses and possibility of the hazard of falling. Besides the wind gust is always a question of safety to its installer. Moreover the are only limited movements in the working place. Therefore, the design of 'open grates' to put at floor by floor to act as the installer standing platform is advisable and can ease the installation process.
From the above, we have analysed the possible technical challenges of the double skin facade for this propose 10 storey of office. Therefore, the project design team need to consider that technical challenges during the earlier stage of design in order to come out a proper and efficient double skin facade building design in terms of aesthetical pleasing and maintenance capability.
- Construction Technology 5, Heriot-Watt University
- Terri Meyer, Associate Professor, school of Architecture, University of Waterloo