Building Structures and Forms |
Building Structures and Forms
Formwork is the mould to shape the concrete structure. Like food, delicacies require good ingredients. Good concrete cannot be made from a poor mould; thus formwork is very important in producing good concrete. The design for formwork is such that it is able to take the concrete load and workers weight.
Glass reinforced plastic formwork may be used for complex shapes. However, timber and plywood are commonly used for on site concrete work.
Examples of formwork
Once the formwork has been struck, it should be cleaned straight away.
Reinforced plastics are cleaned with wet cloth and a brush.
Oiling should be done when steel forms are to be kept for some time.
Precaution: Ensure that insides of forms are cleaned before concreting. Openings should be provided for inspection where forms are deep.
Steel must be placed correctly or the strength of a structure may be greatly weakened. It consists of 3main parts, that is cover, fixing and bending.
Cover is the distance from the outside face of the concrete to the nearest surface of reinforcing steel. Cover also determines the strength of the reinforced concrete, the longer the cover, the stronger the concrete.
Sufficient cover is needed to prevent steel rusting. When steel rust, strength of it decreases significantly and may be a safety for the building.
Steel rusting also causes the concrete to break open as it expands roughly 2times when it rust making it susceptible to weathering.
Fixing is fastening the steel reinforcing bars so that they stay in the correct place between the spacers and relative to each other. The steel must stay in place while concrete is being placed, while workers are working on it and during compaction.
Junctions of bars are held together by tie-wire. All pieces of tie-wire must be removed from the formwork before concreting otherwise rust-stains soon occur on the finished product.
Too small a diameter weakens the steel. Too large a diameter may cause problems such as lack of anchorage (a form of bond failure) or create difficulties in keeping other steel bars in the correct place. Thus, the steel needs to be bended to make it suitable for the building.
Example of pre-engineered steel construction:
The main objective in placing is to deposit the concrete as close as possible to its final position as quickly and efficiently as you can, so that segregation is avoided and it can be fully compacted.
As mentioned earlier, concrete should be deposited at, or as near as possible to its final position. The concrete should be placed in uniform layers.
In order to get the air out from the bottom layer of the concrete, the layers should not be more than 45cm thick as the weight will compress the air very tightly.
Vibrators are mobile items of mechanical plant used to vibrate (shake) air out of fresh concrete are only used for thickness of concrete slabs more than 150-200mm thick.
Place the concrete as quickly as possible, but not faster than the compacting method and equipment can cope with. In order to make the layers ‘knit’ together, make sure that each layer of concrete has been fully compacted before placing the next. It must be placed when underlying layer still responses to vibration.
Reasons for Removing Air
In order for concrete to be strong and durable, it needs to be fully compacted. Voids reduce the strength of the concrete. For every 1 % of entrapped air, the strength falls by about 5% to 6%.
Voids reduce the contact between the concrete and the reinforcement and other embedded metals; the required bond will then not be achieved and the reinforced member will not be as strong as it should be.
The best and quickest method to remove air is vibration.
With a properly designed cohesive mix, segregation and bleeding will be minimised. An over-wet mix will result in the larger pieces settling during compaction. A weak layer of laitance will finish up on the surface; if this does happen, the laitance must be removed.
Precautions: Someone experienced in the construction of formwork, preferably a tradesman, should always be standing by when the concrete is being placed.
He should have a supply of suitable materials such as props, bolts etc. to handle dangerous situations.
The extensive use of prefabrication is especially good when it comes to labour saving. It make use of labour saving fixing technology.
- Mass production of units
- Economical in terms of costs and time
- Exterior finishes
- Demountable structures
- Pre-casting before site is available
a. Components are manufactured units, made to pre-determined sizes, to be used in building. Design and use are controlled by the dimensional co-ordination.
b. Mass produced or custom made prefabricated units are fitted onto the structural frame. The tying up between the structure and cladding units is important, only very limited degree of tolerance may be allowed for either. Prefabricated wall cladding panels will closely govern the storey heights and the length of the building, or part of a building where they are used.
c. Building elements comprise of joining different components. Dimensional co-ordination between all the components concerned is essential, and for this method of building it is necessary that this co-ordination shall be based upon a suitable module.
Two sorts of variation or deviation must be taken into account, variation in size and variation in shape. Both variation in size and shape are to be allowed within acceptable limits. The difference between two maximum and minimum limits of size between which the actual size must lie represents the tolerance allowed.
d. The Joint
The major problem in industrialized system building is the joint. External problems include those of weather related and fire resistance problems. The joint width will vary with the work size (size specified for its manufacture) + deviation (variation in size and shape) for each component.
The greater the accuracy of the components to be joined, the less width will the joint require, subject to a minimum gap to allow for movement of the components into position and for filling material (e.g. mastic) if required.
Prevention of weather related problems is done by ensuring the joint is self draining, or water proof based materials such as plastic or rubber.
Fully mechanised public housing construction in the late 1990's involves the following features:
The central core is typically constructed ahead of the residential wings by using a climbing formwork system. The wings then "spiral" around the central core.
Various design considerations should help keep the cost of energy down. Below are the design elements and best practices that building owners and managers of skyscraper can adopt.
Cost of going Green. Studies and research in the United States has showed that although the average premium for green buildings is about 2% more than conventional buildings, they are 28% more energy efficient
It is also noted that if green building features are incorporated during the earlier design stage, the cost will be much lower.
Singapore - According to a report from National Climate Change Strategy, energy use in buildings made up 16% of Singapore’s energy demand in 2004, especially air-conditioning which forms a large part of energy consumption due to Singapore’s tropical climate.
Rising oil prices and rising electricity cost. The graph below shows the electricity tariff from Feb 2000 till Jan 2006. The electricity tariff is reviewed quarterly and it will be adjusted according with the fluctuation of oil price. This is due to the fact that fuel cost makes up about 50% of the cost of electricity. The recent few quarters’ sharp increases in fuel oil have caused the electricity cost to increase sharply. It has become more important for a building to be energy efficient.
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The Envelope Thermal Transfer Value (ETTV) is key to better energy efficiency in air-conditioned buildings.
Heat absorbed from the environment (especially solar heat) by the walls, windows and roof of a building makes up a large portion of a building’s cooling needs. Hence, it is important to minimize this heat gain when designing the architecture.
Air-conditioned commercial buildings should be designed to have a low ETTV. The “Guidelines on Envelope Thermal Transfer Value for Buildings” issued by the Commissioner of Building Control states that the ETTV of air-conditioning building shall not exceed 50W/m2.
Currently, there is a software design tool named Building Energy Standards (BEST) which can be used to calculate ETTV, heat loads and energy consumption of air-conditioned buildings. BEST was developed by National University of Singapore (NUS) in collaboration with BCA.
Energy Efficiency Index (EEI) is used to gauge the energy consumption in buildings. To enhance the energy performance, it is important to have detailed energy performance data and indicators. The estimated energy efficiency index of building can be calculated using the BEST software mentioned earlier.
A study conducted by the Centre for Total Building Performance (CTBP), a joint venture of the National University of Singapore (NUS) and BCA had collected energy consumption data for 104 office buildings in Singapore. The study shows that the energy performance of a building can be assessed using an energy efficiency index regardless of a building’s size, height or age.
According to the study conducted on Energy Efficiency of Office Buildings in Singapore , only the top 10% of the office buildings have EEI of 150KWh/m2/yr and below.
Design long facades with windows facing north-south orientation
Minimize surface areas and windows on east & west facades
A skyscraper’s orientation has a significant impact on the building’s ability to reduce cooling load, the extent of natural ventilation and utilization of daylighting. Here are some good examples are shown below.
Design long facades with windows facing north-south orientation
Minimize surface areas and windows on east & west facades
Design windows on west facades with low emissivity (low-e) coatings to reduce solar heat gain
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Sun-Shading
Sun-shading should retain its aesthetic value while allowing enough daylight to the rooms. Sun-shading should be provided for facades facing east and west to partially shade the building from direct sunlight to minimize solar heat gain. Some good examples of sun-shading are shown below.
To minimize solar heat gain, inter-block shading can be considered where applicable. This allows facades with sun shading to use less expensive glass and day lighting
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Facade Materials
Facade materials also impacts energy efficiency of the building. Some examples of reducing solar heat gain by designing appropriate glazing and insulation of building envelope are as follows:
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The use of day lighting should be incorporated into the building as the preferred mode of interior illumination. It provides the highest quality light with significant health benefits and feeling of well-being. It reduces lighting load and operation costs. Some examples below highlight the uses of day lighting.
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Natural ventilation should be designed wherever possible to minimize the cooling load required and save energy. Some examples below highlight areas suitable for natural ventilation such as lobbies, courtyard, car park, etc., where requirements of comfort level are not so crucial as to affect occupant’s satisfaction and productivity adversely.
For some of the areas with large variations of occupants such as multipurpose or sport hall, hybrid ventilation i.e. combination of natural and mechanical ventilation modes could be considered to save energy. For example, most of the time, natural ventilation can be used to conserve energy and mechanical ventilation can be activated only when the occupancy rate is high.
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In Singapore’s tropical climate, 52% of electricity consumed in buildings goes towards air-conditioning and refrigeration
Although the majority of air-conditioning power is being consumed at the chiller or air-con compressor, other components like distribution pumps and fans should not be neglected. When examining air-conditioning system efficiency, a whole system approach should be considered instead of addressing each component of the system individually.
Software tools are available to help a designer analyze the building air-conditioning requirement. The cooling load pattern and profile of the building should be analyzed and developed so that suitable combination of equipment can be selected to achieve optimum system efficiency at all times.
District Cooling
District cooling provides cooling needs to several buildings on the same site. A centralized chilled water plant can provide the cooling requirements of these buildings more effectively.
District cooling are suitable for projects with several buildings in close proximity. In Singapore, district cooling are available at some business development areas such as Changi Business Park and Biopolis.
Benefits:
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Chiller Efficiency
Select appropriate number and size of chillers according to the cooling load profile of a building to ensure that the chiller systems are always running at their optimum system efficiency at all times. Appropriate sizing is critical to achieving energy efficiency. Many existing systems are oversized. An oversized chiller not only cost more to purchase, it also costs more to operate.
While the chiller must be sized for peak load, it is also important to be sure that it operates efficiently at part load conditions because it is at part load that the chiller operates most of the time.
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VSDS On Chilled Water Pumps
Use variable speed drives (VSDs) to ensure that chilled water pumps are performing at maximum efficiency at part-load conditions. Use VSDs to maximize energy saving at building part load conditions. Varying the speed of pumps allow the efficient circulation of chilled water at building part load.
Use VAV System With VSDS On Fans
VSDs save energy for electric motors driving pumps when capacity is reduced. It must be noted that the power consumption of motor varies approximately with the cube of the motor speed. This means that a reduction of speed by 20% will result in reduction of power consumption by a half i.e. 50% saving. Since most air-conditioning system seldom runs at full load, significant energy saving can be made with these VSDs.
An Energy Conservation Project at NTU shows that by implementing VSD control on chilled water pumps, an average saving of 18% on chiller plant power was recorded.
Variable Speed Cooling Tower
Use cooling tower with variable speed fan to achieve closer match to the actual cooling load required.
The variable speed operation varies the airflow through the cooling tower in accordance to the load variation on the system. By varying the airflow through the cooling towers, it achieves a maximum total chiller plant efficiency based on building load and ambient wet bulb temperature variation. As the fan speed reduces to meet load requirements, the fan input power required will be substantially reduced.
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FCU |
VRV |
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Use fan coil units (FCUs) to serve small areas that have irregular operation hours. VAV type single zone |
Use inverter-controlled variable refrigerant volume (VRV) packaged units for rooms that require 24-hours |
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BENEFITS |
BENEFITS |
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FCU allows the individual small areas to operate independently on-demand. Energy consumption is also minimized as the FCUs serve discrete rooms. |
VRV packaged units avoid the need of operating cooling plants to support small load, especially after normal working hours, thereby saving energy. |
Motion Sensors
Design air conditioning systems that automatically turn on and off by motion sensors in areas where usage are low and irregular such as at meeting rooms and gymnasiums.
Chiller Plant System Control
Design optimum sequencing of chillers, pumps and cooling towers to match the exact cooling load requirements to minimize chiller/pump/cooling tower running time.
Sequencing control of multi-chiller plant is required because the air-conditioning system generally does not operate at full capacity all the time. Effective sequencing control operates optimum number of chillers to meet the varying load and hence save energy.
Control and monitoring facilities should be incorporated in the systems. Adequate monitoring and control enable regulation and tuning of air-conditioning systems to operate at optimum efficiencies with minimum energy consumption. The savings can be very significant. For example, by simply optimizing the operation of air-conditioning system, savings of up to 17% electricity consumption can be achieved.
A reduction in lighting load reduces electricity used and also reduces heat gain in the space, which in turn results in reduction in air conditioning load.
Lighting design should not only consider energy consumption but occupant satisfaction and productivity. It is important to design a lighting system that minimizes contrast ratios. Where applicable, integrate natural and artificial light sources and employ advanced lighting controls.
Enery Efficient Lamps
Energy efficient light fittings should be used with energy efficient lamps such as “T8” or “T5” fluorescent lamp
Table 2 compares the luminous efficacy of lamp types commonly used. Lamp luminous efficacy is the ratio of the lumens emitted by the lamp to its power consumption (watts). The efficacy of the lamps in the table below is based on the lumens output when the lamp is new and the power taken by the lamp only. It includes the power taken by the ballast where the ballast is built into the lamp.
Table: Comparison of luminous efficacy of common types of lamps
Lamp Types |
Lumens per Watt |
Average Life (operating hours) |
Incandescent |
12-15 |
1,000 |
Tungsten-halogen |
15-25 |
2,000-5,000 |
Mercury vapor |
30-50 |
24,000 |
Compact fluorescent |
40-80 |
8,000-12,000 |
Tubular fluorescent |
50-100 |
10,000-15,000 |
Fluorescent tube “T8” |
90 |
12,000 |
Fluorescent tube “T5” |
105 |
17,000 |
High pressure sodium |
60-110 |
24,000 |
Low pressure sodium |
70-180 |
18,000 |
LED |
70 |
40,000 |
High Frequency Electronic Ballast
Install fluorescent light fittings with high frequency electronic ballasts for energy efficiency. Operating the fluorescent lamps at higher frequencies can significantly enhance the lumen watt of the lamp output as fluorescent lamps are sensitive to the operating frequency.
High frequency electronic ballast |
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Potential saving |
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The efficacy of the lamp can be improved by about 10% when fluorescent lamps are operated at higher frequencies. |
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Occupancy Sensors
Use occupancy sensors such as motion sensors to detect occupant motion and light the space only when it is occupied for spaces that have highly variable and unpredictable occupancy patterns, such as staircases, toilets, gyms, etc.
Energy is wasted when lights are left on in unoccupied rooms
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Scheduling
For spaces where lighting needs are predictable and predetermined, use automatic scheduling controls to switch on and/or off the lights for energy savings.
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Use of Dimmers
Lighting in an area can be dimmed if all employees have common lunch hours.
Design a sensor that measures the lighting level in a space to allow a dimmer to adjust the light output to the required lighting level. This will save energy by reducing lighting to the required lux level. This will be useful for new buildings as lamps provide higher light output at the start of their life when the initial lux level will normally exceed design specifications.
Multiple wiring circuits should be used to facilitate variation of lighting level. Some examples of zoning for special usage are highlighted below.
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Efficient Lifts
Design traction lift with AC variable voltage and variable frequency (VVVF) motor drive for energy efficiency. It is highly recommended that even for lifts with speed under 1 m/s, AC VVVF should always be considered whenever feasible.
Use AC synchronous motor, preferably with permanent magnets to avoid the problem of poor power factor in AC asynchronous motor. Furthermore, torque pulsation is a problem for AC asynchronous motor when operating at low frequency and low speed range.
Design motor drive system with either gearless type or planetary gear type. The elimination of gear improves the energy efficiency as gearless drive has no gear transmission loss.
Appropriate Zoning Arrangement
Design appropriate arrangement of lift zoning which will subdivide the floors of the premises into clusters of stops to be served by different lift cars.
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Sleep Mode for Lift
Design lifts to go into sleep mode where lift will automatically switch off the lights and ventilation fan when the lift is idling. As most of the lift has considerable idling time during off-peak hours, putting the lift into sleep mode will save energy.
Intelligent Lift Control
Design group control system that are able to assign the hall calls to individual lift in the way best suited to the existing traffic in order to achieve the maximum possible handling capacity. The system should be able to adjust the lifts operations in accordance with the peak and off-peak traffic flow.
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Lift Car Decoration
Light weight material should be designed for lift car decoration. The interior decoration in the lift car can lead to energy wastage as extra energy has to be consumed to move it up and down the lift shaft to carry this extra dead weight.
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Efficient Escalator
Design escalators with slow down features such as the motion sensor installed at the foot panel. The escalator will oscillate in a slow-mode when not in use and the escalator will be activated to normal speed when the user steps onto the foot panel of the escalator.
Normally, the motor drive system of escalator is running all the time regardless of the load condition of the escalator. Hence, electricity is continuously consumed even when there is no passenger on the escalator. Much energy is wasted if the number of passengers is widely fluctuating.
Electrical Sub-Metering
To achieve energy efficiency, it is important to ensure that energy used by each operation unit is properly accounted for. Hence, a good sub-metering system should be established.
Sub-metering should be provided for substantive energy uses within the building (greater than 100KVA). For a typical office building, this should include separate metering for car park, chillers, air handling fans, lifts and common area lighting and power.
Greenery such as landscaping, rooftop/sky garden and green roof (intensive, extensive or hybrid system) can alleviate urban heat islands through shading and evaporative cooling. The plants also provide a green and more pleasant environment.
To encourage skyrise greening, authorities can review its guidelines on sky terrace to provide development with additional gross floor area. This incentive will help to offset the cost of constructing sky terraces.
Landscaping
Where possible, landscaping should be designed at vacant places such as at the perimeter of the building, covered walkways etc. Landscaping not only beautifies the place, it also blocks off the heat from the sun and provides a cool shade. Besides that, it also retains rainwater and lowers runoff and this will reduce discharge into drainage system.
Rooftop and Sky Gardens
Rooftop and sky gardens could be used to enhance aesthetics of buildings and also to serve as solar and thermal insulation. Rooftop gardens are also known as intensive roof gardens. They are developed to be accessible for use. Regular garden maintenance such as mowing, fertilizing, watering and weeding is required.
A case study conducted in a multi-storey car park and a commercial building in Singapore revealed that the installation of rooftop gardens would significantly improve the thermal environment on building roofs . Some of the measured thermal improvements due to the rooftop gardens are summarized in Table 4.
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Green Roof
Green roofs can be used to enhance aesthetics of buildings and their surroundings. It also reduces glare and provides solar and thermal insulation. Green roofs may be of the following types:
A case study conducted in multi-storey car park in Singapore revealed that the installation of green roofs would significantly improve the thermal environment on building roofs . Some of the measured thermal improvements due to green roofs.
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Hardy Plants Suitable for Green Roofs
