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The structural frame of a building is the most important aspect a contractor needs to consider before construction even begins. A few criteria should be evaluated for the selection of right structural frames and the material used.
- Evaluation of the structural frame :
In Situ Reinforcement Concrete:
The In Situ reinforcement concrete structural frame is common used as a frame for both single and multi-storey buildings such as residential flats and commercial offices. The use of reinforced in-situ cast concrete as a structural material for building is a frame combination with columns, beams, reinforced concrete floors and roof.
- Inside a skeleton frame of continues columns and floors, repetitive floor plans can be formed based on the engineering design of calculation according to the design requirement.
- Shorten of erection duration and build up economic structural frame are achievable due to the uniformity design of formwork and column section.
- By connecting directly to a solid concrete backing, columns, beams and floor slabs can provides a solid and level surface in which the partitions and walls can be built.
- The design of structural and building shape is flexible to suit to the requirement of the occupants.
- Adjustment and alternations in terms of design can be made before or during the construction.
- The entire structure construction activity can be carried out on site.
- As the structure is built, the design can be proceeded.
- With the enhancement factors of safety elements in structural design, the structural concept for disaster resistant such as earthquakes, collision and explosion are achievable.
- It is very difficult and expensive to make corrections when encounter errors or mistake in setting of formwork or falsework once the concrete has been casted.
- Subsequent alteration on the structural is difficult to be made in change of use purpose of the structure.
- Formwork or falsework and reinforcement are tend to be labour intensive. Thus, high number of labour and plant are required on site.
Precast concrete structures are one of the common option used in the range from single storey factory buildings to multi-storey structures for industrial and commercial use. Generally, there are 2 (two) types of precast concrete structures being used effectively:-
- Units relating to the site measurement
- Individual components of structure installed in site
Joints details of the structural components are need to be recognized and analyzed during the pre-planning of production stage.
The fabrication of structural elements in a precast manufacturer is under controlled conditions in which the component elements will normally be columns, beams, walls, floor slabs and staircase units etc. Separate units or components will be delivered to site for erection and installation with other unit.
- A high quality control for the structure components during the fabrication process carried out in the factory.
- High quality of finishing elements according to factory production. Installation duration on site can be shortening.
- Formwork and falsework no more required on site during the installation. Labour and plant on site are reduced significantly .
- The use of scaffolding can be minimized to a high degree with good planning in the installation sequence.
- Site storage space can be minimized especially for the sites which are restricted due to the limitation of space by deliver the off site prefabricated structure upon the time of installation.
- Impossible to do any amendment or adjustment to the structure during the installation stage.
- Design needs to be finalized and completed before casting commences by the manufacturer.
- Not flexible and unsuitable if structural modification or alterations might be needed and considered in the future.
- There is a cost involve in the handling, caring, protection either during the time of transportation and storage on site for the precast products.
- A possibility that the precast structure might be distort during the curing period during the fabrication in the factory.
- Installation schedule for the structure elements on site will be disrupted if one of the element damage due to the sequence of installation affected.
- It could be happened that sometime the use of Prestressed floor panels or beams can encounter the problem like uneven camber in different units.
Structural steel is often being used in industrial plants due to its ability to achieve large open spans in manufacturing areas. The designer has more options in their design criteria because the availability of steel sections in variety of dimensional ranges. One of the additional option nowadays is hollow steel sections which widens the range of the scope of steel in designing lightweight roofs, space frame and other areas where the hollow section is a strong but light means of construction.
- A smaller physical look of the steel column than concrete columns can contribute more useable space and reduce the visual obstructions.
- By using steel, structural such as cantilevers, skewed walls, clear spans, curved framing, sloping surfaces, atria, floor openings, special aesthetic features, and usual loadings are all economically and easily accommodated.
- Steel can be easily reinforced if any additional loads are added to the structure to handle the weight. Likewise, the alternation such as stairwells, new floor openings for elevators, and architectural or mechanical requirements can be easily accommodated due to its flexibility.
- A larger clear spaces is easily and economically achievable by using the type of structural steel frame which can contribute more flexibility with building layouts.
- When come to the fast track construction, steel framing is a good option which it can be rapidly fabricated, rapidly purchased and erected due to its case of design and construction is readily to lends.
- Smaller foundation only required if using steel structural which is normally lighter in weight, thus there is a cost saving in the construction. .
- Structural steel is commonly used in the zone of which there is a possibility of earthquake happened due to its ductility. It is considered a premier structural material for economically resisting earthquakes. It can minimized the recover cost and maximized the life safety during the seismic event.
- Steel is a type of material which can be recycled and most of the steel sold in the market today is produced and recycled from the source of demolition projects.
- Erosion - Steel are easily eroded when freely exposed to air and water, therefore it must be painted thoroughly and periodically. For example, erosion-fatigue failures can occur where steel members are subject to corrosive environments and cyclic stresses. The fatigue strength of steel members can be reduced when the members are contacted with the aggressive chemical environments like coastal area and subjected to cyclic loads.
- Fireproofing or resistant costs - Even though steel structural members are incapable in burning, but their strength is tremendously reduced at temperature commonly reached in fires when the other materials in a building burn.
- Furthermore, steel is an excellent heat conductor, so a non-fireproofed steel members may transmit enough heat from a burning section or compartment of a building to ignite the surrounding materials with which they are in contact in adjoining sections of the building.
- Fatigue - It is another undesirable characteristic of steel which its strength may be reduced if it is subjected to a large number of stress reversals or even to a large number of variations of tensile stress. Normally, fatigue problems will occur only when tension is involved.
- Susceptibility in the buckling process - As the slenderness and length of a compression steel structural member is increased, its danger of buckling increase also. For most of the steel structures, the use of steel columns is very economical because of their high strength-to-weight ratio. Sometimes, however, some additional steel is needed to stiffen them so they will not buckle. Therefore, this tends to reduce their economy aspect.
- Breakable Fracture - Steel may lose its ductility under certain conditions, and breakable fracture may occur at places of stress concentration. It also can be caused by fatigue-type loadings and very low temperatures aggregate the situation. Trixial stress conditions can also lead to breakable facture.
Recommendation of frame material
In situ reinforcement concrete is recommended because it is a most widely used construction materials in the market by its advantages that benefit the proprietor, designer/engineer and developer.
- For the Proprietor, structural build by in situ reinforcement concrete is aesthetically and efficiency in cost. With its material characteristic in high strength, long term durability, and its original thermal mass which results in low maintenance of buildings required, offer high durability and operating energy efficiency.
- For the Developer, in situ reinforcement concrete offers a very competitive building construction solution based on its energy efficiency, cost, long term economic benefits, lower maintenance, as well as for future planned to be reused upon the occupancy of the building changed.
- For the Designer/Engineer, in situ reinforcement concrete can offers unlimited design possibilities in a structural by providing a superior environmental and energy performance. Designs could take advantages by its thermal mass and structural integrity of concrete.
Flexibility in design - In situ reinforcement concrete can be casted into any kind of designed shape by using suitable formwork. This efficiency can provide an customize made design solutions to specific the problems and also aesthetically finishing which often discard the need for further fixings.
Floor systems can be raised due to the use of thin in situ reinforcement concrete floor structures. The concept of raised floors are ideal for the utilities like wiring which is run in the space below. Concrete can meet the requirement due to its flexibility because the increasing wiring installation are considered a very common demands for a new construction.
In situ reinforcement concrete can also allows for longer floor spans with lesser columns to plan around due to its valued load-bearing properties.This offers the flexibility in architectural setting layout by generate more usable space.
Concrete is readily available from many locations. A reinforcement concrete structure can be constructed before the final plans are 100% finalised. By start-up the construction stage earlier, means it can contribute better cash flow for owners and developers.As requests for the changes which are often encounter in any construction stage, the design flexibility of the in situ reinforcement concrete can allows the contractor to conform the design changes after the construction process has begun.
Specification Variety of concrete - concrete can be produced and mixed to an immense range of its mixture specifications to the structural design.This can be achieved by using different kind of mix designs or by the adding of different additive materials such as superplasticizer to speed up the curing process of the concrete during the construction stage.
Material's property of density and mass are one of a major advantage of in situ reinforcement concrete construction for engineered structure. Reinforcement concrete is the good choice by its lateral stiffness, or resistance to horizontal movement when constructing in areas where seismic conditions and high winds are considered. The lateral stiffness can minimized the motion of the building means that occupants of reinforcement concrete towers (high rise building) are less able to experience it.
Energy Efficiency - Having say that most of reinforcement concrete is produced locally or from the nearby batching plant which can minimizing the cost for handling and transportation.In the life cycle cost analysis, the energy efficiency of a structures is a major consideration. Moreover, In situ reinforcement concrete able to reduce the overall building height in construction. This will shortens the vertical length of the mechanical and electrical systems.
Durability - reinforcement concrete is one of the enduring materials in the range of construction material option. Bywell designed and well casted, in situ reinforcement concrete is long life in any structure due to its durability.Such extended life span remain the resources by reducing the maintenance and also if there is a need for reconstruction.
The mass of an in situ reinforcement concrete structure makes it become a thermal reservoir by keeping the large amounts of energy. During the hot or warm period (summer), concrete walls and floors absorb the internal heat during the day, then released back into the space during night time. It is the same principle during the cooling period (winter). This thermal energy allows concrete to maintain and steady the interior temperature. Concrete can delivers energy efficient buildings reducing peak energy demands by releasing and storing the energy required for heating or cooling.
Fire Resistance / Proofing - in situ reinforcement concrete is unique in being an inexpensive and easily available building material which is originally fire resistant, not only with no additional application or treatment for protection from fire, but also having structural and artistic qualities at the same time.
Sound Insulation - sound transmission within the walls of a structure and the external surrounding environment of the building are taken into account in today's structural design criteria.As per tenant requirements to include the isolation of sound transmission, the natural mass of reinforcement wall systems and concrete floors can provides both in vibration control and acoustical resistance.
Shorten Construction duration - in situ reinforcement concrete provides faster construction which means reduced in terms of costs and faster revenue generation. This ease more opportune pay back time of financing charges and faster revenue will be generated for the developer/owner.
Cost effectiveness - in situ reinforcement concrete is always the most economical choice for engineered structures due to its longevity and facilitate in construction. The approach in use of load-bearing concrete exterior walls is to enclose the buildings and carry roof self load and lateral wind loads by eliminate the need to erect separate structural systems and cladding systems.
One of the important structural advantage of concrete is a shallow type of the floor systems, In average, the construction of concrete buildings normally will allow one additional floor to be constructed for each 10 (ten) stories of the traditional building's height, which contributes in more usable space for buildings of its similar size.When encounter with a height limitation, concrete construction is one of the key of consideration by represent the initial construction cost savings and additional income generation.
Re-use or recycle - reinforcement concrete is type of material that is easily recyclable. Old reinforcement concrete which is reached to the end of its structure life can be re-use as aggregate for new concrete mixtures.
- The structural arrangement for the in-situ reinforcement concrete frame referred to the A3 layout as per attached.
- Aspect and alternative frame solutions need to be considered for the increase of building height to 30 stories. The choice of structural system and structural material has a major influence on construction time and cost.
In high-rise buildings, lateral loads become an increasingly dominant parameter which it is considered a significant effect for the planning and design of the whole building. Lateral stability systems are frequently fitted within the central core and services have to be carefully be integrated into the structure. Dominant lateral load are normally come from seismic loading or wind.
Two principal criteria are used in building stability assessment: drift and acceleration. Drift is the ratio of the building deflection over its height, another one is the critical parameter is usually intermediate storey (floor to floor) drift rather than total building drift. Building acceleration is a measure of the speed with which drift occurs, and acceptance criteria are based on human tolerance of movement.
Options for stability
The three most popular systems for providing stability are shear walls, braced tube structures and tube-in-tube structures.
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A shear wall system is the common solution for low- to medium-rise buildings. These can be constructed by either solid concrete walls or braced steel bays, which generally located around the features that remain fixed in plan over many floors. These system depend on linking strong elements at either ends of the wall into a dumb-bell to create a push-pull system, resisting the overturning caused by lateral loads. Depending on the construction budget and the aspect ratio of the building, these systems will often be the most cost-effective option for buildings up to 40 or 50 stories. (as per diagram below)
Braced tubes structures
This type of approach is designed by cross bracing the structural frame with X-bracing over many stories, as shown in the diagram below. To exclude the effects of shear lag in both the flange and web frames, the diagonals of a braced tube are connected to the building columns at each intersection. When come across with the lateral loads, the structures will behaves more like a braced frame by reducing the bending in the structure members of the frames. Hence, it comes to the advantage that the spacing of the columns can be increased and the depth of the beams will be reduced. Therefore, a large size windows can be allowed than in the conventional frame tube structures.
Basically, Tube in tube structures design consists of an outer framed tube together with an internal elevator and service core. In this type of framed structure, braced frame may forms the inner tube which act jointly with the outer frames in resisting both gravity and lateral loading.(commonly used in steel-framed buildings). To play a dominant role for the whole structure, Outer tube usually will be designed with much greater structural depth. Sometimes, this type of structures also call as Hull (Outer tube) and Core (Inner tube) structures. A typical Tube in tube structure is shown as below:-
Double-skin facades are assuming importance in modern building practice. There is an increasing demand for higher quality office or commercial buildings. Stimulating and comfortable working environment in office building are concerned by the occupants and developers nowadays.
Double-skin facades are appropriate when buildings are subject to surrounding environment factors such as a great external noise (sound pollution) and wind loads (natural forces). Double-skin facades can be adopted architecturally for a great advantage because it have a special aesthetic of its own. Further area of application is in refurbishment work, when existing facades are unable be renewed, or where this is not desirable.
- The benefits by adopting Double Skin Facades
For example, if the building is located in a heavily polluted area with high external noise levels (sound pollution) for example beside the expressway, then a multi storey double skin façade type is often suggested. In this approach, the outer layer does not have any openings, in order to reflect the noise by avoid any transmission. (from outdoor to indoor).
Thermal Insulation :
During the cooling period (winter), the external additional skin provides improved insulation. The increased temperature of the air inside the cavity and the reduced air velocity will lower down the heat transfer rate on the surface of the glass which leads to reduction of heat losses.The box window type can be suggested when the energy demand for heating is high since the preheated air inside the cavity can be channel into the offices by maintain the thermal comfort and low energy use.
During hot period (summer), the warm air inside the cavity will be extracted by mechanical, fan supported or natural ventilation. This concept highly depends on the location of the building, on climatic parameters and on daylight availability.
This is the one of the main advantages of the double skin facade system is that they can allow natural (or fan supported) ventilation when possible. The selection of double skin façade type is depends on the aspects of temperatures, air velocity, and the quality of the introduced air inside the building.
By well design and planning, the natural ventilation can reduce the energy consumption during the occupation stage and improve the comfort of the occupants. For example, the interior spaces can easily be overheated during the hot summer day. In this situation, it may be energy saving in pre-cool the offices during the night by using natural ventilation. The indoor temperatures will then be lower down during the early morning hours providing thermal comfort and improved air quality for the occupants.
Reducing environmental impacts and energy saving:
In principle, double skin facades can save energy when properly designed and adopted, usually when the conventional insulation of the exterior wall is insufficient, the savings that can be obtained with the additional skin can be important.
Better protection for the lighting or shading devices:
Since the lighting or shading devices are placed inside the intermediate cavity of the double skin façade, they can be well protected both from wind and rain.
Wind pressure reduction effects:
The double skin facades around high rise buildings can be designed to reduce the effects of wind pressure during the extreme weather.
Emergency escape during fire:
The glazed space of a double skin facade may be used as the fire escape during the emergency.
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- Technical challenges and the planning for proposing a double skin facades
When adopting a double skin facade, there are several common technical issues need to be highlighted and the solutions are as below:-
- Constraints - The basic constraints regarding the feasibility of window ventilation should be evaluated. In this manner, the possibilities and limitations of different system alternatives are determined, as well as the necessary requirements on the double skin façade. This could also give an idea as to the costs of the proposed system and building, compared with an alternative with a single skin façade.
- Type of construction - At this stage it should be known whether window ventilation or any other adoption will be used or not, as well as the level of sound insulation requirements on the facade. Therefore, a performance specification evaluation report should be established in which to help in determining the type of double skin facade construction. Since the starting point for the following dimensioning is determined at this stage, this step is very important.
- Good fresh air supply - For example, if the decision has been made to apply window ventilation of the building, then the dimensions of the openings and air flow routes into the rooms can be planned to ensure a good fresh air supply in the building.
- Avoiding overheating of the cavity of the double skin facade - The dimensions of the openings should be evaluated to limit heat gains in summer in the cavity, in order to ensure thermal comfort in the rooms and to avoid increased the cooling loads.
- Optimizing the air flow - The following parameters are important for the natural ventilation and temperatures of a double skin facade:
- Exploiting the construction to the full aspects - It is important that the facade planner or consultant should be involved at an early stage to participate in discussions of air flows and where appropriate the optimization of the aerodynamic design. Only with a coordination of concepts from the beginning stage can detailed solutions, which are practical to construct and economically acceptable, be developed. This coordination is to ensures that the conclusions made regarding the aero physics will result in a construction, which will work in practice. Another benefit can be that standardized details can be designed enabling prefabrication, which can result in a less expensive construction.
- Maintenance and durability - It is obviously that a double skin façade has two more surfaces need to be cleaned compared to a traditional curtain wall. The cavity surfaces, air in and outlets may be difficult to reach, increasing the maintenance time.
- Recommendation for the type of double skin facade
For multi-story facades, there is a risk that the temperature deposition in the cavity which can burden the cooling loads in the rooms at the top floor. Measures need to be taken to minimize the exhaust air temperature from the cavity and the warm air buffer of the cavity, for days with warm weather.
The size and position of the openings
Appropriate aerodynamic design of the narrowest cross sections through which the air flow has to pass.
In most cases natural ventilation should be sufficient if the air flow resistance of the cavity is kept low. Wind forces can assist, but often in central and northern Europe the wind speeds are low on hot days. For fan assistance to matter, the fans have to be powerful.
Operation conditions - In winter the double skin facade should provide thermal insulation and also for all the year round sound insulation. This means that the openings in the façade should be small, whereas they should be forms the opposite during periods with warm weather.
Therefore, the size of the openings should be adjustable, variable and closable. For this, the control system of the openings has to be intelligent. Means the controls have to be optimized for the different occurring conditions during a year. To integrate the system, it has to be a link to the other functions of the building, which has to be recognized in good time. It is usually very costly to install sensors and a control system afterwards.
The interval or frequency of the cleaning of the glazing and the shading device will influence the costs of the double skin façade comparing with a traditional curtain wall. The frequency of maintenance depends on the ventilation type of the double skin either it is ventilation with air coming from inside or outside OR the environment polluted or not in case of ventilation with outside air.
Some concepts of double skin façade are ventilated with indoor air. This air is coming via the room from the ventilation system of the building. This kind ventilator system filtering the air for dust particles, by which less particles enter the cavity. As a result the cavity gets less filthy and needs less maintenance. The filters of the ventilation system however need to be cleaned periodically.
A full sized double skin façade incorporating a sun shading blind was recommended, as presented in Fig. 1 (refer to attachment), is fixed of two aluminium frames. The double skin façade also includes four openings for ventilation (height of 0.04 m).
The openings cover the whole width of the façade, one at the top and another one is for the bottom of each of the two panels. Thus, various configurations of the ventilation of the double skin can be examined by simple obstruction of these openings.
The sun shading devices are using "Venetian" type blinds placed in the middle of the double skin façade cavity. The double skin façade is mechanically ventilated.
The connection details are shown in Fig. 2 (refer to attachment)
"Source : Streicher W., 2005. Bestfacade - Best Practice for double skinfacades EIE/04/135/S07.38652. WP1 Report State of Art"
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Unit 7 - Flat Roof Construction
- Evaluation of 4 different roof strategies
Flat roofing is considered one of the economical choice in the form of a building perspective. Basically, in order to maintain a flat roofs, it must be covered with a good waterproof material and also be fitted with a good drainage system. Below are some of the most popular flat roofing systems being used in commercial application.
1. TPO Roofing Systems
Thermoplastic (TPO) roofing is a reliable and ideal commercial roofing system in terms of cost effective, environmentally friendly use, easy to maintain and easily comply the various roofing situations.
2. PVC Roofing Systems
PVC roofing is also one of the option in selection a roofing system due to its reliable, environmentally friendly, easy to install and easy to maintain characteristic.
The layer of PVC roofing is consist of :-
PVC roofing systems are more resistant to tears, puntures that other single ply membranes and also impacts. Below are some of the advantages by adopt PVC roofing systems:-
3. EDPM Roofing Systems
EDPM roofing systems is suitable for large scale application because it is considered one the most economical roofing system. EDPM normally is produced in large sheet widths which can lower the installation costs if compared with PVC and TPO roofing.
EDPM is made of :-
- Diene Monomer (small amount)
EPDM roofing systems is reinforced for increasing its puncture resistant, means it is highly resistant in punctures, tears, wind, wear and fatigue. By adopt EPDM roofing systems, it will benefits :-
- Highly reliable and economical
- Long life expectancies
- Resistant to Ozone, weathering, and abrasion
- Flexible in low temperature
- Highly wind resistant
- Superior resistant to extreme heat and fire
4. BUR & modified Bitumen Roofing system
Built Up Roof (BUR) roof system is formed by multiple layers or plies of roof felts laminated together with bitumen which also commonly referred as "tar and gravel' roofs.
Modified Bitumen roofing is a type of Class A, UL fire resistant membrane that can provides the maximum puncture, split and tear resistant. In addition, it is a tough, resilient membrane with strong polyester mat core which is coated by either a flexible APP or SBS polymer modified asphalt. Modified Bitumen Roofing can be maintained well by apply with hot asphalt, adhesive or heat welding methods.
Surfacing options for BUR and Modified Bitumen roof systems can be selected from the below:-
- Aggregate such as gravel, slag, glass-fiber, minerals, hot asphalt, aluminium coatings and elastomeric coatings.
- Multiple surfacing option
- Easy to maintain
- Economical an reliable
- Highly durable and puncture resistant
This type of roofing system are not only highly durable and puncture resistant, but it is also available in an assortment of surface coatings colours.
Some of the benefits by adopt BUR & Bitumen Roofing systems are:-
- Recommendation of one roof strategy
After considered all the 4 types of flat roof system as above, Built Up Roof (BUR) or Modified Bitumen Roofing System (MBS) is recommended in conjunction with the development of 'Green Roof" as concerned by developer for being environmentally aware. With this recommendation, it provides the features of a built-up roof with added tensile strength and elongation of a modified bitumen cap sheet, as well as the quality assurance of in-plant membrane fabrication uniformity and control and reduce the labour requirements for installation.
The recommendation of BUR/MBS is based on the following aspects and benefits:
- Energy costs could be reduced in hot urban environments or hot season.
- Able to reduce the overflow of storm water, by reducing capacity on urban sewer systems and decreasing the overflow related to pollution of natural waterways
- Dust or suspended particles control (reduction)
- To trap rain-dissolved pollutants from air in the vegetated roof's soil and neutralized it.
- Sound control (pollution reduction)
- soil, plants and the trapped layer of air can be act as a insulator to insulate for sound. Sound waves that are produced by machinery, airplanes or traffic can be absorbed, reflected or deflected. The sub base could help to block lower sound frequencies whereas the plants can block the higher frequencies.
- Improve air quality
- by decreasing the flowing of thermal air movement and filter the air moving across it.
- Flat roof design can be in more flexible and creativity forms such as recreational or decorative designed spaces, solar panel easier installation, etc.
- it also can help to address the lack of green space in urban areas.
- Difficulties during the installation can be minimized due to its flat structure design.
- Low maintenance required for flat roof due to its original structure with no any slopes or valleys thus it can be easily cleaned.
- 'Heat Island effect' can be prevented and minimized by adopt the concept of 'Green Flat Roofs' which absorbing the heat and at the same time evaporating water as a way to eliminate it from the roof.