Explain in terms of scientific principles, why:
a) It is advantageous to shade the south facing windows of a modern office block with louvres, and specify, giving, a suitable material for their construction.
b) It is necessary to incorporate a vapour barrier in the construction of a flat roof of a multi storey office block.
a) Only surfaces facing South receive sun all year round, however, South facing windows lose heat in the winter and gain heat in the summer as the sun rises and sets at different times as well as taking different paths across the sky. The reason the sun is stronger in the summer than the winter is because the sun is higher and therefore the rays from the sun tend to be more direct hence more intense. Similarly in the winter the sun is lower and seems to move faster as it appears closer, this is illustrated in the diagram below:
Glass is a poor insulator and when the sun falls, struggles to maintain the heat it has received from the suns rays particularly in the winter when days are much shorter. Louvres help improve insulation in buildings, and reduce heating and gas bills, doing their bit for the environment as you can close them when darkness falls.
Louvres can be made of various materials, for example, metal, timber and even tinted glass. Timber is sustainable and generally the cheapest solution, however the downside to using timber is that it may begin to rot if it is not looked after, therefore I would select metal although it is normally more expensive, it would require less maintenance, and in the long run may be cheaper. Glass the third alternative is the most expensive and least common however the advantage this offers is that you can see out of it when it is closed as the tint emits light however allow visibility.
b) In a multi story office block with a flat roof a vapour barrier would be necessary to allow natural convection of warm air rising to escape through the roof and hence prevent interstitial condensation.
The vapour barrier is likely to be installed in the warm side of the roof (internal) so that condensation in the cold side (external) can escape. If a vapour barrier is not present continual build up of condensation would occur in the ceiling and ceiling space, this would dampen materials and if not controlled, lead to problems such as decay in timber or corrosion in metal fixings.
Flat roofs can cause a problem as there is a lack of airspace for the hot air to escape and the taller the building is, the more the flow of moist air increases as hot air rises because it is less dense than cold air. The flat roof also brings both internal and external temperatures closer together and this affects thermal transmittance. The vapour barrier is an impermeable membrane that helps the warm air in the building escape thus reducing the likeliness of any potential moisture problems.
Other factors, which affect thermal resistance in airspaces, are ventilation and surface emissivity; for example, a concrete deck roof generally has a higher vapour resistance in comparison with a metal deck roof therefore concrete copes better emitting water vapour.
Give an outline description of how a modern 3-storey office block could be constructed as a “high thermal mass” building, and state the advantages of this method of construction for the temperature control and ventilation of the building.
A high thermal mass building (HTM) incorporates building materials which have good thermal storage properties because they absorb, store and release heat when necessary ensuring the internal temperature within the building is more constant throughout all seasons of the year. Heavy building materials such as concrete generally have higher thermal masses in comparison to lighter building materials e.g. timber. Thermal capacity of a wall is often enhanced by painting it a dark colour as it absorbs the heat. HTM delays heat flow through the building envelope therefore making the building warmer in the winter and cooler in the summer.
If constructing a 3-storey office with HTM, it may be an idea to use precast concrete for both the floor and roof; and concrete blocks to form the external wall. As mentioned in question (1a), louvres may also be incorporated into the design; these could be controlled by a building management system (BMS), a computer that automatically monitor and controls the building conditions automatically.
HTM is particularly useful for temperature control as it helps maintain a constant internal temperature all year round, less heating is therefore required making it more economical. A building with HTM uses night ventilation, which provides passive cooling. This is well explained by Nick Barnard of FaberMaunsell:
“Outside air is circulated through the building where it comes into contact with and cools the building fabric. The cooling that is stored in the building fabric is then available to offset heat gains the following day and keep temperatures within comfort levels.”
The higher the thermal mass of the building the more cooling it can store.
Barnard goes on to say that it is most suitable for offices and when using night ventilation in conjunction with HTM, you reduce the requirement for mechanical cooling, yet again making the building more economical and environmentally friendly
a.) Cellular beams have an advantage, to the Structural Engineer, in having a higher strength to weight ratio than a standard UB section.
What benefit does their use give to the Services Engineer?
b.) A services shaft makes a convenient means of routing M & E and waste disposal services to each floor of a multi-storey building for the Services Engineer. What benefit can this be to the structural Engineer, particularly if integrated with the lift and stair wells?
a.) Ever since cellular beams were introduced in 1987, service engineers have benefited as they allow services such as lighting and electrical, to be passed through the circular openings in the beam. Universal beam (UB) sections do not have openings and are therefore less convenient for services engineers as they need to pay more consideration in how they are going to arrange the services around them. The diagram below illustrates both types of beam. A suspended ceiling may be used to hide the services, when a universal beam is used. This would be another expense involved in the project and in a multi-storey design can reduce the amount of storeys, due to the reduced floor to ceiling height.
b.) Structural engineers benefit when a service shaft is incorporated in the lift and stair well of a multi-storey building as it makes designing and construction less challenging as they do not have to consider the location where they are going to integrate the service shaft i.e. preventing the necessity of drilling holes in ceilings or floors etc. for services as the void provided by the lift or stair well, is ideal. A services shaft will also reduce any conflict between the designs of the structural and services engineer as they both know where the services are going to be located and conveyed from on each floor. Maintenance is also easier to carry out as the service shaft is in one specific place, and not all over the building.