4CN002 Construction Technology & Materials

Transcription

1 4CN002 Construction Technology & Materials April 23 A technically and mathematically accurate report demonstrating knowledge of interaction and integration of structural and non-structural building components in relation to a 20 No. build housing development for a housing association by Jonathan Baker; ID# FAO: Ant Hatfield

2 Contents Given Specification... 1 Construction Choice : Foundations... 3 Strip Foundation Pile Foundation Raft Foundations Conclusion : Walls... 9 Brick & Block Cavity Wall Timber Frame Structural Insulated Panels : Roof Type : Thermal Loss a) Heat Loss from proposed building b) Improvement to reduce heat loss c) Modification to reduce CO 2 emissions References: Bibliography: Appendix A: Foundation Choice Appendix B: , Director of Keller Foundations Appendix C: Stone Columns Appendix D: Wall Technologies Appendix E: Thermal Calculations... 29

3 Given Specification INVESTIGATIVE FINDINGS REGARDING SITE AND BUILD MATERIALS Site investigation and ground conditions 1. The Greenfield site consists of a gently sloping site. 2. Trial pit logs show that the site consists of very soft clay with the pockets of compressible fill materials to a depth of 2 metres encountered across the whole site. House dimensions and materials specifications Dimensions: Length 6.5m, Width 5m, Height 5m Wall Construction: 25mm Render (k=0.94 W/m K); Brickwork (k=0.77 W/m K); 50mm clear cavity (R=0.180 m 2 W/m K); 50mm Insulation board (k=0.020 W/m K); 115mm Aerated block (k=0.11 W/m K); 20mm Internal lightweight plaster (k=0.18). Internal boundary layer resistance (Rsi) = m 2 K/W; External boundary layer resistance (Rso) = m 2 K/W; Windows: 4 no. (one in each wall) size: 1m x 2.5m (U value = 5.8 W/m 2 K) Door: 2 no. size 1.75m x 0.8m (U value = 2.0 W/m 2 K) Floor rate of heat loss = 313W Roof rate of heat loss = 628W Internal Temperature 23 C External Temperature 9 C Rate of air change: 1.5/hour 1 P a g e

4 Section A Construction Choices Construction Choice Construction Technology around the world is continually evolving along with stricter environmental and sustainability criteria. The prime outcome for a developer of any structure is profit or investment. The following section of the report discusses the most common form of construction technology for energy efficient housing association units. We will cover Foundations, Walls & the Roof types with recommendations. 2 P a g e

5 1: Foundations The function of any foundation is to safely sustain and transmit to the underlying ground on which it rests the combined dead, imposed and wind loads in such a manner as not to cause any settlement or movement in the house above which may impair its stability or cause damage to any part of the building (Chudley, 2008, p206). 3 P a g e

6 The three most commonly used types of foundation utilised for low-rise, cavity wall construction are: Strip Foundation. 1.1 These are suitable for most subsoil s and light structural loadings such as those encountered in low to medium rise dwellings where mass concrete can be used. Reinforced concrete is usually required for all other subsoil s. 1.2 If the width of the strip foundation doesn t need to be too much wider than the wall being supported then a strip of plain (unreinforced) concrete can be used as is typically the case in the UK where foundation widths are between 300mm and 600mm (Bryan, 2005). 4 P a g e

7 1.3 A deep fill strip foundation is known as a Trench Fill foundation. 5 P a g e

8 Pile Foundation. 1.4 Foundation solutions to depths of up to 4500mm are useful for sites with problems associated with shrinkable clays. An alternative method to the trench fill is to provide piles to take the loads down to the safe bearing soil with a beam across the top of the piles. Up to depths of around 4500mm this can be achieved with what are known as short bored piles (Bryan, 2005). (Emmitt and Gorse, 2005) 6 P a g e

9 1.5 An alternative system would be to use the Vibro Replacement / Stone Column Technique, significantly reducing the amount of consolidation settlement that can occur; stone columns may typically be 3 4 meters in length for a low rise building (Friel Construction Ltd, 2012) (Appendix C) Raft Foundations. 1.6 These combine the ground floor and the foundations into one continuous structural unit. All of the loads combine and are spread out under the raft so that the entire footprint of the house is the foundation bearing area. These can be used for lightly loaded buildings on poor soils. 7 P a g e

10 Conclusion. 1.7 After analysing the site conditions (Appendix A), it would seem most cost effective (Taylor, 2012) (Appendix B) to use vibro-replacement piling to create reinforced stone columns within the ground spanned by stepped reinforced strip foundations whereby the ground floor construction of beam and block technology could then be installed (see images below). (Stepped, stone column foundations prior to 25mm weak mix concrete blinding and reinforced strip foundations) Photo by Jonathan Baker - 17/April/ P a g e

11 2: Walls The most common form of external wall used in UK domestic properties is the masonry brick and block cavity wall common from the 1920 s. Two alternative types of residential wall are the timber framed wall and the structural insulated panel (SIP s) wall, originating in Scandinavia and North America since the 1960 s. There are many others (Appendix D) that are not considered for this project. 9 P a g e

12 Brick & Block Cavity Wall. 2.1 The predominant factor in the design of a wall is whether it is load-bearing or serves only as an enclosing feature. All walls need to be sufficiently strong to carry the self-weight of the structure, together with imposed loads and loads from wind. 2.2 The external brick work acts as an aesthetically pleasing surface and also operates as part of the load bearing structure for the dead and live loads of the property and acts as the main barrier to environmental elements such as rain, wind, and snow. Advantages: Very good fire resistance Restriction of moisture passing through the wall Good thermal insulation Skills required to build are widely available across the UK Provides a feeling of solidity High level of acoustic mass helping deaden outside noise Provides solid fixing for built-in furniture, curtain rails, pictures, etc. Disadvantages: Corrosion of wall ties Damp caused by careless building practice Dense concrete blocks are a poor insulator & require insulation to meet building regs Insulation is generally limited to 100mm and comes with damp penetration concerns Requires time to dry out between block work lifts, thus slower than timber construction, susceptible to freeze thaw Risk of cavity bridging and mucky ties from mortar drops Cannot be constructed during heavy rain or freezing conditions (National Energy Services Ltd, 2008). 10 P a g e

13 Timber Frame 2.3 Factory made panels are based on a stud framework of timber, an outer sheath of plywood, particleboard or similar sheet material, insulation between the framing members and an internal lining of plasterboard. An outer cladding of brickwork weather proofs the building and provides a traditional appearance (Chudley, 2008). (Timber frame construction with brick outer skin) 11 P a g e

14 Timber Frame Advantages: Aesthetically pleasing Low thermal conductivity Little or no disposal problem Renewable/sustainable High strength to weight ratio Easily worked and joined Strong joints can be formed with modern adhesives Good fire resistance Disadvantages of Timber frame: Variable Dimensional changes due to corresponding changes in moisture content Subject to creep under load Combustible in untreated state Subject to organic attack (fungal/insect) 12 P a g e

15 Structural Insulated Panels 2.4 SIPS panels are similar although they are pre-fabricated and remove the timber framing and thus any cold-bridges. SIPS panels are very energy efficient, timber is not particularly good at retaining heat, so the fact that SIPS have fewer timber studs, timber sole plates and other timber members mean that overall heat loss is reduced. Because the SIPS jointing system provides a good thermal break this also reduces heat loss, thus more heat is retained inside the building (Building & DIY, 2012). 2.5 SIPS consist of two parallel faces - usually Oriented Strand Board, sandwiching a rigid core of Polyurethane (PU) foam or Expanded Polystyrene (EPS), making them lightweight, quick to erect and free from problems of compression shrinkage and cold bridging associated with other forms of construction (UKSIPS, 2012). 2.6 Some self-builders have now specified homes without conventional central heating systems, relying on the insulation and airtightness of SIPs, combined with mechanical ventilation system with heat recovery. Supplementary heating is usually specified in just the bathrooms. SIPS panels can be erected with fairly unskilled tradesmen, even DIY enthusiasts (CIOB, 2012). 13 P a g e

16 SIPS Advantages: Habitable roof space Low wastage Quicker construction High level of air tightness Limited cold bridging High thermal performance Can be constructed during heavy rain and freezing conditions Disadvantages: Usually a waiting period of 12 weeks There cannot be any on-site changes; the client has to make all the decisions about the design before the build begins. Oriented strand boards may be constructed with formaldehyde based resins (Kincaid, 2009) Subject to organic attack (fungal/insect) if untreated 14 P a g e

17 3: Roof Type The Roof should provide resistance to excessive heat loss, at least to the standards set out in Approved document Part L of the building regulations. The Roof structure should also provide resistance to failure due to overstressing. It must be able to support its own weight and that of wind loads, and imposed loads such as snow (Chudley, 2008). 15 P a g e

18 Trussed Rafters 3.1 The roof of the proposed properties will be of the trussed rafter type in a queen post trussed configuration. This choice can be designed and ordered from manufacturers. The roof trusses will be prefabricated and delivered to site when required and hoisted by crane onto the superstructure. The Queen Post truss shape is suitable for spans up to 6 meters on domestic type structures (Minera, 2012). 3.2 The pre-fabricated trusses are spaced out at 600mm centres and fixed into place by a roofer with bracing supports ready to be felted, battened and tiled. 3.3 This method of construction means that trusses can be fully erected and spaced in 2-3 days in good weather conditions, with a further 2-3 days to finish off with felt, battens and tiles depending on the number of labourers. 3.4 Final design of the roof should also take into consideration the environmental elements such as weather. Areas of Britain are categorised into average windspeed regions, thus limiting the heights of some building types (Approved Document A), and would be useful to reference in consideration to local wind speeds, and the negative vertical loads imposed upon the roof element. 3.5 Roofs are typically tied down to the superstructure using metal straps spaced at maximum 2 metre intervals (NHBC, 2010). 16 P a g e

19 Image Elements: 1. Wall Plate 2. Min. 1000m Roof Straps (1500mm) 3. Trussed Rafter 4. Internal lightweight block wall (support wall) 5. Internal dense block party wall 6. Gang nail plate 7. Truss clip 17 P a g e

20 Section B Thermal Losses 4: Thermal Loss Heat is a form of energy associated with the continual movement of the molecules in a solid; liquid or gas. Heat flows from hot materials to cold materials, i.e. for heat flow to occur there needs to be a temperature difference ( ). The greater the the faster the rate of heat flow, and vice versa. 18 P a g e

21 4.1 Calculations for all thermal loss are shown in Appendix E. A brief summary of the results follow below. a) Heat Loss from proposed building The U Value for the proposed building is: U = W/m2K Total fabric heat loss for the building is: watts Total ventilation heat loss for the building is: watts The total building heat loss rate for the specified units is watts Total heat loss over the 20 plots would equate to: 66,004 watts, or 66kW. b) Improvement to reduce heat loss 4.2 Improve the thermal efficiency of building elements by using better insulation or better glass. Areas of a home that can be insulated are the loft, walls, and floors providing lots of choice for heat loss reduction. This is also one of the cheapest options to implement in the design stage. 4.3 Should the client be willing to make a large change to the building construction plans then SIPS technology would be highly recommended. Any extra cost in materials is usually more than saved in labour and the time to construct. c) Modification to reduce CO 2 emissions 4.4 Install Solar Photovoltaic Panels upon the roof to reduce the amount of electricity used within the unit, thus reducing its overall Carbon footprint. 4.5 I would recommend this technology as a home developer should get reduced rates on bulk purchases, and the government is currently implementing a feed in tariff scheme whereby home owners are paid for every unit of their own electricity that they produce and use enhancing the market value of the units built. 19 P a g e

22 References: BRE. (2012) The Barratt Green House The first code for sustainable homes level 6 house built by a major homebuilder. [online]. [Accessed 2 February 2012]. Available at: < Building & DIY (2012). SIPS (Structural Insulated Panel Systems). [online]. [Accessed 14 March 2012] Available at: < -sips/sips-structurally-insulated-panel-systems-429/local> Chudley, R. and Greeno, R. (2008) Building Construction Handbook. 8 th ed. Oxford: Elsevier. CIOB, (2012) CPD Event: Innovations in Concrete. University of Bolton, Bolton 2 nd February. Camberley: The Concrete Centre Emmitt, S., and Gorse, C. (2005). Introduction to Construction of Buildings. Oxford: Blackwell Friel Construction Ltd (2012) Foundation & groundworks. Barratt Homes, The Lyng, West Bromwich. 3 rd April Walsall: Friel Construction Ltd. Kincaid, L. (2009) Easy ways to reduce formaldehyde from building materials. [online]. [Accessed 23/April/2012]. Available at: < Minera (2012) Truss Shapes / Profiles Minera Roof Trusses. [online]. [Accessed 22 April 2012]. Available at: < National Energy Services Ltd (2008) Cavity Walls. [online]. [Accessed 15 April 2012]. Available at: < Taylor, D. (2012) to J. Baker, 3 rd April UKSIPS. (2012) What are Sips? [online]. [Accessed 14/March/2012]. Available at: < 20 P a g e

23 Bibliography: American Lung Association (2012) Formaldehyde. [online]. [Accessed 23 April 2012] Available at: < EHSO (2011) Formaldehyde In the Home. [online]. [Accessed 23 April 2012]. Available at: < 21 P a g e

24 Appendix A: Foundation Choice An important factor in determining the foundation type is the founding depth, or the depth below ground where the foundation is resting on sufficiently stable soil mass. The frost line is typically 500mm, and the influence of seasonal moisture variations in clay is 1000mm (Bryan, 2005). Another consideration for us to look at is the sloping site. This will affect foundation depths also, the specification depth below ground will have to be for the lowest part of the site, and so if the whole strip is laid at the same level, the depth of the foundation will increase at the rate that the level of the ground raises (Bryan, 2005). As can be measured from the image below, the ground level rises/falls at a Horizontal angle of P a g e

25 We can therefore determine that in a property of size 6.5m length, the difference between ground level for foundation depths can be calculated as follows: tan6.8 tan Using Pythagorean Theorem and Trigonometry we can determine that the height of O is mm, or 0.78m. Therefore for a foundation to be at the founding level of 2000mm we know that the deepest side of the foundation will be at mm and the other end of the foundation will be at 2000mm to create a level strip foundation. To limit the depth on a sloping site, the foundations can be stepped when there is sufficiently firm subsoil. Deep Strip / Trench Fill Foundation. Deep Trench Fill foundations can be placed up to 4500mm. If the trenches can avoid spalling during excavation and pouring of concrete then this is a good solution. Otherwise consideration needs to be given for machinery type required; extra room in the trench for working at depths and temporary earthwork supports. These methods add additional health & safety costs and financial cost, but ultimately production conditions are the key. 23 P a g e

26 Pile Foundation. Foundation solutions to depths of up to 4500mm are useful for sites with problems associated with shrinkable clays. An alternative method to the trench fill is to provide piles to take the loads down to the safe bearing soil with a beam across the top of the piles. Up to depths of around 4500mm this can be achieved with what are known as short bored piles. By limiting the length to around 4500mm and the diameter to around 350mm, piles can be achieved with relatively small rigs. If greater depths and/or diameters are required then full piling rigs are required. Precast beams have to span the bored piles to support the walls. If the foundations are in shrinkable clay with the beam near the surface, then shrinkage and heave could take place under the beam and around the top of the pile (Bryan, 2005). The alternative to providing deep foundations is to investigate the use of surface floating rafts. Raft Foundations. These combine the ground floor and the foundations into one continuous structural unit. All of the loads combine and are spread out under the raft so that the entire footprint of the house is the foundation bearing area. This gives a safe solution on soils with a limited bearing capacity and has a potential to accommodate differential settlement without transferring the strains into the superstructure. Factors affecting Choice. With the extra depth and the subsoil types involved one needs to consider the economics of foundations. If the production process can be carried out from the surface with little risk, they may be less expensive than a raft with formwork, reinforcement and concrete making up the major cost. For the strip foundation options, the most likely solution will be a deep trench strip foundation to founding level of 2 metres. In this type of foundation production is the key. The trench has to be able to be excavated accurately (line, level plumb and width) without the risk of collapse. This has to be achieved in a reasonable time to excavate sufficient length of foundation, confirm the founding depth and pour the concrete to return the ground to a safe condition. 24 P a g e

27 These conditions can be fulfilled in most clay soils, but the time factor may limit the depth to which this form of foundation can be undertaken as the longer the trench is left open the greater the risk of collapse as the moisture content of the clay changes. As long as the trench can be cut cleanly and accurately the quantity of concrete can be controlled, limiting the financial as well as the health and safety risks (Bryan, 2005). Other factors affecting choice may include the chemical constituents of soils and their reactiveness to concrete. For example, water soluble sulphates in soils such as plastic clay can react with ordinary cement and in time will weaken concrete (Emmitt and Gorse, 2005). Technology choice for foundation will also be dependent upon site type, for example an extension or small satellite building at the rear of a domestic property with limited access would not use short-bore piles, or vibro replacement stone columns because of the limited access for machinery and plant, therefore trench fill foundations would be more cost effective. 25 P a g e

28 Appendix B: , Director of Keller Foundations. 03 April :06 Jonathan, I can give you some guide costs: Typical for Housing is as follows- Detached houses Terraced 1000 each Industrial Units per m2 All of these plus 3k mobilisation Hope this helps Kind Regards Derek Derek Taylor Director Keller Foundations Mobile *Removed for Data Protection* *Removed for Data Protection* Web 26 P a g e

29 Appendix C: Stone Columns In wholly granular soils the effect of the vibrations is to produce a marked improvement in the relative density of the soils, thus significantly improving the allowable bearing capacity and settlement characteristics. In contrast, cohesive or clay type soils are unaffected by the vibrations and constructing the stone columns does not appreciably alter the shear strength of the clay in the short term. The clay soils do, however, confine the stone column and this passive resistance allows the stone column by virtue of its dense granular nature to develop a high bearing capacity relative to the surrounding clay. In this way an overall improvement in bearing capacity is obtained and at the same time the stiffness of the stone columns reinforces the clay therefore significantly reducing the amount of consolidation settlement that can occur. On most sites underlain by made ground, the fill is generally a mixture of these extreme granular and cohesive soil types, but nevertheless the formation of dense stone columns at appropriate intervals beneath the applied loadings strengthens the soil mass and generally ensures that total and differential movements are well within acceptable limits. The stone columns are usually constructed directly beneath main load bearing foundations, either in single or multiple rows beneath strip foundations or in groups beneath pad foundations. Beneath floor slabs the stone columns are usually installed on a rectangular grid with a triangular pattern sometimes being adopted beneath large areas of high intensity loading such as oil storage tanks. The spacing and arrangement of the stone columns is dependent upon the soil conditions, the size of the foundations, the applied loadings and the settlement criteria for the structure to be supported on the treated ground. 27 P a g e

30 Appendix D: Wall Technologies Many other methods of constructing walls exist and are being used around the UK at the moment from Steel frame to Hemp walls. Some construction methods are more adoptable to hotel and apartment structures, there is no real reason they could not be used in smaller residential dwellings, other than production cost. Thin Joint Technology focuses on much larger aerated blocks or very large storey high aerated panels. Instead of a 10mm mortar joint between all bricks, the blocks are set on a 10mm mortar mix against the ground floor slab, and then are fixed with a 2-3mm jointing compound, almost like glue. The thinner joints allow less thermal loss and moisture ingress than a traditional 10mm joint. Also, because of the reduced number of joints in a 1m2 area, the thermal efficiency is much greater. This method was used on the Gaunt Francis/Barratt s Green Home at the BRE Innovation Park and achieved a Code 6 on the government s code for sustainable homes (BRE, 2012). Another new technology is called Insulating Concrete Formwork. These are like large polystyrene Lego bricks. They consist of an inner cavity that once in place are filled with concrete. The hollow brick like structures act as formwork that is left in place, and doubles as an insulating barrier from thermal loss. An external block work wall can be placed outside and tied in, allowing the external surface to be rendered or clad in a number of materials. Other wall technologies reserved for hotels, prisons, and large blocks of accommodation/apartments are Cross wall construction and Tunnel form construction. 28 P a g e

31 Appendix E: Thermal Calculations Several methods can be adopted to calculate the total rate of heat loss. Firstly we need to calculate the thermal conductivity of building elements and their constituent parts, and then we can measure the thermal energy efficiency or U value. Using the U Value, the heat flow rate can be calculated and with this value and the ventilation heat loss rate we can calculate a buildings total heat loss rate; as follows ( ) Firstly we need to ensure that we have all of the R Values. Some are given, and some need to be calculated. The value of (R) is calculated by dividing its thickness (L) by its thermal conductivity ( ). Calculated Values: 102mm Brickwork (R): = m 2 K/W; 25mm Render (R): = m 2 K/W; 50mm Insulation board (R): = 2.5 m 2 K/W; 115mm Aerated block (R): = m 2 K/W; 20mm Internal plaster (R): = m 2 K/W; Given (R) Values: 50mm clear cavity: R = m 2 K/W; Internal boundary layer resistance (Rsi) = m 2 K/W; External boundary layer resistance (Rso) = m 2 K/W; Windows: 4 no. (One in each wall) size: 1m x 2.5m (U value = 5.8 W/m 2 K); Door: 2 no. size 1.75m x 0.8m (U value = 2.0 W/m 2 K); 29 P a g e

32 Therefore, U can be calculated as follows for the walls Element Thickness (m) Thermal Conductivity (k) Thermal Resistance (R) Internal boundary layer (Rsi) Internal Plaster Aerated Block Insulation Board Clear Cavity Brickwork Render External Boundary Layer (Rse) Total Resistance: Wall Construction ) U = W/m 2 K Next we can use the U values, and temperature difference with the areas of each element to calculate the heat loss through each individual element before calculating the total fabric heat loss. Building Element Area (m 2 ) U Value (w/m 2 K) Temperature Difference Heat Loss (watts) Wall (28.6) Wall (28.6) Wall (22.5) Wall (22.5) Window Window Window Window Door Door Roof Floor Total Fabric Heat Loss: P a g e

33 To calculate Ventilation Heat Loss Rate (Pv) we combine heat capacity and seconds in an hour to give a factor of (0.33). 0.33) ( )= 0.33 x 1.5 x x watts So therefore the Total Building Heat Loss Rate: { ( )} + {0.33 ( )} Total Building Heat Loss Rate = watts 31 P a g e