BUILDING CONSTRUCTION I

Transcription

1 1 UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION REVITALISATION PROJECT-PHASE II NATIONAL DIPLOMA IN BUILDING TECHNOLOGY YEAR I- SE MESTER I THEORY/PRACTICAL Version 1: December 2008 BUILDING CONSTRUCTION I COURSE CODE: BLD103

2 2 TABLE OF CONTENTS WEEK 1: WEEK2: WEEK3: WEEKK4: WEEK5: WEEK6: WEEK7: WEEK 8: WEEK 9: BUILDING COMPONENTS (1.1) Building components (1.2) Major building components (a) Foundation (b) Floor (c) Wall (d) Door (e) Windows (f) Roof PRELIMINARY SITE ACTIVITIES (2.1) Site activities that precede Actual building construction (2.2) Provision of facilities of on site SITE ORGANISATION AND LAYOUT (2.3) Site layout and organization SETTING OUT OF BUILDING (2.4) Setting out of building EXCAVATION (3.1) Method of Excavation (3.2) Tools used in manual Excavation (3.3) Equipment used in mechanical Excavation EARTHWORK SUPPORT (3.4) Method of Earth (3.5) Foundation FOUNDATION (3.6) Importance of foundation to building (3.7) Types of foundation soil (3.8) Simple calculation of the area of concrete foundation TYPES OF FOUNDATION (3.9) Types of foundation and their application TYPES OF FOUNDATION (CONTINUED) WEEK10: WEEK11: METHODS OF REINFORCEMENT IN SUBSTRUCTURES (3.10) Methods of reinforcement in Substructures CONSTRUCTION OF FOUNDATION

3 3 (3.11) Method of construction of foundation WEEK12: WEEK13: WEEK14: WEEK15: DAMP PROOFING (4.1) Rising damp and seepage of ground water in building (4.2) Damp proof course (DPC) and damp proof membrane (DPM) (4.3) Function of damp proof courses MATERIALS USED FOR DAMP PROOF COURSE (4.6) Materials used for damp proof course BASEMENT TANKING (4.4) Tanking in basement work HARDCORE (4.7) Hardcore layer (4.8) Blinding (4.9) Termite Treatment

4 4 WEEK 1: BUILDING COMPONENTS (1.1) Building Components A building acts as an enclosure for the activities that of on within, it building will protect the occupants, equipment or goods housed within from the various of he external climate (rain, wind, sun etc). For a building to act as enclosure, it must have external walls and be covered by a roof. The roof will normally rest on the walls and be support by them. The walls in turn, will need a firm base or foundation to be built upon, which will transfer their weight and that of the roof to the ground beneath. To make the building usable, the internal space enclosed by the external walls and roof may need to be sub-divided into room by the introduction of horizontal dividers between storey s, the floors, and vertical dividers between rooms, the internal walls. Stairs or lift can provide access between storeys. Doors can provide access to the building and to each room within the building. Daylight and ventilation can be introduced into the building by the provision of windows in the external walls or roof. All these part of the building are referred to as building component or elements (1.2) Major Building Components The major building components are as listed below. Foundation Floor Wall Door Window Fenestration (Other openings) Roof Ceiling Foundation The function of the foundation is to transmit the load from the building finally to the soil. This, it must do without excessive settlement and compression of the supporting soil layer.

5 5 Functional Requirements of Foundation Ability to carry loads with a minimum movement Adequate width to safely transmit the loads on it to the supporting soil layer Functions of Foundations Foundation provides suitable support and stability for building Transmits to the ground all the loads that come on the building over a sufficient area of subsoil Prevents the failure of the building or uneven settlement. The selection of foundation types is influenced by The type of building The nature of the loading The site condition Foundations are of many types but the more common ones include Strip foundation Pad foundation Pile foundation Raft foundation Fig.1.1.Strip foundation

6 6 Floor Floor can be defined as the horizontal structure which carries imposed and live loads in a building and divides a building into storeys. It plays an important role in a building. The most common material used for the construction of floors that will meet the requirement of building regulations and local bye-laws are concrete and wood. Functional Requirements of Floor Adequate strength and stability to support the loads that comes on it. Resistance to sound penetration Resistance to moisture penetration Thermal resistance Fire resistance Durability Hard wearing Wall This is usually the vertical continuous part of a building which encloses or protects the building or divides the building into rooms and compartments. It is made up of blocks, concrete, mortar, stones, metals etc. types of walls include: Internal, External, Buttress, Sleeper, party, parapet partition and cavity walls.

7 7 Fig 1.2 Internal and external walls Functional Requirements of Walls Openings for daylight and ventilation Fire resistance to provide security and stability in the event of fire Adequate strength to resist being crushed by the loads from floors and roofs they support Durability to withstand the condition under which it will function Adequate stability to resist other forces such as wind pressure and roof loads. Door A door is a movable barrier placed across an opening in a building that provides access into the building or between spaces within the building. Functional Requirements of Door Weather resistance Durability Fire resistance It must be easy to slide open and close

8 8 Fig 1.3 Example of a Paneled door Windows Windows provide natural light and ventilation to the interior of a building while excluding rain and insects. Windows are usually made of timber, steel. But other materials such as plastics (upvc) and aluminium are also popular. Each material has its own advantages and disadvantages. Functional Requirements of Window The minimum area of window in a habitable room should be 10 percent of the floor area The minimum opening area of the window in a habitable room should be 5 percent of the floor area. Adequate security against intruders Adequate resistance to external weather elements Provide privacy.

9 9 Fig. 1.4 Parts of a window Roof This is the topmost covering in a building. It is done during the finishing of a building. It prevents a lot of sound (minimizing incoming sound), dust, wind and rain and also it helps the occupants cover their privacy. Fig. 1.5 The main parts of the structure of a pitched roof. Functional Requirements of Roof Strength: of roof depends on the characteristics of the materials from which it is constructed and the way in which they are put together in the form of a flat or some form of triangular frame.

10 10 Stability: a roof is constructed to support the dead load of the roof structure and its covering, insulation and internal finishes, snow loads and pressure of suction due to wind without undue deflection or distortion. The dead load can be calculated from the unit weight of materials with which it is covered, varying from the continuous impermeable layer of asphalt covering that can be laid horizontal to exclude rain, to the small units of clay tiles that are laid overlapping down slopes so that rain runs rapidly to the covers. Weather Resistance: A roof excludes rain through the materials with which it is covered; varying from the continuous impermeable layer of asphalt converging that can be horizontal to exclude rain to the small units of clay titles that are laid overlapping down slop so that rain runs rapidly to the covers. Durability: The durability of a roof is dependant largely on the ability of the roof covering to exclude rain and snow. Persistent penetration of water into the roof structure may cause or encourage decay of timber, corrosion of steel or disintegration of concrete. Fire Safety: The requirements for control of spread of fire in schedule of the building regulations for dwelling houses limit roof construction relative to the proximity of boundaries of the site of the building by reference to the materials of roof covering. Thermal Resistance: The materials of roof structures and roof covering are generally poor insulators against the transfer of heat. It is usually necessary to use some materials which are good insulator. Examples of such materials include light weight boards, mats or loose materials. This is to provide insulation requirements to meet the building regulation for the insulation of roofs of dwellings is a standard value of 0.25 where the SAP ceiling is over 60. Sound Insulation: The resistance of a roof to the penetration of airborne sound is not generally considered unless the building is close to a busy airport. The mass of the materials of a roof is the main constructions in the reduction of airborne sound. Test Questions i. List the basic building components. ii. State the functional requirements of four building components.

11 11 WEEK 2: PRELIMINARY SITE ACTIVITIES When a builder takes possession of a building site, he it usually provided with a site lay-out plan and the drainages necessary for the erection of the building. Having taken over the site, the task of preparing for and setting out the building can be started. Taking over the site includes having the providing access road to the site to allow the movement men, machines and materials to the site. (2.1) Site Activities that Precede Actual Building Construction The following activities precede actual building construction on site Provision of access road Site clearance Provision of site offices and storage facilities Provision of site services Site Clearance The preliminary works on a construction project site usually begin after the sit facilities have been set up. Clearing the site is essential. First, the vegetation such as bushes and shrubs should be removed. The roots of trees and bushes must be dug out and cleared away. Site clearance also may involve the demolition of existing buildings. demolition is a skilled occupation and should be tackled by experts in that area. The top soil should also be removed up to a depth of at least 150mm to remove any plant life and decaying vegetable. The presence of vegetation and decaying materials means that the top soil is easily compressible and cannot support building foundation. Top soil is however valuable as a top dressing for gardens and may be disposed of in this manner The site needs to be cleared of rocks and boulders in the area where the building will be set out. If they are too large, then the boulders or rocks must be broken into smaller pieces and taken away. Site clearance is done by a combination of manual and mechanical means. The method adopted will be determined by the overall economics which may be influenced by the scale of development ant consideration for any adjacent buildings.

12 12 (2.2) Provision of Facilities on Site A well managed site should have facilities and services which will make the site functional and convenient. On a building construction site, the following facilities and services should be provided. Temporary services: These include the provision of such services as water supply and, electricity supply. Water is required on construction site for drinking and for the works. Where the site is close to the public water mains, the water can be connected directly from the public water mains to the site. The stand pipe should be located close to where the mixing of concrete and mortar will take place. Where the site is far from the public water mains, water can be supplied to the site with use of water tankers. The water can be stored in reservoir provided on the site for such a purpose especially for the works. An electrical supply for power tools, electricity can be supplied from the mains or a petrol generator. A telephone line should be provided which is secured so that it can only be used for official or authorized calls. Temporary access road: This should be provided to the site for the purpose of providing access to vehicular traffic that will be bringing men and materials to the site. The access should be constructed so that vehicles can enter the site in all weather. The access road can become part of the permanent site services in the final design. Site accommodation A site should have an office and sheds for the workers on site to change their clothes and to take rest on site during breaks from work and also have their meals. Site accommodation and similar facilities provided on a site depends on the number of people that are working on the site. Units of accommodation come usually in two forms Sectional timber huts Mobile caravans or cabins

13 13 Sectional timber huts are prefabricated for ease of dismantling and assembly to facilitate the reuse on other sites. Huts of this nature should be designed, constructed and maintained with the same care as permanent buildings to ensure their use for many years on a number of different construction site. A well designed sectional hut should permit the addition of more bays to increase the modular size by length and/or width. The anticipated use of each hut will govern the construction and facilities required. Offices need to be weatherproof, provided with artificial lighting, equipped with furniture that might be required on the site. Similar basic construction can be used for other units of accommodation such as meal rooms, and toilets should be provided and equipped with the basic facilities. Caravan and mobile cabins are available in a wide variety of sizes, styles, and application. The construction is most times of a plywood clad timber frame suitably insulated and decorated. They are usually made of modular system so that by using special connection unit any reasonable plan size and shape is possible. The caravan and cabin are fully equipped with all the necessary furniture light and heating units. The toilets can be connected to site services or be self contained. Material Storage: The type of storage facilities required on a construction site for any material depends on the following factors Durability ie whether it will need protection from the elements Vulnerability to damage Vulnerability to theft Cement, plaster and lime supplied in bags form require a dry store free from draughts which can introduce moist air and cause air set of the material. These materials should not be stored on the site for long period of time on site; therefore provision should be made for rotational use so that the material being used comes from older stock. Aggregates such as sand and gravels require a clean firm base to ensure that foreign matter is not included when extracting materials from the base of the stock pile. Different materials and grades should be kept separated so that the ultimate mix batches are consistent in quality and texture. Care must be taken to ensure that the stock piles are not used as refuse dump. The moisture content of the aggregates should also be taken into consideration if it is exposed to the

14 14 elements like rain, so as to allow for it in deciding the water cement ratio of the mix. Bricks and blocks should be stacked in stable piles on a level and well drained surface in a position where double handling is reduced to a minimum. Facing bricks and other coloured bricks should be covered with tarpaulin to protect them from being discoloured by the weather elements. Blocks should be stacked in such a way as to allow for air to flow freely through the stack. Timber absorbs water easily. To prevent undue moisture movement it should be stored in such a manner that its moisture content remains fairly constant. A rack scaffold tubulars with a sheet roof covering should be used to store timber. The sheet roof protects from rain and the various sizes allow for free flow of air round the timber. Ironmongery, hand tools and paints are some of the most vulnerable materials on site. Some materials such as locks, power tools and cans of paint should be kept in a locked shed or inside any of the completed rooms in the building under construction. Test Questions i. List the activities that precede actual building construction on site. ii. What are the temporary services needed on construction site? iii. How is storage provided for the basic materials on construction site?

15 15 WEEK 3: SITE ORGANISATION AND LAYOUT (2.3) Site Layout and Organisation The building site can be considered a temporary factory, where the building is produced this activity to take place the builder requires men, materials and plants. All these have to be carefully controlled so that the men have the right machine in the most adequate position, the materials stored so that they are readily available and not interfering with the general site circulation a building and the total size of the site on which the building is to be erected. Therefore is no standard size ratio between the free site spaces required considered as a separation problem in terms of allocating space for men, materials and plant. To obtain maximum efficiency there is an optimum way of laying out the site and also a correct amount of expenditure to support the proposed site layout. Any planned layout should be reviewed periodically and adjusted to suit the changing needs of the site activities. A careful consideration of planning and control of this aspect of the building construction will reflect in the progress and profitability of the building project. Factors to Be Considered in Site Layout Before any specific considerations and decisions can be made regarding site layout a general appreciation should be obtained by conducting a thorough site investigation to formulate how the job will be executed. This will involve the assessment of the plants and equipment that will be utilized to execute the work. Specifically the considerations include the following: Access Consideration: this must be considered for both on and off site access. Routes to and from the site must be checked as to the suitability for transporting all the requirements for the proposed work. Access on site for deliveries and general circulation must also be carefully considered so that vehicles delivering materials to the site will do so without difficulty or delay. If is anticipated that large vehicles will be operating on the site it will be necessary to consider the road surface required. If the road and the paved areas will form part of the permanent work there should be constructed earlier in the work. If the anticipated traffic at the end of the work is lighter than the one expected during construction enough protection should be given to the road against the effect of the heavy traffic load. Storage Considerations: the amount and types of material to be stored, security and weather

16 16 protection requirements, allocation of adequate areas for storing materials and allocating adequate working space around storage areas required, siting of storage areas to reduce double handling to a minimum without impeding the general site circulation and/or works in progress. Accommodation Consideration: number and type of site staff anticipated, calculate size and select units of accommodation and check to ensure compliance with the minimum requirements of the relevant construction regulations. Select siting for offices to give easy and quick access for visitors and at the same time giving a reasonable view of the site. Select site for resting sheds and toilets to reduce walking time to a minimum without impeding the general site circulation. Temporary Services Considerations: what, when and where are they required? Possibility of having permanent services installed at an early stage and making temporary connections for site use during the construction period. Coordination with the various service providers is essential. Plants Considerations: the type and nature of plants and where they will be required on the site are important. Whether the plants will be static or mobile. If static the most appropriate position should be selected and hard standing should be provided. If mobile the circulation routes should be checked for optimum efficiency and stability. Provision of space and hard surface for plant maintenance should also be considered. Fencing and Hoarding Considerations: distinction should be made between what is mandatory and what is desirable. These will depend on vandalism record in the area, type of fence or hoarding required possibility of using fencing which will form part of the permanent work by erecting it at the early stage of the work. Safety and Health Considerations: it should be ensured that all the above considerations comply with the relevant construction safety regulations. On taking over the site one of the first jobs is to layout the site boundaries as they are marked out

17 17 on the drawings. The security fence should be set up around the site so as to control the movement of people and materials. The boundary fence should have only one access so that someone can check people in and out of the site. The site office should be located close to the entrance into the site together with the site sheds where meals will be taken. The toilet on the site should be located at one corner away from where it will constitute a nuisance and to also enhance privacy. The aggregates should be stored close to where the mixing will take place which in turn should be located close to where provision has been made for water storage. Generally materials should be stored to close to where they will be put to use. The site should be laid out in such a way that there will be free movement of vehicles around the site in case it will become necessary to move materials using vehicles around the site. Lay out of Construction Site The layout of every site could be divided into Administrative areas Construction areas Administrative area: this will be the location of offices, stores, sub-contractors huts, canteen and similar accommodation. Construction area: this is the actual site of the building to be constructed, and it will be located close to the consumable stores adjacent to the various buildings and equipment required for the construction purpose. The layout of both these areas form an essential part of the early planning in every construction work, the neglect of which will lead to delay in the initial progress of the job leading to extra wastage of resources on the

18 18 Fig 3.1 A typical site layout Test Question i. Discuss the factors considered in a construction site layout.

19 19 WEEK 4: SETTING OUT OF BUILDING (2.4) Setting out of Building This is the transfer of information on the building drawing to the ground with high degree of accuracy. When the site clearance is completed the setting out of the work may begin. It is necessary to have a good knowledge of geometry in order to ensure accurate work. The first tasked in setting out a building is to establish a base line from which the whole of the building can be set out. The position of this line must be marks on site so that it can be re-established at any time. The building line is frequently determined by the highway authority and in urban areas it is often 8m from the back of the public foot path. If other buildings have been erected at the area the building line can be determined from these existing buildings. After the base line has been established, marked and checked the main lines of the building can be set out, each corner being marked with stout peg. A check should then be made of the setting out lines for right angles and correct lengths. The method of establishing of the right angle is what the setting out sets out to establish in addition to the correct length. Setting out Equipment Measuring tape: before setting out any work the tape would be carefully checked for accuracy. Metallic lines tapes tend to stretch after they have been in use for sometime. The tape is to mark the measurement on the profiles. It should be ensured that each measurement is taken from the extended ring at the end of the tape. Profiles: When setting out a building, it is an advantage if the line can be secured so that they are well clear of the building line. The trenches can then be dug without interfering with the lines. Timber profiles erected for this purpose consist of pegs driven into the ground and boards nailed across them. The lines can then be stretched above the ground level well clear of any obstruction and may easily be checked for accuracy. Datum Pegs: Before starting the actual setting out of the building, it is essential to establish a level on the site to which references pertaining to the levels of elements of work may be made in the course of executing the work.

20 20 In planned cities and town ordinance bench marks are established where a bench mark is far from the proposed site, a levelling instrument such as covering, level can be used to transfer the datum level to the site from where it can be distributed around the proposed site, Datum pegs must be located where it would not be disturbed by the operation on the site. Methods of Setting out a Building There are three methods of setting out on small building sites. These are Using 3,4,5 method Using the builder s square Using the theodolite Setting Out Using the 3,4,5 Method 1. Mark out the building line from the road by measuring the required distance or by stretching a line along the existing buildings to the proposed site. The building line is then represented by the line shown as the ranging line, GG1 (in figure above). 2. Mark out the over all length of the building by driving in pegs at A and B along the ranging line. 3. Produce two steel tapes measured and mark out four equal distances on the ranging line starting from the corner peg at B (4m). 4. Pull a tape measure from point B to C and ask an assistance to hold it ready with a hammer and peg. 5. Pull the second tape from the fourth mark at D on the ranging to point E on the first tape. 6. The distance 5m on tape DE should coincide with point 3m to tape BEC to prove that the angle B is 90 O (from Pythagoras theorem). 7. Repeat the same procedure to obtain the right angle from BAF, and mark out the overall width of the building. 8. Establish corner pegs and erect profiles. 9. Mark the position or partition walls on the profile with either nails or saw cuts. Ranging lines are stretched through these nails and the corner peg to establish the ground to indicate the line of excavation for the foundation trenches.

21 21 Fig. 4.1 Setting out of a building using the 3,4,5 method Setting out a building using the builder s square method 1. Find out the distance from the site boundary to the building line on the working drawings. Use the tape measure to measure the same distance from the site boundary to the location of the building line on the ground. Select a corner on the ground to be corner A. 2. Place a peg in the ground at corner A and hammer a nail into the top of the peg. 3. Repeat these steps to place a peg in the ground for corner B

22 22 4. Tie the string between pegs A and B. Measure the distance between A and B. Check that the distance on the ground is the same measurement as the distance on the drawing. 5. Repeat these steps for corners C and D 6. Measure the diagonals A-D and B-C (the two diagonals should be equal) 7. Use the builder s square to check that the corners are at right angles. Fig.4.2. Setting out using the builder s square Test Questions 1. List the three methods of setting out a building 2. List the equipment used in setting out a building

23 23 WEEK 5: EXCAVATION (3.1) Methods of Excavation Excavation is done to receive the foundation that will be constructed for a building. This is done after the setting out and marking out. Excavation can be done using two methods: Manual method and Mechanical method The choice of the method of excavation depends on the size of work. In small construction, it is more economical to use the manual method, while large works will be economical to be executed using the mechanical method of excavation. (3.2) Tools Used in Manual Method of Excavation The manual method of excavation involves the use of the following tools pick-axes used to dig up the soil shovels to remove the dug up soil spades to level the bottom of the excavation and to load into the wheelbarrow, Wheelbarrows to convey the excavated soil away from the excavation area. (3.3) Equipment Used in Mechanical Excavation The mechanical method of excavation involves the use of mechanical plants which are capable of doing more than one task. Examples of these are as follows: Bulldozer: This is used to push the soil layer by layer to one side and pile it up nearby. A bulldozer does not dig out or lift out the soil. Backacter: This equipment digs down with a bucket on a jointed boom and scoops the soil towards itself. Since the bucket is narrow it is useful for forming trenches. It can also deposit soil on trucks or Dumpers. Mechanical Auger: This digs pile holes. It is a large piece of equipment which has a large drill mounted on a platform. The auger drills a hole in the ground and lifts out a column of soil. Dump Truck: Used for the movement of soil over short distances. The body of the dump truck tips forward and deposits the soil in the required position.

24 24 Tipper Truck: This is a road vehicle used to remove large amounts of excavated materials to locations away from the site. The body of the tipper truck tips up and empties the soil at the back of the vehicle. Fig 5.1 Plants used in mechanical excavation Test Questions 1. What are the factors that determine selection of excavation method. 2. State the uses of two mechanical excavating plant.

25 25 WEEK 6: EARTHWORK SUPPORT (3.4) Methods of Earthwork Support It is important to access how long an excavation can safely remain open without support for the sides. If the weather is very dry then the lack of moisture may cause the soil to shrink, crack and fall in. Generally the looser the soil the more it needs to be supported. If the soil is very wet then the sides of the excavation may become unstable. In both cases it is better to provide temporary supports to the sides of the excavation. Excavation on a confined site may also need support in case heavy loads are placed or driven too close to the edges of the excavation. Earthworks supports retain the size of excavation collapsed during the time the excavation will remain open. The process of providing temporary support to the sides of an excavation is referred to as timbering. It is sometime planking and strutting. Timbering is to: Protect the operatives while working in excavation Keep the excavation open by acting as a retaining wall to the sides of the trend. Prevent drainage to adjacent structures that could be caused by excavation Enable work to proceed within the excavation without interruption. Components of Timbering Timbering is made up of the following parts: Polings: These are vertical planks supporting the soil. In sand or gravel, they should be placed close enough to form a continuous timber wall. Depending on the soil type and working conditions, they may be placed about 900 mm apart. The purpose of the wailing is to keep the soil on the sides of the excavation from falling in. Wailings: They are horizontal timber strips supporting the polings Struts: These are the timbers that span across the trench between the wailings. The struts hold the opposite walls of the excavation in place. Wedges: These are the pieces of timber used to maintain the pressure of the polings against the soil. If the soil expands or shrinks while the work is carried out, the wedges may need to be adjusted again.

26 26 Factors to be Considered in Providing Supports to Excavation The nature of the soil Generally non-cohesive soils require more support than cohesive soil The depth of the excavation Shallow excavations need less support than deep excavation The width of the excavation The wide excavation needs to be supported in a different way than narrow excavation. The type of work to be carried out- Operation within the excavation will require working space the amount required will depend on the operation involved. The moisture content of the soil- soils require different amount of support as changes in their moisture content occur. The length of time the excavation will be left open- cohesive soils, in particular may dry out and star to crumble if the excavation is left open for long periods of dry weather The method of excavation hand excavation will require more support than machine excavation The support system used- different methods of excavation support can be installed before, during or after the excavation. The removal of the support system- different support systems can be removed either before or after the excavation. Moving materials into excavation the working space will need to consider the materials being moved into and out of the excavation as well as the operating being carried out within the excavation The proximity use of the land adjacent to the excavation for stacking materials over loading of the ground by stacking materials close the excavation can cause stress on soils at the side of the excavation. Similarly, large vehicle should be prevented from driving too close to the excavation Vibration of soils from construction operation or vehicle using adjacent road- excessive vibration can cause soils to move, making the sides of excavation potentially less stable.

27 27 (a) Timbering in hard soils (b) Timbering in firm soil (c) Timbering in dry loose soils Fig 1.7 Timbering in various types of soil (d) Timbering in loose wet soils Another means of retaining the sides of an excavation without support by sloping the sides of the excavation to the angle of repose of the soil, which is the natural angle at which the soil will remain stable without additional support. When a soil is tipped into a mound it settles to its natural angle of repose. This angle will alter according to the type of soil and its moisture content. Because the angle of repose can rather be shallow, this method of excavation support does, take up a large amount of space on site and is therefore not frequently adopted. However, this technique can be use on talk excavation.

28 28 (3.5) Foundation The function of any foundation is to safely sustain and transmit to the ground on which it rests the combined dead, imposed and wind loads in such a manner as not to cause any settlement or other movement which would impair the stability or cause damage to any part of the building. Test Questions 1. List 4 consideration for providing earthwork support 2. Draw a typical earthwork support to a loose soil.

29 29 WEEK 7: FOUNDATION (3.6) Importance of Foundation to Building The foundation provides anchorage for the building carries the load of the building and safely transmits it to soils of better bearing capacity. provides a wide surface area to spread the load evenly across soil that can support the load. (3.7) Types of Foundation Soil The choice of foundation depends among other factors on the type of soil in which the foundation is to be constructed. Rock this is a very hard foundation soil. This kind of soil has a high load bearing capacity. It can support strip foundation for a load bearing wall and a pad foundation for isolated loads as those transmitted by columns and similar components. Gravel and sand compact; suitable for pad, and strip foundation Clay stiff; suitable for pad and strip foundation. The depth of the foundation should be taken to a depth where it will not be affected by swelling and shrinkage which sometimes occur in clay soil. Sandy clay Firm; suitable for strip and pad foundation. The foundation should be made wide enough taking into consideration the load bearing capacity of the soil. Clay and silt Soft; suitable for raft foundation because of the low load bearing capacity of the soil. Soils are also classified into two categories which are Cohesive soils in which the particles stick together and examples include silt, clay, clay and silt Non Cohesive soil in which the particles do not stick together but are loose especially when they are dry. Examples include rock, gravel and sand Factors that Influence the Choice of Foundation. The choice and design of foundation for building for building depends mainly on three factors The total load of the building

30 30 The nature and beaming capacity of the soil The amount of settlement produced by the loading. The total load on the building The total on a building is made up of dead load, line load and super imposed load. It is assumed that a building imposes a uniform loading all around its perimeter. This is not strictly correct as line load form floors and superimposed loading from the roofs will only be carried by the walls that support these elements. To reduce the complexity of calculating individual loadings for section of the external walls, it is easier to take the worst case scenario (i.e. the wall experiencing the greatest sum of loads) and assume that all other walls are similarly loaded. If the load is assumed to be uniform, then it is not necessary to add up all the loads for the entire perimeter of the building. If a representative 1 metre length of the external wall is taken, then need the total load on that section of wall can be determined and the foundation needed to support that section of the wall can be determined and the foundation needed to support that section of wall can be calculated. This design can then be applied to the rest of the foundation for the house. The nature and bearing capacity of the sub-soil The nature and bearing capacity of the subsoil ( the soil beneath the top soil) varies with the types of soil, it is the degree of compressibility and he amount of moisture in the soil. Also, cohesive soils, clays specification, can be subject to seasoned movement up to a depth of 1metre. These soils exhibit shrinkage/contraction and swelling/expansion during dry and wet seasons respectively. The amount of settlement produced by the loading Soil is compressible to varying degrees. As load is applied to the foundation the soil beneath the foundation will be compressed, the water and air in the voids between the soil particles will be squeezed out and the foundation will settle. This consolidation process will continue until the forces between the particles are equal to the applied load. The speed of the consolidation is determined by the speed of the migrating of water and air from between the soil particle. Foundation built on sand on sand settles relatively rapidly, while foundation built on clay soil settles more slowly and can last for a number of years.

31 31 Soils that are close to the surface are likely to be more compressible than those at greater depth, as deeper soil have been compressed by the weight of the overlaying soil. If the applied load on a clay soil is reduced due to excavation, water tends to move to the unloaded areas and swelling the soil will occur. Peat and other soils containing a lot of organic matter shrink and swell easily as their water content changes. They are very compressible and settle reality even under light loading. Made up ground behaves in a similar manner unless the materials is well graded, carefully placed and properly compacted in thin layers. Shallow foundation should not be used on sites consisting of made up ground. Slight settlement should not cause problems to the structure of the building. Excessive settlement may cause shear failure of the soil. Settlement must also be uniform throughout the building; otherwise damage may result from different settlement. The amount of different movement between parts of a building must be kept within acceptable limits. (3.8) Simple calculations of the area of concrete foundation The size of foundation depends on two factors namely Load being transmitted Bearing capacity of the soil under the proposed foundation. The bearing capacities of soils are obtained from tables or from soil investigation reports. Bearing Capacity of Strip Foundation A strip foundation consists of a strip of concrete under a continuous wall that carries a uniformly distributed load (i.e. the load from the wall to the foundation is uniform throughout the length of the wall). The load on the foundation is therefore considered as load per metre run of the load bearing wall. Width of foundation required = load per metre run of wall (kn/m) Bearing capacity of soil (kn/m 2 ) Example 1 Calculate the minimum width of a strip foundation for a house where the total load from the wall

32 32 is 50 kn/m and the soil bearing capacity is 150 kn/m 2. Solution Total load from the wall = 50 kn/m Soil bearing capacity = 150 kn/m 2 Minimum width of the strip foundation = load per metre run Soil bearing capacity = = 0.333m Bearing capacity for pad foundations A concentrated load may be supported by a column or free-standing brick pier on a pad foundation. The size of the foundation should be related to the load carried by the column or pier and the soil bearing capacity. Example 2 The load on a brick pier that supports the load from part of the upper floor, wall and roof of a two-storey building is 70 kn. The soft clay ground has a bearing capacity of 52 kn/m 2 Area of foundation in m 2 = Bearing pressure in kn Bearing capacity of soil in kn/m 2 = 70/52 = m 2 Test Questions 1. What are the factors that determine the size of a foundation? 2. Calculate the minimum width of a strip foundation that supports a wall that transmits a load of 65 kn/m, given that the soil bearing capacity is 120 kn/m Calculate the minimum plan area of a pad foundation that carries a column supporting a total load of 250 kn when the load bearing capacity is 200kN/m 2.

33 33 WEEK 8: TYPES OF FOUNDATION (3.9) Types of Foundation and Their Application Foundations are classified into two broad categories: Shallow foundations Deep foundations Shallow Foundations Shallow foundations transfer load of the building to a sub-soil at a level close to the surface. They are nearly always the cheapest to construct and generally used where sufficient depth of a strong soil exists near the surface of the ground. The foundation needs to be designed so that the soil is not overstressed so that the pressure on the subsoil beneath the foundation is equal at all points in order to avoid unequal settlement. The common types of shallow foundations are Strip foundation Pad foundation Raft foundation. Strip Foundation Reinforced concrete strip foundations are used to support and transmit the loads from heavy walls. The minimum thickness of a strip foundation is 150 mm. This may be of course, greater where the projection of the edge of the foundation from the base of the wall is greater than 150 mm. A typical strip foundation is as shown in figure 8.1. The width of the strip should be at least equal to three times the thickness of the wall it supports.

34 34 Fig. 8.1 Strip foundation The thickness of the foundation should be at least equal to the projection of the foundation from the base of the wall so that the load transmitted from the wall will not be dispersed over an area outside the width of the foundation as illustrated in figure 8.2. This is to prevent the shear failure of the foundation. (a) Lines of shear failure on wide and thin foundation (b) design of foundation to prevent shear failure Fig. 8.2 Design of strip foundation to avoid shear failure

35 35 The effect of the wall on the relatively thin foundation is to act as a point load and the resultant ground pressure will induce tension on the underside across the width of the strip. Tensile reinforcement is therefore required in the lower face of strip with distribution bars in the second layer running longitudinally especially in soft soils and in wide strip foundation. The reinforcement arrangement for a wide strip foundation is as shown in figure 8.3 Fig. 8.3 Wide strip foundation In firm clays the sub-soil is capable of carrying substantial loads and may only require a foundation to be slightly wider than the wall it is supporting. The foundation still needs to be deep enough to overcome the problems of seasonal changes in moisture content of the soil. A deep, narrow foundation, of about 350mm in width and up to 1.50m in depth as shown in figure 8.4 could be constructed.

36 36 Fig. 8.4 Deep strip foundation On a sloppy site it is necessary to step the foundation parallel to the slope of the ground. This is called a stepped foundation. To prevent differential settlement in a stepped foundation, the height of the step should not exceed the thickness of the foundation. At each step the higher foundation should overlap the lower foundation for a distance equal to the thickness of the foundation. An illustration of a stepped foundation is given in figure 9.5 Fig. 8.5 Stepped foundation

37 37 Test Questions 1. Describe foundation under these headings a. Shallow foundation b. Deep foundation 2. Sketch 4 types of foundation

38 38 WEEK 9: TYPES OF FOUNDATION CONTINUED Pad foundation This type of foundation is used to support and transmit the loads from piers and columns. The most economic plan shape is a square but if the columns are close to the site boundary, it may be necessary to use a rectangular plan shape of equivalent area. The reaction of the foundation to the load and ground pressures is to cup, similar to a saucer, and therefore main steel is required in both directions. A typical example of a reinforced pad foundation is shown in figure 9.5. Fig. 9.5 Pad foundation Raft foundation The principle of any raft foundation is to spread the load over the entire area of the site. This method is particularly useful where the column loads are heavy and thus requiring large bases or where the bearing capacity is low, again resulting in the need for large bases. Raft foundation can be considered under three headings: solid slab rafts, beam and slab rafts, and cellular rafts.

39 39 (a) Reinforce concrete solid slab raft foundation Fig 9.5 Types of raft foundation (b) Cellular raft foundation Pile Foundation Pile foundation is used to transfer heavy load to a soil layer of adequate strength located far below the ground surface that it will be uneconomical to carry out excavation to reach such a depth. This is used where the soil condition is poor close to the earth surface. Pile foundations could be made using concrete, steel or timber. The concrete could be precast or cast insitu. Piles are classified by their method of construction into Bored or Replacement Piles which are concrete cores poured into holes in the ground at measured intervals. Driven or Displacement Piles which are driven into the ground at spaced intervals. By the way they carry their loads pile foundations are classified into two types. These are End bearing pile, and Friction pile (a) End bearing piles are those piles that are driven down to soil layer of adequate strength in such a way that the end of the piles actually bear on this particular soil layer.

40 40 (b) Friction piles are used in situations where the soil layer of adequate strength is located far below the ground surface such that it becomes uneconomical to produce piles that will go to such a depth. This type of piles carries their load by the friction that exists between the shaft of the pile and the surrounding soil particles. The surface of the friction piles are made rough so that the desired frictional force could be produced as the pile is driven into the ground. Test Questions 1. Use drawing to describe a pad foundation 2. State the principle behind raft foundation

41 41 WEEK 10: METHODS OF REINFORCEMENT IN SUBSTRUCTURES (3.10) Methods of Reinforcement in Substructure Reinforcement is provided in concrete structures to enhance its tensile strength. Therefore in all structural elements, the reinforcement is provided in the region of the element that will be subjected to tension. Reinforcement details as provided in some sub-structures are illustrated below. Substructures include the following: Foundations Ground beams Pad foundations: The reinforcement in isolated pad foundation is provided as the bottom of the foundation slab. This is because the point load coming from the column or pier subject the bottom part of the foundation to tension. The reinforcement detail is as shown in figure 10.1 Fig 10.1 Reinforcement details of some of a pad foundation Raft foundation: In raft foundation both the top and bottom faces of the slab are subjected to both tension and compression depending on the position of the columns and other structural components that are carried by the raft. The reinforcement is therefore provided both at the top and bottom of the slab as shown in figure 10.2

42 42 Fig 10.2 Reinforcement details for solid slab raft foundation Ground beams: Ground beams are provided to support the foundation wall in place of a strip foundation in poor soil conditions. They are reinforced to enhance their resistance to the loads that might be coming on them from the wall. There are situations where the ground beams form component parts of some types of raft foundation. Typical reinforcement arrangement in ground beams is shown as an integral part of a raft foundation in figure 10.3 Fig 10.3 Reinforcement details for ground beams in a raft foundation Test Questions 1. What is the main function of reinforcement in foundation 2. State 2 characteristic of a ground beam

43 43 WEEK 11: CONSTRUCTION OF FOUNDATIONS (3.11) Method of Construction of Foundation Strip Foundation Reinforced concrete strip foundations are used to support and transmit the loads from heavy walls. The minimum thickness of a strip foundation is 150 mm. This may be of course, greater where the projection of the edge of the foundation from the base of the wall is greater than 150 mm. A typical strip foundation is as shown in figure The construction entails first of carrying out the excavation of the foundation trench up to a depth of at least 1m. This depth is to ensure that the foundation is constructed at a depth where it will not be affected by the seasonal soil movement that will cause its failure. After the excavation is concluded the concrete is then placed and compacted in the trench usually in one single layer. Fig Strip foundation If the nature of the soil will be such that a wide strip foundation will be used, the reinforcement