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UNIT III
BUILDING COMPONENTS AND STRUCTURES
3 .1 Introduction A building is a structure with a roof and walls that is constructed to serve a specific purpose, such as residential, commercial, industrial, or institutional use. Buildings provide shelter, security, and functional spaces for human activities. They can range from simple one- room to complex multi-storey building and infrastructure. Foundation A foundation is the structural element of a building or infrastructure that transfers the loads from the superstructure safely to the ground. It ensures stability, prevents excessive settlement, and resists external forces such as wind, earthquakes, and soil movement. The selection of a foundation type depends on soil conditions, load requirements, and environmental factors. Types of Foundations Foundations are broadly classified into two categories:
1. Shallow Foundations A shallow foundation, also known as a spread or open foundation, is a type of building foundation placed near the ground surface, typically within a depth of about 3 meters (10 feet). It transfers the building loads directly to the underlying soil. This type of foundation is used when the surface soils have sufficient bearing capacity to support the structural loads. a) Strip Foundation (Wall Footing) A strip foundation is a continuous strip of concrete that spreads the load of a structural element (such as a wall) along the length of the strip. It is designed to transfer the weight of the building to the ground below, providing stability and support. Syllabus Foundations: Types of foundations – Bearing capacity and settlement – Requirement of good foundations. Civil Engineering Structures: Brick masonry – stonemasonry – beams – columns - lintels – roofing– flooring – plastering – floor area, carpet area and floor space index. Types of Bridges and Dams: water supply – sources and quality of water – Rainwater harvesting – introduction to high way and railway.
Types of Strip Footing I. Deep Strip Footing The most common type of foundation is the deep strip foundation, which is also the cheapest provided the soil conditions are suitable. A reinforced concrete strip supports the walls. The trench can be of any depth, but it should be at least 40 inches deep and 24 inches broad. Concrete should have a minimum depth of 9 inches. II. Wide Strip Footing Wide Strip Foundation is seen in areas having soils with low load-bearing capacity. The regular strip foundation cannot be implemented in low-quality soils. The excessive widening and deepening of the construction to prevent wall shearing is not economically justified. The use of reinforced concrete to construct a foundation can be a proper solution. Reinforcing bars give the foundation a tensile property, allowing the entire structure to bear strain and compression. Advantages of Strip Footing It is less expensive than other types of foundation, such as a deep foundation. Strip footing is also quicker in construction It is easy to install strip footing on the field. Strip Footing provides an economical alternative. Strip Footing can be used on sites with poor soil conditions. Disadvantages of Strip Footing Strip Footing is not as strong as other types. It is not suitable for all buildings. it can be susceptible to damage from flooding or other water issues due to its few feet depth. b) Pad Foundation (Isolated Footing) The pad foundations are shallow foundation that take and spread point loads to the soil safely. The pad foundation is preferred if the soil at the site have sufficient strength and is not too deep to construct. The thickness of pad foundations is generally uniform. In some situation
Fig.3: Combined pad foundation The existence of a service or a utility may restrict the extension of the pad foundation. As shown in the figure above, the pad of the external column will be connected to the internal column pad. This will help to counterbalance the loads coming.
4. Continuous Pad Foundations Continuous pad exists when pads and the columns they support are fairly closely spaced. Extending the reinforcing between pads ensures longitudinal stiffness. This technique helps in resisting the differential settlement. Fig.5: Continuous pad foundations 5. Pad and Ground Beam Pad and ground beam comprises smaller isolated pads are connected by ground beams to provide structural rigidity.
Fig.6: Pad foundation with continuous beam The technique helps in improving the integrity along with balancing the eccentric loads. Selection of Type of Pad Foundation Among different pad foundations explained, the selection is based on the following factors: The safe bearing capacity of the soil The load to be supported The Column arrangement Site conditions Accessibility Subsurface conditions The depth of water table Cost Conditions S. No. Feature Strip Foundation Pad Foundation 1 Description Continuous strip of concrete Isolated concrete pad 2 Load Support Continuous wall loads^ Individual^ point^ loads (columns) 3 Construction Excavation along wall, continuous formwork Localized excavation, individual formwork 4 Uses Residential and small commercial buildings Framed structures with concentrated loads 5 Load Distribution Even load distribution along the wall Load concentrated at specific points 6 Advantages Economical, easy to construct^ Economical for point loads, flexible layout 7 Soil Suitability Adequate bearing capacity near surface Adequate bearing capacity near surface 8 Reinforcement Often reinforced with steel bars Often reinforced with steel bars 9 Examples Supporting continuous walls, boundary walls Supporting individual columns, piers 10 Flexibility Less flexible in layout adjustments More flexible in the placement of columns
e) Strap Footing A strap footing is a component of a building’s foundation. It is a type of combined footing, consisting of two or more column footings connected by a concrete beam. This type of beam is called a strap beam. It is used to help distribute the weight of either heavily or eccentrically loaded column footings to adjacent footings. Advantages of Spread Footing
- This type of foundation can be made in different shapes like rectangles, squares, rounds, etc. as per requirement.
- Labour costs for the construction of spread footing are cost-effective.
- Construction of spread footing can be done easily and quickly.
- Spread footing allows the building load to be transferred over a wide area in soil with low load-bearing capacity.
- Spread Footing requires less skilled labour. It can be constructed using unskilled labour.
- Materials used for spread footing can be easily found in nearby available.
- The risk of structural failure is very low as spread footing is constructed in a simple and large area.
- Damage against natural disasters like frost can be minimized.
- Differential settlement is less likely to be in Spread Footing.
- To provide more stability to the structure of the building through spread footing, it transmits and distributes the load over a large area of soil.
- The risk of failure of spread footing is much lower than other types of footing.
- Spread footing is widely used in the construction of residential houses. Disadvantages of Spread Footing
- Spread footing requires more shuttering material.
- spread footing is limited to some soil formations. And spread footing cannot be used for every soil.
- In spread footing, if a foundation-like raft is used, a large quantity of concrete is required. Arch foundation Inverted arch foundations are provided in the places where the SBC (safe bearing capacity) of the soil is very poor and the load of the structure is through walls. In such cases inverted arches are constructed between the walls. End walls should be sufficiently thick and strong to withstand the outward horizontal thrust due to arch action. The outer walls may be provided with buttress walls to strengthen them. Figure shows a typical inverted arch footing. 2. Deep Foundations These foundations are known as Pile foundations. A pile is a slender column made of wood, concrete or steel. A pile is either driven into the soil or formed in situ by excavating a hole and then filling it with concrete. A group of piles are driven to the required depth and are capped with R.C.C. slab, over which super structure is built. The pile transfer the load to soil by friction or by direct bearing, in the latter case, piles being taken up to hard strata. This type of foundations is used when top soil is not capable of taking the load of the structure even at 3 – 4 m depth. Types of Pile Foundations Pile foundations can be categorised based on function or use, materials used, including other criteria. The various types of pile foundations used are given as under: Based on Function or Use
- Sheet Piles
- Load Bearing Piles
- End bearing Piles
- Friction Piles
- Soil Compactor Piles Based on Materials and Construction Method
- Timber Piles
- Concrete Piles
- Steel Piles
- Composite Piles
Friction Pile The friction pile uses the frictional force between its surface and the soil surrounding it to transfer load from the structure to the earth. Based on the underlying layers, friction may develop over the entire pile length or up to a specific length of the pile. In general, the entire surface of the pile facilitates the transmission of loads from the structure the bottom. Friction pile The capacity of friction piles can be obtained by taking the product of the surface area and the safe friction force acting per unit area. While constructing a skin friction pile, it is important to consider an appropriate safety factor as well as the skin friction that will occur at the pile surface. In addition, to improve the capacity of the pile, the pile dimension, number, and surface roughness should be increased. Soil Compactor Piles
Soil Compactor piles, in contrast to other pile foundations, does not support direct loads. This pile is pushed at precise intervals for compressing the soil and enhancing its bearing capacity. Based on Materials and Construction Method Timber Piles Pile foundations used below the water level are made of wood piles. Timber piles can last for 30 years. The piles have a square or round shape, and their diameter lies between 12 and 16 inches. The length of the pile is 20 times the width of the top. Timber piles are made to transfer loads up to 20 tons. To enhance their strength, fish plates can be bolted to the side of the piles. Advantages of Timber Piles There are regular-sized wooden piles. Economical in comparison to other pile types. Easy to set up. Not prone to damage. Timber piles can be cut to any length in case of any alterations are required during placement. These piles have high mobility and can be easily transported.
These piles offer poor mobility and cause difficulty in transportation and handling. These piles are difficult to be driven and are uneconomical. Precast piles are not readily available. When these piles are moved or driven into the ground, there is a likelihood that they may break or get damaged. Cast-in-situ Concrete Piles In the case of the cast-in-place concrete piles, the soil is dug out to the desired depth, and then freshly mixed concrete is put in the hole and left to dry. These pile foundations are made by either driving a metal shell into the ground and filling it with concrete. In cast-in-place piling, round piles are most commonly used. Advantages of Cast-in-situ Concrete Piles Foundation Since the shells are light, they are easy to move around. The length of piles can be easily altered. The shells can be put together at the site. These piles are not liable to break while installing. Procurement of additional piles is easy due to their easy availability. Disadvantages of Cast-in-situ Concrete Piles For this type of pile foundation, the installation needs to be closely monitored, and proper quality checks need to be ensured. There needs to be enough space on-site in order to store the materials. When underground water flow is strong, it is hard to build cast-in-place piles. The bottom of the stack might not be even. If the pile isn't reinforced, it can break under tension. Steel Piles
Steel piles can be either I-shaped or hollow. The diameter of such piles can be anywhere between 10 and 24 inches, and the thickness is 0.75 inches. The piles are easy to drive because they only take up a small area. These piles can be employed as end-bearing piles. Advantages of Steel Piles Steel piles can be installed pretty easily. These piles can attain sufficient depth in comparison to other types of piles. Steel piles can easily penetrate hard soil strata. These piles can be conveniently spliced. Can sustain heavy loads. Disadvantages of Steel Piles Steel piles can get damaged due to corrosion. These piles can deviate during installation. Steel piles are costly. Composite Piles In a composite pile, two different materials, like wood and concrete, are placed in the ground on each other, and both piles work together. In this type of pile, the characteristics of wood and concrete are subsumed together, and both contribute to the overall strength of the pile. The lower part of such a pillar is made up of wood, and the remaining portion is composed of concrete. Advantages of Composite pile
Allowable Bearing Capacity: The maximum load per unit area that the ground can support considering settlement criteria and safety factors. Factors Affecting Bearing Capacity
- Soil Type : Different soils have different bearing capacities (e.g., rock has a higher capacity compared to clay).
- Foundation Depth : Deeper foundations generally have higher bearing capacities.
- Water Table Level : High water tables can reduce the bearing capacity of the soil.
- Load Distribution : Uniformly distributed loads are better supported than concentrated loads. Methods to Determine Bearing Capacity Analytical Methods : Using formulas like Terzaghi's bearing capacity theory. Field Tests : Such as the Standard Penetration Test (SPT) and Plate Load Test. Bearing Capacity of Soil Calculation Analytical Method Analytical method is based on shear criteria. There are 3 types of failure as far as shear is concerned. They are described below: General Shear Failure General shear failure occurs in soil possessing a Brittle-type shear stress curve. Dense sand, silt, over consolidated clay etc., i.e., in the soil of low compressibility. In this case, the failure pattern is well-defined and a sudden shear failure is experienced with the bulging (heaving) of the ground surface adjacent to the foundation at both sides. Generally, shear failure occurs in soil having a relative density greater than 70%. Local Shear Failure In local shear failure, there is considerable compression of soil under the footing and there is only partial development of the state of plastic equilibrium. Failure surface does not reach the ground surface and only slight heaving of soil adjacent to the foundation occurs. Generally, occurs in soil having a somewhat plastic stress-strain curve. E.g. Loose sand with Relative Density between 30-70%. Punching Shear Failure Punching shear failure occurs when there is relatively high compression of soil under the footing. It is accompanied by shearing in the vertical direction around the edge of the footing. Generally, occurs in very loose sand with a relative density of less than 30%.Shallow foundations in loose sand and Deep foundations generally have punching shear failure. Terzaghi's bearing capacity theory
Terzaghi's Theory of Bearing Capacity is a classic and widely used method in geotechnical engineering for estimating the bearing capacity of shallow foundations. Here are the key concepts and details: Basic Concepts Terzaghi's theory assumes the soil beneath the foundation is homogeneous, isotropic, and behaves according to Mohr-Coulomb failure criteria. It considers three components of bearing capacity: Shear Resistance: The soil's ability to resist shear stress. Overburden Pressure: The weight of the soil above the foundation. Cohesion and Friction: The intrinsic properties of the soil. Bearing Capacity Equation The ultimate bearing capacity quq_u is calculated using Terzaghi's bearing capacity formula: 𝑞𝑢 = 𝑐𝑁𝑐 + 𝑞𝑁𝑞 + 0. 5 𝛾𝐵𝑁𝛾 Where: 𝑐 – Cohesion of the soil 𝑞 – Overburden pressure at the foundation level 𝛾 –^ Unit weight of the soil 𝐵 –^ Width of the foundation 𝑁𝑐, 𝑁𝑞, 𝑁𝛾 –^ Bearing capacity factors that depend on the angle of internal friction (φ) of the soil. Bearing Capacity Factors The bearing capacity factors are derived from empirical and theoretical studies and depend on the soil's angle of internal friction (φ). They are given as: 𝑁𝑐 =
tan φ 𝑁𝑞 = 𝑒𝑥𝑝 [𝜋tan(φ)𝑡𝑎𝑛 2 ( 45 °^ + φ 2
)]
𝑁𝛾 = 2 (𝑁𝑞 + 1 ) tan φ Assumptions and Limitations Shallow Foundations: The theory is primarily for shallow foundations where the depth of the foundation is less than or equal to its width. Homogeneous Soil: Assumes soil properties are uniform. Rigid Foundations: Assumes the foundation is rigid and does not deform. General Shear Failure: The failure mode is assumed to be general shear failure. Field Test Method Standard Penetration Test
3. Adequate Depth The foundation must be placed at a depth below the frost line to prevent heaving due to freeze-thaw cycles. The depth should also account for soil conditions and load-bearing capacity. 4. Uniform Settlement The foundation should settle uniformly to avoid differential settlement, which can cause structural damage. Proper soil compaction and site preparation can help achieve uniform settlement. 5. Load Distribution The foundation must distribute the load of the structure evenly to prevent overloading any part of the foundation. Reinforcement, such as steel bars, can help distribute loads effectively. 6. Resistance to Soil Movements The foundation should be designed to resist lateral movements and pressure from the surrounding soil. It should also account for potential soil expansion and contraction. 7. Proper Drainage Adequate drainage systems must be in place to prevent water accumulation around the foundation, which can lead to soil erosion and foundation weakening. Site grading and drainage pipes can help achieve proper drainage. 8. Compliance with Building Codes The foundation must comply with local building codes and standards to ensure safety and structural integrity. Professional supervision and adherence to engineering practices are crucial. Steps to prevent foundation failure 1. Site Preparation Conduct thorough soil testing to understand the soil type and its bearing capacity. Clear the construction site of any organic materials, such as roots, trees, and debris. 2. Proper Drainage Ensure proper site grading to direct water away from the foundation. Install a drainage system around the foundation to prevent water accumulation. 3. Foundation Design Choose the right foundation type (slab, crawl space, or basement) based on soil conditions and building requirements. Use reinforced concrete to enhance strength and durability. 4. Quality Materials
Use high-quality construction materials, such as concrete and steel, to ensure the foundation's longevity. Ensure that the concrete mix has the appropriate strength and consistency.
5. Construction Techniques Follow proper curing practices to allow concrete to achieve its maximum strength. Use adequate reinforcement to resist tensile stresses and prevent cracking. 6. Regular Maintenance Inspect the foundation regularly for any signs of cracking, settlement, or moisture infiltration. Address any issues promptly to prevent further damage. 7. Professional Supervision Hire experienced engineers and contractors to oversee the foundation construction process. Ensure that all construction practices adhere to local building codes and standards. Civil Engineering Structures Masonry Masonry is the craft of building structures from individual units, which are often laid in and bound together by mortar. These units can be made from materials such as brick, stone, concrete, and more. Brick masonry Brick masonry is built with bricks bonded together with mortar. For all permanent buildings, cement mortars are used. But for temporary sheds mud mortar may be used. Brick masonry strength depends on the type of bond and materials used for construction. They play an important role in providing strength, stability, and durability to the brick masonry. TYPES OF BRICK MASONRY The arrangement of bricks in wall construction is called brick bonds. Types of bonds in brick masonry wall construction are classified based on laying and bonding style in walls. The bond in brick masonry is developed by the mortar filling between layers of bricks and in grooves when bricks are laid adjacent to each other and in layers in walls. The various types of bonds generally used in brick masonry are: Stretcher Bond: This bond is sometimes known as running bond. This bond is the simplest bond that is used today, this bond is not suitable as a stand alone structural wall and a structural wall built directly behind it, fixed with wall ties would be needed. Stretcher Bond is normally used a farcade for the main structural building.
