Essential Building Construction Techniques and Material Properties
Methods of Underpinning
Underpinning involves strengthening or increasing the depth of an existing foundation. The primary methods include:
- Pit Method
- Pile Method
- Miscellaneous Methods
1. Pit Method of Underpinning
In this method, the existing wall is divided into suitable sections, typically measuring about 1.20 m to 1.50 m in width.
Holes are made in the existing wall. Needles with bearing plates are then inserted through these holes and supported on jacks (as shown in diagrams). The pit is excavated, and the existing foundation is taken up to the required level.
Precautions for the Pit Method:
- Only one section should be excavated at a time.
- Alternate sections should be taken in succession.
2. Pile Method of Underpinning
In this method, piles are driven along both sides of the existing wall. Needles, often in the form of pile caps, are then provided through the existing wall (as shown in diagrams). This process relieves the existing wall of the loads coming onto it.
The Pile Method is particularly useful in:
- Clayey soils.
- Water-logged areas.
- Walls carrying heavy loads.
For underpinning very light structures, the piles are driven along the structure, and then brackets or cantilever needles are provided to carry the structure.
Negative Effects of Dampness in Buildings
Building materials such as bricks, timber, and concrete naturally contain moisture. Dampness occurs when the moisture content rises beyond an acceptable level, becoming visible or causing deterioration. The acceptable limit of moisture differs significantly between materials. For instance, 10 percent water content by weight in timber might be harmless, but the same level could saturate a brick or cause plaster deterioration.
Dampness severely affects the structure and occupants. The prominent effects are:
- Health Hazards: A damp building creates unhealthy conditions, giving rise to the breeding of mosquitoes and germs of dangerous diseases such as tuberculosis and rheumatism.
- Corrosion: The metals used in the construction of the building are corroded.
- Aesthetic Damage: Unsightly patches are formed on wall surfaces and ceilings.
- Timber Decay: Timber decay occurs rapidly due to dry rot in a damp atmosphere.
- Electrical Deterioration: Electrical fittings deteriorate, potentially leading to leakage of electricity and the danger of short-circuiting.
- Flooring Damage: Materials used as floor coverings are seriously damaged, and floorings may loosen due to reduced adhesion when moisture enters through the floor.
- Pest Infestation: Dampness promotes and accelerates the growth of termites.
- Plaster Failure: It results in the softening and crumbling of the plaster.
- Decoration Damage: Materials used for wall decoration are damaged, leading to difficult and costly repairs.
- Efflorescence and Disintegration: The continuous presence of moisture in the walls may cause efflorescence, which can result in the disintegration of stones, bricks, tiles, etc., thereby reducing the strength of the wall.
Raking or Inclined Shores
A raking shore consists of a wall plate, needles, cleats, rakers, bracing, and a sole plate (as shown in the figure). The wall plate is placed against the wall and secured by means of needles which penetrate into the wall for a distance of about 150 mm. The wall plate distributes the pressure evenly. The needles, in turn, are secured by cleats which are nailed to the wall plate. The rakers are interconnected by struts, braces, or lacings. The set of rakers are stiffened by similar bracing and/or hoop iron, and they are connected with the sole plate by means of iron dogs.
Design Considerations for Raking Shores:
- The center-line of the raker and of the wall should meet at the floor level.
- The rakers prevent the outward movement of the wall and partly deflect the roof and floor loads.
- A large factor of safety should be adopted in the design of inclined shores as it is difficult to assess the actual loads.
- The rakers should preferably be inclined at 45° with the ground. However, in actual practice, the angle of inclination may vary from 45° to 75°. The top raker should not be inclined steeper than 75° with the horizontal.
- The length of the top raker can be reduced by providing a rider raker (as shown in fig. 14-6).
- The sole plate is usually embedded in the ground, and the legs of the rakers rest on the sole plate (as shown in fig. 14-6). The sole plate should be long enough to accommodate all rakers and a cleat on the outside. In case of soft ground, the sole plate is placed on a timber platform to distribute pressure over a greater area.
- It is not desirable to use wedging as it would damage the building which is already in an unstable condition.
- The necessary permission of the concerned owner of the adjacent property in which the raking shores are to be erected should be obtained.
Flying or Horizontal Shores
In this arrangement (shown in figures 14-7 and 14-8), horizontal supports are given to parallel walls that have become unsafe due to the removal or collapse of the intermediate building.
A single flying shore consists of a wall plate, needles, cleats, struts, straining pieces, and folding wedges (as shown in fig. 14-7). The flying shore should have a depth not less than one-thirtieth (1/30th) of the clear span and a width not less than one-fiftieth (1/50th) of its length.
Key Points for Flying Shores:
- The center-line of flying shores and struts, and those of the walls, should meet at the floor levels. If the floor levels of the two buildings are at different heights, the framework may be suitably designed and made unsymmetrical.
- A large factor of safety should be adopted in the design of flying shores as it is difficult to assess the actual loads.
- The struts are generally inclined at 45°, and in no case should the angle of inclination exceed 60°.
Factors Affecting Building Ventilation
Many factors affect ventilation from the viewpoint of comfort and should therefore be considered carefully:
- Air Changes
- Humidity
- Quality of Air
- Temperature
- Use of Building
Air Changes
Where people are working, the air must be moved or changed to ensure proper ventilation of the premises. The minimum and maximum recommended rates of air change per hour are one and sixty, respectively. If the rate is less than one per hour, it will not create any appreciable effect. If the rate exceeds sixty per hour, it may result in discomfort due to high air velocities. For practical considerations and effective working of the ventilation system, the desired rate of air change is typically three, and preferably five, per hour.
The required rate of air change depends upon the volume of the structure, the type of activity in the premises, and the number of occupants. It also depends on the velocity of incoming fresh air and the quantity of heat, moisture, and odor present in the room. Fans may be used to increase air movement. The ventilating system as a whole should ensure smooth movement of air currents and prevent air stagnation at any spot in the room.
Humidity
The criteria of relative humidity (RH) also affect the ventilating system of the structure. For working at a temperature of 21°C, a range of 30 to 70 percent RH is desirable. The value of relative humidity is obtained by comparing dry-bulb and wet-bulb temperatures. For higher temperatures, low humidity and greater air movements are necessary for removing a greater portion of heat from the body.
Quality of Air
The purity of air plays an important role in the comfort of persons affected by the ventilation system. The air should be free from odors, organic matter, inorganic dust, and unhealthy fumes or gases such as carbon monoxide, carbon dioxide, and sulfur dioxide. These impurities depend on the habits of occupants, the volume of the room, and the source of ventilating air.
The ventilating system should be designed to provide comfort by supplying pure air. Therefore, the entry point of ventilating air should not be situated very near to latrines, kitchens, urinals, stables, or chimneys. The existence of pure air in buildings improves the health of occupants, assists in the perfect combustion of fuel, and preserves the material of which the building is constructed.
Temperature and Effective Temperature
It is evident that the incoming air for ventilation should be cool in summer and warm in winter before it enters the room. The usual difference between inside and outside temperature is kept at about 8°C to 10°C.
With regard to human comfort, the term Effective Temperature is used. It is an index that combines the effects of air movements, humidity, and temperature. It indicates the temperature of air at which the sensation of the same degree of cold or warmth will be experienced as in quiet air fully saturated at the same temperature. If two rooms have the same effective temperature, no change of temperature will be experienced by a person when they suddenly leave one room and enter the other. The value of effective temperature depends on the type of activity, geographical conditions, age of occupants, etc. (The popular values of effective temperature in winter are typically provided in charts).
Paint Characteristics and Common Defects
Paint characteristics encompass properties like color, gloss, drying time, and durability, while painting defects refer to issues that compromise the paint’s appearance or performance, such as blistering, chalking, or flaking. Common paint defects include those caused by external factors like weather and those resulting from improper application techniques.
Essential Paint Characteristics:
- Color: The hue and shade of the paint.
- Gloss: The shine or reflectivity of the painted surface.
- Drying Time: The time it takes for the paint to dry completely.
- Durability: The paint’s resistance to wear, damage, and weathering.
- Adhesion: The ability of the paint to stick firmly to the substrate.
Common Painting Defects:
- Blistering:
- The formation of blisters on the paint surface, often due to moisture or improper surface preparation.
- Chalking:
- A powdery, chalky appearance on the paint surface, indicating degradation of the binder.
- Flaking:
- The peeling or shedding of paint layers, often due to poor adhesion or weathering.
- Sagging:
- The paint flowing or drooping on vertical surfaces, usually caused by applying too much paint or painting in humid conditions.
- Wrinkling:
- Wrinkles or folds in the paint film, often due to using the wrong type of paint or improper application.
- Cracking:
- The formation of cracks in the paint film, often due to stress or changes in temperature.
- Blooming:
- A dull, cloudy appearance on the paint surface, often due to moisture or poor ventilation.
- Brush Marks:
- Visible streaks or ridges from brush strokes, often due to improper application techniques.
- Saponification:
- A chemical reaction that can soften or degrade the paint film, leading to defects like blistering or flaking, especially on alkaline surfaces.
- Efflorescence:
- The formation of white, crystalline deposits on the painted surface, often due to salts migrating from the substrate.
- Fading:
- The loss of color or vibrancy in the paint film, often due to UV exposure.
- Grinning:
- A situation where the underlying surface or a primer coat is visible through a thin coat of paint, often due to improper application or insufficient coverage.
Fire Resistance Properties of Building Materials
Fire resistance refers to a material’s ability to withstand fire exposure, delaying the spread of flames and heat, and maintaining structural integrity. This is often quantified by fire resistance ratings, which indicate the duration a material can withstand a specific fire test.
Factors Influencing Fire Resistance:
- Material Composition: The inherent properties of a material, such as its ability to burn, melt, or release toxic gases, play a crucial role.
- Thickness and Density: Denser materials generally offer better resistance to heat penetration.
- Insulating Properties: Materials with good insulating properties can slow the spread of fire by minimizing heat transfer.
- Fire Retardant Additives: Some materials can be treated with chemicals to reduce flammability, such as flame retardant paint.
- Durability and Maintenance: The ability of a material to maintain its fire resistance over time, even with exposure to various conditions and maintenance, is important.
Common Fire-Resistant Materials:
- Concrete: Highly fire-resistant due to its non-combustible components and its ability to act as a thermal barrier.
- Brick and Stone: Known for high heat resistance and ability to withstand fire exposure.
- Steel: Particularly when encased in concrete, steel offers excellent fire resistance, maintaining its structural integrity even during fire.
- Timber: Timber can be fire-resistant, especially when large masses are used, allowing the outer layer to char, shielding the core from the flame.
- Glass: Fire-resistant glass is designed to withstand high temperatures and can act as a barrier to fire spread.
- Cast Iron: Highly fire-resistant and non-combustible, making it a good choice for various applications, including drainage systems.
- Gypsum Products: Gypsum plaster and plasterboards are known for good fire protection due to the unique behavior of gypsum in fire (releasing water vapor).
Behavioral Changes in Materials Under Fire Exposure:
Construction materials behave differently under fire exposure, exhibiting changes in their chemical, physical, mechanical, and thermal properties. These changes can include decomposition, charring, softening, melting, spalling, loss of strength, and increased thermal conductivity. For example, concrete can undergo spalling, while steel loses strength and may soften or creep.
1. Chemical Changes:
- Decomposition and Charring: Wood, for instance, decomposes and chars when exposed to fire, losing its original properties.
- Melting: Some materials, like plastics, can melt at high temperatures.
2. Physical Changes:
- Density Variation: Materials may change density as they are heated.
- Softening: Materials like steel can soften and become more pliable at high temperatures.
- Spalling: Concrete and masonry can experience spalling, where pieces of material break off from the surface, often due to rapid temperature changes and internal pressure.
3. Mechanical Changes:
- Strength Reduction: Fire exposure significantly reduces the strength of materials like concrete and steel.
- Stiffness Reduction: Materials can become less stiff and more flexible under fire conditions.
- Thermal Expansion: Materials can expand and contract as they heat up, which can lead to stress and cracking, especially in concrete.
4. Thermal Changes:
- Thermal Conductivity: The ability of a material to conduct heat can increase or decrease depending on the material and the fire conditions.
Caissons and Cofferdams in Construction
Caisson
- Definition:
- A watertight, box-like structure that is permanently fixed within the water table of an engineering project.
- Purpose:
- To provide a stable foundation for structures built on top of it, often in deep water or soft soil.
- Usage:
- Used for foundations of bridges, piers, and other large structures.
- Types:
- Box caissons, open caissons, and pneumatic caissons.
- Construction:
- Sunk into the ground, filled with concrete, and used as a foundation.
Cofferdam
- Definition:
- A temporary watertight enclosure built within a body of water.
- Purpose:
- To allow the enclosed area to be pumped out, creating a dry working environment for construction.
- Usage:
- Used for construction or repairs in submerged areas, allowing work to be done on dry surfaces.
- Types:
- Braced, earth-type, sheet pile, and cellular cofferdams.
- Construction:
- Built from materials like
Common Flooring Types: Tile and Concrete
Tile Flooring
Materials:
- Ceramic, porcelain, natural stone (like marble or granite), glass, and mosaic tiles.
Advantages:
- Durable and resistant to scratches, dents, and damage.
- Water-resistant and easy to clean.
- Wide variety of colors, textures, and designs, including mimicking wood or stone.
Disadvantages:
- Can be slippery, especially when wet.
- May crack or chip if not properly installed or if subjected to heavy impact.
Applications:
- Entrances, kitchens, bathrooms, and other high-traffic areas.
Concrete Flooring
Materials:
- Cement, aggregates (like gravel or crushed stone), water, and sometimes additives.
Advantages:
- Extremely durable and can withstand high pressures, temperatures, chemicals, and abrasion.
- Versatile and can be used indoors or outdoors.
- Cost-effective for high-traffic areas and commercial spaces.
Disadvantages:
- Can be prone to cracking or chipping if not properly installed or if exposed to harsh weather conditions.
- May require sealing to protect against