14. Construction Tech and Environmental Sciences Flashcards
How would you determine what foundations are required?
I would discuss with a structural engineer. However, note there are a wide range of foundation types. These vary depending on:
- Nature of load requiring support
- Ground conditions
- Presence of water
- Space availability
- Accessibility
What are the main types of foundations?
The main foundation types are categorised as shallow foundations or deep foundations.
What are shallow foundations?
Typically, these are used when the loads imposed by the structure are low relative to the bearing capacity of the surface soils. They include:
1) Strip foundations - These provide a continuous strip to a linear structure such as a wall.
2) Pad foundations - These are rectangular or circular pads used to support localised loads such as columns.
3) Raft foundations - These are slabs that cover a wide area, often the entire footprint of the building. These are suitable when ground conditions are poor and settlement likely.
What are deep foundations?
These are necessary when the bearing capacity of the surface soils is not adequate to support the loads imposed by a structure and so these need to be transferred to deeper layers with higher bearing capacity.
Piles - These are generally classified as:
1) End bearing piles (where most of the friction is developed at the toe of the pile, bearing on a hard layer).
2) Friction piles (where most of the pile bearing capacity is developed by shear stresses along the sides of the pile, suitable when hard layers are too deep)
3) Sheet piles / Diaphragm walls - These are made by excavating a deep trench that is prevented from collapsing by being filled with engineering slurry such as bentonite. The trench is filled with reinforced concrete panels. The joints between which can be watertight.
Can you name and describe different types of floors?
1) Solid ground - These require little maintenance and less prone to movement. Built up using:
- Sub-base - Well compacted rubble or loose stone based material
- Hardcore - Material to make the require level and create a solid base
- Damp proof membrane - Impervious layer such as heavy duty polythene sheeting
- Concrete bed - Providing a solid level surface
- Insulation - To limit heat transfer with the ground
- Screed - Sand and cement mix laid to prepare for installation of floor covering
- Floor finish - Such as vinyl or carpet
2) Suspended timber floor - Usually constructed using timber joists suspended from bearing walls.
- Joists are typically laid across the shortest span.
- Ventilation provided to the void between the floor by placing air bricks (or bents) to external walls
- Can cause draughts. However, can be avoided by installing an airtight breather membrane to maintain an airtight seal.
3) Suspended concrete floors - These are similar to timber suspended floors. It can span greater lengths and offers better sound insulation.
- Voids can be created by beams or ribs, to house services as well as providing support for suspended or attached ceilings.
4) Ribbed floors - The use of narrow-space shallow ribs, rather than wide spaced deep beams. Troughed floors are ribbed in one direction, whereas coffered floors are ribbed in two directions. Ribbed floors have greater span and load potential per unit weight than flat slab construction.
5) Hollow Pot floors - This is a ribbed cast in-situ floor with permanent formwork in the form of hollow clay or concrete pots. This creates a flat soffit, allowing the direction application of a plaster finish or drylining. The pot voids can be used to contain small diameter services within the overall slab depth. The most common form is a one way spanning floor, although two way spanning is also possible.
Can you name and describe different types of walls?
1) Cavity wall - A wall constructed from two skins of masonry. The outer skin can be brickwork or blockwork. The inner skin is generally blockwork. Separated by a cavity to prevent the penetration of moisture and to allow for the installation of thermal insulation.
2) Internal load-bearing wall - A wall that is also required to transfer loads from other parts of the structure to the foundations.
3) Partition wall - Non-load bearing wall that separates the internal spaces of a building.
4) Curtain wall - Non-structural cladding system for the external walls of a building.
5) Parapet wall - Uppermost reaches of a wall that extends above the roof level and provides a degree of protection to roof, gutters, balconies and walkways.
6) Party wall - A wall that stands on the land of 2 or more owners or a wall that is on one owners land but is used by 2 or more owners to separate their buildings.
Can you name and describe the different types of roofs?
1) Pitched roofs
- These are generally split into warm and cold style construction.
- Timber joists are sized using Approved Doc A and TRADA span tables. These are either C16 or C24.
- Joists are strapped down to the wall plates.
- Ventilation needs to be considered if insulating at ceiling joist level. Vapour Control Layer if at rafter level.
- Insulation is required to hit the required U-Values of 0.16 as illustrated in Approved Doc L.
2) Flat roofs
- These are generally split into warm and cold system construction. There is also an inverted flat roof system.
Explain the construction of a warm flat roof?
A warm deck is modern and preferable.
1) Insulation is placed above the joists and main deck. This means the whole roof structure is insulated.
2) The benefit of this is that the chance of condensation to the roof structure is reduced.
3) Unlike a cold deck, there is no need to vent the roof void.
4) If you are installing a warm roof over an existing roof, it is possible to leave the original roof covering in place to act as the vapour barrier as long as it is in good condition.
Ceiling
Joists
Timber deck
Vapour control layer
Insulation
Selected roof covering
Explain the construction of a cold flat roof?
A cold roof is more common and traditional.
1) Insulation is placed directly above the ceiling boards.
2) A cavity is left between this and the roof deck.
3) The air in this area is essentially outside and cold. This must be well ventilated to avoid condensation occurring.
Ceiling
Insulation (between the joists)
Joists
Timber deck
Selected roof covering
Explain the construction of an inverted roof?
The key difference between an inverted roof and other roofing systems is the position of the waterproofing membrane.
The ‘upside down’ sequence means the deck is waterproofed first, then insulated and finally finished with paving or ballast.
The waterproofing membrane is between protected from the elements and other factors such as UV radiation and rapid changes in temperature.
Explain the construction of a cold pitched roof?
Roof joists - Support the weight of the roof.
Breather membrane - Keeps out the rain and offers a secondary protection to any water that gets through the outside layer of the roof.
Battens - These are fixed to the joists with nails.
Tiles
Explain the construction of a warm pitched roof?
Roof joists
Breather membrane
PIR Insulation (or another type of insulation)
Battens
Counter battens (to provide a void if necessary)
Tiles
Name 3 different defects that may occur on a pitched roof?
1) Slipped/broken tiles
2) Tiles supported on timber battens can rot (if exposed to excessive moisture and rain)
3) Roof spread - Caused by overloading the roof with a heavier material
4) Missing mortar
5) Moss on roof / blocked valleys
6) Detailing around openings may fail (such as around chimney breasts, vent or overflow pipes)
7) At junctions such as valleys and perimeter walls where flashings can fail
Name 3 different defects that may occur on a flat roof?
1) Poor detailing - Perimeter flashings etc. may become vulnerable.
2) Poor design - Weak points where services pass through a covering
3) UV Degradation - Sign of an ageing roof which can lead to crazing or ‘alligatoring’
4) Blistering
5) Damage caused by foot traffic or maintenance activity
6) Moisture retention
7) Crazing - Which is small cracks
Rougham - You mentioned the roof suffered from several defects, what were these and what would the remedial action be?
Rougham had a bitumen felt flat roofing system. It suffered from ponding water that would suggest poor falls. These can be solved several ways:
1) Joists cut to falls with flat soffits
2) Joists laid to falls with slopping soffit
3) Firings (tapered strips fixed above the joists) with joist run
4) Firings against joist run
What types of flat roof material are there?
1) Asphalt Roofing
2) Liquid Waterproofing
3) Single plywood
4) Bitumen Felt
What are the advantages of each roofing system?
Liquid Waterproofing
- Lightweight and flexible
- Can be used to overcome complex details