Factors Affecting Selection Of Superstructures Construction Essay

The developers intend to develop a number of new sites to provide low and medium rise luxury housing accommodation for private sale in the current economic market. As part of the planning gain it may be necessary to negotiate other accommodation.

You have been successfully appointed as the ‘technical development manager’ to co-ordinate the design and the construction of the new and proposed developments, together with the after sale and maintenance of this new building stock.

Task 2:

Evaluate with the help of sketches and comments the various forms of substructures that could be used to safely transfer the superstructure loads to the ground conditions identified in the soil report.

Foundations are designed to distribute all of the weight of a structure / building equally over the ground.

During the construction of a building the weight is spread over certain points such as piers and columns and this requires the foundations to be spread evenly under a building so that it reduces the weight of that building.

Foundations are either classified as shallow foundations or deep foundations.

Shallow foundations are used to transfer the load of a building to the soil at a level closest to the floor of a building.

Deep foundations use piles and different types of piers and transfer the load to the soil a lot deeper from the bottom of the building compared to that of shallow foundations.

Foundations must also comply with buildings regulations please see diagram overleaf.

Shallow foundations are pads, strips or rafts.

Shallow foundations are close to the ground surface and are less than the width of the footing normally less than 3m. This is where the surface loading or other surface conditions affect the bearing capacity (the stress on the soil) of a foundation and is called shallow.

Shallow foundations are used when surface soils are strong and able to support the imposed loads. These types of foundation are not suitable in weak or highly compressible soils.

Deep foundations are Piers and caissons.

Deep foundations are those which are deep below the ground surface so their bearing capacity won’t be affected by any surface conditions, normally at depths of 3m below ground level.

Deep foundations are used to transfer the load to a deeper depth if unsuitable soils are found nearer the surface.

Piers are foundations used for the carrying of heavy structural loads and are built in situ in deep excavations.

Caissons are a deep foundation and are built above ground level and once built is then sunk and used for excavating or the dredging of materials from within the caisson.

Foundations are designed and built to construct buildings which will stand up to the forces of nature and these forces are:

Frost heave this is a common problem in sandy soils and this is where the water in the ground freezes and pushes upwards then expands and turns into ice.

The opposite of this problem is Drought and this can cause the shrinkage of the land and is very common in clay soils.

These problems can affect the buildings foundations and in most cases will disturb and crack them. This can be easily avoided if you dig deep enough and in most cases at least two thirds of a meter will suffice.

These forces can affect the structure of buildings so when constructing foundations care must be taken to prevent settlement or movement as this may cause damage to the building once constructed.

It is also necessary to prevent against damage caused by the wind or atmospheric conditions.

Foundations are also known as structural members and are just like giant beams. They carry loads like structural steel beams and their job is similar to that of foundations.

Foundations can be constructed from a number of materials this is so the foundations are capable of carrying the heavy load of a building.

The two types of load imposed on buildings are:

Wind Loads:

Loads imposed by wind.

Dead loads:

These are permanent non moving loads such as the roof, walls and floors.

Imposed loads:

They can consist of anything that isn’t permanent such as furniture, people and snow loads.

Foundations can be made from concrete, stone, concrete block, wood and steel but the best way to construct foundations is by using strong, durable, water resistant materials.

The designing of foundations is a technical job and is done by an Architect or a Structural Engineer they will properly size the foundations and specify the materials from which it should be constructed.

Foundations can only be designed and constructed once a soil test is carried out.

The outcome of the test/s will tell us what we can and can’t build on our site and the type of foundations we can lay.

This will also tell us approximately how much the development will cost and how long it should take to complete.

Once we have the results back from the soil sample and know what we can build. We would need to prepare our site before any building construction work takes place.

Then our site would have to be prepared as this is an important part of this process. This would include the clearing of trees and the placing of stakes to mark the boundaries of our site.

The clearing of trees is an important part of preparing our site as the roots from trees could damage the foundations as they grow, this is due to the large amount of water that trees takes out of the land and around it. As this can change the balance of the subsoil and if any large trees need to be removed we would need to allow at least a year for the land to settle before any work is started on the foundations.

Please see overleaf a diagram on the effects due to shrinkage and ground swell.

The removal of approximately 300mm of the topsoil would need to be done as this part of the ground wouldn’t be strong enough to support the structure of a building.

The topsoil may also contain vegetation and plant growth which could damage the foundations and the structure of the building once built. This also helps to even out the work surface and flatten the ground.

The excavation and laying of foundations would include digging a hole for the foundations and setting and pouring a strong durable material into the hole.

Then the trench would need to be dug and once it has been excavated care must be taken as the walls of the trench may fall in and the moisture in the soil may drain or dry out.

Consideration must be taken when digging trenches from the time it takes to dig to the weather conditions. The building of the walls in the trench means that wet weather may hamper the process and if the trench floods, its walls may collapse and it’s recommended that the work is completed during dry weather.

The digging of trenches can be done by hand or machine depending on the size of the structure and this is done by using the design drawings, leveling instruments and boning rods.

These tools are used to confirm and identify the depth of the trenches for the required foundations.

Below are the possible types of foundations we may use in the construction of our building projects.

Strip foundations (Deep Strip or Trench Fill, Traditional Shallow Strip and Wide Strip). Please see diagram of strip foundations overleaf.

Shallow Strip

Strip foundations are the most common type of foundations used this is because they are strong, cheap to construct and can only be used when the ground conditions are good.

The width of the foundation will depend on the soil type, but are usually 450mm wide for single storey buildings and 600mm wide for two storey buildings and both will normally have a thickness of more than 200mm.

These types of foundations are used to support a line of loads like load bearing walls or if a row of columns require support and are ideal for 1 & 2 storey houses.

Strip foundations are made by using a concrete strip with a reinforced steel mesh as this will support the walls of the trench. The trench depth can vary but should be at least 1000mm with a width of 600mm. The concrete should then have a minimum depth of 225mm. This is to avoid any damage to the foundations caused by any changes in the topsoil.

The trench would have to be excavated / dug until the clay has been found and the floor of the trench would have to be covered with at least 150mm of concrete.

Wide Strip

This is used when the load of the structure is high in relation to the weight bearing capacity of the subsoil.

Wide strip foundations are laid the same way as standard strip foundations except that the trench is much wider and reinforced concrete is used.

Deep Strip or Trench Fill

Deep strip or known as Trench fill is an example of a shallow foundation and is constructed by digging a trench and then filling it with concrete or rubble. It is also used when firm or shrinkable clays are present.

These foundations have smaller narrower trenches and are filled with concrete to within two brick courses of the finished ground level as it’s important to be accurate if the walls are to be central.

The costs of the materials used for this type of foundation are more expensive than the above but it takes less time to build and so labour costs below ground level are less expensive and the job is easier to do.

Deep strip foundations can also be reinforced and can be an alternative to wide strip foundations in soft clay subsoil and should be at least 400mm wide and 900mm deep.

Pile foundations

Pile foundations can be used to support different types of structural loads and are used when the ground is soft and for small structures. These are often used because adequate bearing capacities can’t be found at shallow enough depths to support the structural loads.

Piled foundations are made up of a group of columns built and inserted into the ground and this type of foundation is designed to transmit the load of a building further into the subsoil.

Pile foundations are used when the surface soils are very soft and when designed consideration should be taken so that the pile caps are tied together with beams or with a reinforced concrete slab. This is because they can work in tension and compression and the foundations can act as a single unit.

This type of foundation is used to support buildings where conditions of the subsoil has shrinkable clays or with a high water table.

They are also used when other types of foundations can’t be used this is because the depth would be too deep and it wouldn’t be economical to use.

Pile foundations are made up of a series of columns which sit on a load bearing layer of the soil up to 4m below the surface. Any deeper than this would be too expensive and can’t be used for small building projects.

Pile foundations would consist of scaffold poles or timber and the bottom end of the timber would have an iron shoe with a point on.

This is done so that they can be easily driven into the ground and the top of the timber would also have an iron band round them. This would prevent the timber from splitting when their being driven into the ground.

The digging of foundations would normally be done by machine normally by a crab especially if there are large boulders in the ground. This would be done by hoisting a heavy weight called a monkey.

The monkey is hoisted up in the air and released and then will fall with force and drives the pile into the ground.

The monkey can weigh anything from weights of 100kg to 1120kg and is normally dropped from heights of around 4.5m to 14m.

The piles are then driven under all the walls or under the piers as this is where the weight of the building will be concentrated.

Please see diagram of pile foundations overleaf.

Pad or slab foundations

Pad foundations are suitable for most sub soils except loose sands, gravels and filled areas and are used for buildings over five stories high.

This type of foundation is usually used for office blocks with a steel framed construction and are usually constructed with reinforced concrete and where possible are normally square in plan.

Pad Foundations are designed to span in two directions with main bars and the pad is equal at all points under the soil this is to avoid any unequal settlement.

This is done by designing the centre of gravity of the foundations in line with that of the imposed loads.

Raft

Raft foundations are the biggest type of foundations you can get and are designed and constructed to spread the load of a structure over the whole area. These are used on soft or loose soils with a low bearing capacity so the loads can be spread over a greater area.

Raft foundations can be used when the loads are close together and are also ideal for areas where there is lots of water as the soil bearing capacity is treated like water.

A raft foundation is made up of a concrete slab which will cover all of the loaded area. It may all so be stiffened by ribs or beams which may be incorporated into the foundation. They are also used to help reduce any differential settlement as the concrete slab will resist differential movements between loading positions.

Raft foundations require reinforced steel at the top of the raft as this helps with cracks and more reinforced steel is required at the bottom under the walls to resist any tensile stress.

In soils with low compression reinforcement will be needed at both the top and bottom of the raft. The foundations would need to be extended by at least 300mm past the external walls to help protect the soil from possible frost and to help spread the load.

When constructing raft foundations on sand we would need to use a down stand as this will help to protect against soil erosion. But when building on shrinkable clays the soil under the foundations should be protected by extending the edge of the raft between 125mm & 150mm past the external wall. This is to protect it from any shrinkage or expansion of the clay.

Source of information

Roy Chudley & Roger Greeno

Building construction Handbook 2005

Jack Stroud Foster

Mitchell’s Structure & Fabric Part 1 6th Edition 2000

Mitchell’s Structure & Fabric Part 2 6th Edition 2000

Information also provided by course lecturer Harish Patel