Lecture 3 - Building substructure and foundations PDF

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Building substructure and foundations...

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Building design and Appraisal Week 3 Building substructure and foundations !

- Functions of foundations ! - Design principals! - Types of foundation and when they are used !

Building Structure terms

- Substructure - elements of structure below ground ! - Superstructure - Above ground !

Foundations - function

- Safely transmit building loads (live and dead loads) to the ground ! - What does ‘safely transmit’ mean ?! - Below topsoil layer - Upper 40 - 60 cm which contains organic content which decomposes, therefore the structure will sink !

- Bearing capacity of soil is not exceeded - loads into the foundation doesn’t exceed the capacity otherwise parts of the building would move and crack !

- No seasonal movement - To a depth where its not exposed to seasonal movement eg water freezing and expanding !

- Typically taken as 1m below ground for houses in UK and Canada is 2-2.5m deep ! - And also … ! - Without movement caused by external factors (EG Trees) ! - Foundations on clay soil - Shrinks when moisture is removed from it (EG From trees !

- Trees and foundations ! - THIS IS ON THE QUIZ !

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What are two factors that a designer can alter to provide a safe foundation - Go deeper ! - Or Spread the load wider ! - Foundations spread the loads outwards !

Foundation design depends on four factors:

- A - Loading (from building) ! - Factors affecting the load transmitted :! - Size of building - Larger the mass and the dead load ! - Materials used - Some materials are stronger than others and some are heavier than others !

- Use of building - Residential > Offices > Education > Entertainment > Industrial use ( large -

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dead and active load and also dynamic loads)! - Going up in load ! B - Type of building structure ! - Framed - Places a larger more concentrated load ! - Load bearing masonry! C - Type of soil ( and soil bearing capacity ! - Clay ! - Sand - Made up of Silica - Affected by water movement ! - Chalk - Low load capacity - ! D - Local conditions ! - Trees ! - Flat/Sloping site ! - Depth fo water table ! - Vibration!

- You can make a steeped foundation which is more expensive than !

Foundations - Design principles - Summary Foundation design depends on : ! - Load of building ! - Types of building structure ! - Type of soil and soil bearing capacity ! - Local conditions - trees, flat/sloping site, water table, vibration ! Design has two choices : Depth and type of foundation !

Types of foundations

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Strip foundation ! Raft foundation ! Pad foundation ! Piled foundation !

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Strip foundation

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Traditional strip or deep strip ! Simple - cheapest foundation ! Difficult if more than 2m deep (stability of excavation) ! Not good if varying ground conditions (made ground) ! Not good for point loads ! Problems near trees + clay soil!

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Raft foundation

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Reinforced concrete slab ‘floating’ on the ground ! Spreads load over larger area than other types ! Used for poor ground conditions ! Used for lightweight buildings (garages) !

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Pad foundation

- Concrete pad foundation ! - Used for framed construction ! - Most suitable for most sub soils except loose sand, loose gravel and filled areas !

Piled foundations

- Foundation for high loads, poor ground conditions or near trees ! - 2 Parts : ! - Pile ! - Ground beam !

- Bored pile (CFA) !

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- Vibration pile !

- Ground beam !

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- Deep pile low ground...