3) Foundation Systems

Foundations and footings

Foundations and footings

Foundations:

They are the structural elements that transfer the whole loads (dead and live loads) of the building to the ground. The lower portion of a building structure that transfers building loads to the soil below.

Footings:

They are a foundation under the base of a wall or column

Functions of the foundation

1. Load bearing : to transfer loads to the ground

2. Insulation (thermal, water, damp, acoustical) : water and damp insulation are important because of the corrosion problem of the steel elements under the ground. The concrete of the foundation surface should be covered with bituminous insulation materials

Requirements of foundations

Strength and stability

Having this would prevent movement of the structure due to the shrinkage and swelling of subsoil, and it would resist lateral forces due to the soil movement

Factors to consider when selecting a suitable footing

• loads from the building

• Depth of soil

• Type of soil (gravel, sand, silt, clay)

• Site topography

• Type of superstructure

• Type of structure in neighbourhood/its impact on adjacent properties

• Subsurface and groundwater conditions

• Building code requirements

• Construction method and risk

Types of foundation systems

1. Historic/bearing foundation

   • Stone foundations

   • Rubble trench foundations

1. Modern/skeleton foundation

   • Shallow foundations

   • Deep foundations

Historic/bearing foundation: stone foundation

Historic/bearing foundation: rubble trench foundation

• A shallow trench is filled with rubble and stones

• These foundations may have a drain pipe that helps ground-water drain away

Types of modern/skeleton foundations

1. Shallow foundations

   • Column footing

   • Strip foundation

   • Raft foundation

1. Deep foundations

   • Pier foundation

   • Pile foundation

   • Caissons

Modern/skeleton foundations : Shallow foundations

• They transmit structural loads to the near-surface soil

• Used for most residential buildings (small & light) or buildings with moderate height on soil that has enough bearing capacity (enough strength) at reasonable depth

• Used for economical consideration

• Place below the lowest part of a substructure

• Transfers building loads to the supporting soil by vertical pressure

Shallow foundations : Pad foundations

• also known as individual/isolated/column footing

• Used in high soil bearing capacity

• Used at the skeleton systems with less columns and long spans

• Pad foundations should sit at the same level

• Isolated footings are the individually spread footings that support freestanding columns

• Can be square, rectangle , or circle

• Is central or symmetrical

• Should be connected with tie/bond beams to provide horizontal loads

Modern/skeleton foundation: Deep foundation

• Used when the soil under a foundation is unstable or has low bearing capacity

• used for larger structures

• They extend through unsuitable soil to transfer building loads to a more appropriate-bearing stratum of rock of dense sands below the superstructure (they don’t excavate, they drill since it’s deep)

2 types of soil for foundations

• High bearing capacity soil

• Low bearing capacity soils

Types of pad foundations

1- stepped footing

2- sloped footing

3- simple footing

Encasement in footings

In order to prevent waste material, the corners of the footings are poured with a slope (45 deg)

Shallow footing: pad foundation LABELING

1- footing

2- column

30 tie/bond beam

Shallow footing: minimum dimensions

• in practice (25-30 cm)

When do we use pad foundations

Used for low-ride but large buildings built on rocky sand

Pad footing labeling

1- vertical bars

2- footing

Difference between Stepped - Sloped - Simple Footings

• Stepped footing : in the olden days, they maintained the required depth at all points around a building.

• Sloped footing : trapezoid top, slope of 45 deg from all sides, saving the cost of concrete and steel.

• Simple footing : pad, flat, and plain

Shallow foundations: Strip foundation / continuous wall footing

• it is a continuous concrete footing extended to support a row of columns, bearing walls, and shear walls.

• individual footings are replaced by a continuous strip footing that supports more than two columns in a row

• it’s objective is to spread the loads to a wider strip.

• easy to build and doesn’t require expensive tools.

• durability is less.

Strip foundations : when are they used?

1. Ground and subsoil is weak

2. Heavier and taller buildings are constructed

3. Pad/column foundations are dense

4. When the bearing capacity of the subsoil is low so that large bearing areas become necessary

Shallow foundations : types of strip foundations

• one- way strip foundation

• two-way strip foundation

Shallow foundations : continuous footing v.s strip footing

Continuous footing:

• reinforced concrete footing extended to support a row of columns

Strip footing:

• continuous spread footings of foundations walls

Shallow foundation: Strip foundation forms/types

A. Pad foundation (strip foundation with constant depth)

B. Stepped foundation (Strip foundation with steps)

C. Sloped foundation (strip foundation with various depths

One-way strip foundation

They should be connected to the parallel structure in order to unify the whole system gains earthquake loads

Two-way strip foundation

More suitable for taller buildings

3. Shallow foundations : raft / mat foundation

• it is a large foundation that extends over a great area, frequently an entire building or (a thick, heavily reinforced concrete slab that serves as a single footing for a number of columns or an entire building)

• they are used when the allowable bearing capacity of foundation soil is low.

• When the total area of the strip foundation is more than half of the whole building area, the mat foundation is more economic.

• This foundation spread the dead loads more homogeneously/evenly.

• it has no tie bonds and no beams.

• may be stiffened by a grid of ribs, beams, or walls.

Raft / mat foundation : when do we use it?

1. when the allowable bearing capacity of foundation soil is low relative to building loads

1. when interior column footings become so large that it becomes more economical to merge them into a single slab.

Types of Raft Foundations

1. Raft / mat foundations without bond / tie beams

2. Raft / mat foundations with bond / tie beams

3. Mushroom raft / mat foundation

4. Raft / mat foundation with shear walls

Raft / mat foundation labeling

Type of deep foundations

• Pier foundation

• Pile Foundation

• Caisson

Deep foundations : Pier foundation

• support and transfer the large loads to firm strata below

• can be placed above the ground

• has footing

• useful when building on steep slopes and in areas subject to flooding

Deep foundations : Pile foundation

• used in order to reach a stronger (high-density) ground level

• a ground-level platform is built on very weak grounds (pile cap), it distributes the loads equally among the piles.

• the columns fit on this platform, and are placed below the ground (piles), usually two or more

• doesn’t have footing

• pre-fabricated piles are the main types (saves time, effort, cost…)

• cost of pile foundations is higher than most foundations

When do we use pile foundations?

1. When soil is weak

2. When the groundwater table is high

3. Soil erosion

Pile foundation: what is the difference between A and B’s pile foundation?

A) has less density because there is less distance between the piles

B) has more density because there is more distance between the piles

Deep foundations : Caisson / well foundation

• well foundations are constructed in rivers, lakes, bridge structures, and Barbour constructions.

• used when the high-density ground level is deep and bigger foundations are needed

• more suitable four deep foundation underwater

• doesn’t have footing

• cost is very high

Caisson foundation : in situ v.s pre-fabricated caisson foundation

• In Situ Caisson foundation: provided/made on-location

• Pre-fabricated caisson foundation: pre-made off site

Caisson foundation: labeling

1. Caisson

2. Reinforcement and Additional Resistance

3. Base of Caisson

4. Bearing Stratum of Soil or Rock

Shallow foundation v.s Deep foundation

Shallow: less than 3m

Deep: more than 3m

Pad foundation v.s Pile foundation

Pad: stops at footing in regular soil

Pile: stops at pile in bed rock

Pad foundation v.s Strip foundation

Pad: uses columns and is isolated (one at a time)

Strip: uses walls and is a whole strip

Why do Pad Foundations sometimes need tie-beams?

Because it helps balance the tilting effect due to different turning moments

Retaining Walls

• vertical structured designed to retain material on one side, preventing it from collapsing, slipping, or preventing soil erosion.

• they many be independent structures or may be part of a wider construction work, such as a building.

• they may include a parapet that extends above the height of the retained material for safety reasons

Main use of retaining walls:

To help prevent soil erosion

Different types of retaining walls:

1. Gravity wall

2. Piling wall

3. Cantilever wall

4. Anchored wall

Waterproofing

• To prevent water passage

• Used where groundwater is plain

• Used when the need to protect sub-structure space from moisture is critical

What is the next most important factor in building a construction after structural integrity?

Waterproofing

What is the source of the vast majority foundation problem?

Water

Wet soil beneath a foundation can swell or lose strength

Waterproofing systems protect against water intrusion with the goal of:

• Maintaining long-term structural durability

• Surface protection

• Occupant comfort in buildings

Commonly used materials for waterproofing in buildings:

1. Cementitious material

2. Bituminous material

3. Liquid waterproofing membrane

4. Liquid membrane

Which part of a building is the most important place to waterproof?

• below grade waterproofing

• roof waterproofing

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