01 foundations
DESCRIPTION
iitm ce2330TRANSCRIPT
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Building Construction1. Foundations
CE 2330Civil Engineering Materials and Construction
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Function of foundation
Transmit loads to sub-soil in such a way that• Settlements are within permissible limits,
without causing cracks in the superstructure, and
• Soil does not fail in shear
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Foundations…
• Reduce load intensity (by distributing over large area)
• Distribute loads evenly (minimize unequal settlements)
• Provide level surface• Provide lateral stability• Protect against undermining and soil
movements
Can you draw some foundations on your own???
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Some examples from the past..
What was the function of this building???
Next time you visit an old temple, try to figure out how the foundation system works!
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Broad Classification• Shallow foundations – when depth <= width (in open
excavation, sometimes w>d is also allowed)- Spread footings- Combined footings- Strap footings- Mat foundations
• Deep foundations - Piles- Piers- Wells
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Shallow Foundations
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Depth of footing• Footings should be carried below top (organic)
soil, miscellaneous fill, abandoned foundation, debris, or muck
• According to NBC of India, D depends on:- Securing adequate bearing capacity- How deep shrinking and swelling soils exist (clays)- How deep is the frost penetration (in sand and silt)
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Strip footing design
Width of footing B = W/qs, where W = total load, and qs = safe bearing capacity
d depends on type of concrete, projection of block and soil bearing pressure
Design of isolated column footings similar to strip wall footingsUsed for continuously running walls
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Construction of strip footing
Excavation complete; reinforcement laid in position
Concreting in progress for the strip footing
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Isolated footings
• Used for columns
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Grillage foundations
• Used when load is heavy and bearing capacity is poor
• Primarily for steel columns (primarily industrial structures)
• Consists of two layers of steel I-sections in transverse directions, encased in concrete (to prevent corrosion)
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Combined footings
• Why? – Columns too close, bearing capacity low, end column near property line
• Typically, rectangular or trapezoidal plan• Rectangular CG of footing = CG of
combined loads of the two columns• Trapezoidal – Unequal loads on two
columns
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Continuous footings for more than 2 columns similar to strip footings
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Raft foundations
Used when:• Soil bearing capacity is low• Building loads are heavy• Spread footings cover large area• Difficult to control differential settlement• Reduce settlement on highly compressible
soils (Weight of structure + raft ~ Weight of soil excavated)
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For heavier loads, thickened bands may be provided along column lines in both directions
2 way grids with cellular construction may also be used for heavier loads
Also, basement walls could be used as deep beams
Rafts
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Design of raft footings
• Rafts are designed as RC flat slabs, and the weight of the raft is not considered in the design (carried by the soil)
• However, for differential settlements (related shear and moment) rafts are reinforced more heavily than required by analysis
• Rafts may undergo large settlements without showing differential settlement (double settlement allowed for rafts)
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Rafts – some examples
Reinforcement for 1.5 m thick raft
Slab on grade
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Machine foundations• For heavy machines• Dynamic loads• Rough guide: ratio of weight of foundation to
machine weight may be kept between 2.5 – 3.5• Permissible bearing pressure under dynamic
loads may be taken as ½ to ¼ of pressure for static loads
• Covered in IS 2974
Get some pictures of machine foundations from our labs!
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Deep Foundations
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Why Deep Foundations?
• Heavy loads• Top soil has poor bearing capacity• High water level, with large fluctuations• Tough to retain soil in open excavations• Structure close to sea (scouring may occur)• Foundation close to canal or deep drainage lines• Top soil of expansive nature
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Piles
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Classification of piles• Concrete piles
- Precast- Cast in situ
Driven piles (cased/uncased)Bored piles (pressure, under-reamed, and compaction piles)
• Timber piles• Steel piles
- H-pile- Pipe pile- Sheet pile
• Composite piles- Concrete and timber- Concrete and steel
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Cased cast in situ pile(a) Raymond standard pile
(tapered)
Taper of 1 in 30
L: 6 – 12 m
Dia: 40 – 60 cm (top); 20 – 30 cm (bottom)
Mandrel is withdrawn, leaving shell, which is filled with concrete
(b) McArthur cased pile (no taper)
An additional steel casing of heavy gauge is driven with the core. The core is withdrawn, replaced by shell. Concrete is placed in the shell can casing is removed
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Uncased cast in situ pileSimplex Pile (for both soft and hard soils)
After being driven in, steel tube is removed and space is filled with concrete
New shoe needed for every pile
Alternatively, alligator jaw point could be used; opens up as concrete is poured, and withdrawn with steel tube
Soil needs to be firm around setting concrete
Vibro-piles: Similar to Simplex, except that while extracting tube, upward extracting and downward tamping blows given; used in soft soils
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Bored PilesProcedure:
Ground is bored with auger
Concreting is done with/without casing
Pressure piles:
Compressed air forces concrete down, while steel tube is withdrawn; Pile dia > dia of bore; hence, soil is compressed; better friction
3 sizes: Dia 340 mm, 440 mm, and 500 mm
Depth < 25 m
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Piles – some examples
Excavation for pile in progress
Piles being driven
Alaskan residence on piles
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Sheet piles
Used for supporting earth load in an excavation
Can also be used to dam water for construction
New Orleans after Hurricane Katrina
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Under-reamed pileBored cast in situ pile having one or more bulbs formed by enlarging the borehole for the pile stem using an under-reaming tool
Bulbs increase bearing capacity
Diabulb ~ 2.5 Diastem
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Pile cap and pile group
• Pile cap: Rigid, deep RC slab acting monolithically with the pile group
• Piles in a group are arranged symmetrically about the axis of the column
• Pile cap has uniform thickness, and extends at least 10 – 15 cm beyond the edge of exterior piles
• Piles in the group are embedded at least 15 cm into the pile cap
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Piles and pile cap
Transmission tower -superstructure
Pile cap
Piles
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Well Foundations or Caissons
• French ‘Caisse’ meaning a chest or box• 3 types:
- Box caisson- Open caisson (well)- Pneumatic caisson
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Box caissonBuilt on land and floated to site where it is sunk in position
Used when load bearing stratum is available at shallow depth, and where loads are not very heavy
Mainly used for breakwater and seawall
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Open Caissons
Very popular for bridges in India
Many shapes possible; easy and uniform sinking for circular sections
Important features:
1. Well cap
2. Top plug
3. Sand filling
4. Steining
5. Curb
6. Cutting edge
7. Bottom plug (no reinforcement)
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CaissonsExtensively reinforced caissons under construction
Completed bridge – showing caissons supporting piers
Next time you travel by train / road, try to see if you can spot some caissons
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Well sinking operations
• Laying the well curb• Masonry in well steining• Sinking:
- Excavate material from inside the curb- As well sinks deeper, upward force due to skin friction kentledge required- Uniform dredging (avoid any tilts)- Tremie concreting for bottom plug
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References
• Various web sources• Wikipedia• Building Construction by B C Punmia• Building Design and Drawing by Balagopal
T S Prabhu et al.