rclect17 foundations 12

8/17/2019 Rclect17 Foundations 12

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Reinforced Concrete 2012 lecture 17/1

Budapest University of Technology and Economics

Department of Mechanics, Materials and Structures

English coursesReinforced Concrete StructuresCode: BMEEPSTK601

Lecture no. 17:

FOUNDATIONS


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Content:

Introduction1. Basic design principles2. Classification of foundations3. Classification of soils

4. Special design problems of shellow foundationsStrip foundationsPad foundationsSlab foundations

5. Retaining walls6. Deep foundations

Box foundationsWell foundations

Pile foundationsSlurry wall foundations


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Introduction

„The structure stands on soil, which is not reinforced concrete!”Improper design of the foundations may cause severe damages:

-uneven settlements-swimming up-sliding-loss of stability due to tilting

Foundations transmit generally compression originated by verticalloading, but horizontal actions (wind, earthquake) may also producetension in anchorage members (piles) of the foundation.


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1. Basic design principles

Possibly uniform stress distribution to avoid uneven settlements.

Foundation depth:-on loadbearing soil layer-under freeze depth (at about 80 cm in Hungary)


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2. Classification of foundations

According to foundation depth:

Shallow foundationsStrip foundationsPad foundationsSlab foundations

Deep foundationsBox foundationWell foundation

Pile foundationsSlurry walls


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3. Classification of soils

f u,soil kN/m2

Cohesionless soils:

Sand 200-500Gravel 500-700

Cohesive soils:

Clay, siltsoft 100-300hard 300-500

filling 20-50

Classification of soils according to bearing capacitry:Good soils f soil,d =0,07 kN/cm2

Medium soils 0,03Bad soils 0,01


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4. Special design problems of shallow foundations

u,soild,soilf A

Fασ=≤=σ α= 0,25 to 0,75

It is recommended that half of the contact area be working (That is the

eccentricity is smaller than B /4)


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Stepping down to foundation level of the neighbouring building

Strips arranged in perpendiculardirection

Periodical underpinning of theneighbourung foundation

concreting


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Danger of swimming up by high water table

ground water insulationlevel

provisory sinking

of the water tableduring constructionQ: only selfweight

buildingwatermA3,1Q ≥ 1,3: safety factor


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5.1 Strip foundations

mass concrete

d,ct

d,soil

f

3tan

σ=α

reinforced concrete

yd

2d,soil

szf 2

aA

σ=

transverse steel longitudinalreinforforcement


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Strip foundation along neighbouring building

or: combined footing in perpen-dicular direction

rc. wall strip footing


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5.2 Pad foundations

starter bars of the column

Calculation of x c by succesive approximations


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Partially prefabricated pad foundation:

- prefabrcated sleeve- monolithic rc pad on- mass concrete block


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Reinforcementsystem of eccentricallyloaded padfoundation

starter bars


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5.1 Slab foundations

-By high ground water level-Danger of uneven settlements-To reduce settlement by higher load intensities

Characteristic settlements under a 10 storey building-strip foundation: 4 to 6 cm-pad foundation: 2 to 4 cm-slab foundation: 1 to 2 cm

Calculation of moment distribution as elastically supported slab„Thumb rule”:foundation slab thickness = no. of floors supported x10 cmbut min. 25 cn

3 m thick slab foundation was constructed under the HungarianparlimentFoundation slab under „Clasp”

lightwight structural system (1080-ies):


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6.Retaining walls

drenage

slip surfice

angle of internal frictionclay: ≈20° Ep: passive earth pressure sand: ≈30° (uncertain, better not consider) gravel: ≈35°

Ea: active earth pressure

)2

45(tan2

HE 2

2

aϕ

−γ

= o


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Examples for active earth pressure

H (m) Ea (kN)2 40

4 1606 3608 640

Checks for: 1 sliding2 overturning (around point A)3 bearing


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buttresses

steel tube

buttresse


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Thumb rule of prof. Jáki (1930-ies):

Cohesionless soils: B ≥ 3

H

Cohesive soils: B ≥ 2

H

H ≈ 8 m

Soil anchores with prestressed

tendons


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7. Deep foundations

Box foundations

Applied in bridge construction,for oil drilling towers


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Well foundations

Sinking by excavation

Cutting edge

Bearing capacity: 1000 ti 10000 kN


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Pile foundations

Ways of force transfer between piles and the soil


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Floating piles and end-bearing piles


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Prefabricated rc piles

Monolithic rc piles

dropped weight(Franki piles)

withdrawn steel tube

concrete block

pile cap concretedon reaching the final

depth


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Pile cap (rc. beam) on top of piles

Inclined piles to bracethe pile system againsthorizontal forces


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Franki piles

Diameter 60 cmConcrete: C8-C12Reinforcement:

Ø16, Ø8 links Capacity:1000 to 3000 kN


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Use of spirel drill for mon. piles


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Jet grouting: „soil concrete” pile

drilling injection ready jet repetitioncolumn


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Jet grouting under existing foundation


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Construction of pile-wall


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7.4 Slurry wall foundations


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Phases of constructionof slurry wall foundation


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One of the first applications:

construction of the Metro Milan


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rclect17 foundations 12

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