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A RCHITECTURAL S TRUCTURES : Foundation F ORM, B EHAVIOR, AND D ESIGN ARCH 331 the engineered interface between the D R. A NNE N ICHOLS earth and the structure it supports that S PRING 2018 transmits the loads to the soil or rock lecture

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F2009abn

twenty seven

concrete construction: foundation design

Foundations 1 Lecture 27 Architectural Structures ARCH 331

lecture

Bright Football Complex www.tamu.edu

ARCHITECTURAL STRUCTURES: FORM, BEHAVIOR, AND DESIGN

ARCH 331

  • DR. ANNE NICHOLS

SPRING 2018

Foundations 2 Lecture 27 Foundations Structures ARCH 331 F2008abn

Foundation

  • the engineered interface between the

earth and the structure it supports that transmits the loads to the soil or rock

Foundations 3 Lecture 27 Foundations Structures ARCH 331 F2008abn

Structural vs. Foundation Design

  • structural design

– choice of materials – choice of framing system – uniform materials and quality assurance – design largely independent of geology, climate, etc.

Foundations 4 Lecture 27 Foundations Structures ARCH 331 F2008abn

Structural vs. Foundation Design

  • foundation design

– cannot specify site materials – site is usually predetermined – framing/structure predetermined – site geology influences foundation choice – no site the same – no design the same

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Foundations 5 Lecture 27 Foundations Structures ARCH 331 F2008abn

Soil Properties & Mechanics

  • unit weight of soil
  • allowable soil pressure
  • factored net soil pressure
  • shear resistance
  • backfill pressure
  • cohesion & friction of soil
  • effect of water
  • settlement
  • rock fracture behavior

F2008abn Foundations 6 Lecture 27 Foundations Structures ARCH 331

  • compressibility

– settlements

  • strength

– stability

  • shallow foundations
  • deep foundations
  • slopes and walls

– ultimate bearing capacity, qu – allowable bearing capacity,

Soil Properties & Mechanics

S.F. q q

u a 

finehomebuilding.com Foundations 7 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • strength, qa

Soil Properties & Mechanics

F2008abn Foundations 8 Lecture 27 Foundations Structures ARCH 331

Bearing Failure

  • shear

slip zone punched wedge

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Foundations 9 Lecture 27 Foundations Structures ARCH 331 F2008abn

Lateral Earth Pressure

  • passive vs. active

active (trying to move wall) passive (resists movement)

Foundations 10 Lecture 27 Foundations Structures ARCH 331 F2008abn

Foundation Materials

  • concrete, plain or reinforced

– shear – bearing capacity – bending – embedment length, development length

  • other materials (piles)

– steel – wood – composite

Foundations 11 Lecture 27 Foundations Structures ARCH 331 F2008abn

Basic Foundation Requirements

  • safe against instability or collapse
  • no excessive/damaging settlements
  • consider environment

– frost action – shrinkage/swelling – adjacent structure, property lines – ground water – underground defects – earthquake

  • economics

Foundations 12 Lecture 27 Foundations Structures ARCH 331 F2008abn

Generalized Design Steps

  • calculate loads
  • characterize soil
  • determine footing location and depth
  • evaluate soil bearing capacity
  • determine footing size (unfactored loads)
  • calculate contact pressure and check

stability

  • estimate settlements
  • design footing structure* (factored loads)
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Foundations 13 Lecture 27 Foundations Structures ARCH 331 F2008abn

Types of Foundations

  • spread footings
  • wall footings
  • eccentric footings
  • combined footings
  • unsymmetrical footings
  • strap footings

Foundations 14 Lecture 27 Foundations Structures ARCH 331 F2008abn

Types of Foundations

  • mat foundations
  • retaining walls
  • basement walls
  • pile foundations
  • drilled piers

Foundations 15 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • spread footing

– a square or rectangular footing supporting a single column – reduces stress from load to size the ground can withstand

Shallow Footings

Foundations 16 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • stress distribution is a function of

– footing rigidity – soil behavior

  • linear stress distribution

assumed

Actual vs. Design Soil Pressure

RIGID sand RIGID clay

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Foundations 17 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • net allowable soil pressure, qnet

– – considers all extra weight (overburden) from replacing soil with concrete – can be more overburden

  • design requirement

with total unfactored load:

Proportioning Footings

) ( h q q

s c f allowable net

    

net

q A P 

Foundations 18 Lecture 27 Foundations Structures ARCH 331 F2008abn

Concrete Spread Footings

  • plain or reinforced
  • ACI specifications
  • Pu = combination of factored D, L, W
  • ultimate strength

– 0.75 for shear

  • plain concrete has shear strength

– 0.9 for flexure

    :

n u

M M     :

c u

V V

Foundations 19 Lecture 27 Foundations Structures ARCH 331 F2008abn

Concrete Spread Footings

  • failure modes

shear bending

Foundations 20 Lecture 27 Foundations Structures ARCH 331 F2008abn

Concrete Spread Footings

  • shear failure
  • ne way shear

two way shear

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Foundations 21 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • reinforcement ratio for bending

– – use as a design estimate to find As,b,d – max  from steel  0.004 – minimum for slabs & footings of uniform thickness

Over and Under-reinforcement

bd As  

bars grade bars grade bh As 60 0018 . 50 / 40 002 .  

Foundations 22 Lecture 27 Foundations Structures ARCH 331 F2008abn

Reinforcement Length

  • need length, ld

– bond – development of yield strength

F2008abn Foundations 23 Lecture 27 Foundations Structures ARCH 331

Column Connection

  • bearing of column on footing

– 0.65 for bearing – confined: increase x

  • dowel reinforcement

– if Pu > Pb, need compression reinforcement – min of 4 bars and 0.005Ag

 

1

85 . A fc    

n u

P P   2 A A

1 2

A1 A2

2 1

Foundations 24 Lecture 27 Foundations Structures ARCH 331 F2008abn

– continuous strip for load bearing walls – plain or reinforced – behavior

  • wide beam shear
  • bending of projection

– dimensions usually dictated by codes for residential walls – light loads

Wall Footings

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Foundations 25 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • footings subject to moments

– soil pressure resultant force may not coincide with the centroid of the footing

Eccentrically Loaded Footings

e P M=Pe P by statics:

Foundations 26 Lecture 27 Foundations Structures ARCH 331 F2008abn

Differential Soil Pressure

– to avoid large rotations, limit the differential soil pressure across footing – for rigid footing, simplification of soil pressure is a linear distribution based on constant ratio of pressure to settlement

M P

Foundations 27 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • boundary of e for

no tensile stress

  • triangular stress

block with pmax

Kern Limit

N wpx volume   2 wx N p 2

max 

Foundations 28 Lecture 27 Foundations Structures ARCH 331 F2008abn

– want resultant of load from pressure inside the middle third of base (kern)

  • ensures stability with respect to overturning

– pressure under toe (maximum) qa – shortcut using uniform soil pressure for design moments gives similar steel areas

5 1

g

  • verturnin

. M x R M M SF

resist

   

Guidelines

M P x R

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Foundations 29 Lecture 27 Foundations Structures ARCH 331 F2008abn

– supports two columns – used when space is tight and spread footings would overlap or when at property line – soil pressure might not be uniform – proportion so pressure will uniform for sustained loads – behaves like beam lengthwise

Combined Footings

Foundations 30 Lecture 27 Foundations Structures ARCH 331 F2008abn

– rectangular – trapezoid – strap or cantilever

  • prevents overturning of exterior column

– raft/mat

  • more than two columns
  • ver an extended area

Combined Footing Types

Foundations 31 Lecture 27 Foundations Structures ARCH 331 F2008abn

– uniform settling is desired – area is proportioned with sustained column loads – want the resultant to coincide with centroid

  • f footing area for uniformly distributed

pressure assuming a rigid footing

Proportioning

P1 P2 R = P1+P2 y

a max

q q 

Foundations 32 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • purpose

– retain soil or other material

  • basic parts

– wall & base – additional parts

  • counterfort
  • buttress
  • key

Retaining Walls

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Foundations 33 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • considerations

– overturning – settlement – allowable bearing pressure – sliding – (adequate drainage)

Retaining Walls

Foundations 34 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • procedure

– proportion and check stability with working loads for bearing, overturning and sliding – design structure with factored loads

Retaining Walls

  • Fx

R W

2 5 1

g

  • verturnin

   . M M SF

resist

2 25 1   

. F F SF

sliding resist horizontal

Fresist

Foundations 35 Lecture 27 Foundations Structures ARCH 331 F2008abn

Retaining Wall Proportioning

  • estimate size

– footing size, B  2/5 - 2/3 wall height (H) – footing thickness  1/12 - 1/8 footing size (B) – base of stem  1/10 - 1/12 wall height (H+hf) – top of stem  12”

H B hf t b

Foundations 36 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • design like cantilever beam

– Vu & Mu for reinforced concrete – 0.75 for shear – 0.9 for flexure

Retaining Walls Forces

    :

n u

M M     :

c u

V V

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Foundations 37 Lecture 27 Foundations Structures ARCH 331 F2008abn

Retaining Wall Types

  • “gravity” wall

– usually unreinforced – economical & simple

  • cantilever retaining wall

– common

Foundations 38 Lecture 27 Foundations Structures ARCH 331 F2008abn

Retaining Wall Types

  • counterfort wall
  • buttress wall
  • bridge abutment
  • basement frame wall (large basement areas)

very tall walls (> 20 - 25 ft)

Foundations 39 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • usage

– when spread footings, mats won’t work – when they are required to transfer the structural loads to good bearing material – to resist uplift or overturning – to compact soil – to control settlements of spread or mat foundations

Deep Foundations

Foundations 40 Lecture 27 Foundations Structures ARCH 331 F2008abn

– piles - usually driven, 6”-8” , 5’ + – piers – caissons – drilled shafts – bored piles – pressure injected piles

Deep Foundation Types

drilled, excavated, concreted (with or without steel) 2.5’ - 10’/12’ .

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Foundations 41 Lecture 27 Foundations Structures ARCH 331 F2008abn

Deep Foundation Types

Foundations 42 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • classification

– by material – by shape – by function (structural, compaction...)

  • pile placement methods

– driving with pile hammer (noise & vibration) – driving with vibration (quieter) – jacking – drilling hole & filling with pile or concrete

Deep Foundations

Foundations 43 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • timber

– use for temporary construction – to densify loose sands – embankments – fenders, dolphins (marine)

  • concrete

– precast: ordinary reinforcement or prestressed – designed for axial capacity and bending with handling

Piles Classified By Material

lift hooks

Foundations 44 Lecture 27 Foundations Structures ARCH 331 F2008abn

  • steel

– rolled HP shapes or pipes – pipes may be filled with concrete – HP displaces little soil and may either break small boulders or displace them to the side

Piles Classified By Material

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Foundations 45 Lecture 27 Foundations Structures ARCH 331 F2008abn

Piles Classified By Function

– end bearing pile (point bearing) – friction piles (floating)

“socketed” soft or loose layer

for use in soft or loose materials over a dense base

Rp

common in both clay & sand

Rs =ƒ(adhesion) P P T N

tapered: sand & silt

a p a

f A P   

P

R

Foundations 46 Lecture 27 Foundations Structures ARCH 331 F2008abn

Piles Classified By Function

– combination friction and end bearing – uplift/tension piles

structures that float, towers

P

– batter piles

P

1:12 to 1:3 or 1:4 angled, cost more, resist large horizontal loads

Rp Rs

Foundations 47 Lecture 27 Foundations Structures ARCH 331 F2008abn

Piles Classified By Function

– fender piles, dolphins, pile clusters – compaction piles

  • used to densify loose sands

– drilled piers

  • eliminate need for pile caps
  • designed for bearing capacity (not slender)

large # of piles in a small area

Foundations 48 Lecture 27 Foundations Structures ARCH 331 F2008abn

Pile Caps and Grade Beams

– like multiple column footing – more shear areas to consider