• ELEMENTS OF ARCHITECTURAL STRUCTURES:


  •   
  • FileName: lect26.pdf [preview-online]
    • Abstract: ELEMENTS OF ARCHITECTURAL STRUCTURES:FORM, BEHAVIOR, AND DESIGNARCH 614DR. ANNE NICHOLSSPRING 2012lecturetwenty sixconcrete construction: www.tamu.edufoundation design

Download the ebook

ELEMENTS OF ARCHITECTURAL STRUCTURES:
FORM, BEHAVIOR, AND DESIGN
ARCH 614
DR. ANNE NICHOLS
SPRING 2012
lecture
twenty six
concrete construction: www.tamu.edu
foundation design
Foundations 1 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Foundation
• the engineered interface between the
earth and the structure it supports that
transmits the loads to the soil or rock
Foundations 2 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
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 3 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
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
Foundations 4 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
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
Foundations 5 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Soil Properties & Mechanics
• compressibility
– settlements
• strength
– stability
• shallow foundations
• deep foundations
• slopes and walls
– ultimate bearing capacity, qu finehomebuilding.com
qu
– allowable bearing capacity, qa 
S.F.
Foundations 6 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Soil Properties & Mechanics
• strength, qa
Foundations 7 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Bearing Failure
• shear
slip zone punched wedge
punched wedge
Foundations 8 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Lateral Earth Pressure
• passive vs. active
active
(trying to
move wall)
passive
(resists movement)
Foundations 9 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Foundation Materials
• concrete, plain or reinforced
– shear
– bearing capacity
– bending
– embedment length, development length
• other materials (piles)
– steel
– wood
– composite
Foundations 10 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
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 11 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
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)
Foundations 12 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Types of Foundations
• spread footings
• wall footings
• eccentric footings
• combined footings
• unsymmetrical footings
• strap footings
Foundations 13 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Types of Foundations
• mat foundations
• retaining walls
• basement walls
• pile foundations
• drilled piers
Foundations 14 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Shallow Footings
• spread footing
– a square or rectangular footing supporting
a single column
– reduces stress from load to size the ground
can withstand
Foundations 15 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Actual vs. Design Soil Pressure
• stress distribution is a function of
– footing rigidity
– soil behavior
RIGID RIGID
sand clay
• linear stress distribution
assumed
Foundations 16 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Proportioning Footings
• net allowable soil pressure, qnet
– qnet  qallowable  h f (  c   s )
– considers all extra weight (overburden)
from replacing soil with concrete
– can be more overburden
• design requirement
with total unfactored
load: P
 qnet
A
Foundations 17 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Concrete Spread Footings
• plain or reinforced
• ACI specifications
• Pu = combination of factored D, L, W
• ultimate strength
– Vu  Vc :   0.75 for shear
• plain concrete has shear strength
– Mu  Mn :   0.9 for flexure
Foundations 18 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Concrete Spread Footings
• failure modes
shear
bending
Foundations 19 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Concrete Spread Footings
• shear failure
one way shear two way shear
Foundations 20 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Over and Under-reinforcement
• reinforcement ratio for bending
As
–
bd
– use as a design estimate to find As,b,d
– max  from steel  0.004
– minimum for slabs & footings of uniform
thickness As  0.002 grade 40 / 50 bars
bh
 0.0018 grade 60 bars
Foundations 21 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Reinforcement Length
• need length, ld
– bond
– development of yield strength
Foundations 22 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Column Connection
• bearing of column on footing
– Pu  Pn   0.85 f cA1 
  0.65 for bearing A1
A2 1
– confined: increase x A2 2 2
A1
• dowel reinforcement
– if Pu > Pb, need
compression
reinforcement
– min of 4 - #5 bars
(or 15 metric)
Foundations 23 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Wall Footings
– 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
Foundations 24 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Eccentrically Loaded Footings
• footings subject to moments
P P
e
M=Pe
by statics:
– soil pressure resultant force may not
coincide with the centroid of the footing
Foundations 25 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Differential Soil Pressure
– to avoid large rotations,
limit the differential soil P
pressure across footing M
– for rigid footing,
simplification of soil
pressure is a linear
distribution based on
constant ratio of pressure to settlement
Foundations 26 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Kern Limit
• boundary of e for
no tensile stress
• triangular stress
block with pmax
wpx
volume  N
2
2N
pmax 
wx
Foundations 27 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Guidelines
– want resultant of load from pressure inside
the middle third of base (kern)
• ensures stability with respect to overturning
P
M resist R x M
SF    1.5
M overturning M
x R
– pressure under toe (maximum)  qa
– shortcut using uniform soil pressure for
design moments gives similar steel areas
Foundations 28 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Combined Footings
– 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
Foundations 29 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Combined Footing Types
– rectangular
– trapezoid
– strap or cantilever
• prevents overturning of exterior column
– raft/mat
• more than two columns
over an extended area
Foundations 30 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Proportioning
– uniform settling is desired
– area is proportioned with sustained column
loads
– want the resultant to coincide with centroid
of footing area for uniformly distributed
pressure
P1 P2
assuming a
rigid footing
qmax  qa y
R = P1+P2
Foundations 31 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Retaining Walls
• purpose
– retain soil or other material
• basic parts
– wall & base
– additional parts
• counterfort
• buttress
• key
Foundations 32 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Retaining Walls
• considerations
– overturning
– settlement
– allowable bearing pressure
– sliding
– (adequate drainage)
Foundations 33 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Retaining Walls
• procedure
– proportion and check stability with working
loads for bearing, overturning and sliding
– design structure with factored loads
M resist
SF   1.5  2
W M overturning
Fx
Fhorizontalresist
o
Fresist
SF   1.25  2
R F sliding
Foundations 34 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
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” t
H
b
hf
B
Foundations 35 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Retaining Walls Forces
• design like cantilever beam
– Vu & Mu for reinforced concrete
– Vu  Vc :   0.75 for shear
– Mu  Mn :   0.9 for flexure
Foundations 36 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Retaining Wall Types
• “gravity” wall
– usually unreinforced
– economical & simple
• cantilever retaining wall
– common
Foundations 37 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Retaining Wall Types
• counterfort wall
very tall walls (> 20 - 25 ft)
• buttress wall
• bridge abutment
• basement frame wall (large basement areas)
Foundations 38 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Deep Foundations
• 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
Foundations 39 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Deep Foundation Types
– piles - usually driven, 6”-8”  , 5’ +
– piers
drilled, excavated,
– caissons concreted (with or
– drilled shafts without steel)
– bored piles 2.5’ - 10’/12’ 
.
– pressure injected piles
Foundations 40 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Deep Foundation Types
Foundations 41 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Deep Foundations
• 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
Foundations 42 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Piles Classified By Material
• timber
– use for temporary construction
– to densify loose sands
– embankments
– fenders, dolphins (marine)
• concrete lift hooks
– precast: ordinary
reinforcement or prestressed
– designed for axial capacity
and bending with handling
Foundations 43 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Piles Classified By Material
• 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
Foundations 44 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Piles Classified By Function
– end bearing pile (point bearing)
Pa  A p  fa
for use in soft or
soft or loose loose
layer
materials over a
dense base
“socketed”
Rp
– friction piles (floating)
P common in both P tapered:
clay & sand sand & silt
Rs =ƒ(adhesion) T
N
RP  0
Foundations 45 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Piles Classified By Function
– combination friction and end bearing
P
– uplift/tension piles
structures that float, Rs
towers
Rp
– batter piles
angled, 1:12 to P
cost more, 1:3 or 1:4
resist large
horizontal loads
Foundations 46 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Piles Classified By Function
– fender piles, dolphins, pile clusters
large # of piles
in a small area
– compaction piles
• used to densify loose sands
– drilled piers
• eliminate need for pile caps
• designed for bearing capacity (not slender)
Foundations 47 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614
Pile Caps and Grade Beams
– like multiple column footing
– more shear areas to consider
Foundations 48 Elements of Architectural Structures S2012abn
Lecture 26 ARCH 614


Use: 0.4506