confined masonry buildings: design and construction deficiencies
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Svetlana Brzev, Ph.D., P.Eng.British Columbia Institute of Technology,
Vancouver, Canada
Confined Masonry Buildings: Design and Construction
Deficiencies Observed in the 2010 Maule, Chile Earthquake
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TopicsTopics
Confined masonry: key conceptsConfined masonry: key concepts
Lessons learned from the 2010 Chile Lessons learned from the 2010 Chile earthquake earthquake
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Confined Masonry: Background and Key Design Concepts
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Why Confined Masonry?Why Confined Masonry?
Poor performance of unreinforced masonry and Poor performance of unreinforced masonry and
nonductilenonductile reinforced concrete (RC) frame reinforced concrete (RC) frame
construction caused unacceptably high human and construction caused unacceptably high human and
economic losses in past earthquakes economic losses in past earthquakes
This prompted a need for developing and/or This prompted a need for developing and/or
promoting alternative building technologiespromoting alternative building technologies
The goal is to achieve enhanced seismic performance using technologies which require similar (preferably lower) level of
construction skills and are economically viable
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CONFINED MASONRY:CONFINED MASONRY:
an opportunity for improved seismic an opportunity for improved seismic
performance both for unreinforced performance both for unreinforced
masonry and reinforced concrete masonry and reinforced concrete
frame construction in lowframe construction in low-- and and
mediummedium--rise buildingsrise buildings
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Confined Masonry Construction: Confined Masonry Construction: An Alternative to An Alternative to RC Frame ConstructionRC Frame Construction
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Confined Masonry Construction: An Alternative Confined Masonry Construction: An Alternative to to Unreinforced Masonry ConstructionUnreinforced Masonry Construction
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Confined Masonry: BeginningsConfined Masonry: Beginnings
Evolved though an informal process based on its satisfactory
performance in past earthquakes
The first reported use in the reconstruction after the 1908
Messina, Italy earthquake (M 7.2) - death toll 70,000
Practiced in Chile and Columbia since 1930’s and in Mexico
since 1940’s
Currently practiced in several countries/regions with high seismic risk, including Latin America, Mediterranean Europe, Middle East (Iran), South Asia (Indonesia), and the Far East (China).
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Confined Masonry and RC Frame Confined Masonry and RC Frame Construction: Performance in Recent Construction: Performance in Recent
EarthquakesEarthquakes
January 2010, Haiti January 2010, Haiti
M 7.0M 7.0
300,000 deaths300,000 deaths
February 2010, ChileFebruary 2010, Chile
M 8.8 521 deaths M 8.8 521 deaths
(10 due to confined masonry (10 due to confined masonry construction)construction)
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Global Confined Masonry InitiativeGlobal Confined Masonry Initiative
An International Strategy Workshop on the Promotion of Confined Masonry organized in January 2008 at Kanpur, India
Confined Masonry NetworkConfined Masonry Network established as a project of the World Housing Encyclopedia with two major objectives:
To improve the design and construction quality of confined masonry where it is currently in use; and
To introduce it in areas where it can reduce seismic risk.
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The Guide was developed by an international volunteer
committee consisting of masonry experts from 13
countries
Recommendations based on experience from countries
and regions where confined masonry construction has
been practiced for many decades, including Mexico, Peru,
Chile, Argentina, Iran, Indonesia, China, etc.
Seismic Design Guide for Confined Masonry BuildingsSeismic Design Guide for Confined Masonry Buildings
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Draft guide available online at
www.confinedmasonry.org
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Guide in a NutshellGuide in a Nutshell
Chapter 1 IntroductionIntroduction
Chapter 2 General Requirements General Requirements
Chapter 3 Guidelines for NonGuidelines for Non--Engineered Confined Masonry Engineered Confined Masonry BuildingsBuildings
Appendix ASimplified Method for Wall Density Calculation in Simplified Method for Wall Density Calculation in LowLow--Rise BuildingsRise Buildings
Appendix B Guidelines for Special Inspection of Confined Guidelines for Special Inspection of Confined Masonry ConstructionMasonry Construction
Appendix CSummary of Relevant International CodesSummary of Relevant International Codes
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Components of a Confined Masonry BuildingComponents of a Confined Masonry Building
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Confined Masonry Confined Masonry vsvs RC Frames with RC Frames with InfillsInfills –– Key Differences Key Differences
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Confined Masonry: Construction SequenceConfined Masonry: Construction Sequence
Masonry wall construction in progress
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Confined Masonry Construction: Confined Masonry Construction: ToothingToothingat the Wallat the Wall--toto--TieTie--Column InterfaceColumn Interface
Toothing enhances interaction between masonry walls and RC confining elements
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Location of Confining Elements is Very Location of Confining Elements is Very Important!Important!
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Wall DensityWall DensityA key parameter influencing the seismic performance of
confined masonry buildings
Confined masonry buildings with adequate wall density were
able to sustain the effects of major earthquakes without
collapse
The required d value depends on seismic hazard, soil type,
number of stories, building weight, and masonry shear
strength.
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How to Determine Wall Density? How to Determine Wall Density?
d= Aw/Ap
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Recommended Wall Density Values Recommended Wall Density Values
The Guide recommends d values from 1 to 9.5%
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Simple Approach for Seismic Design of Confined Masonry Components
Wall + RC Wall + RC confining confining elementselements
WallWall
= shear= shear
due to Vdue to V
== ++Confining elementsConfining elements
=tension/compression =tension/compression due to Mdue to M
V
V M
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Seismic Design Considerations for RC Confining Elements
TensionCompressionShear (to be discussed later)M
T C
d1
MMr=0.9*A=0.9*As**φφs**FFy*d*d1
As= total steel area in a tie-column
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Mechanism of Seismic Response in a Confined Masonry Building
Masonry walls
Critical region
Diagonal cracking
Source: M. Astroza lecture notes, 2010
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Failure Mechanism: Key Stages
Masonry walls
Damage in critical regions
Onset of Diagonal cracking
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Seismic Design Objectives
RC confining elements must be designed to
prevent crack propagation from the walls
into critical regions of RC confining elements.
This can be achieved if critical regions of the
RC tie-columns are designed to resist the
loads corresponding to the onset of diagonal
cracking in masonry walls.
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This condition should be avoided!
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Seismic Performance of Confined Masonry Buildings in the February
27, 2010 Chile Earthquake
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BackgroundBackground
Magnitude 8.8 earthquakeMagnitude 8.8 earthquake
521 deaths 521 deaths
5 collapsed buildings5 collapsed buildings
100 severely damaged buildings100 severely damaged buildings
Approximately 1% of the total building Approximately 1% of the total building
stock in the earthquakestock in the earthquake--affected area either affected area either
damaged or collapseddamaged or collapsed
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Confined Masonry (CM) Construction in Chile
Widely used for construction of low-rise single
family dwellings and medium-rise apartment
buildings (up to four-story high).
CM construction practice started in the 1930s, after
the 1928 Talca earthquake (M 8.0).
Good performance reported after the 1939 Chillan
earthquake (M 7.8) and this paved the path for
continued use of CM in Chile.
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Confined Masonry Construction in Chile (Cont’d)
Good performance track record in past earthquakes based on single family (one- to two-storey) buildings.
Three- and four-storey confined masonry buildings exposed to severe ground shaking for the first time in the February 2010 earthquake (construction of confined masonry apartment buildings in the earthquake-affected area started in 1990s).
Modern masonry codes first issued in 1990s – prior to that, a 1940 document “Ordenanza General de Urbanismo y Construcción” had been followed
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LowLow--Rise Confined Masonry ConstructionRise Confined Masonry Construction
Single-storey rural house
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LowLow--Rise Confined Masonry Construction Rise Confined Masonry Construction
Two-storey townhouses (semi-detached): small plan dimensions (5 m by 6 m per unit)
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Performance of Confined Masonry Construction Performance of Confined Masonry Construction
By and large, confined masonry buildings performed
well in the earthquake.
Most one- and two-story single-family dwellings did
not experience any damage.
Large majority of three- and four-story buildings
remained undamaged
A few buildings suffered severe damage, and A few buildings suffered severe damage, and two threetwo three--story buildings collapsedstory buildings collapsed
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Damage Observations: Topics Damage Observations: Topics
Masonry damage (in- and out-of-plane)
RC tie-columns
Tie-beam-to-tie-column joints
Confining elements around openings
Construction materials
Collapsed buildings
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InIn--plane shear failure of masonry plane shear failure of masonry walls at the base level walls at the base level -- hollow clay hollow clay
blocks (blocks (CauquenesCauquenes))
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InIn--plane shear failure of masonry plane shear failure of masonry walls at the base level (contwalls at the base level (cont’’d)d)
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InIn--plane shear failure: hollow clay plane shear failure: hollow clay block masonryblock masonry
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Same failure mechanism as infill masonry in RC frames!
Diagonal Tension (INPS-2), FEMA 306 p.207
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InIn--plane shear failure: clay brick masonryplane shear failure: clay brick masonry
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Same failure mechanism as infill masonry in RC frames!
Bed Joint Sliding (INPS-3), FEMA 306 p.208
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OutOut--ofof--Plane Wall DamagePlane Wall Damage
An example of out-of-plane damage observed in a three-storey buildingThe damage concentrated at the upper floor levels The building had concrete floors and timber truss roofThe same building suffered severe in-plane damage
Damage at the 2nd floor level
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OutOut--ofof--Plane Damage Plane Damage (cont(cont’’d)d)
Damage at the 3rd floor level
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Floor and Roof Diaphragms
Wood floors in single-family buildings (two-storey high)Concrete floors in three-storey high buildings and up (either cast-in-situ or precast) Precast concrete floors consist of hollow masonry blocks, precast RC beams, and concrete overlay (“Tralix” system)
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Discussion on Out-of-Plane Wall Failure
Source: Confined Masonry Guide, Jan 2011 draft
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Tie-Column Failure
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Buckling of a RC TieBuckling of a RC Tie--Column due to the Column due to the Toe Crushing of the Masonry Wall Panel Toe Crushing of the Masonry Wall Panel
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Shear Failure of RC TieShear Failure of RC Tie--Columns Columns
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Same failure mechanism as columns in RC frames with infills!
Column Snap Through Shear Failure (INF1C1), FEMA 306 p.211
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How to prevent buckling and shear failure of RC tie-columns?
All surveyed buildings in Chile had
uniform tie spacing 200 mm
Tie size 6 mm typical, in some cases 4.2
mm (when prefabricated cages were
used)
Closer tie spacing at the ends of tie-columns (200 mm regular and 100 mm at ends)
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How to Prevent Shear Failure?
Must check shear capacity of tie-columns!
Vp ≥ Vr/2
Vr = wall shear resistanceSame approach like RC frames with infills!
Note: an increase of tie-column length may be required in some cases!
Vr
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Inadequate Anchorage of TieInadequate Anchorage of Tie--Beam Beam ReinforcementReinforcement
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Inadequate Anchorage of Tie-Beam Reinforcement (another example)
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Tie-Beam Connection: Drawing Detail
TieTie--Beam Intersection: Plan ViewBeam Intersection: Plan View
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Tie-Column-to-Tie-Beam Connection: Drawing Detail (prefabricated reinforcement)
TieTie--columncolumn
TieTie--beambeam
Note additional reinforcing bars at the tie-beam-to-tie-column joint
(in this case, prefabricated reinforcement cages were used for tie-beams and tie-columns)
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Tie-Column-to-Tie-Beam Reinforcement: Anchorage
Alternative anchorage details involving 90°hooks (tie-column and tie-beam shown in an elevation view) – note that no ties in the joint area were observed
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Deficiencies in TieDeficiencies in Tie--BeamBeam--toto--TieTie--Column Joint Reinforcement DetailingColumn Joint Reinforcement Detailing
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RC Tie-Columns: Absence of Ties in the Joint Area
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Tie-column Vertical Reinf and Tie-Beam Longitudinal Reinforcement
It is preferred to place beam reinforcement outside It is preferred to place beam reinforcement outside the column reinforcement cagethe column reinforcement cage
NONO YESYES
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Tie-Column Reinforcement: Drawing Detail (Chile)
Note prefabricated tie-column reinforcement: 8 mm longitudinal bars and 4.2 mm ties at 150 mm spacingAdditional ties to be placed at the site per drawing specifications
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Absence of Confining Elements at the OpeningsAbsence of Confining Elements at the Openings
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InIn--Plane Shear Cracking Plane Shear Cracking –– the Effect the Effect of Confinementof Confinement
Unconfined openings Confined openings
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Recommendation…
Unconfined and confined openings - criteria specified in the Guide
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Building Materials: Hollow Concrete Blocks ???
Concrete blocks are widely used for masonry
construction in North America and
The quality is very good due to advanced
manufacturing technology
Quality of blocks in other countries often not
satisfactory due to low-tech manufacturing
technology and an absence of quality control
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A severely damaged confined masonry A severely damaged confined masonry concrete block wall in Chileconcrete block wall in Chile
In spite of poor seismic performance, it is impossible to avoid the use of concrete blocks for masonry construction in many countries…
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Masonry Units Masonry Units –– Confined Masonry GuideConfined Masonry Guide
The guide permits the use of concrete blocks, but restricts the percentage of perforations and minimum compressive strength: 4 MPa (bricks) and 5 MPa (blocks-gross area)
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Haiti Blocks
Block D(215 psi)
Block C(1000 psi)
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A Two-Storey Concrete Block Masonry Building, Santiago, Chile
A part of the A part of the community community (200 buildings) (200 buildings) built in the 1910s built in the 1910s using using CMUsCMUs imported imported from the UKfrom the UK
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Block Testing
Typical block dimensions:
501 mm x 138 mm x 251 mm
Block testing setup
Average compressive strength 25 Average compressive strength 25 MPaMPa
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Engineered Confined Masonry Buildings Engineered Confined Masonry Buildings –– Evidence of CollapseEvidence of Collapse
Two 3-storey confined masonry buildings
collapsed in the February 2010 Chile
earthquake (Santa Cruz and Constitución)
Most damage concentrated in the first
storey level
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A Possible Collapse Mechanism for MultiA Possible Collapse Mechanism for Multi--storey Confined Masonry Buildingsstorey Confined Masonry Buildings
7272
Seismic Performance of Confined Masonry Buildings: Seismic Performance of Confined Masonry Buildings: ShakeShake--Table StudiesTable Studies
Shake-Table Testing of a 3-storey Confined Masonry Building at UNAM, Mexico (Credit: Sergio Alcocer and Juan Arias)
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AB
C
N
Building #1: Building Complex in Constitución (Cerro O’ Higgins)
Note a steep slope on the west and north sides!
74collapsedcollapsed
Three Building Blocks: A, B and CThree Building Blocks: A, B and C
A B C
damageddamaged
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Building Plan – Collapsed Building
RC Tie-Columns:P1= 15x14 cmP2 = 20x14 cmP4 = 15x15 cmP5 = 70x15 cmP6 = 90x14 cm
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Building C CollapseBuilding C Collapse
Building C collapsed at the first floor level and moved by approximately 1.5 m towards north
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Building C Collapse (contBuilding C Collapse (cont’’d)d)
C
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Probable Causes of CollapseProbable Causes of Collapse
1.1. Geotechnical issues: a localized influence of Geotechnical issues: a localized influence of
the unrestrained slope boundary and the unrestrained slope boundary and
localized variations in sublocalized variations in sub--surface strata surface strata
caused localized variations of horizontal caused localized variations of horizontal
(and possibly vertical) ground accelerations(and possibly vertical) ground accelerations
2.2. Inadequate wall density (less than 1% per Inadequate wall density (less than 1% per
floor)floor)
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Building #2: A ThreeBuilding #2: A Three--Storey Building in Storey Building in Santa CruzSanta Cruz
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Collapsed ThreeCollapsed Three--Storey Building Storey Building
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Probable Causes of CollapseProbable Causes of Collapse
Poor quality of construction (both brick and concrete block masonry)Low wall density (less than 1% per floor)
Note: only one (out of 28) buildings in the same complex collapsed !
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Key Causes of Damage Key Causes of Damage
1.1. Inadequate Inadequate wall density wall density 2.2. Poor quality of masonry materials and Poor quality of masonry materials and
constructionconstruction3.3. Inadequate detailing of reinforcement Inadequate detailing of reinforcement
in confining elementsin confining elements4.4. Absence of confining elements at Absence of confining elements at
openingsopenings5.5. Geotechnical issuesGeotechnical issues
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Acknowledgments
Earthquake Engineering Research Institute (SPI Projects Fund)Maximiliano Astroza and Maria Ofelia Moroni, Professors, Department of Civil Engineering, Universidad de Chile (members of the EERI team)Roberto Meli and other co-authors of the Confined Masonry Guide
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