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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