Confined masonry overview

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Training on

Retrofitting Techniques and

Correction/exceptional Manual

29 May - 01 , June 2017, Kathmandu , Nepal

CONFINED MASONRY (CM)

Kuber Bogati

Structural Engineer

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ObjectivesAs a result of this session, you should be able to:

• Understand about Confined Masonry Building : Key Concepts

• Know why Confined Masonry in Nepal

• Compare Reinforced Masonry and Confined Masonry

• Compare RC Frame with URM Infill Vs. CM,

• Know Seismic Performance of CM Buildings

• Understand How CM Resists Earthquake Effects

• Damages in Confined Masonry in Past Earthquakes

• Know General Planning and Design Aspects

• Know Guidelines for Non-engineered CM Buildings

• Introduce with Minimum Requirement & Inspection Form of CM

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What is Confined Masonry (CM) Construction

• CM construction consists of masonry walls and horizontal and vertical RCconfining members built on all four sides of a masonry wall panel.

• Masonry walls : made either of clay brick or concrete block units

• Confining Elements :• Vertical ties : (Tie-columns or Practical columns)• Horizontal ties : (Tie-beams)

Key Components of a Confined Masonry (CM) Building

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• CM construction consists of masonry walls and horizontal and vertical RCconfining members built on all four sides of a masonry wall panel.

• Masonry walls : made either of clay brick or concrete block units

• Confining Elements :• Vertical ties : (Tie-columns or Practical columns)• Horizontal ties : (Tie-beams)

Key Components of a Confined Masonry (CM) Building

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CONFINED MASONRY

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Confined Masonry Construction

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CONFINED MASONRY

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Structural Components of a Confined Masonry (CM) Building

• Confining Elements : Provide restraint to masonry walls and protect them fromcomplete disintegration even in major EQs.

• Confining members are effective in• Enhancing the stability and integrity of masonry walls for IP & OOP EQ

Loads• Enhancing the strength (resistance) of masonry walls under EQ loads• Reducing the brittleness of masonry walls under EQ loads

• Masonry walls : Transmit the gravity load from the slab(s) above down to thefoundation. The walls act as bracing panels, which resist horizontal EQ. forces.Must be confined by concrete ties

• Floor and Roof Slabs : acts as diaphragms, transmit gravity and lateral loads tothe walls

• Plinth Band : Transmits the load from the walls down to the foundation.

• Foundation : Transmits the loads from the structure to the soils

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Construction Masonry (CM) – Global Context

• Evolved through informal process based on performance in earthquake• Practiced in central and south American countries since as early as 1930’s and

40’s• Currently practiced in several countries of high seismic risks- Latin, America,

Mediterranean Europe, Iran, Indonesia, China and in India (late comer)• CM if Properly built, shows satisfactory performance in severe earthquakes in

the past

• 1985 Mexico earthquake (M8.0)• 2001 La paz earthquake (elsalvador) (M7.7)• 2004 Sumtra earthquake (Indonesia) (M9.0)• 2007 Pisco earthquake (Peru) (M8.0)• 2010 Chile earthquake (M8.8) and others

• Confined masonry network established in 2008 under WHE with two objectives• To improve design and construction of CM where is currently in use• To introduce CM in areas where it can reduce seismic risks

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World Wide Practices

1. Chile2. Colombia3. Mexico4. Peru5. Argentina6. Eurocode7. Algeria8. China9. Iran10.Indonesia11.India

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Confined Masonry in Nepal - Context

• Construction of reinforced concrete frame and masonry wall is trending in citiesand towns

• Heavy damage observed in those construction in the last earthquake even in lowPGA and spectral acceleration

• Non-ductile RC frame construction• Unreinforced masonry walls vulnerable to lateral loading

• The presence of wall is in RC construction is not utilized as well as theconsequence of irregularity is overlooked

Confined masonry construction provides opportunity for improved performance in earthquake utilizing constriction from both RC and masonry components [technologies which require similar (preferably lower) level of construction skills and are economically viable]

Its simple in design and analogues to conventional construction of RC frame with walls (EXTENSIVE ENGINEERING INPUT NOT REQUIRED)

Reinforced Masonry vs. Confined Masonry

• Reinf. Enhance strength, Stability

• Corners, T-junction, additional Location

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Reinforced Concrete Frame Construction

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RC frame with URM infill vs. Confined Masonry

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RC frame with URM infill vs. Confined Masonry

Integrity of wall and frame

Construction sequence

Frame first, Wall later Wall first, Column/Beams later

Source : Tom Schacher

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Seismic Performance of CM

Confined masonry construction is found in countries/regions with very high seismic risk,

• Latin America (Mexico, Chile, Peru, Argentina),

• Mediterranean Europe (Italy, Slovenia),

• South Asia (Indonesia), and the Far East (China).

• In some countries (e.g. Italy) for almost 100 years

• If properly built, shows satisfactory seismic performance

EXTENSIVE ENGINEERING INPUT NOT REQUIRED!

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Oaxaca quake,September 1999

Tecomán earthquake,January 2003

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Confined masonry construction has been exposed to several destructive earthquakes:

• 1985 Lloleo, Chile (magnitude 7.8)

• 1985 Mexico City, Mexico (magnitude 8.0)

• 2001 El Salvador (magnitude 7.7)

• 2003 Tecoman, Mexico (magnitude 7.6)

• 2007 Pisco, Peru (magnitude 8.0)

• 2003 Bam, Iran (magnitude 6.6)

• 2004 The Great Sumatra Earthquake and Tsunami, Indonesia (magnitude 9.0)

• 2007 Pisco, Peru (magnitude 8.0)

• 2010 Maule, Chile earthquake (magnitude 8.8)

• 2010 Haiti earthquake (magnitude 7.0)

Confined masonry buildings performed very well in these majorearthquakes – some buildings were damaged, but no humanlosses

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A six-storey confined masonry building remained undamaged in the August 2007 Pisco, Peru earthquake (Magnitude 8.0) while many other masonry buildings experienced severe damage or collapse

Confined Masonry Performed Very Well in Past Earthquakes

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How Confined Masonry Buildings Resist Earthquake Effects

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Toothing : Monolithic action Horizontal Reinforcement

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Mechanism of shear resistance for a confined masonry wall panel

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Confined Masonry Building : Vertical Truss Model (left) and Collapse at theGround Floor Level (right)

Masonry : Diagonal Struts RC : Tension/compression Cracking at G.F. (soft story) : horizontal reinforcement

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Figure 8. Critical regions in a confined masonry building: a) a general diagram showing critical regions in the RC tie-columns

Masonry : Diagonal Struts RC : Tension/compression Cracking at G.F. (soft story) : horizontal reinforcement

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How Confined Masonry Buildings Resist Earthquake Effects…

Failure modes characteristic of CM Walls :

• Shear Failure Mode (due to IP Seismic Loads)

• Flexural Failure Mode (due to OOP Loads)

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How Confined Masonry Buildings Resist Earthquake Effects…

Shear Failure Mode (due to IP Seismic Loads)

Flexural Failure Mode (due to OOP Loads)

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Key Factors Influencing Seismic Resistance of CM Structures

• Wall Density : Strength

• Masonry Units and Mortar : (Stronger)

• Tie -Columns : (Ductility & Stability)

• Horizontal Wall Reinforcement

• Openings : 10% , Load path,

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Key Factors Influencing Seismic Resistance of CM Structures…

Horizontal Wall Reinforcement

Failure modes in the confined masonry walls with openings

The walls with largeropenings develop diagonalcracks

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Damages in Confined Masonry in Past Earthquakes

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Damage Observation: Topics

1.Masonry damage (in-and out-of-plane)

2.RC tie-columns

3.Tie-beam-to-tie-column joints

4.Confining elements around openings

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In-plane shear failure of masonry walls at the base level - hollow clay blocks (Cauquenes)

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In-plane shear failure of masonry walls at the base level - hollow clay blocks (Cauquenes)

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Out-of-Plane Wall Damage

Damage at the 2nd floor level

• An example of out-of-plane damage observed in a three-storey building

• The damage concentrated at the upper floor levels

• The building had concrete floors and timber truss roof

• The same building suffered severe in-plane damage

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Tie-Column Failure

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Buckling of a Tie-Column due to the Toe Crushing

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Shear Failure of RC Tie-Columns

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Inadequate Anchorage of Tie-Beam Reinforcement

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Deficiencies in Tie-Beam – to - Tie – Column Joint Reinforcement Detailing

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Absence of Confining Elements at the Openings

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In-Plane Shear Cracking – the Effect of Confinement

Unconfined openings Confined openings

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Key Causes of Damage in CM

1.Inadequate wall density

2.Poor quality of masonry materials and construction

3.Inadequate detailing of reinforcement in confining elements

4.Absence of confining elements at openings

5.Geotechnical issues

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General Planning and Design Aspects

• Architectural Guideline

• Construction Guideline

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Architectural Guideline

1. Plan Shape : Rectangular

NOYES

IRREGULARSYMMENTRICAL

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2. Plan Shape : Length-to-width ratio less than 4 times

NOYES

POORLY PROPORTIONED PLAN WELL PROPORTIONED PLAN

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3. Walls should be in a symmetrical

NO YES

INADEQUATE PLAN : LAYOUT ADEQUATE SHAPE

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4. Walls should be continuous up the building height

NO YES

LOAD PATH NOT CLEAR LOAD PATH CLEAR

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5. Opening : same position up the building height#Vertical ties : At both sides ( if opening <1.5 Sq.m.) (Toproduce diagonal Strut Action)

NO YES

POOR LOCATION OF WINDOW AND DOOR OPENING

GOOD LOCATION OF WINDOW AND DOOR OPENING

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6. Confining Elements : Tie-beams at 3m vertical spacing: Tie-columns at 4m: Wall to wall intersection: Free end of a wall

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7. Walls : At least three fully confined walls should beprovided in each direction

Inadequate Wall Distribution Adequate Wall Distribution

NOYES

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8. Walls Density : At least 5 % in each of two orthogonaldirection

𝑾𝒂𝒍𝒍 𝑫𝒆𝒏𝒔𝒊𝒕𝒚 =𝑻𝒐𝒕𝒂𝒍 𝑿 − 𝒔𝒆𝒄𝒕𝒊𝒐𝒏𝒂𝒍 𝑨𝒓𝒆𝒂 𝒐𝒇 𝒂𝒍𝒍 𝒘𝒂𝒍𝒍𝒔 𝒊𝒏 𝒐𝒏𝒆 𝒅𝒊𝒓𝒆𝒄𝒕𝒊𝒐𝒏

𝑺𝒖𝒎 𝒐𝒇 𝑭𝒍𝒐𝒐𝒓 𝒑𝒍𝒂𝒏 𝒂𝒓𝒆𝒂 𝒇𝒐𝒓 𝒂𝒍𝒍 𝒇𝒍𝒐𝒐𝒓𝒔 𝒊𝒏 𝒂 𝒃𝒖𝒊𝒍𝒅𝒊𝒏𝒈

Eurocode 8 (1996)a) At least 2% for a site with a design ground accln up to

0.2g (corresponding to seismic zone II of Indiab) At least 4% for a site with a design ground accln up to

0.3g (corresponding to seismic zone III of Indiac) At least 5% for a site with a design ground accln up to

0.4g (corresponding to seismic zone IV of India

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9. Building Height : Low-to medium-rise (Eurocode 8,1996)

Eurocode 8 (1996)a) Up to 4-story high for a site with a design ground

accln up to 0.2g (corresponding to seismic zone II of India

b) Up to 3-story high for a site with a design ground accln up to 0.3g (corresponding to seismic zone III of India

c) Up to 2-story high for a site with a design ground accln up to 0.4g (corresponding to seismic zone IV of India

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Guidelines for Non-Engineered CM Buildings

𝑺𝒕𝒐𝒓𝒆𝒚 ∶ 𝑶𝒏𝒆 𝒐𝒓 𝑻𝒘𝒐

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𝟏.𝑾𝒂𝒍𝒍 𝑫𝒆𝒏𝒔𝒊𝒕𝒚𝒂.𝑴𝒂𝒔𝒐𝒏𝒓𝒚𝑾𝒂𝒍𝒍𝒔

Walls Density : At least 5 % in each of two orthogonal direction

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Guidelines for Non-Engineered CM Buildings2. Openings𝒂.𝑴𝒂𝒔𝒐𝒏𝒓𝒚𝑾𝒂𝒍𝒍𝒔

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3. Wall Spacing𝒂.𝑴𝒂𝒔𝒐𝒏𝒓𝒚𝑾𝒂𝒍𝒍𝒔

• Building with Flexible floor should not exceed4.0 m in high seismic region

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4. Wall Dimensions and H/t ratios Restrictions

𝒂.𝑴𝒂𝒔𝒐𝒏𝒓𝒚𝑾𝒂𝒍𝒍𝒔

• Minimum wall thickness 110 mm

• H/t : less than 25, for one or two storey

• H/L : Should not less be than 0.5

• Maximum wall height : 3 m

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5. Parapets and Gable Walls𝒂.𝑴𝒂𝒔𝒐𝒏𝒓𝒚𝑾𝒂𝒍𝒍𝒔

• Parapets

• RC tie column should extent to the top : (1.2m)

• Otherwise Parapet height : 0.5m

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Guidelines for Non-Engineered CM Buildings6. Toothing at the Wall – to – tie-column interface

Toothing in confined masonry walls: a) machine-made hollow units, b) hand-made solidunits, and c) provision of horizontal reinforcement when toothing is not possible.

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Guidelines for Non-Engineered CM Buildings6. Toothing at the Wall – to – tie-column interface

Toothing applications: a) recommended construction practice (S. Brzev), and b) not recommended - absence of toothing in concrete block construction (C. Meisl).

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𝟏. 𝑺𝒑𝒂𝒄𝒊𝒏𝒈

𝒃. Confining Elements (Tie-Columns and Tie-Beams)

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𝟏. 𝑺𝒑𝒂𝒄𝒊𝒏𝒈


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2. Minimum Dimensions


• Tie – column size : (Depth x Width) : 150 mm x t

• Tie-beam Size : same as tie-column size or tx150 mm

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3. Reinforcements


• Minimum 4 reinforcing bars for tie - column , 2 tie - beam

• Bar size : 12 mm dia (Fe 500 or Fe 415)

• Stirrups/C-hooks : 6 mm dia @150 mm at center

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It is preferred to place beam reinforcement outside the column reinforcement cage

YESNO

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3. Reinforcements


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3. Reinforcements


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4. Construction issues


• Confining elements must be carefully constructed

• Slump : 125 mm recommended

• Concrete can be cast in three lifts when continuous is not possible

• RC tie-columns should not be cast above the completed portion of the wall

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Guidelines for Non-Engineered CM Buildings5. Foundation and Plinth Construction• Similar as traditional masonry construction

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Guidelines for Non-Engineered CM Buildings5. Foundation and Plinth Construction• Similar as traditional masonry construction

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𝒄. Additional Requirements for Building with Flexible Diaphragms

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𝒅. 𝑪𝒐𝒏𝒔𝒕𝒓𝒖𝒄𝒕𝒊𝒐𝒏 𝑸𝒖𝒂𝒍𝒊𝒕𝒚

• Construction quality has a significant bearing in seismic performance of CM building

• Properly designed and built CM buildings performed well in past earthquakes in most cases

• Poorly built ones experienced damage

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An Example Illustrating Wall Density Calculation

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Storey : twoSeismic Zone : VWall thickness : 110 mm

Typical Floor Plan of a Confined Masonry Building

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1. Floor area per floor = 4*9.2 = 36.8 m^2Total floor area for 2 floorsTOTAL FLOOR AREA = 2*36.8 = 73.6 m^2

2. Wall density in the longitudinal directionWall area ( walls 1 & 2 only) :

Wall Area = [9.2+(9.2-1.2)]*(0.11) = 1.9 m^2

𝑾𝒂𝒍𝒍 𝑫𝒆𝒏𝒔𝒊𝒕𝒚 =𝑾𝒂𝒍𝒍 𝑨𝒓𝒆𝒂

𝑻𝒐𝒕𝒂𝒍 𝑭𝒍𝒐𝒐𝒓 𝑨𝒓𝒆𝒂

=𝟏. 𝟗

𝟕𝟑. 𝟔= 𝟎. 𝟎𝟐𝟔 = 2.6 %

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3. Wall density in the Transverse directionWall area ( walls A, B & C) :

Wall Area=[4.0+(4.0-1.2)+4.0-1.2]*(0.11) = 1.1 m^2

𝑾𝒂𝒍𝒍 𝑫𝒆𝒏𝒔𝒊𝒕𝒚 =𝑾𝒂𝒍𝒍 𝑨𝒓𝒆𝒂

𝑻𝒐𝒕𝒂𝒍 𝑭𝒍𝒐𝒐𝒓 𝑨𝒓𝒆𝒂=

𝟏. 𝟏

𝟕𝟑. 𝟔= 𝟎. 𝟎𝟏𝟓 = 1.50 %

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THANK YOU

Engineering

Confined masonry overview