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Svetlana Brzev, Ph.D., P.Eng.

British Columbia Institute of Technology, Vancouver, Canada

Confined Masonry Buildings: Construction Practice and Seismic

Design Concepts

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…and they can be very destructive

EARTHQUAKES HAPPEN

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Recent Deadly Earthquakes in the World

➢ 1993 Latur, Maharashtra, India – 8000 deaths

➢ 1999 Ducze, Turkey – 20000 deaths

➢ 2001 Bhuj, Gujarat, India – 14000 deaths

➢ 2003 Boumerdes, Algeria – 3000 deaths

➢ 2004 The Great Sumatra Earthquake and Tsunami in Indonesia, Thailand, Sri Lanka, and India -270,000 deaths

➢ 2005 Kashmir Earthquake in Pakistan and India -100000 deaths

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CONFINED MASONRY:

an opportunity for improved seismic

performance both for unreinforced

masonry and reinforced concrete

frame construction in low- and

medium-rise buildings

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Confined Masonry Construction: An

Alternative to Reinforced Concrete Frame

Construction

An example from Chile (Source: Ofelia Moroni)

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Confined Masonry Construction: An Alternative

to Unreinforced Masonry Construction

An example from Indonesia (Source: C. Meisl, EERI)

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Earthquake Performance

Confined masonry construction has been practiced in countries/regions with very high seismic risk, such as

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

➢Mediterranean Europe (Italy, Slovenia), ➢ South Asia (Indonesia), ➢Middle East (Iran) and ➢ the Far East (China).

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Earthquake Performance (cont’d)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 (magnitude 8.8)

Confined masonry buildings performed very well in these major earthquakes – some buildings were damaged, but no human losses

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Confined Masonry and RC Frame Construction: Performance in Recent

Earthquakes

January 2010, Haiti

M 7.0

300,000 deaths

February 2010, Chile

M 8.8

521 deaths

(10 due to confined masonry construction)

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Confined Masonry Construction: a Definition

Confined masonry is a construction system

where the walls are built first, and RC

columns and beams are cast afterwards.

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A difference between the confined masonry and reinforced concrete frames = construction

sequence

Confined Masonry

– Walls first

– Concrete later

Reinforced Concrete Frame

– Concrete first

– Walls later

Source: Tom Schacher

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Reinforced Concrete Frame Construction

15Confined Masonry Construction

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Key Components of a Confined Masonry Building

Key structural components of a confined masonry building are:

➢Masonry walls made either of clay brick or concrete block units

➢Tie-columns = vertical RC confining elements which resemble columns in reinforced concrete frame construction.

➢Tie-beams = horizontal RC confining elementswhich resemble beams in reinforced concrete frame construction.

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Components of a Confined Masonry Building

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Confined Masonry: Construction PracticeAn example from Chile

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Indonesia

Mexico

Pakistan

Peru

Confined Masonry Under Construction in Earthquake-Prone Regions of the World

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Location of Confining Elements is Very Important!

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Location of Confining Elements is Very Important!

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Wall Density

➢ A 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

➢ Wall density index (d) is a ratio of the total wall area in each orthogonal

direction and the floor plan area

➢ 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?

d= Aw/Ap

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Recommended Wall Density

The Guide recommends d values from 1 to 9.5%

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How to Distribute Seismic Forces to Walls ?

Wall i

(area Ai)

Vi

V = Total seismic force

at a floor level

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Seismic Force Distribution

F= 1 if H/L1.33

OR

F=(1.33 L/h)2

if H/L>1.33

This is based on the Mexican Code NTC-M 2004

=

=N

i

ii

iii

AF

AFVV

1

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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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Mechanism of Seismic Response in a Confined Masonry Building

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 masonry walls into critical regions of 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 seismic performance should be avoided!

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Seismic Design of Confined Masonry Components = Wall + RC Confining

Elements

Wall + RC confining elements

Wall

= shear

due to V

= +Confining elements

=tension/compression due to M

V

V M

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Seismic Design of Walls for Shear

V

Use CSA S304.1 Cl.10.10.1.1

Vr=Vm

Consider masonry shear resistance only

See page 2-11 of Seismic Masonry Guide (Anderson and Brzev)

Diagonal crack is caused by shear stresses (same as beams)!

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Design Walls for Shear

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In-plane shear failure: hollow clay block masonry

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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 (cont’d)

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Design of RC Confining Elements

Find T and C forces due to M and design according to CSA A23.3 concrete code (same as RC columns)

MT C

d1

Mr=0.9*As*s*Fy*d1

As= total steel area in a tie-column

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Tie-Column Failure

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Buckling of a RC Tie-Column due to the Toe Crushing of the Masonry Wall Panel

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How to prevent buckling and 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 buckling failure of RC tie-columns?

Source: Confined Masonry Guide, Jan 2011 draft

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Shear Failure of RC Tie-Columns

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How to Prevent Shear Failure?

Vp Vr/2

Vr = wall shear resistance

Same approach like RC frames with infills!

Vr

Source: M. Astroza lecture notes, 2010

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Confined Masonry: Construction Details

Good connections are of critical importance!

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Inadequate Anchorage of Tie-Beam Reinforcement

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Inadequate Anchorage of Tie-Beam Reinforcement (another example)

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Tie-column Vertical Reinf and Tie-Beam Longitudinal Reinforcement

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Draft guide available online at

www.confinedmasonry.org