confined masonry buildings: construction practice and seismic …€¦ · confined masonry...

1

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

British Columbia Institute of Technology, Vancouver, Canada

Confined Masonry Buildings: Construction Practice and Seismic

Design Concepts

2

…and they can be very destructive

EARTHQUAKES HAPPEN

3

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

4

5

CONFINED MASONRY:

an opportunity for improved seismic

performance both for unreinforced

masonry and reinforced concrete

frame construction in low- and

medium-rise buildings

6

Confined Masonry Construction: An

Alternative to Reinforced Concrete Frame

Construction

An example from Chile (Source: Ofelia Moroni)

7

Confined Masonry Construction: An Alternative

to Unreinforced Masonry Construction

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

8

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).

9

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

10

11

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)

12

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.

13

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

14

Reinforced Concrete Frame Construction

15Confined Masonry Construction

16

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.

17

Components of a Confined Masonry Building

18

Confined Masonry: Construction PracticeAn example from Chile

19

Indonesia

Mexico

Pakistan

Peru

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

20

Location of Confining Elements is Very Important!

21

Location of Confining Elements is Very Important!

22

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.

23

How to Determine Wall Density?

d= Aw/Ap

24

Recommended Wall Density

The Guide recommends d values from 1 to 9.5%

25

How to Distribute Seismic Forces to Walls ?

Wall i

(area Ai)

Vi

V = Total seismic force

at a floor level

26

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

27

Mechanism of Seismic Response in a Confined Masonry Building

Masonry walls

Critical region

Diagonal cracking

Source: M. Astroza lecture notes, 2010

28

Mechanism of Seismic Response in a Confined Masonry Building

Masonry walls

Damage in critical regions

Onset of Diagonal cracking

29

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.

30

This seismic performance should be avoided!

31

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

32

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)!

33

Design Walls for Shear

34

In-plane shear failure: hollow clay block masonry

35

In-plane shear failure of masonry walls at the base level - hollow clay

blocks (Cauquenes)

36

In-plane shear failure of masonry walls at the base level (cont’d)

37

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

38

Tie-Column Failure

39

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

40

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)

41

How to prevent buckling failure of RC tie-columns?

Source: Confined Masonry Guide, Jan 2011 draft

42

Shear Failure of RC Tie-Columns

43

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

44

Confined Masonry: Construction Details

Good connections are of critical importance!

45

Inadequate Anchorage of Tie-Beam Reinforcement

46

Inadequate Anchorage of Tie-Beam Reinforcement (another example)

47

Tie-column Vertical Reinf and Tie-Beam Longitudinal Reinforcement

48

Draft guide available online at

www.confinedmasonry.org