Design of Reinforced Masonry

Prof. A. R. SanthakumarVisiting Professor

IIT Madras

Design Principle

• Steel reinforcement can be placed to enhance load resistance

• It also makes the wall ductile

• Steel may be placed in grouted cavities in hallow blocks are inserted in specially made holes in bricks

• It can also be provided in specially made bonds

Basis of Design

• Masonry is designed for a specific action

• It is also common to have reinforcement for one action and unreinforced for another action. Example for shear and compression

• Design is carried out using the same principle as RCC

• Equilibrium and strain compatibility should be considered

Type of reinforcements

• Main reinforcements for compression, bending and shear

• Secondary reinforcement for shrinkage and temperature

• Reinforcement should either be grouted or enclosed in lean concrete

• AS 3600 and AS 3700

Recommendation of codes

• There are no recommendations in IS

• Australian code gives detailed recommendations

• AS 3700

• Limit State method is followed with suitable reduction factors for shear and to avoid brittle buckling failure

Monadnock Building, Chicago, 1891, Burnham and Root, architects

Concept of Shear Wall System

Hanalei Hotel, San Diego

Types of Masonry Construction

2Thickness 15” min

Thickness

(a) Hollow concrete block prisms

Concrete Masonry Compression-test prisms

High-Rise Concept in Block Masonry

Holiday Inn Motel, a Round Bearing-wall Multi-storey Structure

2 d

ays

at 1

6m =

30m

5 days at 8m=40m

Plan

N

4m 8m 3m 2.25m 2.5m 2.25m 1m 3m 4m

30m East wall elevation

Diaphragm level

7.5m

4m

1.5m

1.5m

One Storey Commercial Building

BEARING WALLS

BEARING WALLS

BEARING WALLS

TRANSVERSE SHEAR WALLS

Some Examples of Two-directional Bearing/shear wall layouts

BEARING WALLS

BEARING WALLS

TRANSVERSE SHEAR WALLS

Examples of multi-directional bearing/shear wall layouts

t

h

Conventional load bearing wall

Continuous support

t

Deep wall beams

t

h

Deep wall beam

Columns or rootings

WALL SPAN

300 or 450 PLAN

100

mm

100

mm

400

WALL SPAN

WA

LL H

EIG

HT

P

anel

Wal

l R

einf

orci

ng S

teel

Required Embedment

400 or 600 Dia.

ELEVATION

100

PA

NE

L W

ALL

R

einf

orci

ng S

teel

W

ALL

HE

IGH

T

SECTION

Brick pier-and-panel garden walls

WALL PLAN

(A) BRICK WALL

DE

PT

H

1.3m

LENGTH 6m

TO

FR

OS

T

DE

PT

H

MIN

IMU

M

MA

XIM

UM

HE

IGH

T 1

.6m

100

WALL SECTION t

Serpentine walls

WALL SECTION

GRADE IN BLOCK OR CAST-IN-PLACE CONC. BELOW GRADE

300

450

150

TO

FR

OS

T D

EP

TH

MIN

IMU

M

MA

XIM

UM

HE

IGH

T 1

5 x

t

t

WALL SECTION

SHORT RADIUS AT FREE END

1m RAD

3m

RA

D

3m R

AD

PILASTER AT FREE END

WALL PLAN

1m

DE

PT

H

LENGTH 6m

(B) CONCRETE BLOCK WALL

Lateral load design of masonry walls and their behavior

There are three types’ failure modes that define seismic behavior of structural masonry walls when subjected to in-plane seismic loads. The mechanism depends on the geometry of the wall (height/ width ratio) and quality of materials, and the type of load transfer.

Sliding shear failure

• In the situation of low vertical load and poor quality mortar, seismic loads frequently cause shearing of wall causing sliding of the upper part of the wall at one of the horizontal mortar joints.

Shear failure

• It is a typical mode of failure of masonry wall subjected to seismic loads, and it takes place where the principal tensile stresses, developed in the wall under a combination of vertical and horizontal loads exceeds the tensile strength of masonry.

flexural mode of failure (flexural compression).

With the improved shear resistance and high moment/shear ratio, crushing of compresses zones at the ends of the wall usually take place,

1. There are several different types of structural systems employed to resist the lateral forces which carry the loads from the various floor levels to the foundation.

2. The vertical structural elements used to transfer lateral forces are 1. Shear walls 2.braced frames 3.moment resisting space frames, 4.combination of above.

3. The horizontal structural elements which distribute these forces to the vertical resisting elements are the floor and roof diaphragms.

Floor and roof diaphragms

Buildings Resist Horizontal Earthquake Forces

1. Horizontal Parts:Roof & Floor Structures Diaphragms

2. Vertical PartsSpan horizontal elements Shearwalls

House Element Resist Horizontal Forces

Two-story building

Arrows on left of figure are the seismic forces based on the weight of the building.

Arrow at the roof: represents the seismic force from both the roof weight and one-half of the weight of the walls between the second floor and roofline (F1).

Arrow at the second floor: represents the seismic force of half the second floor weight and one-half of the weight of the first and second story walls (F2).

Arrow at the first floor: represents the force at the first floor that is similarly calculated (F3).

Arrow at the foundation level: Sum of all these forces that must be transmitted safely into the ground. This is why the foundation and cripple wall are so important (FSum=F1+F2+F3)

No Shear Wall at Garage

House Elements Resist Gravity

•What part of a building resists the horizontal earthquake forces?

•Both horizontal and vertical parts of the building resist horizontal earthquake forces.

•Horizontal parts: roof and the floor structures. These parts are called diaphragms.

•Vertical parts that span between the horizontal elements. These walls are called shear walls.

Seismic Force Distribution

The diaphragms are classified into three groups of relative flexibilities:

rigid, flexible, and semi rigid.

 It is assumed to tribute the horizontal forces to the vertical resisting elements in direct proportion to the relative rigidities of those elements.

This premise stems from the fact that under a symmetrical loading, the rigid diaphragm, which in it self does not deform appreciably will cause each vertical element to deflect the same amount.

Rigid diaphragms are capable of transferring lateral and torsional forces to the walls.

Rigid diaphragm

It may be likened to a series spans extending between very rigid supports, (i.e. vertical resisting elements).

It is assumed here that the relative stiffness of these non yielding supports is very great compared to that of the diaphragm, which therefore deflects as a beam.

This beam, having no appreciable continuity across the supports, thus develops no negative moment over them which would affect the distribution of lateral load

Flexible diaphragm:

These exhibits significant deflection under load, and also have sufficient stiffness to distribute a portion of their load to the vertical elements in direct proportion to the rigidities of those elements.

Semi rigid diaphragm

Horizontal forces at any floor or roof level may be transferred to the foundation through the strength and rigidity of the side walls, called as shear walls.

The design strength of shear walls is often governed by flexure.

However, in low walls, the governing design criterion may be shear, Masonry shear walls can be described not only in terms of types of masonry used , but also as load- bearing , non load bearing , reinforced or unreinforced, solid or perforated rectangular or flanged and cantilevered or coupled.

Vertical stability elements

Moment Shear

Deflection of walls due to bending and shear deformations

P

P

Ph P

h

c=m+v 

mm

c AG

Ph

IE

Ph 2.1

3

3

L

h

L

h

tE

p

m

c 343

Rigidity of the pier =Rc =

c1

L

h

L

hp

tEm

343=

Δf

PPh/2

Ph/2

PP

Moment

Shear

Deflection of walls due to bending and shear deformations

Rigidity of the pier = Rf =

L

h

L

hp

tEm

33

Effect of aspect ratio on deflection due to shear 

Aspect ratioh/L

Percentage deflection due to shear

Cantilever wall Fixed end wall

0.25 92 98

1 43 75

2 16 43

4 5 16

8 1 4.5

1For squat walls (h/L < 0.25), rigidities based on shear deformations are reasonably accurate.2For (0.25<h/L<4) intermediate cantilever walls both deflections components should be include ‘d’ in the calculation of relative rigidities.For high (h/L) the effect of shear deformation is very small and rigidity based on flexural stiffness is reasonably accurate.

SEISMIC RESISTANCE OF RAT-TRAP BOND WALL AND

FILLER SLAB SYSTEM

FIGURE 2 TYPICAL CROSS SECTION

The Building System

Rat – Trap Bond Masonry

Typical Corner Joint

Method of Construction

Advantages

Validation

Experimental setup 

EXPERIMENTAL SET-UP

225230

150230

920230

150

230

660

150

230

# #

#

##

#

#

#

#

#

One layer brick on edge

15 MB 300

15 MB 300 15 MB 300

16

2

59

7

4 83

10

1.2

6

5.97

10

8

4.3

WALLUNDERTEST

ELEVATION END VIEW

1. STRAILS

2. DEFORMAIONS

3. LOADS

4. FAILURE PATTERN

Data for 1 cubic metre 

Earthquake Resistance 

Test Procedure

Load vs Moment

Load Application

Specimen Details

S.No.

Name Load (N) Moment (N mm)

Failure Between

1 450M1 650 292500 Brick and Concrete surface at the bottom level

2 450M2 18431 310500 Brick and Concrete surface at the bottom level

3 340M3 1440 597600 II and III level Bricks

4 340M4 6143 601750 II and III level Bricks

5 450M5 4733 647400 I and II level Bricks

6 450M6 7973 9337500 I and II level Bricks

7 340M7 90000 0 Vertical cracks on all four sides

Practical Case

Practical Case

Safe Moment(Nmm)

Axial Load (N)

Safe Span of the Cantilever (L1)(mm)

1000 200000 574.9891

2000 325000 759.8557

3000 450000 911.9664

4000 575000 1044.287

5000 685000 1149.326

6000 775000 1229.175

7000 885000 1320.751

8000 940000 1364.414

Safe Cantilever Spans for Limiting Tension in Brickwork