ISBN 978-9937-0-9019-3

Seismic Strengthening Of Stone Masonry

Wall Using Horizontal Bands

Paras Khati

Department of Earthquake Enginering

IOE Thapathali Campus

Kathmandu, Nepal paraskhati@gmail.com

a) Abstract— This paper introduces a technically

feasible and economically affordable horizontal

bandage retrofitting technique for low

earthquake resistant masonry structures in

developing countries as Nepal. An unreinforced

masonry wall section has been tested for lateral

strength before and after introduction of

horizontal bands. The wall thus seismic

strengthened by introduction of horizontal bands

using section modifier. The Stresses developed in

wall of retrofitted model are decreased by 50-

90% than its original modal. The analysis

demonstrates introduction of horizontal bands

can mitigate the risk to unreinforced masonry

wall buildings in future scenario earthquakes.

This method improves the in-plane and out-of-

plane mechanism of unreinforced masonry wall.

Keywords— Horizontal bands, Retrofitting, Stone

Masonry, Section modifier.

I. INTRODUCTION

Nepal lies in earthquake prone zone. Nepal has

encountered many earthquake in past. Nepal has

suffered 16 major earthquakes, including recent

Gorkha Earthquake of 25 April 2015 in which more

than six lakhs houses were fully damaged, three lakh

houses were partly damaged among which most were

traditionally constructed unreinforced stone masonry

with mud mortar. Unreinforced stone masonry with

mud mortar is mostly used construction materials in

the Nepal. Here the construction practice of masonry

system is non-engineered. It is also most vulnerable

against earthquakes. The unreinforced masonry is a

brittle material. Hence, if the stress state within the

wall exceeds masonry strength, brittle failure occurs,

followed by possible collapse of the wall and the

building. Based on post-earthquake damage surveys,

the major types of masonry failure modes have been

identified as:

a. Failure of In-Plane Walls

It depend on the geometry of the wall

(Height/Width ratio), quality of materials

and on boundary restraints and loads acting

on the wall. In-plane loads usually exhibit

three modes of failure:

Sliding Shear: In case of low vertical

load and poor quality mortar.

Shear: Wall loaded with significant

vertical load as well as horizontal

forces can fail in shear with diagonal

cracking. Diagonal cracking of piers

either start from corners of openings or

in solid walls, from the wall ends

Bending: this type of failure can occur

if walls are with improved shear

resistance. Crushing of compression

zones at the ends of the wall usually

takes place

b. Failure of Out-plane Wall

Occur due to the lack of adequate wall

ties, bands or cross walls. When ties are

adequate, failure mode is out of plane

bending horizontally. Slender walls are

more likely to suffer this type of

damage.

c. Corner Separation

Failure is due to lack of lateral support

at two ends of the wall during out of

plane loading. Separation of orthogonal

walls due to in-plane and out-of-plane

stresses at corners.

d. Delamination of Walls

Caused by lack of integrity of two

wyths of the wall.

Openings in the masonry walls will form short piers,

which will experience concentrations of shear stresses

and hence diagonal cracks. The corners of the

openings, tension cracks may appear due to the

reverse cyclic stress induced by lateral loading.

Major deficiencies of masonry buildings:

i. Lack of Strength and Stiffness

ii. Lack of Integrity between Walls

Roof and Floor

iii. Absence of Vertical and Horizontal

Bands

KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”

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iv. Lack of Cross Walls

v. Asymmetric Configuration of

buildings

vi. Inadequate Gap between Adjacent

Buildings

vii. Construction Deficiency

Most of the Human casualties due to earthquake are

due to structural damage of low strength masonry

buildings (mainly with mud mud mortar). This can be

mitigated by improving seismic performance of the

existing low strength masonry structures. A suitable

retrofitting technique for Stone masonry structures

with mud mortar in developing countries should

guarantee not only its efficiency in terms of

improvement of the seismic resistant characteristics

of the structure, it should also be considered that; the

used material is economical and locally available and

the required labor skill is low. For this introduction of

horizontal bands in low strength masonry wall plays a

vital role in strengthening the masonry structure. A

main objective of this paper is to evaluate seismic

performance of stone masonry wall before and after

introduction of horizontal bands. Introduction of

horizontal bands technique is to take the lateral load

in the structure thus decreasing the stresses developed

in masonry wall preventing the collapse of the

structure.

The advantages of the horizontal bands retrofitting

method compared to the other earthquake

strengthening technology are as follows:

• The method is simple and inexpensive.

• The method does not require high skills.

• The materials required for the method can are

cheap and easily available.

II. METHODOLOGY

The analysis and design of the Stone Masonry wall

with mud mortar is done using SAP Analysis and

manually. First of all manual calculation is done and

the wall is checked for stresses whether it is under

permissible stresses or not .The stresses developed in

wall are greater than the permissible stresses. Here

the retrofittig technique (ie., introduction of

horizontal bands) is adopted .The analysis and design

presented for the calculation of the reinforcement to

be provided in Lintel & Sill bands here is

approximate and is in very simplified approach. A

typical common style, single stone masonry wall with

mud mortar 300 mm thick, 3300 mm height and

10800mm long with opening 1200mm*1500mm 2-

windows and 1200mm*2100mm 1-Door is

considered for the study. Parameters adopted for

stone masonry wall with mud mortar modeling are:

Unit Weight = γ = 19 KN/m3

Poisson’s Ratio = ν = 0.1(for mud mortar 0.1 to 0.15)

Modulus of Elasticity (E) =700 N/mm2

Slenderness ratio (SR) = Maximum of and =

11 or 46.5 = 46.5 taken

Eccentricity of wall = 0

Height to width ratio = = 0.305 < 0.75

Fig. 1. 3D Model of wall in SAP2000

The wall was modeled using SAP 2000 and analyzed

for stress concentration and distribution. The resultant

stresses: shear, axial compression and tension were

checked against permissible stresses and upon failure,

the strengthening measure was designed.

Assuming Masonry Unit with Crushing Strength 10

N/mm2 and mortar type L2 (mud mortar), referring to

Table 8 of IS 1905-1987,

Basic Compressive strength (fb) = 0.53 N/mm2

Stress reduction factor (Ks) = 0.43

Area Reduction Factor (Ka) = 0.7 + 1.5 A =1

for A being greater than 0.2 m2

Shape Modification Factor (Kp) =1

Permissible Compressive Strength (Fa) = 0.53 x

0.43 x 1 x 1 = 0.228 N/mm2

KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”

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Check against S11 in model: 0.826N/mm2 > 0.228

N/mm2

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Allowable Shear Stress Calculation:

τc = 0.1 + fd/6 ,

Where fd = compressive stress due to dead load

in N/mm2 (from Analysis)

τc = permissible shear stress in N/mm2

τc = 0.1 + 0.038/6 = 0.106 N/mm2

Check shear stress against S12 in model: 0.412

N/mm2 > 0.106 N/mm

2

The stresses induced in the wall are beyond its

strength.

Hence, strengthening measured is needed.

Resisting Bending Moment, M = T x Z

T=C= Allowable stress x steel area

The allowable stress is increased by 25% for

earthquake loading

Allowable stress in steel = 1.25 x 0.56 x fy

Bending moment (M) = q x ℓ2 /10

M = 1.25 x 0.56 x fy x Ast x Z

Ast is calculated and used in bands using section

modifier in SAP 2000.

Here RCC horizontal Bands are used as retrofitting

technique in stone masonry structure with mud

mortar.

The wall thus retrofitted using horizontal bands. The

dimension for Lintel and Sill bands from manual

calculations (i.e. lintel band- 400*65*10800 and sill

band- 350*65*10800) are used in SAP 2000 as frame

section with section modifier. The wall was modeled

using layered shell design providing equivalent

thickness of steel. The resultant stresses induced after

strengthening were analyzed and checked against

permissible stresses for the composite section.

Fig. 2. Section Design of Lintel band in SAP 2000

The permissible stresses check were maintained as

per IS 1893: 2000. The permissible stresses for

retrofitted section were computed as :

Permissible Compressive Strength (Fretrofittd wall)

Fretrofittd wall = Fwall + Fsteel (1)

F wall only = fb x Ks x Ka x Kp (2)

Fsteel = fs x Ast provided (3)

Assuming Masonry Unit with Crushing Strength 10

N/mm2 and mortar type L2 (mud mortar) , referring

to Table 8 of IS 1905-1987,

Basic Compressive strength (fb) = 0.53 N/mm2

Stress reduction factor (Ks) = 0.43

Area Reduction Factor (Ka) = 0.7 + 1.5 A =1 for A

being greater than 0.2 m2

Shape Modification Factor (Kp) =1

Similarly, permissible Shear Strength of retrofitted

wall (τc (retrofitted wall) ) was computed as,

τc (retrofitted wall) = τc original wall only + τc (steel) ) (4)

τc original wall only = 0.1 + fd/6 (5)

Where,

fd = compressive stress due to dead load in N/mm2

τc = permissible shear stress in N/mm2

τc (steel) computed from Table 23 of IS 456:2000

The wall increased capacity in terms of Axial

compression and Shear Stress and the induced

stresses were also safe within its limits.

III. RESULTS AND DISCUSSION

The analysis of stone masonry wall with mud mortar

was done using SAP 2000 and manual calculation

was done for design of bands. The seismic design

included determination of dimension of horizontal

bands and reinforcement in bands. The seismic

analysis of the wall was carried out considering

earthquake in two directions. The design forces for

retrofitting were determined by considering direct and

torsional forces due to lateral loads, axial load.

TABLE I. MODAL COMPARISION TABLE

Parameters Unit

Original

Model

Retrofitted

Model Remarks

Length m 10.8 10.8 lintel band-

(400*65*10800)

and sill band-

(350*65*10800)

are used in SAP

2000 as frame

section with

section modifier

Height m 3.3 3.3

Thickness m 0.3

0.43 at

bands &

0.3 at all other parts

Permissible

compressive

stresses N/mm² 0.288 3.288

Capacity has

increased

Permissible

Shear Stress N/mm² 0.106 0.551

Shear Strength has

increased.

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TABLE II. OUTPUT COMPARISION TABLE FROM SAP 2000

Fig. 3. Axial Stress Distribution S11 in Original Wall

Fig. 4. Axial StressDistribution S11 in Retrofitted Wall

Fig. 5. Shear Stress Distribution S12 in Original Wall

Fig. 6. Shear Stress Distribution S12 in Retrofitted Wall

a) The Stresses developed in wall of retrofitted

model are decreased by 50-90% than its

original modal.

b) The permissible compressive stresses and

shear stress of retrofitted wall are increased by

11.4 and 5.2 times of original model

respectively.

c) Base shear is increased provided by the added

layer of steel and concrete in retrofitted model.

Limitations of the study:

a) The basic crushing strength of masonry unit

was assumed to be 10N/mm2.

b) The study only focuses on local strengthening

of wall elements at lintel and sill level.

c) The analytical models using SAP 2000, uses

equivalent steel methodology for determining

the reinforcement in Lintel and sill bands.

d) The modeling is done only for a single wall

structure.

Case

Type Stress

Original

Model

Retrofitted

Model

Remarks

Envelope

s11 0.982 0.099

Stresses in

retrofitted

model are

decreased

s22 0.826 0.052

s12 0.412 0.123

Dead s11 0.038 0.016

s12 0.035 0.033

Base Shear(KN)

86.27 113.67

Base shear

is

increased

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IV. CONCLUSION

In Nepal construction practice of masonry system is

non-engineered which are most vulnerable against

earthquakes. This can be mitigated by improving

seismic performance of the existing low strength

masonry structures. A suitable retrofitting technique

for Stone masonry structures with mud mortar is

introduction of bands which, increases it's

compressive and shear strength significantly and

provides safety against failure in compression and

shear. It shows:

1. Increase in permissible compressive stress

and shear stress in wall by 11.4 and 5.2

times of original model respectively.

2. Improvement of the in-plane and out-of-

plane strength of the wall by introduction of

horizontal bands.

REFERENCES

[1] Agrawal, P., & Shrikhande, M. (2013). Earthquake Resistant Design of Structures. Delhi: PHI Learning Private Limited.

[2] Bothara , J., & Guragain, R. (March 2004). Seismic retrofitting of low strength unreinforced masonry non-engineered school buildings. Bulletin of the New Zealand Society for Earthquake Engineering.

[3] Center of Resilient Development (CoRD), MRB Associates. (2016). Seismic Retrofitting Guidelines of Buildings in Nepal: Masonry. DUDBC/UNDP/Comprehensive Disaster Risk Management Program.

[4] (Program, May 2011).

[5] Earthquake Risk Reduction and Recovery Preparedness Program. (May 2011). Engineer's Training on Earthquake Resistant Design of Buildings Volume II. Department of Urban Development and Building Construction/UNDP.

[6] Earthquake Risk Reduction and Recovery Preparedness Programme for Nepal. (2016). Seismic Vulnerability Evaluation Guideline for Private and Public Buildings. Department of Urban Development and Building Construction/ UNDP.

[7] Indian Institute of Technology. (2005). IITK-GSDMA Guidelines for Structural Use of Reinforced Masonry. Kanpur: Gujarat State Disaster Mitigation Authority.

[8] IS 1905:2002 Code of Practice for Structural Use of Unreinforced Masonry, Third Revision. (2002). Bureau of Indian Standard.

[9] IS 456:2000, Plain and Reinforced Concrete-Code of Practice. (2005). Bureau of Indian Standards.

[10] Manandhar, V., Marasini, N., Prajapati, R., Guragain, R., & Chaulagain, R. (2020). Experimental investigation of low cost steel wire mesh retrofit for stone masonry in mud mortar. 17th World Comference on Earthquake Engineering. Sendai, Japan: The 17th World Conference in Earthquake Engineering.

[11] NBC 205. (n.d.).

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