Post on 09-Feb-2022
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”
Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal
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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”
Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal
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.
KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”
Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal
KEC Conference 2021
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ISBN 978-9937-0-9019-3
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
KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”
Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal
KEC Conference 2021
223
ISBN 978-9937-0-9019-3
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.).
KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”
Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal
KEC Conference 2021
224