SEISMIC LIMIT STATES AND RESISTANTMECHANISM OF THE SOIL-CEMENT BRICKSCONFINED MASONRY
Nicolas GUEVARA1
MEE07156

Supervisor: Hiroshi FUKUYAMA2

ABSTRACT
Performance based philosophy is proposed to verify the seismic design of the soil-cement confined
masonry. Experimental wall data considering in-plane and out-of-plane behaviors have been used topropose three seismic limit states: Serviceability, Reparability and Safety. Besides that, two design
seismic forces, one given in the current special normative for designing and construction of housing andanother taken from recorded earthquakes, have been applied to the basic constructed house modulus toverify the safety as well as to verify its seismic design using current normative where the soil cement is
not considered as infill element.
Keywords: Performance based, soil-cement brick, confined masonry

INTRODUCTION
Burnt clay bricks have been traditionally used in El Salvador as unreinforced masonry and as infill
element in confined masonry. However, its use has been pointed out as harmful for the environment. The
crystallization process through firing the bricks using firewood has been devastating the forest in the
country. To reduce this growing problem, bricks made of soil and cement mixture have been currently
used as infill element in confined masonry instead of clay bricks. Initially, research studies were focusedon finding out the best soil-cement proportion without study the seismic behavior of the masonry system.
Due to the earthquakes in January and February of 2001, one cooperation project started, sponsored by theJapanese International Cooperation Agency (JICA). As part of this project “Enhancement of thetechnology for the construction and dissemination of popular earthquake resistant housing”, known as
Taishin Project, which has been already executed in its first phase; the soil-cement bricks system has beenstudied recently under the objective of to find out its mechanical properties. Besides that, the necessity of
upgrading the design philosophy, the Performance Based Design has been currently included as target forsecond phase of the mentioned project. The experimental research on the soil-cement has been alreadycarried out, covering differences stages since the survey of soil around the country until decide the bestsoil-cement mix, construction of walls and experimental test, which is used to reach the objectives of thisstudy. Therefore, the present study is devoted to propose performance levels or seismic limit states like
Serviceability, Reparability and Safety, by using the experimental wall data obtained during the researchstudy on soil-cement. Since the Collapse of masonry houses should be prevented, two seismic designforces are applied to the basic house modulus, one demand is taken from the recommendations given by

1
2

University Professor, University of El Salvador (UES), El Salvador.
Chief Research Engineer, Department of Structural Engineering, Building Research Institute (BRI), Japan.

1the special normative for designing and construction of houses, and another is taken from recorded pastearthquakes.

DATA
This research study is based on the experimental study executed to the soil-cement system. Beforehand,sixteen mixtures of soil-cement were widely studied to find out the best mix, taking into account for theselection, parameters like strength, crack pattern, etc. Such proportions of soil-cement are shown in table 1.
Table 1. Soil-cement proportions
Percentage of volume
Sand
Silt
100
0
75
75
50
50
25
75

Proportions
Cement-soil ratio
1:8, 1:10, 1:13, 1:16
1:8, 1:10, 1:13, 1:16
1:8, 1:10, 1:13, 1:16
1:8, 1:10, 1:13, 1:16

Around 70 brick units with dimension of
7x14x28cm were manufactured for each
proportion, this means around 1120 units
in total. The manufacturing procedure
followed was the one proposed by
FUNDASAL. The laboratory test carriedout for prism and bricks are shown intable 2.

Table 2. Laboratory test of brick unit and prisms
Besides, four prisms for compression
and four for diagonal compression were
manufactured for each mix; 128 prisms
Bricksin total. The mortar used to bond thebricks has a sand-cement ratio of 3:1,
Prismswhich is popularly used to bond bricks.
MortarSeveral scanning procedures were used
to select the best soil-cement proportion; even though, the last evaluations were done for four proportions,shown in table 3. Taking into account mechanical properties observed as well as the facility to measurethem in field, the proportion using 1:16 cement-soil and 50:50 sand-silt was selected to construct the wall
specimens shown and described in table 4.
Description

Test
Water absorption
Compression
FlexureCompression
Diagonal compression
Compression

No of specimen
80
80
80
64
64
36

Table 3. Experimental results in preliminary study

Description

BricksPrismsMortarLaboratory Test
Water absorption (%)
Compression (MPa)
Flexure (MPa)Compression (MPa)Diagonal compression(MPa)
Compression (MPa)

Cement:soil ratio (1:13)
Average
Averageresult 25-75
result 50-50
28.9
21.8
3.20
7.1
0.09
0.18
4.80
4.70

Cement:soil ratio (1:16)
Average result
Average25-75
result 50-50
21.2
34.1
3.60
5.20
0.07
0.13
4.80
3.60

0.81

0.67

0.81

0.55

----

----

22.7

12.7

The reinforcement of the columns and the beams was similar and consisted of 4 longitudinal bars with
9mm diameter, with stirrups of 6mm diameter. The interval of the stirrups was constant in both, beams

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and columns, and equal to 150mm. The dimensions of concrete sections were 150x150mm for thecolumns and beams. The panel dimensions were 3x3m approximately and were formed by 36 courses ofbricks with 10 units per course. In case of SPP specimen, three walls formed the unit, the lateral walls
were similar to the other models and the main panel had beam at the height of 3m.
Table 4. Wall specimens

Wall specimen SPM

Wall specimen SPC

Wall specimen SPCI

Wall specimen SPP

Besides, there are soil-cement houses already constructed before and after the current research. The basicmodel of popular houses has rectangular dimensions of 6.04x4.21m, measured from the exterior edges. It
has two doors and two windows as openings in the larger direction. The structural system of the roof
consists of channels “C” with 4x2 inches and steel bars with 9mm diameter. The steel bars are welded tothe channels to avoid the slipping. Those channels are supported by the ridge beam. In practicalconstruction, the ridge beam is fabricated welding two channels “C”. The roof consists of fibrocement
sheets placed over the steel bars and channels. The confinement elements with dimensions of 0.15x0.15m
and reinforcement as explained above.

THEORY AND METHODOLY
The last decade has witnessed a clear trend towards “performance-based” seismic design, which can bethought of as an explicit design for multiple limit states or performance levels. Analyzing structures forvarious levels of earthquake intensity and checking some local and/or global criteria for each level has
been a popular academic exercise for the last couple of decades, but the crucial development that occurred
relatively recently was the recognition of the necessity for such procedures by a number of practicing
engineers influential in code drafting. The approach involves the definition of one or more limit states andtheir associated performance criteria and design checks in a precise and quantitative manner. Seismic limit
states recognized in structural design are defined by the damage pattern, which depends on the
deformation level reached for structural and nonstructural elements. Therefore, their definition for thissystem requires the study of parameter like drift, stiffness, resistance and functionality of the structure,
which becomes thresholds when the buildings are being evaluated. Three seismic limit states are being
proposed in this study: Serviceability, Reparability and Safety. In order to give the best definition for thissystem, experimental data of in-plane and out-of-plane tests will be analyzed to find out the damage
sequence as well as its severity level. Seismic limit states are proposed for each specimen but also
proposed for the masonry system. Besides that, two seismic design forces will be applied to the basic
house modules with the main targets of checking the seismic design and the prevention of collapse limit
state.

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RESULTS AND DISCUSSION
The seismic limit states proposed for each specimen has been established through the evaluation of thedamage level for all specimens. The results shown in table 5 are similar, SPM and SPC specimens havethe same drift level for the serviceability and reparability limit states; however, due to the application ofload in only one direction, the collapse drift level is higher for the SPM. Higher drift for each limit state is
proposed when the mid-beam is used in construction.
Table 5. Summary of performance levels
SPM

SPC

SPCI

SPP

Seismic Limit States in Drift (%)
Serviceability
Reparability
Safety

SPM
0.05
0.20
0.55

SPC
0.05
0.20
0.40

SPCI
0.10
0.40
0.80

SPP
0.20
0.80
1.60

With this element the structure has twice the ductility of SPC, which means that the structure will be lessdamaged reaching large displacements. In case of the out-of-plane behavior, the drift levels are higher
than the drift for in-plane behavior; however, due to differences in definition of both drift, any comparison
can be carried out. Even so, it can be said that the lateral failure will occur first, which also agree with thedesign philosophy where the out-of-plane failure is avoided. After having proposed the seismic limit states,
the verification of the collapse house is necessary. To do that, two design seismic forces have been
imposed in the in plane and out of plane direction, by using the base shear coefficient provided for the
special normative for housing and by using high acceleration observed in past earthquakes. The soilcement system has shown accepted behavior for both motioned seismic forces in-plane of walls, see table
6. Even by applying the acceleration of 1g, the capacity of the system is enough to withstand the input

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motions. Damage could be expected for seismic motion with 1g acceleration, but the structure will keep
its functionality and the damage will be repaired in short time. In case of the out-of-plane behavior, theserviceability limit state is exceeded by using the recorded acceleration of 1g acceleration. The structure
will remain functioning, even when some cracks could have developed in the masonry. Using the baseshear coefficient of 0.3 provided for the normative, the structure will behave elastically, within theserviceability limit state.
Table 6. Summary of collapse house prevention
IN-PLANE

OUT-OF-PLANE

CONCLUSIONS
Regarding the Performance Based Design philosophy, three performance levels or seismic limit states
were defined through out this research, which are: Serviceability, Reparability and Safety. Serviceability
limit state has been defined as the stage where the structure keeps behaving elastically, minor damage canbe expected without harmful effect in the building functionality. In case of Reparability limit state, one
particular definition has been chosen for this study, which has to do with the development of steady crack
pattern with the first yielding. It is well known that the definition for the reparability limit state several
factors have to be evaluated, and that is why its definition range depends on the selected parameters. The
Collapse limit state has been defined as the stage where the structure has lost its total capacity, lateral andvertical. However, during the test, this level was not reached due to the existing risk when the wall falldown, then this level has been defined as the stage where the maximum capacity has decreased around20%, which also involves yielding and severe crack pattern in masonry. The last part of this study was
devoted to the verification of the design and prediction of collapse limit state. It can be stated that thehouse fulfills the requirements established in the normative and also that the house will not reach thecollapse for neither of the considered earthquakes. When the house is designed using the seismic demand
given by the normative, its in-plane and out-of-plane behavior will keep elastically within theserviceability limit state. However, when strong earthquake struck the structure, both behaviors will keep
within the reparability limit state, which means that the collapse will not be reached and also that thedamage developed will be economically repairable. The capacity obtained from laboratory test has been
higher than both seismic forced, which indicates that the system is strong enough to withstand strong
earthquakes.

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RECOMMENDATIONS FOR FUTURE RESEARCH
Since the confined masonry system with soil-cement bricks is popularly constructed without technicalsupervision; booklet shall be prepared to specify simply construction method, which considering reduction
factors for designing to take into account the effect of construction quality to keep the house seismic
behavior within the margin provided by the different between the capacity/demand ratio observed inlaboratory test. During the wall test, differences between the real and predicted capacity were shown up,
which have been initially attributed to the workmanship factor as well as the dispersion of quality ofmaterials. Therefore, a properly study shall be carried out to state equations which take into account suchvariables. Finally, the Performance Based Design philosophy herein explained, shall be applied to theothers popular systems as well as the reinforced concrete and masonry building used in El Salvador.

AKNOWLEDGEMENTS
The author wants to express sincere gratitude to Dr Sugano for his support, guidance and friendship during
my study. All the investigation presented herein was made possible by the kind cooperation of the Taishin
Project staff by providing the necessary experimental data to fully achieve the aims of this study.

REFERENCES
Alvarez -Botero J.C. and López Menjívar M.A., 2004 ,Technical Report
ASIA ,1997, Engineer and architect association of El Salvador.
Astroza I.M. and Schmidt A.A, Revista de ingeniería sísmica No 70 59-75.
ATC-40, 1996, Seismic Evaluation and Retrofit of Concrete Buildings (Volume 1).
CEN, 2003, prEN 1998-1, Eurocode 8.
Crisafulli, F.J. 1997. University of Canterbury, New Zeland.
FEMA273, 1997, NE HRP Guidelines for the Seismic Rehabilitation of Buildings.
FEMA356, 2000, Prestandard and Commentary for Seismic Rehabilitation of Buildings.
FUNDASAL, Salvdorean Foundation for de Development and Minimum Housing, El Salvador.
H. Akiyama, M. Teshigawara, H. Fukuyama, 2000, 12WCEE2000.
Jaramillo, J.D. 2002, Revista de Ingenieria Sismica No. 6753-78. Universidad EAFIT, Colombia.
M. Tomazevic, 1999. Slovenian National Building and Civil Engineering Institute.
Ministry of Public Works (MOP) 1994, El Salvador.
Paulay T and M.J.N Priestly, 1992, United States of America.
SEAOC, 1995, Structural Engineering Association of California, Sacramento, California, US.
T. Paulay and M.J.N. Priestley, 1992. New YorkÑ J. Wiley.
T.P. Tassios, 1984. CIB symposium on Wall Structures, Warsaw.
Taishin Project, 2007. JICA Cooperation project in El Salvador.
Timoshenko S. and Woinowsky-Krieger, 1959, Mc Graw Hill, United States of America.
U.C. Berkeley, 1995. An Action Plan.
Whittier, CA, 1994, 1997, California Office of Emergency Services.

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