12TH INTERNATIONAL

BRICK/BLOCK Masonry c O N F E R E N C E

Cas

RECENT EXPERIMENTS WITH CONFINED CONCRETE BlOCK MASONRY WAllS

E. Castilla 1, A. MarinillP

' Instituto de Materiales y Modelos Estructurales, Facultad de Ingeniería, Universidad Central

de Venezuela . Apartado Postal 50361 - Caracas 1050-A, Venezuela

ABSTRACT

This paper summarizes the results of tests performed at the Instituto de Materiales y Modelos Estructurales (IMME) at the Universidad Central de Venezuela on nine ful/­scale confined masonry load-bearing wal/s subjected to alternating and increasing ho­rizontalloads. Variables, such as the type of block, aspect ratio and gravity load we­re considered. The results obtained are given in terms of capacity to dissipa te energy by non-linearity of the materiais, stiffness 1055, ductility and drift limits . The effective­ness of this component was 0150 measured against that of elasto-perfect-plastic sys­tems. Some design proposals that could possibly be used as standards for structural masonry are 0150 presented.

Key words: Confined masonry wal/s, earthquake resistant design, ful/-scale tests, ductility, energy dissipation, stiffness degradation.

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1. INTRODUCTION

Standards for the design and construction of structures built on the basis of ma­sonry load-bearing walls commonly used in Venezuela date back to 1955 (Mi­nisterio de Obras Públicas, 1955). This is a reason for concern as probably mo­re than 50% of the people in the country use this makeshift system to build their houses, without considering the detailing requirements for seismic-resis­tant structures.

The fragility of this type of building became extremely clear following recent earthquakes in Venezuela - EI Pi lar in 1986 and Cariaco in 1997- with the partial or total destruction they caused and the consequent losses in terms of lives and property.

With the aim of reducing the· risks involved in the use of load-bearing masonry while keeping it as an alternative type of building system, the Instituto de Mate­riales y Modelos Estructurales (IMME) at the Facultad de Ingeniería at the Univer­sidad Central de Venezuela undertook a research project with the participation of research professors, technicians and both undergraduate and graduate students, to provide guidelines for the use of this type of structure (López et aI., 1985). The work described in this paper is part of that project.

Confined masonry is unquestionably the most commonly used system for buil­d ing throughout Latin America and, especially, in Venezuela. That is why prio­rity has been given to studying the behavior of such buildings and producing practical results in the short termo It is commonly believed that structural ma­sonry walls are extremely vulnerable to seismic action, and some engineers re­fuse to use this system . Nevertheless, the recent earthquakes mentioned abo­ve, showed that properly designed masonry structures can behave as required . Load-bearing masonry must be ca librated to fit the local characte­ristics; therefore standards or regu lations from another area might not be ap­propriate.

Although some experiments have been performed elsewhere in the world on the capability of confined masonry to withstand lateral loads (Meli, 1985; Ga llegos, 1989), little has been done in Venezuela. In this regard, complete studies exist on confined masonry with hollow clay blocks (Castilla, 1991). Another study was al­so begun to assess the behavior of confined solid clay-brick walls (Carrillo and Molina, 1997).

Far less work has been done on hollow concrete blocks, and this is the reason behind these tests. Castilla and Pose (1995) have presented the results of tests performed on two full-scale confined walls subjected solely to lateral loads. The same was done with two other internally reinforced walls . A comparison was then made between the responses of walls to monotonically increasing lateral loads and walls subjected to alternating lateralloads. It was found that the results of sta­tic tests are not representative of the behavior of walls subjected to alternating 10-

ads. Neither the effects of the axial load nor those of the aspect ratio of the walls were considered.

The fundamental goal of this experiment was to assess the behavior of nine, full­scale confined masonry walls, made of hollow concrete blocks, when subjected to increasing alternating lateral loads. For this purpose, factors closely related to earthquake behavior, i.e. lateral ductility, 1055 of stiffness and energy dissipation capacity, were ali studied. From the practical standpoint, drift limits were found and strength properties were suggested for use when designing bearing walls.

2. GENERAL ASPECTS

One of the main characteristics found when using structural masonry as a load­bearing element in earthquake-prone areas is its fragility when subjected to co­planar lateralloads. This causes tension stresses along the wall diagonais and sub­sequent cracking, causing 1055 of stiffness and strength throughout the entire structural system.

This same tension fragility is also found in concrete, although the problem has be­en satisfactorily solved by including reinforcing steel within the elements. In the same way, reinforced masonry uses reinforcing steel inside the load-bearing walls to achieve tension strength. There is a great deal of experience with this type of construction method throughout the world (Schneider and Dickey, 1980) and it has not been considered in this study.

Another widely used solution, especially among lower-income groups, is confined masonry. This involves placing elements, generally reinforced concrete, at the ends of the walls . The main function of these elements is to delay excessive dia­gonal cracking of the load-bearing walls. Additionally, the confining elements are supposed to absorb the tensions generated by the overturning moments produ­ced by the lateral loads along the wall planes.

As the reduced capacity to withstand tension has been found to be masonry's main weakness, the factors that would accelerate or delay this situation must be considered . The former is discussed in great detail by Johnson and Thompson (1967) and by Gallegos and Casabonne (1985), who have proven that the strength of masonry diminishes when the direction of the applied load approaches that of the layer of mortar. This property conditions the behavior of confined ma­sonry, especially when considering adjoining walls with different aspect ratios.

Another important aspect is the presence of accidents inside the walls, such as openings (windows, doors, etc), utility pipes, or the lack of vertical mortar joints between the pieces. These accidents cause premature weakening of the load-be­aring capacity and the stiffness of the walls when subjected to lateral loads. Me­asures must be taken at the design and construction stages to reduce these effects.

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3. DESCRIPTION OF THE PROTOTYPES TESTED

A set of nine (9) full-scale confined walls was prepared at the IMME, to be tested against lateral loads and axial load. The basic components of the masonry walls were hollow concrete blocks, mortar with a volume ratio of 4:1:1 (sand, lime and cement); the confining elements were made of reinforced concrete.

The walls were built using hollow concrete blocks, with nominal measurements of 40x15x20 cm -Iength, width, height (Figure 1). Three types of tests were used to determine the mechanical properties of the masonry: five (5) blocks were com­pression tested until failure, with an average strength of fp = 135 kg/cm 2 (referred to the gross area of the blocks); five (5) axial compression piles with an average strength of fm = 80 kg/cm 2

; and five (5) 1 xl m wall segments tested for diagonal compression with as average shear of Vm = 7.64 kg/cm 2

•

The confining elements consisted of a large 30x50 cm cross-section foundation be­am with vertical 15x15 cm elements and a top beam measuring 15x20 cm. The ver­tical elements and the top beam were, in general, reinforced lengthwise with four (4) No. 4 bars and transversally with No. 3 stirrups at 6 cm intervals at the ends of the elements, and at 12 cm intervals in the remaining portions. In one of the walls (with a 0.74 aspect ratio and no axial load) the vertical elements were reinforced lengthwise with No. 3 bars to determine the possibility of inelastic energy dissipa­tion by yielding of the reinforcement. The concrete used had average strengths of 300 kg/cm 2 and 360 kg/cm 2 for the foundation beam and the other confining ele­ments, respectively. The reinforcement steel used had an average yield stress of 4437 kg/cm 2

, and was thus classified as type N-60 (fy = 4200 kg/cm 2) .

Three of the walls had an aspect ratio (a = height/length) of 0.74 (a < 1), in th­ree the ratio was 0.97 (a approximately equal to 1) and in three the aspect ratio was 1.26 (a > 1). Ali nine walls were 2.40 m high, measured from the foundation beam, although the actual height to the point where lateral loads were applied was 2.30 m.

Figure 7.

In ali the walls the vertical joints were properly filled with mortar due to the cha­racteristics of the pieces. In each one of the specimens, the masonry was painted white so as to make it easier to observe the cracking of the walls during the tests.

4. lESllNG PROCEDURES

Each one of the walls was tested with alternating lateral load applied in its plane and located at the top, until the limit-state was reached. In addition each wall was subjected to a constant axial load to simulate gravity effects, which was taken as a fraction of the strength of the piles (fm). The walls with aspect ratios of 0.97 and 1.26 were tested with axial loads of O f"" 0.05 fm and 0.10 f,,,; in the case of the walls with an aspect ratio of 0.74 the axial loads were O f"" 0.05 fm y 0.075 fm•

Ali the walls were tested at the Banco Universal de Ensayos of the Instituto de Ma­teriales y Modelos Estructurales (IMME) at the Facultad de Ingeniería at the Uni­versidad Central de Venezuela. lhe foundation beam for each wall was fixed to the floor of the banco using prestressed tension rods so as to ensure that the spe­cimen was properly anchored. lhe lateral and vertical loads were applied using hydraulic jacks.

lhe nine walls were tested with alternating and increasing controlled displace­ment cycles until the limit state of the walls was reached. Each cycle was repea­ted as many times as necessary to achieve stability in the hysteresis loop. Figure 2 shows general features of the tests.

Figure 2.

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Figure 3.

~ ..... ,.~"' ...... ., .~._ .. _ .. ,."" .. ..,

i ! I f'

____ . . ',... .. _ 1 _ _'~

,c:t::;j 1t:I~

l~: _ .... , .~ , ',.. ... ,,) I . , ..

. ... -' _.~j ' .

ElJ· _ ..... " .,,, . -,,~ ............. ,) :lO · CJ.i:J .~ I' , __ . !~ '. --_._"...

The parameters measured during the tests were:

1. Load appl ied by the jacks. 2. Relative displacements at six different points: three recording the difference in

horizontal displacement of the wall in comparison with the foundation. The ot­her three recording the difference in horizontal displacement between the ma­sonry panels and one of the vertical confining elements.

Whi le the tests were being performed, a graph was made of the lateral load ver­sus relative lateral displacement at the top of the wall. Ali the information gathe­red was recorded and processed in the IMME's data acquisition system. Figure 3 shows the hysteresis loops obtained during the tests. In ali cases the formation, history and size of the diagonal cracks was carefully followed.

5. TEST FINDINGS

The positive and negative envelopes of the histeresis loops, dissipated energy for each of the cycles, and the elasto-perfect·plastic equivalent system with equal energy dissipation (Bertero) were found for each of the test walls. Based on this information, the yield displacement, the yield support capacity, and ultimate dis­placement for which there was no significant loss of strength were ali calculated.

When analyzing the energy dissipated by the test walls, the benefits of causing increasing damage to the walls with increased lateral deformation, reflected in a

Figure 4.

45

,,"O E

~35 E E 30 I ~ '-'25 O

"O

8.20 'Cii '6 ,Q 15 01 L.

~ 1 0 w

5

Energío disipada por unidad de desplazomiento

2 4 {; 8 10 12 14 DESPLAZAMIENTO (mm)

<HHHH> Muro 1 ~MUfO 2 .......... Muro 3 -Muro 4 -Muro fi -Muro 6 ~Muro 7 ~Muro 8 --- Muro 9

16 18 20

greater dissipation of energy, was observed (Figure 4). In general it was found that a greater axialload helps increase the dissipation of energy. The explanation for this finding is based on the great friction that develops in the horizontal cracks that formed in the test walls.

8ased on the energy dissipated by each hysteresis loop, an equivalent damping coefficient was determined, showing that the latter is independent of displace­ment and generally ranges between 8% and 10% of the criticaI.

The ductility that an elasto-perfect-plastic equivalent system should develop so as to have it dissipate the same energy as the cycle analyzed was calculated (Figure 5). The ductility facto r never exceeded 2, with 1.5 as a reasonable mean.

When observing the hysteretic cycles in the nine walls, stiffness degradation cau­sed by increased lateral deformation was found (Figure 6). This property has one important advantage from the standpoint of earthquake behavior, as these sys­tems are able to dissipate energy without having to reach yield strength.

When strength is analyzed in terms of the force achieved by each one of the walls, it was found that when the aspect ratio is kept constant, strength increases with an increase in the axial load. It was also found that when axial stress is kept cons­tant, strength increases with a drop in the aspect ratio .

A graph of the shear stress strength for each aspect ratio against the f/fm ratio shows that these points form a straight line. When the walls with a 0.74 aspect

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Figure 5 .

5

4

Ductilidoddel sistema vs. Ductilidad equiva lente elasto- plástica

- Muro 1 ~Muro 2 H+H Muro 3 ............... Muro 4 +++++ Muro 5 - Muro 6 ~ Muro 7 ~ Muro 8 - Muro 9

Oi1-------------,2-------------.3-------------.4-------------,

DLJctiJidad dei sistema

Figure 6.

Pérdida de rigidez lateral

~Muro 1 --....... Muro 2 ........... Muro :3 -Muro 4 ............ Muro 5 ~Muro 6 -.......-. Muro 7 ~Mup"<)8 .-.........". Muro 9

OiO----~2r---~4----!6-----8,----lTO----'12-----1'4--~1T6----'18----~20 DESPLAZAMIENTO (mm)

ratio were analyzed, the test wall with no axial load was not taken into account since the longitudinal reinforcement of the concrete vertical elements was diffe­rent from the other walls.

Figure 7.

Graded 45º cracking was found in ali the walls due to the size of the blocks and the offset used in building the walls (Figure 7). This feature prevented any measurement of the effect of the aspect ratio on the strength of the walls. In some cases the cracks propagated in the vertical concrete elements, as shown in Figure 8. The strength (in terms of shear stress) was analyzed considering the results from the eight walls si­multaneously. Figure 9 shows that the points form a straight line whose equation is:

Vm = 22.3 f/fm + 5.23 (kg/ cm2) (1 )

An equation of a straight line associated with 10% failure probability can also be obtained:

Vm = 22.3 f/fm + 4.88 (kg/ cm2) (2)

In both equations, the shear stresses are a function of the gross area of the walls.

Figure 9 also shows the strength found in the testing of the wall segments' strength to diagonal compression, finding a close fit to the average straight line.

The contribution of friction to lateral strength in the walls can be identified by the term depending on f/f"" which does not vary significantly when concrete walls with typical mortars are used. The strength quality of the masonry is better re­flected in the independent term of the equations obtained, which can be related to the results of the diagonal compression tests on the wall segments.

Earlier works (Castilla, 1991) have found that the lateral drift of walls and their as­pect ratio are proportional. Based on this, and considering the yield displace-

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Figure 8.

Figure 9.

Resultado de ensoyos y propuesta de diseno 10.---------------------------------------------~

9 ........ C'I

E B o "--"'­'-'7 W

!z 6 ;:: ~ 5 o o 4 N o:: ~ :3 t...... (f) w 2

A

-----------.--

"" XXX ~O=0 . 975) *** , ... 0=1.26) "A4l.4l.4l. 0= 0.74) ---- - espuesto ens~os ---- Propuesto diseno ••••• Muretes

0+----T----r----r---.----~--_.----._--_r--~~--1 0.00 O. 1 0.02 0.03 0.04 O. 5 ·0.06 0.07 0.08 0.09 0.10

COMPRESION (f/fm)

ments for the elasto-perfect-plastic systems and the ultimate non-critical displa­cements, the following lim its can be proposed:

Ll/ h ~ 0/350 (3)

LVh ~ 0/765 (4)

where L1y is the equivalent yield displacement, .1u is the ultimate displacement, and h the height of the story. The yield displacement considered is associated with the displacement that prevents severe damage and allows development of the strength of the walls.

The influence of the aspect ratio could not be measured in this study, due mainly to the widespread 45º cracks found consistently in the nine walls; the low leveis of vertical load applied were unable to change the crack pattern. Walls with as­pect ratios significantly below one are extremely unlikely in practice, except in the case of walls with severa I concrete columns. There is no evidence that this ten­dency cannot be influenced by trying to cross an intermediate concrete column. Due to this, tests are needed on walls consisting or more than one panel, prefe­rably with different aspect ratios that will allow for conclusive findings in this re­gard.

Although further details are not included in this paper, the analysis of the de­formations of the walls during the testing show that the general behavior of the walls was governed by shear deformations. These deformations came from the relative horizontal displacements of the walls in comparison with their founda­tions . Furthermore, based on the recordings of the rei ative horizontal displace­ments between the masonry panels and the concrete columns in the walls, it can be said that there were no significant relative lateral deformations between them.

It is necessary to continue insisting on confined structural masonry and its ability to dissipate energy. This argument is important beca use, in general, design standards for seismic-resistant structures tend to allow the reduction of seismic actions due to inelastic response. Implicit in this reduction is the fact that the system has properties for the stable dissipation of energy, a con­dition that is generally achieved by means of proper hysteretic behavior of the materiais.

The test walls showed very little energy-dissipation capacity, although overall be­havior showed a non-linear response to the lateral loads. This dissipation of energy was generated mainly by the friction occurring in the mortar joints. The experiments with these walls showed clearly that no earthquake-resistant energy­dissipation properties were found in this type of masonry.

6. CONClUSIONS ANO RECOMMENOATIONS

The conclusions to be reached from the interpretation of the results of tests per­formed on nine confined masonry walls made of hollow concrete blocks, subjec­ted to horizontal loads until the limit-state are:

1. In general it was found that the lateral deformation governing the behavior of the walls was better described by shear deformations.

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2. The relative horizontal deformations between the panels of masonry and the concrete vertical elements were not significant. Any action taken in an effort to improve the bond between these elements would help improve the overall be­havior of the walls.

3. The diagonal cracking of the walls was closely related to the size of the struc­tural blocks. The influence of the aspect ratio of these walls in this regard was not fully studied. In this regard, further testing is needed on walls consisting of more than one panel and, naturally, more than two concrete vertical confining elements.

4. The energy dissipation recorded in each of the tests came mainly from the fric­tion generated within the horizontal and vertical mortar joints. This informa­tion made it possible to determine equivalent viscous damping coefficients of 8% to 10% of the criticaI.

5. In ali the cases it was found that, as the lateral deformation of the walls incre­ased, they lost effective lateral stiffness. The stiffness degradation causes flexi­bility in the system, although this behavior has the advantage that these sys­tems can dissipate energy without achieving yield strength.

6. The ductility of these walls for elasto-perfect-plastic systems did not, in gene­ral, exceed 2, leaving the walls beyond repair. From a practical standpoint, this ductility shou ld be limited to no more than 1.5

7. There is no question whatsoever that the inelasticity of these materiais should not be used for earthquake-resistance purposes.

8. 50 as to reduce damage to load-bearing confined masonry walls made of con­crete blocks, and achieve reasonable strength to lateral loads, the relative drift of the walls should be no more than:

!J.jh ~ 0/35 O (3)

9. The maximum strength capacity of confined walls can be simplified for design purposes to the following expression:

Vm = 22.3 flfm + 4.88 (kg/cm2) (2)

10. The tests performed show that diagonal compression testing of wall seg­ments would be extremely useful for design purposes.

11 . Although not part of the scope of this study, a 0.2 fm testing for the com­pression stress generated by gravity loads should not be exceeded. Any stress above this levei could activate fragile mechanisms on the walls when they are subjected to alternating horizontal loads.

ACKNOWLEDG EM ENTS

The authors wish to thank the Consejo de Desarrollo Científico y Humanístico at the Universidad Central de Venezuela; the Instituto de Materiales y Modelos Es­tructurales (IMME); Siderúrgica dei Turbio (SIDETUR); Concretera Lock Joint Con­solidada, C.A. and to Coordinación de Investigación of the Facultad de Ingenie­ría at the Universidad Central de Venezuela for the funding of this research project. The authors also wish to thank Mrs. Francine Jácome for the transcription in English of this paper.

REFERENCES

Bertero, V., "The Role of Ductility in Seismic-Resistant Design of Structures", University of Cali­fornia at Berkeley, Berkeley, California .

Carrillo, V., Molina, A., (1997), "Evaluación dei Comportamiento de Muros Confinados de Blo­ques Macizos de Arcilla con Diferentes Relaciones de Aspecto ante Cargas Laterales Alternantes", thesis for Civil Engineering Degree, Facultad de Ingeniería, Universidad Central de Venezuela, Ca­racas.

Castilla, E., (1991), "Experiencias Recientes en Mampostería Confinada Sismorresistente", thesis for appointment as Associate Professor, Facultad de Ingeniería, Universidad Central de Venezue­la, Caracas.

Castilla, E., Pose, M., (1995), "Evaluación dei Comportamiento de Muros de Mampostería de Concreto ante Carga Horizontal", Boletín Técnico deIIMME, 33(1), Caracas.

Gallegos, H., (1989), "Albafíilería Estructural", Pontificia Universidad Católica dei Perú, Fondo Editorial, Lima.

Gallegos, H., Casabonne, c., (1985), "Resistencia a la Tracción de Albafíilería de Arcilla y Sílico­Calcárea" , International Seminar: Construcción de Vivienda Económica en Zona Sísmica, Escue­la Superior Politécnica dei Litoral, Guayaquil.

Johnson, F. , Thompson, N ., (1967), "Development of Diametral Testing Procedures to Provide a Measure of Strength Characteristics of Masonry Assemblages", International Conference of Ma­sonry Structural Systems, Austin.

López, OA, Castilla, E., Genatios, c., Lafuente, M., (1985), " Una Proposición para el Estudio de Edificaciones de Mampostería en Venezuela", Taller - Normativa y Seguridad de Construcciones en Zonas Sísmicas, IMME, Caracas.

Meli, R., (1985), "Consideraciones sobre el Disefío Sísmico de Edificaciones de Mampostería", Taller - Normativa y Seguridad de Construcciones en Zonas Sísmicas, IMME, Caracas.

Ministerio de Obras Públicas, (1955), "Normas para el Cálculo de Edificios", Dirección de Edifi­cios e Instalaciones Industriales, Caracas.

Schneider R., Dickey, W., (1980), "Reinforced Masonry Design", Prentice Hall, New Jersey.

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