INVESTIGATION OF THE INFLUENCE OF CHB WALLS IN THESEISMIC PERFORMANCE OF LOW-RISE REINFORCED CONCRETEFRAMES USING THE EQUIVALENT STRUT THEORY AND SAP2000

Rodolfo P. Mendoza Jr.1, 3, Edgardo S. Cruz1, 2, Delia B. Senoro1, 21School of Civil, Environmental and Geological Engineering, Mapua Institute of Technology, Intramuros, Manila,

Philippines, 10022Sustainable Development Research Office, School of Civil, Environmental and Geological Engineering, Mapua

Institute of Technology, Intramuros, Manila, Philippines 10023Engineering Research and Development for Technology, Department of Science and Technology, Taguig,

Philippines, 1631

ABSTRACT: This paper shows the in-plane effects of CHB walls in the seismic performance oflow-rise reinforced concrete frames. The construction of low-rise reinforced concrete (RC)buildings with infilling CHB wall is a common practice in the Philippines. Conventionalanalysis of these frames treats the CHB walls as non-structural elements. However, the inherentin-plane strength and stiffness of CHB walls allows them to interact with compositing framesduring seismic activity. These conditions deviate from the expected response of buildingsanalyzed as bare reinforced concrete frame. In order to account for these effects, the CHB wallswere modeled as equivalent pin-jointed compression struts at two opposite ends of thecompositing frame forming a cross-bracing-like system. Hence, the primary objective of thestudy is to investigate the influence of these CHB walls during seismic activity once consideredin the frame analysis. The study was divided into two stages: the first stage is the experimentalinvestigation of the mechanical properties of CHB masonry and the second stage is analyticalmodeling of frames taking into account the presence of CHB infill. Load-bearing and non-loadbearing CHB prisms with four types of mortar mixture proportion were tested to determine thelocal CHB infill properties based on ASTM procedures. The determined properties of CHBmasonry were used to model the infill strut using the FEMA 356 procedures. A non-linearpushover analysis using SAP2000 was conducted in a bare frame model and sixteen frames withdifferent CHB infill properties. It was found that CHB walls can significantly influence the in-plane seismic performance of a low-rise RC frames by increasing both the strength and stiffnessof the frame by as much as 26.5% and 12.7% respectively.

KEYWORDS: CHB, Infill; Pushover Analysis; SAP2000

1. INTRODUCTION

Reinforced concrete (RC) moment resisting frame is a common type of structural systemused in low-rise buildings in the Philippines. Concrete Hollow Block (CHB) walls are usuallyprovided within these frames which are commonly considered as non-structural elements.However, in actual construction practice, these CHB walls were integrated through the infillingframe because of the presence of reinforcing dowels which connect the CHB wall to the frame.Under lateral loads, the frame tends to separate from the infill wall near windward lower andleeward upper corners. This causes compressive contact stresses which are developed betweenthe frame and the infill at the other diagonally opposite corners. The high in-plane rigidity of themasonry wall provides additional stiffness to the frame. Additional stiffness reduces the naturalperiod of vibration which in turn leads to increase in accelerations and inertia forces (Charleston

2008); such actions change the structures mode of behavior and the forces in the frame. Theneglect of infills in seismic design can be attributed to the common misconception that masonryinfill in frames can only increase the overall lateral load capacity; hence, beneficial to seismicperformance (Paulay and Priestly 1992). However, serious structural damages, as recorded inthe 1990 Luzon earthquake, were traced to the modification of the structural frame due to thepresence of infilled CHB walls. The exclusion of CHB walls in the analysis of these buildingshas resulted in unintended soft storey failures and induced torsional eccentricities.

On the other hand, several researchers over the years have proven the significant positiveimpact of masonry infilled walls in the seismic performance of RC frames. The observedbehavior of infilling walls clearly illustrates their significant structural implications such as anincrease in structural stiffness and strength relative to RC bare frames (Mondal et al. 2008).Asteris (1996) has concluded that properly designed infills can considerably reduce theprobability of collapse, even in cases of defective frames. Bertero and Brokken (1983) quotedthat an infill that is properly designed and connected to the frame offers conceptual and practicaladvantages, particularly if the basic structural system is a moment resisting frame. Theintroduction of the compression strut theory paved way for the designers and researchers toinvestigate the effects of masonry walls in the performance of concrete frames. The compressionstrut theory was based on experimental observations; i.e., when frame is acted by in-plane forces,the frame tends to separate from the infill near windward lower and leeward upper corners of theinfill panels. This action caused compressive contact stresses developed between the frame andthe infill at the other diagonally opposite corners. The equivalent compression strut concept wasfurther evaluated by Holmes (1961) in which, he developed a pin-jointed diagonal strut havingthe same material and thickness of the infill and with a width equal to one-third of the infilldiagonal length. Similarly, Smith (1967) has introduced a relationship between the width of thediagonal strut and an infill-frame stiffness parameter ().

Currently, foreign government agencies such as the US Federal Emergency ManagementAgency (FEMA 273 1998) and Applied Technology Council (ATC 40 1999), haverecommended modeling procedures to incorporate the effects of masonry infill in the behavior ofbuilding frames. The use of simple modeling approach in the analytical investigation of masonryinfill was integrated with non-linear static pushover analysis. Pushover analysis is an inelasticanalysis method in which the structure is subjected to constant gravity loads and push tomonotonically increasing lateral force or displacement pattern. Pushover method of analysistends to simulate the effects of inertia forces by the application of static forces along the heightof the structure. The evaluation of new and existing buildings have preferred the use of pushoveranalysis due to its simplicity, intuitiveness and wide availability of reliable and user-friendlystructural engineering software such as SAP2000. The observed positive and negative impacts aswell as the founding of simple analytical modeling procedures for infill walls and the birth ofreliable and user-friendly pushover analysis software have prompted designers and researchers toinvestigate the effects of masonry walls in the seismic performance of concrete frames.However, none of the past researches have investigated the effect of masonry properties on theinfill-frame behavior. The geographical variations in masonry properties may pose aninconsistency in the results of past investigation. Therefore, the need to investigate local masonryproperties is evident and important. The current study aims to investigate the influence of localCHB masonry walls in the seismic performance of low-rise RC frames. A two-stage study was

initiated; that is, to determine the properties of local CHB masonry, and to adopt these propertin the investigation on the influence ofseismic performance of low-rise RC frames.

2. METHODOLOGY

The methodology on the investigation of the influence of CHB walls in the seismic performanceof low-rise RC frames is described in detail on the subsequent sections.

2.1 Testing of CHB Units, Mortar

The machine-built CHB units were supplied by a CHB commercial manufacturerCompression test of twenty-fournon-load bearing CHB units was conducted to compare the CHB manufacturer reportedcompressive strength and the actual compressive strength of CHB units. Loadload bearing CHB units were designated as (Scompression test was also conducted on twelve 2of mixture proportion namely: Type M (1:3), Type S (1:4.5), Type N (1:6) and Type O (1:9).Grading requirements for theanalysis. Cement-to-water ratio was recorded to be 1.05 for Type M, 0.65 for Type S, 0.46 forType N, and 0.32 for Type O mortar. The compressive strength of CHB masonry was determinedusing the prism test method. Threethickness of CHB units (4-inch and 6for non-load bearing), and (3)specimens were labeled accordingof mortar used (e.g. 4S-M). The prism specimens were sawASTM C1314 or the Standard Test Method for the CompressForty-eight CHB prism specimensconstructed inside a moisture-hours of curing.

Figure 1. CHB Prism specimens

; that is, to determine the properties of local CHB masonry, and to adopt these propertin the investigation on the influence of CHB walls modeled as compression infill strut in the

rise RC frames.

The methodology on the investigation of the influence of CHB walls in the seismic performancerise RC frames is described in detail on the subsequent sections.

Mortar and CHB Prism

built CHB units were supplied by a CHB commercial manufacturerfour CHB specimen consisted of 4-inch and 6

load bearing CHB units was conducted to compare the CHB manufacturer reportedcompressive strength and the actual compressive strength of CHB units. Load

CHB units were designated as (S) and (T) type units, respectivelycompression test was also conducted on twelve 2-inch cube specimen of mortar from four typesof mixture proportion namely: Type M (1:3), Type S (1:4.5), Type N (1:6) and Type O (1:9).Grading requirements for the aggregate used in mortar were verified by conducting a sieve

water ratio was recorded to be 1.05 for Type M, 0.65 for Type S, 0.46 forType N, and 0.32 for Type O mortar. The compressive strength of CHB masonry was determined

prism test method. Three variables were considered in the experimentinch and 6-inch), (2) strength of CHB units (S for load bearing and T

(3) the type of mortar used (Type M, S, N, and O)according to the thickness of the units, strength of the unitM). The prism specimens were saw-cut to meet the requirements of

ASTM C1314 or the Standard Test Method for the Compressive Strength of Masonry Prism.eight CHB prism specimens were tested under compression of masonry.

-tight bag shown as Figure 1 and sealed after the initial forty

CHB Prism specimens during curing using a moistu

; that is, to determine the properties of local CHB masonry, and to adopt these propertiesCHB walls modeled as compression infill strut in the

The methodology on the investigation of the influence of CHB walls in the seismic performance

built CHB units were supplied by a CHB commercial manufacturer.inch and 6-inch load bearing and

load bearing CHB units was conducted to compare the CHB manufacturer reportedcompressive strength and the actual compressive strength of CHB units. Load-bearing and non-

, respectively. Similarly,inch cube specimen of mortar from four types

of mixture proportion namely: Type M (1:3), Type S (1:4.5), Type N (1:6) and Type O (1:9).verified by conducting a sieve

water ratio was recorded to be 1.05 for Type M, 0.65 for Type S, 0.46 forType N, and 0.32 for Type O mortar. The compressive strength of CHB masonry was determined

in the experiment; i.e., (1) thestrength of CHB units (S for load bearing and T

the type of mortar used (Type M, S, N, and O). The sample, strength of the units and the type

to meet the requirements ofive Strength of Masonry Prism.

were tested under compression of masonry. The prisms wereand sealed after the initial forty-eight

using a moisture-tight Bag.

All prism specimens were tested at an age of 28 days. The testing was conducted at the UTMCenter of Mapua Institute of Technology. Speed of loading was maintained at 15mm/min.Modulus of elasticity was determined using the secant modulus method in which the slope of theline for the modulus of elasticity is taken from 0.05 to a point on the curve at 0.33 ,where is the ultimate compressive strength2.2 Building Model Considered

The building model considered was an office three-storey reinforced concrete buildingwith typical floor plan and elevation shown in Figures 2a and 2b, respectively. The lateralresisting elements were located along the perimeter of the frame both on the N-S and E-Wdirection.

(a) (b)

Figure 2. Typical Floor Plan (a) and Frame Elevation (b)

CHB masonry walls enclosed the perimeter of the building. The building is located in aseismic zone four region. Column sections are 500x500 mm, typical floor beams sections are 500x 300 mm and roof beam sections are 400 by 250 mm. Concrete compressive strength istaken as 21.0 MPa and yield strength of reinforcing steel strength is taken as 276 MPa.

2.3 Structural Modeling and Analysis

A two dimensional frame modeling and analysis was conducted for the frame along gridA using the commercial software SAP2000. Flexural rigidity for columns and beams weremodeled considering the cracked section properties taken as 0.7EcIg for columns and 0.5 EcIg forbeams (FEMA 356 2000). Design loads were referred to the minimum design load tables ofNSCP C101-01. Imposed loads for the 2nd and 3rd floor level were computed as 13.8 KN/m, and

2.4 KN/m for roof beam levels. Lateral loads were computed using the Equivalent Lateral ForceProcedures by NSCP C101-01. Lateral load distribution was analyzed considering the effect ofaccidental torsion. Column nodal loads, shown in Table 1, were distributed in proportion to theirlocation along grid A. Torsional analysis was conducted by moving the center of mass of thestructure by 5% of the least horizontal dimension of the structure. From the analysis, a factor of1.027 was derived and added to the computed direct story shears. Nonlinear hinges weremodeled using the FEMA 356 default hinges properties. For beams, M3 plastic hinges wereapplied at member endpoints or on their dissipative zones whereas, bi-axial (PM3) hinges wereapplied at column end joints.

Table 1. Column Nodal Forces for Analysis

2.4 Modeling of CHB Masonry Infill

The CHB masonry walls were modeled as a pin-jointed strut in which resistance waslimited to compression forces only. The stiffness contribution of CHB masonry infills isrepresented by equivalent compression strut connecting windward upper and leeward lowercorners of the infilled frame. The Applied Technology Council (ATC 40 1999), and the Pre-standard and Commentary for the Seismic Rehabilitation of Buildings (FEMA 356 2000) haverecommended similar modeling procedures in order to incorporate the effects of masonry infill inthe behavior of the building frames. According to this standard and pre-standard documents, theelastic in-plane stiffness of masonry panels can be represented by an equivalent diagonalcompression strut of a width, , as given in Equations (1) and (1a).

= 0.175(). (1) = (1a)

where;

= column height between centerlines of beams (in); = height of infill (in) = expected modulus of elasticity of frame material (ksi); = expected modulus of elasticity of infill material (ksi); = moment of inertia of column, (in4); = Length of infill panel (in); = diagonal length of infill panel (in);

Level Story Forces(KN)

End ColumnNode Forces

(KN)

InteriorColumn NodeForces (KN)

With Accidental TorsionEnd ColumnNode Forces

(KN)

InteriorColumn NodeForces (KN)

Roof 537.7 33.61 67.21 34.52 69.023rd 425 25.56 53.13 27.28 54.562nd 211.3 13.21 26.41 13.57 27.12

Total 1174

= thickness of infill panel and equivalent strut (in); = angle whose tangent is the infill height to length aspect ratio (radians); and = coefficient used to determine equivalent width of the infill strut

The width is related to the stiffness parameter () as described in equation 1a. Theequivalent diagonal strut shall have the same thickness and modulus of elasticity as that of theCHB infill that it represents. The infill struts were placed concentrically across the diagonal ofthe frame. Structural performance level for the masonry infill was monitored using the FEMA356 (2000) drift criteria given as 0.2% for Immediate Occupancy, 0.6% for Life Safety and 1.5%for Collapsed Prevention. The compression struts were modeled as axial elements with non-linear axial hinges applied on endpoints of the strut member.

3.0 RESULTS AND DISCUSSION

Twenty-four CHB units were tested under compression to compare reported compressivestrengths of CHB units to the actual experimental values. The relative percentage difference(RPD), shown in Table 2, between the reported and actual compressive strength were reported tobe as high as 288% for the non-load bearing CHB units and as high as 147% difference for loadbearing CHB units. Table 2 also shows that the non-load bearing units have higher RPDscompared to the load bearing CHB units. This shows that the properties of non-load bearingCHB units are highly variable compared to the load-bearing CHB units. The higher values ofcompressive strength of non-load bearing CHB units may be affected by the unreported age ofthe units. Hence, such factors must be addressed in further studies.

Table 2. Compressive Strength of CHB Unit: Reported vs Experimental Values

The results of the compression test of the 2-inch cube mortar were tabulated in Table 3. Type Mmortar had the highest recorded compressive strength while the lowest compressive strength wasrecorded for Type O mortar. The compressive strength of the Type M mortar is significantlyhigher by approximately 320% than that of a Type S mortar. While the relative percentdifference of the strength between the other mortar types ranged from 109% to 120%.

Figure 3 shows the generated mean stress-strain diagram from compression testing of twelvetypes of CHB Prism. It can be seen that the maximum compressive strength was attained byprism type 4S-M or the prism with 4-inch load bearing CHB unit with Type M mortar while thelowest stress level was recorded for prism type 4T-S. The differences among specimens wereobserved at strain levels between 0.01 to 0.02.

CHB Unit TypeReported

CompressiveStrength (MPa)

Actual CompressiveStrength based onexperiment (MPa)

RPD (%)

4-inch load bearing (4S) 4.8 10.02 1094-inch non-load bearing (4T) 2.76 10.71 2886-inch load bearing (6S) 4.8 11.86 1476-inch non-load bearing (6T) 2.76 10.4 277

Table 3. Compressive Strength of 2-inch Cube Mortar Specimen

Figure 3. Average Stress-Strain Diagram for all CHB Prisms

For the 6-inch CHB prism, 6S-M and 6T-M had higher stress levels corresponding to 0.01 to0.02 levels of strain while 6T-O had the lowest recorded stress level. For the 4-inch CHB units,4S-M had the highest stress level while the lowest stress level was reported for 4T-S. Thevariability of the compressive strength results was evaluated by computing the standard error ofall the stress means. It can be seen in Figure 4, that among all the specimens, 6T-M had the mostvariable stress level, followed by 4T-S for strain levels 0.01 to 0.02. In general, a more variablestress level was observed in non-load bearing CHB units. This is consistent with the observeddisparity in the compressive strength of non-load bearing CHB units as discussed above.

Mortar Type MixtureProportion Cement-to-water RatioCompressive

Stength (Mpa)

M 1:3 1.05 11.52S 1:4.5 0.65 2.75N 1:6 0.46 2.61O 1:9 0.32 1.25

Figure 4. Average Stress-Strain Diagram for all CHB Prisms

From the determined experimental values of compressive strength and modulus of elasticity ofCHB masonry, a relationship between the constant a (reciprocal of modulus of elasticity) andcompressive strength of CHB prism is shown in Figure 5.

Figure 5. Relationship between the Constant and Compressive Strength of Masonry

Based on the best-fit curve generated, a polynomial relationship may be used todetermine the relation between the modulus of elasticity and the compressive strength of CHBmasonry. From this graph Equation 2 was derived relating the modulus of elasticity andcompressive strength of CHB masonry. This equation did not show a good agreement with thecode-based equation for modulus of elasticity which was attributed to the relatively weaker unitsused in this study.

= 100 . (2)

Figure 6 shows the Pushover Capacity Curve (PCC) of RC bare frame model and sixteenframe models with different infill properties. The figure illustrates the significant influence ofinfill which increased both strength and stiffness of the frame analyzed as bare frame model. Byconsidering the presence of infill in the analysis, the strength of bare frame was increased by asmuch as 26.5% while the stiffness of a bare frame was increased by 12.7%. The highest baseshear was recorded for frame with 4T-S infill. This infill may be considered as a relatively weakinfill with a lowest recorded modulus of elasticity of 221 MPa. Albeit, the frame with 4T-S hadresulted to a higher strength and stiffness, it did not show a good performance as localizedsudden failure was observed in modeled infill struts.

Figure 6. Base Shear vs Roof Displacement

Figure 7 shows the hinges formation in bare frame, frame with relatively weak infill (Em Em 510 MPa) shows a better performance. Gradual formation of hinges was observed in the infillstruts and beam members. Also, there are no formation of collapsed hinges observed in beamsand columns. This is in addition with the sustained beam mechanism (strong column-weakbeam) behavior of the frame as illustrated in Figure 7.

Figure 7. Plastic Hinges Formation

4.0 CONCLUSIONS

Based on the results of this analytical and experimental study, the following conclusions weredrawn; The consideration of CHB walls in the analysis of low-rise RC frames had significantly

influenced the behavior of the frame under seismic loading. The results from the analyticalinvestigation showed that considering the effects of CHB walls in the seismic performance oflow-rise RC frame can increase the over-all strength and stiffness of the frame by as much as26.5% and 12.7%, respectively. However, it was observed that the ductility of the frame wasconsiderably reduced except for frames with relatively weak infill properties.

The over-all performance of the structure was investigated through formation of plastichinges and it was observed that frames with relatively strong infill behave more suitable as agradual formation of hinges was observed with no formation of collapsed hinges. On thecontrary, a sudden formation of collapsed hinges was observed in the frame with relativelyweak infill.

The reported compressive strength of CHB units may not represent the true properties of theCHB units. The non-load bearing CHB units showed a higher RPD values indicating a highervariability in their properties. This is consistent with the observed behavior of prismconstructed with non-load bearing CHB units which had the highest variable stress level.

The relationship between the compressive strength of CHB masonry and modulus ofelasticity can be expressed in terms of a polynomial expression. This demonstrates that thereis a non-linear relationship between compressive strength and the elastic modulus ofmasonry. The derived equation did not show a good agreement with the code-based empiricalequation for masonry compressive strength of modulus of elasticity and was attributed to therelatively weaker units used in the study.

REFERENCES

Asteris, P.G. (2008) Finite Element Micro-Modeling of Infilled Frames. Electronic Journal ofStructural Engineering, Volume (8), 1-1

Bertero, V.V., and Brokken, S. (1983) Infills in seismic resistant buildings. ASCE Journal of theStructural Division, ASCE, Vol. 109, ST6, 1337 - 1361.

Charleson, A (2008) Seismic Design for Architects. Elsevier Ltd.

Federal Emergency Management Agency 356 (2000) Prestandard and Commentary for the SeismicRehabilitation of Buildings. Second Edition, Federal Emergency Management Agency, WashigntonD.C.

Holmes, M. (1961) Steel frames with Brickwork and Concrete Infilling. Proceedings, The Institutionof Civil Engineers, Vol. 19: 473 -478.

Mondal G. and Jain S. K. (2008) Lateral Stiffness of Masonry Infilled Reinforced Concrete (RC)Frames with Central Opening. Earthquake Spectra, Earthquake Engineering Research Institute, Vol. 24,701723

Paulay, T and Priestley,M.J.N (1992) Seismic Design of Reinforced Concrete and Masonry Buildings.John Wiley & Sons, Inc.

Smith, B.S. (1967) Methods for Predicting the Lateral Stiffness and Strength of Multistorey InfilledFrames. Building Science, Vol. 2: 247 -257.

ABOUT THE AUTHORS

Engr. Rodolfo P. Mendoza Jr. is a graduate of BSCE from the Don Honorio Ventura Technological StateUniversity in 2008. He ranked 2nd Place in the November 2008 Board Examination for Civil Engineers.Currently, he is a candidate for MS in Structural Engineering at the Mapua Institute of Technology. He isalso a scholar of Engineering Research and Development for Technology (ERDT), Department ofScience of Technology (DOST). He can be contacted at mendoza.structuralconsulting@gmail.com.

Engr. Edgardo S. Cruz, is a cum laude graduate of BSCE from the University of Santo Tomas in 1997. Heranked 14th Place in the November 1997 Licensure Examination for Civil Engineers. He finished his MSdegree major in Structural Engineering at the University of the Philippines Diliman in 2006. Currently, heis a permanent full time faculty member at the School of Civil, Environmental and GeologicalEngineering at Mapua Institute of Technology. He is also the current faculty research associate of theSustainable Development Research Office (SDRO) and Coordinator for Community Extension of theInstitute. He can be contacted at engredgar2k@gmail.com.

Dr. Delia B. Senoro, has been a practicing civil engineer for more than 2 decades and is a doctor inenvironmental engineering, acquired the graduate study degrees from the University of the Philippines,Diliman in collaboration with Chia Nan University in Tainan, Taiwan. Currently, she is the ProgramCoordinator for Environmental Engineering (undergraduate and graduate studies), a Professor and In-charge of the Sustainable Development Research Office (SDRO) of the School of Civil, Environmentaland Geological Engineering, Mapua Institute of Technology. She is the Philippine representative forSwedish International Development Cooperation Agency (SIDA) International Training Program (ITP)on Strategies for Chemical Management (SCM) and Education for Sustainable Development (ESD) for2011. She is also the Philippine representative for the Asian Network of Environment Research andEnergy (ANERGY) starting 2011. She can be contacted through dbsenoro@mapua.edu.ph.

ACKNOWLEDGEMENTS

The authors would like to express their sincerest gratitude to the Engineering Research andDevelopment for Technology (ERDT), Department of Science of Technology (DOST),Philippines for funding this study.