The influence of infill walls on rc frames under seismic excitation

Architectural Engineering November 2014, Volume 2, Issue 4, PP.68-72

The Influence of Infill Walls on RC Frames under Seismic Excitation Lu Liu #, Ziyan Wu, Hongbin Sun School of Mechanics and Civil Engineering & Architecture, Northwestern Polytechnical University, Xi‟an 710129, China #

Email: liulu649@163.com

Abstract Infill walls are considered as nonstructural components, but still involved in resisting seismic loads. The paper addresses pertinent issues on the evaluation of target seismic performance levels of infilled RC frames, establishing a nonlinear finite-element scheme on the software OpenSees and adopting improved equivalent slant strut modeling infill walls. The deterministic limit states can be expressed as thresholds of interstory drift ratios (IDR) under specific ground motions. Numerical simulation has been performed using incremental dynamic analysis (IDA) in order to obtain the representative nodes‟ maximum displacement responses. The paper shows three performance objectives of infilled RC frames, and the IDRs are compared with bare RC frame‟s and criterions‟.

From the data analysis, it can be verified that if not taking into account the influence of infill walls, the IDR may be non-conservative. It can be concluded that the existence of infill walls will significantly improve stiffness of global structure, and substantially raise seismic performance.

Keywords: Target Performance Levels; Infilled RC Frames; Improved Equivalent Slant Strut; Incremental Dynamic Analysis

1 INTRODUCTION In past decade, the investigations of earthquake hazard proved that infill walls would develop strong interaction with the main frames under seismic excitation. However, infill walls were often considered as nonstructural components and overlooked in the structural analysis. In order to complete performance-based design on RC frames, some experiments and numerical simulations were performed: Fardis and Panagiotakos[1] and Kappos et al.[2] evaluated the influence and configuration of infill walls on RC frames under seismic loads; Dolšek and Fajfar[3] studied the effect of masonry infills on the seismic response of a four-storey reinforced concrete frame based on pushover analysis and the inelastic spectrum approach; Celarec et al.[4] and Dymiotis et al.[5] analyzed the influence of uncertainty on seismic capacity of infilled RC frames based on sensitivity and fragility analyses. The literature survey presented here is to highlight the influence of infill walls and give strong laboratory and field evidence that infill walls shouldn‟t be neglected.

This paper establishes a numerical model on the software OpenSees for assessing seismic behavior and target performance levels of infilled RC frames. IDA method has been performed to obtain target IDRs. The comparison of thresholds of three performance levels among the infilled RC frame, bare RC frame and criterions proves that infill walls play an important role in buildings.

2 TARGET PERFORMANCE LEVELS OF INFILLED RC FRAMES A performance objective is defined as a given level of performance under ground motion. Buildings‟ target performance levels are discrete damage states selected from the infinite spectrum of possible damage states that buildings could experience during an earthquake[6]. „„Enhanced Rehabilitation Objectives‟‟ of FEMA-356[6] and SEAOC 2000[7] give different deterministic IDR thresholds of RC frames, as following Table.1 TABLE 1 IDR THRESHOLDS CORRESPONDS TO DIFFERENT PERFORMANCE LEVELS Criterion FEMA-356 SEAOC 2000

Immediate Occupancy 1% 0.5%

Life Safety 2% 1.5%

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Collapse Prevention 4% 2.5%

Following the definition of target performance levels, the corresponding seismic hazard levels must be determined to establish the performance objectives. Ground motion can be represented as time histories, acceleration response spectra, displacement response spectra, drift demand spectra, or by other means. In this work, peak ground acceleration (PGA) is used to represent the intensity of ground motion[6]. Chinese seismic design code[8] defines the three-level performance objectives and RC frames will satisfy the demands of “No Damage in Minor Earthquakes”, “Mendable in Moderate Earthquakes” and “No Collapsing in Strong Earthquake”. This paper combines domestic and international norms, and divides target performance levels of infilled RC frames into three degrees: immediate occupancy, life safety, collapse prevention, combining the limit states of both the main frame and infill walls. The corresponding PGA values have been listed in Table.2.

TABLE 2 THREE-LEVEL SEISMIC FORTIFICATION INTENSITY[8] Seismic Fortification Intensity PGA

6 degree 0.15g

7 degree 0.30g

8 degree 0.45g

3 MODELING OF INFILL WALLS Modeling infill walls has been a major challenge. This paper uses an improved equivalent slant strut based on FEMA 356 guidelines, modified by Kadysiewski S and Mosalam K M[9]. Table.3 lists the material properties and area of improved equivalent slant strut.

TABLE 3 EQUIVALENT STRESS-STRAIN RELATIONSHIP

f mo (GPa)

17e-3

 mo

0.00278

f mu (GPa)

1.99e-3

 mu

0.00406

The effective width of equivalent slant strut is an important parameter in this model. Chen Yinsong[10], from Chongqing university, built five simplified models of infilled RC frames and carried out finite element analysis. By comparing these results with four tests, it can be verified that the effective width formula proposed by Chrysostomou[11] can better simulate the interaction between infill walls and the main frame.

4

w  0.175( h) d ,   -0.4

EW tw sin(2 ) 4 Ec I c H in

(1)

The effective width of equivalent slant strut is 516 mm calculated by formula 1 in this paper. Physical parameters of infill walls are given in Table.4.

TABLE 4 SIMPLIFIED MODEL PARAMETERS OF INFILL WALLS

Ew (Gpa)

Ec (Gpa)

I c (mm4 )

H in (mm)

tw (mm)



h(mm)

d (mm)

5.034

19.4

2.13e+9

2800

120

0.676

3300

4883.6

4 CASE STUDY This paper establishes a six-story infilled RC frame and bare RC frame, six bays in X direction and three bays in Y direction on the software OpenSees. Figure.1 shows the three-dimensional models.

Ground motion is one of the most critical factors on the evaluation of target performance levels. The magnitude of ground motion, its frequency content and duration are very difficult to predict. According to the concept proposed by Jack Baker and Cornel[12] (Conditional Mean Spectrum, CMS), 30 seismic excitations are selected and modulated into 7 PGA: 0.05g, 0.15g, 0.25g, 0.30g, 0.35g, 0.45g, 0.55g. In this work, IDA has been carried out and the maximum displacement responses of respective nodes are collected. In Fig.1, it can be seen that the control nodes are N25, N53, N81, N109, N137, N165, N193. Fig.2 plots the maximum interstory drifts (MID) of each story under 7 PGA values corresponding to 30 seismic excitations.

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(a)

(b)

FIG.1 THREE-DIMENSIONAL MODEL (a) INFILLED RC FRAME; (b) BARE RC FRAME

Fig.2 shows that the MID of six stories increase along with the increase of PGA values. It‟s clear that the 2nd story‟s MID is usually larger than others. That can also be verified next. By comparison, lower floors‟ displacement responses are usually larger than higher floors. So the paper considers the 2nd story as the critical story of global frame and its performance levels are defined as the global performance limit states.

According to the research report of Cimellaro G.P[13], the IDR can be assumed to be lognormally distributed. From Fig.2, IDRs of each story will be figured out. By means of the method of maximum likelihood, the mean and standard deviation of each story are worked out and plotted in Fig.3 and Fig.4, respectively. A large number of points provide reasonable results to obtain the thresholds of target performance levels.

FIG.2 MAXIMUM INTERSTORY DRIFTS (MID)

FIG.3 MEANS OF IDRS

FIG.4 STANDARD DEVIATIONS OF IDRS - 70 http://www.ivypub.org/AE

In Fig.3, it is verified that the 2nd story is the critical story. In Fig.4, the standard deviations of the 2nd story are largest, which means greater discreteness and more danger.

According to Chinese seismic design code[8], the PGA values of minor, moderate and strong earthquake can be taken as 0.15g, 0.30g, 0.45g. As a result, the IDR thresholds of three target performance levels can be obtained from Fig.3.

TABLE.5 IDR THRESHOLDS BY IDA Three performance levels PGA threshold

Immediate Occupancy 0.05g 0.45%

Life Safety 0.30g 1.17%

Collapse Prevention 0.45g 1.94%

Owing to the limit of paper length, only part of analysis is presented here. This paper omits the modeling and IDA on bare RC frame, and comparing thresholds of target performance levels among infilled RC frame, bare RC frame and criterions, as shown in Fig.5.

FIG.5 COMPARISON OF PERFORMANCE OBJECTIVES‟ THRESHOLDS

From Fig.5, it‟s clear that the thresholds of infilled RC frame are much smaller than the bare RC frame‟s. For immediate occupancy state, reducing 8%; for life safety state, reducing 10%; for collapse prevention state, reducing 35.3%. That‟s to say that the existence of infill walls can significantly improve stiffness of global frame, raising its seismic performance. If not taking into account the influence of infill walls, the drift ratio thresholds may be non-conservative.

5 CONCLUSION Combining with the damage characteristics, research status and performance-based seismic design of infilled RC frames, this paper presents a nonlinear finite-element modeling scheme for assessing three target performance objectives. The main findings can be summarized as follows: 1, Based on the software OpenSees, improved equivalent slant struts are used to model infill walls. And a six-story infilled RC frame is built as the nonlinear finite-element scheme, taking into account the influence of infill walls.

2, Combined with the definitions of criterions about the target performance levels, this paper divides the seismic performance of infilled RC frames into three levels: immediate use, life safety, and collapse prevention, and carries out IDA to obtain IDR of each story. The target performance levels consist of limit states of both the main frame and infill walls under specific PGA. 3, Comparing the thresholds generated by the infilled RC frame, bare RC frame and criterions, it‟s easy to find that IDRs of infilled RC frame are clearly smaller. The existence of infill walls contributes to the stiffness of the main frame and this structural contribution will increase following the addition of PGA.

6 ACKNOWLEDGMENTS The authors would like to acknowledge the funding support by the National Natural Science Foundation of China under Award Number 51278420, Graduate Starting Seed Fund of Northeastern Polytechnical University under the Grant Number Z2014114.

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REFERENCES [1]

Fardis M N, Panagiotakos T B. Seismic design and response of bare and masonry-infilled reinforced concrete buildings. Part II: infilled structures[J]. Journal of Earthquake Engineering. 1997, 1(3): 475-503

[2]

Kappos A J, Stylianidis K C, Michailidis C N. Analytical models for brick masonry infilled R/C frames under lateral loading[J].

Journal of earthquake engineering. 1998, 2(1): 59-87

[3]

Dolšek M, Fajfar P. The effect of masonry infills on the seismic response of a four-storey reinforced concrete frame—a deterministic assessment[J]. Engineering Structures. 2008, 30(7): 1991-2001

[4]

Celarec D, Ricci P, Dolšek M. The sensitivity of seismic response parameters to the uncertain modelling variables of masonry-infilled reinforced concrete frames[J]. Engineering Structures. 2012, 35: 165-177

[5]

Dymiotis C, Kappos A J, Chryssanthopoulos M K. Seismic reliability of masonry-infilled RC frames[J]. Journal of Structural Engineering. 2001, 127(3): 296-305

[6]

Fema P. Commentary for the Seismic Rehabilitation of Buildings[J]. Federal Emergency Management Agency, Washington, DC. 2000

[7]

Vision S. Performance based seismic engineering of buildings[J]. Structural Engineers Association of California, Sacramento, Calif. 1995

[8]

GB. Code for Seismic Design of Buildings[S][Z]

[9]

Kadysiewski S, Mosalam K M. Modeling of Unreinforced Masonry Infill Walls Considering In-Plane and Out-of-Plane Interaction[M]. Pacific Earthquake Engineering Research Center, 2009

[10] Chen Yin Son. The Simulation Method and Effect Law for Masonry Infill Walls in Nonlinear Seismic Response Analysis of Reinforced Concrete Frames[Z]. Chongqing University, 2011 [11] Chrysostomou C Z. Effects of degrading infill walls on the nonlinear seismic response of two-dimensional steel frames[J].

Dissertation Abstracts International. 1991, 51(12): 348 [12] Baker J W, Allin Cornell C. A vector‐valued ground motion intensity measure consisting of spectral acceleration and epsilon[J].

Earthquake Engineering & Structural Dynamics. 2005, 34(10): 1193-1217 [13] Cimellaro G P, Reinhorn A M. Multidimensional performance limit state for hazard fragility functions[J]. Journal of engineering mechanics. 2010, 137(1): 47-60

AUTHORS 1

2

degree of civil engineering in Northwester

northwestern

Lu Liu, born in 1991,earned his bachelor

Ziyan Wu, born in 1962, Professor, responsible professor at polytechnical

university

civil

engineering

n Polytechnical University,Xi‟an, China. H

discipline,

er research area covers applied fragility of

construction of Shan Xi provincial institute, director of vibration

structures.

managing

director

of

civil

engineering

and

institute of Shan Xi branch, China project management research committee (PMRC) standing committee, China international project management certification (IPMP) appraiser. Her research area covers applied Structural health monitoring and reliability

3

Hongbin Sun, born in 1991, earned his bachelor degree of civil

assessment. By now, she has published the papers: In-plane

engineering in Northwestern Polytechnical University, Xi‟an,

dynamic response analysis of curved pipe conveying fluid

China. His research area covers applied Structural health

subjected to random excitation. Nuclear Engineering and Design,

monitoring and reliability assessment.

2013,

256:

214-226.

(EI:

20130615987085,

SCI:

000316522800022); The dynamic reliability analysis of pipe conveying fluid based on a refined response surface method.

Journal of Vibration and Control.(SCI), etc.

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