effect of urm infills on seismic

7/31/2019 Effect of Urm Infills on Seismic

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4 t h I n t e r n a t i o n a l Co n f er e n c e o n E a r t h q u a k e E n g in e e r i n gTaipei , Taiwa n

October 12-13, 20 0 6

P a p e r N o . 0 6 4

EFFECTOFURMINFILLSON

SEISMICPERFORMANCEOFRCFRAMEBUILDINGS

Yogendra Singh1

Dipankar Das2

ABSTRACT

Use of unreinforced masonry infills as partitions, in frame buildings is a common practice in India

and many parts of the worlds. The infills are, generally, not considered in the design and the buildings

are designed as bare frames. This study investigates the effect of masonry infills on the seismic

performance of RC frame buildings. For this purpose, three buildings, 4 storey, 8 storey and 16 storey

tall, having identical plan, have been considered. Linear analysis and design of each bare frame

building has been performed as per the relevant Indian codes of practice. The effect of infills on

dynamic characteristics, yield patterns and seismic performance has been studied with the help of

Non-Linear Push-Over Analysis. It has been observed that infills contribute to a large increase in the

stiffness and strength of the structure while the deformation capacity of the structure gets reduced. In

case of uniformly infilled frame buildings designed as per codes, it has been observed that infills failbefore frame elements. It has also been observed that due to strut action of infills, high axial forces

get generated in columns of uniformly infilled frame buildings and as a result, columns fail earlier

than those of bare frame buildings.

Keywords: Earthquake, Masonry infills, Reinforced concrete frame, Non-linear modeling, Pushover

analysis

INTRODUCTION

In India, masonry infilled reinforced concrete frames are a common structural system for buildings.

Masonry infills are used for functional or aesthetic reasons, rather than as structural elements. Their

presence is generally ignored by the designers and no consideration is given to their own seismic

safety or their effect on the performance of the structure. Although, these are considered as non-

structural elements, they interact with the frame when the buildings are subjected to lateral loads. The

masonry infills act as diagonal struts between the frame joints. This diagonal strut results inconsiderable increase in stiffness and strength of the frame. The increased stiffness, in tern, attracts

increased inertia force during earthquake. Due to this frameinfill interaction, the failure modes of the

global structure may get changed. Four types of failure modes have been identified (Pauley and

Priestley, 1992) in case of infilled frame buildings: (1) Tension failure of the tension side column

resulting from the applied overturning moments in infilled frames with high aspect ratio, (2) Sliding

shear failure of the masonry along horizontal mortar bed joint causing shear hinges in the columns

due to short column effect, (3) Compression failure of the diagonal strut, and (4) Diagonal tensile

cracking of the panel.

1Assistant Professor, Department of Earthquake Engineering, IIT Roorkee, Roorkee-247 667, India, [email protected] Post Graduate Student, Department of Earthquake Engineering, IIT Roorkee, Roorkee-247 667, India


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During earthquake, the infill itself, is subjected to in-plane, as well as, out-of-plane forces. In in-plane

action, it may fail in any of the last three modes, described above. In case of slender infills, the failure

may also occur due to buckling. In out-of-plane action, the infill fails in bending tension in case of

panels with high h/t ratio, while an arching mechanism is developed, in case of panels with relatively

low h/t ratio (FEMA 356). Generally, the infills first crack due to in-plane action and then fail, with or

without arching action, due to out-of-plane forces. The overall phenomenon is quite complex to be

handled in totality. In the present study, the in-plane strength of infills and their effect on seismic

behaviour of RC frame buildings have been studied.

MODELING OF URM INFILLS

The analytical modeling of infilled frames is a complex issue, because these structures exhibit highly

nonlinear inelastic behavior, resulting from the interaction of the masonry infill panel and the

surrounding frame. The available modeling approaches for masonry infills can be grouped into Micro

models and Macro models. Micro models capture the behavior of infill and its interaction with the

frames in much detail, but these models are computationally expensive. On the other hand, macro

models try to capture the gross behavior of the infill, are approximate but computationally efficient. A

number of models using the two approaches have been proposed by various researchers.

Mallick and Severn (1967) suggested the first finite element approach to analyze infilled frames. The

infill panels were simulated by means of linear elastic rectangular finite elements, with two degrees of

freedom at each of the four corner nodes. Interface between frame and infill was modeled and contact

length was calculated. The slip between frame and infill was taken into account by considering

frictional shear forces in the contact region using link element. Each node of this element has two

translational degrees of freedom. The element is able to transfer compressive and bond forces, but

incapable of resisting tensile forces. Axely and Bertero (1979) suggested two finite element

approaches, namely exact scheme and constraint scheme, to find the stiffness of frame-infill system.

In constraint scheme, frame-infill system is uncoupled and the system is modeled by separate

assemblage of finite elements for the frame and infill. The separate stiffness matrices are formulated

and the stiffness of the infill alone is reduced to the boundary degrees of freedom, using condensation.A constraint relation is assumed between the 12 degrees of freedom of frame and the infill boundary

degrees of freedom. To validate the results, exact scheme with refined assemblage of beam and plane

stress elements has been adopted. This scheme considers coupled system.

Liauw and Kwan (1984) have proposed a plastic theory in which three different failure modes were

identified, related to the relative strengths of the columns, the beams and the infill. Those models can

capture corner crushing with failure in columns and beams, and diagonal crushing of the infill. Rivero

and Walker (1984) developed a nonlinear model to simulate the response of infilled frames to

earthquake motions. In this model, the infill panels are modeled by means of triangular elements,

characterized by homogeneous, isotropic, linear elastic constitutive relations, representing several

bricks and mortar joints. Nonlinearities are considered through the inelastic behavior of the frame, the

interaction between frame and the wall and the cracking of the concrete. A gap element and a jointelement were introduced to model the interface between frame and infill. The gap element is used to

represent the space between frame and infill for no-contact conditions; while the joint element is used

when the two members are in contact. Zhuge and Hunt (2003) have proposed Distinct Element

approach to model masonry infills. This approach considers masonry as an assembly of discrete

blocks. The mortar joints are represented numerically as contact surfaces formed between two block

edges. This model can also capture the behavior of separation and sliding.

The micro models described above are capable of modeling the behaviour of masonry infill and the

frame-infill interaction in considerable detail, but these are computationally not feasible for nonlinear

modeling of the global behaviour of a large building. There are some macro models available, which

simulate the effect of infills with sufficient accuracy and are computationally efficient. Smith (1968)

proposed the width of the equivalent diagonal strut as one fourth of diagonal length of infill panel.

Klinger and Bertero (1976) suggested the model of diagonal strut with hysteretic behavior. The model


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is able to simulate the stiffness degradation due to cyclic loading. Two diagonal struts are required to

model infill panel and width of diagonal strut is same as proposed by Stafford Smith.

Thiruvengadam (1985) proposed multiple strut model of infill panel by considering reciprocal

stiffening effect. The model consists of a moment resisting frame with a number of pin-jointed

diagonal struts in both the directions. FEMA 356 has proposed the diagonal strut model of infill by

considering deformation controlled action with specified properties. This model can capture the frame

infill interaction in global sense. Expected shear strength is considered as the control action and drift

of the infill as corresponding deformation parameter. It has zero stiffness in extension and bending. It

also has zero out of plane stiffness. This model has been used in the present study to investigate the

effect of infill on behaviour of frame buildings.

BUILDINGS CONSIDERED

To observe the effect of infill on the global behavior of frame buildings, a 4 storey, an 8 storey and a

16 story building, with identical plan, as shown in Fig. 1, have been considered. The overall plan

dimensions are 25.28m x 11.90m, measured from the centre line of the columns. The height of the

ground floor is 4.5m and inter storey heights are 3m. A solid slab of 150 mm thickness has beenconsidered for all storeys. Live load intensity on floors and roof has been taken 4kN/m

2and 2kN/m

2,

respectively. Slab loads have been distributed to frame elements according to yield line pattern. The

thickness of exterior infills has been considered as 230 mm and that of interior infills as 150 mm,

consistent with the prevailing practice in India. 30% reduction in the weight of infills has been done to

take into account the openings. However, the effect of opening on stiffness and strength has been

ignored.

Y

X

CR

5.

95

2.

29

3.6

6m

CL

Figure 1. Plan of Buildings

The plan of the three buildings is symmetric about transverse axis and slightly asymmetric about

longitudinal axis. There are no transverse beams over the corridor. This results in frames with high

aspect (H/B) ratio in transverse direction. The selection of such a plane has been made to study the

effect of widely varying aspect ratio, on the behaviour of infilled frames. All the buildings have been

designed as per the provisions of IS: 1893 (2002) and other relevant Indian codes of practice, by

considering the structure as bare frame, with and without the correction factor for reduction of time

period due to presence of infills. The correction factor is applied to the design base shear, in the formof a multiplication factor equal to the ratio of the spectral acceleration corresponding to the time

3.16m 3.16m 6.32m 6.32m 3.16m 3.16m


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period obtained from empirical formula for infilled frames to the spectral acceleration corresponding

to the time period obtained from bare frame model.

NONLINEAR MODELING AND ANALYSIS

In the present study, frame elements have been modeled by FEMA chord rotation model. Two stiff

zones have been considered at the ends of the elements, to model the finite size of joints. All the

nonlinearity has been concentrated at the faces of the joints and elements by introduction of plastic

hinges. The properties of plastic hinges have been defined as per FEMA 356. Between two hinges at

the ends, the beam portion has been considered as elastic. Floor slabs have been modeled as rigid

diaphragms and joints have been considered as infinitely stiff.

Masonry infill panels have been modeled by two strut elements along the two diagonals. Nonlinear

gap elements have also been used, which are active in compression only. Nonlinearity has been

concentrated in each strut element by providing axial hinges. The force deformation properties for

hinges have been derived from the drift limit states given in FEMA 356. Nonlinear link elements

behave as a normal bracing element (active in both compression and tension) in linear analysis. So the

elastic properties of link elements have been selected in such a way that it will not change the elasticstiffness of the infill panel.

First, a Linear Dynamic Analysis (Response Spectrum Analysis) has been performed as per IS: 1893-

2002 for each bare frame building and the design of RC members has been performed for calculated

forces using Limit State Design approach. Then, Nonlinear Static Analysis (Push-over Analysis) has

been performed using the capacity spectrum method (FEMA 440) in SAP 2000 Nonlinear software.

Parabolic and uniform vertical distributions of lateral loads along the height of the building have been

considered.

PARAMETRIC STUDY

Dynamic Characteristics

A comparison between the dynamic characteristics of bare frame buildings and uniformly infilled

frame buildings has been presented in Tables 1 and 2. The time periods have been obtained from

modal analysis of the analytical model, described above, and using the following empirical

relationship, as per IS:1893-2002,

B

0.09HT = (1)

Where, H is the height of the building and B is the base dimension in the direction under

consideration.

It can be observed from Table 1 that the time periods get reduced, drastically, due to inclusion of

infills in models. A comparison between time periods obtained from analytical models and those

obtained from empirical codal provisions shows that empirical formula gives much lower time periods.

The analytical time periods shown in the Table have been obtained considering the infills to be solid.

In reality, some of the infills have openings, which is expected to reduce the stiffness, and hence, to

elongate the time period of the buildings. The lower period estimated from the empirical formula,

imposes larger base shear on the building, and results in conservative design. It has been also

observed that the difference of time period from empirical formula and that from analytical modelling

is increasing with the increase of the building height i.e. for tall buildings, the codal formula is more

stringent.


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It can be seen from Table 2 that in case of uniformly infilled frame buildings, relative participation of

first mode is decreasing. This is in confirmation with the expected behaviour of infilled frames, where

bending mode is having more predominant role as compared to shear mode, resulting in larger

participation of higher modes.

Table 1. Fundamental time periods of bare frame and uniformly infilled frame buildings

Fundamental Time

Period (sec)

(From Analysis)

Time Period (sec)

(From IS: 1893-2002)No. of

Storeys

Frame

Configuration

X Y X Y

Bare 1.16 1.42 0.53 0.53

4 Uniform

infill0.48 0.57 0.24 0.35

Bare 1.62 2.12 0.85 0.85

8 Uniform

infill 0.86 1.06 0.46 0.67

Bare 2.91 3.96 1.40 1.40

16 Uniform

infill1.70 2.47 0.89 1.29

Table.2 Modal mass participation factors for bare frame and uniformly infilled frame buildings

Modal Mass Participation Factor

Bare Frame Uniformly Infilled frameNo. of

StoreysMode

X Y X Y1 58.69 88.64 69.17 79.16

2 3.36 8.65 3.40 2.384

3 1.31 2.10 1.00 0.70

1 68.93 79.14 59.64 72.16

2 8.84 11.88 6.58 9.428

3 2.53 4.47 2.11 2.99

1 68.48 72.96 60.97 64.82

2 13.36 15.54 7.46 14.1416

3 3.52 4.14 3.00 4.06

Axial Force and Bending Moment Distribution

The effect of infills on the distribution of column axial forces and bending moments has also been

studied. Figs. 2 and 3 show the typical variation of axial force along the height of columns CL and CR,

respectively, under combined gravity and earthquake load. CL represents the column on the tension

corner of the building when the earthquake loads are applied along positive X and Y directions.

Similarly, CR represents the column on compression end. It can be observed from the Figs. that axial

force due to earthquake gets increased and bending moment gets reduced in the columns in infilled

frame buildings for a particular level of earthquake. The column axial forces in infilled frame, due toearthquake forces, are large enough to cause net tension in columns on tension side and the failure of


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columns may occur due to tension. Similarly, on compression side, the column axial load increases,

considerably, due to presence of infills. This increase in axial load may result in failure of columns at

a lower moment and it considerably reduces the ductility of columns. This may also result in yielding

of columns prior to yielding of beams.

0

2

4

6

8

10

12

14

16

-300 -200 -100 0 100 200

Axial Force

StoreyHeight

Bare Frame

Uniformly

Infill Frame

0

2

4

6

8

10

12

14

16

-800 -600 -400 -200 0

Axial Force

StoreyHeight

Bare Frame

Unifomly Infill Frame

Figure 2. Variation of axial force in column CL Figure 3. Variation of axial force in column CR

0

2

4

6

8

10

12

14

16

-250 -200 -150 -100 -50 0 50 100

Bending Moment

StoreyHeight

Bare Frame

Unifomly Infill Frame

0

2

4

6

8

10

12

14

16

-300 -200 -100 0 100

Bending Moment

StoreyHeight

Bare Frame

Uniformly Infill Frame

Figure 4. Bending moment in column CL Figure 5. Bending moment in column CR

Figs. 4 and 5 show the bending moment along the height of the same columns. The load transfermechanism of the building shifts from frame action to truss (or braced frame) action, due to presence

of infills. Accordingly, the bending moment in the columns reduces.

Figure 6. Failure mechanism of a long direction frame of 4 storey building


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Yield Pattern

A study of yield pattern of the various buildings has been made to understand the inelastic behavior of

the buildings under monotonically applied incremental static loads in different directions. It has been

observed that in case of uniformly infilled frame buildings, hinges form firstly in infills and then in

frame elements (Figs. 6 and 7). As explained above, due to strut action of infills, high axial forces get

generated in columns of uniformly infilled frame buildings and as a result, columns fail earlier than

those of bare frame buildings.

Figure 7. Failure mechanism of a short direction frame of 4 storey building

At collapse level, hinges form in ground storey columns, also, depicting weak storey phenomenon.

However, this phenomenon has been observed in case of frames with low aspect ratio, as this was

observed only in case of four storey building. In taller buildings, this phenomenon was not observed.

A comparison of Fig. 6 and Fig. 7 also depicts this behaviour. Along the longer direction of the

building (Fig. 6) the frames have lower aspect ratio relative to the frames along shorter direction of

the building (Fig. 7). It can be observed that soft storey phenomenon is more pronounced in frames

along the longer direction of the building.

Drift demand gets substantially reduced due to incorporation of infills in bare frame buildings. It has

also been observed that drift predominant portions along the height of the bare frame buildings get

shifted downwards due to inclusion of infills.

Capacity Curves and Performance Levels

A comparative study has also been made for different building models with respect to their capacity

curves and performance levels. Figs. 8-13 show the capacity curves and performance points for DBE

and MCE levels of earthquake forces for different buildings. It can be seen from the capacity curves in

Figs. 8-13 that the strength and stiffness of the uniformly infilled frame buildings are increased,

considerably, but ductility is reduced than that of the corresponding bare frame buildings. The overallperformance of the buildings, in terms of drift at performance point, has improved due to inclusion of


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infill. However, the same may not be true regarding the plastic deformations in individual frame

members. It has also been observed that more ductility is available in frames with higher aspect ratio.

This can be observed by comparing the capacity curves of buildings with different heights and the

curves along X and Y direction of the same building.

0

2000

4000

6000

0 0.1 0.2Roof Displacement (m)

BaseShear(kN)

0.3

Uniform Infill

Bare Frame

Figure 8. Capacity curve of 4 storey building in long direction

0

1000

2000

3000

4000

5000

0 0.1 0.2 0.3

Roof Displacement (m)

BaseShear(kN)

Bare Frame

Uniform Infill

Figure 9. Capacity curve of 4 storey building in short direction

0

2000

4000

6000

8000

0 0.2 0.4 0.6 0.8


BaseShear(kN)

Uniform Infill

Bare Frame



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0

2000

4000

6000

0 0.2 0.4 0.6 0.8


BaseShea

r(kN)

Uniform Infill

Bare Frame


0

2000

4000

6000

8000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8


BaseShear(kN)

Uniform Infill

Bare Frame


0

2000

4000

0 0.2 0.4 0.6 0.8 1


BaseShear(kN)

Uniform Infill

Bare Frame



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CONCLUSIONS

Effect of unreinforced masonry infills on seismic behaviour of RC frame buildings of different

heights, designed as per current Indian codes of practice, has been studied by performing Push-Over

analysis. It has been observed that masonry infills have significant effect on dynamic characteristics,

stiffness, strength and seismic performance of buildings. IS: 1893-2002 gives highly conservative

time period formula for infilled frame buildings. In case of uniformly infilled buildings, the

contribution of higher modes is increased. Axial force in columns increases due to inclusion of infills

in the frames and some of the columns develop net tension. This alters the yield pattern considerably

and building with smaller aspect ratio may develop a column sway mechanism in ground storey. In

case of uniformly infilled frame buildings, strength capacity increases than that of bare frame

buildings but ductility capacity is reduced. This effect reduces with the increase of the height of the

building.

REFERENCES

Axley J.W. and Bertero V.V. (1969). Infill Panels: Their Influence on Seismic Response of Buildings,

Earthquake Engineering Research Center, University of California, Berkeley, CA, Rep. UCB/EERC-79/28.

FEMA 356. (2000). Guidelines for Seismic Rehabilitation of Buildings, ASCE, Reston VA.

FEMA 440, (2005). Improvement of Nonlinear Static Seismic Analysis Procedures, Federal Emergency

Management Agency, Washington, D.C.

Klinger R.E. and Bertero V.V. (1969). Infilled Frames in Earthquake Resistant Construction, Earthquake

Engineering Research Center, University of California, Berkeley, CA, Rep. UCB/EERC-76/32.

Liauw T.C. and Kawn K.H. (1984), New Development in Research of Infilled Frames, Proc. 8th World Conf.

on Earthq. Engng., San Francisko, 4, 623-630.

Mallick D.V. and Severn R.T., (1967), The Behaviour of Infilled Frames under Static Loading, The Institutionof Civil Engineers, Proceedings, 39, 639-656.

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

Sons Inc., New York.

Rivero C.E. and Walker W.H. (1984), An Analytical Study of The Interaction of Frames and Infill Masonry

Walls, Proc. 8th World Conf. on Earthq. Engng., San Francisko, 4, 591-598.

Stafford Smith B. (1968), Model Test Results of Vertical and Horizontal Loading of Infilled Frames, ACI J.,

65(4), 618-624.

Thiruvengadm V. (1985), On the Natural Frequencies of Infilled Frames, Earthquake Engineering and

Structural Dynamics, 13, 401-419.

Zhuge Y. and Hunt S. (2003), Numerical Simulation of Masonry Shear Panels with Distinct Element

Approach,Journal of structural Engineering and Mechanics, 15, 477-493.

effect of urm infills on seismic

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