refined pheriomenological model for masonry infill panels … · 2014. 5. 16. · 526 earthquake...

Irregularities in the seismic response of R/C

buildings due to the presence of masonry infills

C. Michailidis, A.J. Kappos, K. Stylianidis

Department of Civil Engineering, Aristotle University of

Thessaloniki, Thessaloniki 54006, Greece

Abstract

A refined phenomenological model for masonry infill panels surrounded by R/Cframes and subjected to transient cyclic loading is used to investigate theinfluence of the presence of regular and irregular patterns of masonry infills onthe seismic behaviour of multistorey frames. It is seen that regular patterns ofinfills (not necessarily in all bays) can lead to a behaviour generally superior tothat of corresponding bare frames. Open storeys in infilled buildings lead todamage concentration in these storeys and failure can only be avoided throughheavy confinement in R/C members, as that specified in Eurocode 8.

1 Introduction

One of the most common types of irregularity in buildings is caused by theuneven distribution of infill walls, traditionally considered as "non-structural"in the design process, which however have long been known to significantlyaffect the stiffness and strength of the buildings when they are subjected toseismic loading (Klingner & Bertero\ Bertero & Brokken'). This interactionbetween infill walls/panels and the surrounding frames is recognised in moderncodes, such as the Eurocode 8 (CEN TC 25(f), and some empirical rules aregiven to account for adverse effects, while the positive role of the infills inincreasing the strength and energy dissipation capacity of buildings (Penelis etal.\ Penelis & Kappos ) is generally ignored.

The main objective of this paper is to investigate the influence of thepresence of masonry infills with symmetric and unsymmetric arrangements onthe seismic behaviour of multistorey reinforced concrete (R/C) frames. A

Transactions on the Built Environment vol 20, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509

526 Earthquake Resistant Engineering Structures

refined pheriomenological model for masonry infill panels surrounded byreinforced concrete (R/C) frames and subjected to transient cyclic loadingpreviously developed by the authors (Michailidis et al.~) is used to this purpose.

2 Structures analysed

A ten-storey R/C frame was designed (Kappos & Antoniades ) to the Eurocode8 provisions for a design ground acceleration Aj = 0.25g and ductility class"M" (behaviour factor q=3.75). In the design no account for the presence ofinfills was made, in order to focuss on possible detrimental effects due to theirpresence and to be able to compare the calculated behaviour with that of actualstructures which have been designed in a similar way. Note that this approach isalso permitted by the Eurocode for the case of uniformly distributed infills,while a magnification factor depending on the strength reduction betweenadjacent infilled and open storeys is required in the irregular configurations.

A number of different arrangements and quality of brick masonry infillpanels (running bond), reflecting several situations that may be found inpractice, were then studied, including structures with an open ground storey("pilotis") or with an open intermediate bay (coupled-wall like structure). Withregard to mechanical characteristics, the two combinations shown in Table 1were considered (low-strength LS, and high-strength HS masonry). Thefollowing seven different structures resulted (Fig. 1), with the correspondingfundamental periods:# Bare frame (BF) T^ =0.98 sec• Fully infilled frames

* LS infills (IF 1) TO =0.51 sec* HS infills (IF2) T^ =0.42 sec

# Infilled frames with open lower storeys* 1 storey (IF1P) T,,=0.58sec(LS)

(IF2P) To =0.51 sec (HS)* 2 storeys (IF2P2) T,, =0.64 sec (HS)

# Frame with infills at the exterior bays only (IF2M) T<, =0.49 sec (HS).

Table 1. Mechanical characteristics of the brick masonry infills

Code

LSHS

Compressivestrength (MPa)

1.53.0

Cracking•y (%) T (MPa)0.0785 0.1010.0555 0.271

Ultimatey (%) T (MPa)0.3568 0.2690.2523 0.381

It is seen that for these nominally identical structures (as far as design isconcerned) the ratio of periods T^* / T^ = 0.98/0.42 = 2.33 and thecorresponding ratio of stiffnesses k^ / k^ = 2.33 = 5.44, that is the presenceof infills affects considerably the dynamic characteristics of the building.


Earthquake Resistant Engineering Structures 527

6.0 -

(b)

4.0 *

(C) (d)

Fig. 1 Structural configurations studied: (a) Fully infilled frame (IF); (b) Oneopen storey (IFF); (c) Two open storeys (IFP2); (d) "Coupled-wall" (IFM).



3 Modelling considerations

Based on the concept that to obtain the contribution of the infill wall, it ispossible to subtract the shear carried by an identical bare frame from the shearcarried by the infilled frame (at the same deformation level), Valiasis et al.have developed a rather refined pheriomenological model for relatively weakbrick masonry infill panels surrounded by R/C frames; the hysteretic behaviourof the infill was determined on the basis of tests on single-storey infilled R/Cframes of various configurations, carried out by Valiasis & Stylianidis . Themodel was subsequently extended by Michailidis et al/ to transient cyclicloading, including small amplitude inelastic cycles. The main hysteresis rulesfor the model are shown in Fig. 2, where it is seen that proper account is takenof all basic features of inelastic seismic response, including:• Different pre-cracking and post-cracking stiffness• Strength degradation during cycling at a constant amplitude of

deformation• Pinching behaviour due to shear and slippage• Stiffness degradation increasing with the amplitude of deformation• Slip under non-zero constant lateral load.

cracking5;

•" 10

LoadLng-unloading-reloading path:0-1-2-3-4-T-5-6-7-T-S-9-10-U-T-12-13-I4-I5-16-T

Fig. 2 Hysteresis rules for masonry infill wall

An isoparametric F.E. with 2 D.O.F. per node has been developed forimplementing the aforementioned model. Only shear deformation is taken intoaccount (shear panel), and hysteresis rules define the shear stress (T) - shearstrain (y) behaviour at the centroid of the element. A standard displacementtransformation is used to relate y at the centroid to the 8 nodal displacements. Inthe structures shown in Fig. 1 each of the four nodes of a shear panel isconnected to the adjacent beam-column joint nodes of the surrounding frame.

R/C members are modelled using standard point hinge member-typemodels with bilinear Takeda hysteresis for members with axial load N~const.,



and bilinear non-degrading with My-N interaction for members (exteriorcolumns) with considerable variation of axial load (see Kappos & Antoniades^and Penelis & Kappos ).

4 Selection and normalization of input motions

As the calculated dynamic response of inelastic structures is sensitive to thecharacteristics of the input accelerogram (Penelis & Kappos , Michailidis etal/), a total of six records from the earthquakes that caused serious damage,including collapses and casualties, in Greece during the years 1975-1990 wereused, including the earthquakes of Volvi, 1978 (motion recorded atThessaloniki city centre), Alkyonides, 1981 (motion recorded at Korinth citycentre), and Kalamata, 1986 (motion recorded at Kalamata city centre). Allthese records are characterized by the fact that they come from surfaceearthquakes with small epicentral distances (these are the typical destructiveearthquakes in Greece). For comparison purposes, the well-known SOOEcomponent of the El Centro 1940 record was also included in the study.

The resulting 7 records were normalised to fractions of the intensity of thedesign earthquake (Aj=0.25g), using a modified Housner technique (see Penelis& Kappos ), whereby the scaling factors are derived by considering the areasunder the velocity response spectra of the actual records and of the designearthquake, between appropriately defined bounds of the natural period. A 10sec duration was used for all records, but the integration time step was differentin the bare frame (0.02 sec) and in the infilled frames (0.005 or 0.01 sec).

5 Discussion of results

Although the seven motions had been normalised to the same spectrumintensity, the maximum calculated response was found to be up to 180% higherthan the corresponding minimum response in some locations, while themaximum response (irrespective of location) was up to about 100% higher.Nevertheless, the distribution of critical response quantities along the height ofthe buildings was reasonably uniform. The discussion which follows is basedon the average calculated response for the seven motions.

An important global index of the seismic performance of the structures isthe normalised (with respect to the storey height h) interstorey drift Ax/h;average values of this index for all the structures studied are plotted in Fig. 3. Itis first pointed out that the effect of masonry strength (and stiffness) isrelatively minor (compare IF1 and IF2, and IF IP and IF2P in Fig. 3a) for therange of strengths studied. As expected, the drifts of the frames infilled with theweaker masonry walls are smaller than those of frames with strong walls; notethat the differences in the natural periods are minor, hence the seismic input isgenerally the same in both weak and strong masonry infilled structures.



storey level storey level

0 0.5 1drift ratio Dx/h (%)

-*- BF -o- IF1 -*- IF2 -o- IF1P -

(a)

-IF2P

0.5 1drift ratio Dx/h (%)

-BF -0-IF2M -+-IF2 -O-IF2P2

(b)

1.5

-IF2P

Fig. 3 Interstorey drift ratios for bare and infilled frames. Average values for 7motions normalised to Aef=0.25g.

Fully infilled frames (IF1 and IF2) are characterised by significantly lowerdrifts than the corresponding bare frame (Fig. 3a), exhibiting a tendency ofgradually decreasing drift with the height of the building, which agrees wellwith the picture obtained from inspection of actual buildings damaged byearthquakes [7]. As this is a drift pattern different from that calculated for thebare frame which exhibits a rather uniform distribution with maximum driftsoccurring at the seventh and ninth storeys where a stiffness taper occurs in bothexterior and interior columns, it is attributed to the higher infill to framestiffness ratio in the upper storeys, where the geometry of the infills remainsunchanged, while beams and columns have smaller dimensions than at thebottom of the structure.

With regard to the open storey(s) structures, it is seen from Fig. 3 that theyexhibit very high drift values at the open storeys (up to 1.3% in IF IP) anddrastically reduced drifts in the infilled storeys. This is a well-known featurewhich clearly indicates the concentration of damage in the open ("soft") storeys.The structure (IF2P2) with the open lower two storeys has a maximum drift ofabout 1%, slightly lower than the corresponding value for the pilotis building(IF2P), but significant damage (under the design earthquake) is also expected atthe second storey, hence the cost of repair can be higher in this case. Thestructure with no infills in the intermediate spans (pseudo-coupled-wall, IF2Min Fig. 3b) is characterised by moderately increased drifts with respect to thefully infilled frame (IF2) along the entire height of the building, which is an



indication that no tendency for damage concentration in weak regions appearsin this case.

Shown in Fig. 4 are the rotational ductility factors (u ) for the columns ofthe various infilled structures studied (strong masonry case). With regard to theinterior column ductility (Fig. 4a), it is seen that its distribution along the heightof the building is rather uniform (no damage concentration) for both the bareand the regularly infilled cases (IF2 and IF2M); note that for the latter two casesthe response of columns is very close to elastic conditions (|He<l).

storey level10

0 1 2 3 4 5 6 7internal columns ductility

_*-BF -0-IF2M -+-IF2 ->-IF2P2

(a)

- IF2P

1 2 3 4 5 6 7external columns ductility

-BF -o- IF2M -+-IF2 -o- IF2P2 -IF2P

(b)

Fig. 4 Column ductility factors for bare and infilled frames: (a) Interiorcolumns; (b) Exterior columns.

Consistent with the drift distributions of Fig. 3, the column ductility demands inthe open storey(s) structures are quite high, with (average) values of p^exceeding 6 in the pilotis building. With regard now to exterior columnductilities, a somewhat different picture appears in Fig. 4b, the u^ values in thelower storeys of the regularly infilled structures being closer to those of thebuilding with two open storeys. This is somewhat misleading in the sense thatthese increased values (compared with those for the interior columns) aregenerally not due to the higher plastic rotations (6p) in the exterior columns, butrather due to lower yield rotations (6y) caused by different axial loading. Tensileaxial loads with values up to 90% the tensile yield strength developed at theexterior columns, resulting in quite low yield moment and rotation values,hence at increased rotational ductility factors. It is pointed out that no tensile



forces were recorded in the bare frame, therefore the different axial loads areattributed to the frame-infill interaction.

Despite the relatively high rotational ductility demands in the irregularlyinfilled structures, it has been found using a methodology previously suggestedby the authors (see Kappos & Antoniades^ and Penelis & Kappos^) that theavailable plastic rotation capacity of the columns was higher than thecorresponding demands, even in the case of the most critical input motion. Thiswas the result on the one hand of heavy confinement of the column end regionsaccording to ECS provisions (double 10 mm hoops at 90 mm centres), and onthe other hand of relatively moderate axial load levels (below the balancedconditions) in the columns. In the absence of conclusive test data, no attemptwas made to estimate the shear capacity of the columns subjected to alternatingtension and compression and compare it with the corresponding demands.

r level ' level

0.5 1 1.5 2beam ductility

- BF -o- IF1 -*- IF2 -*- IF1P -+- IF2P

1 4 ----- L-

0 0.5 1 1.5 2 2.5 3 3.5beam ductility^_________

-IF2 • -IF2P2 • IF2P

Fig. 5 Beam ductility factors for the structures studied

The beam ductility factors (average over 7 motions) calculated for thevarious structures studied are shown in Fig. 5; the values shown refer to"negative" yielding at the supports (top reinforcement in tension), which aregenerally lower than the values corresponding to "positive" yielding but havebeen found to produce more critical capacity/demand ratios due to the T-beameffect in the latter case. It is seen that beam ductility demands are quite low inboth the bare and the infilled frames, with the exception of the two open storeysbuilding (IF2P2), where increased beam ductilities (exceeding 3) are recorded atthe first-storey level. It is pointied out that the "coupling beams" in structure



IF2M do not exhibit higher ductility demands than those of the bare frame.As shown in Fig. 6, more energy is dissipated in the infilled, than in the

bare frames due to the higher strength of the former and the different dynamiccharacteristics. More importantly, the energy dissipation mechanism is differentin each structure. In the fully infilled frames (IF1, IF2) more than 90% of thetotal energy dissipation takes place in the infills (about 80% in the pseudo-coupled-wall). On the contrary, in the pilotis buildings the largest amount ofenergy is dissipated in the columns of the open storey(s), while in IF2P2 thebeams of the first storey also contribute significantly.

MEAN VALUES MEAN VALUES

BF IF2M IF2 IF2P2 IF2PIF2M IF2 IF2P2 IF2P

(c) (d)

Fig. 6 Energy dissipation in the structures studied (average of 7 motions)

6 Conclusions

1. The calculated response of infilled frames appears to be in agreement withboth test results and observations of seismic damage in actual structures ofthis type, which is quite different from damage in bare frames.

2. The present study indicated a superior performance of structures withcontinuously arranged masonry infills (fully infilled or with regularlyarranged openings) with respect to that of corresponding bare frames. Infillwalls can provide the main energy dissipation mechanism in structures



subjected to the design earthquake, provided weak storeys are avoided.3. The ECS design procedure appears to be effective in providing sufficient

ductility and energy dissipation capacity in R/C columns, propertiesparticularly needed in the case of irregular (weak storey) buildings.

4. Finally, the complexity and the cost of the analysis used is such that apossibility for practical application may be envisaged, at least in the case ofparticularly important structures, and also for code calibration purposes.

References

1. Bertero, V.V & Brokken S. Infills in seismic resistant buildings, Journal of, 109(6) 1983, 1337-1361.

2. CEN Techn. Comm. 250 / SC8. Eurocode 8: Design provisions forearthquake resistance of structures - Part 1: General rules (ENV 1998-1-1/2/3), CEN, Brussels, 1994.

3. Kappos, A.J. & Antoniades, K. "Seismic performance assessment of R/Cbuildings designed to the 1994 Eurocode 8", Proceed. 5th SEC ED Conf. onEuropean Seismic Design Practice, Chester, U.K., Oct. 1995, pp. 349-357.

4. Klingner, R.E & Bertero V.V. Earthquake resistance of infilled frames.SYrwcfwro/ D/Wfmrz, /(SCE, 104(ST6) 1978, 973-989.

5. Michailidis, C.N., Stylianidis, K.C. & Kappos, A.J. Analytical modelling ofmasonry infilled R/C frames subjected to seismic loading, pp. 1519-1524,Vol. 3, Proceed. 10th European Conference on Earthquake Engineering,Vienna, Austria, Aug. - Sep. 1994, Balkema 1995.

6. Penelis, G.G. & Kappos A.J. Earthquake Resistant Concrete Structures, E &FN SPON (Chapman & Hall), London, 1996.

7. Penelis, G.G., Sarigiannis D., Stavrakakis E. & Stylianidis K.C. A statisticalevaluation of damage to buildings in the Thessaloniki, Greece earthquake ofJune, 20, 1978, VII, pp. 187-192, fmcfWmgj q/ Pf& fFor&/ Co/?f 0/7Earthq. Engng., Tokyo-Kyoto, Japan, Aug. 1988, Maruzen, Tokyo, 1989.

8. Valiasis, T.N. & Stylianidis K.C. Masonry infilled R/C frames underhorizontal loading - Experimental results, European EarthquakeEngineering, 111(3), 1989, 10-20.

9. Valiasis, T.N., Stylianidis K.C. & Penelis G.G. Hysteresis model for weakbrick masonry infills in R/C frames under lateral reversals. EuropeanEarthquake Engineering, VII(l), 1993, 3-9.


refined pheriomenological model for masonry infill panels … · 2014. 5. 16. · 526 earthquake...

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