792 CYCLIC BEHAVIOUR OF INFILLED R/C FRAMES KOSMAS C. STYLIANIDIS Lecturer, Dept. of Civil Eng., University of Thessaloniki P. O. Box 482, Thessaloniki 540 06, Greece ABSTRACT The results of an experimental programme carried out at the R/C Lab. of the Arist . Univ. of Thessaloniki are presented here . It included investigation of single - story one-bay 1/3 scale infilled R/C ductile frame models, the in- fill being an unreinforced masonry wall not connected to the bounding frame, under cyclic quasistatic horizontal loading. The parameters under investiga- tion were the level of the axial compressive load of the columns, the wedging conditions of the masonry against the internal surface of the frame, the quality of the mortar and the presence of a concrete lintel beam at the mid- height of the infill. NOTATION LIST H: lateral loading ó: horizontal displacement y: angular distortion ko: initial stiffness ductil i ty index A: energy dissipated per cycle A/2ó: reduced energy dissipated per cycle FB/FBN: bare f r ame model without/with axial load on the columns infilled frame model without/with axial load on the columns m (subscr i pt) : infilled frame b (subscript): bare frame INTRODUCTION The signif i cant change of the dynamic characteristics of the bare basic str u- ctural system by the incorporation of infills is a fact stated by many authors [1],[2],[3],[4],[5] . As a result, recent seismic codes recommend either an effect iv e structural i solation of the infills from the surrounding frames so that their structural effects can neglected or , alternatively,
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792
CYCLIC BEHAVIOUR OF INFILLED R/C FRAMES
KOSMAS C. STYLIANIDIS Lecturer, Dept. of Civil Eng., University of Thessaloniki
P. O. Box 482, Thessaloniki 540 06, Greece
ABSTRACT
The results of an experimental programme carried out at the R/C Lab. of the Arist . Univ. of Thessaloniki are presented here . It included investigation of single - story one-bay 1/3 scale infilled R/C ductile frame models, the infill being an unreinforced masonry wall not connected to the bounding frame, under cyclic quasistatic horizontal loading. The parameters under investigation were the level of the axial compressive load of the columns, the wedging conditions of the masonry against the internal surface of the frame, the quality of the mortar and the presence of a concrete lintel beam at the midheight of the infill.
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Figure 2. Lateral load-displ acemen t curve - Infilled frame FI .
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Figure 3. Lateral load-displacement envelopes.
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Figure 4. Ratio of infilled frames strength to bare frame strength. Models without axial force(left), models with axial force(right).
TABLE 2 Ratio of infilled frames initial stiffness* to bare frame ·: nitial stiffness*
O 2 4 6 8 1012141618202224 y(., .. ) o 2 4 6 8 1012141618202224 y(., •• )
Figure 6 . Ratio of infilled frames energy dissipation to bare frame energy dissipation . Models without axial force(left) , models with axial force (right).
plastic system without strain hardening. In the corresponding envelopes of
the infilled frames , after reaching the critical distort i on, a descending
branch follows. This branch tends asymptotically to the bare frame strength
provided that shear failure is prevented (Fig.3, Fig.4).
- The presence of the infill leads to lower critical distortions (Fig . 3).
- The energy dissipation of the infilled frames is much greater than that of
the bare frames and is significant especially at low distortions, because
the system dissipates energy through friction across the infill cracks (Fig.
5 , Fi g. 6) .
- The ductility of the infilled frames is satisfactory (Fig.7 , Table 3).
The parameters investigated have the following effects on the behaviour
of the infilled frames: - The axial load on the columns increases strength , initial stiffness and energy
dissipation and slightly decreases ductility and equivalent viscous damping.
798
- Good wedging inc reas es strength, initial stiffness and energy dissipation.
The increases are significan t at low levels of distortion and tend to zero
at distortions higher of the criti ca l. Ouctility is not significantly affected.
- The use of strong morta r leads to a slight increase in strength, initial
stiffness and energy dissipation while ductility is not affected.
- The presence of a concrete lintel beam decreases strength, initial stiff
ness and energy dissipation at low levels and increases them at high levels
of distortion. Ouctility is not affected either. In some cases slip f ailure
of the infill acros s the mortar joints between the lintel beam and the neigh
boring series of bricks occured. This failure mode of the infill leads to the
formation of two diagonal struts, i nstead of the one usually expected, which
in turn lead to the formation of plastic hinges at column midheight. Conse
quently diagonal cracking of the columns occurs due to their low slenderness ratio.
The conclusions above are indicated in Table 4, where the plus sign de
notes the positive effect, the minus sign denotes the negative effect and
the zero sign denotes the neutral effect of the parameter.
REFERENCES
[lJ Klingner, R.E. and Bertero, V.V., Infilled Frames in Earthquake-Resistant Construction. EERC Report No.76-32, Earthquake Engineering Research Center, University of California, Berkeley, Oec.1976.
[2J Axley, J.W. and Bertero, V.V., Infill Panels: Their Influence on Seismic Response of Buildings. EERC Report NO.79-28, Earthquake Engineering Research Center, University of California, Berkeley, Sept.1979.
[3J Bertero, V.V . and Brokken, S., Infills in Seismic Resistant Building. ASCE, Journal of Str. Engineering, V.109, No.6, Jun.1983, pp.1337-61.
[4J Smith, B.S. , The Composite Behaviour of Infilled Frames. In Proceedings of the Symposium on Tall Buildings , University of Southampton, Apr . 1966 , pp.48l-95.
[5J Liauw, T.C., An Effective Structural System Against Earthquakes-Infilled Frames. In Proceedings of the Seventh Wor 1 d Conference on Earthguake Engineering, V.4, Istanbul, Sept.1980, pp.481-5.
[6J Tiedemann, H., A Statistical Evaluation of the Importance of Non-Structural Oamage to Buildings. In Proceedings of the Seventh World Conference on Earthguake Engineering, V.6, Istanbul, Sept.1980, pp.617-24 .
[7J Parducci, A. and Mezzi, M., Repeated Horizontal Oisplacements of Infilled Frames Having Different Stiffness and Connection Systems-Experimental Analysis . In Proceedings of the Seventh World Conference on Earthguake Engineering, V.7, Istanbul, Sept.1980, pp . 193-6 .
[8J Sugano, S. and Fuj i mu ra, M. , Ase i smi c Strengthen i ng of Ex i st i ng Reinforced Concrete Buildings . In Proceedings of the Seventh World Conference on Earthquake Engineering , V.4 , Istanbul , Sept.1980, pp.449-56.
[9 J Govi ndan, P., Lakshmipathy, M. and Santhakumar, A. R., Oucti 1 ity of Infilled Frames. ACI Journal, Proceedings, V.83, No.4, Jul.-Aug.1986, pp.567-76 .
H
----,~- -- - -- ----
I I
,
799
r-===Il= Y2 (II=Q8-;-0.9) r--
Yl
Figure 7. Definition of the ductility indexo
TABLE 3 Ductility index ~ of infilled frames
(3=0.9 (3=0.8 Frames Axial force
y
Scatter Average Scatter Average
F1"'F8 F1N"'F8N
No Yes
2.8d5.4 6.1 2.6"' 9.0 4.9
TABLE 4
5.7733.2 5.4 "' 29.1
14.9 13.9
Influence of the parameters invest i gated on the mechanical behaviour of the infilled frames
Parameters lnitial Ductility Distortion Strength Energy investigated stiffness index 1 evel dissipation