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.

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

F1~F8/F1N~F8N : 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 stru­

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 ive structural i solation of the infills from the surrounding frames so

that their structural effects can correctly ~be neglected or , alternatively,

L

793

a tight placing of the infills so that their interaction with the frames

should be properly considered in the design, detailing and construction,

especially for seismic excitations. However, this latter recommendation of

the proper consideration of the infill-frame interaction strikes against the

fact that, inspite of the efforts made so far, infilled frames are difficult

to model analytically bacause of structural uncertainties and computational

complexities.

The open research field of the infilled frames behaviour in combination

with the economical interest on the non-structural damage [6] and with the

experience gained from the 1978 earthquakes in Thessaloniki area, where the

significant role of the infills was widely recognized , led the R/C Lab. of

the Arist. Univ. of Thessaloniki to the decision to carry out a wide experi­

mental and analytical programme in the field of infilled frames, placing

emphasis to materials and construction techniques used in Greece. This paper

includes the results of the first stage of the experimental programme.

MATERIALS ANO METHOOS

The experimental models consisted of a series of sixteen single-story one­

bay 1/3-scale infilled R/C ductile frames, the infill being an unreinforced

masonry wall not connected to the bounding frame. Two bare frames were used

as reference models . While the R/C frames and the thickness of the infill

remained constant for all the frame models (columns 15x15cm, beam 10x20cm,

aspect ratio 1/h =150cm/100cm=1.5, reinforcement ratio for the columns and

the beam p=1%, infill thickness 6cm), the influence of the following para­

meters was investigated:

- The level of the axial compressive load of the columns . Two levels were

chosen, one of zero load and another of 14% of the ultimate strength of the

columns.

- The wedging conditions of the masonry against the internal surface of the

frame. Half of the models had a surrounding mortar joint against the i nterna 1

surface of the frame representing good wedging, while the other half had a

1mm wide space between the infill and the upper beam of the frame represent~

ing defective wedging.

- The quality of the mortar. Two types of mortar were used, one strong(type

S) and one week(type O, conforming to ASTM C270-73).

- The presence of a concrete lintel beam. Half of the infills had a slightly

reinforced concrete beam of 6x6cm in section at he midheight of the infill,

794

not connected to the bounding frame, .whilethe other half had nolintel beam.

Lateral loading was applied by two single acting jacks and included full

reversals of gradually increasing displacements. At every displacement level

two reversals were applied. The exeperimental was terminated when the angular

distortion reached the value of 3.0% for the bare frames and 2.4% for the in­

filled frames. The final output of the experimental investigation was one

force-displacement curve for every frame. The assessment of the behaviour of

the frames was performed on the basis of strength, initial stiffness, ductili­

ty and energy dissipation [lJ,[7J,[8J,[9J.

The mechanical characteristics of the materials used are given in Table 1.

TABLE 1 Mechanical characteristics of the materials used (MPa)

Material

Concrete Clay bricks Mortar (type S) Mortar (type O) Masonry (type S) Masonry (type O) Steel bars 08.0mm Steel wire 02.7mm

Compressive strength

25.2 5.8

10.7 3.7 3.0 1.9

Shear strength

0.33 0.26

RESULTS

Yield strength

348 271

Ultimate strength

457 395

The main conclusions from the comparison of the behaviour of bare and in­

filled frames are:

- The failure modes of the infilled frames are almost the same as those of

the corresponding bare frames, with the exception of the formation of plastic

hinges at column midheight in some cases, where slip failure of the infill

across a horizontal mortar joint occured.

- The hysteretic loops of the bare frames are rich, typical to the inelastic

rotation of the plastic hinges (Fig.1). On the contrary, pinching effects

occur at the force-displacement curves of the infilled frames, typical to

brittle behaviour due to infill cracking (Fig.2) .

- The strength and the initial stiffness of the infilled frames are much

greater than that of the bare frames (Fig . 3, Fig.4, Table 2) .

- The force-displacement envelopes of the bare frames are that of an elasto-

795

FRAM E FB H( KN )

22 A:::: r -~IT r-c--

·~J[.·.· .•••••. · •• ·.·r~-I 20 ~-''r--- V9 18 -f~ + 17' v:tc- ~ 16 - /Í - -vs ~ 14

e- f ~ ~ v; ~V: !?!~~ V I ___ ~ 12

10 UM:. ~~ .~~ . ~!___ ~_ r- , - r-- r -- r - ,8

f- t--- ~~ 1-"-~~c-~ , H ;r--, - t - t?:~~ V I I I ~ V ' k--:::: i-:::~ L::: [ 2, - t--~ --- I-- t-

" ... J_--p"" ~ t;:::' f;2' ~ L JX~' i -I- 6(mm)

0'" ''''''' '0 •• ': W 'v '"t ~B "' 1 T-o-p:'~~p!6 r ° '''~ ~o Ô ° ,(·' .. 1 t--- I- ~ ~ ti' I I . ~P':~v ~r--- t 11 ri -

I/! 1- ..m r;; ~t---- J i 1 '-I 1. 1 8 ~-"-- _L . .J

j vi-:::~ l W lJI1 10

li 1// ... _J"'-lL 1Yi.-"'í~.& 1l'. 12

V/ rr ~ 10?~~ II 14 J. '!J f::7 I V, I . If i/

16 1// I/}'/ if V I .Lr---- t-- -18

~p"V rr V~ ri' Á '/ [ 20 r--

V r--- f-" V 22 ,-, _.~

Fig ure 1. La tera l load- dis placement cu r ve - Bare frame FB.

H( K N )

go

FR AME F 1 45

~ { ( :;~;::.Et- -f E ~!:!i:) ~ .. ..... H(.) 6(-) 40

. :-".<:.-:. ····>?i:,-- - 3 5

od wedgi~.: 30

rtar ty pe S L ™ C 270) 1< .......

mo (AS ... ·i'.: j

11 , 1/ . - IL ' t- . ~ 25 ' ~j ' _ li V Ll _

I ri ]L I I I I I

, 1/ -~~f -l( L~ ~ , 20

1 5

~J V r1 v -l I

=r I I -1 1 1~ !-r.. 1?:::: ~r v/ V l W • ././ --"'v v V" [

2422 20 1!l 16 1~2 10 ~ 6 "l":2 ~~ 8 10 12 14 16 18202224

~ /f v~~ l;UIIJI_1 I I I IJ t---1l _'1L~ r1 ~ L VL 15

, ~ I/l/ I

~-::{t1 ~ ~ V 20

25

30 /Í /"' -../V ,..-I /

I - li 35

L_L_-LL-40

~- 45

Figure 2. Lateral load-displ acemen t curve - Infilled frame FI .

H mm)

Y (0/ •• )

H(KNJ

4

- 1/ r--... ~. t-.....

MClQTl -.alu<l O 1

LL~ f-- - 1"1 - ::- f,4...

50

/~ r--.. - -v I / -3

2 o V JEID. 0 1

796

!

I

~

! ,

o O

O

O

O

O

O

20

O

H(KNJ

í ...

I ' I, /

'1/ ./

I1 V ri

F1N+FBI'-I]

1" -~~U<l - - t" - 1.. - -

1--...... :®El I I

6(rrm1 r I I I I I I I I I IO\TT1m. O 2 4 6 8 101214 16 18 2022 ~y~/ •• l O 2 4 6 8 10121416182022 24y("t •• l

Figure 3. Lateral load-displacement envelopes.

t Hm/Hb

)

, I\:

I I'\. ... , r- f-. ...

, ~ -- -f- - ._-

.5

~~ltRllJII IIII ~ 1.5

,., ~5

I e6'nml ~~+---+--+I I I-+--+--I 1 +-+-+1 I I----+-+-I 1 +-+---lI I l 66'nml~ ~ O 2 4 6 8 101214115 1820 2224 y(Olool O 2 ,., 6 8 101214 16182022 24y (·I •• l

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*

kom/kob

Frames Axial force Scatter Average

Fl+F8 No 3.55+5.05 4.5 FIN+F8N Yes 3.86+7.62 5.3

* Stiffness calculated at low distortion (y=O.l%)

797

~(KNm) 20 m

~(KNm) 20 m

O Fl';'F]l

. ~ ........ MClcn w)u e , '/...:i ..... t>(

5 r - ,/ ...... r.::: - -;,' --r-

V /'

/' ,

l.F~ 5 V

5 O(rnm)

IF1N';'F8NjI_

I fr-...MClCn vdue ~ _. --f. r-. r-I--,..- _)o-.. J ~ -

V i"o-

/ / ,/ f-- "" FBNJ - r--

./ V

l-r-O(mm)

2~ 50

22 45

200 40

1~ 35

150 30

12' 25 100 20

7. 15

~ 10

2. 5

O 2 4 6 8 1012 14 16 182022 24 y~/ • .J o 2 4 6 8 10 12 14 16 1820 22 '4!,. y~/ •• )

.0 6

5.

4.

3

2

u

. 0

. 0

.0

Am/Ab

I/' ,. ""-... _t'--...

Figure 5. Reduced energy dissipated per cycle.

Am/Ab

7 O

6

5

4.

.0 ..,.

.0 ~

O ~~ '" ... ""'-...

3

- - 2 - ... ... t-o -'--. - - - -- 6(mm) 1 .0 66'nm)

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

Axial Low + + load + High + +

Good O Low + + wedging + High O O

Mortar O Low + + strength + High O O

Lintel O Low beam High + +