masonry infill analysis

7/26/2019 Masonry infill analysis

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INFILLS

IN

SEISMIC

RESISTANT BUILDING

3

By Vitelmo Bertero,

1

F. ASCE and Steven Bro kken

2

ABSTRACT:

This paper summarizes studies in which the effects of masonry and

lightweight concrete infills onR/Cmoment existing frame buildings were stud

ied experimentally and analytically. The experimental investigation consisted

of a series of quasi-static cyclic and monotonic load tests on

1/3-scale

models

of the lower 3-1/2 stories of an 11 story-three bay reinforced concrete frame

infilled in the outer two bays. Different panel material and reinforcement com

binations were tested. For reasons of economy, ease of construction, favorable

mechanical properties, and efficiency of different types of masonry infill, it was

concluded that the most promising panel configuration consisted of solid brick

laid in mortar reinforced with two mats of welded wire fabric, one bonded to

each side of the wall in a layer of cement stucco (mortar). The implications of

these experimentally obtained results are analyzed by investigating how the

infills affect the dynamic response of

R/C

moment resisting frame buildings,

as well as considering the effect of these implications on design of new build

ings, and retrofitting of existing buildings located in regions with differing seis

mic risk.

INTRODUCTION

Recognit ion that

the

dynamic characteristics

of the

bare basic struc

tural system

are

significantly changed

by the

incorpo rat ion

of

infills

has

led to the formulation of two building design phi losophies in seismic

resistant design.

One

philosophy requires that

the

infills

be

effectively

isolated structurally from

the

structural system

so

that their structural

effects cancorrectlybeneglected. The second considers theinfillsto be

tightly placed,

and,

therefore, their interaction with

the

structural sys

tem

to

resist

the

effect

of all

k inds

of

excitations sh ou ld

be

proper ly con

sidered

in the

design, deta i l ing,

and

construction.

The authors bel ieve that

the

second philosophy offers more concep

tualand practical advantages, particularlyif the basic structural system

is moment resisting frame. This

is

because

a

main principle

for

seismic-

resis tan t des ign i s : A void unnecessary ma sses ,

and, If a

m a s s

is

nec

essary,use itstructurally to resist seismic effects (3).T h us if wallsand

partit ions

are

needed

and the

economical material

is

m a s o n ry

or

con

crete, attempts should

be

m a d e

to use

these infills

as

structural ele

men t s .Thep rope ruse of infill elementscan be of great practical value

This paper

is

dedicated

to Dr.

Bruno Thurliman

on his

60th anniversary

as a

tributeto histeachingandresearchin theareaofinelastic behavior.

'Prof, ofCiv. Engrg., Univ.of California, Berkeley,Calif.

2

Design Engr., URS/JohnA.B lume & A ssoc, Engrs.

Note.Discussion open until November

1, 1983. To

extend

the

closing date

one month,

a

written request m ust be filed with

the

ASCE Manager

of

Technical

and Professional Publications.Them anuscriptfor this paperwassubm ittedfor

review

and

possible publication

on

February 18, 1982. T his paper

is

part

of the

Journal

of

Structural Engineering,

Vol. 109, No. 6,

June,

1983.

ASCE, ISSN

0733-9445/83/0006-1337/$01.00. Paper No. 18059.

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in strengthening and stiffening the usually very f lexible moment resist

ing bare frame. The connection details between infi l l and frame are also

simplified, bu t beca use , of the inte rac ting effects, the infills ca n be s ub

jected to deformations and stress beyond their elastic resistance and pro

duce semibr i t t le types of fai lure when unreinforced masonry and con

crete panels are used. This is not a ser ious disadvantage with respect to

isolated panels, however, because i t is recommended that the infi l l panels

contain adequate reinforcement even in this case (10).

When the panel inf i l ls are t ightly placed in the frame, the problem of

avoiding prem ature failure raises the qu est ions : (1) H ow sho uld these

panels be reinforced; and (2) how should they be connected to their sur

roundings? A comprehensive review of the l i terature avai lable on these

problems to 1974 (6) revealed the need for further research, and so an

integrated experimental investigation was init iated in 1974 at the Uni

versity of California, Berkeley.

Resul ts obtained to 1978 have been repor ted in Refs . 2 , 5 , and 6 . A

second series of experiments on a 3-1/2 story and 1-1/2 bay subassem-

blage of an 11-story apartment building (Fig. 1) have been recently com

pleted. Eighteen tests were conducted to investigate the relative perfor

mance of various types of infi l l ing materials and construction techniques

(4). The effects of infills on the seismic resistant

R/C

cons t ruc t ion were

studied analytically and have been reported in detail (4).

T his pa pe r is pre se nte d w ith the following objectives: (1) T o su m

marize the experimental investigation and the results obtained; (2) to

evaluate these results and to assess the practical use of infills in sites

located in regions with differing seismic risk; (3) to formulate recom

mendat ions for the design of new seismic-res is tant bui ld ings with in

filled frame structural systems, and for the retrofitting of existing build

ings having R/C moment res is t ing f rames as a s t ructural system.

DESCRIPTION OF EXPERIMENTAL INVESTIGATION AND RESULTS

Specimens.The specimens were s imilar to those used in the f i r s t

series of stud ies (5,6) a nd are illustra ted in Fig. 1 an d 2. Four different

types of infi l ls were used. Two infil ls consisted of hollow-unit masonry:

clay (Fig.

2(b))

an d co ncrete block. T he third type of m as on ry infil ls w ere

split brick with exterior welded wire fabric (WWF) reinforcement (Fig.

3). The wires of the WWF mat were spliced to dowels lef t anchored in

the confined regions of the bounding frame members (Fig. 3) so that the

panel was f i rmly at tached to the bounding f rame. The four th type of

infi l l was l ightweight concrete panels.

REPAIR, STRENGTHENING AND RETROFITTING OF SPECIMENS

R epair M ethod.A fter an infil led f rame loading prog ram w as com

pleted, i t was found that severe panel damage was general ly conf ined

to one level and so panel replacement was necessary at only one level .

The damaged panel was removed , wi th care taken to r e ta in the r e in

forcing steel (or WWF) protruding from the frame which had.been cast

in p lace for panel reinforcement anchorage. Cracks in the beams and

columns were repaired by epoxy injection. If crushing of concrete had

occurred, al l loose concrete was removed from the frame members, leav-

1338

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( e )

COLUMNS IB X18

BEAMS = 12 X 24

); and

{b)

Reinforced Hollow Brick Infill

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]

FIG. 3.Third Type of Test Specimens: Solid Brick Infills Reinforced with WWF

at Each Face

Strengthening Method.

During some tests the spiral transverse steel

was observed to fracture in critical inelastic regions of the columns in

the first story, causing immediate shear failure at that location in the

column. A ny type of repair becam e difficult an d re nd ere d this story level

useless in subsequent tes t ing. I t was , therefore, decided to s t rengthen

this story so that panels in other stories could be tested. Strengthening

was achieved by placing a rather substantial amount of reinforcing steel

in the pane l op enin g a nd casting this story solid (5 in. thick) in c oncre te.

Retrofitting Method.To retrofit inf i l l panels into an existing bare

frame, this frame was dril led to attach an anchorage system for the panel

reinforcement. This anchorage system consisted of steel plates attached

to the beam s with anch or bolts at 8 in. O .C. (200 m m ) an d to the col

umns with bol ts a t 4 in . O.C. (100 mm). Wedge anchors were used in

the columns and the third-story beams. The f irst- and second-story beams

were dr i l led completely through, threaded rods were inser ted to secure,

by means of nuts, on both sides of the beam, anchorage plates for welded

wire fabric reinforcement anchorage (see Fig. 4).

TESTING OF SPECIMENS

The models were loaded as shown in Fig . 2(a) (6) . The rat io between

the lateral force and corresponding over turning moment was calculated

by a dyna m ic elastic analysis of the entire fram e. A nalyse s we re co n

ducted on both the bare frame and the infi l led frame. Overturning mo

ment from stories above the subassemblage, as calculated from analysis,

was applied automatically using a preset transfer between the lateral and

axial jacks through a servocontrol system.

In the first series of studies (5,6), four tests Were conducted. In the

second series, a total of 18 tests we re perform ed. M ain results are sum

m arized in T able 1 an d som e typical load-deform ation relatio nsh ips for

the specimens tested are i l lustrated in Figs. 5-10.

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b) e)

FIG. 4.Details of WWF Reinforced Infi l l Used to Retrofit ExistingR/C Bare Frame:

(a) Frame-Panel Anchorage System;(b) Deta i l B (See Fig . (a)) , Threaded Rod P ro

vid ing Posi t ive Anchorage Bol t ing Complete ly through Beam; (c) Deta i l C (See

Fig. (a)) , Wedge Anchor Fastening WWF to Column;

(d)

Detail A (See Fig. (a)),

Wedge Anchor Fastening WWF to Beam; and (e) Sect ion X-X (See Fig . (d))

E V A L U A T I O N O F T E S T R E S U L T S A N D T H E I M P L IC A T I O N S O N D E S I G N

A N D R E T R O F IT T I N G O F S E I S M I C - R E S I S T A N T B U I L D I N G S

Infills no t on ly m odify th e available (supp lied) stiffness, str en gt h

(yielding and ul t imate) , damping, hysteret ic behavior and deformation

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T A i L E 1.Summary o f Specime ns Tested and Their

Test

specimen

number

D

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

Model

number

(2)

1

1 ,R1

3

2

1, R2

1, R3

2 , R l

3 , R 1

3, R 2

3,

R 3

3,

R 4

1,R4

2, R2

2 , R3

4

5

5, Rl

4 , R l

Loading

program

(3)

Monotonic

Cyclic

Monotonic

Cyclic

Monotonic

Cyclic

Cyclic

Cyclic

Cyclic

Cyclic

Monotonic

Monotonic

Cyclic

Monotonic

Cyclic

Cyclic

Monotonic

Cyclic

First-story panel

(4)

Clay brick p = 0%

Clay brick p = 0%

Con crete brick p = 0.6%

Clay brick p = 0.6%

6 in . R /C

6 in . R /C

Clay brick p = 0.15%

Con crete brick p = 0.6%

No panel

6 in . R /C

6 in . R /C

6 in . R /C

6 i n . R / C

6 in . R /C

No panel

Split brick 90 = 0.4%



Second-story panel

(5)

Clay brick p = 0.6%

Clay brick p = 0.6%

LWC p = 0.6%

Clay brick p = 0.6%

Clay brick p = 0.6%

Clay brick p = 0.6%

Clay brick p = 0.6%

LWC p = 0.6%

LWC p = 0.6%

LWC p = 0.6%

LWC p = 0.6%

Clay brick p = 0.15%

Clay brick p = 0.6%

Clay brick p = 0.6%

No panel




a

l kip = 4.45 kN.

Factored by 2.0 in. /2 .5 in.

Note: +1 K/ in . = 0 .175 kN /m m .

capacity of the building structure, but these changes also introduce mod

ifications in the demands of these same response parameters to any given

ear thquake ground motion.

The addition of infi l ls br ings an increase in the building mass. This

increase in mass has two main effects : (1) The reactive mass, M, is in

creased; an d (2) the p erio d,

T,

of the structure is increased. Furthermore,

while the addition of the infills by virtue of its mass increases the period

T, i t also introduces an increase in st iffness and thus decreases the T.

EFFECTS OF INFILL ON THE SUPPLIED LATERAL STIFFNESS, K

AND ON THE PERIOD, T

The lateral st iffness of the subassemblage tested, based on the inter-

story drift, is given in T able 1. B ecause th e initial tang en tial stiffness

deteriorates very quickly at the service lateral load, an effective inters-

tory lateral stiffness, K*, at service load level has been evaluated and

introduced. In interpreting the signif icance of these values regarding the

lateral stiffness of the prototype frame, K?, it has to be considered that

the interstory lateral stiffness of the model frame Kf can be considered

as twice that measured in the tests of the subassemblage and that the

K1

is equal to the

Kf

multiplied by the length scale L, i .e. ,

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Maximum Resistance and interstory Lateral Stiffness

-~-~ ...

Third-story panel

(6)

Clay brick p = 0.6%

Clay brick p = 0.6%

LWC p = 0.6%

Clay brick p = 0.6%

Clay brick p = 0.6%

RC p = 0.6%

Clay brick p = 0.6%

LWC p = 0.6%

LWC p = 0.6%

LWC p = 0.6%

LWC p = 0.6%

RC p = 0.6%

Clay brick p = 0.6%

RC p = 0.6%

No panel




Max

load

H. in

thousands

of

pounds

3

(7)

55.2

35.3

67.9

54.5

68.6

80.0

39.2

46.7

27.4

92.7

100.0

63.2

76.0

83.0

12.5

70.7 56.6

b

61.3 49.0

b

57.3 45.8

b

Location

of failure

(8)

First story

First story

First story

First story

Third story

Second story

First story

First story

First story

Second story

Second story

Second story

Third story

Second story

To ta l mechan ism

First story

First story

Combined

mechan ism

Maximum

Initial

tangent

(K/ln.)+

(9)

1,090

1,090

585

920

195

271

780

725

103

990

1,500

494

178

203

65

1,250

834

960

Interstory Lateral Stiffness

Effective

at

service

K? (K/ln.)+

(10)

206

236

212

187

195

238

195

250

60

358

409

167

176

210

35

292 (234)

b

118 (94)

b

203 (162)

b

Relative

Kf/Klf

(11)

5.89

6.74

6.06

5.34

5.57

6.80

5.57

7.14

1.71

10.23

11.69

4.77

5.03

6.00

1.00

8.34 (6.69)

b

3.37 (2.69)

b

5.80 (4.63)

b

K? = K?L

S

= 2KfL, (1)

T his inters tory lateral stiffness K? will be u se d as rep res en tativ e of the

lateral stiffness of the prototype.

Lateral Stiffness of Infilled

Frames versus Bare Frame.Compar ing

the values given in Table 1, i t can be seen that considering average of

Kf for infills of the same type, the smallest of all lateral stiffness of in

filled frame, (Kf)^, (obtained for the solid brick panels reinforced with

w elded w ire fabric) was 4.66 that of the bare frame (Kf)^ . T he largest

of all the (K^)ifcorrespo nding to the re inforced l ightw eight concrete w as

10.94 times the (Kf)fc/ and in the average the (Kf),/ was 6.31 times the

(Kf)v-

Effect of (K^)if on Period, T, of Building.

Although in genera l the

addit ion of an infi l l decreases the period,

T,

the specific amount of de

crease depends upon how the tota l mass of the bui lding, M, changes

relative to the stiffness with the addition of infill . Depending on the

assumpt ion of how the M changes , the di fferent resul ts are summarized

in Table 2, where two bounds regarding the changes in M have been

evaluated: Upperbound,all 11 fram es of bu ildin gs of Fig. 1 are infilled;

and lowerbound,only 4 of the 11 frames are infilled. F or ea ch of th ese

two bou nd s tw o cases w ere c ons idered, one -in w hich th e

M

is assumed

the same as when the s t ructure i s cons idered as a bare f rame, and the

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TABLE 2.Effects of Infills on the Period,T

if

, of the Prototype Building

Degree of

changes

and type

of infill

(D

Lowest (so l id

b r i ck w i t h

W W F )

Average (hol

l ow

masonry)

H ighest ( l igh t

we igh t

concrete)

A L L 1

UP P E R B O UND

FRAMES ARE INFILLED

SAME MASS

(2)

0.46

0.40

0.30

n

in Seconds,

f o r 7 V ,

in Seconds

1.30

(3)

0.60

0.52

0.39

1.01

(4)

0.46

0.40

0.30

Infill

Adds

Mass

(5)

0.49

0.42

0.32

L O W E R B O UND

ON LY 4 OF 11 FRAME S ARE

SAME MASS

r

r

-/r

(6)

0.65

0.58

0.47

in Seconds,

for TV,

in Seconds

1.30

(7)

0.84

0.75

0.61

1.01

(8)

0.66

0.59

0.47

INFILLED

Infill

Adds

Mass

(9)

0.66

0.59

0.48

Tyis the period of the prototype building with bare frame structure.

other in w hich th e infill add s m as s. A nalysis of the resu lts obtaine d re

veals that any of the infill, even the softest, will produce significant change

in the

T

of the bui lding. Furthermore, the effect of the added mass due

to infills on the

T,

is very small and can be neglected.

Per iod of the Prototype B ui ld ing , T .To hav e the values of T in sec

for the prototype building, i t is necessary to est imate i ts period where

a bare frame structure building is used, T

bf

. T h i s Ty can be analytically

computed or est imated from the experimental results . The analytical ly

computed value was 1.30 sec (6). Using the experimental s t iffness of the

subassemblage and applying Eq. 1 , cons ider ing as the prototype mass

the estimated one of 23,144 kips (102,945 kN), the T

bf

resul ts to be equ al

to 1.01 sec. Using these two values as an est imation of the period of the

bare frame building, i t is possible to compute the period for the infi l led

frame building,

T

if

.

These values are given in Table 2.

EFFECTS OF INFILL ON THE SUPPLIED STRENGTH TO THE BUILDING

These effects are again evaluated on the basis of the results obtained

in the tes t specimens , making di fferent assumpt ions regarding the num

ber of frames that are infi l led in the real building. The evaluation of the

strength is based on the est imation of the base shear s trength,

V

n

, that

the model of the bui lding could have res is ted. This es t imat ion in turn

will be based on the measured lateral resis tance of the specimen tested,

(V)

s

,w hich is equ al to the m axim um lateral force

H

plotted in the dia

grams of Figs. 5-9 and summarized in Table 1.

Base Shear St rength of Bare Frame,

{V)

hl

.

Considering tha t the

maximum measured la tera l res is tance H of the specimen in Tes t 15 and

12.5 kips (55.6 kN ), the total lateral resistan ce of the m od el of the co m

plete building, i f the only resis t ing structural element were the 11 bare

frames, would amount to 275 kips (1,224 kN).

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TABLE 3.Effects of Infills on Supplied Maximum Strength of the Prototype

Building,(V)

if

Typa of

Infill and

reinforcement

D

Unreinforced

masonry

Reinforced

hollow

masonry

Solid brick

reinforced

withWWF

Reinforced

lightweight

concrete

p . as

a per

centage)

(2)

0

0.15

0.60

0.40

0.60

Upper B ound

All 11 Frames are Infilled

V ) h

In

thousand

pounds

(3)

Lower 35,3

Lower 39.0

Lowest 46.7

Average 65.0

Highest 83.0

Lowest 57.3

A verage 63.1

Highest 70.7

Lower 92.7

Higher 100.0

V ) ? / /

V ) f r

(4)

2.82

3.14

3.74

5.20

6.64

4.58

5.05

5.65

7.42

8.00


10/25

4 0

2 0

C L

x 0

= VY

H

y x 2]L

S

2

(2)

W hen o nly 4 of the 11 frames are infil led, the d eterm ina tion of Vre

quires analysis of the load-de form ation relat io nsh ip of the infi lled frames

and that of the bare f rame, (Figs . 5-9) , and an assumpt ion regarding

the in-plane flexibility of the floor system (diaphragm). To simplify the

discuss ion, i t wi l l be assumed that the diaphragm is r igid and that no

tors ion is developed.

DISPLACEMENT, A ( IN)

(a)

DISPLACEMENT, A ( IN)

(b)

FIG.

7.Load-Deflection Relationship for Unreinforced Clay Brick Infill

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DISPLACEMENT, A (IN)

DISPLACEMENT, A (IN)

(a)

(b)

FIG.

8.Load-Deflection Relationship for Reinforced Concrete Block

Infill:

(a)

Monotonlc Test: Specimen 3;(b) Cyclic Test: Specimen 8

A s il lustrated in Fig. 10, th e infilled frame re ach es its pe ak ela stic

strength at a displacement ( interstory drif t ) somewhat smaller than the

one a t w hich the bare frame reaches it s m axim um la tera l s t ren gth . T hus

the elast ic st rength of the bui lding cannot be obtained adding the peak

stren gth of th e ba re frame to that of the infilled frame . F or each different

type of infi l l i t would be necessary to analyze the load-deformation of

the infil led frame together with that of the bare frame. From inspection

of the resul ts obtained, i t has been concluded that a lower bound of the

strength can be obtained by considering that when the infi l led frame

80

60

40

,

H

K

I

P

S

)

o

-J

-20

40

-60

_

1

L

^

+ H

. S P UT

BRIM.

p.0.*

y

iv

i i

/f^^V

f

1 1

y

Y

i i i i

DISPLACEMENT, &0NI

DISPLACEMENT, a (IN)

(b)

FIG. 9.Load-Deflection Relationship for WWF Reinforced Brick Infill

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H ( KIPS )

0 A y en?- | 2 3

DISPLACEMENT A

IM T

(IN)

INT

FIG.10.Lateral Load-lnterstory Drift Diagrams for Some Specimens Tested

reached i ts peak elas t ic s t reng th , the bare f rame had develope d equa l

to half of i ts maximum strength, i .e. , that the [(V

n

)ff]{h

if

)

ma>

. = 2 x [1/2

(V)l

f

] =

12.5 kips (55.6 kN ). A s sho w n in T able 3 , a l thou gh the un -

reinforced masonry infill resulted in the lowest lateral resistance, it still

w as 2.82 t imes the resistance of the bare frame w he n 11 frames w ere

infilled, an d 1.34 times w h e n o nly 4 of th e 11 frames w ere infilled. T he

largest increase in lateral resistance w as obtain ed for the reinforced l ight

weight concrete infi l l , amounting to on the average, 672 and 212 percent

increases, depe nd ing on w he the r 11 or only 4 of the frames w ere infilled.

ESTIMATION OF DEMANDS: EFFECTS OF CHANGE IN T

The dynamic response depends no t on ly on the dynamic charac ter

istics of the building

(T ,

,

V

n

a n d

\i),

but also on the dynamic charac

ter is tics of the g rou nd m otion s . A n easy wa y to obtain an idea of the

potential effects of the changes in

T

over the response is by analyzing

the response spectra of the cr i t ical ground motions. In doing so the fol

lowing two cases have to be dist inguished: l inear elastic and inelastic

response. Before discussing these two cases, i t is necessary to define the

following:

Mass, M, of the Building.Because the two main effects of the change

in mass are small for this particular building, i t will be assumed that the

mass is the same 23,144

k/g

(102,990 kN /g ) w he th er the structure of the

building is considered as bare frame or infilled frame.

Per iod, T, of the B are Frame B ui lding. T o i l lus t rate how the in i tia l

stiffness of the bare frame can affect the influence of infills, the two

following periods of the bare frame will be considered: the

Ty

est imated

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T, PERIOD (SECS.I

FIG.

11.Smooth Linear Elastic Response Spectra for an Effective Peak Ground

Acceleration of 0.5g and a Damping Ratio = 5% after Newmark and Hall (8).

Illustration of Effects of Changes inTon Force and Deformation Demands

from test results equals 1.01 sec, and the one obtained analytical ly, i .e . ,

1.30 sec.

D a m p i n g R a t i o , .Although the addit ion of infi l ls may introduce

considerable change in , usually increasing i t for large deformations,

(values of = 12% hav e bee n m eas ure d) for s im plici ty 's sak e, the for

the infi l led frame building is assumed to be the same as for the bare

frame bui lding un de r s t rong gro un d m ot ion s , i .e . , = 5%.

Linear Elas t ic Response.A l inear e las t ic response spect ra as sug

gested by Newmark and Hall (8) , for a maximum effective peak accel

eration of 0.5

g

(Fig. 11), has been selected for discussion.

Effect of Changes in T on Se i smic Force Demands , V.Table 4

summarizes this effect . Because of the decrease in

T

ind uc ed by th e ef

fect of the infills from 1.30 to 0.39 sec (in the case of the largest de

crease), when al l the frames are infi l led the demands in design seismic

forces increase ab ou t 1 41% . Figure 11 i l lustrates th is increa se. For s im

plici ty i t is assumed that the total seismic force demand is direct ly given

by the f i rs t mode response, i .e . , the response of the s t ructure i s cons id

ered as that of a s ingle degree of freedom having the total mass M of

the bui lding and the per iods computed in Table 3. In the case that

T

bf

= 1.01 sec, the addition of infills changes this value to 0.40, 0.46, and

0.30 sec for the average, lowest, and highest decreases. This change causes

an increase in seismic force demands of 86%. Table 4 shows the est i

m ated increase w h en only 4 of the 11 frames are infi lled; the m in im um

increase is 56% . Inc rease s in seismic forces of the o rd er of 56% to 141%

are very significant and cannot be neglected. I t is clear that for the type

of ground mot ions represented in the se lected e las t ic response spect ra ,

the more flexible the bare frame, the larger the increase in the seismic

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TABLE 4.Increase In Linear Elastic Seismic Force Demands,V, Due to

Con

sideration of Infills as Structural Elements

D e g r e e s

of change

in T, and

type of infill

(D

Lowest (solid

brick with

WWF)

A verage (hol

low

masonry)

Highest (light

weight

concrete)

Note: Compa

T

v

, in

seconds

(2)

1.30/1.01

1.30/1.01

1.30/1.01

rison of V

Upper Bound


T j . i n

seconds

(3)

0.60/0.46

0.52/0.40

0.39/0.30

? with seis

v?/v

(4)

2.41/1.86

2.41/1.86

2.41/1.86

Increase,

as a per

centage

(5)

141/86

141/86

141/86

Lower Bound

Only 4 of 11 Frames are Infilled

TjF,In

seconds

(6)

0.84/0.66

0.75/0.54

0.61/0.47

vim

(7)

1.56/1.57

1.76/1.86

2.41/1.86

inic force demands based on the building

Increase,

as a per

centage

(8)

5 6 / 5 7

76/86

141/86

>are frame

structure,

Vy,

and for same mass.

forces attracted by the addition of the infill .

Effect of Ch ang es in T on D eform at ion D em and s . Figu re 11 i llus

t ra tes how the maximum disp lacement decreases 82% when the

T

bf

of

1.30 sec is reduced by the addition of the infill to a

T,f

of 0.39 sec. Table

5 summ ar izes the decrease in d i sp lacement de m an ds . I t should be noted

that even when only four frames are infi l led, the decreases vary from

33% to 60%. These decreases in deformation are very significant and

have beneficial effects : The smaller the deformation the smaller the dam

age, ei ther to the s tructural or nonstructural components , and the smaller

the P-A effects , which are two of the main drawbacks in the use of just

bare moment res is t ing frame.

TABLE 5.-Decrease In Linear Elastic Displacement Demands,

slderatlon of Infills as Structural Elements

V

Due to Con-

Degrees

of

change

i n r ,

and type

of infills

(D

Lowest (so l id

br ick wi th

WWF)

Average

(ho l

l o w

masonry)

Highest ( l ight

we igh t

concrete)

T ^ , i n

seconds

(2)

1.30/1.01

1.30/1.01

1.30/1.01

Upper Bound


Tjf, in

seconds

(3)

0 .60 /0 .46

0 .52 /0 .40

0 .39 /0 .30

8/B

(4)

0 .44 /0 .34

0 .34 /0 .24

0 .18 /0 .15

Decrease,

as a per

centage

(5)

5 6 /6 6

6 6 /76

8 2 /8 5

Lower Bound


T>,

in

seconds

(6)

0 .84 /0 .60

0 .75 /0 . 5 4

0 .61 /0 .47

(7)

0 .6 6 /0 . 6 7

0 .6 0 /0 . 4 9

0 .4 6 /0 . 4 0

Decrease,

as a per

centage

(8)

3 4 /3 3

4 0 / 5 1

5 4 / 6 0

Note: Comparison of 5 with the displacement demands based on the building bare frame

structure, S.

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Overal l Effect of Inf i l l s on Strength Demand and Strength Supply:

Intens i ty of M ot ion s tha t Inf i l led Fram e B ui lding C an R es is t Elas t i-

cally.B ased on the resul ts sum m arized in T ables 3 and 4, the following

observations can be m ad e reg ard ing th e overall effect of infill o n stren gth s,

w he n the behav ior rem ains in the e las t ic rang e: (1) W hen all the bare

frames of the building are infi l led, the increase in supplied strength con

s iderably exceeds the increase in s t rength demands; (2) in cases where

only 4 of the 11 frames are inf il led w i th panels hav ing a p > 0.4%, the

increase in suppl ied s t rength is larger than the increase in demanded

st rength.

From the s tand po int of e las t ic s t reng th, i t app ears that the use of

al l types of infi l ls (considered in the Berkeley invest igation), when prop

erly re inforced w i th p & 0.4%, is adva ntag eo us , in com parison to the

behavior of bare f rame bui ldings . The only thing remaining is to es t i

mate what intens i ty of ground mot ions the suppl ied e las t ic s t rength wi l l

be capable of resis t ing. The main results of this est imation are sum

marized in Table 6. From comparison of resul ts obta ined between in

fi l led frames and bare frame buildings, the fol lowing observations can

be made .

Case W here A l l Fram es are Inf i lled. Un reinforced m aso nry infills

could be used advantageously ( i .e . , e las t ic s t rength suppl ied larger than

elast ic s trength demands) in seismic regions in which the peak effective

acceleration

a

ep

is < 0.12

g,

whi ch , a cco rd i ng t o t he A T C recommenda

tions (1), is for most of the U.S. (areas 1, 2, and 3). In the case of rein

forced lightweight concrete infills, these infills could be used in seismic

regions in wh icha

ep

< 0.32g , wh ich m eans they could be used in reg ions

of very severe ea r thquake ground mot ions . The maximum va lue spec i

fied by A T C (1) for a

ep

is 0.40 g.

Case W here O nly 4 of the 11 Fram es are Inf i l led. Un reinforced m a-

TABLE 6.Building Seismic Resistant Coefficient, C = (V)/Wand Effective Peak

Acceleration,

a

That it can Resist Elastically

Type of

infill and

reinforcement

D

None bare

frame

Unreinforced

masonry

Reinforced

hollow

masonry

Solid brick

reinforced

with WWF

Reinforced

lightweight

concrete

p, as

a per

centage

(2)

0.%

0.15%

0.6%

0.4%

0.6%

Upper

Bound


V, in

thousands

of

pounds

3

(3)

2,475

6,989

(L)

7,762

(L)

12,870

(Av)

12,494

(Av)

19,107

(Av)

C

(4)

0.11

0.30

0.34

0.56

0.54

0.83

T, in

seconds

(5)

1.30

0.52

0.52

0.52

0.60

0.39

a

f/

(6)

0.10

0.12

0.13

0.22

0.21

0.32

Lower B ound


V,in

thousands

of

pounds

8

(7)

2,475

3,329

3,610

5,468

5,330

7,735

C

(8)

0.11

0.14

0.16

0.24

0.23

0.33

T, in

seconds

0)

1.30

0.75

0.75

0.75

0.84

0.61

q>/g

(10)

0.10

0.07

0.08

0.13

0.14

0.17

*1

kip =

4.45

kN.

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sonry could be used in se ismic regions where the

a

ep

0.07

g,

i.e. , in

regions located in the U .S. area classif ied by A T C (1) as 1 an d 2 . T he

solid split bricks reinforced with welded wire fabric could be used ad

vantageously with respect to bare f rame in regions where a

ep

0.14 g

(i.e., for a l l 1 , 2 , and 3 areas according to ATC map) without danger of

suffering serious damage. Similarly, reinforced lightweight concrete in

fil l could be used in areas where a

ep

0.17g, i.e . , ATC areas 1-4.

I t can be concluded that inf i l l ing moment resis t ing f rames with prop

erly reinforced panels offers advantages when designed so that the frames

would remain in the e last ic range dur ing the most severe ear thquake

ground motion that can occur . But what would happen if these inf i l ls

were subjected to deformations larger than those corresponding to i ts

max imum elastic streng th? Ca n th e infilled frame survive suc h defor

mations without severe damage? In a t temping to answer i t is necessary

to analyze the inelastic behavior of infills in the infilled frames, and how

this behavior affects the performance of the frames.

Effect of Infil l on the Inelas tic R esp on se of th e B uildin g. In the

analysis of this effect is is convenient to distinguish the following cases:

DuctileMoment ResistingFrame Infilled with UnreinforcedM asonry.

Un

der cyclic loading, (Fig. 7(b)) as soon as the panel reaches i ts maximum

strength (which occurs with very small amounts of inelast ic deforma

t ions, approximately 1.5 times that which will correspond to linear elas

tic behav ior, given a disp lacem ent ductil ity ratio, (JL

8

, of about 2.5), there

is a reduction in strength to a value of about 23 kips (102 kN) that is

c lose but somewhat higher (10%) than that observed in the exper iments

conducted with a first soft story frame (Specimen 9, Fig. 6), and then

an increase up to a value of about 30 kips (133 kN) up to a |x

6

of about

39.

It should be noted that after a

(JL

S

of 2.5, som e po rtion s of the un

reinforced infill started to spall out. If an analysis using an inelastic re

sponse spectra derived from the linear elastic spectra of

Fig.

11 according

to the rules given in Ref. 8 for a (x

6

= 2.5 is conducted, the increase in

strength de m and du e to the decrease in T from 1.30 sec to 0.52 sec is

found to be 138%, wh ile exper iments sho w that the increase in the s up

plied strength is 182% for |A

6

up to 2.5. T herefore, rega rding s treng th,

it appears that ductile moment resistant frame with unreinforced infil ls

can be used advantageously in regions where

a

ep

is =0.26

g

if all the 11

frames are infilled, or

a^

0.22

g

if only 4 of the 11 frames are infilled.

T he real pro blem w ith th is kind of infill is no t initial stiffness or str en gt h,

but that with panels having large dimensions, as those under s tudy, as

soon as maximum strength is reached the masonry uni ts can shat ter and

large portions of the infil l spall out. In earthquake response, this is l ike

an explosive failure with shedding of large portions of unreinforced ma

sonry all around. This type of explosive failure of unreinforced masonry

infills has been typically observed after moderate to severe earthquake

ground m otion. In general it is inadvisable to use u nreinforced m aso nry

infills except in cases where the response demands will not exceed the

elastic range, and where out-of-plane failure of the infills can be restrained.

Nonductile Moment ResistingFrameInfilled with UnreinforcedMasonry.

This case is s imilar to the previous one but even more dangerous be

cause the explosive type of failure of the infill leads the infilled frame

to behave like one soft story frame with very large demands in shear

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and plas tic ro ta tions in the columns and /o r the beam s or beam -column

joints adjacent to the failed infilled pa ne l. A s thes e elem en ts ha ve no t

been designed to resist such demands, the explosive fai lure of the un-

reinforced m aso nry us ual ly will lead to the collapse of the frame. T hu s

this system should not be used except for cases where the bui lding can

resist e lastically th e effect of the m ost sev ere earth qu ak e gr ou nd m otion ,

i .e . , should be l imited to regions where a < 0.12g if all the frames are

infilled or a s 0.07 g if only 4 of the 11 frames are infilled.

Properly Designed Ductile Mom ent R esistantFrameInfilled with Reinforced

Masonry orConcrete Panels. 1)

Reinforced masonry infi l ls . Experiments

show that |x

s

at the average peak strength of the reinforced masonry

infill, V)f, is at least equal to two. Therefore, the reinforced masonry

infil led frame b uilding on th e average can resist seismic gr ou nd m otio ns

(of the types given a design response spectra as that of Fig. 11) having

the following peak accelerations: If all 11 frames are infilled

a

ep

= 0.40

g

for T = 0.52 sec and a

ep

= 0.38 g for T = 0.40 sec; if 4 of the 11 frames

are infilled a

ep

= 0.26 g for T = 0.75 sec and a = 0.18 g for T = 0.54

sec.

In the case where the infil l consisted of solid split bricks reinforced

with two layers of WWFsince the infil led frame can develop a |A

8

=

4.2 with a reduction of only 14% in strength (Figs. 9 and 10), i t becomes

evident that this type of st ructural system can resist earthquake ground

motions have the fol lowing a : If all 11 frames are infilled a

ey

= 0.77 g

for T = 0.60 sec and a

ep

= 0.59 g for T = 0.46 sec; if 4 of the 11 frames

are infilled

a^

= 0.55

g

for

T =

0.84 sec and

a

ep

= 0.44

g

for

T

= 0.66

sec.

In the case of a building with bare ductile framefor a

T

if

= 1.30 sec

i t would require developing a (x

s

s 6.1 to be able to resist a g ro un d

mot ion wi th an a^ = 0.55g, and for a Ty = 1.01 sec it would require a

|i,

6

5.6 to resist an

a

ep

= 0.44

g.

Since exper iments have shown tha t

the bare frame structure can develop a JJL

S

= 6.1 w ith ou t an y significant

loss in strength, i t would appear that there is no advantage in using

infills except when the majority of the frames are infil led. However, i t

should be recognized that for a bare frame structure to develop a jx

s

=

6.1, i t would have to undergo lateral displacements considerably larger

than that needed for an infi l led frame building to develop JJL

S

= 4.2. Fur

thermore, while in the case of the infil led frame, most of the damage

wil l be developed in just one or two stories where the inelast ic defor

mations are concentrated; in the case of the bare duct i le moment re

sist ing frame, the damage wil l spread throughout the whole height .

In the case of solid split bricks reinforced with WWF, the specimens

were deflected, producing an interstory drift of 2.4 in. at the story where

inelast ic deformation was concentrated. This drif t , which means an in

terstory drift ratio of 0.07, was achieved without any significant spalling

of debris. This interstory drif t , when translated in duct i l i ty displace

ment , means a |x

8

= 14 w hich w as at tained w ith a redu ct ion of st ren gth

of 32 pe rce nt (Figs. 9 an d 10). T herefo re, this sp ecim en cou ld resist th e

following a w ith ou t da n ge r of failure (collapse): If all 11 frame s are in

filled

a

ep

= 2.05

g

for

T

= 0.60 sec and

a^ =

1.54

g

for T = 0.46 sec; if 4

of 11 frames are infilled

a

ep

= 1.62

g

for

T

= 0.84 sec anda

ep

=

1.31

g

for

T

= 0.66 sec.

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In conclusion i t can be stated that the use of specially designed mo

ment resistant frame infi l led with reinforced masonry, particularly solid

spl i t br icks with WWF, can be used advantageously for even the most

severe seismic regions of the U.S.; provided the number of stories is

limited to about

11.

This l imitation is necessary because the inelastic de

formation in this type of structure is usually concentrated in one or two

stories, the larger this number of stories of a building the larger will be

the demand in the story in which this inelastic deformation is concen

trated. Fur thermore, the f rame has to have very duct i le members be

cause the inelastic demands at the story in which the inelastic defor

mat ions concentrate , would be very large. This problem has been

discussed by Park and Paulay (9) , who show that the required column

curvature ductility factor

U

ti/ycii can t>

e

typically expressed as

U

ci/yci

= 12.54r - 3.2 where r is the number of the story to the top of which

the deflections are to be measured.

(2) R einforced Lig htw eigh t C onc rete Infills. T his type of infilled frame

is capable of dissipating energy with a ducti l i ty somewhat larger than

two without any loss in strength. However, for a |x

g

just larger th an

three the s t rength reduces rapidly to a value somewhat h igher than the

strength corresponding to the soft story frame. Considering a (x

s

= 2, it

has been estimated that buildings with this type of infi l led frame can

resist ground motions with the following a

ep

: If all 11 fram es are infilled

a

ep

< 0.54

g

for

T =

0.39 sec and

a^

< 0.54

g

for

T

= 0.30 sec; if 4 of 11

frames are infilled

a^

< 0.31

g

for T = 0.61 sec and

a^

s 0.25

g

for T =

0.47 sec. Considering the value at which strength appears to be stabi

l ized, 42 kips (187 kN), which is considerably higher than the 27.4 kips

(122 kN) w hic h is th e m ax im um lateral resistan ce of a ba re fram e soft

story, and that the inelastic deformation at this level gives a m = 6.6,

the following values of

a

ep

can be ob tain ed : If all 11 frames are infilled

tie,,

< 0.64

g

for T = 0.39 sec and

a^

< 0.48

g

for

T =

0.30 sec; if 4 of the

11 frames are infilled

Ugp

^ 0.37

g

for T

0.61 sec and u

e

p ^ 0.28

g

for

T

= 0.47 sec.

From analysis of the above results i t can be concluded that

R/C

bare

frame buildings of the type investigated can be advantageously infi l led

with reinforced l ightweight concrete for even the most severe seismic

regions of the U.S. if all the frames are infilled, and for the ATC Map

areas 1, 2, 3, 4, and 5 if only 4 of the 11 frames are infilled.

Nonductile Moment Resistant Fram e Infilled with Reinforced Pane ls.In

general this type of construction is not advisable if significant inelastic

deformation is expected. In infilled frames the inelastic deformation is

concentrated within a few stories, usually the lower ones, so ducti l i ty

demands on the frame members of these stories can be very large, con

sequently these members should be ducti le. Because of this type of be

havior a designer could be tempted to design as duct i le only the mem

bers of the story or stories in which inelastic behavior of the infill is

expected. To design in this manner appears logical and economical,

however , the designer must be aware that the resul ts obtained in th is

investigation, as well as in others, clearly show that for such a design

to work i t must be assured that the inelastic deformations will actually

concentrate in the weakest spot, i .e. , the story that is designed as duc

tile.

This is not an easy task. The uncertainties involved in predicting

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the cri t ical seismic response of buildings are so large that conservative

precaut ions should a lways be taken. Fu rtherm ore, the s t reng th, s tif fness

and deformation capacity of masonry infi l ls are very sensit ive to quali ty

control of the materials and workmanship. To believe that i t is possible

to control exac tly w he re inelast ic defo rm ations can occur in a real

building is too optimistic.

CONCLUSIONS

In view of the relat ively small amount of experimental data on which

the fol lowing conclusions are based, and the idealizat ions, s implifica

t ions,

and assumpt ions made in the numerical analys is conducted, i t i s

convenient to c lear ly recognize the cons t ra ints surrounding the val idi ty

of the conclusions so that they wil l not be misused. These l imitat ions

are summarized regarding the fol lowing parameters :

1. Type of Frame. A special ly designed R/C moment res i s t ing space

frame and 3 bays and 11 stories .

2.

Type of Infi l ls . Unreinforced and reinforced masonry units (hollow

and solid bricks, and concrete blocks) and lightweight reinforced concrete.

3. Qual i ty control of M ater ia ls . A l thou gh the m aso nry un i ts use d in

construction were carefully selected and the grout , mortar, and concrete

carefully designed, mixed, placed, and cured, considerable variat ions in

the mechanical characteris t ics of these materials were observed. The re

sults indicated that the behavior of the infill is very sensitive to vari

at ions in the quali ty of material and, therefore, good quali ty control of

all material is a must for infills, particularly masonry infills.

4. Workmanship. Some weaker , s t i f fer , and premature types of ine

las t ic behavior and pat tern of cracking and/or crushing were a t t r ibuted

to lack of uniform workmanship in laying the masonry uni ts and in the

anchorage of the infi l l to the frame; thus excellent workmanship is

required.

5.

Infill Panel A rrang em ent . T he two external bays of the 3 bay frames

were ful ly infi l led, i .e . , without any opening, and formed what could

be called a cou pled infil led fram e.

6. T ype of B ui lding Co nsidered in the A ssessm ent of the Im pl ications

of Resul ts Obtained. Regular bui ldings having a rectangular plan con

sisting of 11 frames of 3 ba ys a nd of 11 stories hig h w h er e th e fram es

are fully infilled, as described in item five, and the locations of these

infi l led frames are such that no significant torsional forces are induced

during the se ismic response of the bui lding. The importance of this l im

i ta t ion cannot be overemphasized.

7. Idealization of the A ctual Lateral Load -Deform ation R elat ionsh ips

of the Bare and Infi l led Frames. The analytical assessment of the impli

cat ions of the experimental resul ts regarding behavior of the bui lding

have been made ideal iz ing the actual experimental re la t ionship by a l in

ear elast ic-perfectly plast ic model using different yielding strengths and

ductility levels.

8 . Dynamic Character is t ics of Bui lding Si te and of Ground Motions .

I t is assum ed that the bui lding is on firm gro un d an d a r igid found a

t ion can be cons t ructed, a nd tha t a ll the gro un d m ot ions that can occur

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have dynamic characterist ics similar to those included in the derivation

of the smoothed linear elastic (Fig. 11) and inelastic design response

spectra suggested by Newmark and Hal l (8) . The impor tance of the l im

i tat ions imposed by these assumptions in conjunct ion with the ideal i

zation pointed out in i tem seve n shou ld be em phasized , particularly w he re

significant inelastic behavior is involved in the response. The effects of

ground motions containing severe acceleration pulses (higha

ep

) of long

duration should be investigated before the conclusions from these re

sul ts are appl ied to the design of new bui ld ings and/or to ret rof i t t ing

of existing buildings. The interacting effects of the observed significant

deformation softening after reaching peak lateral resistance, with long

accelerat ion pulses inpu t , can lead to deformation de m an ds considerably

higher than those predicted by a l inear elasti-perfectly plastic idealiza

tion (7).

9. R eliabil ity of the A nalytical R esults . In view of all th e as su m ptio ns ,

ideal izat ions , and uncer taint ies involved in the conducted analyses , the

numerical values obtained should be considered as approximate and in

dicating trends, rather than an exact representation of what can be ex

pected in specific cases.

CONCLUSIONS

Conclusions Regarding Overall Behavior of the Infilled Specimen

Tested.

1.

The addition of either unreinforced or reinforced infi l l to moment

resisting frame increases significantly the lateral stiffness and lateral re

sistance of the frame.

2.

As soon as cracking occurs , which happens very ear ly , a t service

lateral load level, the initial tangential lateral stiffness decreases

signif

icantly, up to 80 percent, to a value that remains practically constant for

a long range of lateral load. To represent this behavior an effective in-

terstory stiffness at lateral service load has been defined.

3.

The ins tantaneous lateral s t i f fness and s t rength depends on the

previous loading history. Under monotonically increasing load these two

characterist ics depend on the type of infi l l . These characterist ics do not

depend upon how the panel is reinforced but they are sensi t ive to the

quali ty control of the materials and to how well the infi l l is made, par

t icularly to the workmanship along the interfaces of the infi l ls and the

boundary f rame elements .

4.

Hysteret ic behavior depends upon the type of inf i l l , the amount

and ar rangement of reinforcement , the way that the panel is a t tached

(anchored) to the frame, and the loading history. The cyclic loading,

including force reversals of unreinforced infills, leads to considerable de

ter ioration in st iffness and strength when compared with the values ob

served under monotonic loading; the largest deter iorat ion occurred un

der cyclic loading with full reversal of deformations. This deterioration

is due to propagation of infi l l damage that usually concentrates in one

story. The peak strength under cyclic loading, which is smaller than that

obtained under monotonical ly increasing load, deter iorates as the se

verity of deformation and number of cycles increases, but remains some-

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what larger than the s t rength of a f rame wi th a sof t s tory corresponding

to the s tory in which damage of the infi l l concentrates . Excellent hys-

tere t ic behavior has been obta ined wi th the use of sol id br ick masonry

infi l ls externally reinforced with welded wire fabric covered with cement

mortar.

5. A l thoug h the inters tory displacem ent ducti li ty un de r pea k s t reng th

is smal l , about 2 , large values are obta ined under reduced s t rength. In

the case of solid brick externally reinforced with welded wire fabric, this

duct i l i ty was 4.2 under 86% of the peak s t rength, and reached the value

of 14 un de r 68% of pe ak stren gth .

6. Except for very few specim ens (Specimen 18 an d o ne rep orte d in

R ef. 6) w ho se failure m ech anis m s invo lved tw o stories , in all oth er spec

imens the damage concentra tes in one s tory, consequent ly the f inal

m echanism of failure is w ha t can be defined as a som ew hat s t rength

ene d soft s tory fram e. T hu s the energy diss ipated by an inf il led

R/C

frame should be larger than a bare soft s tory frame.

7. Fai lure of unreinforced masonry inf i l l s was accompanied by pro

duction of substantial debris containing hazardously large pieces of ma

sonry. The amount of debris in reinforced infi l ls was smaller and most

was contained in the plane of the infi l l , part icularly in solid brick ma

sonry reinforced externally with welded wire fabric.

8. The initial effective viscous damping coefficient of the virgin spec

imen s is smal ler than 2 % . A s soon as cracking dev elops th e value of this

damping coefficient increases up to 12%.

Conclusions from Comparison of Behaviors of Infilled Frames and

Bare Frame.

1. The initial tangential interstory lateral stiffness of the virgin infilled

frames wa s m or e th an 10 tim es the similar stiffness of th e ba re fram e.

2.

The effective interstory lateral stiffness of virgin infilled frames was

5.3 to 11.7 t imes the lateral s tiffness of the bare frame de pe n di n g o n the

type of infill. In case of repaired infills and retrofitting of repaired frames,

this effective lateral stiffness w as a t least 3.4 time s th at of the virg in ba re

frame.

3.

The maximum lateral resis tance of virgin infi l led frames was 4.8 to

5.8 t imes that obtained for the bare frame. For cases of repaired infi l ls

and retrofi t t ing of repaired frames the maximum lateral resis tance was

2.8 to 8.0 times that of the bare frame.

4.

The interstory displacement ducti l i ty rat io of the infi l led frame is

smaller than that of a bar e frame b u t larger th an th at of a ba re soft s tory

frame. For what can be considered a maximum acceptable interstory drift

index, say 0.02 or even for values of this index up to 0.07, the hysteret ic

behavior of the solid brick masonry externally reinforced with welded

wire fabric was superior ( large energy absorption and energy dissipation

capacities) to that of the bare frame.

5. T he add it ion of infil ls introd uce s s ignificant ch ang es in the dy nam ic

characteristic of the bare moment resisting frame. In the linear elastic

range the fundamental per iod is decreased more than 54%, whi le the

m ass is increased in not m ore th an 10%. T he effective viscuou s da m pin g

coefficient is increased considerably, up to 500%. In the inelastic range

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the pat tern of la teral deformations changed fundamental ly because most

of the significant inelastic deformations concentrate in one, or at the most,

two stories.

CONCLUSIONS DRAWN FROM ASSESSMENT OF THE IMPLICATION

OF

EXPERIMENTAL RESULTS OBTAINED REGARDING

TH E

SEISMIC RESISTANT DESIGN OF BUILDINGS

1. The addition of infil l into the moment resisting frames of a build

ing introduces significant changes in the dynamic characteristics of the

bui ld ing which should be considered in i t s design. These changes de

pend upon the number of frames that are infi l led as well as the locat ion

of these frames.

2.

T he m ass is increased; how ever, eve n w he n all the transverse frames

of the building under consideration (Fig. 1) are infil led, the increase with

respect to a bare frame building is only about 10%, and those two main

effects of this increase are negligible.

3. The stiffness of the building is increased significantly in the case

where all the frames are infil led, the increase varies from 366% to 994%.

If only four of the frames are infilled the increase varies from 136% to

353%.

4. If the 11 frames are infil led the d ecre ases in the fun da m en tal pe

riod varies from 54% to 70%. If only four frames are infilled, the decrease

varies from 35% to 53%.

5. The value of the effective viscous damping ratio for the whole

building increases w he n com pare d w ith a bare frame structu re.

6. Stre ng th Sup ply . A dd ition of infills to the frames in creas es th e

available (supplied) strength of the bare frame building significantly. If

all the 11 frames are infil led the lateral stren gth in the tra nsv erse direc

t ion of the bui lding is increased in 182% up to 700%, depending upon

the typ e of infills. In the ca se w h e re on ly 4 of the 11 fram es are infilled,

the increase varies from 34% to 255%.

7. Strength Demands. For l inear elast ic behavior the addit ion of in

fills to the bare frame increases the stre ng th de m an ds in 86% u p to 141%

w he n all the frames are infilled, a n d in 56% to 141% w h e n on ly 4 of th e

11 frames are infilled.

8. Supplied Strength vs. Demanded Strength in the Case of Elast ic

Behavior. From comparison of values given in the above conclusions 6

and 7, i t can be con clud ed that, excep t for cases of unreinfo rced infills

in which only 4 of the 11 frames are infil led, the increase in supplied

st rength is larger than the increase in the d em an de d s t rength , thu s f rom

the viewpoint of strength it is beneficial to add infil ls.

9. Deformation Demands in the Case of Elast ic Behavior. The addi

tion of the infil ls decreases the demands on maximum displacement with

respect to that corresponding to the bare frame building. The decreases

vary from 56% to 85% in cases where all the frames are infil led, and

33% to 60% in cases where only 4 of the 11 frames are infil led. This

decrease in displacement demand is a significant advantage in the use

of infills.

10. From conclusions 8 and 9 it is obvious that if i t is possible to de-

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s ign the bui ld ing to rem ain in the e las t ic rang e , then it i s adv anta

geous to add any of the types of infi l ls , even unreinforced masonry, i f

all the frames are infilled and

a

ep

^ 0.12

g.

In cases where only 4 of the

11 frames are infilled, i t is ad va nta ge ou s to a d d an y ty pe of infills re in

forced with p 2: 0.4% that have been considered in this study. While a

bare frame bu ilding can resist e lastical ly gr ou nd m otion s similar to th ose

considered in the d erivat ion of the res po ns e spectra of Fig. 11 w ith an

effective peak acceleration of a = 0.10 g, the addition of infills of solid

bricks reinforced external ly with wire welded fabric al lows the bui lding

to resist an

a

ep

= 0.21

g,

i.e. , an increase of 110% in intensity of ground

m otio ns if all th e frame s are infilled. If only 4 of th e 11 fram es ar e infilled

it can resist an

a

ep

=

0.14

g,

i.e. , an increase of 40%. By infil l ing all the

frames with reinforced l ightweight concrete i t is possible to resist e las

t ical ly ground motions with an a

ep

s 0.32g, which means tha t they can

be used in all the seismic regions of the U.S. except those classified as

area 7 in the A T C m ap area classification.

11.

For bui ld ings which can res is t the ext reme ground mot ion ex

pected at the site through large inelastic deformations, the use of infil ls

like that of solid bricks reinforced externally with welded wire fabric of

fers considerable advantage over the use of just bare frame. Because these

infi l led frames can develop an interstory displacement duct i l i ty jx

s

= 4.2

with a reduct ion in strength of only 14%, the bui lding can resist ground

mot ions wi th an

a^

s 0.44

g

ev en if on ly 4 of th e 11 fram es are infil led.

To be able to resist a similar ground motion the bare frame building wil l

need to develop a

JJL

S

> 5.6 with significant ly larger disp lacem ent , an d

consequent ly more damage throughout the whole bui ld ing.

CONCLUSIONS D R A W N FROM ASSESSMENT OF THE IMPLICATION

OF EXPERIMENTAL RESULTS OBTAINED REGARDING THE

REPAIR

AND RETROFITTING OF EXISTING BUILDINGS

1. For bare frames that have been damaged (cracking and spal l ing of

unconfined concrete) due to considerable yielding, developing interstory

displacement duct i l i ty of four, the fol lowing repair technique gives good

resul t : removal of any crushed and loose concrete and recast ing of i t ,

and injection of cracks with epoxy.

2. Undamaged, or damaged bare frames after their repair , can be ef

fectively retrofit ted for seismic resista nt pu rp os es b y th e ad dit ion of rein

forced infil ls that are properly attached (anchored) to the frame. Of all

the infil ls studied, the one that offers the greatest potential to retrofit

st i ffness, st rength and energy dissipat ion capaci ty to exist ing bui ldings

is the one based on use of sol id bricks reinforced external ly with welded

wire fabric covered with cement mortar and anchored to the frame, as

illustrated in Fig. 4.

ACKNOWLEDGMENTS

This paper is dedicated to

Prof.

Dr . B runo Thur l iman on h i s 60 th an

niversary as a tribute to his teaching and research in the area of inelastic

behavior.

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APPENDIX I .REFERENCES

1. T entative Provisions for the D evelo pm ent of Seismic R egulations for B uild

i ngs , A pplied T echnology Counci l Publ ica tion A T C 3-06, Nat ional B ureau

of Standards, June, 1978.

2.

A xley, J. W ., an d B ertero, V. V., Infill Pan els: T heir Influence o n Seismic

R esponse of B ui ldings , Report

No. EERC 79-28,

Ear thquake Engineer ing Re

search Center, Un iversity of California, B erkeley, Calif., Sept., 1979.

3.

B ertero, V. V., Seismic Performance of R einforced Con crete Stru ctures ,

Amales

de la

Academia

Nacional de

Ciencia

Exactas,

Fisicas

y Naturales, Buenos A i re s ,

A rgentina, Vol. 31, 1979, pp . 75 -144.

4.

B rokken, S. T ., an d B ertero, V. V., Stu die s on Effects of Infills in Seismic

R esi stan t R /C Cons t ruc t ion , Report No. EER C 81-12,Ear thquak e Engineer

ing Research Center, University of California, Berkeley, Calif., Oct., 1981.

5.

Klingner, R. E., an d B ertero, V. V., Earthq uake R esistance of Infilled Fr am es,

Proceedings,

ASCE, Vol. 104, No. ST6, June, 1978.

6. Klingner, R. E., an d B ertero, V. V., Infil led Fram es in Earth qua ke R esistant

Construc t ion, Report No. EERC 76-32, Ear thquak e Engineer ing R esearch

Center, University of California, Berkeley, Calif., Dec, 1976.

7.

M ahin, S. A . , an d B ertero, V. V., A n Evaluation of Inelastic Seismic Design

Spectra, Proceedings, ASCE, Vol. 107, No. ST9, Sept. , 1981.

8. Ne wm ark, N . M ., and Hall , W. I . , Pro ced ures an d Criteria for E arthqu ake

Resis tant Design, Building

Standards

for

Disaster

Mitigation, Nat ional Bureau

of Standards, Building Science Series 46, Feb., 1973.

9. Park, R. , and Paulay, T. ,

Reinforced Concrete

Structures,John Wiley and Son s ,

New York, N.Y., 1975.

10.

Priestley, M. J . N . , M aso nry , Design ofEarthquake Resistant Structures, E.

Rosenblueth, ed. , John Wiley and Sons, New York, N.Y., 1980, pp. 195-222.

APPENDIX I I .NOTATION

The following symbols are used in this paper:

a = a c c e l e r a t i on ;

K = la te ra l s t i f fness ;

L = l e n g t h ;

M = m a s s ;

P = axia l lo ad ;

r = n u m b e r o f t h e s t o r y t o t h e t o p ;

T = p e r i o d ;

V = b a s e s h e a r s t r e n g t h ;

p = pe r c e n t a g e o f m a i n r e in f o r c ing s t e e l ;

= c u r v a t u r e ;

t, = e ff ec ti ve v i s c o u s d a m p i n g r a t i o ;

J U L ,= duc t i l i t y r a t i o ; a n d

A = l a te r a l d i s p l a c e m e n t .

S u b s c r i p t s

bf = ba r e f r a m e ;

ep =

effec tive p ea k ;

I = i n t e r s t o r y ;

; / = inf i l led f ram e;

rif = re info rced inf i l led f ram e;

s = sca le ;

uci = u l t im a t e c u r v a tu r e a t s e c t i o n /;

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yd = yielding curv ature at sect ion i; and

8 = displacement .

Superscripts

D

=

demands;

m

= m odel frame;

p

= p rototy p e frame; and

s specimen.

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masonry infill analysis

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