seismic strengthening of rc frames with shear … · seismic strengthening of rc frames with shear...

Proceedings of the 6th International Conference on Mechanics and Materials in Design, Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

-1427-

PAPER REF: 5669

SEISMIC STRENGTHENING OF RC FRAMES WITH SHEAR WALLS Georgios Tsionis

1(*), Fabio Taucer

2, Artur Pinto

1

1European Commission, Joint Research Centre (JRC), Institute for the Protection and Security of the Citizen (IPSC), European Laboratory for Structural Assessment 2European Commission, Joint Research Centre (JRC) (*)

Email: [email protected]

ABSTRACT

Experimental testing and numerical analyses of elements and structures have demonstrated the feasibility and effectiveness of shear walls for the seismic retrofit of existing reinforced concrete frame buildings. The paper presents a literature review and discusses the effect of the most important parameters. It is shown that shear walls improve the global response of existing buildings in terms of stiffness, strength and displacement demand and that their effectiveness is increased for walls with larger dimensions. The design and detailing of the dowels and anchorage bars used for the connection between new and existing elements is a critical issue. The results available in literature, complemented by parametric numerical analyses, may provide the basis for the development of design guidelines with emphasis on strength and stiffness characteristics and on detailing of the connections.

Keywords: seismic strengthening, RC frames, shear walls.

INTRODUCTION

The seismic engineering research community has dedicated significant efforts in developing retrofit measures for existing buildings with insufficient seismic performance, aiming at the reduction of casualties and economic losses in the event of earthquakes.

There are two main objectives in seismic retrofit, i.e. to reduce demand or to increase capacity, and three key properties to examine: strength, stiffness and deformation capacity. Among the strengthening measures developed for reinforced concrete (RC) frame buildings, this paper focuses on RC shear walls. This technique offers a number of advantages, the most important being the reduction of storey drifts, the prevention of storey mechanisms and possibly also the reduction of irregularities in height and plan.

Lateral loads are resisted mainly by the new walls that are appropriately designed to carry them while the existing elements are expected to play a secondary role. Shear walls may be constructed as new elements, by RC infilling or using precast panels. Furthermore, they may be designed and constructed to rock at their base or for uplifting of the foundation from the soil and combined with energy dissipating devices.

A literature review of experimental and numerical research on the use of shear walls for the strengthening of RC frame buildings is presented in the following. The results of previous research are discussed with a view to identifying the most important parameters and the way they affect the effectiveness of the retrofit in terms of strength, stiffness and deformation capacity. The literature review makes it also possible to point out the issues that need to be refined.

Symposium_10 Seismic Behaviour Characterization and Strengthening of Constructions

-1428-

NEW RC SHEAR WALLS

A RC wall may be added around an existing column, as a new element to the exterior of the frame or in the form of a buttress, as schematically presented in Fig. 1. The first option entails higher disturbance to the occupants and more intensive secondary interventions. The two others minimise disturbance, but in turn require more space outside the building, which might not be available. Whatever the location of the new wall with respect to the existing frame, its foundation is a major issue as a new element needs to be constructed, possibly incorporating existing ones. This is particularly demanding when the wall is constructed at the border between adjacent properties.

(a) (b) (c)

Fig.1 - View and cross-section above the foundation of RC frames strengthened with walls: (a) around a column, (b) external to the frame, (c) as buttress

Two- and three-storey scaled RC frames strengthened with new walls built around the existing columns (Bush et al, 1991), following a buttress scheme (Kaltakci et al, 2008) or constructed on the exterior side of the frames (Kaplan et al, 2011) have been tested under cyclic loading. The strengthened frames showed higher strength (three to four times) and stiffness (up to seven times) than their as-built counterparts. In all specimens, a monolithic connection was achieved between the new and existing elements, but detailing of reinforcement in order to avoid congestion of rebars and facilitate implementation emerged as an important practical consideration. The new walls were successful in changing the failure mechanism to a beam-sway one. Horizontal cracking was observed at the cross-section of one wall where the starter bars from the foundation were terminated, while another specimen failed because of shear sliding at the wall base.

ROCKING WALLS

Rocking walls are intended to suffer no damage – thus they maintain their strength and stiffness – and no residual deformation thanks to the self-centring offered by vertical post-tensioned tendons. This was verified in cyclic tests of scaled wall specimens (Restrepo and Rahman, 2007). The specimens with dog bones (mild steel rebars with reduced diameter in the middle) purposely placed at the base of the wall showed a notable capacity of energy dissipation.

Cycling testing of a scaled three-storey wall-frame specimen (Mori et al, 2008) confirmed that when the wall was free to uplift, the global force-displacement curve exhibited a flag shape with lower energy dissipation and smaller residual deformation than the building with the fixed wall, see Fig. 2. Furthermore, damage was concentrated at the ends of the beams in the first case, whereas it extended throughout the wall and at the base of the corner column in the second.


-1429-

Fig. 2 - Force-drift curves of a wall-frame building: (left) free uplift of wall and (right) fixed wall (Mori et al, 2008)

Nonlinear dynamic analysis was performed to design the retrofit of an 11-storey frame building with pin-supported walls (Qu et al, 2012). The walls imposed a uniform height-wise deformation and reduced drifts by almost 50 % with respect to the existing building. Similarly, nonlinear static and dynamic analyses were also used to investigate the effect of uplifting of the walls used to strengthen one prototype and two real irregular buildings (Fardis et al, 2013). Rocking walls were protected from damage and did not significantly affect the response of the existing members of the regular building. It was demonstrated that high irregularities or constraints related to the appearance and functionality of the building may limit the effectiveness of shear walls. Both studies showed that, in practical applications, the demand on existing members can be significantly reduced, but some members might need upgrading.

A numerical study of a prototype 20-storey structure subjected to near-fault earthquakes (Alavi and Krawinkler, 2004) showed that new shear walls fixed at the base were effective in reducing drift demands for stiff frames, but less so for flexible ones because of the specific characteristics of the near-fault ground motion. On the other hand, rocking walls were found to be advantageous independently of the frame stiffness as regards deformations and the wall shear and moment demands. It was also shown that exceedance of the wall shear capacity during an earthquake will significantly reduce the effectiveness of fixed walls and only slightly affect that of rocking walls.

EXPERIMENTAL STUDY OF SQUAT RC INFILL WALLS

RC infilling consists of transforming a bay of the existing frame into a shear wall by filling it with reinforced concrete (shotcrete, cast in situ or made up of precast elements). The connection to the existing frame is achieved through dowels anchored in the existing beams and columns and embedded in the web of the new wall. RC infilling has been experimentally studied since the early 80s, mostly for squat walls, i.e. walls that have a height-to-length ratio less than 2 (CEN, 2004).

One of the earliest experimental campaigns examined one-storey one-bay RC frames strengthened with large precast concrete panels in various configurations (Higashi et al, 1980). A monolithic wall cast together with the frame resulted in the highest increase of stiffness and strength and was practically equivalent to a wall made up of precast concrete panels vertically connected to each other. Similar specimens were tested to investigate the influence of the amount of column vertical reinforcement, the type of connection between the wall and the frame and the presence of openings in the wall (Hayashi et al, 1980; Aoyama et al, 1984). Regular and high-strength chemical anchors resulted in similar strength, almost


-1430-

equal to the strength of a monolithic wall and slightly higher than that of a wall with mechanical anchors. As expected, specimens with a higher ratio of longitudinal reinforcement in the columns exhibited higher strength but smaller ductility. Generally, specimens with infills were more ductile than those where the wall was cast monolithically with the frame.

Other important parameters are the presence of lapped splices at the base of the columns and the level of damage before strengthening (Altin et al, 2008; Sonuvar et al, 2004; Teymur et al, 2008; Turk et al, 2003). The improvement both in terms of strength and energy dissipation increased and drift reduction was higher for longer overlapping length or continuous longitudinal reinforcement and for no initial damage. It is noted that most of the strengthened specimens failed due to sliding at the cross-section where the starter bars from the foundation were terminated (Altin et al, 2008; Sonuvar et al, 2004).

Cyclic tests on single- and two-storey frames (Anil and Altin, 2007; Kara and Altin, 2006) demonstrated that partial infilling results in a smaller increase of strength, stiffness and energy dissipation. The position of the wall within the bay had a slight effect on the strength and a more pronounced one on stiffness. In fact, higher stiffness was measured for the specimen were the infill was connected to the top and bottom beams and to a column on one side than for a specimen were an infill of the same length was placed in the middle of the bay and connected only to the top and bottom beams. On the other hand, the position of window-like openings did not affect the effectiveness of infills (Aoyama et al, 1984). The increase in strength, stiffness and dissipated energy is generally higher for longer walls and larger boundary elements (Sonuvar et al, 2004). Longer walls exhibited more rapid post-peak degradation of strength.

Infills made of full-height panels are difficult to construct, whereas the geometry and connection details are important for small panels. Cyclic tests of frames strengthened with four types of panels (Baran et al, 2011) showed that epoxy mortar provided sufficient connection between the panels and that shear keys and welded connections may be considered unnecessary. Both types of connection resulted in similar enhancement of the strength. However, panels with shear keys performed much better as regards energy dissipation.

EXPERIMENTAL STUDY OF SLENDER RC INFILL WALLS

While squat walls have been extensively investigated, there are significantly less studies on slender ones, which correspond to the majority of practical applications on multi-storey buildings. The force-displacement curves and the crack patterns at the end of cyclic tests of three-storey and single-storey frames retrofitted with precast concrete panels are compared in Fig. 3 (Higashi et al, 1980; 1984). The three-storey specimens failed in flexure whereas the squat walls failed in shear. RC infilling of the three-storey frame resulted in a higher strength increase than in the single-storey one. Furthermore, as a ductile failure mode was activated, the retrofitted three-storey specimen showed also a significant improvement of the deformation capacity.

The effectiveness of RC infills, FRP strips and precast concrete panels applied on one face of masonry infills was examined through cyclic (Erdem et al, 2006) and pseudo-dynamic tests (Kurt 2010) on scaled three-bay and two-storey frames. As evidenced by the force-displacement envelopes shown in Fig. 4, RC infills and precast panels were capable of increasing the strength and stiffness and reducing significantly the deformation demands. RC infills provided a higher increase in strength and stiffness than precast panels. However, the precast panels performed better in terms of deformation (displacement ductility of 4.9, compared to 3.2 for the RC infills).

Proceedings of the 6th International Conference on Mechanics and Materials in Design,Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26

Fig.3 Force-displacement response and damage of threeand one-storey (bottom

Bas

e sh

ear

(kN

)

Fig.4 Force-displacement

Pseudo-dynamic tests were also performed on linfilled frames (Strepelias, 201height (more reinforcement for specimen SW1 and less for SW3) existing frame was implemented by anchorage bars and short dowels or by long dowels of larger diameter. The response of the 5. All specimens failed in a flexuresection was influenced by the presence of lapped splices and the amount of web reinforcement. The critical crossthe base of the columns and hence, FRP jackets and SW3. Specimen SW2 failed above the spliced rebars at the base of the second storey, while SW3 failed at the base because of the re


-1431-

displacement response and damage of three-storey (top – Higashi storey (bottom – Higashi et al, 1980) frames strengthened with precast panels

Drift ratio (%)

Top displacement (mm)

displacement envelopes of bare and strengthened RC frames (Kurt,

dynamic tests were also performed on large-scale four-storey 2012). The amount of wall reinforcement was reduced a

(more reinforcement for specimen SW1 and less for SW3) and the connection to the existing frame was implemented by anchorage bars and short dowels or by long dowels of larger diameter. The response of the three specimens was practically the same. All specimens failed in a flexure-dominated mode, but the location of the critical cross

section was influenced by the presence of lapped splices and the amount of web reinforcement. The critical cross-section of SW1 was at the termination of the starter bars at the base of the columns and hence, FRP jackets were placed around the column base of and SW3. Specimen SW2 failed above the spliced rebars at the base of the second storey, while SW3 failed at the base because of the reduced moment resistance due to the smalle

Precast concrete panels

Reference frame

RC infill

FRP strips

Higashi et al, 1984)

rengthened with precast panels

Kurt, 2010)

storey specimens of RC ). The amount of wall reinforcement was reduced along the

and the connection to the existing frame was implemented by anchorage bars and short dowels or by long dowels of

same, as seen in Fig. dominated mode, but the location of the critical cross-

section was influenced by the presence of lapped splices and the amount of web rmination of the starter bars at

were placed around the column base of SW2 and SW3. Specimen SW2 failed above the spliced rebars at the base of the second storey,

duced moment resistance due to the smaller


-1432-

amount of web reinforcement. The response of the two types of connection was similar in specimen SW1 and therefore only the simpler one was implemented in SW2 and SW3. Only slight cracking was observed at the interface between the infill and existing members, but did not appear to affect the global behaviour of the specimens.

Ba

se m

om

ent

(kN

m)

2nd

storey drift (%)

Fig.5 Moment-drift response (left) and crack pattern (right) of four-storey frames strengthened with RC infills (Strepelias, 2012)

The most recent study involved pseudo-dynamic tests on a full-scale four-storey frame (Chrysostomou et al, 2012). The tested structure comprised two parallel three-bay frames connected by an RC slab, with infills placed at the central bay. The two types of connection described above (Strepelias, 2012) were implemented between the infill and the existing elements. The infills in the two frames had different amounts of horizontal and vertical reinforcement, with the north one being stronger. Minor cracking was observed after the test for an earthquake with peak ground acceleration equal to 0.10g. No significant damage was evident after the test for the life-safety input motion (0.25g): horizontal cracks developed above the lapped splices at the base of the walls but there was no relative movement at the interface between the infill and the existing frame. The two frames exhibited essentially the same response for both tests. As seen in Fig. 6 that compares the global force-displacement results, the frame with the higher amount of reinforcement in the infill showed a slightly higher base shear resistance. Overall, the specimen satisfied the intended performance requirements for each level of earthquake excitation.

Fig.6 Force-displacement response of a four-storey frame structure strengthened with RC

infills (Chrysostomou et al, 2012)

0.10g 0.25g


-1433-

SUMMARY OF EXPERIMENTAL STUDIES

Previous research available in literature examined the influence of a number of parameters on the response of frames strengthened with shear walls. These parameters include the geometry (height, hw, length, lw, and width, tw) of the wall, the amount of longitudinal and transverse reinforcement of the existing and new elements, the amount of dowels at the interface, the dimensions and reinforcement of boundary elements, and the location and area of openings in the wall.

The results of the experimental investigations discussed in the previous sections of the paper are summarised in Table 1, where bc is the column width, lb is the length of the infilled bay and ρs,d is the dowel reinforcement ratio, i.e. the area of dowels along the perimeter of the new element divided by the area of the interface between the new wall and the existing frame. The symbols k, f, um and uu denote respectively the experimental values of stiffness, strength, displacement at maximum force and ultimate displacement of the strengthened specimens normalised to the corresponding values of the as-built ones.

The experimental results show a very large scatter of all the measures used to quantify the effectiveness of retrofit. Overall, it is confirmed that walls with larger dimensions (thickness, length, boundary elements) offer higher increase in strength and stiffness and larger reduction of drifts, both at peak strength and at failure.

RC infills that span the whole bay and are connected on all four sides to the existing beams and columns behave almost as well as walls cast monolithic to the frame. Infills that are connected only to the beams or are made of precast panels without connection through their interface evidently provide smaller enhancement of strength and stiffness. Most of the tested specimens were squat and therefore failed in shear, for which continuity and proper anchorage of horizontal reinforcement is critical.

The examined parameters affect mainly the stiffness and deformation properties and much less the strength of retrofitted frames. As a matter of fact, the medians of f and k are respectively 4.4 and 13.4, while their coefficients of variation are 0.7 and 1.3.

The results of several experimental campaigns show that the presence of spliced bars with insufficient lap length reduces the effectiveness of this strengthening technique. In most of these specimens, failure occurred at the cross-section above the starter bars at the base. However, steel or FRP jackets around the base of columns with short lap length were able to prevent this failure mode.

Lastly, it is noted that the majority of past experimental tests has been performed on single-storey walls, while practical applications involve mostly mid- or high-rise buildings. Relevant research has demonstrated the different failure modes of squat and slender walls: the behaviour of walls with high aspect ratio is mainly flexural and therefore associated to stable hysteretic response, improved capacity of energy dissipation and higher ductility.


-1434-

Tab

le 1

Sum

mar

y of

exp

erim

enta

l re

sult

s on

the

use

of

RC

inf

ill

wal

ls

Co

mm

ents

Sho

tcre

te

3 pa

nels

con

nect

ed t

o be

ams

only

3 pa

nels

con

nect

ed t

o be

ams

and

at t

heir

int

erfa

ce

2 pa

nels

pla

ced

at th

e ex

trem

ity

of t

he b

ay

2 pa

nels

pla

ced

at th

e ce

ntre

of

the

bay

4 pa

nels

wit

h m

orta

r at

the

ir in

terf

ace

4 pa

nels

wit

hout

mor

tar

at t

heir

int

erfa

ce

Mon

olit

hic

wal

l

Mon

olit

hic

wal

l

Con

cret

e sh

ear

keys

and

bin

der

Ste

el a

ncho

rs a

t to

p be

am o

nly

Ste

el a

ncho

rs o

n al

l si

des

Ste

el a

ncho

rs o

n al

l si

des,

rou

ghen

ing

of c

oncr

ete

Infi

ll i

n gr

ound

sto

rey

only

Pan

els

wit

h m

orta

r at

the

ir i

nter

face

Pan

els

wit

hout

mor

tar

at t

heir

inte

rfac

e

Mon

olit

hic

wal

l

Con

tinu

ous

long

itud

inal

rei

nfor

cem

ent

Con

tinu

ous

long

itud

inal

rei

nfor

cem

ent

Lap

len

gth

= 4

Φ

Lap

len

gth

= 1

5Φ

Lap

len

gth

= 4

Φ, u

ndam

aged

fra

me

Con

tinu

ous

long

itud

inal

rei

nfor

cem

ent

Lap

len

gth

= 1

2.5Φ

, ste

el j

acke

t at

col

umn

base

Lap

len

gth

= 1

2.5

Φ

Lap

len

gth

= 1

2.5

Φ, b

ound

ary

elem

ent

thic

knes

s =

tw

Lap

len

gth

= 1

2.5

Φ, b

ound

ary

elem

ent

thic

knes

s =

bc

uu

0.4

1.2

0.6

1.7

1.3

0.4

2.1

0.5

0.7

1.8

2.3

1.6

0.7

um

0.5

f 3.7

3.1

4.3

1.4

1.3

3.6

1.4

5.2

6.9

3.7

4.9

4.3

4.9

4.4

1.6

3.0

2.7

4.4

8.6

9.1

10.2

8.3

13.9

13.2

16.2

10.6

20.1

22.0

5.2

7.6

2.7

4.4

k

20.7

6.2

8.7

5.4

3.1

19.0

5.5

23.1

12.4

20.7

17.2

12.4

20.7

25.0

2.2

7.5

9.2

25.8

11.7

21.7

17.3

58.9

29.2

14.3

129.

8

6.1

14.6

ρs,

d (

%)

0.4

0.8

0.8

0.8

1.5

0.5

0.5

0.2

0.9

0.9

0.9

1.2

1.2

1.0

1.0

1.0

1.3

1.3

1.0

1.0

1.2

1.0

hw/l

w

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.6

0.6

0.6

0.6

0.6

2.0

2.0

2.0

2.0

2.0

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

1.2

2.3

1.2

4.2

2.4

l w/l

b

1.0

1.0

1.0

0.5

0.5

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

0.3

0.5

t w/b

c

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.3

0.3

0.3

0.3

0.3

0.4

0.4

0.4

0.4

0.4

0.6

0.3

0.3

0.3

Sp

ecim

en

2PW

3C3

4C3C

62C

2A

7C2B

8C4

9C40

13F

W

W2

W3

W4

W5

W6 2 3 4 5 8 A2

A4

A6

A8

A10

B4

B6

B8

B10

B12

S1 2 3 4

Ref

eren

ce

Hig

ash

i et

al,

198

0

Ha

ya

shi

et a

l, 1

908

Hig

ash

i et

al,

198

4

Tu

rk e

t a

l, 2

00

3

So

nu

va

r et

al,

20

04

Erd

em e

t a

l, 2

00

6

Ka

ra a

nd

Alt

in,

200

6


-1435-

Tab

le 1

(co

nti

nu

ed)

Sum

mar

y of

exp

erim

enta

l re

sult

s on

the

use

of

RC

inf

ill

wal

ls

Co

mm

ents

Tw

o w

alls

pla

ced

at t

he e

xtre

mit

y of

the

bay

Sin

gle

wal

l pl

aced

at

the

cent

re o

f th

e ba

y

Tw

o w

alls

pla

ced

at t

he e

xtre

mit

ies

of t

he b

ay

Sin

gle

wal

l pl

aced

at

the

cent

re o

f th

e ba

y

Con

tinu

ous

long

itud

inal

rei

nfor

cem

ent

Bou

ndar

y el

emen

t th

ickn

ess

= t

w

Bou

ndar

y el

emen

t th

ickn

ess

= b

c

Wel

ded

spli

ces

Wal

l ex

tern

al t

o th

e fr

ame,

ρl,

c =

1.3

%

Wal

l ex

tern

al t

o th

e fr

ame,

ρl,

c =

2.3

%

Rep

aire

d fr

ame

Und

amag

ed f

ram

e

Pre

cast

con

cret

e pa

nels

RC

inf

ill

107×

82 m

m p

anel

s, s

hear

key

s an

d ep

oxy

mor

tar

248×

50 m

m p

anel

s, s

hear

key

s an

d ep

oxy

mor

tar

107×

82 m

m p

anel

s, e

poxy

mor

tar

248×

50 m

m p

anel

s, e

poxy

mor

tar

107×

82 m

m p

anel

s, e

poxy

mor

tar

248×

50 m

m p

anel

s, e

poxy

mor

tar

107×

82 m

m p

anel

s, e

poxy

mor

tar,

lap

len

gth

= 2

0Φ

248×

50 m

m p

anel

s, e

poxy

mor

tar,

lap

len

gth

= 2

0Φ

Wal

l ex

tern

al t

o th

e fr

ame

uu

um

0.6

0.6

1.1

0.5

0.6

0.9

1.0

0.6

0.1

0.2

0.1

0.1

0.2

1.0

0.5

f 6.7

6.4

4.2

11.5

9.3

3.7

5.9

7.5

6.0

5.1

6.9

10.6

5.6

8.1

9.3

11.6

3.8

4.2

1.7

1.6

1.3

1.7

2.4

2.3

2.5

2.9

3.6

4.0

2.3

3.1

3.3

k

28.3

20.6

10.1

47.3

37.7

4.7

10.9

17.2

14.4

10.4

20.4

88.7

29.0

52.7

138.

0

83.3

6.1

5.2

3.3

3.2

3.1

2.9

5.2

5.5

2.7

4.7

7.2

ρs,

d (

%)

0.4

1.2

1.0

1.0

1.0

1.0

1.0

0.9

1.2

1.0

1.0

0.8

0.8

0.6

0.8

0.8

0.7

0.7

0.4

0.4

0.1

0.2

0.2

0.4

0.2

0.3

0.2

0.4

hw/l

w

1.7

1.5

2.8

0.7

0.7

2.5

1.4

1.0

0.9

1.6

0.7

1.2

1.2

1.2

1.2

1.2

1.0

1.0

2.2

2.2

0.6

0.6

0.6

0.6

0.6

0.6

0.6

0.6

l w/l

b

0.8

0.8

0.5

1.0

1.0

0.3

0.5

0.8

0.8

0.8

1.0

1.0

1.0

1.0

1.0

1.0

0.8

0.8

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

t w/b

c

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

0.3

1.0

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.6

Sp

ecim

en

5 6 7 2 3 4 5 6 7 8 9 2 3 4 5 6 S3

S4

S1

S2

CA

4

CB

4

CC

4

CD

4

CC

4-2

CD

2

LC

4

LD

4

Ref

eren

ce

Ka

ra a

nd

Alt

in,

200

6

An

il a

nd

Alt

in,

20

07

Alt

in e

t a

l, 2

008

Ka

lta

kci

et

al,

200

8

Tey

mu

r et

al,

20

08

Ku

rt,

20

10

Ba

ran

et

al,

201

1

Ka

pla

n e

t a

l, 2

01

1


-1436-

NUMERICAL STUDIES

Experimental research examined the influence of geometry and of the connection between new and existing elements. Numerical analyses may be employed to extend the scope to a broader range of parameters, further to the design of interventions as described previously.

For instance, the performance of post-tensioned braces, X braces and RC infills was compared on the basis of static and dynamic numerical analyses of low-, mid- and high-rise prototype RC frames (Pincheira and Jirsa, 1995). All retrofit strategies increased the strength and stiffness of the existing buildings. RC walls performed satisfactorily in all the examined cases, while the two bracing systems resulted in increased internal forces and high inelastic deformations for some of the existing elements and were not always successful in reducing the storey drifts to acceptable levels, particularly for high-rise buildings.

Parametric numerical analyses may be used to design the intervention for the desired balance between strength and stiffness. For this purpose, one-storey one-bay frames were studied using a hysteretic model calibrated on the experimental data from 55 specimens and varying the wall thickness, reinforcement ratio and area of anchors (Phan and Lew, 1996). Higher wall thickness was verified to increase the strength and decrease the deformation demand. The amount of wall reinforcement did not appear to have influence neither on the strength nor on the drift of the strengthened walls. Finally, increase of the area of anchors resulted in slightly higher strength and a rapid reduction of drift demand.

Attempts to simulate the experimental response showed that elaborate numerical models are more accurate, particularly until attainment of the maximum resistance, but are not always successful in simulating the post-peak response, while results are sensitive to the model parameters (Altin et al, 2008; Kaplan et al, 2011; Restrepo and Rahman, 2007; Strepelias, 2012; Teymur et al, 2008). Besides, existing design expressions for strength and stiffness can be used provided that the connection to the existing elements is (almost) monolithic.

CONCLUSIONS

Extensive experimental testing and numerical analyses have demonstrated the feasibility and effectiveness of shear walls for the enhancement of the seismic performance of existing RC frame buildings in terms of stiffness, strength and displacement demand. The latter is important for the reduction of non-structural damage, a requirement that has gained growing attention. However, architectural and functional constraints in real buildings may impose limits on the location and geometry of shear walls and therefore on the improvement of the global behaviour.

Shear walls can increase the strength and stiffness of existing buildings more than ten times. They impose a uniform height-wise distribution of deformation, thus preventing storey mechanisms. Experiments on multi-storey specimens have demonstrated that different failure modes and locations need to be considered, depending on the deficiencies of the as-built structure and the distribution of strength along the height of the new wall. Experimental and numerical results on the effectiveness of different solutions applied on the same structure clearly show that RC infilling provides the highest increase in strength and stiffness.

The impact of the selected retrofit measure on the existing structural elements requires consideration. Numerical analyses of real buildings strengthened with RC walls have shown that some columns and beams may need to be strengthened after the addition of the wall.


-1437-

The development of models and their implementation in analysis software is essential for the wider use of shear walls for strengthening. Existing design formulas provide conservative estimates of resistance but are unable to accurately reproduce the experimental post-peak behaviour. The codified design rules can be used, if a monolithic connection with the existing frame is achieved.

The design and detailing of the connection between new and existing elements is indeed an important issue. A number of solutions, mostly designed without following an established or documented procedure, have been tested. They provide adequate strength, but often are labour-intensive and result in reinforcement congestion. On the other hand, experimental evidence exists for the satisfactory behaviour of lighter connections.

The results available in literature, complemented by parametric numerical analyses, may provide the basis for the development of design guidelines focusing on the strength and stiffness characteristics and the connection between new and existing elements.

REFERENCES

[1]-Alavi B, Krawinkler H. Strengthening of moment-resisting frame structures against near-fault ground motion effects. Earthquake Engineering and Structural Dynamics, 2004, 33, p. 707-722.

[2]-Altin S, Anil Ö, Kara ME. Strengthening of RC nonductile frames with RC infills: An experimental study. Cement and Concrete Composites, 2008, 30, p. 612-621.

[3]-Anil Ö, Altin S. An experimental study on reinforced concrete partially infilled frames. Engineering Structures, 2007, 29, p. 449-460.

[4]-Aoyama H, Kato D, Katsumata H, Hosokawa Y. Strength and behaviour of postcast shear walls for strengthening of existing R/C buildings. 8th World Conference on Earthquake Engineering, San Francisco, 1984.

[5]-Baran M, Okuyucu D, Susoy M, Tankut T. Seismic strengthening of reinforced concrete frames by precast concrete panels. Magazine of Concrete Research, 2011, 63, p. 321-332.

[6]-Bush TD, Wyllie LA, Jirsa JO. Observations on two seismic strengthening schemes for concrete frames. Earthquake Spectra, 1991, 7, p. 511-527.

[7]-CEN. EN 1998-1 Eurocode 8: design of structures for earthquake resistance – Part 1: General rules, seismic actions and rules for buildings. European Committee of Standardization, Brussels, 2004.

[8]-Chrysostomou CZ, Kyriakides N, Kotronis P, Poljanšek M, Taucer F, Roussis P, Kosmopoulos A. Seismic retrofitting of RC frames with RC infilling. 15th World Conference on Earthquake Engineering, Lisbon, 2012.

[9]-Erdem I, Akyuz U, Ersoy U, Ozcebe G. An experimental study on two different strengthening techniques for RC frames. Engineering Structures, 2006, 28, p. 1843-1851.

[10]-Fardis MN, Schetakis A, Strepelias E. RC buildings retrofitted by converting frame bays into RC wall. Bulletin of Earthquake Engineering, 2013, 11, p. 1541-1561.

[11]-Hayashi T, Niwa H, Fukuhara M. The strengthening methods of the existing reinforced concrete buildings. 7th World Conference on Earthquake Engineering, Istanbul, 1980.


-1438-

[12]-Higashi Y, Endo T, Ohkubo M, Shimizu Y. Experimental study on strengthening reinforced concrete structure by adding shear wall. 7th World Conference on Earthquake Engineering, Istanbul, 1980.

[13]-Higashi Y, Endo T, Shimizu Y. Experimental studies on retrofitting of reinforced concrete building frames. 8th World Conference on Earthquake Engineering, San Francisco, 1984.

[14]-Kaltakci MY, Arslan MH, Yilmaz US, Arslan HD. A new approach on the strengthening of primary school buildings in Turkey: An application of external shear wall. Building and Environment, 2008, 43, p. 983-990.

[15]-Kaplan H, Yilmaz S, Cetinkaya N, Atimtay E. Seismic strengthening of RC structures with exterior shear walls. Sādhanā, 2011, 36, p. 17-34.

[16]-Kara ME, Altin S. Behavior of reinforced concrete frames with reinforced concrete partial infills. ACI Structural Journal, 2006, 103, p. 701-709.

[17]-Kurt EG. Investigation of strengthening techniques using pseudo-dynamic testing. MSc thesis, Middle East Technical University, Ankara, Turkey, 2010.

[18]-Mori K, Murakami K, Sakashita M, Kono S, Tanaka H. Seismic performance of multi-storey shearwall with an adjacent frame considering uplift of foundation. 14th World Conference on Earthquake Engineering, Beijing, 2008.

[19]-Pincheira J, Jirsa JO. Seismic response of RC frames retrofitted with steel braces or walls. ASCE Journal of Structural Engineering, 1995, 121, p. 1225-35.

[20]-Phan LT, Lew HS. Strengthening methodology for lightly reinforced concrete frames. 11th World Conference on Earthquake Engineering, Acapulco, 1996.

[21]-Qu Z, Wada A, Motoyui S, Sakata H, Kishiki S. Pin-supported walls for enhancing the seismic performance of building structures. Earthquake Engineering and Structural Dynamics, 2012, 41, p. 2075-2091.

[22]-Restrepo JL, Rahman A. Seismic performance of self-centering structural walls incorporating energy dissipators. ASCE Journal of Structural Engineering, 2007, 133, p. 1560-1570.

[23]-Sonuvar MO, Ozcebe G, Ersoy U. Rehabilitation of reinforced concrete frames with reinforced concrete infills. ACI Structural Journal, 2004, 101, p. 494-500.

[24]-Strepelias E. Strengthening of existing frame structures by infilling into RC walls - experimental and analytical investigation. PhD Thesis, University of Patras, Patras, 2012 (in Greek).

[25]-Teymur P, Yuksel E, Pala S. Wet-mixed shotcrete walls to retrofit low ductile RC frames. 14th World Conference on Earthquake Engineering, Beijing, 2008.

[26]-Turk M, Ersoy U, Ozcebe G. Retrofitting of reinforced concrete frames with reinforced concrete infills walls. fib Symposium on Concrete Structures in Seismic Regions, Athens, 2003.

seismic strengthening of rc frames with shear … · seismic strengthening of rc frames with shear...

Documents