chapter 4 experimental investigations on exterior beam...

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CHAPTER 4

EXPERIMENTAL INVESTIGATIONS ON EXTERIOR

BEAM-COLUMN JOINTS

4.1 GENERAL

Buildings subjected to seismic forces may undergo several

reversals of stresses in the course of an earthquake. Beam-column joints are

the critical region in moment resisting frames. The failure of reinforced

concrete structures in recent earthquakes caused concern about the

performance of beam-column joints. Due to the restriction of space available

in the joint block, detailing of reinforcement assumes great significance.

Confinement of joint is one of the ways to improve the performance of beam-

column joints during earthquakes. This chapter describes an experimental

study of exterior beam-column joints with two non-conventional

reinforcement arrangements. One exterior beam-column joint of a six storey

building in seismic zone III of India was designed for earthquake loading

(Ingle and Jain 2005). The transverse reinforcement of the joint assemblages

were detailed as per IS 13920:1993 and IS 456:2000 respectively. The

proposed non-conventional reinforcement was provided in the form of

diagonal reinforcement on the faces of the joint, as a replacement of ties in the

joint region for joints detailed as per IS 13920 and as additional reinforcement

for joints detailed as per IS 456. These newly proposed detailing have the

basic advantage of reducing the reinforcement congestion at the joint region.

In order to study and compare the performance of joints with different

detailing, four types of one-third scale specimens were cast (two numbers in

52

each type). The main objective of the present study is to investigate the

effectiveness of the proposed reinforcement detailing. All the specimens were

tested under reversed cyclic loading, with appropriate axial load.

One of the basic assumptions of frame analysis is that the joints are

strong enough to sustain the forces (moments, axial and shear forces)

generated by the loading, and to transfer the forces from one structural

element to another (beam to column in most of the cases) (Subramanian and

Rao 2003). The assumption of rigidity of joints often ignores the effects of

high shear forces developed within them. Actually, the shear failure is always

brittle in nature and cannot be deemed as an acceptable structural performance

especially during earthquakes. Thus, a proper understanding of the joint

behaviour is imperative in designing earthquake-resistant joints.

In Indian Code of practice for plain and reinforced concrete (IS 456

: 2000), the joints are usually neglected for specific design and attention is

restricted to the provision of sufficient anchorage for beam longitudinal

reinforcement. This may be acceptable when the frame is not subjected to

earthquake loads. Often, the poor design of beam column joints is

compounded by the high demand imposed by the adjoining flexural members

(beams and columns) in the event of mobilising their inelastic capacities to

dissipate seismic energy. Unsafe design and detailing within the joint region

jeopardises the entire structure, even if other structural members conform to

the design requirements. Among the Indian codes, IS 13920 : 1993 deals with

the ductile detailing of reinforced concrete structures subjected to seismic

forces. However, despite the significance of the joints in sustaining large

deformations and forces during earthquakes, specific guidelines or

recommendations on beam-column joints are not included in the Indian code of

practice (IS 456 : 2000).

53

According to the capacity design philosophy, beam hinging (while

avoiding column hinging and joint shear failure) is the most desirable mode of

failure to guarantee high energy dissipation during earthquakes, through

significant inelastic deformation and without overall reduction of strength.

The implementation of design philosophy presents serious problems

particularly in quantifying the different types of damage (structural and non-

structural) and what constitutes frequently minor, sometimes moderate, and

rarely major earthquakes. Plastic hinges are “expected” at locations where

structural damage can be allowed to occur due to inelastic actions involving

large deformations. Hence, in seismic design, damage in the form of plastic

hinges are accepted to form in beams rather than in columns (Uma and

Meher Prasad 2006). Proper confinement of joints can be done to rectify the

column hinging or joint shear failure. To satisfy the strong column-weak

beam collapse mechanism, it is important to understand the progress of

damage and failure pattern in joints under seismic loads. This can be obtained

by the reversed cyclic loading test.

In the past three decades extensive research has been carried out for

studying the behavior of joints under seismic loading (Jisra 1991). Various

international codes (ACI 352 R, Euro Code 8, NZS 3101 etc.) have been

subjected to periodic revisions. The role of transverse reinforcement and

mechanism of shear transfer in the joints for seismic resistance is the subject

of much debate (Hwang et al 2005). Experimental results on performance of

beam-column sub assemblages of modern structures indicate that current

design procedures could sometimes lead to excessive damage of the joint

regions (Tsonos 2007). IS 13920 : 1993 assumes that the role of hoops is to

confine the joint core. The real function of hoops may be both to confine the

joint core and to carry shear as tension tie and hence to reduce the width of

the crack. The special confining reinforcement (IS 13920 : 1993) serves three

purposes. It provides shear resistance to the member. It also confines the

concrete core and thereby increases the ultimate strain of concrete, which

54

gives greater ductility to the concrete cross section and enables it to undergo

large deformations. Again it provides lateral restraint against buckling of the

compression reinforcement. Experimental studies reveal that the usage of the

rectangular spiral reinforcement significantly improves the seismic capacity

of external beam-column connections (Karayannis et al 2005).

The present study aims at developing an innovative methodology to

confine the joint region. The confinement of joints is achieved by the

inclusion of inclined bars at the joint region. An external beam-column joint

from an actual six storey building has been considered for the present

investigation. The details regarding the design of joint region and

experimental studies on the same are discussed in the following sessions.

4.2 ANALYSIS AND DESIGN OF RC BEAM-COLUMN JOINT

A six storey RC building located at Chennai, India (in seismic zone

III as per IS 1893 (Part 1): 2002) on medium soil was analysed. The joint of

column marked “A” in Figure 4.1 was considered for design. The length of

the column was 3 m and the cross section was 450 mm deep by 300 mm wide.

The longitudinal and transverse beams were 450 mm deep by 300 mm wide.

A live load of 3 kN/m2 and floor finish of 1 kN/m2 were taken for analysis.

The thicknesses of peripheral and internal walls were taken as 250 mm and

150 mm respectively. M30 grade concrete and Fe 415 grade steel were used

for design. The plane frames constituting the joint regions were analysed. The

design was carried out based on the proposed amendments to IS 1893 and IS

13920 (Ingle and Jain 2005). Four types of detailing for beam-column joints

were considered for the present study, such as (i) joint detailed as per IS

13920 : 1993, (ii) joint detailed as per IS 13920 : 1993 with the proposed

non-conventional detailing. The non-conventional detailing comprises of two

cross inclined bars as a replacement of ties in the joint

55

(a) Plan of the building

(b) Elevation of longitudinal frame (c) Elevation of transverse frame

Figure 4.1 Details of the frames analysed

3m

3m

3m

3m

3m

3m

A A B

Exterior joint

3.6 m 5.1 m 2.7 m 5.1 m 3.6 m

A

B

Exterior joint

3.6 m 5.1 m 2.7 m 5.1 m 3.6 m

A

B 3.

6 m

3.

6 m

5.

1 m

X

Y

56

region. (iii) joint detailed as per IS 456 : 2000 with additional U bars as per

SP : 34(S & T)-1987, (iv) joint detailed as per IS 456 : 2000 with additional

U bars and the proposed non-conventional detailing. The non-conventional

detailing comprises of two cross inclined bars as additional confining

reinforcement in the joint region.

The shear forces, bending moments and axial forces around the

exterior beam-column joints due to the induced earthquake loading were

estimated. Seismic analysis was performed using equivalent lateral force

method given in IS 1893: 2002 and its proposed amendments (Jain and Murty

2005a).

4.2.1 Design of Beam

Plane frames were analysed and the force resultants for various

load cases such as dead load, live load and earthquake load in beam AB of

short frame were estimated and are shown in Table 4.1.The force resultants

for different load combinations considering earthquake load in Y direction

which is predominant, is shown in Table 4.2. The beam region of the joint

was checked for axial stress, member size, minimum and maximum

reinforcement to design as a flexural member. The design moment and shear

force from the critical load combinations for the beam AB were 160.05 kNm

and 112.8 kN respectively. Longitudinal reinforcement details of transverse

beam and longitudinal beams near the exterior joint are shown in Table 4.3,

Figure 4.2(a) and Figure 4.2(b). Spacing of ties on beam AB near joint was

calculated as per IS 456 : 2000 and as per IS 13920 : 1993. Two legged ties of

8 mm diameter were provided at a spacing of 100 mm centre to centre near

the joint and at 150 mm centre to centre near the mid span. The beam bars

were provided with adequate development length to take care of the pull out

force under cyclic loading.

57

Table 4.1 Force Resultants in Beam of Short Frame for Various Load

Cases

Load Case

Left end(A) Centre Right end (B)

Shear (kN)

Moment (kNm)

Shear (kN)

Moment (kNm)

Shear (kN)

Moment (kNm)

Dead Load 24.90 -12.70 -3.38 9.69 31.60 24.70

Live Load 8.10 -4.70 -1.62 4.04 11.30 10.50

Earthquake Load EQY

-50.30 94.00 -50.30 3.46 50.30 87.00

Table 4.2 Force Resultants in Transverse Beam for Different Load

Combinations

Sl. No.

Load Combinations

Left end Centre Right end

Shear (kN)

Moment (kNm)

Shear (kN)

Moment (kNm)

Shear (kN)

Moment (kNm)

1 1.5DL+1.5LL 49.50 -26.10 -7.51 20.62 64.35 52.80

2 1.2(DL+0.25*L

L+EQY) -28.05 96.15 -64.91 17.00 101.67 137.19

3 1.2(DL+0.25*L

L-EQY) 92.67 -129.45 55.81 8.70 -19.05 -71.61

4 1.5(DL+EQY) -38.1 121.95 -80.53 19.74 122.85 167.55

5 1.5(DL-EQY) 112.8 -160.05 70.37 9.36 -28.05 -93.45

6 .9DL+1.5EQY -53.04 129.57 -78.50 13.92 103.89 152.73

7 .9DL-1.5EQY 97.86 -152.43 72.40 3.54 -47.01 -108.27

58

Table 4.3 Details of Main Reinforcement in Beams Near the Joint A

Beam Top Main reinforcement

Bottom Main Reinforcement

Transverse beam near joint A 4- 20 Φ bars 4- 20 Φbars

Longitudinal beam left to joint A 3- 20 Φ bars 2- 20 Φbars

Longitudinal beam right to joint A 3- 20 Φ bars 2- 20 Φbars

Figure 4.2(a) Reinforcement Details of Transverse Beam Near the

Exterior Joint

59

Figure 4.2(b) Reinforcement Details of Longitudinal Beams Near the

Exterior Joint

4.2.2 Design of Column

The column was designed for earthquake loading in both X-direction

and Y-direction. Hence, all 13 load combinations as shown in Table 4.4 were

considered and critical combination is selected. Since the joints are subjected

to large shear force during earthquakes; joint shear strength should be

checked. Hence adequacies of depth and width for the column are checked as

per the proposed provisions for beam-column joints. The column is checked

for flexural member considering the lower axial force.

60

Table 4.4 Forces in Column for different Load Combinations

Col

umn

end

Forc

e/M

omen

t (k

N/k

Nm

)

1.5(

DL

+LL

)

1.2(

DL

+LL

+EQ

X)

1.2(

DL

+LL-

EQX

)

1.2(

DL

+LL

+EQ

Y)

1.2(

DL

+LL-

EQY

)

1.5(

DL

+EQ

X)

1.5(

DL

-EQ

X)

1.5(

DL

+EQ

Y)

1.5(

DL

-EQ

Y)

(0.9

DL

+1.5

EQ

X)

(0.9

DL

+1.5

EQ

X)

(0.9

DL

+1.5

EQ

Y)

(0.9

DL

+1.5

EQ

Y)

AB

Axi

al L

oad

-839 -748 -594 -458 -885 -782 -590 -419 -953 -508 -315 -145 -678

AT 647.9 572.4 464.2 369.1 667.4 596.4 461.1 342.3 715.2 384.9 249.6 130.8 503.7

BB -648 -572 -464 -369 -667 -596 -461 -342 -715 -385 -250 -131 -504

BT 456.6 398.3 332.3 276.5 454.1 412.8 330.3 260.6 482.6 264.2 181.7 111.9 333.9

AB

Mom

ent M

x

-14.6 46.68 -70 -11.6 -11.6 61.5 -84.3 -11.4 -11.4 66.06 -79.7 -6.84 -6.84

AT -12.9 44.76 -65.4 -10.3 -10.3 58.95 -78.8 -9.9 -9.9 62.91 -74.8 -5.94 -5.94

BB -14 45.96 -68.3 -11.2 -11.2 60.45 -82.4 -11 -11 64.83 -78 -6.57 -6.57

BT -14.1 36 -58.6 -11.3 -11.3 48 -70.2 -11.1 -11.1 52.44 -65.8 -6.66 -6.66

AB

Mom

ent M

y

-13.5 -10.8 -10.8 50.28 -71.9 -9.9 -9.9 66.45 -86.3 -5.94 -5.94 70.41 -82.3

AT -12.9 -10.3 -10.3 48.72 -69.4 -9.45 -9.45 64.35 -83.3 -5.67 -5.67 68.13 -79.5

BB -12.9 -10.3 -10.3 51.72 -72.4 -9.45 -9.45 68.1 -87 -5.67 -5.67 71.88 -83.2

BT -13.1 -10.4 -10.4 40.32 -61.2 -9.6 -9.6 53.85 -73.1 -5.76 -5.76 57.69 -69.2

AB

Shea

r Fo

rce

F y 9.3 7.44 7.44 -34.3 49.2 6.75 6.75 -45.5 58.95 4.05 4.05 -48.2 56.25

AT -8.7 7.44 -6.96 33.36 -47.3 -6.3 -6.3 44.1 -56.7 -3.78 -3.78 46.62 -54.2

BB 8.7 6.96 6.96 -33.4 47.28 6.3 6.3 -44.1 56.7 3.78 3.78 -46.6 54.18

BT -8.7 -6.96 -6.96 29.88 -43.8 -6.3 -6.3 39.75 -52.4 -3.78 -3.78 42.27 -49.8

AB

Shea

r Fo

rce

F x 9.9 -31.6 47.4 7.92 7.92 -41.6 57.15 7.8 7.8 -44.7 54.03 4.68 4.68

AT -9.3 30 -44.9 -7.44 -7.44 39.45 -54.2 -7.35 -7.35 42.39 -51.2 -4.41 -4.41

BB 9.3 -30 44.88 7.44 7.44 -39.5 54.15 7.35 7.35 -42.4 51.21 4.41 4.41

BT -9.3 25.44 -40.3 -7.44 -7.44 33.75 -48.5 -7.35 -7.35 36.69 -45.5 -4.41 -4.41

The exterior column was designed for an axial load of 953 kN and a

moment of 86.3 kNm, which were the critical values obtained from the

thirteen different load combinations. Eight numbers of 20 Φ bars were

provided with bars distributed equally on all faces of column. The joint was

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designed for strong column-weak beam conditions for earthquake in

Y direction and earthquake in X direction as per the draft revision of IS 13920

(Jain and Murty 2005b). Special confining reinforcement were provided in the

joint region for detailing as per IS13920 : 1993. In the detailing as per Indian

code of practice for plain and reinforced concrete IS 456 : 2000, joints were

not provided with tie, but U bars were provided for confining the joint core as

given by SP 34 : 1987.

4.2.3 Design of Exterior Joint

The joint shear strength and strong column weak beam conditions

for earthquake in Y direction and earthquake in X directions were checked as

per draft revision of IS 13920 and found satisfied. The joint shear in exterior

joint while earthquake in Y direction and in X direction are shown in Figure 4.3.

Since equal reinforcement were provided at the top and bottom of

transverse beam, the column shear for sway to right or to left during

earthquake loading in Y direction, colV was obtained as 74.50kN. The force

developed in the top reinforcement of beam 1T was 651.55kN. Thus the joint

shear force, intjoV is obtained from Equation (4.1) as 577.04 kN.

colV1TjointV (4.1)

For the earthquake loading in X direction, the column shear for

sway to right or to left, colV was obtained as 98.50 kN. The force developed in

the top reinforcement of right longitudinal beam 1T was 488.66 kN. The force

developed in the bottom reinforcement of left longitudinal beam 2T was

62

325.78 kN. Thus the joint shear force intjoV is obtained from Equation (4.2) as

715.94 kN.

jo int 1 2 colV T C V (4.2)

where 2C is the compressive force in the left beam which is equal to the

tensile force 2T developed in the left beam.

(a) Earthquake in Y direction (b) Earthquake in X direction

Figure 4.3 Joint Shear in Exterior Beam- Column Joint

The effective width of joint ( jb ) as per draft revision of IS 13920,

is lesser of (i) cbj 0.5hbb and (ii) cbb j where, bb is the width of beam;

ch is the depth of column in the considered direction of shear; cb is the width

of column. Effective shear area of joint jjej hbA : where jh is the depth of

joint which can be taken as depth of column.

Shear strength of the joint ( ckej fA0.1 ) was obtained as 739.43 kN;

which is higher than the joint shear force for the earthquake loading in

Y direction and in X direction. Hence, the joint has adequate shear strength in

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both directions as per proposed revision. The flexural strength ratio of column

to beam for earthquake in Y direction and earthquake in X direction were

obtained as 1.84 and 1.39 respectively. Hence, the requirement of strong

column-weak beam condition was satisfied for the earthquake loading in both

Y and X directions.

Thus the joint sub assemblage considered for the experimental

study was checked for shear strength, and strong column-weak beam theory.

The only difference between the sub assemblages considered for the present

study was in the detailing of transverse reinforcement. The special confining

reinforcement is continued in the joint region for detailing as per IS

13920:1993. Out of the two newly proposed detailing patterns, the first were

detailed as per IS 13920:1993 with the non-conventional confining

reinforcement pattern at the joint area. Here the transverse reinforcement at

the joint region was replaced by two pairs of the X-type confining

reinforcement. One additional tie was provided at the middle of the joint to

avoid buckling of the longitudinal reinforcement. In the second proposed

detailing pattern, the detailing is as per IS 456:2000 including the non-

conventional confining reinforcement pattern at the joint area. The joints with

transverse reinforcement detailing as per the Indian code of practice for plain

and reinforced concrete IS 456:2000, joints are not provided with tie, but the

U bars are provided for confining the joint core as given in SP 34: 1987.

4.3 DESCRIPTION OF SPECIMENS

The specimens were cast as one-third scale models and

classified into four groups with two numbers in each group. The Type 1

specimens (A1-13920 and A2-13920) were cast with transverse reinforcement

detailing as per IS 13920: 1993. The Type 2 specimens (X1-13920 and X2-

13920) were detailed as per IS 13920:1993 and with the newly proposed non-

64

conventional reinforcement. One additional tie was provided at the middle of

the joint to avoid the buckling of longitudinal reinforcement in Type 2

specimens. The Type 3 specimens (A1-456 and A2-456) were detailed as per

IS 456: 2000. The Type 4 specimens (X1-456 and X2-456) were cast with the

detailing as per IS 456:2000 and with the newly proposed non-conventional

reinforcement.

The non-conventional reinforcement was provided in the form of two

inclined bars on both faces of the joint as shown in Figure 4.4. The percentage

of diagonal bars provided was the same as that of confining reinforcement

required at the joint as per IS 13920: 1993. Care was taken to provide the

adequate development length as per the code requirement. All the eight

specimens were tested under constant axial load with cyclic load at the end of

the beam. One of the specimens from each group was subjected to an axial

load of three per cent of column axial load capacity and the other specimen

was subjected to an axial load of ten per cent of column axial load capacity

(Karayannis et al 2008). The value of axial load on the second specimen was

arrived from the axial force on the upper column of the assemblage at the

critical load combination selected for design.

Figure 4.4 Pattern Showing the Cross Inclined Confining Bars

65

The dimensions of the joints, diameter of reinforcement and

alignment of longitudinal reinforcement of column were in accordance with

the proposed amendments in IS 13920 (Jain and Murty 2005b). The

longitudinal reinforcement at the column region and beam region of the

specimens were checked for strong column-weak beam theory. High yield

strength deformed bars were used for the longitudinal reinforcement and high

yield strength bars were used for stirrups. The column was rectangular in

shape with dimensions 100 mm x 150 mm and the beam with dimensions

100 mm x 150 mm (Abrams 1987b). The details of the joint assemblage

specimens are shown in Figure 4.5 to Figure 4.8 and in Table 4.5. The photographs

of reinforcement cages are shown in Figure 4.9 and Figure 4.10.

4.4 CASTING OF SPECIMENS

The specimens were cast using ordinary Portland cement (53 grade)

conforming to IS 12269 : 1987. River sand (medium type) passing through

4.75 mm IS sieve and having a fineness modulus of 2.77 was used as fine

aggregate. Crushed granite stone of maximum size not exceeding 8 mm and

having a fineness modulus of 3.58 was used as coarse aggregate. The mix

design was carried out and the proportion was arrived as 1: 0.87: 1.32 by

weight and the water-cement ratio was kept as 0.48. The 28-day average

compressive strength from 150 mm cube test was 44.22 N/mm2. The yield

stress of reinforcement was 432 N/mm2. All the specimens were cast in

horizontal position inside a steel mould on the same day. Specimens were

demoulded after 24 hours and wet cured for 28 days.

66

Figure 4.5 Reinforcement Details of the Beam-Column Joint Specimen

as per IS 13920 (Type 1)

67


as per IS 13920 with Diagonal Confining Bars (Type 2)

68


as per IS 456 (Type 3)

69


as per IS 456 with Diagonal Confining Bars (Type 4)

70

Table 4.5 Reinforcement details of test specimens

Specimen designation

Column Beam Joint

Remarks Longitudinal Rein-

forcement

Transverse Reinforcement

Longitudinal Reinforcement



A1-13920 and

A2-13920

4 Φ8 and 4 Φ6

Φ3 at 25 mm c/c for a distance of 230

mm at either side of joint and 50 mm c/c

for remaining portion

2 Φ8 and 2 Φ6 (top and

bottom)

Φ3 at 35 mm c/c for a distance of 270 mm from joint and 50 mm c/c for

remaining length

Φ3 at 25 mm c/c Confining reinforcement at joint is as per IS 13920:1993

X1-13920 and

X2-13920

4 Φ8 and 4 Φ6

Φ3 at 25 mm c/c for a distance of 230 mm at either side of joint and 50 mm c/c

for remaining portion

2 Φ8 and 2 Φ6 (top and bottom)


remaining length

1 Φ3 lateral and 2 Φ6 diagonally on two faces

with development length in tension

extended to upper and lower column

Confining reinforcement at joint is as per minimum

requirement in column and additional diagonal bars on

two faces

A1-456 and A2-456

4 Φ8 and 4 Φ6 Φ3 at 100 mm c/c



remaining length

Two U bars, Φ3 are provided with

development length in tension extended to beam

No ties at joint. But two U bars (hairclip-type bend) used

for confinement as per IS: 456:2000 & SP34:1987

X1-456 and X2-456

4 Φ8 and 4 Φ6 Φ3 at 100 mm c/c



remaining length

Two U bar , Φ3 as per IS 456 and 2 Φ6

diagonally on two faces with development length in tension

extended to upper and lower column

No ties at joint. But two U bar (hairclip-type bend) used for

confinement as per IS: 456:2000 & SP34:1987 and additional diagonal bars on

two faces

71

(a) Conventional (Type 1) (b) Non-conventional (Type 2)

Figure 4.9 Photographs showing detailing as per IS 13920

(a) Conventional (Type 3) (b) Non-conventional (Type 4)

Figure 4.10 Photographs showing detailing as per IS 456

72

4.5 EXPERIMENTAL SET UP

The joint assemblages were subjected to axial load and reversed

cyclic load. The specimens were tested in an upright position and static

reversed cyclic loading was applied at the end of the beam. A constant

column axial load was applied by means of 392.4 kN (40 t) hydraulic jack

mounted vertically to the 981 kN (100 t) loading frame to simulate the gravity

load on the column. The axial load was selected as three and ten per cent of

the column axial load capacities for the first and second series respectively.

Axial load for the first series specimen was 15.92 kN (1.62 t) and for the

second series was 53.06 kN (5.41 t). One end of the column was given an

external hinge support, which was fastened to the strong reaction floor and the

other end was laterally restrained by a roller support to allow moment-free

rotation at both ends. A schematic drawing of the setup is shown in Figure

4.11. Cyclic loading was applied by two 196.2 kN (20 t) hydraulic jacks, one

kept fixed to the loading frame and the other to the strong reaction floor.

Reversed cyclic load was applied at 50 mm from the free end of the beam

portion. The test was load-controlled and the specimen was subjected to an

increasing cyclic load up to failure with the load increment of 1.962 kN (200

kg).

Figure 4.12 shows the loading sequence of the test assemblages.

Load cells with least count 0.0981 kN were used. The specimens were

instrumented with Linear Variable Differential Transformer (LVDT) having

least count 0.1 mm to measure the deflection at the loading point. An

inclinometer was fixed on top of the beam element of specimen to measure

the rotation near the interface during loading. Electrical resistance strain

gauges were used to measure the strain in the reinforcement during testing.

The experimental setup at laboratory is shown in Figure 4.13.

73

Figure 4.11 Schematic Diagram of Test Set-Up

Figure 4.12 Sequence of Cyclic Loading for Exterior Beam-Column Joint

74

Figure 4.13 Test Setup in the Laboratory

4.6 RESULTS AND DISCUSSION

In this section, the test results are presented in the form of load-

deformation hysteretic curves, energy dissipation curves, ductility charts,

moment - rotation envelope curves, stiffness degradation curves and the force

- reinforcement strain envelopes. The observations during the test are briefly

described here.

4.6.1 Cracking pattern and Failure Mode

In all the specimens in the first series (with axial load 15.92 kN),

initial diagonal hairline crack in the joint occurred at the second loading cycle

when the load reached 3.924 kN in both positive and negative cycle of

loading. But for the second series (with axial load 53.06 kN), the first crack

appeared only in the third cycle for a load of 5.886 kN in all specimens. The

75

yield and ultimate load for the test specimens are shown in Table 4.6.

The cracking patterns of test specimens in the first and second series are

shown in Figure 4.14 and Figure 4.15. In almost all specimens tensile cracks

were developed at the interface between the column and beam. Extensive

damage at the joint region can be noticed in specimen A1-13920 (Figure

4.14(a)), but when axial load increases the damage gets reduced as seen in

A2-13920 (Figure 4.15 (a)). The specimens failed due to the advancement of

crack width at the interface between beam and column. There was a clear

vertical cleavage formed at the junction of all the specimens. In addition, for

the first series specimens, A1-13920 and A1-456, cracks were developed in

the joint region also (Figure 4.14 (a) and Figure 4.14(c)). But, in the case of

X1-13920 and X1-456, the cracks were concentrated in the beam region only

(Figure 4.14(b) and Figure 4.14(d)). Among the specimens, X1-456 exhibited

the best performance. The second series specimens were tested under

increased axial load. The performance was improved due to the increased

axial load. Figure 4.15 shows the specimen in the second series. Like in the

case of the first series, the specimens had failed due to vertical cleavage at

beam-column joint interface.

Table 4.6 Yield and Ultimate Load of Specimens from Experiment

Designation of specimen

Experimental Yield Load (kN)

Experimental Ultimate Load (kN)

Downward direction

Upward direction

Average (Pye)

Downward direction

Upward direction

Average (Pue)

A1-13920 11.77 11.77 11.77 16.18 15.69 15.93 X1-13920 13.73 13.73 13.73 18.63 17.65 18.14

A1-456 15.69 13.73 14.71 16.67 14.71 15.69 X1-456 13.73 13.73 13.73 19.62 19.62 19.62

A2-13920 15.7 15.7 15.7 17.65 19.62 18.64 X2-13920 13.73 13.73 13.73 18.64 18.64 18.64

A2-456 15.7 15.7 15.7 18.64 18.64 18.64 X2-456 15.7 13.73 14.71 19.62 19.62 19.62

76

(a) Specimen A1-13920

(b) Specimen X1-13920

(c) Specimen A1-456

Figure 4.14 Crack pattern in the Specimens of First Series

77

(d) Specimen X1-456

Figure 4.14 (Continued)

(a) Specimen A2-13920

(b) Specimen X2-13920

Figure 4.15 Crack Pattern in the Specimens of Second Series

78

(c) Specimen A2-456

(d) Specimen X2-456

Figure 4.15 (Continued)

A major flexural crack was developed at the beam-column interface

in all specimens. For the specimens with inclined bars (with non-conventional

detailing) no major cracks were noticed at the joint and the joint remained

intact throughout the test. Here the failure was dominated by tensile failure at

the interface than at the joint failure. The improvement of performance by

developing further cracks away from the joint face to the beam region can be

noticed for the specimens with U-bars and inclined bars (X1-456 and X2-

456). For specimen X2-456 the crack width is also less compared to other

specimens.

79

-20

-15

-10

-5

0

5

10

15

20

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45

Beam end displacement (mm)

Loa

d (k

N)

4.6.2 Hysteretic Loops

The force - displacement hysteretic loops for all specimens are as

shown in Figure 4.16 to Figure 4.23. From Table 4.6 it can be seen that the

ultimate load carrying capacity is higher for specimens detailed as per IS 456

with U bars and inclined bars. In this type of non-conventional detailing, the

stiffness increases with loading up to seventh cycle. Here the ductility and

energy dissipation capacities increased without compromising the stiffness. In

general, specimens with inclined bars perform better than conventionally

detailed counterparts. A relative comparison of overall force-deformation

behavior of all specimens in series 1 and series 2 are as shown in Figure 4.24

and Figure 4.25 respectively. It is clear that specimen X1-456 and X2-456

attained maximum loads with less strength degradation.

Figure 4.16 Load Versus Displacement Curve of Specimen A1-13920

80

-20

-15

-10

-5

0

5

10

15

20

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45Beam end displacement (mm)

Loa

d (k

N)

Figure 4.17 Load versus Displacement Curve of Specimen X1-13920

-20

-15

-10

-5

0

5

10

15

20


Loa

d (k

N)

Figure 4.18 Load versus Displacement Curve of Specimen A1-456

81

-20

-15

-10

-5

0

5

10

15

20


Loa

d (k

N)


-20

-15

-10

-5

0

5

10

15

20


Loa

d (k

N)


82

-20

-15

-10

-5

0

5

10

15

20

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45Beam end displacement ( mm )

Loa

d ( k

N )


-20

-15

-10

-5

0

5

10

15

20

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45


Loa

d (k

N)


83

-20

-15

-10

-5

0

5

10

15

20

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45


Loa

d (k

N)


-20

-15

-10

-5

0

5

10

15

20

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45

Displacement (mm)

Loa

d (k

N)

A1-13920 X1-13920 A1-456 X1-456

Figure 4.24 Load-Displacement Envelopes of Specimens in First series

84

-20

-15

-10

-5

0

5

10

15

20

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45

Displacement (mm)

Loa

d (k

N)

A2-13920 X2-13920 A2-456 X2-456

Figure 4.25 Load-Displacement Envelopes of Specimens in Second Series

0

250

500

750

1000

1250

1500

1750

2000

2250

2500

0 10 20 30 40 50

Cumulative upward displacement (mm)

Cum

ulat

ive

ener

gy d

issi

pate

d (k

Nm

m)

A1-13920X1-13920A1-456X1-456A2-13920X2-13920A2-456X2-456

Figure 4.26 Cumulative Energy Dissipation curves of Specimens

85

4.6.3 Energy Dissipation

Structures with higher energy dissipation characteristics are able to

undergo stronger shaking and exhibit better seismic response. The area

enclosed by a hysteretic loop at a given cycle represents the energy dissipated

by the specimen during that cycle (El-Amoury and Ghobarah 2002). Figure

4.26 shows the cumulative energy dissipated versus cumulative displacement

curve of all specimens. It was observed that specimen X2-456 has the highest

value of energy dissipation of 2478.74 kN mm. The energy dissipation for all

the specimens is shown in Figure 4.27. It was observed that the increase in

axial load is beneficial for improving the energy dissipation characteristics of

specimens with inclined bars and U-bars. The energy dissipated in each cycle

for the test specimens are shown in Figure 4.28 to Figure 4.31. It is clearly

observed that specimens detailed with inclined bars (the non-conventional

detailing) have more energy dissipation than that of conventionally detailed

specimens.

995.54

1295.91376.68

804.86547.21

1438.29

735.65

2478.74

0250500750

1000125015001750200022502500

A-13920 X-13920 A-456 X-456Specimen

Cum

ulat

ive

ener

gy d

issi

pate

d (k

Nm

m)

series-1 series-2

Figure 4.27 Energy Dissipation Chart

86

0

200

400

600

800

1000

1200

C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9Cycle Number

Ene

rgy

Diss

ipat

ed (k

Nm

m)

A1-13920, DETAILED AS PER IS 13920, AXIAL LOAD 15.92 kNX1-13920, DETAILED AS PER IS 13920 WITH DIAGONAL CONFINING BARS, AXIAL LOAD 15.92 kN

Figure 4.28 Energy Dissipation Chart in Each Cycle for Specimens

A1-13920 and X1-13920

0

200

400

600

800

1000

1200

C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-11Cycle Number

Ene

rgy

Diss

ipat

ed (k

Nm

m)

A1-456, DETAILED AS PER IS 456, AXIAL LOAD 15.92 kNX1-456, DETAILED AS PER IS 456 WIT H DIAGONAL CONFINING BARS, AXIAL LOAD 15.92 kN


A1-456 and X1-456

87

0

200

400

600

800

1000

1200

C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10Cycle Number

Ene

rgy

Diss

ipat

ed (k

Nm

m)

A2-13920, DETAILED AS PER IS 13920, AXIAL LOAD 53.06 kNX2-13920, DETAILED AS PER IS 13920 WITH DIAGONAL CONFINING BARS, AXIAL LOAD 53.06 kN


A2-13920 and X2-13920

0

200

400

600

800

1000

1200

C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10Cycle Number

Ene

rgy

Diss

ipat

ed (k

Nm

m)

A2-456, DETAILED AS PER IS 456, AXIAL LOAD 53.06 kNX2-456, DETAILED AS PER IS 456 WIT H DIAGONAL CONFINING BARS, AXIAL LOAD 53.06 kN


A2-456 and X2-456

88

4.6.4 Displacement Ductility

Ductility is the capacity of the structure/member to undergo deformation beyond yield without loosing much of the load carrying capacity. In earthquake resistant design of structures, all the critical regions should be designed and detailed with large ductility and stable hysteretic behaviour. The ductility is generally measured in terms of displacement ductility, which is the ratio of the maximum deformation that a structure or element can undergo without significant loss of initial yielding resistance to the initial yield deformation (Park and Paulay 1975). The displacement ductility for all specimens are presented in Table 4.7. The effect of axial load on the displacement ductility can be seen from the ductility bar chart shown in

Figure 4.32. The series 1 specimen with '0.03f Ac g column axial load (where

Ag is the gross cross sectional area of column) had higher ductility than the

series 2 specimens with gA'c0.1f column axial load. The displacement

ductility of specimens with inclined bars was found to be higher than their conventionally detailed counter parts. The specimens X1-456 and X2-456 (with inclined bars and U bars) exhibited high displacement ductility.

Table 4.7 Displacement Ductility of Specimens

Specimen

Displacement (mm) Displacement ductility Average

ductility

% increase of Ductility

for Non conventional

specimens

Yield Ultimate

Downward direction

Upward direction

Downward direction

Upward direction

Downward direction

Upward direction

A1-13920 4.2 3.8 36.5 18.9 8.69 4.97 6.83 -----

X113920 4 3 43.7 18 10.93 6 8.46 23.86

A1-456 4.4 5.5 22.6 22.1 5.13 4.01 4.57 -----

X1-456 3.9 1.4 29.9 14.5 7.67 10.35 9.01 97.15

A2-13920 2.6 1.8 18.2 11.8 7 6.56 6.78 -----

X2-13920 5.1 1.9 35 14.6 6.86 7.68 7.27 7.23

A2-456 5 5 22.8 14 4.56 2.8 3.68 -----

X2-456 4.1 2.9 40.8 22.6 9.95 7.79 8.87 141

89

Figure 4.32 Comparison of Ductility of Beam-Column Joint Specimens

4.6.5 Joint Shear Stress

The horizontal shear stress of the exterior joint sub assemblage can

be given by Equation (4.3) (Murty et al 2003).

cLcD0.5bL

bdbL

hcoreAP

jhτ (4.3)

where P is the imposed cyclic load at the end of the beam, bL and cL are

lengths of beam and column, respectively, cD is total depth of column, bd is

effective depth of beam and hcoreA is the horizontal cross-sectional area of

the joint core resisting the horizontal shear force. It is necessary to limit the

magnitude of horizontal joint shear stress to protect the joint against diagonal

crushing. The ACI-318 standard limits the horizontal joint shear stress as

'cf0.083γ MPa, where '

cf is the cylinder compressive strength in MPa. The

0

1

2

3

4

5

6

7

8

9

10

A-13920 X-13920 A-456 X-456

Specimen

Disp

lace

men

t Duc

tility

Series-1Series-2

90

factor γ depends on the confinement provided by the members framing in to

the joint; is taken as 20, 15 and 12 for interior, exterior, and corner joints

respectively.

Table 4.8 Comparison of Ultimate Joint Shear Stress with ACI Code

Prescribed Limiting Values

Designation of specimen

Downward ultimate

load

uP kN

hjτ

MPa

% increase for X-

specimen ACI

hj

ττ

Upward Ultimate

Load

uP kN

hjτ MPa

% increase for X-

specimen ACI

hj

ττ

A1-13920 16.18 6.02 --- 1.01 15.69 5.84 --- 0.98

X1-13920 18.63 6.94 15.28 1.17 17.65 6.57 12.5 1.10

A1-456 16.67 6.20 --- 1.04 14.71 5.47 --- 0.92

X1-456 19.62 7.31 17.90 1.23 19.62 7.31 33.64 1.23

A2-13920 17.65 6.57 --- 1.10 19.62 7.31 --- 1.23

X2-13920 18.64 6.94 5.63 1.17 18.64 6.94 --- 1.17

A2-456 18.64 6.94 --- 1.17 18.64 6.94 1.17

X2-456 19.62 7.31 5.33 1.23 19.62 7.31 5.33 1.23

In Table 4.8, 'cf is 35.4 MPa and maximum permissible shear stress

as per ACI is 5.92 MPa.

The ultimate value of joint horizontal shear stress induced in the

joints are almost equal or little higher than the ACI recommended values.

It can be seen from Table 4.8 that the shear resisting capacity is more for non-

conventionally detailed specimens than the conventional specimens. The

increase in axial load also improves the shear capacity of joints.

91

4.6.6 Moment Rotation Relation

The envelope curve for the moment at beam near interface of the

sub assemblage and the rotation of the beam element during downward

loading in each cycle are shown in Figure 4.33 and Figure 4.34. Rotation was

higher for the first series specimens than the second series. The rotation is

higher for X1-456 which attained maximum rotation of 0.158 radians.

0100020003000400050006000700080009000

10000

0 0.05 0.1 0.15 0.2

Mom

ent (

kNm

m)

Rotation at joint interface (radians)

A1-456A1-13920X1-456X1-13920

Figure 4.33 Moment- Rotation Relation of Beam Element of Specimens

in the First Series

92

0100020003000400050006000700080009000

10000

0 0.05 0.1 0.15 0.2

Mom

ent (

kNm

m)

Rotation at joint interface(radians)

A2-456A2-13920X2-456X2-13920

Figure 4.34 Moment - Rotation Relation of Beam Element of Specimens

in the Second Series

4.6.7 Stiffness

The stiffness of the beam-column joint was approximated as slope

of the peak to peak line in each loading cycle. The variations of stiffness in

each cycle corresponding to maximum displacement are calculated for the

first and second series specimens and are as shown in Figure 4.35 and Figure

4.36 respectively. Stiffness was higher for specimen X1-456 in the first

series, but in second series initially the stiffness was higher for specimen

A2-13920, but the specimens X2-13920 and X2-456 showed considerable

displacement without sudden loss in load carrying capacity.

93

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45

Sti

ffn

ess(

kN/m

m)

Displacement (mm)

A1-13920X1-13920A1-456X1-456

Figure 4.35 Stiffness Degradation of Specimens in the First Series

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45

Sti

ffn

ess

(kN

/mm

)

Displacement (mm)

A2-13920X2-13920A2-456X2-456

Figure 4.36 Stiffness Degradation of Specimens in the Second Series

94

4.6.8 Reinforcement Strain

In order to study the effectiveness of non-conventional detailing,

electrical resistance strain gauge data for the first series of specimens were

measured. The locations of strain gauges are shown in Figure 4.37. The

envelopes of beam end force - reinforcement strain curve are shown in Figure

4.38 to Figure 4.41. It can be seen that the column bars of the conventionally

detailed specimens strained to some extent. But for the non-conventional

specimen the strain in column main bars were negligible. The main

reinforcement in the outer face of columns was free from strains. This

indicates that shear damage in the joint region was reduced for these

specimens. Strain readings of gauges confirmed that all column longitudinal

bars remained elastic during testing. The inclined bars remained active during

the entire cycle and resisted the shear stresses. The strains in the beam bars

were higher for all the specimens which show the beam mode failure.

Figure 4.37 Locations of Strain Gauges

95

-20

-15

-10

-5

0

5

10

15

20

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5

For

ce (k

N)

strain(%)1-Beam top

2-Beam bottom3-Column near4-Column far

5-Lateral

Figure 4.38 Force - Reinforcement Strains Envelope of Specimen A1-13920

-20

-15

-10

-5

0

5

10

15

20

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5

For

ce (k

N)

Strain (%)1-Beam Top2-Beam bottom3-Column near4-Column far5-Lateral6-Inclined bar(1)7-Inclined bar(2)

Figure 4.39 Force - Reinforcement Strains Envelope of Specimen X1-13920

96

-20

-15

-10

-5

0

5

10

15

20

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5

For

ce (k

N)

Strain (%)

1-Beam top2-Beam bottom3-Column near4-Column far5-U-bar

Figure 4.40 Force - Reinforcement Strains Envelope of Specimen A1-456

-20

-15

-10

-5

0

5

10

15

20

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5

For

ce (k

N)

Strain (%) 1-Beam Top

2-Beam Bottom

3-Column near

4-Column fa r

5-U-bar

6-Inclined bar(1)

7-Inclined bar (2)

Figure 4.41 Force- Reinforcement Strains Envelope of Specimen X1-456

97

4.7 SUMMARY

The experimental study focused on developing non-conventional

reinforcement detailing of the exterior beam-column joint in order to reduce

the congestion in the joint region. Eight specimens were cast and tested. The

confining reinforcement was detailed as per i) IS 13920 (Type 1), ii) IS 13920

with non-conventional detailing (Type 2), iii) IS 456 (Type 3), and iv) IS 456

with non-conventional detailing (Type 4). All the specimens were tested

under reversed cyclic loading, with appropriate axial load. The experimental

investigation shows that the beam-column joints designed as per IS 456 with

non-conventional detailing performed well. It is observed that the non-

conventionally detailed specimens (Type 2 and Type 4) have an improvement

in average ductility of 16% and 119% than their conventionally detailed

counter parts (Type1 and Type 3). Further, the joint shear capacity of the

Type 2 and Type 4 specimens are improved by 8.4% and 15.6% than the

corresponding specimens of Type 1 and Type 3 respectively. The proposed

non-conventional detailing is simple without any congestion compared to the

detailing as per IS: 13920. It is suggested that the joint detailing of Type 4

specimen i.e., providing inclined bars and hairpin bends as laterals can be

used in the exterior joints of low rise moment resisting frames in low to

moderate seismic risk region.

chapter 4 experimental investigations on exterior beam...

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