Lap splice length and details of column longitudinal reinforcement at plastic hinge region

Hong-Gun Park1) and Chul-Goo Kim2)

1), 2 Department of Architecture and Architectural Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea

1) parkhg@snu.ac.kr

2) these09@naver.ac.kr

ABSTRACT

Longitudinal bars in columns are often lap-spliced at the bottom of first story columns where potential plastic hinges are formed. In the present study, seismic resistance of columns with such re-bar splices were investigated by performing cyclic load tests. The effects of lap splice length and details of column longitudinal bars were mainly evaluated. The test results showed that the flexural strength, deformation capacity, and energy dissipation capacity of columns were significantly affected by the splice details despite the same lap splice length. In the column with bottom offset bar splice, the deformation and energy dissipation capacities were relatively large but the flexural strength was less. In the columns with top offset bar splice or straight bar splice, on the other hand, the flexural strength was high but ductility was less. 1. INTRODUCTION In low and moderate seismic zones, lap splices of column longitudinal bars are used at the bottom of columns where potential plastic hinges form. The seismic performance of columns with such re-bar splices can be degraded due to premature bond deterioration of spliced bars. Thus, for special moment frames in ACI 318-14, lap splices are permitted in the center half of the column height where relatively small inelastic deformation is required.

The flexural strength and deformation capacity of columns with re-bar splices are mainly influenced by the offset bar details shown in Fig. 1. ACI 315-99 specifies two types of offset bar details; bottom offset bar splice and top offset bar splice. In the bottom offset bar splice, the bottom splice bars from the lower story are offset inside and spliced with straight top bars. In the top offset bar splice, the top bars are offset inside and spliced with the straight bottom bars from the lower story. However, in small

1)

Professor 2)

Graduate Student

buildings, alternatively, lap splices of straight bars are used for convenient construction (Fig. 1(c)).

In the present study, cyclic loading tests of columns with three offset bar details

and various splice lengths (ls = 30 ~ 50db) were performed to evaluate the flexural strength, deformation capacity, and energy dissipation capacity of the columns.

Fig. 1 Lap splice details of column longitudinal bars

Fig. 2 Dimensions and reinforcement details of spliced columns

Bottom splice bar

offset inside the

core1:6

offset

Top bar parallel to

bottom splice bar

without offset

bend

(a) Bottom offset bar splice (b) Top offset bar splice (c) Straight bar splice

Top splice bar

offset inside the

core

⇒ Bottom splice bar (●) ⇒ Top splice bar (○)

SD400

D13

SD500

D25

(Unit : mm)

=2400

= 30, 40

50

=165

80

600

=165

80

= 50

(=1250 mm)

40

6 (=78)

400

< Bottom offset bar splice >

= 50

=165

80

< Straight bar splice >< Top offset bar splice >

Bottom offset

bar splice

Top offset

bar splice

Straight bar

splice

SL50S2B SL30S2T

SL40S2T

SL50S2T

SL50S2S

2. TEST PROGRAM The major test parameters include lap splice length and lap splice details. In the specimen names, the letters ‘L30’, ‘L40’, and ‘L50’ denote the length of lap splices, ls= 30db, 40db, and 50db, respectively; the last letters ‘B’, ‘T’, and ‘S’ denote the offset bar details: Bottom offset bar splice, Top offset bar details, and Straight bar splice.

The dimensions and reinforcement details of the columns are presented in Fig. 2. The cross-sectional dimension was 400 mm x 400 mm and the shear span length was 2400 mm (a/h = 6.0). SD500 D25 and SD400 D13 bars (db=25.4 mm and 12.7 mm) were used for longitudinal and transverse reinforcing bars, respectively. The splice lengths ranged from 30db to 50db were equivalent to 56% ~ 108% of required lap splice length ls,ACI in ACI 318-14 using the actual material strength.

The spliced bars were confined by ties with 90º end hooks at a spacing of s = 165 mm (= 0.5d). As shown in Fig. 2, the 90º end hooks was alternated end for end in accordance with the reinforcement detailing provision of ACI 318-14. 3. MATERIAL STRENGTH As the cyclic load tests were performed twice, concrete compressive strength (fc') and the bar yield and tensile strengths (fy and fu, respectively) differed in each test group. The yield strength and tensile strength are fy = 475 ~ 590 MPa and fu = 656 ~ 707 MPa, respectively. Compression tests of concrete cylinders (100 mm x 200 mm) were performed on the first day of testing. The compressive strengths of the concrete were fc' = 27 and 37 MPa. 4. CYCLIC BEHAVIOR OF SPLICED COLUMNS The lateral load − lateral drift ratio relationships of the columns are presented in Fig. 3. The lateral drift ratio δ was calculated by dividing the lateral displacement Δ at the loading point by the shear span (a = 2400 mm). The maximum test load Vu were denoted as white circles, while the predicted strengths Vn (=Mn/a) were denoted as horizontal dashed lines. Moment strength Mn of each column was calculated from section analysis considering the applied axial load N, actual material strengths, and the location of the bottom splice bars in the cross-section at the column base. In the bottom offset bar splice (SL50S2B), since the longitudinal bars are located relatively inside the concrete core, the predicted strength Vn is approximately 10% smaller than other splice columns (SL50S2T and SL50S2S). In the specimens where bond splitting failure occurred, the points at initial vertical cracking and bond failure were marked with white and grey squares, respectively.

Figures 3(a)-(c) show the cyclic behaviors of SL30S2T, SL40S2T, and SL50S2T with top offset bar splice, respectively. In the columns with top offset bar splice, the ductility was limited, particularly in SL30S2T and SL40S2T with short lap splices (ls /

ls,ACI = 0.56 and 0.74). The deformation capacity increased as the lap splice length increased. In SL30S2T and SL40S2T, initial vertical cracking and bond failure along the

lap splice regions occurred at 1st cycle of δ = 2.5~3.5% and 1st cycle of δ = 3.5~5.0%, respectively. In SL50S2T with top offset bar splice and ls / ls,ACI of 1.08, initial vertical cracking and bond failure along lap splice length occurred at 1st cycle and 2nd cycle of δ = 5.0%, respectively

In SL50S2B with bottom offset bar splice and ls / ls,ACI of 1.08, on the other hand, ductile behavior was maintained until δ = 7.0% without significant strength degradation. SL50S2B showed the greatest capacity in terms of ductility and energy dissipation. Bond splitting did not occur, and the test was terminated without failure due to lack of the actuator stroke. In SL50S2S with straight bar splice, the cyclic behavior was similar to SL50S2T with top offset bar splice. The maximum loads (Vu = 175 ~ 187 kN) were 9% greater than the nominal strength (Vn = 166 kN), and bond failure with vertical splitting cracks occurred at 2nd cycle of δ = 5.0%. Despite the different lap splice details in SL50S2T and SL50S2S, the maximum strength, deformation capacity, and failure mode were almost identical because the location of bottom splice bars to resist flexural moment was the same in SL50S2T and SL50S2S.

Fig. 3 Lateral load − drift ratio relationship of spliced columns

5. CONCLUSIONS

The flexural strength and deformation capacity of spliced columns are mainly affected by the location of bottom splice bars and splice length ls. The column with bottom offset bar splice showed relatively lower flexural strength but greater ductility and energy dissipation capacity. On the other hand, the columns with top offset bar splice or straight bar splice showed greater flexural strength but lower ductility and energy dissipation capacity. Furthermore, the nominal flexural strength of spliced

Lateral Drift ratio(%)

-200

-150

-100

-50

0

50

100

150

200

-8 -6 -4 -2 0 2 4 6 8

(e) SL50S2S

= 37 MPa

= 890 kN (15%)

5

−

Straight bar

splice

-200

-150

-100

-50

0

50

100

150

200

-8 -6 -4 -2 0 2 4 6 8

(c) SL50S2T

= 37 MPa

= 890 kN (15%)

Top offset

bar splice

−

Bottom offset

bar splice

Top offset

bar spliceStraight bar

splice

Late

ral lo

ad

(kN

)L

ate

ral lo

ad

(kN

)

: Initial vertical splitting crack at splice zone: Peak load : Bond failure

Lateral Drift ratio(%)

-200

-150

-100

-50

0

50

100

150

200

-8 -6 -4 -2 0 2 4 6 8

(d) SL50S2B

= 37 MPa

= 890 kN (15%)

53 5

− 5

Bottom offset

bar splice

-200

-150

-100

-50

0

50

100

150

200

-8 -6 -4 -2 0 2 4 6 8

(b) SL40S2T

= 27 MPa

= 800 kN (18.5%)

4 5

− 5

Top offset

bar splice

-200

-150

-100

-50

0

50

100

150

200

-8 -6 -4 -2 0 2 4 6 8

(a) SL30S2T

= 27 MPa

= 800 kN (18.5%)

4 3

− 55

Top offset

bar splice

Lateral Drift ratio(%)

columns should be calculated considering the location of the bottom splice bars in the plastic hinge region. REFERENCES ACI Committee 3 5. ( ), “Details and Detailing of Concrete Reinforcement”,

American Concrete Institute, Farmington Hills, MI, USA. ACI Committee 3 . ( 0 4), “Building Code Requirements for Structural Concrete and

Commentary”, American Concrete Institute, Farmington Hills, MI, USA.