Experimental Cyclic Behavior of Precast Hybrid Beam-Column Connections with Welded Components
© The Author(s) 2017
Received: 10 January 2016
Accepted: 31 January 2017
Published: 22 May 2017
Post-earthquake observations revealed that seismic performance of beam-column connections in precast concrete structures affect the overall response extensively. Seismic design of precast reinforced concrete structures requires improved beam-column connections to transfer reversed load effects between structural elements. In Turkey, hybrid beam-column connections with welded components have been applied extensively in precast concrete industry for decades. Beam bottom longitudinal rebars are welded to beam end plates while top longitudinal rebars are placed to designated gaps in joint panels before casting of topping concrete in this type of connections. The paper presents the major findings of an experimental test programme including one monolithic and five precast hybrid half scale specimens representing interior beam-column connections of a moment frame of high ductility level. The required welding area between beam bottom longitudinal rebars and beam-end plates were calculated based on welding coefficients considered as a test parameter. It is observed that the maximum strain developed in the beam bottom flexural reinforcement plays an important role in the overall behavior of the connections. Two additional specimens which include unbonded lengths on the longitudinal rebars to reduce that strain demands were also tested. Strength, stiffness and energy dissipation characteristics of test specimens were investigated with respect to test variables. Seismic performances of test specimens were evaluated by obtaining damage indices.
One-story precast concrete structures constitute a significant part of industrial buildings in earthquake-prone regions in Turkey. Post-earthquake observations revealed that beam-column connections are widely influence the overall seismic response of precast concrete structures (Saatcioglu et al. 2001; Ozden and Meydanli 2003; Senel and Palanci 2013). It is still a challenging subject to develop precast concrete beam-column connections emulating the seismic performance of monolithic systems to maintain advantages of precast construction process for multi-story buildings.
In the literature, joint is defined as the intersection of beam and column elements while connection is the region where precast elements connected with a technique (welding, bolting etc.) during construction. Moment-resisting beam-column connections can be categorized mainly as emulative (wet) and dry connections (e.g. ACI 550.2R 2013). In addition, emulative and mechanical components can be assembled to constitute hybrid beam-column connections (Negro and Toniolo 2012) which are commonly used in Turkey.
In emulative connections, continuity of reinforcing bars is provided by a coupling connector or splicing throughout the designated gaps in precast beam and column elements at a precast construction facility. Precast beams are firstly supported on columns’ cover concrete and then the topping concrete is poured on site to fill the gaps in the column and the top of the beam (Park and Bull 1986; Chen et al. 2012). Im et al. (2013) tested five interior precast beam- column connections with U-shaped beam shells. Main test parameters were seating length of beam to joint, steel angle for cover concrete and installation of headed rebars. It was concluded that increase in effective depth of beam-column connection can be obtained by decreasing the seating length and the beam shell thickness.
Dry beam-column connections are achieved by connecting the precast elements with post-tensioning, welding or embedded rods. Chang et al. (2013) presented experimental results of two full-scale interior beam-column connections with embedded ductile rods within the joint region. Main test parameters were the use of high strength concrete, post-tensioning and high performance reinforcing steel. They were concluded that specimens sustained large drifts without any strength degradation. Experimental studies on post-tensioned connections showed that increase in mild reinforcement ratio results in improved ductility and energy dissipation capacities. However, in some cases premature buckling and rupture of mild reinforcement may occur before reaching the drift ratios that the connection can sustain (Cheok et al. 1993; Priestley et al. 1999; ACI T1.2 2003; Ertas et al. 2006). It is proposed that, unbonding of reinforcing bars over a length of precast beam and column elements in a sleeve can reduce the strain demands and prevent rupture which leads significant degradation in strength (Cheok et al. 1996; Pampanin et al. 2001; Belleri et al. 2012). However, unbonded length should be selected properly to ensure yielding of reinforcing bars without premature rupture (Cheok et al. 1993).
Seismic behavior of precast connections is required to be proved by experimental or numerical studies in terms of equivalent strength and ductility reflecting the monolithic behavior as specified in seismic codes (TEC 2007; ACI 318 2011). There has been a great deal of research by means of experimental and numerical studies on reinforced concrete beam-column joints (Ronagh and Baji 2014; Kim and Hyunhoon 2015; Kassem 2015; Rashidian et al. 2016; Lim et al. 2016). However, there are a limited number of experimental studies on the behavior of hybrid (emulative-welded) connections in the literature. Ertas et al. (2006) tested half-scaled one exterior beam-column connection where the first diagonal crack near the connection was reported at 2.2% drift ratio and the beam bottom rebars ruptured at the first cycle of 3.5% drift ratio. Yuksel et al. (2015) tested five half-scaled exterior hybrid connections including a slab which are subjected to monotonic and cyclic drifts applied at the beam tip. Monotonic and cyclic tests on first three specimens showed that strength degradation occurs due to rupture of welded longitudinal rebars and transverse rebars at the connection. Improved specimens showed an increased energy dissipation capacity while the observed in-cycle degradation was about 50% at a drift ratio of 3%. During the tests, increase in strain demands of beam welded rebars played an important role for overall behaviors of the connections.
Experimental studies on welded ASTM A615 type reinforcing bars (ASTM 1992) revealed that welding process causes heat-affected zone on reinforcements following an embrittlement on the material which is undesirable for a ductile seismic design (Rodriguez and Rodriguez 2006; Rodríguez and Torres-Matos 2013). This kind of rebars showed a decreased tensile strain capacity in the vicinity of the welded region. However, welding is still a common used technique in connecting the steel parts of structural elements in precast industry. As shown in the latter sections in this study that, rebars with low carbon content (e.g. B420C grade steel) defined in TS708 code (2010) could show a sufficient ductile behavior under tensile forces after a welding process.
An experimental research program was carried out to improve the cyclic behavior of hybrid (emulative-welded) beam-column connections. In this study, half scale one monolithic and five precast specimens representing interior beam-column connections were tested under reversed cyclic loading. Strength, stiffness and energy dissipation capacities of test specimens were investigated with respect to welding coefficient and unbonded length as the main test variables. Moreover, damage indices were also obtained to compare the seismic performance of these test specimens.
2 Experimental Study
2.1 Material Tests
Weldability and hence the mechanical properties of a reinforcement after welding depend on the chemical composition of the material defined as carbon content (C) and carbon equivalent (CE) ratios (Atakoy 2014; TS 708 2010; ASTM A706M 2013). Therefore, appropriate type of longitudinal rebars which is compatible with the upper limits specified by the TS 708 code (2010) (C: 0.22%, CE: 0.50%) were installed to beam and column sections except one specimen. The rebars were also satisfying the carbon content and CE allowable limits (C: 0.30%, CE: 0.55%) given in ASTM A706M (2013) for Grade 60 rebars which has similar mechanical characteristics.
Tensile test results for longitudinal and transverse rebars.
2.2 Test Specimens
Main test parameters and properties of precast concrete interior connections.
Table 2 summarizes the welding coefficient (α), corbel length (Lc), transverse reinforcement ratio (ρw), slenderness ratio of longitudinal rebars (sh/db), applied unbonded length (Lu), the carbon content (C) and the CE ratio of the longitudinal bars corresponding to each precast specimen.
2.3 Experimental Set-up
3 Experimental Results
3.1 Damage Patterns
Drift ratios corresponding to initiation of each observed damage state.
Flexural crack (%)
Shear crack (%)
Diagonal cracks in joint panel (%)
Concrete spalling (%)
2.2% (2nd cycle)
2.2% (2nd cycle)
3.5% (2nd cycle)
3.5% (1st cycle)
3.5% (1st cycle)
3.5% (3rd cycle)
3.5% (3rd cycle)
Beam longitudinal rebars of precast specimens welded to end plates buckled and sequentially or subsequently ruptured in most cases except for improved SP3-R specimen. Since SP1 specimen had welded rebars with CE similar to ASTM A615 type rebars (ASTM 1992) as shown in Table 2, rebar rupture occurred at a relatively lower drift ratio (2.2%) compared to revised precast specimen (SP1-R). SP2 and SP3 specimens had lower carbon content and concrete crushing at 2.75% drift (Fig. 14d) followed by rebar buckling and rupture were observed in these specimens In SP1-R specimen, provided additional ties were improved the behavior and prevented early rebar buckling. Moreover, the combination of applying unbonded length and additional vertical ties could postpone the rebar buckling in SP1-R and SP3-R specimens, and subsequently the rebar rupture.
3.2 Lateral Load–Drift Ratio Relationships
In SP1 specimen, maximum strength was attained at 2.2% drift ratio in the pull and push directions as shown in Fig. 16b. 40% degradation in strength for SP1 specimen was observed due to premature failure of beam welded longitudinal rebars. SP2 and SP3 specimens showed similar behavior and maximum strengths were attained at 2.2% drift ratio as shown in Fig. 16c, d.
In SP1-R specimen, tests were performed up to 5% and 3.5% drift ratios in push and pull directions as shown in Fig. 16e, respectively. SP1-R specimen showed a more ductile behavior compare to SP1 specimen. However, shear failures after 3.5% drift ratio led severe pinching as can be seen in load–drift relationship. SP3-R specimen attained maximum strength at 2.2% drift ratio and first degradation in strength was observed at the second cycle of 3.5 drift ratio as shown in Fig. 16f.
3.3 Local Response
4 Evaluation of Experimental Results
4.1 Lateral Strength and Ductility
Lateral strength and ductility of test specimens.
0.75 QM (kN)
Acceptance criteria for moment frames based on structural testing (ACI 374.1 2005) requires that strength of specimens at the third cycle of 3.5% drift ratio (Q 3.5) should not be less than 0.75Q M . Since in-cycle strength degradation was caused by abrupt rupture of welded reinforcing bars for precast specimens, this condition was provided by SP1-R and SP3-R specimens only as indicated in Table 4—Q 3.5 column.
Displacement ductility (µe ) of a connection specimen is the ratio of ultimate drift ratio (Θ U ) to the effective yield drift ratio (Θ e ). Displacement ductility of each specimen is shown in Table 4. SP3-R had higher displacement ductility among the precast specimens after the achieved improvements.
4.2 Lateral Stiffness
Initial stiffnesses of test specimens.
4.3 Energy Dissipation
4.4 Damage Index
During the test of monolithic specimen (MONO), bond-slip of beam reinforcing bars after 3.5% drift ratio caused a pinched behavior in the force–deformation relation and decreased the energy dissipation. On the other hand, precast specimens showed increased relative energy dissipation ratios with the increase of embedment length of beam top reinforcing bars.
Precast specimens which were tested at the first stage (without an unbonded length) didn’t show similar behavior in terms of ductility as the MONO specimen showed. Strain development of beam welded rebars played an important role on the overall behavior of precast connections. SP1 specimen beams reinforced with relatively high carbon content bars showed an abrupt decrease in the strength at 2.2% drift ratio. However, SP2 and SP3 specimens showed a gradual strength degradation up to 3.5% drift ratio. Obtained damage indices for SP1, SP2 and SP3 specimens were close to the index corresponding to collapse state contrast to MONO specimen.
Unbonding of welded rebars within a sleeve could be able to decrease strain demands in the vicinity of the connection. SP1-R specimen showed a higher ductility than SP1 specimen but it showed severe pinching due to shear failures.
SP3-R specimen showed an improved seismic behavior thanks to unbonded length approach and the additional ties to prevent early buckling of longitudinal bars.
Based on the experimental findings and further evaluations, revising the relevant requirements in design codes such as maximum spacing of beam transverse reinforcement, adopting additional vertical ties and the unbonded length approach to resist early buckling and rupture of flexural rebars is proposed.
Experimental study was supported by Turkish Precast Concrete Association and Dokuz Eylul University Scientific Research Program under the Grant No. DEU-BAP 2012.KB.FEN.019. The contributions of Dr. Sevket Ozden, M.Sc. Hakan Atakoy, M.Sc. Gunkut Barka, Dr. Turkay Baran, Dr. Ozgur Ozcelik and M.Sc. Eng. Umut Yucel to the experimental studies carried out in the Structural Mechanics Laboratory at Dokuz Eylul University are gratefully acknowledged. Tensile tests were performed in Mechanic Laboratory at Dokuz Eylul University Metallurgical and Minerals Engineering Department.
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