Tenth U.S. National Conference on Earthquake EngineeringFrontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska 10NCEE

EXPERIMENTAL STUDY ON REDUCED-LENGTH BUCKLING-RESTRAINED

BRACES UNDER SLOW-CYCLIC LOADING

M. S. Pandikkadavath1 and D. R. Sahoo2

ABSTRACT Conventional bucking-type braces (BTBs) suffer from the strength-and-stiffness degradation under the compressive cyclic loading in the event of earthquakes. On the other hand, the use of buckling-restrained braces (BRBs) in a framed structure results in excessive residual drift response in the post-earthquake scenario. The disadvantages of BTBs and BRBs can be eliminated by using a single composite brace in which the elastic and undamaged BTBs provide the sufficient lateral stiffness and the input seismic energy is allowed to dissipate through the yielding of core segments of the reduced-length BRBs (RLBRBs). In this study, two specimens of RLBRBs are tested under gradually-increased reversed-cyclic loading conditions to evaluate their lateral strength, stiffness, energy dissipation and ductility capacity. The only parameter varied in this study is the cross-section of core segments in the specimens. Standard displacement history as per ANSI/AISC 341-10 provisions and a modified loading protocol for increased ductility demand considering the reduction in the (yielding) core length are used in the slow-cyclic testing. The core segments of RLBRBs are restrained by concrete in a detachable steel casing system, instead of a closed casing system as used in the conventional BRBs. RLBRBs showed the excellent hysteretic response, energy dissipation, and displacement ductility. The specimen RLBRB2 exhibited a maximum displacement ductility of 30. However, further tests are required in order to establish the limits of displacement ductility and cumulative plastic deformations of RLBRBs.

1Ressearch Scholar, Dept. of Civil Engineering, Indian Institute of Technology Delhi, New Delhi 110016 (INDIA) 2Assistant Professor, Dept. of Civil Engineering, Indian Institute of Technology Delhi, New Delhi 110016 (INDIA) Pandikkadavath MS, Sahoo DR. Experimental study on reduced-length buckling-restrained braces under slow-cyclic loading. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

Experimental Study on Reduced-Length Buckling-Restrained Braces

under Slow-Cyclic Loading

M. S. Pandikkadavath1 and D. R. Sahoo2

ABSTRACT Conventional bucking-type braces (BTBs) suffer from the strength-and-stiffness degradation

under the compressive cyclic loading in the event of earthquakes. On the other hand, the use of buckling-restrained braces (BRBs) in a framed structure results in excessive residual drift response in the post-earthquake scenario. The disadvantages of BTBs and BRBs can be eliminated by using a single composite brace in which the elastic and undamaged BTBs provide the sufficient lateral stiffness and the input seismic energy is allowed to dissipate through the yielding of core segments of the reduced-length BRBs (RLBRBs). In this study, two specimens of RLBRBs are tested under gradually-increased reversed-cyclic loading conditions to evaluate their lateral strength, stiffness, energy dissipation and ductility capacity. The only parameter varied in this study is the cross-section of core segments in the specimens. Standard displacement history as per ANSI/AISC 341-10 provisions and a modified loading protocol for increased ductility demand considering the reduction in the (yielding) core length are used in the slow-cyclic testing. The core segments of RLBRBs are restrained by concrete in a detachable steel casing system, instead of a closed casing system as used in the conventional BRBs. RLBRBs showed the excellent hysteretic response, energy dissipation, and displacement ductility. The specimen RLBRB2 exhibited a maximum displacement ductility of 30. However, further tests are required in order to establish the limits of displacement ductility and cumulative plastic deformations of RLBRBs.

Introduction Buckling-restrained braced frames (BRBFs) are widely used as primary lateral force-resisting systems in the high-seismic regions. Buckling-restrained braces (BRBs) possess better lateral strengths in tension and compression, lateral stiffness, energy dissipation, and displacement ductility. The entire length of a typical BRB can be primarily divided into three segments, namely, unyielding and unrestrained end (connection) segment, unyielding and restrained transition segment, and yielding and restrained middle (core) segment. The length of core segments varies from 60-80% of the total length of BRB. Extensive studies [e.g., 1-5] have been conducted to investigate the seismic performance of BRBs and the connections. While BRBs offer nearly similar tensile and compressive axial resistance with excellent energy dissipation, the residual drift response of BRBFs under high seismic excitations is significantly higher than the special concentric braced frames (SCBFs) [e.g., 6-7]. In order to overcome this limitation, a

1Ressearch Scholar, Dept. of Civil Engineering, Indian Institute of Technology Delhi, New Delhi 110016 (INDIA) 2Assistant Professor, Dept. of Civil Engineering, Indian Institute of Technology Delhi, New Delhi 110016 (INDIA) Pandikkadavath MS, Sahoo DR. Experimental study on reduced-length buckling-restrained braces under slow-cyclic loading. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

composite brace system can be developed in which the BRBs of reduced lengths are used in series with the conventional buckling-type braces in the same bracing system. Analytical investigation [8] confirmed that the composite brace system had better seismic performance with the reduced residual drift response as compared to the conventional BRBFs. This study is focused on the characterization of hysteretic response of the reduced-length BRBs (RLBRBs). RLBRBs have several advantages, such as, easy replacement, lesser weight, easy and simple erection process, and economical. Several researchers [9-11] have investigated the cyclic performance of BRBs in which the core lengths varied in the range of 20-40% of their total lengths. The maximum plastic strain level in a conventional BRB is usually limited to 1-2% [9]. Since the plastic strain in a BRB is inversely proportional to its yielding length, the ductility demand on a BRBF is increased if the conventional BRBs are replaced with RLBRBs. Hence, it is necessary to investigate the hysteretic, fatigue, and ductility behavior of the RLBRBs under the cyclic loading conditions. Two test specimens of RLBRBs are experimentally investigated under the action of the gradually-increased reversed-cyclic loading conditions. The core segments of specimens are restrained by concrete in two detachable steel casings.

Experimental Study

Figure 1(a) shows a typical application of RLBRBs in a braced frame in which the BRBs and buckling-type braces are connected in series. Fig. 1(b) shows shape and dimensions of the RLBRB specimens used in the component testing. Single-plate core sections are used in both the specimens. The geometric details and fabrications of specimens, test set-up and loading protocol used in this study are discussed in the following sections.

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Figure 1. (a) Schematic representation of a braced frame with the arrangement of reduced-length BRBs, (b) BRB sections used for the component testing in this study.

Geometry of Test Specimens Two reduced-scale RLBRB specimens representing one-third length of a prototype RLBRB that can be fitted in an intermediate story level of a 6-story frame used in previous studies [6-7] were

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considered in this study. As shown in Figure 1(b), the total length of specimens was 600 mm out of which 300 mm (50%) of total length was used as core segment. About 12.5% (75 mm) of the total length was considered as the transition and end segments at both ends of the specimens. This proportioning may viable to change in a full-scale RLBRB component as the prototype RLBRB may require the smaller percentage of the total length as the non-yielding transition and end segments. For the present test program, 8 mm thick steel plate was used as transition, end and core segments of test specimens. Transverse stiffener plates of 12 mm thick were welded in the transition and end segments in order to restrict the inelastic actions limited to the core segment only. Specimen RLBRB1 had a core size of 65 mmx8 mm, whereas the specimen RLBRB2 had a core size of 60 mmx8 mm. The yield strength of core steel was 300 MPa obtained from the coupon test. Additional 25 mm thick steel plates were welded at both ends of specimens in order to facilitate their connections with the test set-up. Fabrication of Specimens In this study, the core segments of specimens were restrained by concrete inside a detachable casing. Instead of single (closed) hollow casings as used in the conventional BRBs, two detachable U-shaped casing with lips prepared by welding 6 mm thick steel plates were used in this study. As shown in Figure 2(a), sufficient bolt holes were left in the lips, which helped in assembling two halves of the casings to act as a single restraint all around the core segment. The concrete used to fill the casing had a characteristic compressive cube strength of 25 MPa. A slot in the shape of steel core was left in the concrete during the casting. The gap between the core and concrete restraint was around 2 mm on either side of the core. Debonding material (grease) was used between the steel core and the hardened concrete. Figure 2(b) shows the final configuration of the specimen after assembling the two U-shaped casings. Generally, the infill concrete and the casing of BRBs do not suffer any damage under the cyclic loading and the steel core may only reach its fracture limit in the extreme events. In case of the detachable casing, it is possible to replace the damaged steel core with an undamaged one using the same casing and concrete. Thus, the detachable casing facilitates the faster, simple and easy replacement of the damaged BRB and allows the post-earthquake inspection of the steel core after a seismic event.

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Figure 2. (a) Detachable U-shaped steel casing used for the preparation of test specimens, (b) Final configuration of RLBRB specimen after assembling of two casings.

Test Setup Figure 3 shows the test set-up used in this study. One end of the specimen was attached to a

reaction block, whereas the other end was connected to a servo-controlled hydraulic actuator. The hydraulic actuator had a force rating of 250 kN and stroke length of 125 mm. The load cell and linearly-varying differential transformer (LVDT) in the actuator were used to monitor the axial load and axial displacement of the test during the testing. The axes of hydraulic actuator and the test specimens were aligned along a straight line in order to avoid the bending action on the end plates of the specimens. Roller bearings and lateral supports were placed at the actuator end of the specimen, which restrained the vertical displacement of the specimens and supported the self-weight of the actuator.

Figure 3. Test set-up used in this study. Loading Protocol Two types of loading protocols were used in the testing of RLBRB specimens. Loading type 1 consisting of both standard and low-cycle fatigue loading requirements as per FEMA 450 [12] and ANSI/AISC 341-10 [13] specifications was applied to the specimen RLBRB1. In this loading protocol, the value of axial yield displacement (Δy) in the displacement cycles was computed based on the geometry and material properties of the specimen. It consisted of 6 cycles of Δy (0.45 mm) followed by 4 cycles of 2.5Δy (1.125 mm), 4 cycles of 5Δy (2.25 mm), 2 cycles of 7.5Δy (3.375 mm), 5 cycles of 10Δy (4.5 mm), 2 cycles of 12.5Δy (5.625 mm), 2 cycles of 15Δy (6.75 mm) and 40 cycles of 7.5Δy (3.375 mm) as shown in Figure 4(a). The first 25 cycles aimed at to meet the maximum ductility demand of 15 (the requirement in the event of DBE level of ground motion), with respect to the axial yield displacement of the RLBRB. The remaining 40 cycles resulted in an equivalent cumulative inelastic displacement of 260. Thus, the entire loading protocol resulted in a total cumulative inelastic displacement demand of 390 on the specimen RLBRB1.

Loading protocol specified by ANSI/AISC 341-10 [13] is applicable to the conventional BRBs in which the length of core segments vary from 0.6 to 0.8 of the total brace length. However, for the reduced-length BRB as investigated in this study, the length of yielding (core) segment varies approximately from 0.2 to 0.3 of the total lengths, assuming the length of BRB in a composite brace is only one-third of total length, all other parameters remaining constant. Thus, the Loading type 1 was later modified based on the expected displacement demand and termed as Loading type 2, which was used in the testing of the specimen RLBRB2. In order to achieve a desired level of lateral drift in the prototype braced frame, the displacement values in the loading history were multiplied by a suitable factor. Loading type 2 consisted of 6 cycles of

Δy (0.45 mm), 2 cycles of 3Δy (1.35 mm), 4 cycles of 7.5Δy (3.375 mm), 4 cycles of 15Δy (6.75 mm), 2 cycles of 20Δy (9 mm), 2 cycles of 25Δy (11.25 mm), 2 cycles of 30Δy (13.5 mm) and 2 cycles of 45Δy (20.25 mm) as shown in Figure 4(b). Thus, this loading protocol was intended to impose a displacement ductility of 45, which was equivalent to a ductility value of 15 for the conventional BRB. Low-cycle fatigue cycles were intended to achieve a cumulative ductility demand of 600.

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Figure 4. Displacement history used in this study; (a) Loading type 1 (b) Loading type 2.

Test Results Slow-cyclic loading as per the loading protocols was applied to the specimens. The real-time load-displacement response of specimens was monitored using an automatic data acquisition system. The main parameters studied are overall performance, hysteretic behavior, load carrying capacity, displacement ductility, and the hysteretic energy dissipation potential of the RLBRB specimens. The observed parameters for each specimen are discussed in the following sections. Overall Behavior As mentioned earlier, the specimen RLBRB1 was subjected to Loading type 1 in which the amplitude of displacement cycles was smaller than Loading type 2. Figure 5(a) shows the final

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state of test specimen RLBRB1 after the test. No damage to the core segment or concrete was observed till the end of the test. Slight buckled configuration of the core segment suggests that the specimen could have undergone a few more cyclic excursions prior to its failure. Figure 5(b) shows the final state of the specimen RLBRB2 after the testing. The specimen behaved as expected till 20th cycles of the excursion without any instabilities. At 21st cyclic excursion level, the fracture of welding connection at the junction of steel casing resulted in the crushing of concrete. The local concentration of compression buckling of the steel core was observed near the transition regions of the specimens as shown in the Figure 5(b). Due to this local instability, the fatigue cycles were not applied to the specimens.

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Figure 5. Overall state and deformed configurations of core segments of specimens after testing;

(a) Specimen RLBRB1 (b) Specimen RLBRB2. Hysteretic Response Axial load vs. displacement response (i.e., hysteretic response) of the specimen RLBRB1 is shown in the Figure 6(a). Specimen RLBRB1 showed slightly asymmetric hysteretic response under the Loading type 1. The specimen yielded in compression during the preliminary load cycles prior to the application of the Loading type 1. Minor pinching in the hysteretic response was noticed for the specimen RLBRB1. The specimen sustained the entire Loading type 1 without any failure. Since the amplitude of displacement cycles was smaller than that in Loading type 2, the maximum strain level experienced by the specimen was 0.67% only. As stated earlier, the specimen could have undergone few more cyclic excursions since no instability was noticed till the end of the tests. The hysteretic response of the specimen RLBRB2 under Loading type 2 is shown in Figure 6(b). Nearly symmetric hysteretic response in tension and compression was noticed. The compression strength of the specimen was about 20% higher than the tension strength. No reduction in the strength was noticed till the ductility level of 30 beyond which the welding failure of casing was noted as discussed earlier. The maximum displacement reached by the specimen prior to failure of casing was 12.5 mm that

corresponds to an axial strain level of 2.1% for a total length of specimen as 600 mm.

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Figure 6. Axial load-displacement response; (a) Specimen RLBRB1, (b) Specimen RLBRB2. Backbone Curves Figure 7(a) shows the comparison of the backbone curves of axial load-displacement response of the specimens. Both specimens exhibited similar bilinear axial load-displacement response. As expected, the specimen RLBRB1 showed the higher axial stiffness as compared to the specimen RLBRB2 because of the larger cross-section. Further, the post-yield stiffness of both specimens was nearly parallel. The maximum axial load carries by the specimen RLBRB1 was 200 kN (compression), whereas the corresponding value of maximum axial load carried by the specimen RLBRB2 was about 240 kN (compression). Energy Dissipation Response The hysteretic energy dissipated by the specimens at various axial displacement levels during the testing in shown in Figure 7(b). The specimen RLBRB1 exhibited a maximum hysteretic energy of 2.76 kNm at a ductility level of 15. Similarly, the RLBRB2 dissipated a maximum hysteretic energy of 7.2 kNm at a ductility level of 30. Both specimens showed excellent hysteretic energy dissipation potential. The total cumulative hysteretic energy dissipated by the specimen RLBRB1 under Loading type 1 is shown in Figure 8 (a). The exponentially-varying segment of the curve represents the energy dissipated during the standard cycles, whereas the linearly-varying segment of the curve corresponds to the energy dissipated in the low-cyclic fatigue displacement cycles. The specimen dissipated a total cumulative energy of 44.73 kNm till the end of testing under the Loading type 1, out of which about 22.3 kNm of energy was dissipated till 27th cycles of the standard gradually-increased reversed-cyclic loading and the remaining energy was dissipated in the low-cyclic fatigue cycles. Figure 8 (b) shows the cumulative energy dissipated by the specimen RLBRB2. Since the low-cyclic fatigue displacement cycles were not applied to the specimen, only exponential segment of curve was noticed. The maximum value of energy dissipated by the RLBRB2 specimen was 39.9 kNm.

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Figure 8. Cumulative hysteretic energy dissipation response; (a) Specimen RLBRB1, (b) Specimen RLBRB2.

Conclusions

Following conclusions can be drawn from the present study: − RLBRB specimen exhibited similar behavior as the conventional BRBs with stable and non-

degrading hysteretic response under cyclic loading. The compression over-strength factor (i.e., ratio of yield strengths in compression and tension) for RLBRBs was about 1.2.

− The RLBRB specimens reached a maximum displacement ductility of 30 without any failure to the steel core. This value can be used in the design process for the proportioning of RLBRB portion of the composite brace.

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− Detachable casings can be used to replace the closed (single) hollow casing to achieve the same hysteretic response of BRBs. However, the design and detailing of detachable casing is extremely important in order to achieve the desired performance of steel core elements.

Acknowledgments

Authors would like to acknowledge the structural engineering laboratory staff for their help in the experiment.

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