Eleventh U.S. National Conference on Earthquake Engineering Integrating Science, Engineering & Policy June 25-29, 2018 Los Angeles, California

AN INNOVATIVE SELF-CENTERING CONNECTION FOR CIRCULAR

CONCRETE FILLED STEEL TUBES

Y. Gao1 and R. Leon2

ABSTRACT The conceptual application of self-centering systems has gained traction over the past few years both because of extensive experimental testing that confirmed the theoretical advantages of these systems and the advent of the Next-Generation Performance Based Seismic Design approach. The latter has freed designers to pursue innovative strategies that fall outside the conventional prescriptive approaches. In this context, this paper explores the combination of a self-centering connection in conjunction with circular concrete filled tubes for use in the design of special composite moment-resisting frames (CSMF). For collapse resistance, the vertical load carrying capacity of the structure must be maintained; circular concrete filled tubes (CCFT) have been shown experimentally to be the toughest structural element available today. For resiliency, the structure should have minimal residual deformations; self-centering connections are an ideal solution for this performance requirement. Conceptually their combination is an ideal structural solution. However, there is an inherent difficulty in connecting circular tube columns to conventional wide flange steel beams as this type of connection requires extensive shop and field welding of and to through or wrap-around plates. To address that issue, this research proposes an innovative bolted self-centering partially-restrained connection (SC-PRC) configuration to connect CCFT columns and wide flange steel girders with shape memory alloy through bolts capable of providing self-centering capacity. A robust 2D simplified spring computational model is described in detail and its effectiveness proven through system analysis, including nonlinear dynamic response history analyses. Results indicate that CSMFs with well-designed SC-PRCs can exhibit excellent self-centering performance, and possess the ability to protect structural components from damage under moderate earthquakes.

1 PE, Ph.D., Hellmuth Obata + Kassabaum, New York, NY 10018 (email: ygao63@vt.edu) 2David H. Burrows Professor, Via Dept. of Civil Engineering, Virginia Tech, 102D Patton Hall, Virginia, VA 24061 Gao Y, Leon R. An innovative self-centering connection for circular concrete filled steel tubes. Proceedings of the 11th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Los Angeles, CA. 2018.

An Innovative Self-Centering Connection for Circular Concrete Filled

Steel Tubes

Y. Gao1 and R. Leon2

ABSTRACT The conceptual application of self-centering systems has gained traction over the past few years

both because of extensive experimental testing that confirmed the theoretical advantages of these systems and the advent of the Next-Generation Performance Based Seismic Design approach. The latter has freed designers to pursue innovative strategies that fall outside the conventional prescriptive approaches. In this context, this paper explores the combination of a self-centering connection in conjunction with circular concrete filled tubes for use in the design of special composite moment-resisting frames (CSMF). For collapse resistance, the vertical load carrying capacity of the structure must be maintained; circular concrete filled tubes (CCFT) have been shown experimentally to be the toughest structural element available today. For resiliency, the structure should have minimal residual deformations; self-centering connections are an ideal solution for this performance requirement. Conceptually their combination is an ideal structural solution. However, there is an inherent difficulty in connecting circular tube columns to conventional wide flange steel beams as this type of connection requires extensive shop and field welding of and to through or wrap-around plates. To address that issue, this research proposes an innovative bolted self-centering partially-restrained connection (SC-PRC) configuration to connect CCFT columns and wide flange steel girders with shape memory alloy through bolts capable of providing self-centering capacity. A robust 2D simplified spring computational model is described in detail and its effectiveness proven through system analysis, including nonlinear dynamic response history analyses. Results indicate that CSMFs with well-designed SC-PRCs can exhibit excellent self-centering performance, and possess the ability to protect structural components from damage under moderate earthquakes.

Introduction The continuing migration from rural to urban areas and the concentration of the latter in areas of great exposure to natural disasters requires that researchers and engineers to work together to look for more effective and economic solutions for promoting community resilience [1]. The Next-Generation Performance Based Seismic Design (PBSD) concepts have been proposed [2, 3], which allow the design of new structures and the retrofit of existing ones to be undertaken under a series of performance targets mutually agreed by the engineer and the stakeholders [4]. Damage mitigation and speedy recovery are two of the most important design objectives consistent with the next-generation design concepts. The implementation of a self-centering systems fits well within the PBSD principles, as a well-designed self-centering system is capable

1PE, Ph.D., Hellmuth Obata + Kassabaum, New York, NY 10018 (email: ygao63@vt.edu) 2David H. Burrows Professor, Dept. of Civil Engineering, Virginia Tech, 102D Patton Hall, Virginia, VA 24061 Gao Y, Leon R. An innovative self-centering connection for circular concrete filled steel tubes. Proceedings of the 11th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Los Angeles, CA. 2018.

of reducing the residual deformations and limit them to well below the 0.5% threshold usually assumed for building. In addition, after a moderate or even severe earthquakes, the probability of losing the building functionality will be reduced and can be recovered rapidly. Currently, the most common mechanisms to achieve self-centering capacity include: (1) post-tensioned wires or bars [5-7]; (2) rocking mechanism [8]; and (3) the combination of the above two [9, 10]. These mechanisms require that the self-centering components themselves must be kept elastic during the entire loading history, especially for the structures with post-tensioned strands. Any yielding in these post-tensioned components could cause the self-centering capacity to diminish or disappear in several bays of the frame at the same time because usually the same post-tensioned strands will go across several girders and connect them together [5]. Another applicable method to obtain the self-centering capacity is to utilize the shape memory alloys (SMA), i.e. Nickel-Titanium based or Nitinol based alloys [11, 12], which are a group of materials exhibiting superelastic behavior. This behavior is characterized by significant deformation capacity with negligible residual deformation after unloading as the deformation is the result of reversible phase transformations between the martensitic and austenitic phases of the material [13]. The phase transformation can be triggered by either applied stress or temperature. Under a constant temperature, a SMA in the austenitic phase can transform to its martensitic phase at a specific stress and reversely transform back to austenitic phase at a lower without obvious residual strain as long as the maximum reached strain less than certain threshold of about 6%~8% [13-15]. During the loading-unloading process, a large plateau and a closed loop, or flag shaped stress-strain curve, will be generated. A strain hardening phase will initiate when the phase transformation to martensite is completed. Research indicate that SMA possesses very good fatigue and corrosion resistance as well [16]. These characteristics, including large deformation capacity with full recovering ability, strain hardening, energy dissipation through hysteresis loops, and stable cyclic behavior, make SMA an ideal material to supply reliable self-centering capacity for the lateral resisting systems in seismic design. Although the application of SMA in civil engineering is still not common due to its high price and the lack of the relevant research, the development and marketing of SMA materials with a lower price is changing this limitation [17] and the feasibility of applications of SMA in civil engineering has already been proved by many researchers for both wires and large diameter bars [14, 15, 18-21]. Since proposed in the early 1900s, concrete filled steel tubes (CFTs) have been considered as an ideal structural component to resist large seismic loadings. Their effectiveness stems primarily from their superior characteristics of combining large axial and flexural strength with high ductility capability, especially for the Circular-CFTs (CCFTs). Compared with their rectangular counterparts, CCFTs can supply much higher confinement effects on the concrete core and increase its compressive strength and ductility significantly [22, 23]. Under cyclic loadings, CCFTs will present a strong and ductile strain-hardening behavior after the peak strength is achieved and be able to provide stable hysteresis curves as compared to a softening post-peak behavior for RCFTs [24]. Furthermore, in addition to the required large stiffness to resist the lateral drift for the mid- to high-rise buildings in the high seismic zones, analysis results indicate that CCFTs possess the best fire resistance performance among all different cross sectional shapes [25]. However, the benefits of CCFTs in seismic design have not been maximized mainly due to the high cost and complexity of connecting conventional structural components onto the curved surface of the CCFT columns. The current connection configurations are usually designed to be fully restrained connections (FRCs) with complete full penetration field welds and accompanying QA/QC issues in field [26, 27]. Connection brittle failures triggered by initial micro-cracking at

welds in the Northridge and Kobe earthquakes had led to significant changes in design and QA/QC procedures and considerably increased the cost [28, 29]. Energy dissipation capacity is another key feature to evaluate the system and component seismic performance. Systems with FRCs can provide only limited energy dissipation from the connections themselves; most of energy will have to be dissipated by yielding the connecting beams or columns due to the limited connection deforming capacity. This will cause considerable damage and residual deformation in structures and result in significant cost and time for repair [30, 31]. A number of researchers have proved that well-designed partially restrained connections (PRCs) can provide equal or even better cyclic behavior than their FR counterparts and exhibit better structural strength reverse capacity with a lower probability of brittle failure [32-35]. In this paper, an innovative self-centering partially restrained connection configuration (SC-PRC) will be proposed for connecting CCFT columns and wide flange steel girders with the application of SMA for the self-centering capacity. The aim is (1) to provide another reliable type of connection for CCFT columns to prevent collapse of structures under severe earthquakes, (2) to enable the structures to recover their original undeformed shape by self-centering means, and (3) to preserve their functionality after small, moderate or even severe earthquakes. Structures with SC-PRCs are likely to come into service immediately after minor or moderate earthquakes. Their use will help to greatly reduce injuries, downtime and economic losses.

Innovative Self-Centering Connection Configuration In order to create a reliable and economic type of connection for the CCFT columns for the US market, the new proposed SC-PRC is designed to have the following characteristics [27]:

• Completely avoid welding on-site to reduce the high-cost due to QA/QC procedure, all required welding work will be finished in shop with a good quality control.

• Minimize the use of any demand critical welds to reduce the probability of brittle failure. • Utilize end plate connection configuration which has been proved to be one of the most

robust types of connection through the tests of the SAC project. • Adopt the reduced beam section (RBS) as a fuse to limit the maximum strength demands. • Combine both SMA rods for self-centering capacity and steel rods for required strength

and stiffness. The use of through rods results in a rapid assemblage for low-cost erection. • Supply large deformation and overstrength capacities by only tensioning the through rods

without cyclic tension-compression deteriorations being introduced. • Control the residual deformation to be less than 0.5% for building demolition under MCE

level earthquakes by ‘tuning’ the relative strength between the connection and the RBS. • Prevent collapse and limit the ultimate story drift by restoring greater strength and initial

elastic stiffness after reaching the maximum design target under the extreme earthquakes beyond the MCE level and shifting the plastic hinges into the RBSs in girders.

• Increase energy dissipation through inelastic mechanisms in connection itself. • Potentially expand into biaxial bending moment connection for 3D-moment frames. • Easily and economically to be fabricated, erected, maintained, and replaced as necessary.

Connection Configuration In Figure 1, both the new proposed interior and exterior SC-PRCs are presented, and a cut-away

view of the interior connection is shown in detail in Figure 2. Generally speaking, the new proposed uniaxial bending connection is composed of several main components as follows:

• A CCFT column as the main structural lateral resisting system component. • A square HSS section to provide a flat surface for the girder with an end plate connection. • (1) or (2) WF steel girders with an extended end plate and RBS. • (4) or (2) SMA or steel through rods based on the connecting girder size on each four rod

layers as the main tension components of the SC-PRC. • Two inner diaphragms and the infilled high strength expansive grout between tubes to

transfer the end plate compression into the CCFT column. • Top and bottom cover plates as the confining components for the infilled grout. • Stiffeners on girder to shift the plastic hinge away from the end plate into the RBS region

and distribute the forces from the girder into the end plate more evenly.

Figure 1. Schematic connection configuration of the interior and exterior SC-PRC for uniaxial

bending [27].

Figure 2. Cut-away view of the interior SC-PRC for uniaxial bending [27].

The assembling procedure of the new proposed connection shown in Figure 2 is relatively simple. Most of the assembling work will be finished in shop and the only two remaining steps on site are filling concrete in the CCFT column and installing the through rods with the desired

pretension. The grout can be filled in the shop prior to shipping the connection to the construction site. Either temporary through rods with any reliable material or permanent nonstructural pipes could be used for making the through holes during the concrete and grout filled later before the permanent through rods are installed. The in-shop work is primarily welding. With only fillet welds with small sizes required [27]. The only potential location for full penetration is between the steel girder and the end plate. The ease of fabrication and rapid erection will reduce the entire project cost greatly because these two tasks are usually the costliest parts of most projects.

SC-PRC Computational Model and Behavior Although computational power is not a major design concern today, it is still worthy to reduce the unnecessary computational cost for not only expediting the research process but also promoting the potential commercial use. In order to reduce the computational cost of the new proposed SC-PRC for structural analysis, a 2D simplified spring model (2D-SSM) has been developed. It is based on two primary deforming mechanisms: (1) end plates rotations and ancillary deformation and (2) elongation of the through rods under the beam end moments. The through rods provide the connection resisting moments by tension forces accordingly shown in Figure 3. To simplify the 2D-SSM, three essential assumptions are made: (1) the end plate is stiff enough to generate a rigid body rotation based on a yield line at the level of the girder flange in; (2) the moment contribution from end plate yielding to the total connection moment capacity is negligible; and (3) the inner diaphragms and infilled grout are able to provide a very rigid support so that the inward displacement under the large end plate compression is insignificant. A detailed preliminary design procedure for the new proposed SC-PRC has been well developed based on the above three assumptions; a comprehensive finite element analysis in ABAQUS has been executed for one SC-PRC designed based on the new proposed preliminary design procedure and the results have indicated the validities of the applied assumptions and the expected deforming mechanisms [27].

Figure 3. Deformation mechanisms for interior and exterior SC-PRC under uniaxial bending

[27]. Figure 4 presents the proposed 2D-SSM for uniaxial bending. Its components are described in detail by Gao [27] and are briefly introduced as follows:

• Rods element: only tensile stiffness is defined to combine both SMA and steel rod behaviors with self-centering and pretension effects; the compressive stiffness is released

to simulate the rod sliding behavior caused by the rod residual deformation after yielding.

• Gap element: assigned with a large compressive stiffness to simulate the rigid support for the end plate from the inner diaphragms and infilled grout, and zero tensile stiffness to simulate the end plate separation from the rectangular tube under the beam end moments.

• Shear element: assigned with a large transverse stiffness to transfer the beam end shear into the CCFT column, and both axial and bending stiffness are released.

• End plates: represented by the two shorter vertical black lines; defined as rigid elements. • Framing components: CCFT column and WF girders defined as elastic elements. • Plastic hinges: simulate the lumped plastic behaviors of the framing components.

Figure 4. 2D simplified spring model (2D-SSM) for uniaxial bending [27].

The two beam end moments shown in Figure 4 don’t have to be equal nor in the same direction all the time; when either end moment becomes zero, the model simulates an exterior connection. Compared with the connection deforming mechanisms in Figure 3, the 2D-SSM is obviously able to capture all the key features of its prototype including (1) the end plate rotation with through rod elongation, (2) through rod self-centering, pretension, and sliding effects, (3) the plastic behavior of the structural framing members, and (4) the three assumptions mentioned above. The only inherent flaw is that the end plate tip where the imaginary compression flange is closed to rotates into the connection slightly, this effect is believed to be inappreciable according to a discussion at full length [27]. Connection Cyclic Behavior In order to verify the robustness of the new proposed 2D-SSM for system analysis, an individual SC-PRC with a simple beam-column model is tested under the two opposite cyclic displacement loadings at two beam ends in SAP2000 (Figure 5). A 14’-0’’ long 24’’x1.375’’ CCFT column is pinned at bottom end and restrained horizontally at the top; two 16’-0’’ long WF W33x130 girders are connected to the middle point of the CCFT column by one SC-PRC, which is composed of eight 1.27’’ Dia. SMA rods with yield strength of 67ksi with 30% pretension and eight 1.128’’ Dia. Gr.50 steel rods with 50% pretension. All rod length is 32.25’’. The connection, as an example, is designed based on the new proposed preliminary design method and each component of the 2D-

SSM has been well defined accordingly [27].

(a) Analysis model (b) Displacement loading history.

Figure 5. Analysis model and loading history for testing the 2D-SSM cyclic behavior [27]. Figure 6 illustrates the cyclic behavior of the 2D-SSM under the loading history above with elastic columns and beams. This connection moment rotation (M-R) behavior is totally new and has never been shown before as far as these authors can tell. It is obvious that this M-R behavior possesses three backbones in either positive or negative moment range. The upper backbone curve represents the strength supplied by both SMA and steel rods; the middle backbone curve represents the strength supplied by only SMA rods during loading; and the lower backbone curve represents the strength supplied by only SMA rods during unloading. The sloped lines between the backbone curves stand for either the loading of the steel rods or the unloading of SMA or steel rods. A much more comprehensive discussion and step-by-step explanation of this M-R behavior has been done by Gao [27]. The six key points shown in Figure 6 are explained briefly as follows:

• A: ultimate capacity determined by the preliminary design procedure. • B: maximum SMA strain reaches the threshold of 5% for no residual deformation. • C: maximum capacity supplied by only SMA rods at the rotation of point B. • D: capacity supplied by only SMA rod pretension with zero rotation. • E: capacity supplied by all rod pretension with zero rotation. • F: capacity supplied by only SMA rods with yielding strength.

Seismic Performance of CSMF with SC-PRCs

The 2D-SSM has been proven to be robust and effective to capture all the key characteristics of the individual connection behavior of the new proposed SC-PRC [27]. In order to promote the application of the new proposed SC-PRC in seismic design, nonlinear dynamic response history analysis has been executed in OpenSees for a 3-span (96’ wide) composite special moment frame (CSMF) under large live load of 100psf with five stories above grade and one story basement (84’ high). Two CSMFs with CCFT columns and SC-PRCs or conventional moment connections have been designed to be consistent with all the current design codes. The scaled Chi-Chi earthquake has been used for the analysis as shown in Figure 7 and part of the corresponding nonlinear dynamic analysis results are presented in Figure 8. The red lines represent the conventional CSMF

responses and the blue lines represent the responses of the CSMF with SC-PRCs. From the roof drift response, one can easily find that the application of the new proposed SC-PRC can effectively reduce the CSMF’s residual deformation. Although a greater roof drift magnitude and a longer oscillation present, other alleviation methods (i.e. dampers) can be combined in use. Further research with this combination needs to be found out in the future. One beam plastic hinge M-R response has also been illustrated in Figure 8 (b). It is clear to see that the application of the SC-PRC can protect the structural girder from severe plastic damage greatly.

Figure 6. Cyclic behavior of the 2D-SSM with elastic structural framing members [27].

Conclusions

This paper introduces an innovative self-centering partially restrained connection for seismic design, consisting of CCFT columns and end plate connections with through SMA rods. The new proposed SC-PRC can provide a more reliable connection without on-site welding and QA/QC procedures required for other types of connections to CCFT columns. Seismic lateral resisting systems with the new proposed SC-PRCs possess desired characteristics including self-centering, collapse prevention, structural component protection, high resilience, rapid erection, and low-cost construction and maintenance. A 2D simplified spring model has been developed for structural analysis and promotion of the potential commercial use. Analysis indicates that the 2D-SSM is robust and effective to simulate its prototype accurately. Nonlinear dynamic response history analysis has been executed for both conventional and self-centering composite special moment frames with CCFT columns. Results prove that the new proposed SC-PRC is able to reduce the structural residual deformation greatly and protect the structural girder from severe plastic damage effectively. Further research may be necessary to alleviate the relative large drift magnitude and the long time oscillation for the self-centering system.

Figure 7. Scaled acceleration history of the east component of Chi-Chi earthquake [27].

(a) Roof drift response (b) One beam plastic hinge M-R response

Figure 8. Roof drift response and one beam plastic hinge moment rotation response [27].

Acknowledgments The financial support of the Via Department of Civil and Environmental Engineering, the David H. Burrows Professorship and the Advanced Research Computing at Virginia Tech for this research are gratefully acknowledged. Any opinions expressed in this study are those of the authors alone.

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