Seismic performance of steel moment frame office buildings with square concrete-filled steel tube gravity columns

Highlights

• A dual moment frame lateral-force resisting system is proposed.

• The system relies on CFT columns to provide additional strength and stiffness.

• 1- to 4-story buildings with the dual CFT system were less susceptible to collapse.

• CFT columns were most effective in 1-story and 2-story office buildings.

Abstract

A dual lateral-force resisting system consisting of a primary lateral-force resisting system and secondary concrete-filled steel tube (CFT) columns placed in the gravity framing is presented in this paper. The dual CFT system concept relies on the primary lateral-force resisting system to supply the main lateral strength, while additional lateral strength and robustness is provided by the CFT columns. To explore the viability of the concept, the predicted seismic performance of 1-story, 2-story, and 4-story office conventional buildings, with perimeter steel moment frames and wide-flange gravity columns, was compared to the performance of the same buildings but employing square HSS columns filled with unreinforced concrete. The analyses predicted that, compared to conventional buildings, buildings with the dual CFT system were 20–83% less susceptible to seismic collapse, depending on the strength and ductility of the primary moment frame, the orientation of the wide-flange columns in the conventional building, and the number of stories. Using high-strength, thick, or slightly larger CFT columns did not significantly improve collapse safety. Buildings with the dual CFT system generally had improved seismic performance, depending on the moment frame design, the number of stories, and the intensity of the ground shaking. Buildings with the dual CFT system had up 45% lower repair costs, up to 64% shorter repair time, and a lower probability that the building would be deemed unsafe.

Introduction

Dual lateral-force resisting systems consisting of moment frames acting with a secondary system, e.g. ASCE 7-16 Table 12.2-1 [1], have two key characteristics that may enhance seismic performance compared to single systems. First, dual systems explicitly can provide increased redundancy [2], allowing load to be transferred through alternate paths that are intended to be part of the lateral-force resisting system. By contrast, a single system may lead to unintentional redistribution of load. In fact, the potential for beams and columns designed to support gravity loads (“the gravity framing system”) to act as dual or “reserve” lateral-force resisting system, intended or unintended, has been recognized for several years [3], [4], [5], [6], [7]. Second, dual systems may provide added safety against collapse and added resistance to structural and non-structural damage compared to a single system [2]. The latter characteristic is the motivation for the dual system explored in this study.

The effectiveness of a dual lateral-force resisting system that utilizes the gravity framing as the secondary element in the system depends on several factors, including building height, structural configuration, location of column splices, beam-to-column connection details, and the seismic zone [3], [4]. Depending on these factors, gravity framing may help reduce residual story drift [5], improve collapse safety [3], [6] and improve serviceability [7]. For a dual system that involves a steel moment frame (MF), a previous study [4] showed that the gravity system needs to be capable of resisting at least 10% of the prescribed seismic forces in order to be effective. Dual systems designed according to ASCE 7-16 provisions are required to resist 25% of the prescribed seismic forces. In many cases, however, the lateral capacity contribution of a conventional gravity framing system is modest (on the order of 10–30% percent of the lateral-force resisting system). Furthermore, in many cases soft-story mechanisms limit the ability of the gravity framing to improve global collapse resistance of the building. Thus, previous work indicated that a gravity framing dual system concept may be most advantageous in moderate-seismic zones, as opposed to high seismic zones.

Concrete-filled steel tube (CFT) columns integrated with the gravity framing system may provide a pragmatic approach to both increase the lateral strength of the gravity framing system and improve the global collapse mechanism. Prior research of moment frames with CFT columns and wide-flange beams and CFT columns acting alone [8], [9] has demonstrated that CFT columns have significant flexural strength, axial compressive strength, and significant strength when subjected to combined axial and flexure loads, while at the same time contributing toward a cost-effective construction method. Perhaps more important in the context of a dual system that utilizes the gravity framing system to deliver additional lateral strength, CFT columns have excellent flexural strength in two orthogonal directions, whereas wide-flange (W-shape) sections have poor flexural strength in the weak direction (minor-axis bending).

The objective of this analytical study was to explore the potential of a dual CFT system, consisting of a primary steel MF and a secondary system utilizing CFT columns in the gravity framing system, for low-rise (1-story and 2-story) and mid-rise (4-story) office buildings located in low and moderate seismic zones. Three types of MFs were considered: (1) a non-ductile steel MF, (2) a ductile steel MF designed for the lowest spectral acceleration in ASCE 7-16 [1] Seismic Design Category (SDC) D, and (3) a ductile steel MF designed for the highest spectral acceleration in SDC D. The focus of the study was on square CFT columns using HSS 14 × 14 × 5/16 with a steel minimum yield stress, $F_y$ equal to 317 MPa (46 ksi) and a concrete compressive strength, $f_c\prime$ equal to 34.5 MPa (5 ksi). The CFT section was selected based on providing a flexural strength that is comparable to the flexural strength of typical wide-flange columns used in the gravity framing system. The effect of high-strength CFT columns, thick CFT, and slightly larger CFT columns was also evaluated for 4-story buildings. Rectangular and circular HSS were not considered in this study. The seismic collapse safety and seismic performance of the buildings with CFT columns in the gravity framing system was predicted and compared to the performance of “conventional” buildings (with wide-flange gravity columns) using adaptations of the FEMA P-695 methodology [10] to evaluate collapse safety, and the FEMA-P58 methodology [11], [12], [13] and companion software, PACT (Performance Assessment Calculation Tool) [14] to evaluate repair cost, repair time and the probability of unsafe placarding after an earthquake.