Steel Moment-resisting Connections Research Articles

Experiments on cold-formed steel moment-resisting connections with bolting friction-slip mechanism

This paper presents an experimental investigation into the cyclic behaviour of cold-formed steel (CFS) moment-resisting (MR) beam-to-column connections utilising a friction-slip mechanism within a web-bolted connection arrangement. The bolting slip back and forth movements are accommodated through slotted holes to dissipate seismic energy within the CFS MR connections. Nine full-scale experiments were conducted on connections with and without slip for comparison purposes. The slip connections were designed to undergo slip prior to the initiation of local buckling in the CFS beam. This avoids premature local buckling, which could significantly degrade strength, particularly for connections with lower thickness beams. The slip connections, as a result, produce a greater energy dissipation capacity and ductility factor by up to 79% and 2.5 times, respectively, than that of the corresponding slip-resistant connections.

Seismic design of widening flange connection with fuses based on energy dissipation

Innovative steel moment-resisting connections with replaceable ductile and dissipative low-yield-point (LYP) steel cover plates provide structural resilience after strong seismic events via their behavior as multifunctional structural fuses. To maximize the structural fuse effect without excessively weakening the bearing capacity, this connection is improved by widening beam flange plates close to the beam-to-column joint. In this study, the influences of key factors on the seismic performance, the damage control mechanism, and the behavior of fuses are evaluated and quantified via extensive numerical investigations. A seismic design procedure based on energy dissipation performance is summarized, and a corresponding design example is presented. Analytical results demonstrate that a larger degree of weakening can effectively extend the active coverage of structural fuses. With the increase of the degree of widening of the beam flange, a desirable fuse effect can be realized with a smaller required degree of weakening. However, once the degree of widening of the beam flange plates is sufficiently large, further widening the beam flange has only a minor influence on enhancing the fuse effect of the LYP steel cover plates. The increase of the plastic resistance moment of the cross-section of the beam or the splicing position results in the requirement of a larger degree of weakening of the connection to achieve an equivalent fuse effect. Consequently, to ensure the acceptable bearing capacity of the connection, a larger beam height requires a relatively smaller splicing position or a larger degree of widening of the beam flange plates, and vice versa.

Steel Structures Research Update: Steel Diaphragm Innovation Initiative

The Steel Diaphragm Innovation Initiative (SDII) is a multiyear academic-industry partnership to advance the seismic performance of steel floor and roof diaphragms in steel buildings is highlighted. The team includes AISC T.R. Higgins Lectureship Award and AISC Milek Fellowship Award winners for topics ranging from developments in long-span composite slabs to buckling-restrained braced frames to continuity plate detailing for steel moment-resisting connections. The team’s motivations for creating SDII stemmed from issues with respect to the knowledge base for steel diaphragm performance, codes and standards, as well as missed opportunities for advancements in seismic performance-based design. The available research on steel diaphragms was primarily focused on the strength of isolated systems; little was known about ductility or whole-building performance. The team developed a five-year case and plan to “advance the seismic performance of steel floor and roof diaphragms utilized in steel buildings through better understanding of diaphragm-structure interaction, new design approaches, and new three-dimensional modeling tools that provide enhanced capabilities to designers utilizing steel diaphragms in their building systems” (SDII, 2017). The work includes providing research support for much-needed revisions to proposed seismic codes and standards for steel diaphragms. SDII is also working on innovative steel diaphragm solutions for efficient, robust and resilient steel building systems.

Development of cold-formed steel moment-resisting connections with bolting friction-slip mechanism for seismic applications

This paper presents an investigation into a friction-slip mechanism in bolted joints in cold-formed steel (CFS) moment-resisting connections suitable for seismic areas. The aim is to achieve higher ductility and energy dissipation capacity through an appropriately designed bolted connection, enabling activation of bolt slip, acting as a fuse mechanism, postponing the initiation of non-ductile local buckling in the CFS beams. By means of validated finite element analysis (FEA), both monotonic and cyclic performance of CFS connections comprising two types of square (SB) and circular (CB) bolt group arrangements, with four types of beam sections, are studied comparatively without and with slip at various levels. It is shown that the connections with slip provide higher energy dissipation capacity by up to 75% compared with that of the connections without slip. A design approach to predict an appropriate level for the connection slip moments to be initiated before the beam buckling moment is introduced using the direct strength method (DSM). On comparing the bolt forces obtained for the SB and CB groups, the CB connections produce a more uniform bolt-group force distribution which is closer to the idealised method; while the SB connection encounter a significant delay of up to 30% in activation of bolt group slip which could lead to unfavourable beam local buckling. Furthermore, to avoid an unfavourable hardening effect due to the bolts bearing action, slotted holes are used with a recommended length obtained for the cyclic movements of the bolts in the range of the designed connections. The instantaneous centre of rotation results show that the connections with slip have less deviation from the idealised centre of rotation than the connections without slip.

Non-linear Behavior of New (FSFN) Moment Resisting Connections in Comparison to the Existing KBB Connections in Steel Frames

After Northridge (1994) and Kobe (1995) earthquakes, several studies have been conducted to improve the seismic performance of steel structures. In this investigation, new steel moment-resisting connections (FSFN) developed by the authors, were studied by the non-linear numerical analysis. These connections were single-sided beam-to-column assemblies that are representative of exterior beam-to-column connections. Seismic performance of FSFN connections in comparison to the W-series of existing KBB connections under standard and near fault load history based on AISC 341 and FEMA 355D were also studied. For this purpose, 32 analytical modeled specimens were evaluated and the results were compared. Results showed that FSFN bracket connection has a seismic performance similar to KBB connection and it is an appropriate connection for the special moment resisting frames both in far and near fault zones. More so, these connections do not have patent and due to their dimensions which are smaller than the Kaiser brackets, they can supported deeper beams up to 100 cm. Therefore, it can be considered as an appropriate alternative for the Kaiser patented connections. This paper is to present the analytical modeling, non-linear behavior and the results of the comparison studies in detail including obtained conclusions.

Cyclic performance of steel moment-resisting connections with reduced beam sections — experimental analysis and finite element model simulation

Reduced beam section (RBS) moment-resisting connections have been developed in order to provide a highly ductile response and reliable performance. Recommendations for the design and detailing of the RBS member were prescribed in EC8, Part 3. However, the effectiveness of these recommendations for a European profile is dubious, due to limited existing data from European research. An experimental program was performed in order to evaluate the proposed values of the geometrical characteristics of the RBS, and the results are presented in the present paper. Two full-scale subassemblages were tested under cyclic loading and the results are compared with those obtained from the theoretical model, using the finite element method. The analysis confirms the need for readjustment of the geometrical characteristics of the RBS in order to apply to European profiles.

Experimental Evaluation of Kaiser Bolted Bracket Steel Moment-Resisting Connections

The Kaiser bolted bracket (KBB) is a new beam-to-column moment connection that consists of proprietary cast high-strength steel brackets that are fastened to the flanges of a beam and then bolted to a column. This fully restrained connection is designed to eliminate field welding in steel moment frame construction. This paper summarizes the development of bolted bracket connections and presents the results of seven full-scale KBB tests. These tests were conducted to evaluate the connection for both the retrofit of existing and the construction of new steel moment frames. More specifically the tests were intended to assess the ductility of the connection under cyclic inelastic loading and to qualify their performance with respect to code requirements.

Numerical models and ductile ultimate deformation response of post‐tensioned self‐centering moment connections

AbstractNew steel moment‐resisting connections that incorporate post‐tensioning elements to provide a self‐centering capacity and devices to dissipate seismic input energy have recently been proposed and experimentally validated. Experimental studies have confirmed that these connections are capable of undergoing large lateral deformations with negligible residual drifts. To facilitate their implementation, accurate modeling of the behavior of systems incorporating post‐tensioned connections must be readily available to designers and researchers. A number of simplified models have been suggested in the literature by researchers trying to capture experimental results at the beam–column connections and thereby to predict the global response of structures incorporating such connections. To provide a clear set of guidelines for the modeling of post‐tensioned steel frames, for practicing engineers as well as researchers, in this paper three types of numerical models of increasing complexity are presented: (i) a sectional analysis procedure, (ii) a lumped plasticity spring frame leveled approach and (iii) a non‐linear solid finite element analysis to predict the response at ultimate deformation levels. The analytical results obtained from the numerical models predict well the structural behavior of these connections when compared with available experimental data. Even at the ultimate deformation level, analytical results are in good agreement with test results. Furthermore, detailing requirements are proposed to assure that flexural hinges form in the beams in order to improve the cyclic response of steel self‐centering connections when drifts exceeding the design drifts are imposed to the system. Experimental and analytical studies demonstrate that steel post‐tensioned self‐centering connections incorporating the proposed detailing in the beams develop an increased deformation capacity and thereby exhibit a ductile response while avoiding a sudden loss of their strength and stiffness. Copyright © 2008 John Wiley & Sons, Ltd.

Cyclic Behavior of Steel Moment-Resisting Connections Reinforced by Alternative Column Stiffener Details II

A study has been conducted to reassess the current AISC seismic panel zone design criteria for the Welded Unreinforced Flange-Welded Web (WUF-W) steel moment-resisting connections and to investigate acceptable minimum seismic panel zone design criteria for moment-resisting connections. Five full-scale WUF-W cruciform specimens were tested in this research, coupled with corroborating nonlinear finite element analysis. All had relatively weak panel zones designed to cause large localized strain concentrations in the girder flange-to-column flange connection region and to exceed the panel zone shear deformation of the 4gy (in other words, four times the panel zone shear yield strain, gy). This paper presents the cyclic panel zone behavior of the WUF-W cruciform specimens and an assessment of the current AISC seismic panel zone design criteria for these connections. Two possible modifications for seismic panel zone design criteria are then proposed and verified against these tests as well as experiments from the literature. One equation provides a more accurate assessment of panel zone nominal strength for steel moment-resisting connections, and the second equation provides a panel zone required strength equation that targets permitting up to 8gy panel zone shear deformation for WUF-W and similar connections.

Cyclic Behavior of Steel Moment-Resisting Connections Reinforced by Alternative Column Stiffener Details I

Experimental and corroborating computational research was performed to reassess the design and detailing of column reinforcement (in other words, continuity plates and doubler plates) according to U.S. specifications for both seismic and non-seismic design and to explore economical alternative stiffener details. The performance of six full-scale girder-to-column cruciform specimens that use the Welded Unreinforced Flange-Welded Web (WUF-W) connection detail was evaluated. The effects on the cyclic behavior of three different doubler plate details, including a back-beveled fillet-welded detail, a square-cut fillet-welded detail, and a box (offset) detail, were investigated. In addition, the performance of a continuity plate approximately half the thickness of the girder flange fillet-welded to both the column flanges and doubler plates was investigated. Current local flange bending and local web yielding design criteria were evaluated and the applicability of the suggested alternative column stiffener details was verified. A companion paper outlines findings with respect to panel zone behavior and design for seismic applications.

Continuity Plate Detailing for Steel Moment-Resisting Connections

The 1994 Northridge, California earthquake resulted in the fracture of complete-joint-penetration groove (CJP) welds connecting girder flanges to column flanges in steel moment-resisting connections in a number of steel frame structures ranging from one to 26 stories in height (FEMA, 2000a). These welds fractured due to a combination of reasons, including the weld material having low fracture toughness; the pre-Northridge connection geometry making the CJP welds more susceptible to high strain and stress conditions; and welding practices that resulted in inconsistent weld properties (FEMA, 2000a). As a result of these fractures, in the subsequent years there has been a tendency to be more conservative than necessary in the design and detailing of steel moment-resisting connections both in seismic and non-seismic zones within the United States. In particular, continuity plates and web doubler plates have often been specified when they are unnecessary and, when they are necessary, thicker plates have been specified than would be required according to the applicable specifications. In addition, the welds of the continuity plates to the column flanges have often been specified as being complete-joint-penetration groove welds, even though the use of more economical fillet welds may have sufficed. This paper has given the background of the local web yielding and local flange bending limit states and the conclusions of previous researchers regarding the behavior of continuity plates and the welds attaching them to columns. It has also summarized the results of nine pull-plate tests and corresponding finite-element analyses studying column-stiffening details. The preliminary conclusions from these tests are focused on monotonic loading applications and will be synthesized with future cyclic loading experiments, with implications for seismic design, in Lee and others (2002) and other ongoing research.

Bolted large seismic steel beam-to-column connections Part 1: experimental study

Two large bolted steel moment-resisting connections were studied by experiments. These connections were single-sided beam-to-column assemblies that are representative of exterior beam-to-column connections, and they were composed of W36×150 Grade 50 beams and W14×283 Grade 50 columns. T-stubs were cut from W40×264 sections of Grade 50 steel. The T-stub stems were welded to the beams and prestressed by bolts to the beam flanges in the shop. Final beam-to-column assembly required no additional welding: the T-stub flanges were bolted to the column and the column shear tab was bolted to the beam web. The specimens had two symmetrically located T-stubs with different stem geometry: Specimen 1 had rectangular-shaped stems, whereas Specimen 2 had U-shaped stems. During the cyclic testing the beam deformation was minimal controlled by active participation of the T-stub flanges: a separation between T-stub flanges and the column flanges was observed. This separation was caused by bending plastic deformation in the T-stub flanges and plastic deformation in the high-strength bolts. This phenomenon allowed energy dissipation and prevented severe buckling of the beam flanges and beam web.

Cover-Plate and Flange-Plate Steel Moment-Resisting Connections

Nonlinear finite-element modeling of steel moment-resisting connections reinforced with steel plates was undertaken as part of the second phase of the SAC project. Five cover-plate and five flange-plate connections of W143176 columns to W30399 beams were studied. Connections reinforced with cover plates or flange plates will perform substantially better than unreinforced connections. Flange-plate connections are likely superior to cover-plate connections. Rectangular reinforcing plates are superior to trapezoidal shaped plates. Three-sided fillet welds should be used to join reinforcing plates to beam flanges. Bracing the beam flanges outside the plastic hinge remote from the column face to delay lateral-torsional buckling is not necessary. Beam web reinforcement at the column face does not affect the transfer of shear force from the reinforced beam to the column. Beam theory should not be used to estimate stresses in reinforced connections.

Analysis of Mid-Rise Steel Frame Damaged in Northridge Earthquake

This paper presents the findings from a computational investigation performed on the Borax corporate headquarters building, a four-story steel frame structure in which 75% of the steel moment-resisting connections suffered brittle fractures during the 1994 Northridge earthquake. A companion paper provides detailed documentation of the forensic investigation and repair design performed immediately following the earthquake on this structure, which was less than one year old when the earthquake struck. This paper describes a series of preliminary analyses conducted shortly after the forensic investigation and repair were completed. The main objectives were to investigate the behavior of the structure during the earthquake, and to determine whether different levels of established, advanced analysis techniques could estimate the distribution an extent of the damage. The results show that a three-dimensional dynamic analysis using a site-specific accelerogram provides strong correlation with the observed damage, while elastic static and dynamic analyses, two-dimensional nonlinear static and dynamic analyses, and three-dimensional nonlinear static analyses show less correlation. In addition, the results indicate that substantial redistribution of forces may have occurred, and that force redistribution most likely had a noticeable effect on the final pattern of failures observed in the structure.

Repairs to Mid-Rise Steel Frame Damaged in Northridge Earthquake

The Borax corporate headquarters building, a low-rise steel moment frame, was less than one year old when it sustained significant structural damage during the 1994 Northridge earthquake. Seventy-five percent of its steel moment-resisting connections suffered severe brittle fractures. This paper presents a detailed documentation of the forensic investigation performed on this structure immediately following the earthquake. A companion paper presents the findings from a subsequent computational investigation performed on the structure. The two primary failure modes at the Borax building were column flange fracture and pull-out of the girder flange from the column. The failures are primarily attributed to a fundamental flaw in the standard code-prescribed welded-flange bolted-web connection and the extreme ground motion at the site. The paper also describes the repairs made to the structure. These repairs were completed before the SAC research that now guides repair and retrofit of these structures was available.