Steel Special Moment Frames (SMF) are regularly used as seismic force-resisting systems due to their excellent ductility and wide accommodation of building floor plans and heights. The current AISC Seismic Provisions require the use of reinforcing continuity plates and dictate their size based on a set of rules conservatively inferred from experimental testing. These rules often result in the unnecessary reinforcement of columns and usually require costly complete-joint-penetration (CJP) groove welds to fasten the reinforcing plates. Full-scale testing of 10 moment frames was performed to investigate the design of these continuity plates and their weldments. Six of these frames were exterior connections utilizing the prequalified Reduced Beam Section (RBS) connection, while the remaining four were interior connections utilizing the prequalified Welded Unreinforced Flange-Welded Web (WUF-W) connection. While violating the current continuity plate requirements, all 10 connections surpassed the 0.04 rad story drift requirement of SMF according to the prequalification criteria of the AISC Seismic Provisions. Experimental testing was also performed to measure, for the first time, the in situ residual stresses of a continuity plate. Detailed parametric finite element modelling and modern fracture mechanics using the Cyclic Void Growth Model for ductile fracture prediction was used to develop an amended set of limit states of reinforced columns. These amended limit states, in conjunction with a newly proposed width-to-thickness requirement, permit the design of column reinforcement based on a rational plastic approach. A fillet weld design that capacity protects the weldments based on a von Mises yield surface is included in this new plastic design method. This new experimentally verified design basis for fillet weldments of continuity plates results in significant fabrication savings. It was also found that sizing the doubler plate weldments for the average developed shear flow according to the relative doubler plate stiffness was adequate to fasten the doubler plate. This results in significant savings over current requirements which currently require welds to develop the shear strength of the doubler plate.

Steel Special Moment Frames (SMF) are one of the most popular lateral force-resisting systems for multistory building construction in high seismic regions due to their architectural versatility. With a significant amount of research that was conducted after the 1994 Northridge, California earthquake, AISC has published design guidelines (AISC 341 and AISC 358) to avoid brittle fracture of beam-to-column welded moment connections that occurred in more than 100 steel buildings. This dissertation addresses two issues related to the moment connection design of SMF.

Unless the column flanges are sufficiently thick, AISC 341 requires that continuity plates be installed, and that expensive complete-joint-penetration (CJP) groove welds be used to connect the continuity plates to the column flanges; the conservative nature of this requirement stems from a lack of procedure that the designer can use to quantify the required seismic forces in the continuity plates such that more economical welds (e.g., fillet welds) can be used. The first objective of this research was to investigate a design procedure and to verify it with full-scale testing of two Reduced Beam Section (RBS) moment connections. It was shown that the proposed design procedure could result in a more economical weld design while developing the ductile response of the moment connection.

AISC seismic design codes implicitly assume that beams are orthogonal to the columns in elevation, but in real-life construction beams are sometimes connected to the columns with a slope. To fill this knowledge gap, both experimental and analytical studies were conducted. Full-scale testing of two additional moment connections with a 25° angle of inclination showed that sloped connections are vulnerable to brittle fracture at the “heel” location, where the beam flange and column form an acute angle. Fracture would initiate from the end of beam web CJP weld and the weld access hole. Guided by finite element simulation, a truss analogy model was proposed to predict the force concentration at the heel location. A Force Concentration Factor was introduced to evaluate when the effect of inclination can be ignored. When such effect needs to be addressed, a solution that introduces a curved slot in the beam web was proposed. The effectiveness of this solution was verified by nonlinear finite element analysis.

Three issues on steel truss bridge evaluation and design are investigated in this research. Part 1 deals with gusset plate buckling and the associated compression strength. Gusset plates are routinely used to connect members at panel points in steel truss bridges and braced-frame buildings. Current design practice is to treat a portion of the gusset plate at the end of the connected member (e.g., diagonal brace) as a compression member. This process requires a determination of the member length, an effective length factor, and the use of a Whitmore section with a proper dispersion angle to define the cross section of the compression member. The method, first proposed by Thornton (1984), was mainly based on engineering judgment. After the collapse of the I-35W Bridge in Minneapolis, MN in 2007 with 13 casualties, significant research efforts at both federal and state levels were made, which resulted in the new design requirements for checking gusset buckling in the AASHTO LRFD Bridge Design Specifications. Yet, the new requirements are still based on the Thornton’s column analogy concept with some minor modifications. In this dissertation, a novel approach that treats gusset buckling as a phenomenon of plate buckling under shear is developed. This proposed design method properly reflects the sway buckling mode, a failure mode that is always observed in laboratory testing but is not reflected in the column-analogy method. Based on finite element simulation, the model developed considers the effects of the free edge length of the gusset plate, the brace connected length, and the angle between the brace and the gusset plate. A correlation with the available test data shows that the scatter of data is drastically reduced when compared with the conventional design method.Part 2 of the research addresses issues faced by bridge designers on the load rating of existing steel truss bridges. Built-up members in many truss bridges have centroids that do not coincide with the working lines, and such eccentricity at panel points produces moments in the members. One common practice is to analyze the bridge as a pin-connected truss, and then the end moments are computed as the axial load multiplied by either 100% or 50% of the end eccentricity, depending on the type of the connection (either pin-connected or gusset-connected) at the panel point. Together with field load testing of a truss bridge in Southern California and the associated finite element simulation, this study concludes that the Caltrans practice of handling eccentricity has no technical basis. Instead, eccentricity should be explicitly included in the finite element models. The last part of the research deals with the behavior and design of steel pins. Using steel pins to connect members at the panel points is common in many existing steel truss bridges. The AASHTO Specifications treat the pin as a beam, and a moment-shear interaction check is used to check the pin strength. Since test data, especially pins with larger diameters, is very scarce, experimental testing of twelve 2-in. diameter pins with two steel grades was conducted. Together with finite element simulation, this research shows that the beam-analogy approach adopted in the AASHTO Specifications can be very conservative. Alternative equations to predict the strength of steel pins are proposed, which would potentially eliminate many unnecessary retrofits of steel pins in existing truss bridges.

Steel Special Moment Frame (SMF) is a preferred seismic force-resisting system for its architectural flexibility and high ductility. To achieve economy in design and construction, there is a growing trend to use deeper columns (e.g., with a section depth larger than 14 in.) to limit the code-enforced story drift requirements in recent years. A deep column has larger slenderness ratios and is more vulnerable to both local and global buckling. Since AISC Seismic Provisions assume that plastic hinging will occur at not only beam ends but also column bases, little research was available on the deep column hinging behavior under axial compression and cyclic drift.

A full-scale test program with thirty-seven deep columns subjected to both axial compression and cyclic drift was conducted. Test variables included sectional and member slenderness ratios for local and global buckling controls, axial force level, axial force type (constant versus variable), boundary condition (fixed-fixed versus fixed-flexible), transvers loading type (monotonic, cyclic, or near-faulty), axis of bending (strong-axis versus weak-axis) and bending type (uniaxial versus biaxial). From the database of this test program and a prior one with shallow (W14) stocky columns, three buckling modes were identified. Significant shortening was common to two buckling modes that were typical in deep columns, but not in shallow and stocky columns.

While the observed buckling modes could be properly simulated by nonlinear finite element simulation, a practical procedure to predict the governing buckling mode was needed for both cyclic modeling and design purposes. One procedure which considered the interaction between flange and web local buckling was first proposed; the accuracy of the procedure was verified by results from both tested columns and a large number of numerically simulated columns. Second, the combined experimental and numerically simulated database was then used in a multi-variate regression analysis to establish expressions for the cyclic backbone curves; these curves are needed for ASCE 41-type performance-based nonlinear response analysis of SMF. Finally, the same database was used to establish the limiting width-thickness ratios for potential adoption by AISC Seismic Provisions. A novel approach which considered the axial shortening as the limit state was proposed to establish these limiting width-thickness ratios for column design.

This thesis investigates nonlinear cyclic responses of deep wide-flange steel beam-columns, which are primarily used in Special Moment Frame (SMF) for their high in-plane, strong-axis moment of inertia to satisfy story drift limits specified in building codes. SMF design principles aim to achieve energy dissipation through plastic hinging of the beams, while flexural yielding of the columns at the base is also permitted. Although behavior of the beams has been extensively researched, that of the columns is lacking especially for deep columns (e.g., W18 to W36). Therefore, cyclic testing of deep columns was conducted to generate experimental database. Due to large width-to-thickness ratios of these sections, test results showed significant web and flange local buckling; some specimens also exhibited lateral-torsional buckling. These local and global instabilities resulted in significant axial shortening and flexural strength degradation. These behaviors differ significantly from those observed in prior testing of shallow W14 columns, featuring excellent ductility capacity at high axial loads.

Additionally, the test matrix was designed to investigate the effects of section depths, varying axial loads, lateral-drift loading sequences, and boundary conditions on the column responses. Inevitably in this testing, the responses were also influenced by flexibility of column-end connections. To eliminate this undesired variable from the responses, a procedure was developed to correct the lateral drift response based on the second-order Timoshenko elastic theory. The effects of boundary conditions were further investigated using high-fidelity finite element software ABAQUS. Results show that fixed-fixed and fixed-rotating column responses can be converted to one another.

Collectors are structural components that play a critical role to transmit inertia forces in the floor diaphragms to the vertical seismic-force resisting system in a building structure. Yet little research has been done on collectors. A three-phase test program was conducted on a half-scale, two-story steel building by using the NHERI@UCSD large high performance outdoor shake table. The main objectives of this project were to investigate the inertial force load path in the floor diaphragms and the seismic behavior of collectors and their connections. Phase 1 tests were performed as a “single-story phase” with only the first story with a composite slab constructed. An innovative experimental technique was developed such that the absolute acceleration history response of any floor in a multi-story prototype building experiencing nonlinear response and higher-mode effects could be simulated by using a re-usable single-story specimen through a transfer function approach. Test results validated this testing technique. Phase 2 tests were conducted after a second story with a bare steel roof deck was added to the test building; the conventional testing method with the scaled historical ground acceleration as the input motion was used. In Phase 3, two buckling-restrained braces were added to the second story to modify the building dynamic characteristics and the collector seismic load path. Earthquake simulation tests were conducted again until the failure of side-lap connections of the roof deck occurred. Test results showed that the current collector design would overestimate the axial forces in the roof collectors because it neglects the effect of flexural rigidity of the collector connections, which would mobilize gravity columns to transfer some inertial forces to the story below. An improved design method for estimating the roof collector axial forces that considers the flexural rigidity of the collector connections was proposed. Test results also showed that the unintended moment demand produced by the connection rigidity would cause the steel connections in composite collectors to experience axial forces higher than that assumed in design. Recommendations including connection design requirements and collector width-to-thickness ratios were also made.

Steel moment frames are commonly used in seismic regions, with wide flange steel columns serving as essential elements in their design. Compliance with drift limit requirements updated after the 1994 Northridge earthquake has led to the frequent selection of deep and slender columns, which maximizes the moment of inertia of the section for more economical design. However, such columns are susceptible to experiencing local and global buckling and subsequent axial shortening when subjected to high axial forces and cyclic lateral loads. This study presents the development and experimental observations of a full-scale testing program of cruciform beam-to-column subassemblies through quasi-static and advanced hybrid simulation.A novel framework for hybrid simulation is presented that employs a mixed displacement and force control strategy. A key feature of this framework is a controller-based displacement-to-force transformation that enables compatible displacements between the numerical and experimental model for force-controlled degrees of freedom. The framework can be applied within conventional displacement-based time-integration algorithms allowing for implementation across a variety of software and hardware architectures; it is demonstrated here using OpenSees software with detailed nonlinear numerical models. This study includes numerical verification of the proposed force control strategy and the later implementation for the hybrid simulations as part of the experimental program. In the hybrid tests, the experimental column axial load is applied in force control due to the large stiffness and when combined with lateral seismic loads, result in column buckling with significant axial shortening. The hybrid simulation results verify that the proposed mixed displacement and force control strategy effectively enforces displacement compatibility and force equilibrium between the numerical and experimental structures at the boundary degrees of freedom. Additionally, hybrid tests include an online model updating algorithm focused on the modeling parameters of plastic hinge elements representing the reduced beam sections in the numerical model. A smooth plasticity model is utilized for beam plastic hinges with updating parameters identified from on-line experimental data through a modified version of the unscented Kalman filter. The hybrid simulation shows that the numerical beam hinges based on simple hysteretic model with updated parameters are able to capture the characteristic behavior observed in experiments even for cases involving fracture. A selective updating concept is proposed to allow for updating multiple numerical component accounting for asymmetric behavior and variability in the response. The selective updating method is validated through virtual hybrid simulations that are better able to identify and isolate the effects from different simulations. The combination of results from physical and virtual tests highlights the benefits of model updating on the local and overall system-level response. Data from the present study and previous tests were employed to develop a column hinge macro-element. It allows for simulating the moment-rotation response of deep columns through fiber-based elements, while the vertical direction is modeled via an empirical expression based on the measured axial shortening. The macro-element is included in a steel moment frame model to assess the axial shortening for different seismic intensities.