Seismic Performance of Exposed Column-Base Plate Connections with Ductile Anchor Rods

This paper presents full-scale experiments and computational analyses on exposed column-base plate connections with ductile anchors. The aim is to examine the seismic performance of these connections for their prospective use as weak bases, wherein steel moment frames are designed to concentrate inelastic rotations in the base connections rather than in the connected columns. The connections feature upset thread anchor rods in which the threads are milled to a smooth shank, providing a designated stretch length over which inelastic deformations may be concentrated. The shank is frictionally isolated from the footing using polyethylene tape. The four full-scale experiments investigate the effects of axial force, rod diameter, and rod material grade. The test specimens withstand (without anchor rod failure) the application of two Applied Technology Council-the Joint Venture partners of the Structural Engineers Association of California, the Applied Technology Council, and the Consortium of Universities for Research in Earthquake Engineering (ATC-SAC) protocols applied consecutively (each to drift amplitudes of 5%), followed by additional cycles to 6.5% drift amplitude. Complementary line element-based and continuum finite-element simulations are conducted to examine to what extent the experimentally observed response may be generalized to untested configurations. Implications for design are summarized, along with the limitations of the study

Characterization of the chemical signatures of air masses observed during the PEM experiments over the western Pacific

Extensive observations of tropospheric trace species during the second NASA Global Tropospheric Experiment Western Pacific Exploratory Mission (PEM-West B) in February-March 1994 showed significant seasonal variability in comparison with the first mission (PEM-West A), conducted in September-October 1991. In this study we adopt a previously established analytical method, i.e., the ratio C2H2/CO as a measure of the relative degree of atmospheric processing, to elucidate the key similarities and variations between the two missions. In addition, the C2H2/CO ratio scheme is combined with the back-trajectory-based and the LIDAR-based air mass classification schemes, respectively, to make in-depth analysis of the seasonal variation between PEM-West A and PEM-West B (hereinafter referred to as PEM-WA and PEM-WB). A large number of compounds, including long-lived NMHCs, CH4, and CO2, are, as expected, well correlated with the ratio C2H2/CO. In comparison with PEM-WA, a significantly larger range of observed C2H2/CO values at the high end for the PEM-WB period indicates that the western Pacific was more impacted by "fresher" source emissions, i.e., faster or more efficient continental outflow. As in the case of PEM-WA, the C2H2/CO scheme complements the back-trajectory air mass classification scheme very well. By combining the two schemes, we found that the atmospheric processing in the region is dominated by atmospheric mixing for the trace species analyzed. This PEM-WB wintertime result is similar to that found in PEM-WA for the autumn. In both cases, photochemical reactions are found to play a significant role in determining the background mixing ratios of trace gases, and in this way the two processes are directly related and dependent upon each other. This analysis also indicates that many of the upper tropospheric air masses encountered over the western Pacific during PEM-WB may have had little impact from eastern Asia's continental surface sources. NOx mixing ratios were significantly enhanced during PEM-WB when compared with PEM-WA, in the upper troposphere's more atmospherically processed air masses. These high levels of NOx resulted in a substantial amount of photochemical production of O3. A lack of corresponding enhancements in surface emission tracers strongly implies that in situ atmospheric sources such as lightning are responsible for the enhanced upper tropospheric NOx. The similarity in NOx values between the northern (higher air traffic) and southern continental air masses together with the indications of a large seasonal shift suggests that aircraft emissions are not the dominant source. However, photochemical recycling cannot be ruled out as this in situ source of NOx. Copyright 1999 by the American Geophysical Union

Advancing fracture fragility assessment of pre-Northridge welded column splices

Summary: A refined probabilistic assessment of seismic demands and fracture capacity of welded column splice (WCS) connections in welded steel moment resisting frames (WSMRFs) is presented. Seismic demand assessment is performed through cloud‐based nonlinear time history analysis (NLTHA) for two case‐study structures, i.e., a 4‐ and a 20‐ story WSMRFs. Results from NLTHA are used to derive fracture fragility of WCS connections. To this aim, the study investigates (1) optimal ground‐motion intensity measures for conditioning probabilistic seismic demand models in terms of global (i.e., maximum inter‐story drift ratio) and local (i.e., peak tensile stress in the flange of WCSs) engineering demand parameters of WSMRFs; (2) the effect of ground‐motion vertical components on the longitudinal flange stress of WCS connections and their resulting fracture fragility; and (3) the effect of WCS capacity uncertainties on the fracture fragility estimates of those connections. For the latter case, an advanced finite element fracture mechanics‐based approach proposed by the authors is employed to capture aleatory and epistemic uncertainties affecting fracture capacities. The focus is on pre‐Northridge WCS connections featuring partial joint penetration and brittle materials, making them highly vulnerable to seismic fracture. Fracture fragility results for the case‐study structures are compared and discussed, highlighting the importance of the considered issues on fragility estimates, particularly in the case of high‐rise structures. Findings from the study contribute shedding some light on the influence of seismic demand and capacity uncertainties on the assessment of fracture fragility of WCS connections. These findings can guide similar performance‐based assessment exercises for WSMRFs to inform, for instance, the planning and design of retrofitting strategies for those vulnerable connections

Reliability Analysis and Design Considerations for Exposed Column Base Plate Connections Subjected to Flexure and Axial Compression

Exposed column base plate (ECBP) connections are commonly used in steel moment resisting frames. Current approaches for their design are well-established from a mechanistic standpoint. However, the reliability of connections designed as per these approaches is not as well understood. A detailed reliability analysis of the prevalent approach in the United States is performed in this study by using 59 design scenarios from steel moment frames subjected to combinations of dead, live, wind, and seismic loads. The analysis is conducted through Monte Carlo sampling reflecting uncertainties in the loads, material properties, component geometry, as well as demand and capacity models for the various components (e.g., base plate, footing, anchor rods) of the connection. Results indicate that the current design approach leads to unacceptable and inconsistent probabilities of failure across the various components. This is attributed to: (1) the use of a resistance factor for the footing bearing stress that artificially alters flexural demands on the base plate, and (2) the calibration of resistance factors for the plate and anchors without appropriate consideration of variability in demands. Two alternative approaches are examined as prospective refinements to the current approach. One eliminates the resistance factor for the bearing stress when used to determine flexural demands in the base plate, while the other considers overall failure of the connection rather than failure of individual components within the connection. For both approaches, new resistance factors are calibrated to provide consistent and acceptable probabilities of failure across all limit states and all types of loading. Design and cost implications of these alternative approaches are summarized

Seismic demands in column base connections of steel moment frames

Methods for the seismic design of base connections in steel moment frames are well-developed and routinely utilized by practicing engineers. However, design loads for these connections are not verified by rigorous analysis. This knowledge gap is addressed through nonlinear time history simulations using design-level seismic excitation that interrogate demands in column base connections in 2-, 4-, 8-, and 12-story steel moment frames, featuring base connections that reflect current U.S. practice. The results indicate that: (1) for exposed base plate connections, lower bound (rather than peak) estimates of axial compression are suitable for design because higher axial forces increase connection strength by delaying base plate uplift; (2) even when designed as pinned (as in low-rise frames), base connections carry significant moment, which can be estimated only through accurate representation of base flexibility; and (3) the failure of embedded base connections is controlled by moment, which may be estimated either through overstrength or capacity-based calculations

Fatigue Followed by Seismic Fracture in High-Rise Steel Moment Frames

Steel moment frames in high seismic regions such as California are often designed to resist fracture. This is accomplished by minimizing flaws and cracks in critical regions of the structure, and through the use of toughness rated materials. However, recent simulations show that especially in high rise structures, wind-induced vibrations may contribute to the growth of fatigue cracks that reduce structural reliability during a seismic event. A series of interconnected experimental and simulation studies are presented. These include scaled flume tests of a representative building, which are then combined with structural models of a high rise building to examine the potential for fatigue crack growth, and quantify its impact on structural reliability. Implications for existing and new buildings are discussed, along with strategies for mitigatio

Column splice fracture effects on the seismic performance of steel moment frames

© 2017 Elsevier Ltd The influence of welded column splice fracture on the seismic response of steel moment frames is examined. The study is motivated by pre-Northridge moment frames with welded column splices with crack-like flaws that are highly vulnerable to fracture. Costly retrofit strategies to repair these splices are usually intentioned to preclude splice fracture, without an explicit examination of its effects on global response. This study simulates post-fracture response of splices through a new material model, which is informed by fracture-mechanics based estimates of splice strength, and reproduces phenomena such as gapping and re-seating that occurs in the splices after fracture. Nonlinear response history simulations (incorporating this model) are used to examine the response of 4- and 20-story moment frames. The simulations, using 100 ground motions, and reflecting key aspects of nonlinear response are conducted within a Performance Based Earthquake Engineering (PBEE) framework, to examine global and local structural response in a probabilistic sense. The simulations indicate that neither the collapse potential nor building deformations are significantly affected by splice fracture when compared to benchmark simulations without fracture. This is attributed to a combination of phenomena; these include the mobilization of building rocking due to splice fracture, and the tendency of fractures to cascade upwards through individual columns rather than across a story. The results suggest that splice fracture may not necessarily trigger structural collapse, and retrofit strategies that consider global, rather than local response may be more cost effective. Limitations of the study are outlined