Column Design in Steel Frames under Gravity Loads

In the composite walls stiffness system of reinforced concrete walls with composite boundary members, boundary members resist most of the moment due to seismic loading. In contrast, the reinforced concrete (RC) wall provides shear resistance. The SRCW system comprises partially restrained steel frames with reinforced concrete infill walls. The steel columns and beams act as boundary members to resist gravity loads and most of the overturning moment due to seismic loading, while the reinforced concrete (RC) infill wall acts as shear-resisting web. The RC infills increase the lateral stiffness dramatically compared to a bare steel frame, thus avoiding excessive drift and reducing the seismic demands on the steel frames. In that way, why not use just steel shear frames instead of partially restrained steel frames with reinforced concrete infill walls? This paper presents a numerical study of the behavior of a composite structural system consisting of steel shear frames with reinforced concrete infill walls (SFRCW). The composite interaction is achieved using welded bolts as shear connectors along the steel frame–infill interfaces. Welded bolts were used as shear connectors because they are frequently used in Colombia due to their ease of installation. The SFRCW system may be particularly appropriate for low-to-moderate-height structures. In addition, the steel shear frame will support gravity loads at the construction stage, allowing progress in height. The system can also be used to strengthen existing steel buildings. The relatively light steel frame constructed using shear connections maximizes the system's economy.This study compared the behavior of the SFRCW with columns acting in their weak and strong axes and with different numbers of shear connectors. It also compared the behavior of the bare steel moment frame, bare steel shear frame, and reinforced concrete wall. The numerical models show interesting results.

CURVBRG: A program for analysis of curved I-girder bridges

We present a detailed and comprehensive structural study of molecular models of liquid methanol, ethanol, and 1-propanol that originate from a series of reverse Monte Carlo (RMC), molecular dynamics (MD), and united-atom Monte Carlo (UA:MC) simulations. We compare several modeling approaches: RMC simulations that employ experimental neutron and X-ray diffraction data as sole constraints, RMC with diffraction data complemented with partial radial distribution functions (PRDFs) from MD or UA:MC, and conventional MD and UA:MC simulations. The assessment is done in view of the structural parameters of the hydrogen bond and resulting morphological characteristics of molecular aggregates. To achieve these tasks, a computer program for structural analysis of molecular configurations together with the appropriate aggregate classification scheme has been developed. We have analyzed the morphology of clusters, their probability, and size distributions. Any cyclic structures that appeared in the configurations were extracted and characterized in the same manner. We found that MD and UA:MC simulations resulted in configurations with bulkier, more threadlike aggregates that were not entirely consistent with the experimental evidence from diffraction experiments. A combination of neutron and X-ray diffraction data with PRDFs from MD simulations, simultaneously applied as constraints in the RMC procedure, proved to be a modeling approach with the most conclusive results.

This paper presents a method to define two-dimensional (2D) and three-dimensional (3D) elastic-stiffness matrices for nonprismatic elements (tapered or haunched), based on traditional beam theory and the flexibility method. The proposed formulation includes deformations and the shape of the cross section but neglects warping deformations. Although more rigorous formulations for tapered elements have already been addressed, the proposed procedure is presented so its direct application or implementation in computer programs for structural analysis is straightforward. The procedure is compared against the design tables of the Portland Cement Association (PCA). It is demonstrated that the PCA tables are obsolete for today's state-of-the-knowledge on nonprismatic members because they can lead to significant errors. A new set of design aids for most common cross sections used in building structures have been developed to substitute the PCA handbook of frame constants. Closed-form solutions for linearly tapered elements of rectangular, square, and circular cross sections are provided. Finally, it is demonstrated that the stiffness factors for nonprismatic elements depend on the span-to-depth ratio of the element ( L /h). To the writer's knowledge, no one has proved this fact before.

The use of effective length factors in the design of columns in frames is quite widespread in the USA. Thus, the latest AISC Allowable Stress Design Specification and the recently adopted AISC Load and Resistance Factor Design Specification include effective length factors in the expressions for the strength of columns and beam-columns. This lecture reviews the basis of elastic and inelastic effective length factors for columns in unbraced frames.

In this study, the inelastic load-carrying capacity of the compression diagonal of a typical cross-bracing system used in concentrically braced frames under gravity and wind loads is investigated, taking into consideration its interaction effect with the tension diagonal. Depending on the lateral stiffness of the tension diagonal, the compression diagonal can be fully or partially braced by the tension diagonal at their intersection point. An expression for the transition lateral stiffness, kst, that demarcates the fully and partially braced conditions is derived. When the compression diagonal is fully braced, its maximum load-carrying capacity is a function of its member slenderness, L/r, only. However, when the compression diagonal is partially braced, its load capacity is dependent upon both its member slenderness, L/r, and the lateral stiffness, ks, of the tension diagonal. Once the equations for the maximum load-carrying capacity of the compression diagonal are established, design guidelines are proposed and design examples are given to demonstrate how the proposed guidelines can be used for the design of cross-bracing systems in steel frames. The consideration of the lateral bracing effect will result in a more economical and logical design for such bracing systems.

Top beam flange fractures were observed in the post-earthquake steel frame building inspections. Both seismic loads and gravity loads have effect on top beam flange fractures. The objective was to quantify the variation of stress intensity factor to weld root flaw sizes on top of the beam flange at column face in steel frame connections under gravity loads. Finite-element analyses were used to study fracture toughness in welded beam-column connections. Investigations of fracture behavior mainly focused on the standard pre-Northridge connection geometry. Finite element analysis was performed using the ANSYS computer program. Stress intensity factor was calculated through a J-integral approach. The parametric study was conducted to quantify elastic fracture demands as a function of beam geometries. Results show that the distributions of stress intensity factor tend to be even across the flange width under gravity loads. The thickness of beam web has little effect on stress intensity factors. Stress intensity factor increases nearly linear with the increase of flaw size, and it increases with the increase of depth of beam and width of beam. Stress intensity factor reduces with the increase of thickness of beam flange.

This study addresses the effect of pre-stressed cables on a pre-stressed mega-braced steel frame through employing static analysis and pushover analysis. The performances of a pre-stressed mega-braced steel frame and a pure steel frame without mega-braces are compared in terms of base shear, ductility, and failure mode. The influence of the cable parameters is also analyzed. Numerical results show that cable braces can effectively improve the lateral stiffness of a pure frame. However, it reduces structural ductility and degenerates structural pre-failure lateral stiffness greatly. Furthermore, it is found that 20% fluctuation in the cable pretension has little effect on structural ultimate bearing capacity and lateral stiffness. As comparison, 20% fluctuation in the cable diameter has much greater impact.

Storey-based stability of unbraced steel frames at elevated temperature

This paper evaluated the progressive collapse robustness of steel intermediate moment-resisting frames (MRFs) under different fire-induced column failure scenarios. For this purpose, 3-, 7-, and 12-story intermediate MRFs were modeled numerically and analyzed under different gravity and fire loads. The effects of the seismic design requirements, number of stories, gravity load ratio (utilization ratio), and redundancy and position of the heated column were discussed. Because the investigations included models with different number of stories and different seismic to gravity load ratios, the results can be applied for a wide range of IMFs. The results show that none of the studied frames in the high seismic zone with a low gravity loads ratio experienced progressive collapse under any of the considered fire scenarios. However, by increasing the ratio of gravity loads in frames, the vulnerability to progressive collapse was increased. This suggests that in areas with a lower risk of environmental lateral loads, where MRFs generally are designed for gravity loads and smaller lateral forces, the progressive collapse robustness of MRFs under fire-induced column failure should be given serious consideration. In addition, the results show that the possibility of progressive collapse under the fire-induced column failure scenarios in the first story of low-rise frames is more than that in the first story of high-rise frames, and generally low-rise frames are more vulnerable to progressive collapse than high-rise frames. Furthermore, the possibility of progressive collapse under fire scenarios in the upper stories is more than that in the lower stories, especially for the top story. In this study, a new combined failure mechanism for global collapse of MRFs was identified.

Top-and-seat angle connections (TSACs) exhibit inherently asymmetric and nonlinear moment–rotation behavior, which can significantly influence the global response of steel frames subjected to combined gravity and lateral loading. In this study, a three-dimensional finite element model of an unstiffened TSAC is developed and validated against experimental moment–rotation data from the literature under monotonic loading conditions. The validated model is then used to investigate the influence of key geometric parameters, including top angle thickness, bolt diameter, and beam depth, on the connection’s moment–rotation response in both positive and negative bending directions. Subsequently, the monotonic connection behavior is incorporated into nonlinear static analyses of steel portal frames to examine the effects of asymmetric connection response and moment reversal on frame-level stiffness degradation and capacity. A practical SAP2000 modeling workflow is proposed in which the finite element-derived monotonic moment–rotation curves are implemented using zero-length rotational link elements, allowing combined consideration of material, geometric, and connection nonlinearities at the structural level. The comparisons between Abaqus and SAP2000 results demonstrate consistent frame-level responses when identical monotonic connection characteristics are employed, highlighting the ability of the proposed workflow to reproduce detailed finite element predictions at the structural analysis level. The results indicate that increasing top angle thickness, bolt diameter, and beam depth enhances the lateral stiffness and base shear resistance of steel frames. Positive and negative bending directions are defined consistently with the applied gravity-plus-lateral loading sequence.

A computerized methodology for structural synthesis of kinematic chains: Part 2— Application to several fully or partially known cases

A computerized methodology for structural synthesis of kinematic chains: Part 1— Formulation

Finite element modeling and capacity analysis of post-tensioned steel frames against progressive collapse

Urban structures for lower-cost housing projects