Amit H. Varma

Seismic behavior and modeling of high-strength composite concrete-filled steel tube (CFT) beam–columns

Abstract The behavior of square concrete-filled steel tube (CFT) beam–columns made from high-strength materials was investigated experimentally. The effects of the width-to-thickness ratio, yield stress of the steel tube and the axial load level on the stiffness, strength and ductility of high-strength CFT beam–columns were studied. Sixteen three-quarter scale CFT specimens, which included eight monotonic beam–column specimens and eight cyclic beam–column specimens, were tested. The experimental results indicate that cyclic loading does not have a significant influence on the stiffness or strength of CFT beam–columns. However, it causes a more rapid decrease of the post-peak moment resistance. The moment capacity of high-strength CFT beam–columns can be predicted with reasonable accuracy using the American Concrete Institute (ACI) code provisions for composite columns. Fiber-based models were developed for the CFT beam–column specimens. The uniaxial stress–strain curves for the fibers were derived from three-dimensional nonlinear finite element analyses of the CFTs. The results from the fiber analyses of the monotonic and cyclic beam–column specimens compare favorably with the experimental results.

NS0 cell damage by high gas velocity sparging in protein‐free and cholesterol‐free cultures

Recent developments in high cell density and high productivity fed‐batch animal cell cultures have placed a high demand on oxygenation and carbon dioxide removal in bioreactors. The high oxygen demand is often met by increasing agitation and sparging rates of air/O2 in the bioreactors. However, as we demonstrate in this study, an increase of gas sparging can result in cell damage at the sparger site due to high gas entrance velocities. Previous studies have showed that gas bubble breakup at the culture surface was primarily responsible for cell damage in sparged bioreactors. Such cell damage can be reduced by use of surfactants such as Pluronic F‐68 in the culture. In our results, where NS0 cells were grown in a protein‐free and cholesterol‐free medium containing 0.5 g/L Pluronic F‐68, high gas entrance velocity at the sparger site was observed as the second mechanism for cell damage. Experiments were performed in scaled‐down spinners to model the effect of hydrodynamic force resulting from high gas velocities on antibody‐producing NS0 cells. Cell growth and cell death were described by first‐order kinetics. Cell death rate constant increased significantly from 0.04 to 0.18 day−1 with increasing gas entrance velocity from 2.3 to 82.9 m/s at the sparger site. The critical gas entrance velocity for the NS0 cell line studied was found to be ∼30 m/s; velocities greater than 30 m/s caused cell damage which resulted in reduced viability and consequently reduced antibody production. Observations from a second cholesterol‐independent NS0 cell line confirmed the occurrence of cell damage due to high gas velocities. Increasing the concentration of Pluronic F‐68 from 0.5 to 2 g/L had no additional protective effect on cell damage associated with high gas velocity at the sparger. The results of gas velocity analysis for cell damage have been applied in two case studies of large‐scale antibody manufacturing. The first is a troubleshooting study for antibody production carried out in a 600 L bioreactor, and the second is the development of a gas sparger design for a large bioreactor scale (e.g., 10,000 L) for antibody manufacturing. Biotechnol. Bioeng. 2008;101: 751–760.

Fundamental Behavior of Steel Beam-Columns and Columns under Fire Loading: Experimental Evaluation

This paper presents the results of experimental investigations conducted to determine the fundamental behavior of steel members under fire loading. A total of eleven full-scale steel members were tested under combined thermal and structural loading. First, five A992 steel beam-columns (W10×68) were tested to determine their fundamental moment-curvature responses at elevated temperatures and different axial load levels. The experimental approach involved the use of radiant heating and control equipment to apply the thermal loading, and close-range photogrammetry combined with digital image processing techniques to measure the deformations (curvature) in the heated zone. Next, six A992 steel wide-flange (W8×35 and W14×53) columns were tested to determine their inelastic buckling behavior and axial load-displacement responses at elevated temperatures. A self-reacting test frame was designed to subject the column specimens to axial loading and heating. The thermal loading was applied by using the same type of...

Immunoassays to study prevalence of antibody against GB virus C in blood donors

Immunoassays were developed to determine the seroprevalence of antibody against human GB virus C (GBV-C). The antigenic target in each assay was a 44.6-kDa glycosylated protein representing the first 315 amino acids encoded by the GBV-C E2 gene. Sera or plasma were assayed for E2 antibody using an anti-human EIA format in which antigen-coated polystyrene beads were reacted with sample, and bound antibody was detected by addition of enzyme labelled goat anti-human IgG. The presence of anti-E2 antibody was confirmed using a sandwich EIA format in which samples were reacted with antigen coated polystyrene beads, followed by addition of solution phase biotinylated antigen. Detection of antibody captured biotinylated E2 was accomplished by addition of enzyme-conjugated anti-biotin antibody. Antibody against the E2 antigen was detected in 7.4 and 7.8% of 500 sera and 500 plasma, respectively, from US volunteers donating to a Wisconsin blood center, and in approximately 10.7% of hepatitis and retrovirus marker-negative volunteer blood donors from a Missouri blood center. The rate in 1018 sera from US commercial donors at multiple US blood centers was 36.7%. These results indicated a relatively high prevalence of GBV-C exposure in US volunteer donors, and particularly in commercial donors. The clinical implication of the high exposure rate is unclear. These immunoassays are being combined with nucleic acid detection to assess prevalence of GBV-C world wide and to determine if GBV-C plays a role as an etiologic agent.

Development and validation of fiber model for high-strength square concrete-filled steel tube beam-columns

Steel-concrete composite construction has become popular because it combines the advantages of both steel and concrete materials economically. This article presents a general fiber model that can be used to predict the monotonic and cyclic force-deformation behavior of high-strength square concrete-filled steel tube (CFT) beam-columns. The authors also present a general analytical approach for developing the critical input parameters for the fiber model, that is, the stress-strain curves for the fibers modeling the CFT cross section. Their general approach develops the fiber stress-strain curves analytically from the results of a three-dimensional nonlinear finite element analysis of a corresponding CFT segment. The authors report on the validation of their model with experimental data from 26 high-strength square CFT beam-column specimens that were tested under combined axial and monotonic or cyclic flexural loading. The results from the CFT fiber models compare reasonably accurately with the corresponding experimental results.

Experimental Investigation of Effects of Dowel Misalignment on Joint Opening Behavior in Rigid Pavements

Experimental investigations were conducted to determine the fundamental joint opening behavior of concrete pavements and to evaluate the effects of dowel misalignment on this behavior. The parameters included in the experimental investigations were the number of dowel bars (one, two, three, or five) at the joint, the dowel misalignment type (horizontal, vertical, or combined), misalignment magnitude (0, 1/36, 1/18, 1/12, 1/9 rad), and uniformity across the joint. The effects of these parameters were evaluated for the joint opening behavior and structural distresses observed in the specimens. Forty-seven instrumented laboratory-scale specimens of pavement slabs with doweled joints were tested. The experimental results include the load per dowel-joint opening behavior and observations of structural distress. These results indicate that (a) all pavement joints are initially locked, and opening displacements occur after the load per dowel exceeds 5 to 7 kN or the average bond shear stress exceeds 0.22 to 0.30...

Flexural fatigue behavior of threaded connections for large diameter pipes

Full-scale flexural fatigue tests were conducted to investigate the fatigue behavior of a patented threaded connection for large diameter (0.61 m (24 in) outside diameter, 25.4 mm (1 in) wall thickness) offshore pipes. Fifteen fatigue tests were performed by subjecting the threaded connection to constant amplitude stress ranges (between 69 MPa (10 ksi) and 151.8 MPa (22 ksi) on gross cross section) with zero mean stress. The corresponding measured fatigue lives varied from 45000 to 4852200 cycles. Fatigue failures were in the form of cracks through the thickness of the wall and located at the root of the first full contact thread. The failure surfaces were ‘typical’ with identifiable zones of crack initiation, propagation and fracture. Linear regression analysis of the experimental results, namely the applied stress range (Sr) and the measured number of cycles to failure (N) data, in the log-log domain gave anR2 value of 0.88 and the least-squares best fit equation asSr (MPa)=1573.2N−0.212. The 90% probable fatigue strength prediction equation was estimated asSr=1393.8N−0.212. This equation is recommended for design purposes.

Analysis and Design of Noncompact and Slender CFT Beam-Columns

AbstractConcrete-filled steel tube (CFT) beam-columns are categorized as compact, noncompact, or slender depending on the governing slenderness ratio (width-to-thickness b/t or D/t ratio, λ) of the steel-tube wall. The current AISC specification recommends the bilinear axial force-bending moment (P−M) interaction curve for bare steel members for the design of noncompact and slender CFT beam-columns. This paper compiles the experimental database of tests conducted on noncompact and slender CFT beam-columns, and demonstrates the overconservatism of the AISC P−M interaction curve. This paper also presents the development and benchmarking of detailed 3D finite-element models for predicting the behavior and strength of noncompact and slender CFT members. The benchmarked models are then used to evaluate the fundamental P−M interaction behavior of CFT beam-columns, and the influence of material and geometric parameters such as the tube slenderness ratio (λ), material strength ratio (Fy/fc′), member length-to-sec...

In-plane seismic behavior of rectangular steel-plate composite wall piers

AbstractAn experimental study investigated the behavior of large-scale steel-plate composite (SC) walls subjected to cyclic lateral loading. The testing program involved four rectangular SC wall specimens with an aspect ratio (height-to-length) of 1.0. The specimens were anchored to a concrete basemat with a pretensioned bolted connection that was designed to be stronger than the walls. The design parameters considered in the investigation were wall thickness, reinforcement ratio, stud spacing, and tie bar spacing. The pretest analyses, global force-displacement responses, contributions of the steel faceplates and infill concrete to the lateral resistance, load transfer between the faceplates and infill concrete, and damage to the face plates and infill, are documented. The four SC walls failed in a flexural mode characterized by tensile cracking of the concrete, tensile yielding of the steel plates, crushing of concrete at the toes of the wall, outward local buckling of the steel faceplates, and fracture...

Stability Behavior of Steel Building Structures in Fire Conditions: Role of Composite Floor System with Shear-Tab Connections

This paper presents a qualitative assessment of the influence of the composite floor system and shear-tab connections on the stability behavior of a typical mid-rise (10-story) steel building subjected to corner compartment fires. A ten-story steel building with composite floor systems was designed following the design practices in the US. The building had an interior core of reinforced concrete (RC) shear walls to resist the lateral loads. Effects of gravity loads and fire conditions were simulated using the finite element method and numerical analysis techniques. The concrete material model used in the numerical simulations was benchmarked using experimental data from concrete slab thermal tests. The results from the numerical investigations indicated that at elevated temperatures, the composite beam undergoes elongation, sagging and rotation at the beam ends. This results in additional rotation and compression demands on the connections at the ends. The shear-tab connections provided significant negati...