VTechWorks Repository :: Browsing by Author "Murray, Thomas M."

Browsing by Author "Murray, Thomas M."

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Active control of floor vibrations
Hanagan, Linda M. (Virginia Tech, 1994-12-15)
The active control of structures is a diverse field of study, with new applications being developed continually. One structural system, which is often not considered a dynamic system, is the floor of a building. In many cases the dynamics of a floor system are neglected in the design phase of a building structure. Occasionally, this omission results in a floor which has dynamic characteristics found to be unacceptable for the intended use of the building. Floor motion of very small amplitudes, often caused by pedestrian movement, is sometimes found objectionable by occupants of the building space. Improving an unacceptable floor system's dynamic characteristics after construction can be disruptive, difficult and costly. In search of alternative repair measures, analytical and experimental research implementing active control techniques was conducted to improve the vibration characteristics of problem floors. Specifically, a control scheme was developed utilizing the measured movement of the floor to compute the input signal to an electromagnetic actuator which, by the movement of the actuator reaction mass, supplies a force that reduces the transient and resonant vibration levels. Included in the analytical component of this research is the development of a mathematical model for a full scale experimental test floor. This model is studied, using a matrix computation software, to evaluate the effectiveness of the control scheme. The experimental component of the research serves two purposes. The first is the verification of the system behavior assumed in the analytical component of the research. The second is the verification of control system effectiveness for various excitations, control gains, and actuator locations on the experimental test floor and six additional floors.
Analysis and Design of Steel Deck-Concrete Composite Slabs
Widjaja, Budi R. (Virginia Tech, 1997-10-15)
As cold-formed steel decks are used in virtually every steel-framed structure for composite slab systems, efforts to develop more efficient composite floor systems continues. Efficient composite floor systems can be obtained by optimally utilizing the materials, which includes the possibility of developing long span composite slab systems. For this purpose, new deck profiles that can have a longer span and better interaction with the concrete slab are investigated. Two new mechanical based methods for predicting composite slab strength and behavior are introduced. They are referred to as the iterative and direct methods. These methods, which accurately account for the contribution of parameters affecting the composite action, are used to predict the strength and behavior of composite slabs. Application of the methods in the analytical and experimental study of strength and behavior of composite slabs in general reveals that more accurate predictions are obtained by these methods compared to those of a modified version of the Steel Deck Institute method (SDI-M). A nonlinear finite element model is also developed to provide additional reference. These methods, which are supported by elemental tests of shear bond and end anchorages, offer an alternative solution to performing a large number of full-scale tests as required for the traditional m-k method. Results from 27 composite slab tests are compared with the analytical methods. Four long span composite slab specimens of 20 ft span length, using two different types of deck profiles, were built and tested experimentally. Without significantly increasing the slab depth and weight compared to those of composite slabs with typical span, it was found that these long span slabs showed good performance under the load tests. Some problems with the vibration behavior were encountered, which are thought to be due to the relatively thin layer of concrete cover above the deck rib. Further study on the use of deeper concrete cover to improve the vibrational behavior is suggested. Finally, resistance factors based on the AISI-LRFD approach were established. The resistance factors for flexural design of composite slab systems were found to be f=0.90 for the SDI-M method and f=0.85 for the direct method.
Analysis and Modeling of Snap Loads on Synthetic Fiber Ropes
Hennessey, Christopher Michael (Virginia Tech, 2003-11-07)
When a rope quickly transfers from a slack state to a taut state, a snapping action occurs and produces a large tensile force which is known as a snap load. Energy is dissipated during this snap load, and it is proposed to use synthetic fiber ropes as a type of passive earthquake damper in order to take advantage of this phenomenon. This thesis is the second phase of a multi-stage research project whose goal is to investigate and develop what will be known as Snapping-Cable Energy Dissipators (SCEDs). The experimental data that was collected in the Master's Thesis of Nicholas Pearson was organized and analyzed as a part of this research in order to evaluate the behavior of the ropes during the snapping action. Additional tests were also conducted for this project under more controlled conditions in order to better understand how the ropes change throughout a sequence of similar snap loadings and also to determine the amount of energy that is dissipated. The data from both projects was then used as input parameters for a mathematical model that was developed to characterize the behavior of the ropes during a snap load. This model will be utilized in subsequent research involving the finite element analysis of the seismic response of structural frames containing SCEDs.
Analysis of Buckled and Pre-bent Columns Used as Vibration Isolators
Sidbury, Jenny Elizabeth (Virginia Tech, 2003-12-04)
Vibrations resulting from earthquakes, machinery, or unanticipated shocks may be very damaging and costly to structures. To avoid such damage, designers need a structural system that can dissipate the energy caused by these vibrations. Using elastically buckled struts may be a viable means to reduce the harmful effects of unexpected vibrations. Post-buckled struts can support high axial loads and also act as springs in a passive vibration isolation system by absorbing or dissipating the energy caused by external excitation. When a base excitation is applied, the buckled strut may act to reduce the dynamic force transmitted to the system, thus reducing the structural damage to the system. Several models of buckled and pre-bent struts are examined with different combinations of parameters and end conditions. The models include pinned or fixed columns supporting loads above their buckling load, and columns with an initial curvature supporting various loads. The varying parameters include external damping, internal damping, and stiffness. The columns will be subjected to simple harmonic motion applied at the base or to a multi-frequency base excitation. The response of each model is measured by the deflection transmissibility of the supported load over a large range of frequencies. Effective models reduce the motion of the supported load over a large range of frequencies.
Analytical and Experimental Investigation of Improving Seismic Performance of Steel Moment Frames Using Synthetic Fiber Ropes
Ryan, John C. (Virginia Tech, 2006-09-22)
The presented research investigated the viability of a double-braided synthetic fiber rope for providing improved performance of steel moment frames subjected to earthquake-induced ground motions. A series of experimental tests, including a 1:3-scale dynamic test and 1:6-scale shaking table tests, was conducted using Northridge ground-motion input. A series of nonlinear dynamic analytical studies, using DRAIN-2DX, was conducted to develop the experimental tests. Throughout experimental testing, the ropes exhibited a hyper-elastic loading response and a reduced-stiffness unloading response. A conditioning cycle was defined as a loading cycle induced in the rope above the highest load expected to be experienced by the rope, and was determined to be requisite for ropes intended to be used for the stated objectives of the research program. After experiencing a conditioning cycle, the rope response returned to initial conditions without permanent deformation, demonstrating repeatability of response through several loading cycles below the conditioning load. In the 1:6-scale shaking-table experiments, the ropes drastically improved the performance of the steel moment frames. Maximum and residual drift were reduced significantly, with a corresponding minimal increase to the maximum base shear. Base shear was reduced at several peaks subsequent to the initial pulse of the Northridge ground-motion input. The analytical model developed was excellent for predicting elastic response of the 1:6-scale shaking table experiments and adequate for the purpose of planning shaking table studies. Correlation of peak rope forces between the analytical model and experimental results was poor, and was attributed to limitations of the pre-defined elements used to represent the rope devices in the software program. The inability of the elements to capture the complex unloading response of the rope was specifically noted.
Application of Magneto-Rheological Dampers in Tuned Mass Dampers for Floor Vibration Control
Ritchey, John Kenneth (Virginia Tech, 2003-10-02)
The purpose of this research is to establish the effectiveness of tuned-mass-dampers (TMD) using semi-active magneto-rheological (MR) dampers to mitigate annoying floor vibrations. Annoying floor vibration is becoming more common in today's building structures since building materials have become stronger and lighter; the advent of computers has resulted in "paperless" offices; and the use of floors for rhythmic activities, such as aerobics and concerts, is more common. Analytical and experimental studies were conducted to provide an understanding of the effects of incorporating the semi-active-TMD as a remedy to annoying floor vibration. A pendulum tuned mass damper (PTMD) in which the tuning parameters could independently be varied was used. Closed form solutions for the response of the floor using passive dampers were developed. In addition, a numerical integration technique was used to solve the equations of motion where semi-active dampers are utilized. The optimum design parameters of PTMDs using passive and semi-active dampers were found using an optimization routine. Performances of the PTMD in reducing the floor vibration level at the optimum and when subjected to off-tuning of design parameters using passive and semi-active dampers were compared. To validate the results obtained in the analytical investigation, an experimental study was conducted using an 8 ft x 30 ft laboratory floor and a commercial PTMD. Comparative studies of the effectiveness of the PTMD in reducing floor vibrations using semi-active and passive dampers were conducted.
Application of the Finite Element Method to the Seismic Design and Analysis of Large Moment End-Plate Connections
Mays, Timothy Wayne (Virginia Tech, 2004-08-12)
Due to problems associated with welded moment connections uncovered after the Northridge earthquake, large bolted connections are becoming a much more attractive alternative for design in seismic regions. However, stringent design requirements established by the AISC Seismic Provisions for Structural Steel Buildings (1997) make current moment end-plate configurations and design procedures inadequate for multi-story buildings. This dissertation first examines and critiques current seismic design philosophies as applied to moment end-plate connections. Next, the finite element method is used to develop much-needed design procedures for large moment end-plate connections, and to improve the understanding of the role of geometric parameters (e.g., bolt pitch and stiffener locations) in the response of these connections. Finally, single-story and multi-story frames incorporating large moment end-plate connections with known moment-rotation characteristics are considered under seismic loading to determine the effectiveness of these systems in dissipating energy caused by the ground motion.
Behavior and modeling of partially restrained beam-girder connections
Rex, Clinton O. (Virginia Tech, 1996)
Beams in a typical steel framed floor design are assumed to have pinned supports for purposes of design. In reality, the connections between the beams and girders in a steel framed floor system are not pinned. The design bending moments and deflections of the attached beam could be reduced if the true rotational restraint provided by the beam-girder connections could be included in the design. The connection rotational restraint is characterized by the moment-rotation behavior. Consequently, a method for approximating the moment-rotation behavior of the beam-girder connection is required before the beneficial effects of the true connection rotational restraint can be considered in design. Experimental and analytical research on the moment-rotation behavior of a specific type of beam-girder connection is presented in this dissertation. The primary objective of this research is to develop a component model of the connection that can be used to approximate the moment-rotation behavior. The component model is based on the hypothesis that the connection behavior can be modeled as a combination of the connection component behaviors. The connection components are the fundamental pieces of the connection such as bolts, shear studs, and welds. In general, the component model can be very computationally intensive. Consequently, a secondary objective of this research is to develop a connection model that is simpler to use. Behavior models for each of the connection components are presented and/or developed. These models are derived from a combination of existing literature, experimental and analytical research, and basic mechanics. Next, a method of combining the component behaviors into a connection model that can be used to approximate the moment-rotation behavior is developed. Results from experimental research on the moment-rotation behavior of the beam-girder connection are then used to verify the model. Finally, a simplified model of the beam-girder connection is developed. This model is based on the same hypothesis as the component model; however, through a combination of assumptions, simplifications, and the results of parametric studies the simplified model becomes far less computationally intensive than the full component model.
Behavior and Strength of Simple and Continuous Span Re-Entrant Composite Slabs
Traver, Thomas Mathew (Virginia Tech, 2002-07-29)
This study investigates the further development of the commercially available re-entrant steel deck profile. The effects of various embossments and continuous construction are investigated through three Series of composite slab load tests. The test specimens in this study were constructed to simulate actual field construction of composite slabs as part of reinforced concrete structures. The results of this experimental study are analyzed using methods given in the ASCE Standard for the Structural Design of Composite Slabs. Recommended design procedures for the improved re-entrant profile are given and various future profile modifications are suggested.
Behavior and Strength of Welded Stud Shear Connectors
Rambo-Roddenberry, Michelle (Virginia Tech, 2002-04-08)
The behavior and strength of welded shear studs are subjects of ongoing study. In recent years, research has shown that the American Institute of Steel Construction (AISC) specification equations for shear stud strength are unconservative for studs placed in deck with ribs transverse to the steel beam. Twenty-four solid slab push-out tests, 93 composite slab push-out tests, and bare stud tests were performed to study the effects on stud strength of friction, normal load, position of studs in the ribs of steel deck, concrete strength, and stud properties. Stud diameters ranged from 3/8 in. to 7/8 in., deck heights ranged from 2 in. to 6 in., and both single and pairs of studs were tested. The push-out test results from this study were combined with other studies to propose a new stud strength prediction model. Three new beam tests were performed to study the effect of the stud position in the ribs of the steel deck. The results of these tests, along with 61 other beam tests, were used to verify the new stud strength prediction model. A reliability study was performed to determine resistance factors for stud strength and beam strength.
Behavior of Connection with Beam Bearing on Bottom Flange of Girder
Lee, Wey-Jen (Virginia Tech, 2001-09-20)
An analytical investigation was conducted to study the behavior of a bottom flange bearing beam-to girder connection subjected to patch loading. This connection would be useful with deep deck (thickness greater than 3 in.) composite slabs as well as with commonly used deck where floor-to-floor height needs to be minimized. Five girder specimens were loaded until yielding during the initial phase of the research. The analysis section consists of the yield line theory and finite element study that were used to develop a model to predict the collapse loads of the girder sections. These results from the model were then compared to the experimental loads. A design procedure utilizing the proposed model and future work recommendations are then presented.
Behavior of Diagonal Knee Moment End-Plate Connections
Italiano, Vincenza M. (Virginia Tech, 2001-05-03)
An experimental and analytical investigation was conducted to study the behavior of diagonal knee moment end-plate connections and a multiple row extended moment end-plate connection. Diagonal knee moment end-plate connections differ from typical moment end-plate connections because of the large pitch distance required between the top flange and first row of tension bolts. The large pitch distance is outside of the geometric parameters of all previous research. Design solutions are presented for five moment end-plate connections with provisions added to accommodate these parameters. The analytical investigation focused on the limit states of end-plate yielding and bolt rupture. Yield-line analysis was used to predicted end-plate yielding and a simplified Kennedy method proposed by Borgsmiller and Murray (1995) was used to predict bolt rupture including and excluding prying forces. An experimental investigation was conducted to verify the design solutions. Five knee area specimens and one plate girder specimen were tested in this study. The analytical and experimental results are analyzed and compared. For the test specimens that failed in the connection, the predicted results proved to be conservative. Recommendations are presented at the end of the study as well as sample calculations.
Classification of end plate connections with application to gable frames
Banerjee, Gautam (Virginia Tech, 1990-02-18)
In this study, connection classification system is developed on the basis of previous classifications. Further, flexible connections are modeled by matrix displacement method. The effect of flexible connections are studied on gable frames. Firstly, flush end-plate connections with single row of bolts at the tension flange, are classified. The classification system was developed in this study. End plate connections whose moment-rotation curves are known arc classified on the basis of moments as FR (fully restrained) and PR (partially restrained) connections. Further, the connections are also classified by entering a plot with coordinates - Ratio of Moment at the connection and plastic moment and ratio of corresponding rotation and rotation at plastic moment. Depending on the location the connection can be classified. Secondly, for connections, the rotational stiffness is determined from the moment rotation curves and used in the computer code to implement flexible connections. The effect is studied on gable frames. For the loading and frame used there is not much variation in moments at the flexible joints due to connection flexibility and hence flush end plate connections can be used in gable frames effectively
Comparison and Analysis of the Strength, Stiffness, and Damping Characteristics of Concrete with Rubber, Latex, and Carbonate Additives
Bowland, Adam Gregory (Virginia Tech, 2011-06-27)
This dissertation presents the results of a study performed to investigate methods for increasing the damping capacity of concrete. A variety of additives, both particle and latex based, were added to standard concrete mixtures by replacing up to 20% of the fine aggregate to measure their effects on strength, stiffness, damping, and air content. The additives included rubber particles from recycled tires, calcium carbonate particles, styrene butadiene rubber (SBR) latex, and a commercially available product named ConcreDamp which contains vegetable gum suspended in styrene butadiene latex. An initial investigation resulted in the observation that all of the additives with the exception of the SBR latex would both increase air content and decrease compressive strength. As a result, combinations of additives were investigated to see if both the mechanical and dynamic properties could be improved. The addition of steel fibers to mixtures with ground rubber were found to significantly increase air content which offset any gains in compressive strength. The combination of ground rubber and latex was shown to improve both increase compressive strength and reduce air content. The study advanced to investigate the effects of rubber size on air content, strength, and damping. It was found that for the same volume of rubber, a larger rubber particle would decrease air content, decrease compressive strength, and improve damping. The results of this study show that the best performing additive was the vegetable gum latex which improved the concrete damping by a factor of 2 when added as 15% of the fine aggregate. Additionally, an equation is presented for calculating a strength reduction factor for concrete containing rubber particles of different sizes. Finally, two full scale footbridge laboratory specimens were tested to investigate the effect of increased material damping at the structural level. One footbridge was constructed using a base concrete mixture without damping admixtures. The second was constructed with a concrete mixture that contained a replacement of 15% of the fine aggregate with ground rubber. The results were used to create a finite element model in SAP2000 that was used to predict the effects that high damping concretes would have on the footbridge specimen.
Comparison of P-Delta Analyses of Plane Frames Using Commercial Structural Analysis Programs and Current AISC Design Specifications
Schimizze, Angela Marie (Virginia Tech, 2001-04-23)
Several different approaches to determining second-order moments in plane frames were studied during this research. The focus of the research was to compare the moments predicted by four different commercially available computer analysis programs and the current design specification, the AISC LRFD moment magnification method. For this research, the second-order moments for ten commonly designed frames were compared. An overview of various second-order analysis procedures is presented first. The solution procedure utilized by each computer program and the AISC moment magnification method are explained. Also, the frames considered in the research are described. Next the frames are analyzed and the results between each of the computer programs and the current design specifications are compared. Finally, conclusions are drawn concerning the consistency of the second-order moments predicted by each of the solution procedures and recommendations for their use are discussed. In general, each of the four computer analysis programs evaluated and the AISC moment magnification method can consistently and adequately predict the second-order moments in plane frames.
Computational Study of Tension Field Action in Gable Frame Panel Zones
Wei, Gengrui; Koutromanos, Ioannis; Murray, Thomas M.; Eatherton, Matthew R. (2018-04-01)
A computational study was conducted to evaluate the extent to which the column panel zone in a gable frame knee joint can develop tension field action when subjected to positive bending (top flange of the rafter is in tension). Past testing was reviewed and used to validate finite element models. The validated finite element modeling approach that utilized shell elements, was then used in a parametric study to evaluate the post-buckling strength of the panel zone beyond the shear buckling strength. In addition, a plastic mechanism model was used to derive an equation for the post-buckling shear strength of the panel zone and the derived equation was validated against the results of the parametric study. The result was a proposed equation for tension field action strength of the panel zone when subjected to positive bending that had an average error of 1% compared to the model results.
Connection limit states design teaching aid
Kerr, Robert A. (Virginia Tech, 1994-07-15)
Steel connection design is one area in structural steel design courses that is not always thoroughly addressed. This report attempts to address this area of steel design at a basic level. Its purpose is to be used as a teaching aid for a structural steel design course, and to familiarize students with connection design and its associated strength limit states. Limit states for steel connection design have been covered using both AISC ASD and LRFD Specifications. However, all included connection design examples used only LRFD limit states. Wherever possible all limit state calculations are accompanied by printouts from a knowledge-based expert system, CONXPRT. Typical building connection limit states are covered by way of an accompanying steel structure, which includes many of the connections, in order for students to receive an adequate grasp of both simple framing and moment connections. The purpose of this steel "sculpture" is to actively reinforce the students' understanding of the basic building connections seen in industry today.
Deep Deck and Cellular Deck Diaphragm Strength and Stiffness Evaluation
Bagwell, Jonathan (Virginia Tech, 2007-06-13)
Twenty cantilever diaphragm tests were performed in the Structures and Materials Laboratory at Virginia Tech. The tests included both deep deck and cellular deck profiles with varying structural and side-lap connections. The tests were conducted with three different structural connections: screws, pins and welds and two different side-lap connections: screws and button punch. The tests were conducted and both load and deflection of the diaphragms were recorded. The current International Code Council, ICC, evaluation procedure shows that there are two different methods for measuring diaphragm deflection. The first method was by measuring specific corner displacements and making corrections to remove any rigid body motion. The second method is by measuring the deflection of the diagonals of the diaphragm. In this study both measurements were taken to do a comparison of the results that were obtained. Both strength and stiffness values were calculated based on the Steel Deck Institute (SDI) Diaphragm Design Manual (2004) and modifications described by Luttrell (2005). The paper by Luttrell (2005) only recommends modifications for the calculation of diaphragm stiffness. The data obtained from the tests were compared to the SDI calculations to distinguish any noticeable trends. Modifications are recommended regarding diaphragm strength and further research is suggested to create a better stiffness prediction of diaphragms.
Design and Behavior of Composite Steel-Concrete Flexural Members with a Focus on Shear Connectors
Mujagic, Ubejd (Virginia Tech, 2004-04-06)
This study consists of three self-standing parts, each dealing with a different aspect of design of composite steel-concrete flexural members. The first part deals with a new type of shear connection in composite joists. Composite steel-concrete flexural members have increasingly become popular in design and construction of floor systems, structural frames, and bridges. A particularly popular system features composite trusses (joists) that can span large lengths and provide empty web space for installation of typical utility conduits. One of the prominent problems with respect to composite joists has been the installation of welded shear connection due to demanding welding requirements and the need for significant welding equipment at the job site. This part of the study presents a new type of shear connection developed at Virginia Tech— standoff screws. Results of experimental and analytical research are presented, as well as the development of a recommended design methodology. The second part deals with reliability of composite beams. Constant research advances in the field of composite steel-concrete beam design have resulted in numerous enhancements and changes to the American design practice, embodied in the composite construction provisions of the AISC Specification (AISC 1999). Results of a comprehensive reliability study of composite beams are presented. The study considers specification changes since the original reliability study by Galambos et al. (1976), considers a larger database of experimental data, and analyses recent proposals for changes in design of shear connection. Comparison of three different design methods is presented based on a study of 15,064 composite beam cases. A method to consider effect of degree of shear connection on strength reduction factor is proposed. Finally, while basic analysis theories between the two are similar, requirements for determining the strength of composite beams in Eurocode 4 (CEN 1992) and 1999 AISC Specification (AISC 1999) differ in many respects. This is particularly true when considering the design of shear connections. This part of the dissertation explores those differences through a comparative step-by-step discussion of major design aspects, and accompanying numerical example. Several shortcomings of 1999 AISC Specification are identified and adjustments proposed.
Design of moment end-plate connections for seismic loading
Meng, Ronald L. (Virginia Tech, 1996)
Analytical and experimental research into the seismic response of four-bolt extended moment end-plate connections was conducted. Full-scale connections, ranging in size from moderate to large, were designed, fabricated and tested under cyclic loading until connection failure was observed. The design procedures for minimum end-plate thickness were developed from yield-line theory with prying forces included in the bolt tension forces. Stiffened end-plates, four-bolt wide connections and shimmed end-plate connections were valiations of the four-bolt connection tested. A325 and A490 bolts with internal strain gauges were employed to record and analyze bolt tension forces. Test results demonstrate that the design approach is satisfactory, but several aspects of connection response not previously observed nor reported were encountered. When weld access holes were present in an extended end-plate connection, excessive 3-D stresses developed in the hole region, causing a brittle fracture of the beam flange. In the absence of weld access holes, ductile failure occurred, evidenced by local buckling of the beam flanges and plastic hinge formation. These two responses or failures were exhibited by all connection sizes. The use of end-plate stifieners appeared to provide sufficient stress reduction, as ductile failures were observed in all stiffened, extended end-plate connection tests with weld access holes. Grade 50 steel, four-bolt end-plate connections with built-up beam sections were also tested and demonstrated that inadequate weld strength exists in the beam web-toflange welds. Prior to fracture in these welds, the connections responded in a ductile manner with local beam flange buckling. In conjunction with the full-scale testing, finite element models were created for several connection sizes. When actual material properties of the steel and bolts were modeled, an excellent correlation of test data and the model was noted. When weld access holes were introduced in the models, an increase of flange strain in the hole region was noted. Although not conclusive nor comprehensive for every connection configuration, the four-bolt design appears satisfactory to survive seismic activity. Further research should provide answers to other configurations and eventually provide an acceptable alternative beam-to-column connection for high seismic areas.

Browsing by Author "Murray, Thomas M."

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