Thomas M. Murray

In Search of Suitable Control Methods for Semi-Active Tuned Vibration Absorbers:

The main purpose of this study is to identify a suitable control method for semi-active tuned vibration absorbers (TVAs) in structural vibration applications. Four control policies are considered. The semi-active control schemes include the following: velocity-based, on-off groundhook control (on-off VBG); velocity-based, continuous groundhook control (continuous VBG); displacement-based, on-off groundhook control (on-off DBG); and displacement-based, continuous groundhook control (continuous DBG). A force-excited model that can be representative of many structural systems is adapted as the baseline model for our analysis. Each of the control policies is applied to the baseline model coupled with a TVA. In order to equally evaluate the control policies, the TVA parameters are optimized according to each policy using numerical optimization techniques. The optimal design parameters are obtained based on minimization of peak transmissibility. The performances of each of the optimized cases are then compared along with the equivalent passive model using the peak transmissibility criteria. The results indicate that all of the semi-active peak transmissibilities are lower than those of the passive, implying that the semi-active TVAs are more effective in reducing vibration levels. The results further indicate that on-off DBG performs the best among the considered control polices.

Three-dimensional finite element analysis of double angle connections under tension and shear

Abstract Double angle connections welded to the beam web and bolted to the column flange are considered. They are subjected to axial tensile loads, shear loads, and a combination of these loads. The loads are increased monotonically and steel angles with three different thicknesses are analyzed. An elastic–perfectly plastic constitutive law is assumed. The connection and beam are discretized using three-dimensional finite elements in ABAQUS. Modeling of the separation of the angle from the column and modeling of the contact forces between the bolt heads and the angles are included. Load–displacement and moment–rotation curves for the three connections are generated, as well as stress distributions for the angles. In addition, an experimental test is carried out with axial loading, and two mechanical models are proposed.

Recent developments for the design of moment end-plate connections

Moment end-plate connections are used in the United States primarily in pre-engineered, industrial buildings. Their use in multi-story, moment-resistant frame construction is becoming more common because of advancements in design methods and fabrication techniques. This paper reviews past literature and design methods for both flush and extended end-plate configurations, including column side limit states. Recommended design procedures, which are based on recent analytical and experimental research in the United States, are then presented. Design procedures for two flush end-plate configurations (two and four bolts at the tension flange) are based on the yield-line method, but include tension bolt force predictions. Procedures for two extended configurations (four and eight bolts at the tension flange) are the result of finite element studies. Bolt force predictions are also included in these procedures. Methods to evaluate the three column side limit states (web yielding, web buckling and flange bending) at moment end-plate connections are also presented.

Finite element modeling of large capacity stiffened steel tee-hanger connections

Abstract This paper presents the development of a mathematical model describing the behavior of a stiffened steel tee-hanger connection. The particular tee-hanger connection has two rows of pretensioned high-strength bolts on either side of the tee-stem (eight bolts total). This geometric configuration results in a highly indeterminate problem as the bolt forces cannot be determined directly. Thus, an analytical study with modeling of the connection as an assemblage of finite elements was conducted. Only one-quarter of a symmetric section of the tee-hanger was analysed with eight-noded isoparametric brick elements used for the tee-flange plate elements. Bi-linear stress-strain behavior is considered for both the bolt and plate material. To consider the inelastic steel behavior in each cycle, the elastic moduli of the yielded elements are reset to their secant values. Analytical results are compared with results from tee-hanger connection tests. Based on comparison of experimental and analytical results, it is concluded that the model developed adequately explains the connection behavior. The study is restricted to A36 steel and A325 bolts with maximum diameter of 1 1 2 inches.

Controlling Floor Vibration with Active and Passive Devices

This paper reviews research, conducted by the authors over the last decade, pertaining to the control of ex- cessive floor vibration using active and passive devices. The active device studied uses a proof-mass actuator to deliver the control force to the floor system. Effectiveness and stabil- ity characteristics for a single-input/single-output (SISO) con- trol scheme, using velocity feedback, are explored. The SISO system is shown to increase damping to 40% of critical on an experimental floor when amplitudes remain in the linear range. When implemented on two in-place floors, at least a 70% reduction in vibration amplitudes due to walking was ob- served. Next, the benefits of expanding to a practical single- input/multi-output (SIMO) control system are identified. Ad- ditionally, techniques to optimize the SIMO scheme are presented. Because of the stability characteristics of the controlled system, the improvement noted for the SIMO scheme is most dramatic for floors with fundamental frequen- cies near the natural frequency of the actuator. I na2H zfloor example, a SIMO control scheme provided seven times more reduction than that of the SISO system. The passive device research focuses on the experimental implementation of tuned mass dampers (TMDs) to control floor vibration. Two different configurations are explored. The uniqueness of the first device is that liquid filled bladders are used to provide an economical damping mechanism. When implemented on an office floor, a significant improvement of walking vibration lev- els was observed. Satisfaction with the repair was noted from the occupants. The second device utilizes a configuration that has great flexibility in the field, thus allowing for more eco- nomical mass production. Using two TMDs, a significant re- duction of response was noted for the 5 and 6 Hz modes. Research to improve these active and passive strategies continues and will be reported as significant results are achieved.

Cost‐effective steel building design

In this article, the factors in the cost of steel construction are discussed with emphasis on the dominant components of the total cost. Basic suggestions are given for improving the economy of steel building construction. Subsequently, several recent developments are summarized and the expected impact of these developments on economy is noted. Finally, some of the trends and needs that are critical to the continued advancement of the state-of-the-art in economical steel design and construction are summarized. The focus of this article is steel design and construction in the United States. Although mainly based on American experience in steel design economy, the article has a general value as it addresses the important ultimate goal: quality without costliness.

North American approach to the design of continuous Z- and C- purlins for gravity loading with experimental verification

Abstract The design of cold-formed, through fastened, roof purlins requires a significant number of assumptions. The usual North American practice is to make the following major assumptions: constrained bending, full continuity through the lapped section, and a lap strength equal to the sum of individual purlin strengths. To verify these assumptions, test results of multiple span purlin systems were gathered from 10 laboratories. Of the 45 test results reported, 24 were found to be sufficiently documented for complete evaluation. Comparison of the test results and analysis/design procedures using the above assumptions and AISI specification provisions showed that the assumptions are valid.

Simplified Experimental Evaluation of Floors Subject to Walking-Induced Vibration

AbstractWhen occupants complain of lively floor motion due to human activity, it is often necessary to estimate the dynamic properties of the floor and ascertain the form and level of vibration response toward the goal of designing an effective retrofit solution. The most detailed and accurate experimental method is to determine the natural vibration modes using full experimental modal analysis, which requires substantial equipment including at least one floor shaker, and then determine the acceleration due to walking. However, due to cost and time constraints and possible unacceptable occupant disruption, it may not be possible to perform a full experimental modal analysis. An alternative testing procedure is to use a handheld spectrum analyzer to record floor accelerations due to heel drop and timed walking excitations. These measurements are then used to estimate the natural frequencies and vibration response level, respectively. This paper describes the procedure and compares results with experimental...

Effect of Bottom Chord Extensions on the Static Flexural Stiffness of Open-Web Steel Joists

AbstractAnnoying floor vibrations, a growing issue in today’s world of lighter structural members and low inherent damping because of modern office fit-outs, may also be significant in open web steel joist supported floor systems. In the presence of these vibrations, sometimes bottom chord extensions are installed in an attempt to stiffen the floor and correct the annoying vibrations problem. This study focuses on the effect of bottom chord extensions on the static flexural stiffness of joists under relatively light loads associated with occupant-induced floor vibrations. Extensive analytical and experimental studies were conducted on single-span and three-span joist-supported laboratory footbridges with different bottom chord extension configurations. Three-dimensional finite-element computer models were created to simulate and compare the results of the stiffness tests. Testing was done with (1) the bottom chord extensions installed prior to concrete placement, (2) all or part of the bottom chord extens...

Observations from Vibration Testing of In-Situ Structures

Low-frequency floor and footbridge vibration serviceability problems typically arise when the structure is excited in resonance due to a walking excitation and the resulting accelerations exceed human comfort levels. The measures required to resolve an annoying vibration problem after the structure is constructed can be very difficult and expensive to implement. In most cases, the costs of fixing the problem in-situ are much greater than tackling the problem in the design phase, prior to the structures construction, considering the potential cost to building owners from possible legal expenses, loss of rental revenue, and consultation fees. Design guidance for composite steel framed floor systems and footbridges is available in the AISC/CISC Steel Design Guide Series 11: Floor Vibrations Due to Human Acitivity . Although the current design guidance is generally acceptable, there is a need to continue characterizing the often-complicated vibration behavior of these structures in an effort to refine current design and analysis techniques, particularly as researchers gain a better understanding of behavior by collecting high-quality experimental data on in-situ floor and footbridge structures. This paper presents observations from such efforts to further characterize behavior through experimental modal testing of a large in-situ composite steel office floor and a laboratory constructed multi-span footbridge. While not entirely inclusive, some general observations are noted on the dynamic behavior, problems encountered, and the consistency/reliability of the applied testing/analysis techniques employed by the researchers.