Rocking Response of Free-Standing Blocks under Cycloidal Pulses

This paper examines in depth the transient rocking response of free-standing rigid blocks subjected to physically realizable trigonometric pulses. First, the expressions for the dynamic horizontal and vertical reactions at the pivot point of a rocking block are derived and it is shown that the coefficient of friction needed to sustain pure rocking motion is, in general, an increasing function of the acceleration level of the pulse. Subsequently, this paper shows that under cycloidal pulses a free-standing block can overturn with two distinct modes: (1) by exhibiting one or more impacts; and (2) without exhibiting any impact. The existence of the second mode results in a safe region that is located on the acceleration-frequency plane above the minimum overturning acceleration spectrum. The shape of this region depends on the coefficient of restitution and is sensitive to the nonlinear nature of the problem. This paper concludes that the sensitive nonlinear nature of the problem, in association with the presence of the safe region that embraces the minimum overturning acceleration spectrum, complicates further the task of estimating peak ground acceleration by only examining the geometry of free-standing objects that either overturned or survived a ground shaking.     

Experimental Investigations on the Seismic Response of a Base-Isolated Reinforced Concrete Frame Model

The existing knowledge based on the physical characteristics of rubber-based seismic isolators and the validity of assumptions involved in design can be strengthened with shake table tests and field monitoring during earthquakes. Existing structures can be retrofitted with seismic isolators and successful examples are coming up day by day. This paper presents the results of analytical and experimental investigations of a one-third scaled model of a reinforced concrete soft first-storied structure mounted on natural rubber-based isolators and subjected to uniaxial seismic motion. Translational and rocking responses of the scaled model were measured under four different synthetic time histories, covering a wide range of frequencies. Through base isolation, the time period of the horizontal mode is far removed from the peak spectrum but the rocking modes may well be entrapped in the peak region. Hence, a simplified fail-safe system for protection against overturning was also studied.     

Effects of a vestibular stimulation program on stereotypic rocking behavior

This study measures the effect of a vestibular stimulation program on the stereotypic rocking behavior of three severely mentally retarded persons within both experimental and natural settings. A multiple baseline design was used. Frequency and duration of stereotypic rocking behavior were measured by a partial-interval time sample. Results indicated a statistically significant reduction of both frequency and duration of rocking behavior directly after receiving vestibular stimulation and 1 hour after stimulation. The rocking behavior remained reduced after 6 days without the vestibular stimulation program. It was concluded that vestibular stimulation resulted in a reduction of the stereotypic rocking behavior of these subjects.     

Full-Scale Tests of Seismic Cable Restrainer Retrofits for Simply Supported Bridges

Many parts of the central and southeastern United States have recently begun initiating seismic retrofit programs for bridges on major interstate highways. One of the most common retrofit strategies is to provide cable restrainers at the intermediate hinges and abutments in order to reduce the likelihood of collapse due to unseating. To evaluate the force-displacement behavior of the cable restrainer retrofits, a full-scale bridge setup was constructed based on an existing multispan, simply supported steel girder bridge in Tennessee, that has been considered for seismic retrofit using cable restrainers. Seismic cable restrainers were connected to the bridge pier using steel bent plates, angles, and undercut anchors embedded in the concrete as specified by typical bridge retrofit plans. The full-scale bridge model was subjected to monotonic loading to test the capacity of the cable restrainer system and to determine the modes of failure. The results showed that the primary modes of failure are in the connection elements of the pier and girders, and they occur at force levels much lower than the strength of the cable. Modifications to the connection elements were designed and tested. The new connections resulted in a higher strength and deformation capacity of the cable restrainer assembly.     

Effects of Moment-to-Shear Ratio on Combined Cyclic Load-Displacement Behavior of Shallow Foundations from Centrifuge Experiments

Current design guidelines for shallow foundations supporting building and bridge structures discourage footing rocking or sliding during seismic loading. Recent research indicates that footing rocking has the potential to reduce ductility demands on structures by dissipating earthquake energy at the footing-soil interface. Concerns over cyclic and permanent displacements of the foundation during rocking and sliding along with the dependence of foundation capacity on uncertain soil properties hinder the use of footing rocking in practice. This paper presents the findings of a series of centrifuge experiments conducted on shear wall-footing structures supported by dry dense to medium dense sand foundations that are subjected to lateral cyclic loading. Two key parameters, static vertical factor of safety (FSV), and the applied normalized moment-to-shear ratio (M/(H?L)) at the footing-soil interface, along with other parameters, were varied systematically and the effects of these parameters on footing-soil system behavior are presented. As expected, the ratio of moment to the horizontal load affects the relative magnitude of rotational and sliding displacement of the footing. Results also show that, for a particular FSV, footings with a large moment to shear ratio dissipate considerably more energy through rocking and suffer less permanent settlement than footings with a low moment to shear ratio. The ratio of actual footing area (A) to the area required to support the vertical and shear loads (Ac), called the critical contact area ratio (A/Ac), is used to correlate results from tests with different moment to shear ratio. It is found that footings with similar A/Ac display similar relationships between cyclic moment-rotation and cumulative settlement, irrespective of the moment-to-shear ratio. It is suggested that shallow foundations with a sufficiently large A/Ac suffer small permanent settlements and have a well defined moment capacity; hence they may be used as effective energy dissipation devices that limit loads transmitted to the superstructure.     

Three-Degree-of-Freedom Rigid Model for Seismic Analysis of Cracked Concrete Gravity Dams

A rigid model with three-degrees-of-freedom is proposed for the purpose of seismic analysis of cracked concrete gravity dams. The model considers the geometry of the dam and all its possible modes of motion: sliding, rocking, rock-sliding, and drifting. The governing equations for all the modes are derived with the Mohr-Coulomb friction assumption at the crack, and corresponding conditions to initiate and maintain the modes are also given. For impact that follows rocking and drifting modes, postimpact velocities of the model are explicitly determined according to the momentum principle and the concept of restitution from classical point collision. Studies with the proposed model on rectangular blocks demonstrate two different types of rocking according to the slenderness. Applications to dams indicate that a large coefficient of friction does not necessarily prevent sliding, and rocking and drifting modes should not be neglected in estimating the stability of concrete gravity dams cracked at the base or at a height.     

Experimental Response of Piles in Sand under Compound Motion

This paper describes a fundamental experimental study on the vertical, horizontal, and rocking dynamic behavior of single pile foundations in granular soils. Aimed at generating an extensive experimental database with sufficient parametric variations to clarify a number of issues, multiple series of canonical small-strain forced-vibration centrifuge tests were performed on two model piles using the technique of random vibration and impact loading. Correlated well with vertical- and horizontal-centric dynamic tests within the experimental program, a novel hybrid-mode test method by means of eccentric excitation is validated and employed for the characterization of the foundation responses in general planar motion. A large set of experimental data for different length scales were generated and synthesized in the frequency domain in the form of directional force-response transfer functions. By virtue of the physical measurements, the validity and limitations of two fundamental elastodynamic pile solutions pertaining to the physical dynamic soil-foundation problem are also evaluated.     

Development, Shake Table Testing, and Design of FRP Seismic Restrainers

The development and performance of fiber-reinforced polymer (FRP) fabrics as an alternative to steel for restrainers to reduce bridge relative movements at hinges during earthquakes was explored. Glass, carbon, and hybrid (glass/carbon) restrainers were developed and tested on a representative in-span hinge using a shake table at the large-scale structures laboratory at the University of Nevada, Reno. The components of the study presented in this article are: (1) the FRP restrainer development and testing; (2) comparisons among FRP, steel, and shape memory alloy restrainers; and (3) development and evaluation of a simple restrainer design method and a numerical example. Important findings of the study were that compared to steel restrainers, the FRP restrainers were effective in substantially reducing the relative hinge displacements and pounding at hinges. The method to develop the flexible portion of the FRP restrainers and the bond to superstructure was successful in accomplishing the target performance. The proposed restrainer design method provides a rational yet simple tool to design FRP or other types of restrainers.     

Seismic Restrainer Design Methods for Simply Supported Bridges

Existing restrainer design methods recommended by the American Association of State Highway and Transportation Officials (AASHTO) and the California Department of Transportation (Caltrans) to prevent unseating in simply supported bridges subjected to strong earthquakes were evaluated. Three new methods with different levels of complexity were developed: W∕2, modified Caltrans, and the equivalent linear static design for restrainers. The adequacy of different design methods was assessed using a nonlinear response history analysis computer program. Effects of different earthquakes, substructure stiffness, bearing strength, seat width, and skew angle were studied. It was found that in bridges with reinforced concrete pier caps the inherently wide supports help prevent unseating, and simple methods such the AASHTO and W∕2 method are satisfactory. In bridges with steel pier caps that provide narrow bearings the modified Caltrans method is recommended. The relatively involved equivalent linear static design for restrainers method is a more rational design procedure and leads to considerably fewer restrainers than others.     

In-Plane Seismic Response of URM Walls Upgraded with FRP

Recent earthquakes have shown the vulnerability of unreinforced masonry (URM) buildings, which have led to an increasing demand for techniques to upgrade URM buildings. Fiber reinforced polymer (FRP) can provide an upgrading alternative for URM buildings. This paper presents results of dynamic tests investigating the in-plane behavior of URM walls upgraded with FRP (URM-FRP). These tests represent pioneer work in this area (dynamic and in-plane). Five half-scale walls were built, using half-scale brick clay units, and upgraded on one face only. Two moment/shear ratios (1.4 and 0.7), two mortar types (M2.5 and M9), three composite materials (carbon, aramid, and glass), three fiber structures (plates, loose fabric, and grids), and two upgrading configurations (diagonal “X” and full surface shapes) were investigated. The test specimens were subjected to a series of synthetic earthquake motions with increasing intensities on a uniaxial earthquake simulator. The tests validate the effectiveness of the one side upgrading: the upgrading technique improved the lateral resistance of the URM walls by a factor ranging from 1.3 to 2.9; however, the improvement in the lateral drift was less significant. Moreover, no uneven response was observed during the test due to the single side upgrading. Regarding the upgrading configurations, the bidirectional surface type materials (fabrics and grids) applied on the entire surface of the wall (and correctly anchored) can help postpone the three classic failure modes of masonry walls: rocking (“flexural failure”), step cracking, and sliding (“shear failures”). Additionally, in some situations, they will postpone collapse by “keeping the bricks together” under large seismic deformations. On the other hand, the diagonal “X” shape was less successful and premature failure was developed during the test.     

Seismic Behavior of Batter Piles: Elastic Response

Several aspects of the seismic response of groups containing nonvertical piles are studied, including the lateral pile-head stiffnesses, the “kinematic” pile deformation, and the “inertial” soil-pile-structure response. A key goal is to explore the conditions under which the presence of batter piles is beneficial, indifferent, or detrimental. Parametric analyses are carried out using three-dimensional finite-element modeling, assuming elastic behavior of soil, piles, and superstructure. The model is first used to obtain the lateral stiffnesses of single batter piles and to show that its results converge to the available solutions from the literature. Then, real accelerograms covering a broad range of frequency characteristics are employed as base excitation of simple fixed-head two-pile group configurations, embedded in homogeneous, inhomogeneous, and layered soil profiles, while supporting very tall or very short structures. Five pile inclinations are considered while the corresponding vertical-pile group results serve as reference. It is found that in purely kinematic seismic loading, batter piles tend to confirm their negative reputation, as had also been found recently for a group subjected to static horizontal ground deformation. However, the total (kinematic plus inertial) response of structural systems founded on groups of batter piles offers many reasons for optimism. Batter piles may indeed be beneficial (or detrimental) depending on, among other parameters, the relative size of the overturning moment versus the shear force transmitted onto them from the superstructure.     

Nonlinear Dynamic Response of Piles under Horizontal Excitation

This paper presents test results from cast-in situ reinforced concrete single and group piles subjected to strong horizontal excitation. The tests were conducted for different eccentric moments simulating different excitation levels to obtain the frequency-amplitude response of the pile. Moderate nonlinear behavior is observed in both horizontal and rocking components of vibration. The experimental results were compared with dynamic interaction factor approach using nonlinear solutions. The accuracy of the nonlinear analysis in predicting the dynamic response depends on the choice of parameters that best characterize the response of boundary zone around the pile and the realistic length of pile separation. It is shown in this study that by allowing for boundary zone and separation between pile and soil, close agreement between theoretical predictions and measured response curves can be achieved.     

Effects of Flexibility on Rocking Impedance of Deeply Embedded Foundation

This paper presents a detailed investigation of the influence on the rocking impedance of flexible sidewalls in embedded foundations. The study focuses on vertical and cylindrical foundations embedded in a homogeneous elastic stratum. In order to anticipate the effects of the flexibility, closed-form formulae for rocking impedance are derived based on three-dimensional wave propagation theory and superposition of both the rocking and flexural movements on a single model by assuming complete bonding conditions between the soil and foundations. A function representing the effect of the flexibility is identified from the closed-form formulae, in terms of the height to radius ratio of the foundations and the ratio of the stiffnesses of the ground and foundations. The rocking impedance for a flexible sidewall differs from those evaluated under the assumption of a rigid sidewall. The applicable range of rocking impedance evaluated under the assumption of a rigid sidewall is assessed, and contours to assist in the estimation of the dynamic response of deeply embedded foundations are presented.     

Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Using Nonmetallic Anchor Systems

This paper presents the flexural behavior of reinforced concrete beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) sheets using nonmetallic anchor systems. The developed nonmetallic anchor systems replace the permanent steel anchorage. Nine doubly reinforced concrete beams are tested with various types of nonmetallic anchor systems such as nonanchored U-wraps, mechanically anchored U-wraps, and CFRP sheet-anchored U-wraps. The flexural behavior of the tested beams, including detailed failure modes of each nonmetallic anchor system, is investigated. The study shows that the developed nonmetallic anchors are more effective in resisting peeling-off cracks compared to the permanent steel anchors and the beams strengthened with the nonmetallic anchors provide comparable load-carrying capacity with respect to the steel anchored control beam.     

Seismic Analysis of Segmental Retaining Walls.?II: Effects of Facing Details

This paper describes a finite-element analysis of two 6-m-high segmental walls, subjected to seismic loading, with special attention to connection performance and permanent deformation of the facing. The first segmental wall uses concrete facing blocks with pins, and the second wall uses the same concrete blocks but without pins. The analysis provides some insight into the behavior of segmental walls, particularly the load transfer mechanism between the geosynthetic reinforcement and the segmental facing units duiring earthquake loading.     

Seismic Analysis of Segmental Retaining Walls. I: Model Verification

Block-faced geosynthetic reinforced soil retaining walls, referred to as “segmental” retaining walls, have been extensively used in recent years as permanent civil engineering structures. The disjointed concrete facing blocks are held together through interface friction and concrete keys or mechanical connectors. Because of the disjointed nature of the facing blocks, the design of the segmental wall must consider the available shear resistance between these blocks. Connection capacity must also be considered. Of concern also is the permanent deformation of the segmental wall face following an earthquake. This paper describes a finite-element analysis of a model segmental wall subjected to earthquakelike loading generated by a shake table. The finite-element analysis used the computer program DYNA3D. Results from DYNA3D, using a simple model—Ramberg-Osgood model—to simulate the nonlinear hysteretic behavior of soil, are consistent with observed results from laboratory shake table tests on segmental walls.     

Design of Anchored Slabs in Spillway Stilling Basins

This paper characterizes the temporal behavior of uplift force generated by turbulent pressure fluctuations in spillway stilling basins. Theoretical and experimental analyses are presented that define the magnitude and temporal evolution of the maximum uplift acting on the lining of such basins. Analyses for the dynamic behavior of anchored floor slabs are also investigated. It is concluded that the applicability of the equivalent thickness criterion based on the balance of the forces acting on the slabs in static condition is unsafe for anchored slabs, because this criterion yields an inadequate area for the anchor steel. The results lead to a recommendation to double the area of anchor steel as computed by the equivalent thickness criterion for the design of slabs in stilling basins.     

Queueing network modeling of elementary mental processes.

This article examines the use of reaction time (RT) to infer the possible configurations of mental systems and presents a class of queueing network models of elementary mental processes. The models consider the temporal issue of discrete versus continuous information transmission in conjunction with the architectural issue of serial versus network arrangement of mental processes. Five elementary but important types of queueing networks are described in detail with regard to their predictions for RT behavior, and they are used to re-examine existing models for psychological processes. As continuous-transmission networks in the general form, queueing network models include the existing discrete and continuous serial models and discrete network models as special cases, cover a broader range of temporal and architectural structures that mental processes might assume, and can be subjected to empirical tests. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Flow-Induced Failure of Cable-Tied Blocks

An experimental study of flow-induced failure of cable-tied blocks is presented. The particular failure mechanism studied is overturning and rolling up of the leading edge of a cable-tied block mat. Individual blocks were investigated also. The block size, flow depth, and block (mat) protrusion above the surrounding bed were systematically varied. The results are presented in terms of the critical dimensionless shear stress θc for block (mat) failure. A relationship between θc, block size and block protrusion, and flow depth is given.     

Raked Piles—Virtues and Drawbacks

This technical note examines some of the characteristics of behavior of pile groups containing raked piles, via a simplified and hypothetical example. Three cases are examined: (1) a group subjected to vertical and lateral loadings, with no ground movements; (2) a group subjected to vertical and lateral loadings, but with vertical ground movements also acting on the group; and (3) a group subjected to vertical and lateral loadings, but with horizontal ground movements acting on the group. In each case, the effect of pile rake on typical behavioral characteristics (group settlement, lateral deflection and rotation, and pile loads and moments) are examined. It is found that, while the presence of raked piles can provide some advantages when the group is subjected to applied vertical and lateral loadings, especially in relation to a reduction in lateral deflection, some aspects of group behavior may be adversely affected when either vertical or horizontal ground movements act on the group. Thus, caution must be exercised in employing raked piles when such ground movements are expected to occur.