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1.
The traditional methods for analyzing deformation in structures attempt to solve the partial differential equations of the classical theory of continuum mechanics. Yet these equations, because they require the partial derivatives of displacement to be known throughout the region modeled, are in some ways unsuitable for the modeling of discontinuities caused by damage, in which these derivatives fail to exist. As a means of avoiding this limitation, the peridynamic model of solid mechanics has been developed for applications involving discontinuities. The objective of this method is to treat crack and fracture as just another type of deformation, rather than as pathology that requires special mathematical treatment. The peridynamic theory is based on integral equations so there is no problem in applying the equations across discontinuities. The peridynamic method has been applied successfully to damage and failure analysis in composites. It predicts in detail the delamination and matrix damage process in composite laminates due to low velocity impact, and the simulation results of damage area correlates very well with the experimental data.  相似文献
2.
A sound repair on a 40 year old four-span prestressed concrete girder bridge is performed with an innovative strengthening method using prestressed carbon fiber reinforced polymer (CFRP) sheets. In fact, this application is the first North American field application of its type. An adequate repair design is conducted based on the American Association of State Highway and Transportation Officials Load Resistance Factor Design (AASHTO LRFD) and the Canadian Highway Bridge Design Code. To ensure the feasibility of the site application using prestressed CFRP sheets, tests are conducted and closed-form solutions are developed to investigate the behavior of the anchor system that is necessary for prestressing the CFRP sheets. A full-scale finite-element analysis (FEA) is performed to investigate the flexural behavior of the bridge in the undamaged, damaged, and repaired states. The AASHTO LRFD exhibits conservative design properties as compared to the FEA results. The repaired bridge indicates that the flexural strength of the damaged girder has been fully recovered to the undamaged state, and the serviceability has also been improved. An assessment based on the AASHTO rating factor demonstrates the effectiveness of the repair.  相似文献
3.
The writers report on a simulation study of the performance of the North Tower (WTC-I) of the World Trade Center complex during the impact of American Airlines Flight 11 on September 11, 2001. We discuss impact damage that the structural core might have sustained and its possible behavior under structural and thermal loading. Our simulations indicate that the worst damage to the core structure was in stories 95 through 97 of the tower. We estimate that a core collapse mechanism could be initiated if the tower core column temperatures were elevated to about 700°C.  相似文献
4.
Shot peening involves bombarding the surface of a metal part with small spherical media called shot. It creates a uniform layer of compressive residual stress at surfaces and considerably increases the part life. This study is devoted to an evaluation of the edge (corner) effects in the shot peening process. Geometry variations in metal parts such as edges or corners cause variance in the residual stress profile induced by shot peening. This paper presents the finite-element modeling and scheme that we use to simulate the shot peening process by both single impact and multiple impacts for part geometries with different edge radii. The results achieved show that, after the same shot peening process, a part with a relatively larger edge radius results in a satisfactory residual stress profile which could equal or exceed that expected for a part with a smaller edge radius. The results may enable the automated edge preprocess step for shot peening with an easily machined large radius to reduce cost and improve productivity.  相似文献
5.
Static load tests were performed on support details used at the ends of prestressed concrete pedestrian bridge girders to determine the resistance characteristics of girder supports in the direction perpendicular to the longitudinal axis of the girders. The specimens tested represent support details that have also been widely used in prestressed concrete highway bridges in Minnesota and in other states. Two specimens, one representing the free-end detail and one representing the restrained-end detail were subjected to a combination of vertical and lateral loads. The applied loading was intended to simulate the loading conditions to which the girder ends would be subjected in the event of an over-height vehicle collision with the bridge. The tests revealed two types of lateral load resisting mechanisms depending on the type of support detail. The specimen with the free-end detail resisted the lateral loading through sliding friction between the components of the support assembly. Deformation of this specimen was a combination of shear deformation of the bearing pad and sliding of various support components. The restrained-end detail exhibited larger lateral load capacity than the free-end detail due to the resistance provided by the anchor rods that were intended to prevent the lateral movement of the girder ends. Failure of the specimen with restrained-end detail was due to the concrete breakout and bending of the anchor rods.  相似文献
6.
The establishment of human habitats on the Moon and on Mars will require protecting them from the hazards of near-Earth and interplanetary space. In addition to solar radiation, another hazard to be faced by these habitats is the damage that can result from the high speed impact of a meteoroid on a critical structural component. Therefore, lunar habitats and their accompanying support facilities need to be designed with adequate levels of protection that will allow them to also withstand the damage that can result from a meteoroid impact. In this paper we discuss some approaches to shielding for lunar habitats, focusing on shielding that is intended primarily to provide protection against meteoroid impacts and on shielding approaches that use resources mined or extracted from the Moon. The Moon’s mineralogy is discussed and suggestions are presented for materials and material combinations that can be used to develop shielding for lunar habitats and which are comprised primarily or entirely of lunar materials. Several shielding mechanisms are also presented that could be effective against impacts by meteoroid particles having diameters on the order of that which are likely to strike a fairly large lunar habitat at least one or two times per year. The paper concludes with recommendations for continuing work in optimizing the design of meteoroid shielding for lunar habitats.  相似文献
7.
Bridge piers located in navigable inland waterways are designed to resist impact forces from barges and flotillas in addition to other design considerations (e.g., scour dead, live loads, etc.). The primary design tool for estimating these forces is the AASHTO Guide Specification that provides a simple hand calculation method to determine an “equivalent impact force.” The simplicity comes at a cost of excluding the effect of the pier shape, impact duration, and interaction between barges in a flotilla. The objective of this paper is to present a hand calculation method for determining barge or flotilla impact forces on bridge piers. The primary advantage of this approach lies in its incorporation of pier geometry, interaction between barges, and impact duration. The proposed method is derived from the conduct of hundreds of finite-element dynamic simulations of jumbo hopper (JH) barges and flotillas, made up of JH barges, impacting bridge piers. Results are presented and compared with those derived from the AASHTO method and detailed finite-element modeling.  相似文献
8.
Early time transient waves multireflected in a finite pile, governed by a damped wave equation, are analyzed by a reverberation-ray matrix. The pile is surrounded by compacted soil, and the composite is modeled by elastic springs and viscous dampers distributed along the length and at the tip of an elastic rod. Steady-state waves with complex frequencies and wave numbers that are generated by a source of harmonic time function at the top and reverberated between the top and bottom surface of the pile are sorted in matrix form into ray-groups, arriving at a receiver in successive orders of reflections from the bottom. The steady-state ray groups are synthesized into a series of nonsingular Fourier integrals that can be evaluated accurately with a fast Fourier-transform algorithm. The first integral (zeroth-order) has also been reduced by complex contour integration to the well-known closed-form solution in Bessel functions for a semi-infinite pile. Detailed time records of velocity response received at the top after three reflections are calculated to illustrate attenuation and damping; arrival times and amplitude-phase change on each reflection for various lateral and base supports. The calculated records resemble ultrasonic nondestructive testing data.  相似文献
9.
Bridges that cross navigable waterways may be affected by accidental ship impacts. To better characterize ship impact loads on bridge pier structures, a comprehensive centrifuge model test program involving 48 ship impact tests was performed on a 2×3 pile group and a 3×3 pile group founded in saturated silty sand. These model tests simulated groups of 2-m-diameter by 31.5-m-long pipe piles. The effects of three factors related to the ship (tonnage, speed, and bow structure) and two factors related to the bridge pier structure (superstructure mass and pile-group size) were investigated through these impact tests. The characteristics of the ship impact load were identified and the mechanism of the ship-bridge collision was analyzed. The test results show that the ship impact load was highly dependent on the ship bow structure and the ship impact speed. The test results were compared with other published data and the AASHTO loads. An empirical equation was suggested to relate the ship impact load to the five influencing factors.  相似文献
10.
Results from experimental testing of three permit vehicles are presented in the paper. The selected heavy vehicles, which require permits from state DOTs, included two tractor-trailer systems and a midsize crane. The vehicles were experimentally tested on popular existing speed bumps and on a representative highway bridge. The selected bridge was a reinforced-concrete structure constructed in 1999, located on the U.S. 90 in Northwest Florida. The bridge approach depression, combined with a distinct joint gap between the asphalt pavement and the concrete deck, triggered significant dynamic responses of the vehicle-bridge system. Similar dynamic vibrations were observed and recorded when the permit vehicles were driven over the speed bumps. Time histories of relative displacements, accelerations, and strains for selected locations on the vehicle-bridge system were recorded. The analysis of experimental data allowed for assessment of actual dynamic interactions between the vehicles and the speed bumps as well as dynamic load allowance factors for the selected bridge.  相似文献
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