Journal of Bridge Engineering

This paper presents both numerical and experimental studies of the transient response of concrete I-girders with and without flaws subjected to transverse elastic point impact. These studies were aimed at evaluating the capability of using the impact-echo method for integrity testing of concrete I-girders. Numerical and experimental studies were performed first to gain an understanding of transient stress wave propagation in solid I-girder segments. Experimental studies were carried out on full-size reinforced-concrete I-girder segments containing flaws at known locations. Results obtained from these studies show that the transient response of a solid I-girder subjected to impact on the bottom surface is composed of a number of resonant frequencies caused by cross-sectional modes of vibration. Among these frequencies there is a predominant frequency, and its value depends on the cross-sectional geometry and dimensions of the bottom portion of the I-girder for a given P-wave speed in concrete. It is also shown that the presence of a flaw disrupts the modes of vibration. A shift of the predominant frequency to a lower value is a key indication of the presence of the flaw. In addition, multiple wave reflections between the impact surface and the surface of the flaw produce a large amplitude peak in the spectrum at the frequency corresponding to the depth of the flaw, and thus make it easy to locate the flaw.

Dynamic tests were done on polytetrafluoroethylene (PTFE) sliding bridge bearings to evaluate their frictional characteristics at frequencies above 1 Hz. High frequency seismic responses of PTFE bearings are expected to occur in bridge structures in eastern North America, where the estimated ground motion is well above 1 Hz. Three different PTFE-steel interfaces were tested at frequencies ranging from 0.02 Hz to 5 Hz, with displacement amplitudes of up to ±70 mm and under confining pressures ranging from 5 MPa to 45 MPa. The experimental results showed a significant initial transient frictional response at frequencies above 1 Hz before the dynamic, steady-state, frictional behavior was achieved. This transient response is characterized by an initial high static coefficient of friction that slowly decreases to a dynamic steady-state coefficient of friction after several response cycles. Modifications to an existing mathematical friction model are proposed to take into account this initial transient frictional response.

A three-dimensional dynamic finite element model is established for the Tsing Ma long suspension Bridge in Hong Kong. The two bridge towers made up of reinforced concrete are modeled by three-dimensional Timoshenko beam elements with rigid arms at the connections between columns and beams. The cables and suspenders are modeled by cable elements accounting for geometric nonlinearity due to cable tension. The hybrid steel deck is represented by a single beam with equivalent cross-sectional properties determined by detailed finite element analyses of sectional models. The modal analysis is then performed to determine natural frequencies and mode shapes of lateral, vertical, torsional, longitudinal, and coupled vibrations of the bridge. The results show that the natural frequencies of the bridge are very closely spaced; the first 40 natural frequencies range from 0.068 to 0.616 Hz only. The computed normal modes indicate interactions between the main span and side span, and between the deck, cables, and towers. Significant coupling between torsional and lateral modes is also observed. The numerical results are in excellent agreement with the measured first 18 natural frequencies and mode shapes. The established dynamic model and computed dynamic characteristics can serve further studies on a long-term monitoring system and aerodynamic analysis of the bridge.

Four suspension bridges are studied: the Williamsburg (1903), Bear Mountain (1924), Triborough (1936), and Golden Gate (1937). Safety factors against failure are calculated for the main cables from original specifications as well as from actual cable wire tests performed at the time of construction. Using recent inspection reports, current safety factors against failure are computed for the main cables using a Type I Extreme Value Distribution. Two wire models are used: a more conservative Ductile-Brittle Wire Model (where wires with less than 0.6% elongation were considered brittle and are discounted) and a Ductile Wire Model (where only fractured wires are discounted). The number of fractured cable wires is estimated from the limited number of actual fractures observed; the Type I Distribution is used to estimate the true number of brittle wires. The most conservative current safety factors against failure range from 2.0 for the Triborough Bridge to 3.0 for the Williamsburg Bridge. Additional wire sampling and testing should be performed on the Triborough Bridge's cables.

The historic Rock Island Government Bridge, built in 1896, united the East and West across the mighty Mississippi. It's an ideal example of American technology, creativity, and entrepreneurship. The bridge, located adjacent to Lock and Dam No. 15, consists of two riveted Pratt through trusses, five riveted Baltimore trusses, and a camelback center-pivoted swing truss. The paper discusses the unique problems encountered during construction of the bridge, as well as its upgradings and maintenance. Many projects have been undertaken in the 100 years since the completion of the bridge. In each case, the structure was designed as near state of the art, but with rapidly increasing loading of the equipment operated over the bridge, modifications have been necessary to strengthen the spans and floor systems, as well as nearly all the other components of the structure. Several changes also have been required due to the Mississippi River Navigation projects and the increased highway traffic. Repairs to combat the aging process were done annually as a routine matter. The bridge was the first major accomplishment of Chicago engineer Ralph Modjeski, who subsequently established a reputation as one of the country's leading bridge designers.

Because drying shrinkage cracking is a major source of bridge deck cracking, and because shrinkage compensating concrete (SCC) is known to substantially reduce shrinkage cracking, the use of SCC should be closely examined in an effort to increase the durability and service life of bridge decks. One of the concerns regarding the use of SCC is its more demanding construction requirements for placement time and for curing. This paper addresses relevant material and construction issues that have resulted in the successful use of SCC for bridge decks. The Ohio Turnpike Commission (OTC) has used SCC exclusively for its new and replacement bridge decks for the past 12 years, and they have been very satisfied with its performance to date. The OTC's evaluation of SCC is that it greatly mitigates shrinkage cracking. The OTC's requirements for SCC at the concrete plant, in-transit, and at the job site are included in this paper. The OTC's construction procedures are presented in the form of a chronological sequence of photographs illustrating the construction activities on a new SCC deck placement in 1994.

Managing a complex roadway bridge system requires a comprehensive system of management tools to aid decision making. During the decision making process, a major effort is needed to define which bridge(s) or which element(s) of the bridge(s) needs attention regarding rehabilitation or replacement. Any effective rehabilitation or repair of a given structure demands a proper understanding of its existing condition. This understanding starts with inspection. The bridge inspector's findings can be stored in a comprehensive database system and can be used by the bridge owner to decide which structures should be considered for rehabilitation or replacement. The main objective of this paper is to develop a Uniform Bridge Element Identification System (UBEIS) that can be used in a bridge management system. UBEIS is a generalized multidimensional coordinate system that consists of a string of alphanumeric characters. The string of alphanumeric characters consists of groups of letters and groups of digits to provide a unique global identification for structural elements. The groups of letters define the generalized coordinates and the type of element, and the digits provide the coordinate values. The UBEIS is a self explanatory identification system that does not require any structural detail drawings, such as framing plan, cross sections, etc., to identify a unique structural element. This paper incorporates the current New York State bridge inspection rating system, and also presents a flowchart for a simple database model for a roadway bridge management system based on UBEIS and the bridge inspection rating system. This system can be considered as a practical link between inspection and rehabilitation, and is a warning system against bridge deterioration and potential failure.