Improved Seismic Response of Multisimple-Span Skewed Bridges Retrofitted with Link Slabs

Results of a recent bridge inventory evaluation indicated that about 50% of Turkish highway bridges have more than 30° of skew angle and can be classified as irregular bridges. During the recent major earthquake in Turkey, multisimple-span bridges with continuous decks and link slabs performed well even though these bridges were in the vicinity of the fault line. This study aims to evaluate the improvements in seismic response of skew bridges in terms of forces and displacements when link slabs are added as a retrofit tool. A series of elastic dynamic analyses and nonlinear time history analyses were conducted to investigate the seismic response of various standard highway bridges with different span lengths and skew angles. A new reinforcement design for edge zones of link slabs is proposed for bridges located in high seismic zones. In practice, link slabs can be implemented easily during a regular redecking of a bridge.     

Modeling Early-Age Bridge Restraint Moments: Creep, Shrinkage, and Temperature Effects

An increasing number of bridges are being designed with continuous spans instead of simple spans. By reducing the number of joints in a bridge, the traveling public receives a better riding surface and corrosion caused by leaking joints can be reduced. Also, redundancy is created when the system is made continuous, producing a tougher structure. However, a continuous system is more complicated to design and secondary restraint moments due to creep, shrinkage, and thermal effects can develop at the connection. This paper presents results from an experimental study done to monitor the early age restraint moments that develop in a two-span continuous system made of full-depth precast concrete bulb tee girders. The restraint moments observed were compared to the predicted restraint moments using the RMCalc program . The observed restraint moments were significantly lower than predicted by the program. Expansion of the deck during curing, which is generally not considered in the predictions, significantly influenced the early age restraint moments. A simplified model to predict the restraint moments considering thermal effects is proposed.     

Live Load Distribution Formulas for Single-Span Prestressed Concrete Integral Abutment Bridge Girders

In this study, live load distribution formulas for the girders of single-span integral abutment bridges (IABs) are developed. For this purpose, two and three dimensional finite-element models (FEMs) of several IABs are built and analyzed. In the analyses, the effects of various superstructure properties such as span length, number of design lanes, prestressed concrete girder size, and spacing as well as slab thickness are considered. The results from the analyses of two and three dimensional FEMs are then used to calculate the live load distribution factors (LLDFs) for the girders of IABs as a function of the above mentioned parameters. The LLDFs for the girders are also calculated using the AASHTO formulas developed for simply supported bridges (SSBs). The comparison of the analyses results revealed that LLDFs for girder moments and exterior girder shear of IABs are generally smaller than those calculated for SSBs using AASHTO formulas especially for short spans. However, AASHTO LLDFs for interior girder shear are found to be in good agreement with those obtained for IABs. Consequently, direct live load distribution formulas and correction factors to the current AASHTO live load distribution equations are developed to estimate the girder live load moments and exterior girder live load shear for IABs with prestressed concrete girders. It is observed that the developed formulas yield a reasonably good estimate of live load effects in prestressed concrete IAB girders.     

Behavior of Transverse Confining Systems for Steel-Free Deck Slabs

Extensive research conducted over the past eight years in Canada has led to a concrete deck slab of girder bridges that can be entirely free of any tensile reinforcement. This slab, known as the steel-free deck slab, derives its strength from its internal arching action, which is harnessed longitudinally by making the slab composite with the girders, and transversely by restraining the relative transverse movement of the top flanges of adjacent girders. Two steel-free deck slabs have already been built, in which the transverse confinement is provided by welding steel straps to the girders. This paper presents test results on two other kinds of transverse confining systems, which are applicable to both steel and concrete girders. It is shown that the steel-free deck slab, in addition to being more durable than slabs with steel reinforcement, can also prove to be more economical.     

Experimental Behavior of Bridge Beams Retrofitted with Postinstalled Shear Connectors

A number of older bridges were constructed with floor systems consisting of a noncomposite concrete slab over steel girders. A potentially economical means of strengthening these floor systems is to connect the existing concrete slab and steel girders with postinstalled shear connectors to permit the development of composite action. This paper presents the results of an experimental investigation of this concept. Five large-scale noncomposite beams were constructed, and four of these were retrofitted with postinstalled shear connectors and tested under static load. The retrofitted composite beams were designed as partially composite with a 30% shear connection ratio. A noncomposite beam was also tested as a baseline specimen. Test results showed that the strength and stiffness of existing noncomposite bridge girders can be increased significantly. Further, excellent ductility of the strengthened partially composite girders was achieved by placing the postinstalled shear connectors near zero-moment regions to reduce slip demand on the connectors. The test results also showed that current simplified design approaches commonly used for partially composite beams in buildings provide good predictions of the strength and stiffness of partially composite bridge girders strengthened using postinstalled shear connectors.     

Repair of Slab–Column Connections Using Epoxy and Carbon Fiber Reinforced Polymer

Repair, strengthening, and retrofit of reinforced and prestressed concrete members have become increasingly important issues as the World’s infrastructure deteriorates with time. Buildings and bridges are often in need of repair or strengthening to accommodate larger live loads as traffic and building occupancies change. In addition, inadequate design and detailing for seismic and other severe natural events has resulted in considerable structural damage and loss of life, particularly in reinforced concrete buildings. Numerous buildings and bridges suffer damage during such events and need to be repaired. The use of carbon fiber reinforced polymer (CFRP) composite fabric bonded to the surface of concrete members is comparatively simple, quick and virtually unnoticeable after installation. The use of composites has become routine for increasing both the flexural and shear capacities of reinforced and prestressed concrete beams. Earthquake retrofit of bridge and building structures has relied increasingly on composite wrapping of columns, beams and joints to provide confinement and increase ductility. This paper presents the results of cyclic testing of three large-scale reinforced concrete slab–column connections. Each of the specimens was a half-scale model of an interior slab–column connection common to flat-slab buildings. The specimens were reinforced according to ACI-318 code requirements and included slab shear reinforcement. While supporting a slab gravity load equivalent to dead load plus 30% of the live load, the specimens were subjected to an increasing cyclic lateral loading protocol up to 5% lateral drift. The specimens were subjected to the same loading protocol after they were repaired with epoxy crack sealers and CFRP sheet on the surfaces of the slab. Repair with epoxy and CFRP on the top surface of the slab was able to restore both initial stiffness and ultimate strength of the original specimen.     

Development of a Specification for Bridge Seismic Retrofit with Carbon Fiber Reinforced Polymer Composites

The U.S. Interstate 80 bridge over State Street in Salt Lake City is very near the Wasatch fault, which is active and capable of producing large earthquakes. The bridge was designed and built in 1965 according to the 1961 American Association of State Highway Officials specifications, which did not consider earthquake-induced forces or displacements. The bridge consists of reinforced concrete bents supporting steel plate welded girders. The bents are supported on cast-in-place concrete piles and pile caps. A seismic retrofit design was developed using carbon fiber reinforced polymer (CFRP) composites, which was implemented in the summer of 2000 and the summer of 2001, to improve the displacement ductility of the bridge. The seismic retrofit included column jacketing, as well as wrapping of the bent cap and bent cap-column joints for confinement, flexural, and shear strength increase. This paper describes the specifications developed for the CFRP composite column jackets and composite bent wrap. The specifications included provisions for materials, constructed thickness based on strength capacity, and an environmental durability reduction factor. Surface preparation, finish coat requirements, quality assurance provisions, which included sampling and testing, and constructability issues regarding the application of fiber composite materials in the retrofit of concrete bridges are also described.     

Seismic Retrofit of State Street Bridge on Interstate 80

The State Street Bridge, in Salt Lake City, was designed and built in 1965 according to the 1961 AASHO specifications; the design did not include earthquake-induced forces or displacements since only wind loads were considered. The bridge consists of four reinforced concrete (RC) bents supporting composite welded steel girders; the bents are supported on cast-in-place concrete piles and pile caps. A vulnerability analysis of the bridge was conducted that determined deficiencies in (1) confinement of column lap splice regions, (2) anchorage of longitudinal column bars in the bent cap, (3) confinement of column plastic hinge zones, and (4) shear capacity of columns and bent cap–column joints. Seismic retrofit designs using carbon-fiber-reinforced-polymer (CFRP) composites and steel jackets were performed and compared for three design spectra, including the 10% probability of exceedance in 250 years earthquake. The CFRP composite design was selected for implementation and application of the composite was carried out in the summer of 2000 and 2001, while the bridge was in service. The paper describes the CFRP composite design, which, in addition to column jackets, implemented an “ankle wrap” for improving joint shear strength and a “U-strap” for improving anchorage of column bars in the bent cap; other retrofit measures were implemented, such as bumper brackets and a deck slab retrofit. A capacity versus demand evaluation of the as-built and retrofitted bents is presented.     

Viscoelastic Dampers at Expansion Joints for Seismic Protection of Bridges

This paper presents the result of a study on the use of viscoelastic dampers at expansion joints of highway bridges for preventing superstructure decks from falling off the seats and∕or from colliding with each other in the event of a severe earthquake. The Kelvin and Maxell models, consisting of an elastic spring and a linear viscous damper combined in parallel and in series, respectively, are considered for analysis. A 2D finite-element analysis using bilinear hysteretic models for bridge substructures joints was performed on example bridges constructed with one or two expansion joints. It was demonstrated that the damper is effective in suppressing the relative displacements at the expansion joints without introducing a significant increase in ductility demands for the substructures. The result also showed that the spring component of the Kelvin and Maxwell models has little effect on the performance of the damper component. This study clearly indicated that the use of linear viscous dampers offers a practical solution to the seismic problem that often arises from bridges with expansion joints.     

Strengthening of a Steel Bridge Girder Using CFRP Plates

For bridge owners faced with a rising number of structurally deficient steel bridges, the rehabilitation of steel girders using advanced composite materials offers an attractive solution for short-term retrofit or long-term rehabilitation. Several laboratory studies conducted at the University of Delaware have shown that carbon fiber-reinforced polymer (CFRP) plates can be used to effectively strengthen steel bridge girders. Initial studies focused on several issues including the effect on global stiffness and strength, bond force transfer and development, and environmental and fatigue durability of the CFRP∕steel bond. Once the feasibility of the strengthening procedure had been thoroughly examined, strengthening of an existing steel bridge girder was performed. This paper reviews the research conducted to date, and presents details of a demonstration of this technology performed on a bridge located on Interstate 95 in Newark, Del.     

Live Load Distribution for Steel Girder Bridges

This paper deals with distribution of truck load on girder bridges. Previous analytical studies based on finite-element method indicated that AASHTO code-specified girder distribution factors (GDFs) are inaccurate. In particular, GDFs appear to be conservative for longer spans and larger girder spacing, but too permissive for short spans and girder spacings. Therefore, a field testing program was carried out including about 20 steel girder bridges with spans up to 45 m. For each tested structure, GDFs were determined by measuring strains in the girders under heavy trucks. Test trucks were 11-axle vehicles, loaded to the legal limit in Michigan (over 650 kN). The strains were recorded for a single truck and for two trucks side-by-side. The tests were repeated for crawling speed and normal traffic speed for the location. In all tested bridges, the GDFs determined from the field measurements are lower than code-specified values. In addition, the considered bridges were analyzed using a commercial finite-element software package, ABAQUS. The analytical results were compared with those from field tests. It was observed that the maximum values of the strain and corresponding stress are lower than analytical values obtained using ABAQUS. The reason for this discrepancy is unintended composite action and partial fixity of supports (rather than simple supports).     

Development and Implementation of a Continuous Strain Monitoring System on a Multi-Girder Composite Steel Bridge

This paper describes the implementation and evaluation of a long-term strain monitoring system on a three-span, multisteel girder composite bridge located on the interstate system. The bridge is part of a network of bridges that are currently being monitored in Connecticut. The three steel girders are simply supported, whereas the concrete slab is continuous over the interior supports. The bridge has been analyzed using the standard AASHTO Specifications and the analytical predictions have been compared with the field monitoring results. The study has included determination of the location of the neutral axes and the evaluation of the load distributions to the different girders when large trucks cross the bridge. A finite-element analysis of the bridge has been carried out to further study the distribution of live load stresses in the steel girders and to study how continuity of the slabs at the interior joints would influence the overall behavior. The results of the continuous data collection are being used to evaluate the influence of truck traffic on the bridge and to establish a baseline for long-term monitoring.     

Adhesively Bonded and Translucent Glass Fiber Reinforced Polymer Sandwich Girders

An experimental program has been carried out to investigate the structural behavior of adhesively bonded glass fiber reinforced polymer (GFRP) sandwich girders. The girders, conceivable for main spans up to 20 m, are composed of translucent double sandwich element webs and adhesively bonded pultruded shape flanges. The continuous adhesive connections between web and flanges are loaded in a highly favorable manner without peeling stresses. Despite the complex load-carrying and failure behavior of the girders, simple calculation models can be applied. The successful girder experiments allowed for the development of a material-adapted construction method for new GFRP bridges and buildings. The integration of architectural aspects such as transparency, translucency, lighting, and color, as well as building physical aspects for buildings such as thermal insulation, substantially increases the overall value of the proposed constructions. Together with the lower life-cycle costs, this further justifies these new materials’ higher initial costs as compared with traditional materials.     

Wheel Load Distribution in Simply Supported Concrete Slab Bridges

This paper presents the results of a parametric study related to the wheel load distribution in one-span, simply supported, multilane, reinforced concrete slab bridges. The finite-element method was used to investigate the effect of span length, slab width with and without shoulders, and wheel load conditions on typical bridges. A total of 112 highway bridge case studies were analyzed. It was assumed that the bridges were stand-alone structures carrying one-way traffic. The finite-element analysis (FEA) results of one-, two-, three-, and four-lane bridges are presented in combination with four typical span lengths. Bridges were loaded with highway design truck HS20 placed at critical locations in the longitudinal direction of each lane. Two possible transverse truck positions were considered: (1) Centered loading condition where design trucks are assumed to be traveling in the center of each lane; and (2) edge loading condition where the design trucks are placed close to one edge of the slab with the absolute minimum spacing between adjacent trucks. FEA results for bridges subjected to edge loading showed that the AASHTO standard specifications procedure overestimates the bending moment by 30% for one lane and a span length less than 7.5 m (25 ft) but agrees with FEA bending moments for longer spans. The AASHTO bending moment gave results similar to those of the FEA when considering two or more lanes and a span length less than 10.5 m (35 ft). However, as the span length increases, AASHTO underestimates the FEA bending moment by 15 to 30%. It was shown that the presence of shoulders on both sides of the bridge increases the load-carrying capacity of the bridge due to the increase in slab width. An extreme loading scenario was created by introducing a disabled truck near the edge in addition to design trucks in other lanes placed as close as possible to the disabled truck. For this extreme loading condition, AASHTO procedure gave similar results to the FEA longitudinal bending moments for spans up to 7.5 m (25 ft) and underestimated the FEA (20 to 40%) for spans between 9 and 16.5 m (30 and 55 ft), regardless of the number of lanes. The new AASHTO load and resistance factor design (LRFD) bridge design specifications overestimate the bending moments for normal traffic on bridges. However, LRFD procedure gives results similar to those of the FEA edge+truck loading condition. Furthermore, the FEA results showed that edge beams must be considered in multilane slab bridges with a span length ranging between 6 and 16.5 m (20 and 55 ft). This paper will assist bridge engineers in performing realistic designs of simply supported, multilane, reinforced concrete slab bridges as well as evaluating the load-carrying capacity of existing highway bridges.     

“Underlying” Causes for Settlement of Bridge Approach Pavement Systems

A comprehensive field study of 74 bridges in Iowa was conducted to characterize problems leading to poor performance of bridge approach pavement systems. Subsurface void development caused by water infiltration through unsealed expansion joints, collapse and erosion of the granular backfill, and poor construction practices were found to be the main contributing factors. To characterize the problem, International Roughness Index and profile measurements from several sites were used to show that approach pavement roughness is several times higher than the average roadway condition and are most severe at the abutment-to-approach pavement intersection and transverse expansion joints due to large (5–10?cm) joint widths. Further, a settlement time history was documented at one bridge site by measuring the approach slab pavement elevations periodically after completion of bridge construction, revealing a progressive settlement problem under the approach pavement. To better understand the void development under the approach pavement, laboratory compaction tests were performed on granular backfill materials from various bridge sites to quantify their saturated collapse potential in the postconstruction phase. These tests revealed collapse potential of backfill materials in the range of 5–18% (based on volume) with the high values for poorly graded sandy backfill materials, indicating significant settlement problems. Based on the research findings, some relatively simple design and construction modifications are suggested which could be used to alleviate field problems for similar bridge approach pavement systems.     

Seismic Response of Multiple Span Steel Bridges in Central and Southeastern United States. II: Retrofitted

Part I of this two-part paper evaluated the seismic response of typical multispan simply supported (MSSS) and multispan continuous steel girder bridges in the central and southeastern United States. The results showed that the bridges were vulnerable to damage resulting from impact between decks, and large ductility demands on nonductile columns. Furthermore, fixed and expansion bearings were likely to fail during strong ground motion. In this paper, several retrofit measures to improve the seismic performance of typical multispan simply supported and multispan continuous steel girder bridges are evaluated, including the use of elastomeric bearings, lead-rubber bearings, and restrainer cables. It is determined that lead-rubber bearings are the most effective retrofit measure for reducing the seismic vulnerability of typical bridges. While isolation provided by elastomeric bearings limits the forces into the columns, the added flexibility results in pounding between decks in the MSSS steel girder bridges. Restrainer cables, which are becoming a common retrofit measure, are effective in reducing the hinge opening in MSSS bridges with steel bearings. However, when used with elastomeric bearings, the restrainer cables negate the isolation effect of the bearings.     

Carbon Fiber Shear Retrofit of Forty-Two-Year-Old AASHTO I-Shaped Girders

Sixteen shear capacity tests were performed on eight decommissioned AASHTO prestressed concrete girders that had been in service for over 42 years. These bridge members presented a unique opportunity to investigate carbon fiber-reinforced polymer (CFRP) retrofit schemes to enhance the shear capacity of underreinforced girders that were nonrectangular. Four destructive tests were performed to quantify the in-service strength of the girders and the remaining 12 tests were performed on CFRP retrofitted girders. In all, five configurations of the CFRP reinforcement were evaluated. Two anchoring techniques were investigated that either involved epoxying a horizontal CFRP strip over the vertical strips or a new methodology of epoxying a CFRP laminate into a groove over the vertical strips that was cut at the web-to-flange interface. Two methodologies that predicted the shear contribution of the carbon fiber reinforcement were compared with the test results. A carbon fiber-reinforcing scheme of vertical strips and horizontal anchorage strip was found to be the most effective in resisting the applied shear.     

Full-Scale Test of Continuity Diaphragms in Skewed Concrete Bridge Girders

Continuity diaphragms used in prestressed girder bridges on skewed bents have caused difficulties in detailing and construction. The results of the field verification for the effectiveness of continuity diaphragms for skewed, continuous, and prestressed concrete girder bridges are presented. The current design concept and bridge parameters that were considered include skew angle and the ratio of beam spacing to span (aspect ratio). A prestressed concrete bridge with continuity diaphragms and a skewed angle of 48° was selected for full-scale test by a team of engineers from Louisiana Department of Transportation and Development and the Federal Highway Administration. The live load tests performed with a comprehensive instrumentation plan provided a fundamental understanding of the load transfer mechanism through these diaphragms. The findings indicated that the effects of the continuity diaphragms were negligible and they can be eliminated. The superstructure of the bridge could be designed with link slab. Thus, the bridge deck would provide the continuity over the support, improve the riding quality, enhance the structural redundancy, and reduce the expansion joint installation and maintenance costs.     

Performance of Polymer Modified Asphalt Bridge Expansion Joints in Low-Temperature Regions

Conventional asphalt bridge expansion joints used in low-temperature regions generally show cracking within the first 2?years. To improve the low-temperature performance of these joints, the commercial MEIJIA asphalt binder commonly used in bridge expansion joint construction was modified with two polymers: thermoplastic rubber and rubber. The goal is to find an optimum combination of polymers, binders, and aggregates to improve the performance of asphalt expansion joints in low-temperature regions. The polymer modified binders and mixtures were evaluated for their low-temperature properties using ductility, penetration, indirect tension, and bending tests. The study indicates that performance of these joints at low temperature can be enhanced significantly with the right combinations of polymers, binders, and aggregates. Four expansion joints made with the polymer modified asphalt mixtures were installed on two bridges in a cold region. A construction procedure was also developed to install these joints properly to minimize low-temperature cracking along the interface between the joint and bridge deck. After 7?years of service, the four joints show good performance without any visible cracking or rutting.