Adjacent Girders Research Articles

Shear Diaphragm Bracing of Beams. II: Design Requirements

Light gauge metal decking is commonly used in the building and bridge industries for concrete formwork. In addition to supporting the wet concrete, the metal forms also improve the lateral-torsional buckling capacity of the beams or girders they are fastened to since they behave as a shear diaphragm and restrain the warping deformation of the top flange. The design requirements for shear diaphragm bracing are not well established. This is the second part of a two-part paper. The first part of the paper focused on the general stability bracing behavior for shear diaphragms connected to the top flanges of adjacent beams or girders. This part of the paper outlines the design requirements for shear diaphragm bracing and focuses on both the stiffness and strength requirements for beam stability. Design expressions are presented for the stiffness and strength requirements as well as a model that can be used to estimate the forces in the fasteners that connect the diaphragms to the top flanges of the beams. An example is presented at the end of the paper demonstrating the use of the design expressions.

Shear Diaphragm Bracing of Beams. I: Stiffness and Strength Behavior

Light gauge metal decking is commonly used in the building and bridge industries for concrete formwork. In addition to supporting the concrete, the metal forms also improve the lateral-torsional buckling capacity of the beam or girder they are fastened to as they behave as a shear diaphragm and restrain the warping deformation of the top flange. However, the stability bracing requirements for shear diaphragms are not well established. This is the first part of a two-part paper on diaphragm bracing of beams. The first part of the paper focuses on behavior whereas the second part focuses on general design requirements. The diaphragms in the study were connected along two edges to the top flanges of adjacent girders. The parameters that were investigated include the diaphragm stiffness, load type, and load position. The type of loading has a significant effect on the stability bracing behavior of the shear diaphragm. Other factors that have an effect on the bracing behavior include the web slenderness and the presence of intermediate discrete braces. The results presented in the first part of the paper create the foundation for a diaphragm bracing design methodology that is developed and presented in the second part of the paper.

Erection Procedure Effects on Deformations and Stresses in a Large-Radius, Horizontally Curved, I-Girder Bridge

Special attention is required in the construction of horizontally curved steel I-girder bridges due to coupled effects of primary bending and torsional forces. Misguided steel erection procedures can lead to undesired stresses, deflections, and rotations in these types of bridges, resulting in a structure with misaligned geometry and in an unknown state of stress. Further complicating the issue, little guidance related to curved bridge behavior during construction is provided by current design codes, leaving contractors and designers uncertain as to the most appropriate steps to take to achieve an efficient, safe structure. A horizontally curved, six-span steel I-girder bridge located in central Pennsylvania that experienced severe geometric misalignments and fit-up complications during steel erection was studied to investigate curved girder behavior during construction. The structure was monitored during corrective procedures intended to realign it with the design geometry, and field data used to calibrate a three-dimensional computer model generated via SAP2000. The techniques and assumptions proven in the calibration process were used to create a numerical model of a three-span continuous portion of the bridge, which was the subject of several analyses exploring the effects erection sequencing, implementation of upper lateral bracing, and use of temporary supports had on the final deformed shape of the curved superstructure. Findings indicated that using paired girder erection produced smaller radial and vertical deformations than single girder techniques for this structure, and that the use of lateral bracing between the fascia and adjacent interior girders and the placement of temporary shoring towers at span quarter points are both effective means of further reducing levels of deflection.

Significance of SSI and non-uniform near-fault ground motions in bridge response II: Effect on response with modular expansion joint

This is the second part of the study of soil–structure interaction (SSI) and ground motion spatial variation effects on bridge pounding responses. The first part [Chouw N, Hao H. Significance of SSI and non-uniform near-fault ground motions in bridge response I: Effect on response with conventional girder gap. J Eng Struct. [in press]] concentrated on studying the responses of bridges with a traditional expansion joint between adjacent girders. The main objective of this second paper is to investigate the influence of spatial variation of ground motions and SSI on the minimum total gap that a modular expansion joint system (MEJS) between two bridge frames must have to prevent any pounding between the adjoined girders. This minimum total gap is critical in a MEJS design because it ensures intact expansion joint and adjacent girders. Additional investigations of pounding response with a large total gap of a MEJS are performed to study the characteristics of pounding responses when collision does occur e.g. due to unintended underestimation of the ground excitation magnitude, which may be different from the pounding responses between bridge girders with a small gap of a conventional expansion joint. The spatially varying ground excitations are simulated stochastically based on an empirical near-source response spectrum and an empirical coherency loss function. The investigation reveals that neglecting spatial variation of ground motions and SSI can significantly underestimate the total gap of a MEJS required to avoid pounding between the adjacent bridge girders. The results also show that pounding between girders with a large gap of a MEJS in general causes stronger impact forces. Compared with the results reported in the first part of this study, using a MEJS with a large gap is likely to completely preclude bridge girder pounding, and consequently to prevent local damage at the girder ends. However, a large girder movement results in large bending moment in bridge piers, which compensates the advantages of using MEJS in bridges to resist earthquake loading.

Developing a structural-health-monitoring model to monitor cracking in steel-free concrete deck slabs

The ISIS Canada Networks of Centres of Excellence (NCE) program has focused on two main themes to improve civil engineering infrastructure, namely innovative construction technologies, and structural health monitoring (SHM). The former began with the construction of the first field application of the innovative steel-free concrete bridge deck slab technology at the Salmon River bridge, Nova Scotia, in 1995. Although this bridge has continued to function safely under heavy traffic loads, it has developed characteristic longitudinal cracking of the concrete between adjacent girders due to fatigue. This paper describes the recent research to develop an SHM model for monitoring the impact and stability of this cracking. Theoretical and experimental models were used to examine the change in response as cracking develops. A global load distribution matrix was proposed, and the variation in load distribution values with cracking was used to develop a cracking index that can be employed in monitoring the field structure.Key words: structural health monitoring, bridges, concrete, deck slabs, cracking, load distribution.

Girder Differential Deflection and Distortion-Induced Fatigue in Skewed Steel Bridges

The prevalence of fatigue cracking in steel bridge girders due to out-of-plane web distortion motivates development of procedures to evaluate the effects of distortional fatigue. In a previous study sponsored by the Minnesota Department of Transportation (Mn/DOT) the frequency and magnitude of distortional stresses on a typical skewed, steel bridge with staggered, bent-plate diaphragms were assessed. The results revealed a diaphragm deformation mechanism that causes distortional fatigue in the girder web gap, leading to simple, accurate estimates of fatigue stress if bridge properties and differential vertical deflection between girders are known. In the present study, linear finite element models are used to represent composite steel bridges and identify bridge parameters that influence relative deflection of adjacent girders. Parameters found to have a significant effect on differential deflection include girder spacing, angle of skew, span length, and deck thickness. These results are incorporated in a simple procedure that is intended for use in management schemes for skewed, steel-girder bridges, with staggered, bent-plate diaphragms, susceptible to web gap distortional fatigue.

Comments on Steel Construction…Equipment Management…Conferences…

Contractual relationships often impose barriers to good communication in the construction of steel structures. There are times when the erector works for the general contractor, the fabricator works for the general contractor under a separate contract, and the fabricator sublets the detailing to another firm that is under contract to the fabricator only. It is difficult to break through that contractual morass and have meaningful communication between the erector and the detailer. Difficult, but not impossible. There is at least one fabricator and one detailer that do not stand on ceremony and really push for cooperation and communication. My company was the erector in this particular chain, and we were very happy when this fabricator got their portion of the project. It was a bridge that had its deck transition from one width to another. Some girders stopped at a pier while others in the continuous spans continued. There were serious deflection differentials between adjacent girders. The sublet detailer caught this problem and broadcast it to all concerned. The fabricator gave us free reign to communicate directly, as long as we kept the fabricator informed. The major problem was that the cross-frames would not fit until the deck was poured. We needed some of the cross-frames for stability, and we did not want to erect any of them after the deck was poured. There were a series of faxes and phone calls as the detail evolved with a combination of slotted holes for bolts and round holes for pins. The final tightening of the bolts in these connections was performed after the deck was poured, but the cross-frames were erected as we went along and they rotated in the slots as the deck was poured. The sequence of pins, bolts, and tightening was put on the erector’s project specific erection plan so that everyone at the site knew what was happening and what to do. This was one of the best examples of good communication without letting contractual relationships get in the way. It was done in this case; it can be done on all projects.

Influence of SSI and frequency content of non-uniform ground motions on bridge girder poundings

Ph.D., Professor, School of Civil and Resource Engineering, the University of Western Australia, Crawley WA6009, Australia The study addresses the effect of the characteristics of Japanese design spectra on the pounding potential of two bridge girders. The spatially varying ground motions for soft; medium and hard soil condition are simulated stochastically. The effect of non-uniform ground displacements and accelerations, as well as soil-structure interaction is studied. The common design criterion for pounding mitigation is evaluated. The study reveals that a consideration of uniform ground excitation can strongly underestimate the pounding potential, especially when the ground motions have low dominant frequencies, and the soil is soft. The closer are the vibration frequencies of the adjacent girders, the more prominent is the non-uniform ground motion effect. The design recommendation for adjusting the structural frequencies should not be applied, if the ground is soft and a uniform ground excitation cannot be ensured.

Measurement and Analysis of Distortion-Induced Fatigue in Multigirder Steel Bridges

Fatigue damage to multigirder steel bridges on skew can result from distortion caused by differential deflection of adjacent girders that impose out-of-plane bending of girder web gaps. Existing design procedures give recommendations to mitigate the effects of distortional fatigue but do not directly address secondary, out-of-plane deformations, nor do they provide guidance in determining the magnitude of out-of-plane stresses in girder webs. An experimental study was conducted to (1) implement a field monitoring program for a typical multigirder steel bridge on skew supports; (2) assess the frequency and magnitude of distortional fatigue stresses at web-stiffener connections; and (3) evaluate the impact of these stresses on fatigue life. Measurements from twelve strain gauges were continuously monitored and recorded for a period exceeding three months on Minnesota Department of Transportation Bridge #27734. Web-gap stresses in negative-moment regions were found to be much larger than flange stresses. The results of a detailed finite-element study indicate that actual strains at the web gaps may be much larger than the values measured at the strain gauge locations. This study also revealed the mechanism of web-gap distortion, suggesting an approximate method for predicting web-gap stress based on known girder differential deflection.

Modelling three‐dimensional non‐linear seismic performance of elevated bridges with emphasis on pounding of girders

AbstractUnseating of bridge girders/decks during earthquakes is very harmful to the safety and serviceability of bridges. Evidence from recent severe earthquakes indicates that in addition to damage along longitudinal direction, lateral displacement and rotation of bridge girders caused by pounding to adjacent girders can also lead to unseating. To simulate this effect, 3D modelling of the dynamic performance of whole bridge structures, including pounding, is needed strongly. This paper presents a 3D model that is practically suitable to precisely analyse pounding between bridge girders. Experiments have been conducted to verify the proposed pounding model. The 3D non‐linear modelling of steel elevated bridges is also discussed. A general‐purpose dynamic analysis program for bridges, namely dynamic analysis of bridge systems (DABS) has been developed. Seismic analyses on a chosen three‐span steel bridge are conducted for several cases including pounding as a case study. The applicability of the proposed pounding model is illustrated by the computations. The effects of poundings on the response of bridge girders are discussed and the computation results are given. Copyright © 2002 John Wiley & Sons, Ltd.

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.

Use of High-Performance Concrete for Bridge Abutments: A Case Study

As part of an FHWA program to use high-performance concrete (HPC) in bridges, the state of Ohio constructed an adjacent box girder HPC bridge. HPC was used both for the prestressed and precast girders and for the cast-in-place abutments. The abutment mix was primarily designed for durability by using a low water-to-cementitious material ratio and pozzolanic materials. Before construction, laboratory trial mixes were run by using the actual material that was to be used in the abutments. After verifying the suitability of the mix in the laboratory, a few large trial batches were made at the ready-mix concrete plant to verify the plastic and hardened properties of the mix. During construction of the bridge, several problems occurred. Just before construction began, the coarse aggregate failed the state inspection and a new source of coarse aggregate had to be found. Unfortunately, HPC is sensitive to changes in aggregate, but there was no time to create trial batches before construction began. As a result, the mix had to be adjusted during construction. Soon after the initial abutment pours were made, differential shrinkage cracking was found. This was due to problems with inadequate curing. After a few tries, the mix problems were corrected and proper curing eliminated most of the cracking. Specific recommendations for quality control are made on the basis of the experience gained.

Full-Scale Testing of Shear Keys for Adjacent Box Girder Bridges

Adjacent precast, prestressed box girder bridges are widely used in the United States, but the shear keys between the girders tend to crack and leak. Three different shear key configurations were studied, i.e., a current detail where the shear key is approximately 10 in. (250 mm) from the top of the girder and grouted with non-shrink grout, this same detail grouted with epoxy rather than non-shrink grout, and a proposed mid-depth keyway grouted with non-shrink grout. The tests were conducted on a full scale, four-beam assembly which represented part of the bridge. The results showed that the currently used shear key detail cracks due to thermal stresses generated as the beams deflect upward and downward due to daily heating and cooling. The mid-depth shear key is less susceptible to these stresses and was found to be more resistant to cracking. Loading did not appear to cause new cracking, but rather seemed to propagate existing thermal cracks.

Reliability-based condition assessment of deteriorating concrete bridges considering load redistribution

A wide variety of models have been proposed for estimating the reliability of highway bridges. For reinforced concrete bridges subjected to environmental attack, time-variant reliability methods have to be used. In this study, the condition of reinforced concrete girder bridges is assessed using a time-variant system reliability approach in which both load and resistance are time-variant quantities. Several system models are considered, including failure of any girder (series system) and failure of a specified number of adjacent girders (series-parallel system). Adaptive importance sampling is used to determine the cumulative-time system failure probability. An existing reinforced concrete T-beam bridge located near Pueblo, Colorado, is investigated. The influence of resistance degradation and post-failure load redistribution is included. A comparison of reliability estimates for several system models is given, including the influence of correlation among initial girder strengths. The results can be used as a guide for the selection of system models for bridge reliability analysis, identification of critical girders in a bridge system, and for the development of optimal reliability-based maintenance strategies for reinforced concrete highway bridges.

Behavior and Design of Link Slabs for Jointless Bridge Decks

Maintenance of bridge deck joints is a costly problem. Debris accumulation in the joints can restrain deck expansion, causing undesirable forces in the deck and damage to the structure. Water leaking through the joints is a major cause for the deterioration of bridge girder bearings and supporting structures. Therefore, elimination of deck joints at the supports of multispan bridges will reduce the cost of construction and maintenance. This paper presents the results of a test program to investigate the behavior of link slabs connecting two adjacent simple-span girders, and proposes a simple method for designing the link slab. To illustrate the proposed design method, three design examples are included.

Testing of High-Performance Concrete Single-Span Box Girder

As part of a multistate research program on use of high-performance concrete (HPC) in highway bridges, a bridge originally designed as a three-span adjacent box girder bridge was converted to a single-span bridge by using 70-MPa HPC and 15-mm strands. As part of the research, a test beam was constructed and tested. Instruments placed in the beam before casting were used to measure transfer length, which was found to be approximately 1.22 m, larger than the 50-bar diameters usually used in the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications but consistent with recent studies. After the beam concrete reached the required compressive strength, it was tested to destruction. The beam was able to resist the required AASHTO ultimate moment without failure. It was found that the AASHTO cracking load was conservative for this beam, mostly because the measured modulus of rupture greatly exceeded the value assumed in the AASHTO specifications. The behavior of the beam was successfully predicted using a section analysis.

Nondestructive Determination of Response of Shear Keys to Environmental and Structural Cyclic Loading

Adjacent box girder bridges use grouted shear keys to share loads among beams. These keys tend to crack and leak. A full-scale portion of an adjacent box girder bridge is being used to test the performance of grouted shear keys under environmental and cyclic loads. The pulse velocity method is employed to find cracks in the shear keys. Two separate tests were conducted. In the first, shear keys that were grouted in late autumn cracked soon after casting, before any load had been applied. Data from instruments embedded in the beams and shear keys showed large discontinuities in strain caused by freezing temperatures. The strains were much larger than strains that occurred under loads corresponding to the weight of an HS20-44 truck. The beams were subjected to 41,000 cycles of loading, simulating HS20-44 wheel loads. No new cracking occurred from the loading, but cracks caused by temperature propagated under loads. In the second test, the keys were grouted in summer. Higher temperatures caused by the sun’s heat on the top of the beams again caused large thermal strains, which cracked the shear keys. These keys were subjected to 1,000,000 cycles of load corresponding to an HS20-44 wheel load. As in the first test, the load itself did not cause new cracks but the existing thermal cracks propagated under the load.

Static Behavior of Noncomposite Concrete Bridge Decks under Concentrated Loads

Tests were conducted under a concentrated monotonically increasing static wheel load on 1/6.6-scale physical models of three different types of noncomposite deck structure: (1) the entire deck supported on four steel girders; (2) an isolated deck slab region with a width equal to 1/3 of the bridge span; and (3) a 102-mm-wide transverse “beam” deck strip, both simply supported on two opposite edges on adjacent girders. Three reinforcing arrangements for the noncomposite concrete deck are studied: (1) orthotropic steel reinforcement (AASHTO); (2) isotropic steel reinforcement ( Ontario 1983); and (3) “isotropic-0.2%” minimum steel reinforcement. Flexural steel and concrete strain measurements in the “beam” deck strip models are compared with those in the models of the entire bridge deck where combined membrane compression and bending is present. Depending on the degree of the lateral and/or rotational edge restraint of the deck specimens, the ultimate load-carrying capacity of the models of the entire bridg...

Shear Key Performance in Multibeam Box Girder Bridges

This report describes a series of field tests of multibeam prestressed box girder bridges. The objective of the test was to investigate the in-situ performance of the grouted shear keys, located at the longitudinal joints between adjacent girders. The longevity of these joints can be a problem with this type of bridge; failure of the joint will typically not only compromise the load-sharing mechanism between adjacent girders, but also lead to the failure of the deck waterproofing system, with attendant corrosion problems. The tests consisted of monitoring relative displacements occurring across the intergirder joints, as well as bending strains in the girders themselves during passes of a preweighted tandem-axle dump truck, with axle loads typically in the range of 84.7 kN (19 kips). Relative displacements were measured with an especially designed transducer, having a resolution on the order of .00254 mm (0.1 mil), which was bonded to the underside of the bridge across the intergirder joints. Bending strains in the girders were measured with conventional foil strain gauges, having a gauge length of 38 mm (1.5 in.). All bridges tested exhibited relative displacements across at least some of the joints, which indicated a fractured shear key.

Construction Effects on Bracing on Curved I‐Girders

The placement of the concrete deck and the attachment of adjacent girders and diaphragms is generally not considered in the design of horizontally curved bridges unless the entire bare steel structure has been completely constructed and is an integral unit. The supporting of the individual girders prior to attachment of diaphragms is an important control within the construction phase in that shoring may be necessary for stability. Contained herein are general guidelines, that apply to various popular erection schemes, to prevent overstress during construction. The guidelines are determined via a parametric study that quantifies the effect that the top and bottom lateral bracing has upon the stress levels within the main girder elements. The study examines the response of a single‐span, twogirder, curved I‐girder system subjected to its own self‐weight. Multigirder bridges are also examined to determine the effect that the placement of a concrete deck slab has upon the response of the curved girder with th...