Cross Frames Research Articles

Composite steel-free deck bridges: Numerical modelling and pilot parametric study

During the past decade, composite steel-free deck bridges came to reality in Canada through the construction of five bridges. The new structural system enables the construction of a concrete deck that is totally devoid of all internal steel reinforcement. Traditionally, reinforced concrete bridge decks are designed to sustain loads in flexure. The steel-free deck bridge system develops internal compressive forces "internal arching," which leads to failure by punching shear at substantially higher loads than the flexural design load. The fibre-reinforced concrete deck is usually attached to the steel girders through flexible shear connectors. These steel girders are transversely tied together by steel straps and cross frames. In this paper, the concept of the new bridge system is briefly discussed. The generations of the deck slabs are introduced. Brief outlines of the bridges built to date with this new technology are presented. A three-dimensional finite element model is then proposed to study the behaviour of the main structural component of the new system. The model is verified against previous experimental results and is used to perform a parametric study on some aspects which are thought to significantly affect the behaviour of the new steel-free deck bridge system.Key words: bridges, composite girders, finite element method, steel-free deck, steel straps.

Distortional Loads and Brace Forces in Steel Box Girders

Trapezoidal steel box girders are often used in curved bridges due to the large torsional stiffness that results from the closed cross section. However, the torsional loading on the box girders can cause the cross section to distort from its original shape, which results in distortional stresses in the plates of the box section. The distortional stresses are controlled by internal cross frames that restrain the cross section from distortion. Current design specifications do not account for forces in the bracing members due to box girder distortion. This paper presents an analytical study on the distortional behavior of box girders with a trapezoidal cross-sectional shape. Typical torsional loads on curved box girders are discussed, and the distortional components of these applied torsional loads are studied. The distortional components from different torsional loads on trapezoidal box girders are identified and used to derive expressions for the brace forces in the internal cross frames for quasi-closed box girders. The results from the approximate equations for brace forces due to cross-sectional distortion are verified by three-dimensional finite-element analyses.

Composite Action in Steel Girder Bridge Superstructures Subjected to Transverse Earthquake Loading

Experiments are being performed on a 0.4 scale model of a steel girder bridge superstructure with a reinforced concrete deck to investigate means of improving the seismic performance of typical slab and girder bridges. During the early stages of experimentation, the need to understand the influence of composite action became apparent. Accordingly, an analytical study was undertaken. The first of several finite element analyses on a typical continuous four-span, four-girder bridge model assumed shear studs along the entire length of the bridge to provide full composite action. A second model omitted the shear connectors in the negative moment regions, a common design practice to avoid fatigue concerns in the top flange. It was found that the shear connectors in the fully composite model were sufficient to ensure composite action when subjected to transverse loading and provided an adequate load path through the superstructure and into the substructure. However, the lack of shear connectors in the negative moment regions caused the load to be transferred into the steel girders at the points of contraflexure, resulting in damage to these girders and inadequate formation of the ultimate limit state in the columns. The lack of composite action was also found to change the distribution of transverse shear forces in the cross frames, which is important for design of the end cross frames. Making the top chord of the cross frames composite with the deck, at the column bent locations, proved effective in minimizing damage to the superstructure.

Comparison of Measured and Computed Stresses in a Steel Curved Girder Bridge

Steel curved I-girder bridge systems may be more susceptible to instability during construction than bridges constructed of straight I-girders. The primary goal of this research is to study the behavior of the steel superstructure of a curved steel I-girder bridge system during all phases of construction and to ascertain whether the actual stresses in the bridge are represented well by linear elastic analysis software developed for this project and typical of that used for design. Sixty vibrating wire strain gauges were applied to a two-span, four-girder bridge, and elevation measurements were taken by a surveyor's level. The resulting stresses and deflections were compared to computed results for the full construction sequence of the bridge as well as for live loading from up to nine 50-kip trucks. The analyses correlated well with the field measurements, especially for the primary flexural stresses. Stresses due to lateral bending and restraint of warping induced in the girders and the stresses in the cross frames were more erratic but generally showed reasonable correlation. In addition, it is shown that, for the magnitude of live load applied to the bridge, analyses in which composite behavior is assumed in the negative moment region yield better correlation than analyses in which just the bare steel girders are used (no shear connectors were used on the bridge in the negative moment region). It is concluded that the curved girder analysis software captures the general behavior well for these types of curved girder bridge systems at or below the service load level, and that the stresses in these bridges may be relatively low if their design is controlled largely by stiffness.

Steel Bridges with Double-Composite Action: Innovative Design

Bridge engineers are continually faced with the challenge of providing efficient and cost-effective structures. In particular, the Florida Department of Transportation has recognized the need to develop economical bridge configurations in the medium-span range of 200- to 400-ft (60.96-to 121.92-m) spans and authorized a research project at the University of South Florida to identify and develop innovative design concepts within this span range. The study identified the concept of a steel bridge with double-composite action as an innovative bridge concept with the potential for significant cost savings compared with conventional modes of construction. This bridge type has been used with good success in Europe, but to the authors’ knowledge it has not been used in the United States. In addition to a composite concrete top slab, the double-composite bridge concept utilizes a composite concrete bottom slab in the negative moment regions. The result is provision of a design meeting compact requirements throughout, increased stiffness with corresponding decrease in fatigue stress range and deflections, savings in cross frames, and savings in flange material. The design implications of this system are examined, including redistribution effects due to creep and shrinkage, implications of different construction sequences, and strength and serviceability issues. Trial designs are presented, including both plate and box girder type structures, and design considerations are discussed. A prototype structure is identified for further development of the double-composite concept.

Impact of Diaphragms on Seismic Response of Straight Slab-on-Girder Steel Bridges

Many steel bridges have suffered diaphragm (cross frame) damage during recent earthquakes. Diaphragms provide an important load path for the seismically induced loads acting on slab-on-girder steel bridges, but their impact on seismic response is still unclear in many ways. The relative role played by intermediate and end diaphragms in providing lateral load resistance, along with the consequences of diaphragm damage on bridge seismic response, has not been studied. This paper quantitatively investigates the impact of diaphragms on the seismic response of straight slab-on-girder steel bridges. Typical 20 to 60 m span slab-on-girder bridges with and without diaphragms are considered and studied through elastic and inelastic static push-over analyses. Two hand-calculation analytical models are proposed to evaluate their period, elastic response, and pseudospectral acceleration at first yielding. It is shown that a small end-diaphragm stiffness is sufficient to make the entire superstructure behave as a unit in the elastic range. However, a dramatic shift in seismic behavior occurs once an end diaphragm fractures, with a sizable period elongation, considerably larger lateral displacements, and higher propensity to damage owing to P-Δ effects. It is also found that the presence of intermediate diaphragms does not significantly influence the seismic performance of these types of bridges, in either the elastic or the inelastic range.

Cross-Frame Spacing and Parametric Effects in Horizontally Curved I-Girder Bridges

The finite-element method was used to create detailed models of horizontally curved steel I-girder bridges connected by cross frames. The effects of a number of parameters on the behavior of the curved girder system were established and compared to the effect of these parameters in straight girder systems. The parameters that most significantly affect the behavior of the systems were determined to be the degree of curvature, span length, and flange width. An equation was developed based on a nonlinear statistical regression to provide a preliminary design limit for the cross-frame spacing interval. The results of the equation were compared to those results obtained by previous equations developed by various methods for the same purpose.

Seismic Analysis and Design of Modern Steel Highway Connectors

This paper discusses the seismic analysis and design of modern steel plate girder connectors. Full 3-D finite element analyses were conducted on a two-span two-girder bridge to determine its seismic load path and to establish the seismic performance of the bridge with several cross frame configurations. The results of the study showed that the cross frames play a significant role in the seismic behavior since they transfer the inertia forces to the substructure, a phenomenon not accounted for in the current design bridge codes.

Influence of Cross Frames on Load Resisting Capacity of Steel Girder Bridges

The fifteenth edition of the AASHTO manual requires intermediate cross frames in steel girder bridges with maximum spacing of 25 feet. Although these cross frames may be needed for temporary loads their effectiveness following construction has been a point of debate for short and medium span bridges. Cross frames with different configurations have been utilized in bridge construction. Besides increasing construction costs, many problems in steel girder bridges could be attributed to the presence of cross frames. For instance many states have observed cracking in the girder web of bridges in the vicinity of the cross frame's connection to the beam, especially for details where stiffeners are not rigidly connected to top and bottom flanges.Prior to casting the concrete deck, smaller spacing of cross frames results in smaller laterally unbraced lengths of steel girders and, consequently, could result in smaller sections. In some steel bridges it is therefore possible to have diaphragm spacing of less than 25 ft., the maximum spacing allowed by the AASHTO manual. The need for cross frames in these instances becomes an even more serious question, especially if their presence results in unsatisfactory performance of the bridge (such as developing cracking in girder web). To address this issue, a combination of analytical and experimental investigations was conducted. A summary of the experimental investigation related to the use of cross frames is presented in this paper.

Interaction between Cross Frames and Girders

This paper presents a three‐dimensional finite element algorithm for computing internal forces in intermediate and end cross frames of multibeam composite steel bridges. The developed algorithm was applied to four medium‐span [137 ft (41.75 m)] simply supported multigirder tangent four‐lane bridges, with X‐type intermediate cross frames and end cross frames of the Warren truss with verticals type. One was a right bridge and the others had 20°, 40°, and 60° skew angles. It was found that: (1) Most members of the intermediate cross frames may develop compressive and tensile forces; (2) maximum compressive forces occur in the cross‐frame diagonals attached to the ends of exterior stringers, close to the obtuse angles of the bridge; (3) maximum tensile forces occur in the cross‐frame chords, at midspan; (4) the effect of skew may be neglected for skew angles of 20° or less; (5) the higher the skew angle, the higher the maximum forces induced in the cross‐frame members; (6) the larger the cross‐frame members' ...

Effects of intermediate diaphragms in distributing live loads in beam-and-slab bridges

For small-span to medium-span bridges, the beam-and-slab type of construction is popular, mainly because of its ease of construction, and is preferred in practice even though this form of construction may sometimes exhibit rather poor transverse load distribution qualities. This deficiency is often reduced by the incorporation of one or more diaphragms in the deck construction. Although the use of diaphragms in beam-and-slab bridges is very extensive, their use is almost entirely empirical rather than based upon any logical method of analysis. Thus, the requirements for diaphragms that are found in codes of practice all over the world usually consist of simple and arbitrary statements. It is proposed here to carry out a thorough theoretical study of the structural behaviour of diaphragms in beam-and-slab bridges, with a view to establishing this behaviour on a well-reasoned footing. Key words: beam-and-slab bridge, diaphragm, cross frame, grillage analogy, finite element method, orthotropic plate theory.

Lateral Wind Bracing Requirements for Steel Composite Bridges

The design of moderate multi-span composite bridges requires utilization of cross frame diaphragms between girders and bottom lateral wind bracing. The AASHTO code1 specifies that cross frames or diaphragms shall be provided at each end of the bridge and intermediate frames spaced at intervals not to exceed 25 ft. Previously, AASHTO specifications required that bottom lateral bracing should be provided, located near the bottom of the flange, on spans of 125 ft or longer, if the system has a concrete floor or a floor of equal rigidity. It is the intention of this paper to present the results of a recent study,2 which evaluated the effectiveness of bracing in redistributing wind load and formed the basis for the lateral bracing provisions of the 1982 AASHTO Interim Specifications. This effect has been determined by examining the response of single-span, multi-girder, composite bridges, when subjected to wind load. The bridges examined had span lengths of 80 ft, 120 ft, 180 ft, and 300 ft, with girder spacings of 6 ft, 10 ft, and 14 ft. The resulting response of these bridges indicates that the bottom lateral bracing has minimal effects on the redistribution of the wind load. The important parameter in load redistribution is the diaphragm spacing and span length. The utilization of cross diaphragms, in reducing the flange stresses due to wind loading, are given in a series of design equations.

CURVBRG: A program for analysis of curved I-girder bridges

A computer program for structural analysis of open girder (i.e. I-section) bridge super-structures is described. The program is based on a plane grid idealization, in which the effects of warping torsion are considered rationally, and in which structural components such as diaphragms and braced (truss type) cross frames are idealized by essentially exact procedures. The bridge may be of arbitrary shape in plan, the girders may be of variable cross section along their length, and changes in form of the structure as construction progresses may be considered. Deflections, stress resultants and stresses can be determined for static loads, support settlement effects and moving live loads. The structural idealization procedure and theory are explained, the features of a computer program developed to apply the method are described, and comparisons are made between the computed behavior and that observed in a model test.

プレートガーダの直線継ぎと段違い継ぎの比較研究(第I報)

There are two types in complete splice of plate girder. One type is the splice (joints of both flanges and web) at the same cross section (the straight joint). Another is joints of flanges shift some distances from it of web (the shift joint).The shift joint has been used to apply to the field weld joint of I-beam bridge. As the welding technique has been progressing, the joint strength of the good field welding has become to be never inferior to the steel plate strength in recent days, the straight joint should be adopted and welding works should be rationalized. These two types of joints were compared by welding specimens. By specimens' data, we confirmed that in case of the straight joint was adopted, defects would never take place, and the straight was adopted on the cross frame (members which connect between girders of bridge) of Ayakawaohashi.Details of field joints and how to weld were decided by the data.Principal points in the results of this weld works are as follows:1) Results of measurement were good and no visible deformation remained at every cross frame joints.2) The radiograph inspection performed by sampling procedure. Disqualified parts were found only in two passages and these defects were found in early stage of this works. Welding procedures were immediately improved. It had been successful for the purpose of quality control to adopt this sampling produre.3) Root openinge of flange and web of this works were equal but it of web should have been larger (1 mm) than flanges.4) Numbers of labour engaged in chipping grinding (included compressor operating) to finish weld reinforoecment amounted to 13, 3% of total numbers of labour. To rationalize weld works, it ought to be remained.5) Service plates for setting up were fit on webs, but these plates should be fit outsides of flange.