Proposed Modification to AASHTO Cross-Frame Analysis and Design (2021) / Chapter Skim
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12 Research Approach To address the major objectives of NCHRP Project 12-113, four distinct phases were systematically carried out by the research team. The four phases, as identified in the project RFP, are as follows: (I)
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Research Approach 13 • Task 7: Execute Part 1 of analytical program related to the specific bridges evaluated in the field experiments. • Task 8: Execute the field experiments, as introduced in Task 4.
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14 Proposed Modification to AASHTO Cross-Frame Analysis and Design of four independent computational studies. Table 2-1 summarizes these independent studies as well as the various software platforms used.
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Research Approach 15 2.1 Industry Survey An industry survey was developed and distributed by the research team to gather information from DOTs and consultants in the following areas: (i) commonly used software packages, (ii)
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16 Proposed Modification to AASHTO Cross-Frame Analysis and Design • Cross-frame connection details vary substantially from state to state. Figure 2-2 graphically summarizes the range of cross-frame configurations used in the United States.
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Research Approach 17 supports, a straight bridge with skewed supports, and a horizontally curved bridge. The research team assessed the pros and cons for a variety of existing bridge structures throughout Texas but ultimately elected to study three in the greater Houston area.
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18 Proposed Modification to AASHTO Cross-Frame Analysis and Design on numerous factors including field access, traffic control considerations, the capacity of the data acquisition system (DAQ) , and preliminary analysis.
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Research Approach 19 experiences. Note that the one-month monitoring period is sufficiently long to capture stabilized data as well as hourly, daily, and weekly trends that may deviate from normal or average conditions (Connor and Fisher 2006; Fasl 2013)
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20 Proposed Modification to AASHTO Cross-Frame Analysis and Design load position and Fatigue I and II load factors [which have been calibrated for longitudinal girders only through the research efforts documented in SHRP 2 R19B (Modjeski and Masters 2015)
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Research Approach 21 shells but instead use line elements. While the present study utilized line elements with R-factors, additional studies were also carried out making use of shell-element models.
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22 Proposed Modification to AASHTO Cross-Frame Analysis and Design Note that the vertical offset dimension and the presence of connection plates are further discussed in Sections 2.4 and 2.5, which focus heavily on the approaches traditionally adopted by 3D commercial design software programs. Prior to conducting the parametric study, the accuracy of the assumed truss-element modeling approach and its simplifications outlined above was validated with the field-measured data.
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Research Approach 23 was filtered to generally provide only the cases that are likely most critical. Rules were established to eliminate permutations deemed unnecessary or less important than others.
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24 Proposed Modification to AASHTO Cross-Frame Analysis and Design In other words, it was ensured that the bridges studied represent realistic structures that have been properly designed and detailed. • Aside from the parameters listed in the tables, the research team also conducted spot checks to examine the influence of several parameters including different flange dimensions and the removal of bridge barriers.
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Research Approach 25 Based on the findings of this preliminary study, the research team preselected cross-frames to evaluate prior to running the full parametric study. Given the unique geometries and cross-frame layouts, the selected cross-frames are not necessarily in the exact same location for each bridge.
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26 Proposed Modification to AASHTO Cross-Frame Analysis and Design tasks: (i) develop the 3D FEA models for all geometries, (ii)
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Research Approach 27 In total, the research team produced thousands of influence-surface plots similar to those in Figure 2-5. Influence-surface plots are useful visual tools and provide important insights on the load-induced response of cross-frames, particularly in the case of this study where evaluating over 65,000 plots in an efficient and meaningful way is virtually impossible given the number of parameters investigated and the variability in the response.
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28 Proposed Modification to AASHTO Cross-Frame Analysis and Design More specifically, Section 2.3.2, utilizes the over 65,000 influence-surface plots to assess current AASHTO fatigue loading criteria. Section 2.3.3 expands on that analysis and investigates the effects of measured WIM traffic streams.
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Research Approach 29 magnitudes are smaller, but the response is more complex. Whereas the other two truck passages resulted in one primary force cycle, the centerline passage results in additional secondary cycles of lesser magnitude.
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30 Proposed Modification to AASHTO Cross-Frame Analysis and Design state, permanent (dead) loads and locked-in stresses are not explicitly addressed herein, except as described above.
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Research Approach 31 Note that many of these concepts were previously introduced in Section 1.2 as the major questions to be answered in NCHRP Project 12-113. • Maximum stress ranges are compared for the various bridge geometries, such that the impact of each parameter can be evaluated with respect to cross-frame behavior.
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32 Proposed Modification to AASHTO Cross-Frame Analysis and Design vehicles. The records used for the fatigue study contain approximately 11 million truck measurements after all appropriate filters are applied.
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Research Approach 33 2.3.3.1 Application of WIM Records In order to estimate real live load force effects on cross-frame members, the research team applied the filtered WIM traffic streams to a subset of the analytical testing matrix discussed in Section 2.3.1.2. As the computational effort is significant (discussed in the next subsection)
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34 Proposed Modification to AASHTO Cross-Frame Analysis and Design • Load Configuration 2 – Two Traffic Streams: Similar to Load Configuration 1, a WIM traffic stream is stepped along a bridge deck in a defined transverse position. The script uses a cluster analysis to include the effects of groups of vehicles in any adjacent, user-defined transverse position, provided any tire on the following truck is on the bridge during the time window in which the leading vehicle is still on the bridge.
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Research Approach 35 which is outlined in Appendix E To study the effects of this lower stress range truncation on cross-frames, the research team compiled the results of each loading iteration (as listed above)
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36 Proposed Modification to AASHTO Cross-Frame Analysis and Design For the actual location of each 6-foot vehicle track width (i.e., transverse distance between left and right wheel lines) within the realistic lane, the research team assumed a distribution for which the vehicle is located in the center of the lane 55% of the time; 30% of the time, the vehicle is located plus or minus 1 foot of the lane centerline; 10% of the time, the vehicle is located plus or minus 2 feet of the lane centerline; and 5% of the time, the vehicle is riding along one of the lane edges.
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Research Approach 37 obtained from a variable-amplitude spectrum via rainflow-counting techniques and the use of Palmgren-Miner's rule. In order to compare the WIM force effects to the unfactored AASHTO design truck force effects, it is more appropriate to compare the accumulated fatigue damage, rather than just the effective stress ranges.
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38 Proposed Modification to AASHTO Cross-Frame Analysis and Design indicate the front axles of the passing truck are "ahead of " the rear axles of the drive-lane truck. This provides a smooth, continuous function of clear distances, with increasing clear distances (positive or negative)
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Research Approach 39 truss-element modeling approach for cross-frames, appropriate stiffness modifications for composite conditions, and a proposed alternative approach to model cross-frames in refined 3D analyses (i.e., an eccentric-beam model)
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40 Proposed Modification to AASHTO Cross-Frame Analysis and Design Figure 2-13 divides the deformations into in-plane and out-of-plane deformations. The in-plane rotations depicted in Figure 2-13 are attributed to the in-plane rigidity typically provided by the connection and gusset plates.
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Research Approach 41 for the softening effect of eccentric end connections (i.e., the R-factor is often assigned to reduce the cross-sectional area of the truss element or the elastic modulus of the material)
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42 Proposed Modification to AASHTO Cross-Frame Analysis and Design from more refined to relatively simple. Each variation (i.e., the refined, simplified, and angleonly models)
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Research Approach 43 • The out-of-plane "jumps" in the neutral axis, or the eccentricities, are represented by rigid offsets, which imply that the welded or bolted connections between the plates and angles perfectly constrain these elements together (i.e., no slip is assumed in the bolted connections)
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44 Proposed Modification to AASHTO Cross-Frame Analysis and Design In general, the behavior of a shell-element model is compared to the equivalent trusselement model. The ratio of the corresponding stiffness of a shell-element model to that of a truss-element model represents the R-factor.
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Research Approach 45 bridge structures with various gusset plate thicknesses. More specifically, the sensitivity of the load-induced cross-frame response due to the assigned R-factor was examined.
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46 Proposed Modification to AASHTO Cross-Frame Analysis and Design Rather than compare the stiffness of the models indirectly, it is more appropriate to evaluate the ability of the simplified model to accurately obtain design forces, which is of most importance to designers. A ratio of unity represents perfect agreement between the shell-element model and the simplified models.
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Research Approach 47 from comparing specific commercial design software packages and instead highlighted the modeling approach, which is of more general value to designers and software developers. In total, three distinct types of analyses were performed in Software A as part of this study, including 3D models (as a direct comparison with the 3D truss-element models produced in Abaqus and documented in Section 2.4)
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48 Proposed Modification to AASHTO Cross-Frame Analysis and Design the beam element representing the girder between two adjacent cross-frame intersections on that beam. This is particularly important for horizontally curved bridge systems, which generally model the girders as a series of chorded, straight-line segments (White et al.
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Research Approach 49 and converted back into an idealized truss system for obtaining internal member forces. With respect to this transformation, there are two major questions that often arise when implementing these procedures into 2D models.
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50 Proposed Modification to AASHTO Cross-Frame Analysis and Design force predictions. For completeness, the models and corresponding results presented later in this report include equivalent torsional properties.
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Research Approach 51 between the respective centroids)
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52 Proposed Modification to AASHTO Cross-Frame Analysis and Design 2.5.4 Parametric Study Overview To assess the limitations of these various 2D modeling techniques and modifications, a series of analyses were conducted as part of this Commercial Design Software Study. The results of 3D truss-element models and various 2D models were compared for a variety of bridge geometries, where the 3D models served as the basis for comparisons.
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Research Approach 53 torsional bracing. Regardless of whether a system is categorized as torsional or lateral bracing, adequate stability bracing must satisfy both stiffness and strength requirements.
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54 Proposed Modification to AASHTO Cross-Frame Analysis and Design A review of Eq. 2.2 demonstrates that the total stiffness of the system, bT, is always less than the smallest of the three individual terms on the right of the equation.
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Research Approach 55 stay-in-place forms, commonly used to support wet concrete during deck construction, have connections to the girder that potentially can introduce significant flexibility and therefore are not considered bracing elements in bridge applications (Egilmez, Helwig and Herman 2016)
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56 Proposed Modification to AASHTO Cross-Frame Analysis and Design full unbraced length. In case (iii)
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Research Approach 57 flange restraint, which is representative of the steel section alone during deck construction, (ii) continuous lateral restraint to the top flange, which is representative of noncomposite finished bridges not utilizing shear connectors, and (iii)
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58 Proposed Modification to AASHTO Cross-Frame Analysis and Design supported at the ends. For the cases involving reverse curvature, the girders were still simply supported, but girder continuity was simulated with applied end moments (positive and negative)
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Research Approach 59 ideal stiffness, was subsequently performed to obtain the relationship between internal girder moments and critical bracing moments. An additional discussion on modeling convergence and inferring cross-frame brace forces from the results is provided in Appendix F for reference.
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