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21 Design practice for non-conventional bridges is affected by several factors that differentiate it from the conventions of practice for conventional bridges. â¢ Non-conventional bridges are generally high value investments, lifeline structures, or both. â¢ The structural systems for non-conventional bridges are not the classical beam-column viaduct- type structures that are the core of conventional highway bridges. â¢ The structural systems for non-conventional bridges often include tall towers, long spans with non-redundant framing systems, and flexible deck systems where large displacements and residual drift are far more significant than for the dimensions and scale of conventional bridges. The first of these factors is the basis for limited ductility design as first described in ATC-32. A repairable damage standard is primarily to address the criticality and cost of non-conventional bridges within the highway network. The special behavioral characteristics of some non-conventional bridges require that design- ers review the basic tenets of the weak-column strong-beam design basis of the Guide Spec as they consider structural performance and seismic safety for unique structural systems. Certain long-span flexible bridge systems may relate more closely to the strong-column weak-beam per- formance requirements as applied to the safety analysis of tall buildings. Such a case is reviewed in Case Example 1. The dimensions, scale, and character of non-conventional bridge structural systems are not amendable to the simplified analytical methods for conventional bridges, which has led to the current practice requiring nonlinear analysis. In the case of strain-based design criteria, material nonlinearity is necessary to establish the hierarchy of inelastic demand as well as compliance with the performance limit states for multilevel seismic hazards. As for the wind conundrum described in the Introduction, the practice of strain-based capacity protection for limited ductility design removes the conflict of competing lateral load demands that can occur for large non-conventional bridges. The hierarchy of allowable strain levels for ductile and non-ductile elements is addressed absent any definition of controlling load, so that a wind-controlled design when subjected to the nonlinear dynamic analysis for multilevel ground motions can be qualified based only on the results for demand displacement from the seismic event. The basic design is not affected by the hierarchy of lateral demand. Since detailing is governed by the seismic code provisions in the AASHTO BDS or the Guide Spec, the resulting design satisfies both the safety and limited damage performance objectives for design. C H A P T E R 4 Evaluation and Case Examples
22 Seismic Design of Non-Conventional Bridges Case Example 1âCable-Stayed Bridge with Single Pylon The long-span cable-stayed bridge presented here is an example of a single tower and flex- ible deck system that requires performance-based criteria for multilevel seismic ground motion input (see Figures 4, 5, and 6). This type of bridge is often on deep alluvial soils, which is the case for this example. The criteria included the standard performance limit states typical of those long-span bridges identified in the survey and literature. Beginning with the 475-year return event as the functional event and progressing to the 2,475-year return event as the safety event, the criteria addressed strain limits for each performance limit state. The performance condition assigned to the single tower section was limited ductility at the safety level 2,475-year event. Even at this limited ductility, the nonlinear analysis models resulted in ductility demands at a level that was similar to the strength design condition for wind load- ing (note that for strength design of under-reinforced sections, there is no explicit evaluation of rebar strains, just the section equilibrium assumption that is associated with a bilinear stress strain curve). Wind loading on the elastic model was approximately the same demand as the safety level seismic event on the pylon in the nonlinear model with liquefied foundation condi- tions. When reviewing the effect of nonlinear section models on demand levels, results show that even the limited ductility level of rebar strain provides a significant reduction in section moment at the demand displacement when compared with simple elastic section models. Total pile section requirements were controlled by seismic for the liquefied case since moments in the liquefied condition added to axial demands; however, the axial demands were higher for the non-liquefied case due to higher total response for the stiffer foundation condition (6). The section size of the lower pylon was also affected by the vessel impact on the main in-water pier, which controlled section wall and local reinforcing. The general balance of demands was pro- duced in part by the seismic performance criteria. A classical ductility-based design for the single stem tower could have reduced reinforcing for seismic demand considerably from the final reinforc- ing. However, such a reduction would not have been sufficient for the normal factored demand for wind loading or the local section demand for vessel impact. Once dimensioned for the requirements for wind and vessel impact, application of a full Guide Spec capacity protection type design for the base of the single pylon would have added 40% to the pile foundation requirements (there is a syn- ergistic effect of expanding footing dimensions for additional piles, since the mass of the footing is Figure 4. Long-span cable-stayed bridge with single pylonâPort Mann Bridge.
Evaluation and Case Examples 23 a major contributor to pile demand). And perhaps more significant, even absent wind and vessel impact, a plastic hinge-based full ductility design of the pylon could result in residual drift of the pylon that would render the single pylon bridge useless after the design event. Case Example 2âConcrete Arch Bridge Case Example 2 is that of a concrete arch bridge (see Figures 7 and 8). A concrete arch relies on the integrity of the springing for both vertical and lateral loads. While two hinged long-span arches are viable for vertical loads, this is not the case for lateral loads due to distortion and geometric nonlinearity. For an arch springing supported by rock Source: Transportation Investment Corporation. Figure 5. Sample performance criteria for cable-stayed bridge.
24 Seismic Design of Non-Conventional Bridges Figure 6. Example cable-stayed bridge lateral resistance system. Figure 7. Example arch bridgeâHoover Dam Bypass.
Evaluation and Case Examples 25 foundations, the concept of capacity protection for a foundation was not an issue. However, performance requirements that include repairable damage plastic hinges in the arches for lateral loads are problematic. The example structure was developed during the transition of AASHTO BDS and during early development of the Guide Spec after the proposals to adopt NCHRP 12-49 into AASHTO were rejected by the AASHTO Subcommittee on Bridges and Structures. The project-specific seismic criteria were developed using various sources that included some elements of NCHRP 12-49 but essentially followed the performance-based design principles of California non-conventional bridges for limited ductility critical structures. A single 1,000-year return period was used for design based on a review of site-specific data and the forecast of ground motion levels versus return periods in the study sponsored by the owner (not knowing at the time that both the AASTHO BDS and the new Guide Spec would arrive at a 1,000-year return period for seismic design). The framing of the arch structure was established to allow for the potential of ductile strut elements that could be designed to maintain the limited ductility performance requirements for the arch ribs, similar in function to the shear links on SFOBB Self-Anchored Suspension Bridge. Nonlinear analysis was performed as in Case Example 1; however, full moment-curvature defi- nitions were not utilized in this case due to the level of ground motion and seismic demand levels in the arch ribs. The potentially ductile strut elements between the arch ribs remained elastic under the design event once they were sized for the wind demands on the arch structure but were detailed for ductility. REINFORCEMENT Main Column Bars #11, #14 & #18 0.08 0.03 0.015 Main Column Bars #10 and Smaller 0.12 0.03 0.015 Spirals & Ties #8 and Smaller 0.12 0.05 NA Where: = = ultimate steel strain ductile member "performance goals" i.e., repairable damage = "performance goals" i.e., minimal damage Îµu Îµpg Îµpp Îµu Îµpg Îµpp design level of peak cyclical steel strain for struts and design level of peak steel strain for arch and spandrel Figure 8. Strain performance criteria for concrete arch.