Bridge Structures

The dynamic p-y method (based on the Winkler model) is a useful approach for designing bridges to resist seismic events. Further, this approach facilitates characterization of nonlinear dynamic soil-structure interaction while offsetting time and expertise that would otherwise be required to develop continuum-based models. Herein, the nonlinear Winkler method is validated—within bridge FEA software—and employed to demonstrate computation of dynamic bridge foundation member response to seismic loading. Specifically, piles driven in soft clay sites (with specified shear wave velocities) are investigated when subjected to scaled base accelerations derived from the Taft 1952 and Northridge 1994 earthquakes. Superstructure resistance, soil degradation, soil mass participation, and gapping are incorporated into the numerical models. Additionally, insights are made regarding the effect of selected parameter variations on design-relevant structural response quantities (e.g., maximum displacements). This study highlights extant capabilities in design-oriented FEA software for assessing seismic responses of bridge pier foundations.

Orthotropic steel decks (OSDs) consist of a complex network of stiffeners and the deck plate itself. Working as a whole, it takes part in the structural working of the overall bridge, which in its turn results in a lightweight and durable deck concept. Orthotropic steel decks are nevertheless very sensitive to fatigue damage, because of the large number of welded connections. Innovative research focuses on the application of fracture mechanics as well as the influence of residual stresses on the fatigue lifetime. An analysing tool using Linear Elastic Fracture Mechanics (LEFM) is proposed. Manufacturing processes such as welding cause residual stresses, which are present in most civil structures. Including these results in determining the fatigue life using LEFM leads to improved OSD lifetime. Overall, it can be stated that the OSD remains a valuable bridge concept, especially for larger span bridges, that can be understood better using modern research techniques.

When opened in 1967, the San Mateo/Hayward Bridge crossing the San Francisco Bay south of San Francisco incorporated the United States’ first major orthotropic steel bridge deck. The mile long orthotropic steel deck of the bridge was the largest in the world at the time. The orthotropic deck is included within a 2 mile long steel high rise portion of the 7 mile long bridge. Over 40 materials were evaluated for the riding surface of the original orthotropic deck. Epoxy asphalt was chosen and remained in place well past its life expectancy until it was finally replaced in 2015 by a polyester concrete material. The paper chronicles the factors leading to the selection of the material and construction of the initial and replacement riding surface. The replacement needed to be done on a critical bridge in service connecting the East-Bay Area communities to San Francisco. This made accelerated construction a factor in choosing a replacement and motivated evaluation of polyester concrete as a possibility.

Recent bridge projects have incorporated multiple hazards (i.e., blast, fire, manual sabotage, rocket-propelled grenade/mortar) as independent threats on the system. Traditional design methods to handle each threat separately are expensive and can lead to conflicting requirements. This paper will introduce the framework for a robustness-based design process. The outcomes are articulated through a series of generalized variables; topology (i.e., structural configuration relative to the site or location), geometry (i.e., layout of the structural load bearing elements), damage, and hazard intensity measures. A probabilistic framework permits consistent characterization of the inherent uncertainties through the process.

Rather than consider the global resistance as a sliding scale in relation to a fixed load, the proposed alternative is to consider robustness as a fixed property of the system, that is, Robustness is a function of topology and geometry. Geometry and topology are absolute properties that cannot be changed without modifications to the overall system configuration.

If robustness is held to be an absolute property of the system, then resilience represents the variable property that fluctuates with specific design decisions. If element failure (i.e., collapse) is avoided, damaged elements will still require repair or replacement, resulting in a temporary loss of functionality. Resistance should, therefore, be provided such that potential damage minimizes casualties and reduces the likelihood of catastrophic structural losses.

The proposed expression of resilience is a function of hazard and robustness, or a function of hazard, topology and, geometry. The structural performance associated with a specific system configuration is considered as independent from the contribution of component strengthening to address a prescribed load or hazard. The resulting equation for resilience represents the specific hazard magnitude mitigated by a structural design with an assigned robustness. This definition of resilience allows engineers to quantify resilience and robustness in more certain terms and provides a basis to better assess post-event structural behavior.

As the number of requests for permits to use bridges by heavy trucks increases, there is a concern over the potential for rapid fatigue damage to the affected bridges. The damage may especially become critical for bridges that have been designed for lower truck loads than those currently used in practice. To limit damage, and to indirectly impose a cap on the number of overload permits, a modification factor in the bridge rating equation to incorporate the fatigue damage potentials from overloads is introduced. This factor is derived based on the amount of damage that a specific overload may cause and is found to be mainly affected by the current bridge age and the percentage of overloads in the entire truck load population. For single and continuous span steel girder bridges with welded cover plates, simple forms of this modified equation have been proposed and introduced in the paper.

The seismic retrofit of the Seto-Ohashi Bridges (a group of long span bridges crossing Seto-Inland Sea) was started in 2013 and will be completed by 2020. The seismic performance verification of three suspension bridges (Shimotsui-Seto, Kita Bisan-Seto, and Minami Bisan-Seto Bridges) of the Seto-Ohashi Bridges was started in 2014. These bridges are one of the longest highway-railway combined suspension bridges. Recently, large-scale earthquakes that were stronger than considered in the original design occurred in Japan. Therefore, two types of large-scale earthquake motions coming from plate boundary and inland active faults, expected to occur in the future, were used in the seismic performance verification. All truss members of the stiffening girders were modeled in the analysis to see precise behaviors during earthquakes. As the result of the seismic performance verification, in all three bridges, it is found that main members (such as main towers, stiffening girders, and substructures) will not suffer severe damage and the total number of damaged portions is small. Although some members are damaged, the damage level is found to remain below acceptable level by the supplemental in-depth analysis. As members to be retrofitted are deck bearings only, a retrofit method for road deck bearings was selected from various candidates considering workability and efficiency.