Appropriate characterization of these factors in the model can have a significant impact on the model’s overall accuracy. Performing special studies on influential components of the structural model can help to improve the accuracy of global and local structural models.  TOBIN MEMORIAL BRIDGE  History of the Tobin Bridge.   The Maurice J. Tobin Memorial Bridge carries US Route 1 across the Mystic River, connecting the city of Chelsea and the Charlestown section of Boston, Massachusetts. Construction on the bridge began in 1948 and it was opened to traffic in 1950.  The 2 1/4 mile long structure includes 32 approach spans on the Chelsea side, 36 approach spans on the Boston side, the Little Mystic Span, the Big Mystic Span, and the Toll Plaza. Figure 1 shows a 3D rendered view of the components of the Tobin Memorial Bridge with Boston in the background. The Tobin Memorial Bridge consists of three northbound lanes on the lower level of the structure and three southbound lanes on the upper level. The Big Mystic Span, also referred to as the Main Span, is a three-span cantilevered steel through-truss measuring approximately 1,525 feet in length. The Little Mystic Span is a simply supported Warren truss approximately 439 feet in length. Figures 2 and 3 are photographs of the Big Mystic Span and the Little Mystic Span. Truss members in the Big Mystic Span and the Little Mystic Span are mostly built-up steel sections that are bolted and riveted to steel gusset plates.  Scope of the Project.   The Massachusetts Port Authority (Massport) announced a Request for Qualifications early in 2008 seeking structural modeling and analysis of selected components of the Tobin Memorial Bridge. The scope of work included review of existing construction documents, creation of computer models of selected bridge spans and components, development of a cost effective instrumentation plan, and verification and adjustment of the model based on the measured data. Massport contracted the consultant services of a team led by Fay, Spofford, & Thorndike, LLC, and supported by Tufts University, the University of New Hampshire, and Geocomp, Inc.  BRIDGE COMPUTER MODELS  Modeling approach.   Three dimensional line models were prepared in AutoCAD representing the geometry of structural  components of the Tobin Bridge. Figure 4 shows the AutoCAD model of  the Big Mystic Span. Each structural element is represented by at least one line element in the model. Every crossbeam, stringer, truss member, floor beam, sway frame, diaphragm, and bracing member is included in a “microscopic-level” model. Previous research (Catbas et al. 2007) on a bridge similar to the Big Mystic Span demonstrated that a microscopic-level model could predict the structural responses of a non-destructive test with reasonable accuracy. Modeling at a highly detailed level eliminates the need  to make assumptions about the effective behavior of structural elements as required in a smeared and coarse modeling approach. Smeared modeling is an approach that combines a number of structural elements into a few finite elements with effective properties; microscopic-level modeling involves a geometrically complex model but the calculations and assumptions of effective properties do not need to be made. The commercial analysis programs SAP2000® and GT STRUDL® were chosen for this project. Both programs can be used to generate structural analysis models through their Application Programming Interface (SAP2000®) and Text File Input (GT STRUDL®). So in this approach, the geometry model is used as a database to store pertinent information about the existing structure. Structural models can then be generated in many different programs. Creation of finite element models (FEM) from AutoCAD models is accomplished using a three-step process shown in Figure 5.   Figure 5. Computer model development process   
The geometric AutoCAD models are transferred to an Excel spreadsheet using the AutoLISP programming language. Imported geometry is named and assigned numerical inputs required for structural analysis in the spreadsheet including material and section properties, connection stiffness, and loading conditions. Visual Basic routines create the FE models directly from the spreadsheet data.  Standard 3D frame elements compose the steel members  of the FEM. The concrete deck is represented as a mesh of shell elements with a specified thickness. Figure 6 shows the frame and shell elements that compose the SAP2000® model of the Little Mystic Span.   Figure 6. Frame and shell elements in the Little Mystic Span SAP2000® model  Connections.   Truss design typically assumes that the members are connected by frictionless pins and are free to rotate. Forces are assumed to be present only in the axial direction of connecting members. However, the assumption that joints are free to rotate does not represent the structure’s actual behavior. Friction will always be present at the connections, even those detailed with eyebars and pins. Secondary stresses, due to shears, moments, and torsion, build up in the members due to the rigidity of the connections. In the case  of the Tobin Bridge, truss members are connected by large, riveted gusset plates, probably leading  to introduction of secondary stresses in the members, see Figure 7.   Secondary stresses have been long considered in the design of truss bridges. In 1877, the polytechnic school in Munich offered a prize for the solution of how to calculate secondary stresses in a riveted truss. Heinrich Manderla proposed a method to calculate secondary stresses that won the prize (Manderla 1880). His solution, 波士顿托宾纪念大桥的结构建模英文文献和中文翻译(2):http://www.youerw.com/fanyi/lunwen_67676.html