Michael Baker engineers share their overall load rating procedure for complex projects and insight into other "girder bridge" challenges for unbraced sections. A sample bridge and project case study is used to demonstrate their workflow and subsequent results.
Bridge Rating Factor can be simply stated as a measure of safety of a bridge for a specific loading. The general rule of thumb is to begin as simple as possible with approximate analysis or a line girder approach and check if the line girder rating factor is greater than 1. Then the bridge is at an adequate level of safety, requiring no additional work. However, if the rating factor is found to be less than 1, the engineer has to make a decision: 1) Rehabilitate or strengthen the bridge to improve capacity. 2) Update the bridge parameters to reduce the live load demand. 3) Perform a refined analysis to get a better demand.
Choosing the Right Model Type
Why the emphasis on the modeling? It's because the model type directly affects the bending stresses, forces, and displacements meaning differences of live loads used for the rating factor. Assuming the engineer determined the adequacy of refined approach and determined the requirement of 2D or 3D (from AASHTO LRFD 126.96.36.199.4b); they would check the NCHRP Report 725, in the case of curved and skewed steel bridges. NCHRP Report 725 provides a recommendation for design and construction and also for analysis, making it applicable for load rating; therefore giving types of modeling approaches as well. As a verification of the validity of the recommendation, Michael Baker sampled and compared the Minnesota Bridge 27W02.
What's the Difference
As per NCHRP Report 725, a 2D grid and 1D line girder analysis would adequately compute the major axis bending stress, vertical displacement, and girder layover. However, the 2D grid and 1D line girder analysis would be highly inaccurate for cross-frame forces and flange lateral bending stresses. For this reason, the recommendation was to utilize a refined model for the Minnesota Bridge 27W02. The results concluding NCHRP Report 725 to be accurate.
Finishing the Workflow
Once the model is complete, following the load rating procedure yielded the rating factors through a combination of Excel spreadsheets and Civil outputs.
"Using no more than a spreadsheet on your end, but with powerful 3D FEA information in the background, overload permits for suspension, cable-stay, arch, truss, rigid frame, and other complex bridge types can be processed in less than a minute for many complex structures."
Products Used: midas Civil
midas Civil is an engineering software for the design of bridges and civil structures. It features a distinctively user friendly interface and optimal design solution functions that can account for construction stages and time dependent properties. Its highly developed modeling and analysis functions enable engineers to overcome common challenges and inefficiencies of finite element analysis.
midas Civil allows the full control of modeling the bridge, especially the cross frames.
AASHTOWare BrR is unable to handle the rating for steel bridges with irregular geometry.
midas Civil's load tracer allow to design the static case for the concurrent force outputs for multiple cases.
AASHTO LRFD bending, sheer & torsional strength, member forces & stresses for each construction stage, and max & min stress summation output checking.
Automatic generation of load combinations all in MS Excel format calculation reports.
Watch Part 1 and Part 2 of the Load Rating Series Here: