For structures with stability issues, one way to check which specific link is causing the problem is start with a constrained model and begin relaxing its DOFs gradually. When you are releasing the DOFs and when you are running into stability issues, you would know which link release is causing that instability. 

With more users asking us questions regarding pushover analysis and its applications in midas Civil, we want to share the answers to some of these common questions to our user community. Hopefully, this would help you understand pushover analysis in midas Civil environment a little more, and we want you to be more confident when using midas Civil to perform seismic design and analysis. We invited one of our Midas experts Yong Yang, principal structural engineer from Jacobs, to share some of his experience regarding those questions.

There are times when engineers would have to design and evaluate bridge structures that fall outside of the AASHTO design guideline. Therefore, when do we define a structure as irregular? How is evaluating an irregular bridge different from evaluating a regular bridge? How to minimize errors during the construction of irregular bridges? We invited midas expert Percy Penafiel, Professional Engineer Specialist from Nevada Department of Transportation, to answer some of the frequently asked questions from our users regarding evaluating irregular bridge structures. 

In typical engineering practices, engineers are used to having six degrees of freedom (DOFs) for modeling and analysis, three for rotations and three for translation. However, additional advanced beam elements can include other DOFs to represent the warping of an open thin-walled cross section. Such elements are not commonly available in professional software. (Article 1.2.6, G13.1 Guidelines for Steel Girder Bridge Analysis, AASHTO/NSBA, 2014). This has required engineers to model flanges as plates in order to obtain warping stresses. Midas Civil on the other hand has the 7th DOF warping feature which should save engineers a lot of time and effort and can grant warping related results directly from frame elements. 

In the design project to replace the old Fulton Road arch bridge in Cleveland, OH, Michael Baker Intl engineer Daniel Baxter and his team has designed a 1,568-foot-long replacement structure for the original arch bridge, which retains the original design of six 210-foot-long concrete deck arch spans. For the replacement bridge structure, a precast, post-tensioned concrete arch bridge design was selected. You may be asking yourself, why post-tensioning the arch bridge? Arches usually provide sufficient stabilities to structures and it is not usual for bridge designers to post-tension such structures. In this tip, Daniel Baxter talks about why his team has decided to post-tension the arch bridge and how they have utilized construction stage analysis to design the post-tension process. 

When we talk about prestressed concrete, the things that we are mostly concerned about are the compressive strength gain with respect to time, and the prestressing tendon relaxation with respect to time. Figure 1 shows various time-dependent effects for concrete including creep and shrinkage.

The factors that affect the creep rate include water/cement ratio, age and strength of the concrete when it is subjected to stress, and ambient temperature and humidity. Creep rate also depends on many other factors related to the quality of the concrete and conditions of exposure such as the type, amount, and maximum size of aggregate; type of cement; amount of cement paste; size and shape of the concrete mass; amount of steel reinforcement; and curing conditions (Robert Salca, tech support, midas UK).

For shrinkage, its rate decreases much faster with time compared with creep as shown in figure 1. Finer aggregates and finer gels result in increased shrinkage, the moisture content of the concrete and the relative humidity of the ambient medium have a big influence on carbonation shrinkage, and harder aggregates with higher modulus of elasticity decrease shrinkage. 

Bridge load posting – identifying just how much weight a bridge can bear – is a matter of public safety and a way to safeguard vital transportation infrastructure. However, load posting is less straightforward than it may seem. Not posting a bridge can create safety issues for the motoring public, while posting makes transportation more difficult for large, heavy vehicles and the industries that use them. 

How to Model the Girder and Deck Connection in Composite Bridge?

The way to simulate the connection between the girder and the deck will depend on how we construct the model. In the 2D all-frame composite bridge model shown in Figure 1, all the elements are connected in the grid within the same plane. Because it was modeled with the "all frame" model type in the midas Civil composite bridge wizard, it only consists of a 2-D grid frame composed of beam elements. In this scenario, the software considers the composite section as a lump section that incorporates both the girder section and the deck section. This means that the composite action is transformed into equivalent section properties in midas Civil. 

What is Convergence? What is a Convergence Study? 

It is easy to obtain the result from bridge finite element analysis, but to get more accurate results requires extra effort. Even the most robust finite element analysis solvers adopt the method that approximates the structural behavior, by minimizing the associated error function compared with the complex function that represents the realistic structural behavior. 

What is Strut-and-Tie Analysis? What are "Struts" and "Ties" in Strut-and-Tie Analysis?