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. 

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. 

In the blog article intro to Time-dependent Analysis for Concrete Structures, we have touched upon the importance of construction stage analysis for concrete structures. The material time function can be plotted and inputted into analysis software like midas Civil to simulate their changing material behavior in various stages of the construction. This article will go over the process of calculating various parameters that contribute to the shape and location of the material's time functions. 

What is the Support of Excavation System?

Creep Analysis 6

Creep Analysis 7

Think about a two-span continuous bridge, as shown in Fig 1. Let's calculate the secondary creep moments. Detailed dimensions and creep factors, etc., are not important, and the MIDAS file "Creep 2ndary Check"is attached for the reader's reference.

Creep Analysis 1 

Creep Analysis 3

The blog article of Creep Analysis 3 showed how Dr. Ghali et al. explained the creep analysis by flexibility methods.


To better understand the creep behavior, solve the previous example in a less efficient way. Here, different sign conventions will be applied.

Creep Analysis 3

We have gone through two different approaches by Dr. El-Badry so far. You can find the previous articles via these two links: Creep Analysis 1, Creep Analysis 2.

For creep analysis, the most common problem in the real-world design is continuous girders built as span by span. This example is very well explained by Dr. Ghali et al. (Concrete structures, Stresses, and deformations, 4th ed., CRC press, Example 4-2). Dr. Ghali et al. explained this problem by flexibility methods. The author will solve this same problem by stiffness methods. The programs do the matrix formulation and equation solve, and only the load matrix formulation and post-processing are our concern in the stiffness methods. Two MIDAS files are attached.


First, the author wants to define the sign convention for member forces clearly.

Creep Analysis 2 

Creep Analysis 5 MIDAS Example