Structural design of composite concrete section in RCS (EN)

1 Midas Civil 

Geometry

It is a member model with a span of 10 m, which is prestressed and loaded by three permanent load cases, which are part of the individual construction stages. 

The model consists of four stages. In the stage Formwork, the beam is supported by a nonlinear spring acting in the compression only. At the Prestressing stage is a model at first prestressed and after that is removed the formwork. At the stage of Casting the slab is cast the slab. The end of the design's working life is considered 100 years due to the bridge beam. 

The internal forces for manual input to IDEA StatiCa RCS are taken from Element 5 and Node 7.  It means from the middle of the beam. 

The internal forces for each part of the section and all construction stages are taken from Midas Civil. The tab with typed-in values for node 7 and element 5 is shown below.       

2 New project

Launch IDEA StatiCa (download the newest version) and select the RCS  application. Create a new project and set the code for bridges EN 1992-2-2008-07. Select the Prestressed section design. 

3 Cross-section

The composite section is exported from the DXF file enclosed in the attachment. It is important to create Phase 1 and Phase 2 to program separate the section into two parts. Concrete C30/37 is used for both components by default but can be modified for each part separately by clicking on Modify (pencil) button.   

4 Prestressing

The setting of the prestressing allows putting Pre/Post-tensioned tendon. Put one Pre-tensioned tendon positioned 250 mm from the center of gravity the whole section.   

In Midas Civil was designed one equivalent tendon with an area of 600 mm^2. To get the same area of the tendon, change the material properties of the tendon according to the tab below. 

5 Desing Member 

In Design Member is necessary to set Construction stages according to Midas Civil. The construction stages correspond to the last step in Midas Civil, the first step is neglected. 

6 Action Stages

Action Stages are the most important in terms of correctly entering internal forces from third-party software such as Midas Civil. It is important to note that the internal forces from Midas Civil are related to each part of the cross-section (See the tab in chapter Midas Civil). In RCS, it is possible to enter internal forces into the Entire or Current section(1). Let's explain what each option means. 

The recalculated internal forces should be put if the Entire option is turned on.

1 Entire

Internal forces will be related to the centroid concrete cross-section (without the reinforcement) with the modulus of elasticity at age 28 days. All phases of the cross-section are taken into account in the case of composite cross-section irrespective of the fact that they currently exist. 

2 Recalculation of internal forces from third-party software 

The internal forces are related to each component and have to be recalculated to the center of gravity of the whole concrete section. Here is in detail described the workflow for all stages:

3 The tendon stress at every stage  

The stresses in the tendon at element 5 and node 7 are extracted from the Midas Civil and put to the tab(3).

4 Current

Internal forces will be related to the centroid of transformed cross-section determined from currently existing phases of cross-section and their prestressing reinforcement. Change of modulus of elasticity due to concrete aging is considered in all phases of the cross-section.  

5  Recalculation of internal forces from third-party software 

If we first enter the internal forces to the Entire section and then switch to Current, we can see the calculation of these internal forces to the current phase of the transformed cross-section or the cross-section with a shifted center of gravity due to the prestressing tendon. The center of gravity also changes the location due to the aging of the concrete.  

7 Sections

Create the three copies of extremes and put the time for every construction stage. This setting allows you correctly entering of internal forces from Action Stages.

8 Internal Forces

The internal forces won't be again rewritten but will be extracted from the Action Stages tab.  At first, set the Current Extreme(1), which represents the construction stage. In general, a partial factor of 1.35 is considered for ULS and permanent loads, which is also the default setting in IDEA StatiCa RCS. For our purpose, we choose a factor with a value of 1(2). We will use the All button to extract the internal forces from the Action Stages.   

For the construction stage Casting of the slab, change the Current Extreme to Stage 60  and perform the same steps as above.  

For the construction stage End of design working life, change the Current Extreme to Stage 36500  and perform the same steps as above.  

9 Results

Run the analysis of all extremes if you move the part Results. To verify that the results are correct, let's pick the last construction stage End of design working life, and move to the Stress limitation code-check. We can check if the Linear elastic stress is the same as in the Midas Civil.

Part 1_bottom = -5.2 MPa

Part 1_top = -2.7 MPa

Part 2 = -1.7 MPa 

Let's check the value of linear stress in Midas Civil in element 5 and node 7.

10 Report

At last, go to the Report Preview/Print. IDEA StatiCa offers a fully customizable report to print out or save in an editable format.

You have designed and code-checked a prestressed composite section according to Eurocode in application RCS.