Post Tensioned Concrete Beam Bridge

Introduction

This tutorial deals with a simple 2-span post tensioned concrete beam bridge. The analytical model of the bridge consists only of beam elements (superstructure and columns).

Note

A basic SOFiSTiK knowledge is required for this tutorial. The standard workflow is explained inside the General Workflow description. Inside this tutorial we show only the project specific workflows, which are different from the basic workflow.

Objectives

  • Starting a new project
  • Define Materials
  • Define Cross Sections
  • Define Pre-stressing systems
  • Generate System and Loads inside SOFiPLUS
  • Linear analysis
  • Live load analysis
  • Construction Stages
  • Design

Project Description

In order for you to follow the work-flow more easily, we have split up the data files according to the different chapters. This enables you to start in the middle of the tutorial if necessary. The idea of this tutorial is to guide you through a simple PSC (Pre-stressed Concrete) bridge project and introduce the general work-flow showing the necessary program tools and functions. All steps like modeling, loading, traffic loads, combinations etc. are simplified.

Note

If there are any hints of new tasks that have to be modified manually (new tasks named “Text Editor (Teddy)”) you find further information’s directly in those tasks. Please open data files related to the chapter.

../../_images/bridge-pic1.png

Bridge Axis:

Spans [m] - 0.70 - 26.60 - 26.60 - 0.70 -
Stations [m] 0 - 0.70 - 27.30 - 53.90 - 54.60
../../_images/system-011.png

Bridge Materials:

Number Title Strength
1 Concrete bridge deck C 40/50
11 Concrete pier C 40/50
2 Reinforcement steel B 500
3 Reinforcement steel stirups B 500
4 Prestressing steel Y 1570C

Cross sections

Number Title Dimensions
1 Column b/h=2.20/1.00
../../_images/system-021.png

Construction stages

Stage Number Title
10 Cast of column for middle support
15 Creep + Shrinkage (of Column)
20 Cast of superstructure beam and activation of self-weight
21 Prestress of superstructure
25 Creep + Shrinkage
30 Further permanent loads (for example asphalt)
35 Creep + Shrinkage until traffic handover
45 Creep + Shrinkage from traffic handover until t=infinity

Design code

This tutorial is based on Eurocode DIN EN 1992.

Download Project Files

You will find the data files on our ftp-server.

Starting a new project

First we create a new SSD project and save it inside a project directory on your local computer. For further information see chapter Start New SSD Project in General Workflow description.

Defining materials

Generate all necessary materials listed above. Follow the procedures explained in chapter Material Definition in General Workflow description.

Defining Cross Sections

In this project we have only one standard cross section for the pier. Please generate a new rectangular cross section with the dimensions and material properties listed above. Follow the procedures explained in chapter in Cross Section Definition in General Workflow description.

The main bridge section will be defined graphically inside SOFiPLUS with the cross section editor.

Note

For the main bridge section we want to use a variable for the non effective parts of the cross sections. This variable will be defined within the bridge axis definition in SOFiPLUS. Therefore we recommend to create the bridge axis with all settings first.

Pre-stressing systems

Please generate a new prestressing system number 1, with a VSL 6-12 multistrand system, see picture below.

../../_images/p-system1.png

Follow the procedure explained in chapter Prestressing System in General Workflow description.

System Generation in SOFiPLUS

The bridge geometry will be defined using the CABD concept inside SOFiPLUS. For that we will work through the following steps:

  • Define main bridge axis
  • Define placements along the bridge axis
  • Define variables (will be used for the main cross section)
  • Generate master cross section within Cross Section Editor
  • Generate structure lines representing the main bridge construction
  • Use the Cross Members Editor to generate the support construction containing springs and couplings
  • Make a final check of the system and align elements if necessary

The following videos will show the main steps to generate the system.

Generate Bridge Axis

The following video will show the axis generation workflow.

Generate Master Cross Section

The following video will show cross section generation workflow within the cross section editor.

System Generation

The following video will show the system generation workflow.

Tendon Generation

The following video will show tendon generation workflow.

Loads and Actions

Before we define any load, we must define all necessary actions within SOFiPLUS. In our bridge example we need the following actions and load cases:

../../_images/actions1.png ../../_images/loadcases1.png

After defining the load cases we generate the loads in SOFiPLUS. First we select an element related line load. Defining the load we select the structural lines and define a line load of 9 kN/m. In every support axis we define a settlement wzz = 10 mm and save the loads within the load cases 51 to 53. For the temperature loads we generate 4 loadcases 81 to 84 containing the basic temperature loads

LC Action Designation
81 NONE constant temperature DT = + 20°C
82 NONE constant temperature DT = - 20°C
83 NONE temperature difference DTZ = -12.3°C
84 NONE temperature difference DTZ = +8.0°C

Warning

Load cases which will be used later on inside the CSM should be saved witsin the action container NONE. With this concept the load cases will not be used twice in case the user defines his own combination rules using actions only!

For further processing including the necessary temperature combinations please see the chapter Define Actions and Loads in General Workflow description.

Linear Analysis

The linear analysis will be processed only for the settlement LC 51-53 and temperature loads LC 91-98. Therefore the selection will be done manually. Please see the chapter Linear Analysis in General Workflow description.

Generate Envelope from Traffic Loads

As already explained in the general workflow it is necessary to run at least one linear load case before evaluating the traffic loads with the influence line method. We recommend to do this analysis before the construction stage analysis to use the full developed stiffness matrix from the complete bridge.

Note

The stiffness effects of the elements were already used within the linear analysis. The following influence line evaluation will therefore be processed on that basis. Additional settings are not necessary.

For a general description of the input workflow please see also the chapter Traffic Loads in General Workflow description. For our little example we have a small bridge with a total width of 5.6 m. First you select an existing bridge axis and define the width of the traffic lane and bridge. The program will automatically generate all possible lanes.

../../_images/ella-011.png

Next you define the load trains and the desired results. In our case we are interested only in the beam results for further design of the composite cross sections.

../../_images/ella-021.png

For the results it is necessary to define different load groups and evaluation cases. Usually it is convenient to separate the results for tandem axes and udl loads. The evaluation results will be saved again in a predefined action container.

../../_images/ella-031.png

The loads on the footway will be defined as an extra evaluation case inside the UDL load group. In that case we need special combinations from UDL and Footway loads. Simply use the combination button inside the dialogue and generate the combinations. After the input is correct and finished you may close the dialogue and process immediately the analysis. Again we recommend to add a task “interactive Graphic” to display the important results.

Define Construction Stages

For post tensioned concrete bridges we need the construction stages for the evaluation of different cross section properties based on the open and filled ducts for the tendons as well as for creep and shrinkage analysis. Insert the SSD tasks “CSM” and fill in the stages as defined in table Construction Stages above.

For our example the following input must be done inside the task “CSM - Construction Stage Manager”.

../../_images/csm-011.png ../../_images/csm-021.png ../../_images/csm-031.png

For further explanations please see also the chapter Construction Stages in General Workflow description and our CSM_1.pdf manual.

Combinations

Please follow the explanations of chapter Combinations and Superpositioning from the General Workflow description.

Design

Please follow the explanations of chapter Design Checks from the General Workflow description.

Documentation

For the final documentation, you can collect all single reports and generate a complete document. Please follow the explanations of chapter Generate Report from the General Workflow description.