Post-tensioned bridge

 

This paper describes one typical postensioned prestressed concrete bridge built in construction stages. The traffic load is taken from EN1991-2 with respect of EN1990/A1. The code for check is according to EN1992-2. The structure is defined by standard modelling using in SCIA Engineer. The typical example of bridge used very often in practise is post-tensioned two girders bridge. A bridge cross-section from C35/45 is displayed in the figure on the right side of this page.

The bridge consisting of four spans (20+26+26+20 m) is built using technique span-by-span with overhang cantilever. A total length of the bridge including edge beams above the supports is Ltot = 93.0m. The construction of each span includes two stages. At first, the concrete is casted-in-place to fixed formwork and next prestressing of the span is realised. This procedure repeats for each span and completely 4 groups of the construction stages are modelled. The prestressing is applied incrementally across two spans. The couplers connect the half of the tendons in vertical construction joint of each cantilever overhang.

Construction stages

The whole structure will be defined using construction stages. The graphical presentation in 3D window is following during of construction.

Analysis

The construction stage analysis is performed for calculation of effects of stages with influence of effects of creep and shrinkage (TDA). Effects of creep and shrinkage are performed according to EN1992-1-1 annex B. There is also possibility to run calculation of TDA (creep and shrinkage calculation) according to code EN 1992-2 annex B. Due to the implementation of calculation of the creep and shrinkage of concrete according to EN 1992-2 and special requirements for concrete checks according to EN 1992-2 it is suitable to present new type of concrete material. There are two groups of materials (EN1992-1-1 and EN1992-2).

The frame XZ which is used for the time dependent analysis doesn’t respect effects of torsion in this project. The envelopes of mobile loads for extreme Mx (torsion moment) should be analyzed on different project type frame XYZ. The value of Mx should be defined in project Frame XZ as Not calculated internal force.

The user should defined not calculated internal forces in menu Loads>Not calculated internal forces for selected loadcase.

Check of prestressed concrete - EN1992-2

User defined section only are suitable for fast performing of concrete checks only in user defined section, where is supposed the most loaded structure and extreme results. Check is performed only in those user defined end section and duration of check is shorter. It is available for all concrete checks and design.

The modelled structure can be checked by two ways (i) globally using Member check, (ii) detailed using Single check. Member check is performed from the standard concrete check service. The results are displayed along the selected members. The output table has different output according to extreme.

Almost all concrete checks has single check. It is detailed analysis of one cross-section. Action button to go there is following in the bottom of service.

The check of prestressed concrete is performed according to EN1992-1-1 as well as to EN1992-2. The special checks according to EN1992-2 are implemented based on the material selection.

We can perform the following list of check for prestressed bridges according to Eurocodes.

Allowable concrete stresses

The stress before and after anchoring

Characteristic combination – longitudinal cracks

Quasi-permanent combination – great creep

 

Check of prestressing reinforcement

The stress before anchoring

The stress after anchoring

The crack limitation from SLS characteristic combination during service – cracks and deformation

 

ULS check

Limit strain method

Ultimate capacity using interaction diagram

Shear check

Torsion check

Allowable concrete stresses check

 

SLS check

Decompression

Crack control verification

Lukáš Dlouhý

09/02/2013