Case Study

Automated Design of Railway Culverts

• Automated design/checking of culverts

• Detailed calculation option

• Reinforcement contour plots

Italian LUSAS distributor Alhambra srl worked in conjunction with the engineering company Ingegneri Associati srl to automate the modelling and design/checking of railway culvert structures in accordance with Italian bridge loading and reinforced concrete codes of practice. Automating the analysis of culvert structures using the LUSAS scripting facilities produces significant time savings, improves the quality of the analysis and also produces a detailed analysis report.

The Culvert Design Wizard uses a number of user-defined dialogs to obtain all the data required for a culvert analysis. First, transverse culvert geometry is defined along with water levels (if any) and whether a road is present in the culvert. This is followed by specifying plan dimensions and the angle of any rail lines crossing the structure. Error checking and incompatible data entry is provided on all dialogs. An option is available to undertake a more traditional 2D analysis considering a strip of 1m width or to define a full 3D model of the structure using thick shell elements.

A series of dialogs enable the following data to be entered:

• Number and position of tracks

• Angle between the railway line and the longitudinal axis of the culvert

• Angle between the lateral faces and the longitudinal axis of the culvert

• Concrete properties (linear elastic material)

• Ground/Soil properties (modelled as loads or linear elastic springs)

• Availability of different stiffness for vertical springs under the structure (using LUSAS variations), and of horizontal springs on vertical walls

• Principal or secondary railway line

• Options for curved railway lines, relative transverse difference in height between rails Eb, and train design speeds

• Thermal and shrinkage actions

A LUSAS model file is created from the data entered with the structure having rigid zones at nodes and with all load conditions required by the design code included. All transverse actions are considered to act in the same direction.

The LUSAS Smart Load Combination feature is used to include all load combinations included in the Italian design code for railway bridges and a global envelope is produced. Train loads are considered at three possible positions for each railway line - each edge of the culvert and at the centre of the span.

Nodes on the model where a detailed calculation is required can be selected. Details of concrete and steel design stresses are entered along with details of minimum steel area, planimetric layout, and of steel cover for each member. Using a Wood-Armer approach, the minimum amount of steel area needed to comply with the requirements of the Italian design code is calculated. Steel areas of transverse and longitudinal steel bars can be plotted on selected parts of the culvert.

A Microsoft Word report provides a complete description of the data input and load combinations used in the culvert analysis and is an important contribution to the speed-up process and the quality of the results presentation. It provides a comprehensive listing of all model information and results details and allows the design engineers the opportunity to add their own comments and additional information. A contour plot of all stresses for final maximum and minimum envelopes activated according the most relevant stresses is also included along with a contour plot of steel area needed in each direction and in each part of the structure. If any user-defined nodes have been selected on the model for detailed stress calculations then these appear in the report also.

Carlo Margheriti of Alhambra srl said: ‘The script developed significantly speeds up the analysis of a typical "simple" structure, such as a railway culvert. By modelling the structure with exact 3D geometry and supports, and including all the loading combinations prescribed by the code of practice the quality of the analysis is improved significantly also.’ He continues: ‘ A simplified 2D strip analysis option is also provided to comply with client wishes and to help check or assess third party calculations that use the same design assumptions.’