Bridge analysis, design + assessment

Case Study

Semi-automatic design of composite bridge decks

  • Wizards to speed-up composite bridge deck modelling

  • Composite cross-section property calculation and section libary

  • Design checking to Italian codes of practice

LUSAS distributor Alhambra srl has helped many of its Italian clients to carry-out design and checking of composite bridge decks and bridges using the general features available in LUSAS Bridge. Now, in conjunction with one of its clients, Tecnostrutture srl, it has created a set of Italian bridge wizards for use with LUSAS finite element analysis software to speed-up model building, analysis and checking of these type of structures.

Composite bridge decks can be modelled in several different ways according to the level of the design and the experience of the designer. Methods include:

1. A 2D line beam analysis where a single longitudinal beam is used to model both steel and concrete with equivalent torsional stiffness of the deck.

2. A 3D grillage analysis where the steel beams and concrete slab are modelled longitudinally and transversely with equivalent beams.

3. A composite beam / shell analysis where beam elements are used to model the steel beams and shell elements model the concrete slab. The line beam is located in the surface of the slab and an offset (eccentricity) is used in its section properties. The shown physical distance in the image does not exist in the model and is for visualisation purpose only.

4. Steel beam webs are modelled with shells, each flange is modelled with beams, bracings are  modelled with beams, and the concrete slab is modelled with shells.

5. Steel beam webs and flanges are modelled with shells, bracings are modelled with beams, and the concrete slab is modelled with shells.

In Italy, standard bridges are usually modelled as grillages (methods 2 and 3 above) and, as a result, wizards were created for use with LUSAS Bridge to speed up the modelling of composite bridge decks for these methods.

Model building

Models are built by defining the two longitudinal edge beams of the bridge then selecting the grillage wizard. Using this, a grillage or ribbed plate model, of straight or curved form, with either single or multiple span, can be generated. An option is provided to include surfaces in the model to assign shell elements.

Italian Grillage Wizard


Parallel and divergent transverse beams are allowed. Transverse beams are automatically built and connected to the selected points by constraint equations or by rigid beams. All similar transverse beams can be built at the same time and several types are allowed including single eccentric beams, and X or K transverse bracings with real eccentricities. Bracing members are automatically grouped together for manipulation in Modeller. 

Transverse Beam Wizard


Beam cross-section properties of composite beams are calculated for the four analysis stages:

  • Steel beams only

  • Composite steel and concrete, at long term

  • Composite steel and concrete, at short term

  • Steel beam and reinforced bars (cracked sections)

Geometric attributes are defined with beam fleshing included. Any sections of a variable height are defined in steps. Single beam, and mono or multi-cellular composite sections are allowed with automatic definition of corresponding width of concrete and of equivalent thickness of bracings.

Composite section property calculation


A full set of EU profiles can be chosen in addition to standard supplied LUSAS sections. It is also possible to join together 2 or 4 equal / unequal angle sections and 2 channel sections.

Geometric properties and other additional information is stored in a .csv file and post-processed in a dedicated spreadsheet to provide elastic stress calculation. 

User attributes to allow elastic stress calculation to be carried out in accordance with the Italian Code of Practice are defined. 

A design check is carried out in a separate wizard.

Italian section library dialog


Loading / Combinations

Maximum and minimum stresses due to Eurocode Highway/Railway loading are obtained using a Smart Combination approach. Results are returned at gauss points of selected elements and are available for diagrams and contours.

In general the load type considered is comprised of a heavy load (a number of point loads or a patch load), and a lane load (a uniform patch load). A minimum distance between the heavy load and the lane load can also be defined. The software will not load spans or any parts of structure where the applied loadings would give a relieving effect. Different dynamic factors can be applied to loads on different spans.

Smart Combination defininition 

Design Checking

A wizard is also used to export geometric section properties, node coordinates and user defined sets of results into an Excel spreadsheet to allow design checks to be performed. Axial stresses, shear and torsion are checked for all construction stages. Shear in connectors and web buckling are also examined.

Design checking using Excel spreadsheet

In Excel, the same LUSAS wizard as used to define the original grillage model is provided, so that if a stress check fails then section data can be changed for a quick check in Excel prior to exporting the data into LUSAS for a revised full analysis to be re-run. 

A separate report wizard allows a list of all checks carried out to be created in Word.

Carlo Margheriti, manager of Alhambra, said: "In the last few years many composite bridges have been analysed by Alhambra using LUSAS Bridge, and in doing so the number and quality of the wizards created and used for this purpose has increased so that they have now become a very productive modelling tool." 

He continues: "Other modelling methods have also been used at Alhambra as and when required, for example, by creating steel beam models where webs are modelled with shells. Comparison of the results from the different methods has increased confidence in our modelling of these composite structures."

Italian Report Wizard

"In the last few years many composite bridges have been analysed by Alhambra using LUSAS Bridge, and in doing so the number and quality of the wizards created and used for this purpose has increased so that they have now become a very productive modelling tool."

Carlo Margheriti, Manager, Alhambra srl

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