1. Who is the software addressed to?

The software is primarily addressed to steel bridge erectors, since erection or deck placement sequences can be easily defined within a single model. mBrace3D can for example determine whether deflections and stresses stay within reasonable limits, whether uplift will occur at the supports, or the critical buckling load of a partially-erected structure.

As the version now supports influence surfaces and vehicle load optimization (so far for plate girder bridges only), the software is now also adressed to bridge designers.

Finally, since structural stability has been the primary focus of the software development since the start, the software also targets researchers in steel stability. mBrace3D has been used as a tool in graduate structural stability classes and it is believed that it could be used in graduate steel bridge design classes as well.

2. What type of bridges can the software model?

mBrace3D can model plate girder bridges and tub girder bridges (trapezoidal box girder bridges).

3. What type of geometries can the software model?

mBrace3D can model straight geometries as well as curved geometries. This includes curved bridges having a point of tangency (a portion of the bridge is initially straight before being curved), as well as bridges having a compound curvature (the bridge has a given curvature up to a certain point and then this curvature changes).


However, at the moment mBrace3D cannot model S-type curvatures.

4. How is the bridge information entered?

The bridge geometry and properties are entered via a series of user-friendly forms. When the bridge information is already available in Excel format, it can be copied and pasted directly into the software.

Unlike conventional FE software, modelling the bridge does not imply any type of "drawing" - the bridge is defined parametrically. This parametric modelling allows for very quick modifications, which are constant during the design.

5. What type of analyses can the software perform?

mBrace3D can conduct linear elastic analyses, which provide the engineer with estimations of the bending induced and torsion induced deflections, stresses, support reactions, cross-frame forces, shear and moment diagrams, flange lateral bending diagrams, etc.

In addition, during the erection phase (where only the steel superstructure is modelled), mBrace3D can conduct eigenvalue buckling analyses, to estimate the stability of the structure. For this phase, and only for plate girder bridges, mBrace3D can also perform geometric nonlinear analyses (large displacement analyses). For straight bridges, initial imperfections entered as a fraction of a buckling mode can be simulated.

mBrace3D can also conduct influence analyses for composite steel/concrete plate girder bridges, as well as vehicle load optimizations based on the calculated influence surfaces. This allows the engineer to assess what will be the minimum and maximum shear and moment values along the bridge as vehicle loads circulate on the deck, for example.


Finally, mBrace3D can also conduct eigenvalue frequency analyses, to estimate the natural vibration modes of a structure.

6. What makes the software different from more traditional grid/grillage models?

mBrace3D explicitly models all steel plates with quadratic thick shell elements. Brace members are modelled with bar elements. As shown in various papers in the past two decades, and as recommended by AASHTO and AISC/NSBA, full 3D shell modelling is the most accurate approach for complex, curved and/or skewed geometries. This includes the estimation of the critical buckling load, local buckling modes, cross-frame forces, torsional deformations, etc.


What has prevented the extensive use of 3D shell models is the complexity involved in the modelling process. mBrace3D  and its user-friendly, parametric modelling, aims to precisely fill that gap. mBrace3D is a niche software for plate girders and tub girders.

7. What are the current limits in terms of model sizes?

3D shell modelling comes at the cost of bigger models. While grillage models contain a few hundreds or thousands degrees of freedom, 3D shell models for medium-sized bridge structures typically involve around 100,000 or 200,000 degrees of freedom. mBrace3D is a fully 64-bit program and can therefore handle those large models. On standard laptops, it was shown that models up to 700,000 DOFs can be handled. On powerful desktops, this number can increase to 3,000,000 DOFs. In general though, it is recommended to start with a coarse mesh and gradually refine it where necessary.

8. Are there any known limitations to the software?

No FE software is flawless. At the moment, one limitation is the modelling of the top and bottom chords of K-frame braces with bar elements. This leaves the K-frame middle point with some local instability. A displacement constraint to enforced to prevent the instability but as a result, the K-frame forces are not estimated correctly. It should be noted though that the overall stability or behavior of bridges with K-frames is not impacted by this limitation.

Another limitation consists of the influence analyses being only applicable to plate girder bridges. In the future, those analyses will also include tub girder bridges.

Also, steel and concrete are assumed to be linear elastic materials. mBrace3D is unable at this point to model plastic behavior and in particular, plastic buckling. Geometric nonlinear analyses are able to capture large displacements but small strains only. Fortunately, especially during erection and construction, materials should stay elastic anyways.

9. How was the software validated?

Software results validation is a crucial part of software development in general. mBrace3D was initially validated with ABAQUS. Later on, scripts were automatically generated to import mBrace3D models into LUSAS. This was paramount in testing and validating a wider range of models and geometries. Of course, simple structures were checked against hand calculations, such as the Euler load or classic beam theory.

Some validation studies are for example provided in the "Tutorials" chapter within this website.

10. Does the software make use of existing finite element libraries?

No. All code was written by the developer himself, except for a few functions that help with the visual rendering of the model.

The program was written in Visual Basic .Net. In the background, the finite element algorithms were coded in Fortran 90.