Last reviewed 30 Sept 2026 · 6 min read
Software used in practice
Structural analysis and design software is a tool, not a substitute for engineering judgement. Commonly used packages include STAAD.Pro, MIDAS Civil, CSI Bridge, SAP2000 and ETABS (from the same family as SAP), LUSAS, SOFiSTiK, Robot and general-purpose FEM programs such as ANSYS and ABAQUS. They differ in convenience and focus (frames and buildings, bridges with staged construction and moving loads, or detailed continuum FEM), but the modelling principles are the same.
Choosing the model type
| Model | Description | When used |
|---|---|---|
| Line / beam model ("spine") | The deck is one beam on the centroid axis, with the section properties | Preliminary design, long box girders, longitudinal behaviour |
| Grillage | A grid of longitudinal (girders) and transverse (deck slab, cross-girders) beam elements | Slab and beam-and-slab decks, skew decks; gives the load distribution among girders |
| Shell (plate) model | Deck slab, webs and flanges as shell elements | Wide box girders, shear lag, distortion, local effects |
| 3D solid model | Brick elements | Anchorage zones, pier caps, joints, massive foundations |
| Space frame | Beam elements in 3D | Buildings, pylons, trusses, piers with bracing |
| Combined models | Beams for the girders, shells for local parts, springs for bearings and soil | Realistic bridge models |
The level of detail should suit the question: a more detailed model is not always a better one. A simple model that is checked is better than a complex model that is not.
Modelling a bridge
- Geometry and alignment: the centreline, curvature, camber, skew; nodes at the bearing points, quarter points and at every section change.
- Superstructure: girders as beam elements with the correct section properties (area, moment of inertia, torsional constant, shear areas); the deck slab as shell or through cross-beams; composite action by rigid links or by the transformed section with the right effective width.
- Bearings: springs with vertical and horizontal stiffness (elastomeric) or with fixed/free/guided conditions; rigid links for the eccentricity between the girder axis and the bearing.
- Substructure: piers and pier caps as beam or shell elements, with the true section and the height; abutments with spring supports for backfill.
- Foundations and soil: fixed base for preliminary design; springs (equivalent stiffness) or full piles with soil springs for the final design and seismic analysis.
- Loads: self-weight, superimposed dead load, live-load lanes and moving-load trains (Class A, 70R, etc.), braking, wind, temperature gradient, prestress, seismic.
- Stages: construction sequence for staged bridges.
Prestress in the model
Tendons can be modelled as equivalent loads (a load-balancing approach) or as tendon elements with the profile, friction, slip and time-dependent losses. The second approach follows the actual stressing and captures the secondary moments.
Moving loads
Vehicles are defined as a series of axle loads with lane widths, then the software moves them along the lanes and finds the maximum and minimum of each force at each section (envelopes) using influence lines or surfaces. Lane positions, the number of lanes loaded and the reduction factors must follow the code.
Modelling a building or station
- Frames of beams and columns; slabs as shell/membrane or as a rigid diaphragm; shear walls as shell elements or wide-column frames.
- Basement and foundation: raft as a shell on springs; piles as beams with soil springs; earth pressure on retaining walls.
- Load cases: dead, live, wind, seismic (equivalent static and response spectrum), temperature, construction loads.
- Load combinations from the code (for instance the IS 456 / IS 800 / IS 1893 combinations for a building; IRC:6 for a bridge).
Interpreting results
Software gives numbers — the engineer must give meaning to them.
- Displacements and deflected shape — check against expectations and against the code limits; the shape should be plausible.
- Member forces — envelopes, and the critical combination for each check; note the sign conventions of the software.
- Reactions — bearing loads, uplift, horizontal forces to piers.
- Stresses — smoothed (nodal-averaged) versus element stresses; peaks at a point load or a re-entrant corner are numerical singularities and should not be used directly.
- Dynamic results — periods, mode shapes, mass participation, base shear, drifts.