Introduction
Section offsets are commonly used to define the representative axis of an element relative to its neutral axis in 3D grillage modelling of bridges. Although vertical section offsets are assigned to superstructure elements based on actual physical geometry in most cases, the choice of offset location can significantly influence structural behavior, load path, support reactions, and force distribution in the analysis model.
Engineers typically encounter three types of vertical offsets while defining beam sections: centre-centre, centre-top, and centre-bottom offsets. Each option changes how loads are transferred between the deck, girders, bearings, and substructure.
For example, a centre-top offset for a supported girder resting atop bearings behaves like a portal frame, subjected to horizontal reactions under gravity loading.
This article explains how section offsets work internally in grillage models and demonstrates their influence using a PSC composite deck example. Through simplified frame analogies, we will understand when each of these offsets is appropriate for different scenarios.
Section Definition along the Neutral Axis
Consider a 2D grillage of a PSC composite superstructure with guided POT-PTFE bearings, as shown in Figure 1.
Figure 1: 2D Grillage Layout
The above bearing articulation is adopted in the reference tutorial model 9 PSC Single Span.mcb using point supports.
Figure 2: Rendered Isometric View of a 2D grillage model.
When you switch on the rendered view, as shown in Figure 2, we can see that the transverse deck elements are not aligned with the top of the girder; instead, the representative axis of the girder is along the neutral axis of the composite section, due to the “centre-centre” offset assigned to the PSC composite sections, as shown in Figure 3.
Figure 3: Centre-Centre Offset assigned to PSC Composite Girders
In this 2D grillage model, simply supported behaviour is observed as the support nodes are along the neutral axis. Since the bearings on the right end are free to translate along the longitudinal direction, no horizontal reactions develop in the grillage. Thus, centre-centre offset works well for 2D grillage models.
From the Project > Structure Type tab, we can change the display mode to align the top of the beam with the XY plane without affecting the structural response, as shown in Figure 4.
Figure 4: Align Top of Beam Section with X-Y Plane for Display
However, is the centre-centre offset applicable if we model the entire bridge? If we adopt the centre-centre offset, will the longitudinal effects in the substructure due to braking forces be calculated accurately? Why do we sometimes get horizontal reactions under gravity loads for simply supported PSC composite girders resting on elastomeric bearings?
These are some of the questions we will address in this article using the tutorial model.
Fundamentals of Frame Action
Before discussing how vertical offsets work, let us review the behaviour of a 2D portal frame.
Consider a pin-supported portal frame of length, l and the height, h, assuming both the beam and the columns have identical cross sections, i.e., I_b = I_c.
When subjected to a uniformly distributed load w along the vertical direction, the hog end moment, M develops as shown in Figure 5.
For the classical formula, refer to Three Member Frame (Pin / Pin) - Top UDL
Anju B Sunil
