Why Does My Simply Supported Bridge Have Horizontal Reactions?

Team MIDAS — Tue, 12 May 2026 01:44:56 GMT

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.

MIDAS Blog

Why Does My Simply Supported Bridge Have Horizontal Reactions?

Introduction