Keywords: influence lines, Müller-Breslau principle, consistent nodal forces..

The use of influence lines is a recognized method for determining the critical design load conditions and this paper shows a direct method for applying influence lines in any structural finite element software. The main idea is to equate displacement or angular discontinuities with nodal forces, and subsequently obtain the influence function only applying a single load case without changing the geometry or boundary conditions of the model. The new approach for determining Influence lines is based on the Muller-Breslau principle, but the discontinuous displacement fields are in the new approach equated to consistent nodal forces, which makes it very suitable for implementation in finite element schemes and applicable for all element types, such as shell, plates, beams etc. This paper derives the consistent nodal forces for angular, lateral and axial displacement discontinuities for a Bernoulli-Euler beam element. To exemplify the new approach the response function of the pre-stressing cables for a cable-stay bridge subjected to a truck loading is studied.

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