Automotive Technology - 4.1 Simple Structural Surfaces


1. Introduction
Compared to finite element analysis (FEA) the simple structural surfaces (SSS) method of analysis is comparatively easy to understand. The technique may be used as a preliminary step prior to carrying out FEA. It is particularly useful in assessing possible load paths.
The methodology is based on the idea that the vehicle can be represented (approximately) by sub-assemblies or components that can in turn be represented by plane surfaces. These plane surfaces are held in equilibrium by forces which arise from vehicle loads and which are transmitted to and from adjacent SSS and eventually through to the ground. Adjacent SSS have equal and opposite forces acting on them.

2. Basic Theory
Looking at the diagram below, the thin, flat sheet has a and b much greater than t.
The resistance to bending of the surface is largely determined by the values of Ixx, Iyy and Izz. These are:

Ixx = a t3 /12
Iyy = t b3 /12 and
Izz = b t3 /12
Iyy will be very much greater than Ixx and Izz.
Forces in the plane of the SSS, Fxx and Fzz can be resisted, providing buckling does not occur, and bending about the Y axis can be resisted. The SSS can provide negligible resistance to transverse forces, Fy and bending about the X or Z axes.

3. Modifications Made in Practice
As a thin flat panel is very prone to buckling, it is common practice to reinforce panels by swaging grooves in them or by adding some type of stiffeners at the edges. These may be additional piece of metal attached by spot welds or they may be integral with the panel, formed by folding. The stiffeners should help to distribute the edge shear loads into the panel. The diagram below shows idealised axial force distributions in the reinforcing elements and the resulting shear force and bending moment distributions along the panel.

These assume that the panel itself only carries shear forces and stresses. In practice this total separation in function will not occur and there will be some variation in shear stress (q1 and q2) along the edges of the panel.

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David Grieve, 12th December 2001.