In many cases, the effect of temperature on 3D structures can yield surprisingly different results than those obtained from a simplified 2D model. Having the ability to model a structure with 3D shell finite elements as well as simple 2D frames, as is the case in SAM-LEAP5, can prove extremely beneficial.

Consider the case of a simple, orthogonal, two span portal slab and wall structure. It is required to obtain the in-plane slab forces, in the direction of the span, due to a temperature rise in the slab. This will intuitively result in longitudinal slab compression due to the vertical bending resistance of the walls. This can be verified using a simple plane frame analysis.

If we now consider the same structure modelled with 3D shell finite elements we will find that the compressive force, as derived above, will be severely underestimated along the centre line of the structure and will in fact become significantly tensile at the edges.

This may seem surprising but can quite readily be explained by considering both longitudinal and transverse expansion of the slab acting together. The transverse expansion is unrestrained, except along the lines of the walls. This results in lateral bowing at the edges of the slab which induces lateral bending along the lines of the walls. The resultant distribution of longitudinal axial force is shown in the following contour diagram.

By using the composite member facility in SAM-LEAP5 we can show the axial forces along a number of 1m wide strips, and the tensile forces at the edge of the slab can be clearly seen.

If you would like to know more about this and other modelling considerations you may wish to attend one of our Technical Seminars on "Exploring Bridge Analysis Techniques". Please contact us at the address below for details.