• Question: What makes a particular shape structurally sound?

    Asked by Anon on 31 Dec 2014.
    • Photo: Steve Cox

      Steve Cox answered on 31 Dec 2014:

      Whether a shape is structurally sound depends on how the loads are going to be applied to it. A classic example is an eggshell, where it is amazingly strong if you apply load from end to end (i.e.if you stand the egg up vertically and apply a load to the tip). In this load direction the shape of the shell is very, very good at resisting the compressive stresses that are being applied to it.

      If you apply the same load around the middle you end up with a gooey mess because the shape is very poor at dealing with the compressive stress.

      The summary is to think about the loads being applied to the object first, and then you’ll find the shape with the correct structural performance

    • Photo: Fiona Dickinson

      Fiona Dickinson answered on 5 Jan 2015:

      I’ve been thinking long and hard about this one because I’m not a mechanical or structural engineer. However I will share my thoughts and process.

      How well a particular shape can withstand loads depends on many things, principally how the load is distributed (how well it can spread the force) and other things such as the type of material.

      If we think about a bridge with an arch, because of the way the arch is formed it has no structural strength until the keystone (centre block) is added to the arch, any force is now spread between the two supports at either side, if we instead imagine a sheet spread between the two struts this flexes as a load is applied.

      Now there are basically two classes of materials when thinking about loads, those that are good under compression (such as concrete – cement with little stones in), and those that are good under tension (such as steel), we take the two and make re-enforced concrete which is the fundamental building block (sorry about the pun) of our modern skyscraper lives. The properties of re-enforced concrete are much better than either the concrete or steel alone.

      Concrete works so well because it cracks and has faults, but the stones mixed in stop the cracks spreading the whole way through the material.

      Other things that work well because of a mix are bones, this is a mixture of an inorganic material (basically a rock) and organic material (biological bits) which together make remarkably strong materials for the use, the ratio between the two depends upon the age of the being and the type of the being, so bone is not as simple as bone.

      So some shapes are inherently strong because they can spread a load evenly and well, but the material matters at least as much as the shape use the wrong material and there is no strength there.

    • Photo: Benjamin Males

      Benjamin Males answered on 6 Jan 2015:

      In mechanical engineering a structure created from long elements such as tubes or beams is considered sound if there is no movement.

      A great experiment you can do to find stable shapes is to try and build a tower from drinking straws and pins. By connecting different lengths of straw with a pin at the ends of the straws, different shapes can be created. You soon find out that if there is too much free movement then the shape is not structurally sound.

      Triangles are very stable as each pinned joint is unable to move relative to the others in the triangle shape. In fact in many jointed structures such as buildings and furniture, triangles shapes are commonly used.