On Frocks and Bias

Or why fashionable dresses cling

I like watching movies, but I don’t much care for the process of promoting them or the somewhat cringeworthy awards season that culminates with the Oscars. Writing this in that season, albeit you will be reading this later in the year, I set myself the challenge of finding something Oscars related that involves structural engineering. I assumed that it would likely involve something to do with the staging or set, but eventually I decided to go in a different direction.

Just how do they make those frocks seen on the red carpet hug the body so tightly?

It may not be immediately obvious what figure hugging dresses, of the type worn to award ceremonies, have to do with structures, so your going to need to bear with me. I hope I am not biting off more than I can chew, because I am no fashionista and I don’t know anything about clothes. As always I shall be relying on engineering principles.....

It seems to me that there has never been a time when the human body has not been viewed as art and its form put on display. In this sense not much has changed between the marble sculptures of antiquity to modern day frocks.

That said, one thing that does seem to have changed is that extensive rigging no longer seems necessary to create a tight fit. I suspect that corsets are not terribly comfortable and therefore I expect most ladies welcome their demise.

To understand why we need to understand cloth and how it behaves. We also need to revisit phenomena we have met in a prior post; namely Young’s modulus and Poisson’s ratio. When a material stretches it narrows and when it is squashed it bulges. The amount of narrowing and bulging is directly related to the stiffness of the material; the stiffer the material the less there is. This is why we rarely notice the effect in steel or concrete. Stiffness is the ratio of stress to strain and is known as Young’s modulus, while the ratio of axial stretching or squashing to lateral narrowing or bulging is known as Poisson’s ratio.

Cloth is unlike steel, because its stiffness varies in different orientations. This is due to the way in which it is made. There are two sets of fibres, which are arranged in perpendicular directions. The warp threads are aligned vertically and the weft are weaved above and below. If the cloth is pulled in the direction of the warp there is little stretch in the direction of the tensile force. Similarly the weft threads prevent narrowing in the perpendicular direction. Alternatively, pull in the direction of the weft and again there is little movement in either direction. For this reason cloth is quite stiff in tension, except that is, if it is stretched at 45 degrees to the warp and weft. In this orientation there is considerable stretching and narrowing. Thus, at 45 degrees cloth has a low Young’s Modulus and a high Poisson’s ratio.

A useful analogy would be to compare the stiffness of a structural grid formed of perpendicular members with a lattice formed of diagonal members. Due to rotation at the joints the lattice is poor in tension, but good in shear, while the perpendicular grid is good in tension, but poor in shear.

Now supposing a dressmaker were to arrange and cut the cloth for a dress such that the warp and weft were aligned at 45 degrees to the vertical. I gather this is known as a bias cut. In such circumstances the self weight of the dress would place the cloth in tension, which would cause stretching in the vertical axis and a corresponding contraction in the perpendicular direction. 

Self-evidently the noted contraction would cause the dress to hug more tightly around its wearer. 

I am quite sure that dress makers must have lots of crafty tricks to enhance this effect. Perhaps they use thicker material, folds or stitching to increase the weight of the dress in certain locations or perhaps they vary the composition of the cloth by using threads of different stiffness or looser weave.

I have no idea if these are actual things or not, but they do seem to be reasonable suppositions based on the need to minimise Young’s modulus and maximise Poisson’s ratio. 

I don’t suppose dressmakers communicate in terms of Young’s modulus & Poisson’s ratio, but to make their dresses work as they do I suspect they do have a rather good empirical understanding of cloth. 

All this means that while I have little time for red carpets and awards ceremonies I can at least admire the skill of the dressmaker and their intimate knowledge of fabric. Whether they know it or not the figure hugging designs that have replaced laces and corsets rely on sound engineering principals.