The Engineering Behind that Favorite Cocktail Number

Charles Seim is one of the world’s most renowned bridge engineers. But before he undertook many of the projects that would bring him to the world’s attention, Chuck, as his friends call him, was busy considering a question not often pondered by science — the stress analysis of a strapless gown. In an essay published in 1956, Seim illustrated his thoughts in “A Stress Analysis of a Strapless Evening Gown,” which he published in a book of essays by the same name. Two weeks ago, Neatorama ran this essay, an exquisite application of engineering to a cocktail party favorite:

If a small elemental strip of cloth from a strapless evening gown is isolated as a free body in the area of plane A in Figure 1, it can be seen that the tangential force F1 is balanced by the equal and opposite tangential force F2. The downward vertical force W(weight of the dress) is balanced by the force V acting vertically upward due to the stress in the cloth above plane A. Since the algebraic summation of vertical and horizontal forces is zero and no moments are acting, the elemental strip is at equilibrium.

Consider now an elemental strip of cloth isolated as a free body in the area of plane B of figure 1. The two tangible forces F1 and F2 are equal and opposite as before, but the force W(weight of dress) is not balanced by an upward force V because there is no cloth above plane B to supply this force. Thus, the algebraic summation of horizontal forces is zero, but the sum of the vertical forces is not zero. Therefore, this elemental strip is not in equilibrium; but it is imperative, for social reason, that this elemental strip be in equilibrium. If the female is naturally blessed with sufficient pectoral development, she can supply this very vital force and maintain the elemental strip at equilibrium. If she is not, the engineer has to supply this force by artificial methods.

You can read the whole essay at Neatorama.

Image by JMR_Photography.