What is the primary operating principle of an inflatable structure?

An inflatable structure primarily operates on the principle of being held in place by air pressure that is greater than the outside atmospheric pressure. This means that the defining characteristic of such structures is the internal air pressure that provides support and stability. As air is pumped into the structure, it creates a difference between the internal pressure and the external pressure, allowing the structure to maintain its shape and form.

The effectiveness of an inflatable building relies on this pressure differential to keep the materials taut and secure. The surfaces of the inflatable are designed to be flexible, enabling them to expand and contract in response to changes in internal pressure and external conditions.

This is why the choice regarding the principle of operation related to air pressure being greater than outside pressure accurately describes how inflatable structures function. The other choices reference concepts that do not apply to the nature of inflatable architecture: external beams for support are typical in traditional construction methods, constant air pressure less than outside pressure would undermine the integrity of the structure, and the notion that they are made only of rigid materials contradicts the very definition of an inflatable structure, which relies on flexibility and air for support.