Reddit reviews Mechanics of Materials

The air pressure provided is just a force over an area. For now, forget how the air pressure acts on the steel tube since the forces are almost negligible. The air pressure acts on the rectangular sign, which has a large surface area, (lets assume) uniformly. With a pressure and an area we can say a single force is applied at the center of the sign. That force is acting on the sign's center of mass, as it's positioned in the picture, which is (lets assume) rigidly attached to the the steel tube. The center of mass of the sign is 1.25m away from the center of the steel tube. This system will apply what is called a moment on the steel tube...also known as a torque. That 1.25m distance would be known as the moment arm. Closed cross sections, specifically circular cross sections, are good at resisting moments applied in this direction. One thing to also note is the force at the center of the sign also applies a moment, with a moment arm from the center of the sign to the ground which is 4.5m. This is a called a bending moment when a force is applied in this direction. When designing such a structure, as an engineer, you will need to be conservative and assume both moments will occur at the same time and the same location. So, apply both moments at the base of the steel tube, where it meets the ground. This will be the most critically loaded location on the structure. Read about Statics and Mohr's Circle which you will learn about in mechanics of materials. Pick-up a used copy of 'Mechanics of Materials'. There are a bunch of cheap listings here on Amazon. You should be able to understand how to solve that problem fairly quickly skimming that book.