3. (a) In terms of a rectangular section, e.g. masonry or brickwork, what is meant by the middle third rule? [2 marks] (b) Figure Q4b shows the plan view of a steel column that carries three axial loads. Given that, A (cross sectional area) = 90.4cm, lyy = 7620cm4 and Izz = 2540cm, calculate the stress in the corners marked A and B. [7 marks] 200 90 kN y 200 KN A 50 50 kN 200 Z Z B Fig.4b 125 125 (c) A concrete wall in FigQ4c, is a rectangular in section and has a length of 2 m, a thickness of 1 m and a height of 3 m. It is subjected to a horizontal wind pressure of 0.75 kN/m which can be considered to act as a uniform distributed load acting over the full height of the wall. If the unit weight of concrete is 24kN/m, determine the maximum and minimum base stresses and sketch the stress distribution diagram. [14 marks] 3.0 m 1.0 m 0.75kN/m Fig.4c