I was trying to solve a question where they told me to find the buckling load of a wing strut in a plane that experiences a compressive load along its axis. The strut can be considered to be a pin-jointed at both ends. Dimensions of the strut is as follows: outside diameter is 150 mm and bore is 100 mm and length of 5 m. For finding the buckling load i used Euler's equation for struts P = (Pi^2*EI)/L^2 and I got the correct answer but i dont understand what exactly I have to do in the second part of the question. Here it is: (b) Find the buckling load if the wing is re-designed so that the strut is prevented from moving laterally in all planes at its centre. For this my lecturer had used this equation P = (4*Pi^2*EI)/L^2 So basically 4 times the previous equation. I want to know how he got that equation and when should I use it. Thanks!