You have been asked to design a strut Figure for a new vers

You have been asked to design a strut (Figure ) for a new version of the machine. You are only required to consider the compression capacity of the strut. A detailed load analysis has determined an applied axial load of 1000KN with a load factor = 2.0, to account for the expected fatigue and impact loading, giving a final design load N*=2000KN.
The strut is a 2000mm long hot rolled cruciform profile using a high alloy aluminium. A cruciform profile is required for logistical reasons and is to have all legs the same length. The strut is pined supported each end so that there is no eccentric loading.
The material properties are:
Fy = 655MPa, Fu=683MPa, Density = 2830kg/m3

Q. Explain why the minimum second moment of areas axis are parallel to each leg and not rotated at an angle around the centroid (descriptive and a diagram do use not maths).

Solution

The load is not eccentric and passes through the centroid. The bending will be about the axes through the centres and parallel to the legs.The moment of inertia is hence computed about the axis parallel to each leg. Bending will not take place about any other axis.