Section Modulus calculation and optimization

Section Modulus Calculation

Objective : To calculate the section modulus of a hood along the longitudinal axis and optimize it in order to increase the load bearing capacity of the hood and resistance to bending.

Theory : Section modulus is a geometric property for a given cross-section.It is used to calculate the load bearing capacity and bending resistance of a structural member. It is the ratio of moment of inertia of area of a cross section to the distance between the neutral axis and the extreme fibre. The following formula applies for the calculation of section modulus :

Z=I/y

Where,

Z = Section Modulus.

I = Moment of inertia of cross sectional area.

y = Distance of extreme fibre from the neutral axis.

From the formula it is clear that with the increase in moment of inertia of cross sectional area, section modulus will increase as well which increases the load bearing capacity and bending resistance. In order to increase the moment of inertia, the area of cross section needs to be increased.

Procedure : The calculation of section modulus of the hood along its longitudinal axis is performed in NX CAD. The features used are Intersection Curve, Projection Curve and Section Inertia Analysis.The intersection curve is used to cut the hood along the longitudinal axis to get the section curve, then project curve is used to project the curve onto a plane for editing the section curve and then section inertia analysis is used to calculate the maximum and minimum moment of inertia of the section curve.

Calculation :

Z= I/y.

Moment of inertia for the first analysis = 1.47x10^4mm4.

Distance of fibre from neutral axis = 340 mm.

Section Modulus Z1= (1.47x10^4)mm4/340 mm = 43.24 mm3.

Moment of inertia for second analysis =1.57x10^4mm4.

Distance of fibre from neutral axis = 340 mm.

Section Modulus Z2 = (1.57x10^4)/340=46.18 mm3.

In order to get a greater section modulus the distance between the outer and inner panel of the hood is increased.

Result :

	Section Modulus(Z) in mm3	Moment of inertia (I) in mm4	Distance of extreme fibre(y) in mm
Original section	43.24	1.47x10^4	340
Improved section	46.18	1.57x10^4	340

Conclusion :

• The section modulus is directly dependent on the moment of inertia cross section area.
• Moment of inertia can be increased by increasing the distance between the outer and inner panel of the hood.
• Increased cross sectional area gives a better bending resistance