Roof challenge

Aim: To design a Sheet Metal Roof and to design reinforcements for the same at appropriate positions.

Objective: To design a Roof and Reinforcements in CATIA and then evaluate/analyze the roof with basic prediction formulas.

The tool used: CATIA Generative shape design workbench.

background: An automobile roof or car top is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements. Because the earliest automobiles were designed in an era of horse-drawn carriages early automobile roofs used similar materials and designs.

this same structure acts as a protective element for the passengers as well as the driver and serves as the crumple zone in the case of accidents absorbing energy.

The parts of the roof are

• Outer cover.

• inner flanges

• Front and rear roof rails

• reinforcements

Evaluation of roof:

The evaluation of the roof is done using 2 methods

1. Heat distortion Criteria
2. Snow Load condition

Heat Distortion condition:

The Heat Distortion formula is used to calculate the viability of the roof to withstand the deformation due to changes in temperatures which may vary from very high temperatures to extremely low temperatures. Under high heat, the metal tends to distort and this calculation can check if the placement of the reinforcements is correct and if it meets the criteria then the reinforcements help in withstanding the heat distortion of the roof panel.

`W = [1.73xx 10^(-3) xx L] + [1.85 xx 10^(-8) xx (R^2)/t] + [ 1.10xx10^(-3) xx l] - 2.68`

Where,

Judgement Condition: OK< 2.7> 3.1

L = Roof Length in X-Direction[mm](Roof dimension in 0-Y)

R = Roof curvature

R = 2(Rx*Ry)/(Rx+Ry)

Rx = X curvature

Ry = Y curvature

t = Roof plate thickness [mm]

l = Bow Roof Span [mm]

Calculations for the Heat Distortion Criteria

Section	Rx	Ry	R	l	W	Result
1	538277.034	2558.821	5093.43	480.754	2.0029	Pass
2	460930.022	2561.415	5094.5173	796.827	2.35	Pass
3	380568.26	53332.772	9354.59	1111.744	4.785	Fail
4	369525.9	11463.904	22837.34	1555.261	15.409	Fail

Conclusion: for the regions which are not passing the results, we can reduce the spans or increase the number of reinforcements so that they can hold up.

Snow Load Condition:

The snow load prediction is done especially in countries where there is a lot of snowfall. It is a precautionary calculation done to see if the roof of the vehicle can effectively take the load of the snow that accumulates on top of the roof without causing the roof to deform.

`Qr = [Iy xx t2] / [αs[(Rx + Ry)/2]^2 xx 10^-8]`

Where,

α = My x Lx2 x 10^-12 , My = Y(Ly-Y)

Judgement condition = Qr ≥ 3.1

250 ≤ s ≤ 380

t = Roof plate thickness [mm]

Ly = Distance between the front and rear roof Rails on the Vehicle along with 0Y[mm]

Length of Roof panel with the centre point between Roof rail Front /Rear as the front and rear reference point.

Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm]

The width of the roof panel is exposed on the surface.

Y = Distance front Roof Rail to Roof BOW[mm]

s = Distance for which Roof BOW bears divided load [mm]

s = L1/2 + L2/2

Iy = Geometrical moment of inertia of Roof BOW (Y cross-section) [mm4]

Rx = Lateral direction curvature radius of roof panel Y cross-section on Roof BOW [mm]

Roof panel curvature Radius of the length Lx in Front view

Ry = Longitudinal Direction curvature radius of the Roof panel X cross-section on Roof BOW [mm]

Roof panel X curvature radius of length s in Side view

Iy	t	α	s	Rx	Ry	Rx+Ry	Qr	Judgement
3418	0.75	0.911	484.865	538277.009	15055.972	553333.071	3.66	pass
3955	0.75	1.428	395.2215	524387.54	46724.996	571112.537	4.834	pass
3428	0.75	1.146	480.1405	442979.921	58787.498	501767.419	5.56	pass

MOI:

1.

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3.

Draft Analysis:

1.

The modelling of The roof is done and the design of the roof is validated with Heat distortion criteria and snow loading conditions. if the model doesn't pass the test, we can suggest the model be modified for the requirement.