Challenge

Roof challenge

SSridhara A3 Jun 2022

ROOF DESIGN

An automobile roof or car top is the portion of an automobile that that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements.

The automobile roof consists of flowing parts:

The outer panel
Front roof rail
Centre roof rail
Rear roof rail

FRONT ROOF RAIL:

The front rail is created at the front side of the roof to provide strength to the roof. Headroom clearance and the driver visibility are the two important factors to be considered while designing the front roof rail is connected to the roof outer panel with the help of mastic seal.

BOW ROOF 1 :

It is used to give strength to area between the front roof and the centre roof rail. Also it is created in the area between the centre the centre roof rail and the rear roof rail to provide enough strength. It can crated in any numbers depending upon the roof length but the position of the bow roofs is to be justified by curvature study.

CENTER ROOF RAIL:

It is created at the centre of the roof. It is connected to the roof by mastic seal.

REAR ROOF RAIL:

It is created at the rear end of the roof. The fitment of the backdoor is to be considered while designing the rear roof rail.

ROOF IMAGES

HEAT DISTORTION CRITIRIA:

Heat distortion temperature is the temperature is the temperature at the metal softens and deforms under a specified load. While designing roof, this criterion is taken into consideration. We created bow roofs when the roof is long and flat to increase its strength. The position of the roofs its to be justified and this justification is done by curvature study.

Bow- roof prediction formula Bow–roof prediction Formula

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68

Judgment Condition

OK< 2.7

Where

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]

TO MEASURE L:

TO MEASURE Rx:

TO MEASURE Ry:

TO MEASURE I (BOW ROOF SPAIN):

HEAT DISTORTION CRITIRIA
SL no	Bows	Rx(mm)	Ry(mm)	R(mm)	l(mm)	L(mm)	t(mm)	W	Ok/ng
1	FR-BR-1	2355.77	5739.40	3340.43	337.67	2081.37	0.75	0.45	OK
2	BR-1-CR	6020.57	5015.06	5472.0	462.73	2081.37	0.75	1.73	OK
3	CR-BR-2	10863.32	4715.99	6576.83	519.97	2081.37	0.75	1.5	OK
4	BR-2-RR	15037.0	5273.54	10358.6	301.47	2081.37	0.75	2.5	OK

Here we can see that the roof is completely satisfying the conditions of heat distortion.

SNOW LOAD PREDICATION FORMULA:

Snow load Prediction formula

Qr = [Iy x t²] / [α x s x [(Rx + Ry)/2]²x 10^-8]

Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.

Were

α = My x Lx² x 10^-12, My = Y(Ly-Y)

Judgment 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 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

Snow load is the downward force on the cars roof by the weight of accumulated snow and ice. The roof can fail if the snow load exceeds the weight the roof was designed to shoulder. Or if the roof was designed or constructed.

So there is formula to predict the strength of the roof.

TO MEASURE Lx:

TO MEASURE Ly:

TO MEASURE Ry:

TO MEASURE Rx:

After calculating all those values, the Qr is found out.

SNOW LOAD PREDICATION
Sl no	Bows	Rx(mm)	Ry(mm)	Lx(mm)	Ly(mm)	Y(mm)	s(mm)	My	α	t(mm)	Qr	Ok/NG
1	FR-BR-1	2365.35	4653.16	1126.8	2081.3	337.67	337.6	588795.1	0.74	0.75	63.4	OK
2	BR-1-CR	5096.49	3757.47	1070.5	2081.3	462.6	462.7	748923.9	0.85	0.75	15.26	OK
3	CR-BR-2	4587.68	4298.35	1056.2	2081.3	519.9	519.6	811771.8	0.90	0.75	12.68	OK
4	BR-2-RR	4822.63	3888.8	1045.6	2081.3	301.46		536550.5	0.58	0.75

It is observed that the roof is satisfying the condition. So, in order to strengthen the roof at these areas embosses are created on the outer roof.

SECTION MODULUS STUDY OF THE SECTION OF THE ROOF RAILS:

The section modulus of the cross-sectional shape is of significant importance in designing of structures. It is a direct measure of the strength of the structure A structure that has a larger section modulus than another will be stronger and capable of supporting greater loads.Section modulus (S) = 1/y

Where,

I = Moment of inertia in (mm4)

y = Distance between the neutral axis and the extreme end of the object (mm)

FRONT ROOF RAIL:

SECTION INERTIA ANALYSIS

I=8040.9 mm^4

Y=108.28/2=54.14

S=L/y=148.52mm^3

REAR ROOF RAIL:

SECTION INERTIA ANALYSIS

I=8.144^3 mm^4

Y=106.62mm/2=53.31mm

S=I/y = 10.1mm^3

CENTER ROOF RAIL:

SECTION INERTIA ANALYSIS

I=192.25mm^4

y=86.2/2=43.05mm

S=4.46mm^3

BOW ROOF 1

SECTION INERTIA ANALYSIS

I=125.25mm^4

y=69.97/2=34.95mm

S=I/y=3.58mm^3

BOW ROOF 2

SECTION INERTIA ANALYSIS

I=165.47mm^4

Y=69.99mm/2=34.99mm

S=I/y=4.72mm^3

DRAFT ANALYSIS:

In NX to check whether the sheet metal part we have designed can be deep drawn, there is an option called draft analysis. This type of analysis is performed based on colour ranges identifying zones on the analysed element where the deviation from the draft direction at any point, corresponds to specified values.

FRONT ROOF RAIL DRAFT ANALYSIS