Challenge

Roof Design

AAnjaneyulu Derangula17 Dec 2022

ROOF DESIGN CHALLENGE

Objective :

The objective of this project is to design car roof with essential flanges and reinforcements like ditch area , front roof rail , bow roofs, central roof rail and rear roof rail for the the given styling surface
Perform a curvature study on the roof and perform the calculations for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct positions.
Finding moment of inertia and section modulus for Bow roofs , central roof rail . front roof rail and rear roof rail.
Perform the draft analysis for each reinforcements.

Introduction about Car Roof :

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. Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure and to with stand the over head forces like snow.

Use of Reinforcements :

If the roof is flat it is very weak and cannot able to withstand the forces like snow , so in order to increase the strength of the roof these reinforcements like front roof rail, central roof rail, rear roof rail and the bow roofs are introduced and joined to the flat surface of the roof with the different joining process.

Different types of Reinforcements of the car roof has :

Front roof rail
Bow roofs
Central roof rail and
Rear roof rail

Front Roof rail :

Front roof rail is the one which joins the wind shield glass , body side outer and the inner panel also. Here the front roof rail is designed based up on the master section given.

Rear Roof rail :

Rear roof rail is the one which joins the back door and the body side outer.

Central Roof rail :

central roof rail helps in providing effectively support to the flat area of the roof as it is more susceptible to failure under the action of load . Generally central roof rail is placed at the center of the roof which is connected to the B-pillar support structure. This central roof rail helps in adding the strength to the roof during the rool- over test. The thickness of the central roof rail is 0.75 mm.

Bow Roofs :

The bow roofs are give to improve the torsional stiffness and load bearing capacity of the roof structure . The number of bow roof present is depends on the overall size of the roof. Presently in this project two bow roofs are added.

Joining process of roof with Reinforcements :

Mastics are introduced between the roof outer panel and the reinforcements for joining . where ever the mastic are introduced there is remarkable improve of the strength.

Spot welding is also introduced to join the reinforcements and the outer panel.

Design considerations of A car Roof :

Visibility criteria
Head clearance
Curvature study
Heat distortion and snow load criteria
Draft analysis

Roof Car crush test :

Roof crush test is the design given by the IIHS ( insurance institute of highway safety ) . After analyzing the many accidents IIHS has then fixed some regulations , that how to perform the crush test .

IIHS gives the rating based up on this test . For this they consider two points while giving the rating

How fast the car can reforms to its original shape after the roll over, and
How deep is the protrusion after the roll over.

Goal of Roof crush test :

To minimize the protrusion into the occupant compartment , so that the seat belts and air bags can do there job in protecting the occupant

Different conditions follow for roof crush test :

There are two conditions that should be followed for the roof crush test , they are

1. Initial condition :

Close and lock all the doors
Close all the windows

2. Second condition :

Fix the body sill or chassis frame on a hard lever surface horizontally

Method of applying force :

The load is applied in such a way that 5⁰is tilted from the front end to the rear end and the load will be in 1800 mm in length and 750 mm in width . The load is applied at the position of 25⁰from the flatness of the roof . The load is applied on the roof for almost 120 sec and the crush speed at which the load applied is 13 mm/sec or less than that

Results :

If the displacements goes above the requirements level then we have to increase the bow roof thickness or add more mastics points to increase the strength . the stronger the roof the better it can protect the occupants in the roll over crashes.

Heat distortion on the study of the roof :

The heat distortion study plays a major role in sheet metal usage . heat distortion temperature is a temperature limit above which the material cannot be used for the structural applications . This study is used to predict the heat distortion temperature at where the material starts to soften when exposed to a fixed load at elevated temperature . In order to avoid bending or damages on the roof, based on the heat distortion temperature , this study will predict the bow roof position which helps to strengthen the roof .

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

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]

Judgement Condition : OK< 2.7> 3.1

Apply values in the formula :

In order to find the heat distortion apply all the necessary values from the roof to the give formula

Calculations :

Roof Rails	Rx (mm)	Ry (mm)	*R=[2(Rx Ry)/(Rx +Ry)] (mm)**	L (mm)	t (mm)	I (mm)	W	Judgement Result
Front Roof-Bow 1	5741.04	2458.81	3443	1985.44	0.75	372.86	1.4564	Ok
Bow 1-Center Roof	5006.4	6059.81	5482.96	1985.44	0.75	414.85	1.9518	Ok
Center Roof-Bow 2	4027.18	10329.41	5795.02	1985.44	0.75	413.72	1.5823	Ok
Bow 2- Rear Roof	3290.69	14518.96	5365.33	1985.44	0.75	405	1.9095	Ok

From the above table it can be concluded that all values of W<2.7 and also not W>3.1 so thus infers that current positioning of bow row are good in state as per design and found ok.

Snow load criteria condition :

This test is done to know how is the roof behaving when there is a snow over . normally due to the snow weight the bent will happen. But the roof should be designed in such a way that when the snow is removal the roof should go it its original position . This is the basic requirements for snow load criteria .

Qr = [Iy x t2] / [α x s x [(Rx + Ry)/2]2 x 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 center point between Roof rail Front /Rear as the reference point of the front and the rear.

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

Width of the roof panel 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]

Apply the values in the formula :

In order to find the snow load criteria apply all the necessary values from the roof to the give formula

Calculations :

Roof Rails	Iy (mm)	Lx (mm)	Ly (mm)	L1 (mm)	L2 (mm)	Rx (mm)	Ry (mm)	t (mm)	Y (mm)	S	My	α	[(Rx + Ry)/2)^2]	Qr	Result
Front Roof - BOW 1	355.966	1121.91	1983.99	467	495.35	4617.46	2480.61	0.75	467	481.175	708434.33	0.8916	12595649.43	3.705	Pass
Bow 1 - Center Roof	549.9	1077.87	1983.99	495.35	495.03	3760.31	5995.66	0.75	958.59	495.19	982918.05	1.142	23794737.66	2.298	Fail
Center Roof - Bow 2	364.052	1060.288	1983.99	495.03	547.12	4740.98	10651.58	0.75	1450	521	774624.28	0.87	55447085.84	0.814	Fail

It is observd from the values does not satisfy, when snow load condtions obtained values of QR > 3.1

Section analysis:

Section moment of inertia is used to predict the amount of deformation and deflection of a part. A higher moment of inertia is preferable as it reduces the amount of deflection for a given force. The thickness of the part and addition of flanges and embosses increase the section inertia of the part. Here the master sections of the roof and roof rails and their section analysis are given.

Bow Roof 1 :

Create an intersection curve with the help of the plane and select the bow roof 1