Roof Design

ROOF DESIGN

- MAYANK TIWARI

AIM - FOR THE GIVEN ROOF STYLING :

1. CREATE THE ESSENTIAL FLANGES & REINFORCEMENTS WHILE ANALYZING THE PREPARED MODEL TO CHECK THE MANUFACTURABILITY & FEASIBILITY OF THE DESIGN.

2. DO A CURVATURE STUDY ON NTHE ROOF & PERFORM CALCULATIONS FOR HEAT DISTORTION & SNOW LOAD CRITERIA TO DETERMINE THE POSITIONS OF BOW ROOFS.

3. FINDING MOI & SECTION MODULUS FOR BOW ROOFS & CENTER ROOF RAIL.

4. CREATION OF DITCH AREA & CHECK FOR DRAFT ANALYSIS.

SOFTWARE USED - SEIMENS NX

ROOF INTRODUCTION -

ROOF CRUSH TEST:

OVERVIEW OF THE TEST:

• Specifies testing one side of a vehicle’s roof
• Does not include a headroom criterion
• Specifies testing to a given displacement instead of a given force level
• Specifies setting the vehicle’s pitch angle during testing based on the measured on-road pitch angle.

An overall rating is assigned based on the peak strength-to-weight ratio (SWR) measured within 127 mm of plate displacement.

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This test can be conducted on either side of the roof structure, the front left or the front right side but not on the both sides in one single test and still meet the requirement of the test. The rigid plate shall not move faster than 0.5 inches/second and the force applied on the plate shall not exceed the lesser of 1.5 times the unloaded vehicle weight (UVW) expressed in kilograms multiplied by 9.8 or 5000 lbs (22240N).

Also, the direction of the force shall be downward and perpendicular to the lower surface of the rigid plate and such that it moves in a straight line without rotation. The duration of the test shall not exceed 120 seconds. The longitudinal axis of the plate is pitched forward at a 5’ angle as viewed from the side of the passenger car as shown,

The lateral axis or the roll of the plate is such that it forms a 25’ outboard angle with a horizontal surface as viewed from the front of the passenger car.

REINFORCEMENTS & ITS TYPES-

If the roof is flat, it is weak and cannot able to with stand the external load like snow load, roll over and etc. so in order to increase the strength of the roof, reinforcement like front roof rail, central roof rail, rear roof rail and the bow roof are introduced and joined in the flat surface of the roof with different joining process. In general, there are four types of reinforcement are used in the hard roof top they are,

• Front roof rail
• Central Roof rail
• Rear roof rail
• Bow roof rail

Roof Outer Panel

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:

Just as the front rail roof, the rear rail roof provides reinforcement to the rear part of the car roof. Similar to the front rail roof, the rear roof rail is to be designed considering the rear visibility criteria and the rear passenger’s headroom clearance.

Central Reinf 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 the strength to the roof during the roll-over test.

Bow roof

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

Design Calculation of Roof-

To make sure the roof design passes the Analysis test some of the criteria to be considered before the design of bow rails under the roof. They are,

• Heated Distortion study on the roof
• Snow Load Condition

Heated Distortion study on the roof:

The heat distortion study plays a major role in sheet metal usage. Heat distortion temperature is the temperature limit above which the material cannot be used for the structural applications. This study is used to predict the heat distortions temperature at the material where it start 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 STUDY

W= [1.73x10 ^ (-3) x L] + [1.85 x 10^ (-8) x (R^2)/t] + [1.10 x 10^ (-3) x1] – 2.68

Where,

L = Roof strength 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]

I = Bow Roof span [mm]

Judgement Condition: OK < 2.7 > 3.1

Here we have to find out Rx and Ry values. To find out

• Create a sketch on Top roof and draw a horizontal line at a center.
• Then offset this line below 300mm and called it as _3y line.
• From the _3y line offset 100 mm above and below and convert all these line into reference line.
• Draw the vertical lines at the middle of each reinforcements and offset it at 100 mm both sides.
• Intersection point is created where the lines are met.
• Project there points the outer roof panel.

Over all Roof Length

Radius of Rx values for Heat Disortion Calculation

Radius of Ry values for Heat Disortion Calculation

Table of Heat Disortion Caluculation Result

SL.NO	BOW ROOF	Rx	Ry	R	t	l	L	R^2/t	W	RESULT
1	FR & 1st	5626.257	3407.256	4243.94	0.75	485.806	2030.35	24014702.3	1.81	OK
2	1st & Ctr	4855.642	6665.982	5617.80	0.75	325.727	2030.35	42079569.1	1.96	OK
3	Ctr & 2nd	4067.588	10127.427	5803.36	0.75	325.968	2030.35	44905316.4	2.02	OK
4	2nd & RR	3427.329	13644.400	5478.06	0.75	429.640	2030.35	40012188.1	2.04	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 Condition.

This test is done to know how the roof is behaving when there is a snow over. Normally due to the snow weight the dent 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 basic requirement for snow load criteria.

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

Where

a = My x L x ² 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 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 Roof Rail to Roof BOW [mm]

S = Distance for which roof Bow bears divided load [mm]

S = L ½ + 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]

Here we have to find out Rx and Ry values. To find that

• Create a sketch on Top roof and draw a horizontal line at a center.
• Offset this line to 100 mm on both sides. Convert all these line into reference line.
• Draw the vertical lines at the middle of each reinforcements and offset it at 100 mm both sides.
• Intersection point is created where the lines are met.
• Project there points the outer roof panel.

Rx values for Snow Load Calculation

Ry values for Snow Load Calculation

Lx1 value

Lx2 value

Lx3 value

Table of Snow Load Caluculation Result

S.NO	BOW ROOF	Rx	Ry	Lx	Ly	L1	L2	t	Y	S	My	a	Iy	Qr	RESULT
1.	FRNT & 1	4106.75	3384.45	1120.97	2030	569.31	406.06	0.75	569.31	487.68	831585.42	1.04	1104.4	8.69	OK
2.	1 & CTR	3793.37	6591.97	1083.02	2030	406.06	406.19	0.75	406.06	406.12	659417.07	0.77	1417.6	5.07	OK
3.	CTR & 2	4229.49	10523.36	1060.67	2030	406.19	-	0.75	406.19	203.09	659575.38	0.74	1130.9	4.69	OK

If roof structure pass to the snow load condition. They, must have Q_r values from above 3.1.

Q_r > 3.1

So, we done the curvature study on the roof. Heat distortion and snow load condition my roof is pass the two criteria, so position of the bow roofs and center reinforcement roofs placed at right area.

SECTION MODULUS:

The moment carrying capacity of an object is directly dependent on geometrical property (I) and material property (E) of an object, which is collectively termed as flexural rigidity (EI). Geometry of an object plays an important role in load bearing capacity of an object which is indicated by moment of inertia of a section.

Therefore, section modulus is the predominant factor which evidences the strength of an object and is defined as the ratio of the moment of inertia of the object about its centroidal axis to the distance of the extreme fibers of the object from the neutral axis.

There are two types of section modulus we have,

• Elastic section modulus
• Plastic section modulus

Elastic modulus is defined as,

S = I/Y

S – Section modulus,

I – Second moment of area

Y – distance from the neutral axis to outer most fiber of the object.

BOW ROOF FRONT:

Moment of inertia (Min), Imin = 1104.4 mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y =35 mm

For minimum MOI,

S = 1104.4/35

S_MIN = 31.55 mm³

BOW ROOF REAR:

Moment of inertia (Min), Imin = 1130.9 mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 45mm

For minimum MOI,

S = 1130.9/45

S_MIN = 25.13 mm³

RE - INFORCEMENT CENTER ROOF RAIL:

Moment of inertia (Min), Imin =1417.6 mm⁴

Section modulus = moment of inertia / distance between the neutral axis to extreme end of the object.

S = I/Y

Y = 35 mm

For minimum MOI,

S = 1417.6/35

S_MIN = 40.50mm³

DRAFT ANALYSIS:

The Draft Analysis command enables you to detect if the part you drafted will be easily removed. This type of analysis is performed based on color ranges identifying zones on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.

Minimum draft angle is 7 degree is considered for analysis. Green colour show positive draft angle of the part more than 7 degree.

ROOF OUTER PANEL

FRONT ROOF RAIL

BOW ROOF FRONT RAIL

RE - INFORCEMENT CENTER ROOF RAIL

BOW ROOF REAR

REAR ROOF RAIL

Different Views of Complete Roof Design -

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NOTE - * EARLIER BY MISTAKE I HAD CREATED ROOF FILE IN FEATURE GROUPS INSTEAD OF CREATING IT IN AS A ASSEMBLY,BUT IN THIS ONE I HAD ALL MY CALCULATIONS FOR HEAT DISTORTION & SNOW LOAD.

* I AM ATTACHING TWO FILES,IN WHICH THE MODEL IN GREEN ONE IS THE CORRECT ASSEMBLY FILE.

# CATPART & ASSEMBLY FILE IS ATTACTHED BELOW