Roof Design

ROOF DESIGN

AIM; The aim of the project is to design a roof of a car using the given master sections and datas.

OBJECTIVE

The objective of the project is to design a car roof with essential flanges and reinforcement like ditch area, front roof rail, bow roofs , rear roof rail and central roof rails for the styling surface using the given master section and as per the industrial standards. Also Perform acurvature study on the roof and perform the calculation for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct positions .Perform draft analysis for each reinforcement and finally calculate the moment of inertia of all the roof rails.

INTRODUCTION

An automobile roof is the portion of an automobile that sits above the passenger compartment , protecting vehicle passengers from sun ,rain ,wind and other elements. roof is the structure above the wind shield glass having the reinforcement to provide the strength to the overall structure and to with stand the over head forces like snow. When it is seing from outside it is a just a closing of the top surface of the car , but in the design point of view we want to consider many design consideration for the roof design

DESIGN CONSIDERATION OF ROOF

• VISIBILITY CRITERIA
• HEAD CLEARENCE
• MATERIAL THICKNESS
• HEAT DISTORTION CRITERIA
• SNOW LOAD CRITERIA
• MATERIAL THICKNESS
• DRAFT ANALYSIS
• HEAD ROOM CLEARENCE

TYPE OF REINFORCEMENTS GIVEN FOR THE CAR ROOF

• FRONT ROOF RAIL
• BOW ROOFS
• CENTRAL ROOF RAIL
• 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 upon the master section givenas shown below.

Adisionaly emboses are created on the front roof rail to increase the strength .The front roof rail is joined with the roof outer by two methods ones is using the spot welding and by using mastic sealents.

BOW ROOF RAIL

The bow roofs are provided to improve the torsional stiffness and load bearing capacity of the roof structure. The bow roof provides depends upon the overall size of the roof. Here for the given roof two bow roofs are created.Bow roofs are joined to the outer by using spot welding process and by the usage of mastic sealents.

CENTRE ROOF RAIL

The centre roof rail helps in providing effectively support to the flat area of the roof as it is more susceptile to failure under the action of load. Centre roof rail helps in adding stength to the roof during the rool over test. Generally central roof rail is placed at the centre of the roof which is connected to the b pllar.

REAR ROOF RAIL

Rear roof rails joins the body side outer and the back door, spot welding and mastics are used for the joining process.

ROOF CRUSH TEST

Roof crush test is design given by the insurance institute of highway safety , after analyzing the many accidents they has then fixed some regulation , that how to perform crash test and based on this test they give the ratings to the car or vehicle undergoing the test , the factors that the IIHS consider to give the ratings are how fast the car ca attains to its orginal shape after the roll over and how much deep is the protrusion after the roll over, the aim of the test is to minimize the protrusion into the occupant compartment.

HEAT DISTORTION CRITERIA

The heat distortion criteria indicate what temperature material start to soften when exposed to a fixed load at elevated temperatures , in order to avoid bending , damage to the roof based ont the heat distortion temperature. This study will provide the which position of the bow roof give enough dtrengthen to roof.

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 conditions

OK< 2.7

RESULT

From the above reult we can confirm that our design is satisfying the judgement condition of the W value should be less than 2.7 . and our design obeys the heat distortion criteria.

Snow Load Prediction Formula

Snow load is the downward force on a 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 poorly designed or constructed , so there is a formula to predict the strength of the roof.

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)

t = Roof plate thickness [mm]

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

Length of Roof panel with the center 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

Judgment condition

Qr ≥ 3.1

250 ≤ s ≤ 380

MOMENT OF INERTIA VALUE OF THE SECTIONS

• FRONT ROOF RAIL

• BOW ROOF 1

• CENTRE ROOF RAIL

• BOW ROOF 2

• REAR ROOF RAIL

For the calculation of snow load criteria the minimum moment of inertia value of the bow roof 1, centre roof rail and bow roof 2 are taken respectlively.

In the 3rd case the snow load criteria is not following by our design ,The Qr value is not satisfying in 3rd case. So here we want to redesign our current design , sinceit is a time consuming process for repositioning the redesign the roof rail , for compensating this loss of strngth we are adding additional emboses on the outer roof rail and by this way we can give strenth to the roof.

DRAFT ANALYSIS

• FRONT ROOF RAIL

• BOW ROOF 1

• CENTRE ROOF RAIL

• BOW ROOF 2

• REAR ROOF RAIL

• ROOF

SECTION VIEWS

CONCLUSION

• The results obtained after doing a curvature study on the roof was adequate
• since at some portions the snow load criteria is not satisfying we have used emboses for improving the stength
• Draft anlysis have been conducted over all the roof rails and roof outer almost all the portions are satisfying the positive draft of 7 deg.
• Finally our roof is designed.