Simple Structural Surface Method (VBSS) NITW

NITW SSS PRESENTATION

1. ME56021-Vehicle Body & Safety Systems Simple Structural Surface (SSS) Method

2. Introduction SSS method originated from the work of Pawlowski of the Warsaw Technical University in 1964 The vehicle to be modelled using SSS method will be represented using several plane surfaces or panels. Elementary method is beam method, FEM is advanced method and SSS is intermediate

3. Objectives of SSS method To demonstrate that a car structure can be represented by simple structural surfaces (SSSs). To introduce the load paths in a sedan structure for different load cases. Simple  Structural   Surfaces  (SSS) method is offered as a means of organizing the process for rationalizing the basic vehicle body  structure  load paths. The application of this simplified approach is highly beneficial in the development of modern passenger car  structure  design.

4. An SSS is defined as a plane structural element or sub-assembly that can be considered as rigid only in its own plane and flexible to out of plane load. Elements are assumed to be rigid in its plane Can carry loads in its plane – Tension, compression, shear and bending Loads normal to plane and bending out of plane is invalid and not allowed Simple Structural Surface

5. SSS (examples)

6. Non-SSS (examples)

7. SSS method Each plane surface must be held in equilibrium by a series of forces. These forces are created due to the weight of the components attached to them. The rails that are attached to adjacent plane surfaces would provide reactions to maintain equilibrium. Equal and opposite forces will be exerted to the adjacent members. The loads on each SSS member is propagated through the whole structure from one rail to the other until the overall equilibrium is achieved. This way, any deficiency of the plane structures such as discontinuity in load path can be determined easily.

8. SSS method Any SSS that is not supported adequately due to omission of a suitable adjacent component or panel will be revealed. This in turn indicates stiffness deficiency. The SSS method is used prior to and during the initial design stage can complement computer aided design and synthesis notably. In the beginning stage of a new vehicle design there are normally insufficient data available to develop a complete finite element model for analysis. Model at this stage is generally being represented by sticks or beams and structure surfaces by panels.

9. SSS method Figure below shows a simplified model of the standard sedan, consisting of a ‘closed box’ passenger compartment, comprising floor, roof, side frames, front and rear bulkheads and windscreen.

10. SSS method For simplicity, all these surfaces are assumed to be plane to demonstrate the SSS method.

11. SSS method List of 16 SSSs in the standard sedan 1. Transverse floor beam (front) carrying the front passengers. 2. Transverse floor beam (rear) carrying the rear passengers. 3. and 4. Inner wing panels carrying the power-train and supported by the front suspension. 5. Dash panel–transverse panel between passengers and engine compartment. 6. Front parcel shelf. 7. and 8. Rear quarter panels carrying luggage loads and supported by the rear suspension.

12. SSS method List of 16 SSSs in the standard sedan 9. Panel behind the rear seats. 10. Rear parcel shelf. 11. Floor panel 12. and 13. Left-hand and right-hand side frames. 14. Windscreen frame. 15. Roof panel. 16. Backlight (rear window) frame.

13. Bending Load Case Figure shows the baseline loads for the pure bending case. F pt – powertrain load F pf – front passenger load F pr – rear passenger load F l – luggage load R F – front suspension reaction R R – rear suspension reaction

14. Bending Load Case Figure shows the baseline loads for the pure bending case. F pt – powertrain load F pf – front passenger load F pr – rear passenger load F l – luggage load R F – front suspension reaction R R – rear suspension reaction

15. Bending Load Case The magnitude of the loads is the weight of the component factored by a dynamic load factor. It should be noted that all these loads are applied in the planes of SSSs. It is essential this condition is achieved in order to ensure sufficient strength and stiffness can be provided through the structure. The bending and shear loads on each component can be determined and from these satisfactory stress levels can be determined.

16. Bending Load Case By considering the given structure as a simply supported beam, supported at front and rear axle the reaction forces can be calculated by taking moments about the front and rear suspension mountings.

17. Bending Load Case

18. Bending Load Case Typical other items that may be included for analysis are: front bumper, radiator, battery, instrument panel/steering column, exhaust, fuel tank, spare wheel, rear bumper and distributed loads due to the weight of the body structure. If these are to be included, the positions of all these components as well as their masses must be known. The suspension reactions can be calculated with a similar procedure.

19. Bending Load Case (End and Edge Loads)

20. Bending Load Case (End and Edge Loads) We require edge loads and end loads to ensure all SSSs are in equilibrium. These edge/end loads are indicated by the forces P1 to P13.

21. Bending Load Case (End and Edge Loads) The passenger loads are carried by the two transverse floor beams SSS (1) and (2). These floor beams are supported at each end by the side-frame forces P1 and P2. Note that there is an equal but opposite force acting on the side frame.

22. Bending Load Case (End and Edge Loads) Consider now the inner front wings SSS (3) and (4), the loads acting on these are the loads from the powertrain F pt /2 and from the front suspension R FL . The applied loads F pt /2 and R FL are held in equilibrium by the end loads P4 and P5 and by the edge (shear) load P3.

23. Bending Load Case (End and Edge Loads) The shear load P3 reacts into the dash panel while the end load P4 reacts into the front parcel shelf (6), and P5 into the floor panel (11). These forces can be obtained by the equations of statics, i.e. resolving forces and taking moments.

24. Bending Load Case (End and Edge Loads) Note, the horizontal SSS (6) is necessary in order to carry force P4. Without providing that SSS(6), we will have a weakness or an unsatisfactory load path in the structure.

25. Bending Load Case (End and Edge Loads) By working through the individual SSSs of this model, it will be realized that there are sufficient forces to achieve equilibrium for each component And that all the loads act in the planes of the SSSs.

26. Bending Load Case (End and Edge Loads) Transverse floor beam (front) (1) Resolving forces vertically and by symmetry (loads are assumed to be applied symmetrically about the vehicle longitudinal center line): P 1 = F pf /2 -------- (1)

27. Bending Load Case (End and Edge Loads) Transverse floor beam (rear) (2) Resolving forces vertically and by symmetry P 2 = F pr /2 -------- (2)

28. Bending Load Case (End and Edge Loads) Left and right front inner wing panels (3) and (4) Resolving forces vertically for the left-hand panel (3) P 3 = R FL – F pt /2 ------ (3)

29. Bending Load Case (End and Edge Loads) Left and right front inner wing panels (3) and (4) Taking moments about the rear lower corner P 4 xh 1 = R FL xl 1 – ( F pt /2)x(l 1 +l pt ) ---------(4) Similar equations can be obtained for the right hand panel (4)

30. Bending Load Case (End and Edge Loads) Left and right front inner wing panels (3) and (4) Resolving forces horizontally: P 5 = P 4 ------------ (5)

31. Bending Load Case (End and Edge Loads) Continuing similarly we get For Dash panel (5) P 6 = P 3 ------------ (6) For Front parcel shelf (6) P 7 = P 4 ------------ (7)

32. Bending Load Case (End and Edge Loads) Rear quarter panels (7) and (8) Resolving forces vertically for the left-hand panel (7) P 8 = R RL – F l /2 ------ (8)

33. Bending Load Case (End and Edge Loads) Rear quarter panels (7) and (8) Taking moments about the front lower corner P 9 xh 2 = R RL xl 2 – (F l /2)x(l 1 +l 2 ) ---------(9) Similar equations can be obtained for the right hand panel (8)

34. Bending Load Case (End and Edge Loads) Rear quarter panels (7) and (8) Resolving forces horizontally: P 10 = P 9 ------------ (10)

35. Bending Load Case (End and Edge Loads) Continuing similarly we get For Panel behind the rear seats (9) P 11 = P 8 ------------ (11) For Rear parcel shelf (10) P 12 = P 9 ------------ (12)

36. Bending Load Case (End and Edge Loads) Floor panel (11) Forces P 5 and P 10 act on this SSS. These will not necessarily be equal so additional forces P 13 are required acting at the sides which react on the side frames.

37. Bending Load Case (End and Edge Loads) Floor panel (11) Resolving forces horizontally: 2P 13 = 2(P 10 – P 5 ) ----------- (13)

38. Bending Load Case (End and Edge Loads) Left-hand and right-hand side frames (12) and (13) Both side frames are loaded identically.

39. Bending Load Case (End and Edge Loads) Left-hand and right-hand side frames (12) and (13) Both side frames are loaded identically. The enlarged views are shown here.

40. Bending Load Case (End and Edge Loads) Left-hand and right-hand side frames (12) and (13) The forces acting on the side frames have already been obtained from the previous equations Apply equations of statics to check the equilibrium

41. Bending Load Case (End and Edge Loads) Left-hand and right-hand side frames (12) and (13) Resolving forces vertically and horizontally we have P 6 – P 1 – P 2 + P 11 = 0 ------------(14) P 7 + P 13 – P 12 = 0 ------------(15)

42. Bending Load Case (End and Edge Loads) Left-hand and right-hand side frames (12) and (13) Taking moments about the lower corner of the windscreen pillar where P 6 and P 7 act we simplify the equation by eliminating two terms P 1 l 3 + P 2 l 4 - P 11 l 5 - P 12 (h 2 - h 1 ) – P 13 h 1 = 0 ----------------(16)

43. Bending Load Case (End and Edge Loads) In practice some rounding errors due to difficulties in defining the exact positions of each force may occur. It should now be noted that windscreen frame (14), roof panel (15), and backlight (16) SSSs are not subject to any load for this bending case.