SED302: CAE Bridge Design Optimisation - CAE Design Project Assessment Answers

November 01, 2018
Author : Charles Hill

Solution Code: 1EAA

Question: SED302: CAE Bridge Design Optimisation

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CAE Bridge Design Optimisation

CAE Technical Report

Design Teams: You will need to choose design teams of 4 to 5 people before the end of Week 2. You will self- enrol into your teams online in CloudDeakin. You must keep a diary of your team’s activities throughout the project and capture minutes from your meetings. This will be assessed and is compulsory for all teams. You are encouraged to use the specific group discussion boards which will be created for your team on CloudDeakin.

The Design Brief: Geelong’s famous one-way bridge that spans the Barwon River at Queens Park (shown in Figure 1) has been selected for upgrading and the Geelong City Council is seeking your help as Computer Aided Design Engineers. The Geelong City Council is interested in a low-cost, lightweight and subsequently, a structurally efficient design capable of supporting light vehicle traffic.

Your design team is required to design, analyse and build a small prototype of your new Queens Park bridge and present it through an oral presentation and an accommodating CAE report to the Geelong City Council (in this case it is your lecturing staff and your fellow peers). Balsa wood will be the material used to construct your prototype. The scaled prototype must span a width of 300 mm and have a total length of 360 mm. The width and height must not exceed the dimensions as shown in Figure 2. The bridge must have a smooth surfaced roadbed to allow vehicles to drive over it. A rectangular of 125(L) x 60(W) x 60(H) must be able to pass through your bridge (simulating a vehicle passing through it). Additionally, you cannot design structure below the roadbed as canoes and rowing boats often pass beneath it, similar to the picture show in Figure 1. Your prototype bridge must be able to support a minimum load of 12 kg.

Your objective is to design the most structurally efficient bridge based on a strength-to-weight ratio. Therefore, as light as possible, but be able to support the greatest load. And thus, you must demonstrate optimisation of your design. This will be primarily based on the four-point-bend load case described below, that will be experimentally performed in the practical class of week 10/11. The four-point-bend load case simulates a truck driving over the bridge.

Figure 1. The one-way bridge over the Barwon River at Queens Park, Geelong.

Figure 2. Schematic representation of the design envelope for your scaled prototype and the w-point bending load case.

The Load Cases and Analysis: In order to assure your bridge design will be adequate, the following load cases and structural analysis must be performed via FEA and then your design optimised:

Static 4-point bending: This is the primary load case and will be assessed experimentally in the practical class of week 10/11. Therefore, you must have your balsa prototypes ready for testing by this time. The load case simulates a two point loads applied central to your bridge, as shown in Figure 2. A load will be gradually applied until failure occurs and the maximum failure load determined. The bridge will be simply supported by two cylindrical pipes.

For the experimental tests, a simplified truck will be used to apply the load. See Figure 3 for the schematic representation of the loading truck. A SolidWorks model of the loading truck is available to you (loading_truck.sldprt) to ensure you are able to successfully load your bridge during the experimental trials.

Figure 3. Schematic representation of the loading truck used in the experimental trials.

Static Torsional loading:

Adequate torsional stiffness is required from your design to ensure that if any off-axis loading occurs your design is still suitable. Therefore, you must assess the torsional stiffness of your design accordingly.

Modal Analysis:

Apart from static bending and torsional stiffness, which is assessed by the previous load cases, you must investigate the relative stiffness of your structure by performing a modal analysis. The modal analysis may highlight areas of weakness not shown by the static load cases and therefore, these must be considered in your design also.

Buckling Analysis:

You must ensure your design does not exhibit a buckling failure and, therefore, you must demonstrate the buckling performance of your structure for the static load cases described above.

Note: As with any FE analysis supporting and justifying your analysis results with a numerical convergence study and analytical hand calculations is critical.

Material:

You will use Balsa wood to construct your prototype bridges. You will need to create a material model for your FE analysis in ANSYS and select the appropriate material properties from that data supplied. Further investigation and potential validation of your material model may be needed to ensure your FE prediction is accurate. You will need to think of how you might achieve this. Table 1 gives you the material data for a common tropical balsa wood. Notes: Data is based on small, clear, air-dried samples unless specified. Key Words: Ochroma lagopus; Lumber, Timber.

Table 1. Typical material properties for Tropical Balsa wood.

Balsa wood material list:

For on-campus teams - the school will supply your team with the Balsa wood and the appropriate construction materials. A list of the maximum available Balsa geometry is detailed in Table 2. Upon collecting your Balsa material you must provide a CAD model of the design you are going to construct, along with a list of the required materials (number of each piece needed). Only then will you be issued your materials.

For off-campus students – you can request to have the balsa material sent to you, but you must contact the unit-chair for this to happen and follow the instructions as listed above. Otherwise, it will be assumed you will source the required materials yourself. The Balsa wood and appropriate glue etc. is available at most hobby or hardware stores. In the past, sending balsa wood via mail hasn’t been very successful and often breakages have occurred and there is also postage times to be considered.

Note: There is the potential to have more material if you can clearly demonstrate a unique design that requires the additional material – speak to your lecturing staff about this if you feel you need more materials.

Email your completed material list and CAD model to your unit chair. Please convert you SolidWorks assembly/part file to an eDrawings part/assembly file. This allows us to view it and minimises the file size requirements. Follow the procedure shown in Figure 4.

Table 2. Available Balsa Material

Figure 4:

Converting SolidWorks Parts/Assemblies to eDrawings for easier file transfer.

Practical Class and Design Assessment:

During the practical class (Week 10/11) you will need to have your completed Balsa bridge prototype ready for testing. You should also have completed a thorough design analysis via FEA and have a predicted value for the maximum failure load according to the 4-point bending load case. Your bridge design will be weighed and then tested according to the 4-point bending load case. Your team’s design will be assessed in two areas:

1. Structural design:

You should aim to have the highest strength-to-weight ratio for the 4-point bending test conducted in the practical class. The strength-to-weight ratio is calculated according to:

2. FEA prediction accuracy:

You will also be required to predict the failure load of your design from you FEA, prior to the practical class. How close you are to the physical test will also be assessed. Therefore, you will need to ensure you have an accurate FE model to improve your chance of success in this area.

FEA prediction improvement:

As with any FEA there may be some margin of error, however, you want to ensure this is small as possible. Following, the completion of the experimental test you will need to investigate potential errors in your model and improve the accuracy of them prior to the submission of the final report. You must provide a detailed discussion of this process in your CAE report.

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Solution:

1. IntroductionA Product development process includes Engineering design , CAD or Computer Aided Design, CAE or Computer Aided Engineering which further comprises of FEA or Finite Element Analysis. The report aims at discussing the best possible solution of a case using FEA simulation and analysis

FEA or Finite Element Analysis is a numerical method used to solve any Multiphysics problem. It is considered as branch of Solid Mechanics. The method is used to solve a number of real life problems and analyse the problem in details. It can be used in following cases.

FEA can be applied in following cases

    • Structural Analysis

    • Vibrational Analysis

    • Thermal Analysis

    • Electrical Analysis

    Dynamic Analysis

FEA results works on three basic requirements such as boundary conitions, loading conditions and meshing accuracy. There are number of softwares available to help us with, some of them being ANSYS, ABACUS and Solidworks Simulation.

1.1 Problem Statement

In the Problem Statement, the client has asked for a low-cost, lightweight and a structurally efficient design for a bridge which can support vehicle traffic. It is required to design, analyze and simulate a small prototype of a new Queens Park bridge with the help of CAE. The design must be most structurally efficient and must be based on strength to weight ratio. There are certain constraint which needs to be kept in mind. They have been listed in next heading.

1.1.(A)CONSTRAINTS

The constraints involved with the prototype should

Span a width of 300mm and a length of 360 mm.

Figure 1 the bridge size constraints.

Allow passing of a rectangular block of dimension 125 x 60 x 60 through it.

Not contain any design below roadbed to stop passing of boAts and canoes.

Must be able to support a load of 12 kg

Withstand a four point bend load case simulating a truck driving on the bridge

Be made of balsa wood whose all properties has been listed below

Should be made up of limited material provided as listed below

2 Design Study

A bridge is used to provide a passage between two different terrain such as valleys, rough terrains and obstacles such as bodies of water. The bridge uses a number of natural and man-made materials.the selection of bridge depends upon the distance between the terrains, the length of obstacles and the type of load to be transferred. Based on such observations some types of bridges have been discussed below.

2.1 Types of Bridge

Some common types of bridges are

2.1.1 Girder Bridges

It is most common type of bridge used all over the world and can be easily understood like using a log of wood kept above a creek. These girder bridges are basically of two types namely I beam Girders and Box type Girders. These girders are easy to design and build. They easily withstand torque and force for a curved shape bridge. (Aboutcivil, 2016)

Here in this case a girder bridge can be thought only when we have larger strength materials or a prototype is typically based on the beam strength. As the distance is long and there should be no pillars below this design shall be neglected. Further the beam design might use a large number of construction material.

2.1.2 Arch Bridges

These bridges are classical architecture and one of the oldest bridge form after the girder bridges. The construction of bridge has evolved from ages and is now one of the most beautiful form and a steady architecture. These bridges use the curved structure of building material to provide a high resistance to all type of bending forces. There are four basic arch type bridges they are

    Hinge less arch bridge

    Two Hinged arch bridge

    Three hinged arch bridge

    Tied arch bridge

Such bridge has only drawback that it can only be used where foundation are solid and strong as no horizontal movement can be allowed.

This bridge can be tried as it has comparatively larger bending moment absorbing capacity and a good looking structure.

2.1.3 Cable Stayed Bridge

When any girder bridge gets a support of steel wires from any pier raised over to a height the bridge type become Cable stayed bridges. Steel cables are used in these cases to hold down girders. The material used in this type of bridge comparatively very less. But these typr of bridge combined with any other form of bridges can be a better solution.

These bridges are lightweight and hence are more prone to wind and earthquakes as any single rope failure may case the whole structure to fail. (Aboutcivil, 2016)

The bridge was not selected as the material provided was not enough to build the bridge on this concept.

2.1.4 Rigid Frame Bridges

Sometimes also known as rahmen bridge is like a standard girder type bridge. However it differs from the girder bridge type as here the bridge piers and girders are all solid structure and almost made up of same material. These bridge are build where stress distribution at the corners are not uniform. They together form a giant solid structure. One of those can be seen in the below figure.

2.1.5 Truss bridge

This type of bridges are formed from small straight beam units. The physics of such bridge lies on the load distribution on each and every member of structure. The small beams when together organized can support a large amount of weight. Their erection is simplest as it only requires one type of building block that is steel member. (Aboutcivil, 2016)

There can be a number of truss bridge types. They are listed below

    Warren Truss bridge

    Pratt truss bridge

    Howe Truss bridge

    K Truss bridge

This bridge can also be used for construction as it requires one small building elements of same type. It can carry a great amount of load and does not require piers always.

2.2 Final selection

Finally truss bridge was selected for the following reasons.

    • It can withstand large amount of load.

    • The construction of this bridge is simple.

    • It doesn’t require piers in small range and hence can easily allow the passage of boats below it.

    • Each member carries equal load and is important.

    • Vertical member can be added on the design whenever required to hold larger loads.

    • Calculations can be easily made for the design.

    The height constraint was easily met with this type of bridge.

3 3D modelling

The selected bridge type is truss bridge type. The small elemental member were drawn in solid works and then assembled to get the final design. The material dimension was chosen same as per required constraints.

3.1 Initial Design

The Intial design selected was made in solidworks using a combination of two truss types. But the selected model was rejected on the basis of its heaviness and amount of material being used. Although the design was perfect but other constraints were also required to be met.

Now the challenge was to design a compact truss bridge having almost equal bearing strength and considerably utilizing less material. The design was achieved after repeated iteration and finally the following model was selected. The details of the design has been explained in the below reports.

3.2 Part Modelling

For part modelling all the parts and assembly was done in Solidworks software itself. The part modelling consisted of base design, support design and the roof design.

3.2.1 Base Design of bridge

The base design of bridge has a length of 360 and a width of 80mm. It has been extruded to 6.5 mm. further the base was reduced to a width of 6.5mm as per the material provided. The whole design was made of single structure and base was ready as shown below. Two extra extrusion were done at about 300 mm apart below the base representing the support point of the whole bridge.

3.2.2 Support Design

The supporting pillar truss member was selected for the maximum width size of 6.5 mm.the drawing has been shown below.

3.2.3 The roof design

The roof designing was done with the member having width of 5mm. the drawing has been shown below.

3.3 Assembly

The final Assembly after mating of parts was done looked like below. The weight of the whole model after defining the material was observed to be 2.7429 kg.

4 Finite Element Analysis

4.1 the Geometry featuring and defeaturing

The model was imported as well as made in ANSYS Workbench. The whole bridge was defeatured and made into a bounding box of 0.36m X 0.135m X 0.08 m. The weight of the model in Ansys using the material was found to be 2.7249 Kg.

4.2 FEA setup

After the part was assembled in Solidworks the model was imported in ANSYS for further analysis. Static Structural module was chosen for the analysis.In the whole setup a load of 490 N was applied on the base of bridge. The two fixed support at a distance of 300 was kept and fixed. Further in the setup a Standard Earth Gravity was given to make the situation more realistic. The figure below represents the situation.

4.3 Mesh

The meshing of bridge was done with automatic settings. The reference centre was kept coarse and the mesh sizing was taken to be 4e-03. Medium smoothing and transition was kept fast. The meshed structure however looked like as shown below in the figure.

Mesh Details

  • Total Nodes - 103183
  • Total Elements - 39300
  • Total Active Bodies - 10
  • Mesh Size - 4e-03
  • Mesh Type - Tetrahedral

4.4 Supports

The fixed support was applied at the base of bridge where the two cylindrical rods has been attached. The two supports are about 300mm apart.

4.5 Loading Conditions

The load was applied on the base of bridge shown red in color. The load was applied in the downward direction. The load is defined with components type where the value of Y component is 490 N.

4.6 Results

Some of the results have been shown below.

    Deformation Result – the maximum deformation observed in the drawing is 5.2457e-06 m. the maximum deflection is observed at the centre of the bridge.

    Equivalent Stress – The stress varied in the bridge from 7.1647e5 Pa to a maximum value of 6.2824e6 Pa. the maximum stress is involved near the point of application.

    Safety factor – the safety factor is 15 as observed.

5 Conclusion Finally a bridge was designed as ber following constraints.It can withstand large amount of load.The construction of this bridge was simple and could easily allow the passage of boats underneath the bridge constructed was of truss type hence each member carried equal load.The height constraint was easily met with this type of bridge.the model was made and analysed on ANSYS 16.2 Workbench and appropriate results were obtained. No modification was required as the bridge had met all the constraints.

The final design of the bridge consisted of three main piers with three triangular supports. The load was distributed evenly and well balanced as the maximum stress was hardly 6.8 MPA. The factor of safety as 15 in itself tells the strength of the bridge. The bridge material was kept balsa wood as required and all the calculations were made on its basis. The bridge hence can be tried and tested.

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