Major Assignment Project

1.    General Instructions

This major assignment is a group-based Design and Experiment project. Each group can choose one topic from the following two options to work with:

·    Truss-structured Bridge Model

·    Sloped Track with Friction for Lifting

The formation of each assignment group should be between 2 (minimum) to 4 (maximum) members. The members of a group are expected to discuss and share the

2.     Marking Scheme

Both assignment topics involve some hands-on physical modelling/experimental tasks and computational analysis. At the completion of the assignment project, a group report needs to be developed and submitted for formal assessment. Irrespective which topic you choose for the assignment work, the group assignment report needs to include the following key elements for

assessment:

3.     Project Descriptions

For detailed descriptions of the assignment project tasks please see below and also refer to the sample project reports that have been provided.

Truss-Structured Bridge Model

Under this assignment topic, you are expected to complete three major tasks:

·    Design and modelling task

·    Computational analysis task

·    Experiment task

Design and Modelling Task

In groups design and build a truss-structured bridge. To ensure the convenience of physical modelling, the following materials are recommended for bridge construction:

·    PVA glue

·    Popsicle sticks

·    String

·    4 sheets of A4 card (to act as decking for the bridge)

The vertical sections of the truss structure designed for the bridge model are expected to be light, simple, and effective, so that it is easy for force analysis. You may use the Murray Rail Bridge (see Figure 1 below) to inspire your design. But, in the mean time, do not limit your imagination and creativity!!

Figure 1.  Rail Bridge over River Murray at Murray Bridge

(http://l.yimg.com/ea/img/-/100827/3504396768_2ff5bbf763-167dvdl.jpg)

The bridge model also needs to meet the following specifications:

·    It spans 1 meter, with clear cross section of 200x200mm (recommended for load testing)

·    Maximum weight: 1kg

·    It shall be able to taking external load between 500N (minimum) and 1300N

Computational Analysis Task

The objective for the truss-structured bridge design and modelling is to support the highest possible load with the least self weight.

The following assumptions can be applied for the truss structure analysis:

·    Only analyse the vertical section of the truss structure (as shown in the photo above)

·    The structure is treated as a planar truss. All the forces are applied only at the joints.

·    ALL the joints are assumed to be pinned joints, and ALL the members are two force members

·    A static load is applied as a concentrated force at one position at a time. The MINIMUM static load is 500N.

·    Do not consider the effect of the self weight of the bridge model on the structural members

·    The maximum tension inside a member is Tmax = 1367N and the maximum compression is Cmax= 113N.

You need to use appropriate truss structure analysis techniques for the following tasks:

1)   Identify THREE critical positions on the planar truss structure where a static load will be applied and justify why these positions are chosen for the analysis

2)   For EACH position, perform the following analysis and present the results:

a.    Identify Zero-Force Members inside the structure

b.   Applying a load from 500N and increase with 200N at a time to calculate i.   the force inside each member

ii.   Identify the members have the highest compression an those have the highest tension (to ensure they will not exceed Tmax and Cmax for safety consideration)

3)   Identify

a.    The maximum load the bridge structure can take in theory, and b.   Where in the truss the load can be applied.

Experiment Task

To complete this task, you need to

1)   Conduct load testing on the bridge model to validate the Computational Analysis results from Task 2. The following supports for the bridge model can be applied for the test (Figure 2).

2)   Report the outcomes and discuss the differences observed between the experiment results and computational results

Sloped Track for Lifting with Friction

Under this assignment topic, you are expected to complete three major tasks:

·    Design and modelling task

·    Experiment task

·    Computational analysis task

Design and Modelling Tasks

In this project, you need to design and conduct an experiment on a sloped track for loading a laden box (or crate) to a platform using a cable. The following specifications need to be followed for the design and modelling:

·    The height of the platform is set as 600mm from the level of ground.

·    The laden box/crate is weighted as 200N (or 20kg approximate )

·    The length of the sloped track (L) and the angle formed between the sloped track and the ground (a) are adjustable

For building the sloped track, you can use any materials that strong and safe enough to sustaining a

20kg load and its movement without resulting in breakage or notable deformation.

For the box/crate, it can also be in any material which is strong and safe enough for handling during the experiment.

You may also need to use some auxiliary materials or components for building the track.

A proper design drawing and a design description must be produced to include in the final report

Experiment Tasks

The experiment is to identify both static friction coefficient (μs) and kinetic friction coefficient (μk) for the surface of contact between the laden box and the track. You need to:

1)   Search and design TWO different testing methods for finding the friction coefficients.

2)   Compare differences in the results from the two methods

Also, you need to

3)   Adjust the angle of the slope α and measure the correlation between α and the pulling force P, while keep the box/crate moving up the slope in a constant speed. Produce the α-P diagram

It is necessary for you to choose suitable gadgets for measuring force, time, speed, displacement based on the testing methods to ensure data accuracy. Meanwhile, you also need to ensure the design of the testing process is proper for the reliability of the data collection.

Experiment design and data analysis process must be included in the final report

Computational Analysis Tasks

The objective of this computational analysis is to complete the following tasks:

1)   Assume the box/crate moves up long the slope in a constant speed, validate the test results from Experiment Tasks 3) by using the equilibrium equations to calculate the correlation between α and pulling force P. Produce the α-P diagram

2)   Assume the box/crate moves up long the slope in a constant speed. Determine with which α

value, the maximum efficiency of lifting can be achieved. The objective function for maximum efficiency is defined as having minimum work from the pulling force P along the slope:

3)    Measure the weight and dimensions of the slope and conduct force analysis:

a.    When the motion of the box is impending with the pulling force P and the slope angle α  as identified in Task 2) above, determine the friction forces at ALL contact surfaces/points on the sloped track.

b.   Based on the calculated values of the friction forces, determine whether the sloped

track remains stably at rest.

For the analysis, the following important conditions must be applied

·    The height of the platform (H) is a constant as set

·    For safety reason, the pulling force must not exceed the maximum tension TMAX = 160N (80% of the weight of the box/crate) in any circumstances during the lifting process

·    The length of the slope L changes with the angle α

Note:

It was brought to my notice that some of you haven't done Engineering Drawing (ENR118) before this unit. And, many of you are unfamiliar with 3D modelling and simulation.

If you choose to do the Bridge Design topic, in this case I can let you just focus on the bridge concept design and load analysis based on calculations. You don't need to physically build and test the bridge, nor need to use any software for simulation. Those parts are useful for the project (particularly in terms of having some hands-on work and applying the skills developed from other courses), but not as directly related to the core knowledge and skill sets required by Mechanics as the design and analysis tasks. So, you can have a conceptual design of a truss bridge and then do the analysis to identify the forces inside the members to discuss whether the design will work, how much the maximum load can be, which members can be of safety concern, and how to modify and improve.

Also, another alternative is that you can work on the second assignment topic listed, i.e. Friction Experiments. Although it also needs to have some physical things built and tested, it is less resource demanding than making a bridge model.