MECH3110 Mechanical Design 1

Fasteners Assignment Guidelines (20%)

Name your files in the following format: zID_FastenersAssignment.pdf E.g., z5160675_FastenersAssignment.pdf

Background

In December 2024, Darson visited Christchurch, New Zealand, to present at the Australasian Association for Engineering Education (AAEE) Annual Conference. During an evening stroll, he came across a local landmark—the Christchurch Swings (#chchswing)—a popular installation that integrates large beams and suspended swings, providing both aesthetic appeal and recreational use (Figure 1(a)). Upon closer inspection, he noticed that the beams were connected by fasteners, forming the key structural joints of the installation, as shown in Figure 1(b).

Christchurch is well known for its seismic activity, having experienced several significant earthquakes in recent history. In such an environment, fail-safe design is critical. Fail-safe design refers to engineering strategies that ensure structures remain safe and functional even when components fail. This approach is essential in earthquake-prone regions to prevent catastrophic failures and protect public safety.

Fasteners play a critical role in the integrity of any structure, especially in load-bearing applications like the #chchswing. If a fastener joint fails, fail-safe mechanisms—such as redundancy in the design—must ensure that the structure does not collapse. In this assignment, you will apply your knowledge of fasteners to design the joint system that secures the beams of the #chchswing while ensuring fail-safe principles are incorporated to enhance structural safety.

Your Tasks

In this assignment, your objective is to design a fastener joint for connecting the steel beams of #chchswing, such that the whole structure will not collapse even if another fastener joint fails. The joint must ensure safety, durability, and appropriate load-bearing capacity with a reasonable factor of safety. In this assignment, we will focus on the single swing at the front only, not the twin swing at the back of Figure 1(a).

Remember, your goal is NOT to find the most accurate answer to an engineering mechanics question. Your goal is to come up with a practical and conservative design for the fastener joint to ensure safety.

Figure 1 (a) Darson on #chchswing (single swing at the front). (b) The frame of #chchswing is held together by fasteners. (c) An example of how this public structure can be abused (the twin swing at the back).

Figure 2 (a) Dimension of the single #chchswing at the front. (b) the approximate dimension of the beam cross-section. The exact dimension of this I beam depends on your selection.

Specifically, you are to:

1.   Design the location of the hanger/chains of the swing on Beam BC, ensuring the person is positioned roughly at the centre of the frame. for optimal stability and aesthetics   (i.e. instaworthy).

2.   Select an I-beam from a supplier’s catalogue for the structure. The width and height of the beam cross-section are approximately 250 mm based on visual inspection. Justification of beam selection is outside the scope of this assignment.

3.   Pick a joint (Joint B or Joint C) to analyse. In reality, all joints should be analysed, but for this assignment, focusing on one joint is sufficient.

4.   Design the placement, number of fasteners, and spacing between the fasteners at your selected joint (refer to Figure 1(b)).

5.  Adopt a fail-safe design principle and analyse the safety of your selected joint assuming the other joint has failed due to seismic conditions:

a.   If you choose to analyse Joint B, assume Joint C has failed, and Beam CD has collapsed, leaving only structure ABC under loading.

b.   If you choose Joint C, assume Joint B has failed, and Beam AB is no longer supporting structure BCDE.

6.   Construct a free-body diagram of Beam BC and calculate the normal force, bending moment, and shear force at your selected joint.

a.   Make conservative assumptions regarding loading conditions. Refer to Figure 1 (c).

b.   Will all fasteners at this joint carry the same load? If not, how can we ensure our design is conservative?

7.   Develop a preliminary design of the joint and select a set of preliminary fasteners from a supplier.

8.   Construct the spring model of the joints and determine spring rates, k.

9.   Determine the pretension force for the fasteners.

10. Produce the spring rate diagram for the fasteners.

11. Conduct a failure analysis of the fasteners, including

a.  the factor of safety against fastener yielding (load factor nL).

b.  the factor of safety against joint separation n0 .

c.   Examination  of the  possibility  of  shear failure. Consider all  modes of shear failure. Discuss the role of pre-tension during your shear failure analysis.

12. Discuss your results and conclude whether your proposed design is suitable for the application. If not, re-iterate.

13. For your final design, specify the size, property class, length, and pre-load for the fasteners, as well as the diameter, depth, and threading requirements for the holes required.

Important tips:

1.   The steps above are just suggestions to help guide the design process. They don't have to be followed in a rigid, step-by-step manner.

2.  This assignment is designed to be a practical engineering exercise that emphasises conservative design principles over exact numerical precision.

3.   Feel free to use computer tools, such as the “measure” function in SOLIDWORKS, to help you analyse the geometry, weight, and centroid of the structure.

4.   Make conservative assumptions and justify them wherever needed.

Deliverable

You are tasked with the responsibility of preparing a comprehensive report that outlines the outcomes of your investigation. The report should contain the analysis and decision-making  process.  This professionally formatted document will concisely summarise your findings and provide all the necessary engineering documentation required to verify your conclusions. The report will have, at a minimum, the following sections:

•    Title Page

•    Executive Summary

• Table of Contents

• Introduction

•    Main Body

•    Conclusion

•    Bibliography

•    Appendices

•     Design calculations.

•    Catalogue excerpts.

•     Engineering drawings,

• Design tables and charts from the textbook, etc.

A detailed guide on how to write a design report can be found in “Project Report Guidelines” on Moodle (the guidelines for the project report of your Gearbox Prototyping Assessment).

Formatting

• Formal language: Engineering reports should be written in third-person narrative. Avoid the use of informal and personal language, such as “I think … .” and “We did … .” . Instead, it should be “Something was performed to … ”.

• Figures and tables: Wherever possible, you should use figures and tables to convey information. It is significantly easier to refer to a figure or table than to read half a page trying to describe something. However, whenever you include figures and tables, you MUST always introduce and refer to them. Never put in a figure/table without explaining what it is and its importance to the analysis using text. A small figure is a useless figure. Ensure that the information is concise and easily read. All figures and tables must have proper captions.

o For figures, the caption should be below the images.

o For tables, the caption should be above the table.

o Proper references should be added to the captions of figures and/or tables if necessary.

• Page limit: Your report must not exceed a maximum of 15 pages, from the introduction to the conclusion sections. Your report should only be as long as required to convey all the information needed concisely. Do not write filler or irrelevant material, as it detracts from the professional tone of your report. A short, concise, to-the-point report that details everything you need and nothing more is a joy to read. Marks for this report will be awarded on quality and not quantity of the work.

• Multi-level headings are standard practice in engineering reports and assist in creating the

table of contents. Please be sensible with the number of levels, especially in a short report like this one. Three levels should be ample, e.g., “2.1.3 Preliminary Fastener Selection” .

• Page numbering: The title page should not have a page number, but everything after that

should. All page numbering before the introduction should be in Roman numerals (i.e. i, ii, iii, iv, etc.), with the numbering switching to numbers at the Introduction (the introduction section is  on page 1).

• Referencing: You may choose any referencing system (e.g., Harvard, IEEE, etc.) you like, but please ensure all information sources are referred to, and the referencing style is consistent throughout the report.

File Submission

The assignment is due at 11:55 pm on Friday, Week 8. You must submit your .pdf file to the Moodle submission box before this time.   During submission, name your files in the following format: zID_FastenersAssignment.pdf. E.g., z5160675_FastenersAssignment.pdf.

Work submitted late without an approved extension is subject to a late penalty of five percent (5%) of the maximum mark possible for that assessment item, per calendar day. The late penalty is applied per calendar day (including weekends and public holidays) that the assessment is overdue. There is no pro-rata of the late penalty for submissions made part way through a day. Work submitted after five days (120 hours) will not be accepted and a mark of zero will be awarded.

For example:

• Your course has an assessment task worth a total of 100 marks (Max Possible Mark)

• You submit the assessment on time and you get 60/100 (Awarded Mark)

•    You submit the assessment 1 day late and the late penalty of 5% per day is applied (5% deducted/day from maximum possible mark for that assessment item)

• Your adjusted final score is 55/100.