University College Dublin
School of Mechanical & Materials
Engineering
MEEN10030 Mechanics for Engineers
Spring 2022 Group Assignment
Construct a lightweight space frame or three-dimensional truss
structure that will maintain a nominal mass of 500g at a height
of 0.25m above the ground.
By
Group 20
Dennis Bablyak / Number #21361861
_____________
Max Nolan / Number #21325043
______________
Gordon Ó Luineacháin / Number #21485474 _____________
Andrew Ksenych / Number #21310431
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Patrick Nolan / Number #21397066
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Seosamh Mac Briain/ Number #21462064 ______________
Date: 22 March 2022
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Contents
Design Concepts ...................................................................................................................................... 3
Final Design & Calculations ..................................................................................................................... 8
Materials & Testing ............................................................................................................................... 10
Discussions & Conclusions ..................................................................................................................... 11
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Design Concepts
Design 1
For this structure we looked at the structural strength of trusses and their capability to support
weight. The plan was to have two towers supported by trusses connected by a centre truss and have
the bottle of water suspended from this connection. This was inspired in part by a crane system but
having two sides instead of one to resist the moments at the bottom of one of the structures.
The straws would be bound together using lightweight tweed tied into a clove hitch knot. In theory
the weight on the structure pulling would be resisted by the trusses in both towers and should not
collapse inwards.
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Design 2
The primary purpose of the tripod design is to decrease the number of straws used to create the
space frame and bring the string to the forefront of the design. Furthermore, this design would
suspend the bottle of water below the point that all three beams of the tripod meet. Consequently,
it would be more accessible for our group to adjust the length of the string connecting the water
bottle to the frame to achieve the height of 0.25 meters above the ground.
The three poles would be attached together using two different knots, the clove hitch and the tripod
lashing . The clove hitch is used as a base for the knot, it prevents the rope from moving and allows
you to use adequate force to tie the tripod lashing. Once you have completed the tripod lashing, you
tie another clove hitch to prevent the two previous knots from unravelling.
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Design 3
This design was inspired by ancient Aztec pyramids that were constructed between 1000BC and
1500AD. One of the most important aspects of this design was that the top of the pyramid was
square and flat, which would be the perfect area to place the water bottle. Also, the weight of the
water bottle would be evenly distributed over all four triangular support and would lessen the strain
on each straw and the central shaft of the pyramid.
These are measurements for the truncated square pyramid design. The design requires four of the
pyramid triangles and the square top is the where the four pyramid triangles are connected. It was
important for the group to do some preliminary calculations to test the viability of each design. In
addition, it would indicate how many straws would be required to create this space frame.
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Design 4
This design consists of five triangular prisms with knots to provide tension and hold the structure
together. The equilateral triangle design was based on Warren bridge trusses patented in 1848. This
uses less members than Howe and Pratt trusses and therefore less weight. Diagonal members
alternate between tension and compression, with approximately equal forces acting on all members.
Each straw would need to be at least 14.43 cm in length to reach the minimum height of 25 cm.
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Design 5
This design drew inspiration from the Eiffel tower. Our goal with this design was to create a tall
tower with a triangular base capable of holding the water bottle .25 metres above the ground. This
design was the last to be prototyped and we incorporated many of the lessons learnt from the
previous prototyped. We learnt that the straws were too fragile, and we needed to group them
together to increase the structures overall strength. From the truncated square pyramid design, we
found out from that the more load bearing supports the frame, the higher the likelihood of the
prototype being able to support the weight of the tower. Additionally, from the same design, we
discovered that if we inserted the straws into another straw, it would dramatically increase the
amount of force that an individual straw could withstand.
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Final Design & Calculations
We decided to prototype each individual design to find their respective faults. We could then
improve the designs and eventually produce a viable option that would meet the requirements set
out in the assignment brief.
Design 1:
The issue with this design was that it had too little support in the vertical direction. The trusses
resisted any horizontal motion, but we only had 24 straws holding up the water bottle and
supporting its own weight. This was determined to be insufficient.
Design 2:
When prototyping this design, we discovered that the theoretical saving of straws would not be
realised because the straws that we used could not hold up the water bottle. We would have to use
up to eight straws to provide the support required for the water bottle. Therefore, we could not
achieve the primary purpose of this design, and we decided that this design would not be able to
hold a mass of 500 g.
Design 3:
When we created the first prototype of this design, we found multiple flaws. One of the faults was
the straws that we used were too brittle to retain the shape that we wanted when the weight was
applied to the top of the structure. An additional fault that we discovered was that connecting the
straws together using string would be very time-intensive and it wouldn’t be feasible to make the
design the size that would accomplish the tasks set out by the assignment brief within the time that
we had remaining.
Design 4:
We quickly found that this was not a suitable prototype as it could not support the mass required.
Perhaps this design would be feasible with more rigid straws. However, from our experience, the
straws bent and crumpled under minimal forces. This design could be utilised if we were to tie three
straws together for each truss, but this would increase the mass significantly, resulting in a poor
performance ratio.
Design 5:
We chose this design to base our space frame on because it could hold the required mass. We had
also already prototyped the previous four designs allowing us to amend any errors from previous
builds. In doing so, we improved the design before building it and saved construction time. We also
learned the constraints and strengths of the materials that we were using. For example, we
manipulated bendy straws to provide more structural integrity to the base of the structure by
inserting the end into the next straw. This design was the combination of all the design and
prototype work that we had completed and found it to be our most effective and reliable prototype.
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Dimensions of the final design are as follows: the tower height is 34 cm. At the bottom of the
structure, the distance between the joints of the octagon is 15 cm and the distance from the joints
to the tower is 17 cm. The red plastic straws at the bottom of the structure are 7 cm long. The
circumference of the tower at the bottom is 19 cm and 17 cm at the top.
Calculations:
After completing our final design, using the formula below we calculated our performance ratio (M);
The total mass of our structure= 120 grams
Performance ratio = 4 1/6 = 4.166666667
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Materials & Testing
While building the structure, it was noted that it was difficult to build a structure with the straws we
had available. They were far too flexible and bent when subjected to relatively small forces. More
rigid straws would have been much more suitable. Tying straws together was also an issue. When
knotted tightly, it was found that the straws crumpled and lost structural integrity.
When selecting the straws to be used for our chosen design, we looked at many different materials
along with different types of straws. While testing each straw material we looked for the lightest and
strongest of them all and certain types of straws that would best suit our design. Through trial and
error, we ruled out the use of paper/cardboard straws quickly. We found out that while they had to
be put under more force to bend then the plastic straws, which had much more elasticity, once the
paper straws bent it was nearly impossible to recover them back to their original shape which could
be done a lot easier with the plastic ones. In doing so, the plastic straws also offered much more
sway or mouldability to our design, then that of rigid one with the paper straws, so our water bottle
was able to easily slot into our structure.
The type of plastic straw(s) to be used in our final design was also considered, be it the length, using
longer or shorter straws, the girth, with skinny or fatter straws, and even the flexibility, whether the
straw had a bend in it, for each element of our structure this was all looked at to give us our best
possible design and in turn optimum performance ratio, limiting excess straw weight.
We tested many different designs in the run up to our final design and learned from each one. Bits
and pieces from our previous designs then helped us to create this, such as the base of our final
design was inspired by the pyramidal base of design three, we then put an extra twist on it for added
support.
We also investigated doubling up straws or fitting straws inside of each other by using different sized
straws, this nearly worked out better than using string altogether especially on the shaft of our final
design.
After having chosen our straws, we then needed string, natural or synthetic, which we both tried in
our design process. We realised fishing line would’ve been the best to use as it is extremely light and
tough, however this was not readily available to us, so we chose to use natural string instead which
we had an abundance of.
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Discussions & Conclusions
We realised in undertaking this project that while we were able to produce creative builds in our
design stage, the construction of these designs was quite challenging. Having discovered that using
trusses offered the best performance ratio for our structure we tried to prototype them. However,
using only string and straws to make some of these optimum designs was near impossible.
Therefore, for practical purposes we were forced to sacrifice a high-performance ratio to create a
suitable design.
To reach a final design faster and more efficiently, the sketching part of the process could have sped
up prototyping if more viable designs were in greater detail and entailed more information such as if
the structure would functionally work. As well as that, prototyping with tape instead of using string
would have reduced time wasted on flawed designs. Finally, choosing the right material of straw that
would have more strength and be more flexible could have allowed us to create a design with less
mass and stronger supports for the water bottle.
Also, we believe that excess time was spent on the building process of our structure and not enough
on the design process. We spent too little time designing unsuitable structures and too much time
attempting to build them. If completing a similar project in the future, we would allow more time for
the design stage and question whether structures would practically work. This would significantly
decrease time spent on the project.
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