Span the gap project evaluation
Span the gap project focused on with
finding a way to get from A to B, we were delegated to span the gap with
certain materials. The gap is 1500 mm and no single piece of material is that
long, except for the string. The aim was to build an engineered solution that combines
the materials together which successfully spans the gap and holds fair amount
of weight, testing would be done to signs of failure and destruction, initially
the weights will be applied in the middle of the structure on a compulsory
platform.
First step to successfully solving this
problem was to analyse the materials we were given, their properties and
possible application to the structure. Our group has somewhat managed to look
at the different use of materials and their properties. We have looked at how
we could incorporate the materials; we thought of that the primary material
would be foam board; three pieces together spam across 2400 mm, we immediately
focused on foam board whereas we should have considered the use of other
materials available.
Our group have come up with some initial
ideas in the class, further research was carried out at in our own times. First
idea was based around the use of arch, my initial thought was that arches are
build from the ground upwards and our brief did say that we can only go 150 mm
below the table. Second idea was based around the beam, I figured that it has a
great compression and tension resistance and would hold good amount of weight.
Donnie came up with an A frame like structure. Rest of the group have analysed
the designs and some suggestions were made, for instance bird mouth and
positioning the structures in a way in which they would use the tables to take
some stress from the weigh applied. Another suggestion was concerned with
engineering solution to connect the pieces of foam together. One was the use of
strings and another was to incorporate finger joints. We all decided to go with
two ideas, the arched bridge and the beam. After we all dismissed further
individual research was made. I consider this stage as successful as everyone
was involved and we managed to come up with three good ideas.
Selection of our final design was made
during model making, only when we went on to create prototypes we realised what
will work and what will fail. We delegated different tasks to members of our
group. The beam idea looked very promising on paper but when we went on to make
it everything changed, we came up with issues of connecting the materials, Rhia
suggested we use the thread to pierce through the foam. The beam successfully spammed
the gap although it was visible right away that the structure started to bend
in the centre, it's first sign of failure.
Doug's idea was to incorporate finger joints and use the
thread to lock them. Once the making of this prototype was done we tested it
out with water bottle, it managed to hold the weight pretty well therefore we
decided to put a brick on it, it had some success too however it started to
lean towards one side. Masoud suggested we think about depth of our structure, spreading
it ends out would help. This led us towards making new model and here is where
most issues were exposed.
Myself and Donnie volunteered to build the final model, firstly
our prototype used 4.5 pieces of foam board, we were delegated a specific
amount of materials and we went over the limit. This showed that we didn't really
look at our material list and sizes whilst building the prototype of the arched
bridge. Justification and selection of the design was an unsuccessful stage as
failed to communicate, stick to material list and take on board important tips.
Changes we made to the design included
the heights of the arches and the depth of the structure. We kept the lengths
the same although the centre of the structure would be wide enough just hold
the platform where as the ends would spread, reusing the foam we had, waste we
had when we cut out the arches, almost semi circle looking like pieces, we used
that to spread the ends although on the other hand another issue came up,
hinging of the joints and length of the structure. Form of the structure didn't
have much influence on buckling resistance, previously a brick (roughly 2 kg)
made the first model buckle, our final structure started to buckle at 1.5 kg, but
there is a lot of factors to consider such as amount of materials used. Since
it can't be properly measured the importance of this element is low. What we
didn't consider was the length as the structure has became shorter due to the
fact that we changed its form, it meant that the structure will be go into the
tables 10/20 mm, in case heavy load will be applied onto the centre, the
structure will just collapse due to the fact that it's too short. Therefore we
went on to design bird mouths that would extend the structure by extra 185 mm on
each end. We incorporated bird mouth
although it wasn't on an angle, it would extend the structure. The importance
is low as we didn't really need to make bird mouths as there are other ways to extend
the structure, it would have scored higher if it was on an angle which would
have helped to take some stress away from the structure.
Finger joints were changed, we decided
that cutting the joints to the exact dimensions was wrong, therefore we decided
to cut them with tolerance meaning that the foam board will have to be pushed
in with some force, it made it stronger and there was no need to use the
thread, this principle was also used when we created braces for the bridge,
finger locks came in very hand and I would list that as the most important
technique in our structure. Both score high in terms of importance as the
technology behind the idea was visibly successful during the application of
weigh, they both strengthen the joints and all different elements come together
as one solid structure. This stage was successful, we managed to develop our
technologies and incorporate better solutions.
We had to estimate
the weigh our bridge will be able to hold, we all agreed that 2 kg is reasonable
weight. The structure got to 1.5 kg quite comfortably without any signs of
failure but past this point the joints became very loose, although the bracing
helped to hold it together, at 1700 g the structure began to lean towards one
side, one end was countering the other and it put a lot of stress on the middle
section, the joints didn't break due to the braces, they started to hinge which
was causing the twisting and later on fail of the structure at 1900 g, only 100 g
away from the estimated weigh.
After the test I carried an extra research and read through
few books, the structure was based on the arches and due to the nature of the
foam board it's more resistant to tensions and compression when positioned upwards
beam like. We overlooked the principle of torsion and twist, hence the bridge
started do buckle to one side and eventually snapped in the middle. Some good
points we had were the joints, the use of low tolerance prevented the finger
joints from coming apart and the bracing did help to slow down the hinge
movement that would caused the failure, perhaps more material in the centre
would make it less vulnerable, it was something that was mentioned early on by
our lecturers.
The only material
used was foam board, we only used 5940 cm2 in our structure. Without making any
big changes to the structural form of the design we could have added support to
the middle of the structure as we overlooked a clearly visible weak spot of the
structure. Adding the deck from the beam idea but only in the middle of the
structure and instead of having two elements spreading the ends we should have
incorporated joists, these would help to shape the structure without making it
any more weak. The approach we had to designing the final element should have
been different, we should have considered doubling the bracing on the joints,
this way it would prevent the hinging that appeared later on in the test.
Another point that should have been incorporated the rest of materials, paper
and thread could have helped to support the centre as it was the weakest point,
layering the centre or using the platform more effectively, perhaps making it
an internal part of the structure that would prevent the twisting in the
centre. This was a good experience as it gave us an insight of how important
organization, time management and communications are within our industry.
