Design And Build

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					To Do list:

    
       Introduction
       Background research
       ---Different types of bridge Needs a lot more shit written about it
       Our Bridge Design
       ---Design 1
       ---Design 2
       ---Design 3
       Experimental work
       ---Rolling Tubes
       ---Joints
       ---Testing the strength of tubes
       ---Testing Design 1
       ---Testing Design 2
       ---Testing Design 3
       Calculating expected breaking load
         and faliure point



Design and Build Assignment Group
Project


Underpinning Concepts
As a starting point for the design of the bridge, existing designs of bridges were studied
to discover the most suitable design to use. A bridge often uses several of these
features:

Types of bridges:


Arch bridge

The downward load on an arch bridge is transferred to the abutments. Many arch
bridges just use this effect so only rely on the compressive strength of the materials
used. Wide spans are harder to build.
Constructing a strong arch out of paper may be too complicated

Beam
The beam utilises the tensile strength of the material used, usually wood steel or
reinforced concrete. It is not the most efficient way of carrying a load although the I
beam (rolled steel joist) uses

Trestle

Cantilever

Suspension/Cable stays


Advice from second years
By asking some people who did the bridge construction project last year it was
ascertained that the main trip-up points were from trying to make a bridge that was too
intricate and complicated. As structures




Experimental Modelling


I beam
An attempt was made to build a simple style ‘I’ beam bridge, as shown in the diagram
below. As the top of the beam would be in compression and the top in tension, it would
take advantage of the paper’s high tensile strength at the bottom, with the. The vertical
parts are corrugated to provide rigidity.

The final product was a mess and was a lesson in how incredibly fiddly an intricate
design would be. The structure did not behave how it was supposed to and failed under
a tiny load. A single tight-rolled tube as described below could take much more load
despite using around a fifth of the amount of paper.
The lesson learnt was to use very simple and bulky designs that in no area rely on a
paper member that is just one or two layers thick, and that it is far beyond our skill to
build a bridge that requires many accurate shapes or precise dimensions.
For these reasons, box sections, although probably feasible were not attempted, and the
focus was put onto building a simple structure like an ‘A’ frame, using rolled paper.

Rolled tubes
The first tubes were rolled using a pencil as a dummy to keep the centre hollow. These
tubes were found to be very loose, and therefore very soft and prone to buckling and
denting. Just applying prit-stuck to these tubes would cause them to be dented.


By rolling from the corner of the paper it was possible to roll the paper very tight, as
there are only 2 points on the paper that is ‘joining’ the roll. These tubes were very hard
and strong relative to the previous design, but the very ends of the tube were only one
layer thick, steadily increasing to maximum thickness at the middle.
The ends of the diagonally rolled tubes were weak, and in the design they would be the
points where the load is applied onto the bridge and table, and at nodes. To strengthen
these, tubes were experimented with that had patches applied to the corners that would
end on the outside. Patches were applied each with a decreasing size to try to avoid
lumps and kinks forming but unfortunately when it was rolled the patches would roll at a
different speed to the paper underneath, causing it to clump up, and squeeze glue into
puddles. This is because the paper was rolled so tight. An example of this problem is
shown below:




Futher attempts to make the beams stronger were to add a roll of paper to the insides of
the ends of the beams, and by cutting up the paper before hand into a shape that when
rolled diagonally would contain plenty of paper all the way to the end, albeit shorter.
Joining 2 sheets of A3 together with a small overlap caused the same problem as when
patches were done - the paper would slip and glue become squashed into pockets in the
paper.

The final solution, used in the final design of a bridge was to have sheets of A4 butted
together, but not joined, and to roll them as one sheet, using plenty of glue to ensure the
separate sheets in the roll stayed as one – THIS IS NOT QUITE TRUE!!

Design1:

The first design is shown below. It
Calculating the theoretical maximum load on the
bridge
Calculating the downwards component on the beams

Maximum Compressive Force on the Beams
The tubes for the four main supports were built using the same size piece of paper and
using the same technique as shown below where:
a = XXmm
b = XXmm
c = XXmm
the dotted line shows the direction of the roll

Using the formula supplied in the assignment briefing, the maximum loads were
calculated thus:



where:
E = young’s modulus of the paper = 1.8109
L = 33cm = 0.33mm
As the tube was rolled diagonally, the middle of the tube will be thicker than the ends.
The likely failure will be at the weakest point, which will be at the thinnest part of the
tube, at the end. Therefore the values for the tube’s thickness have been taken from the
end.

Put the maths in here….
F = 492.66N

Maximum Tensile Strength the Beams Can Take

The load is a point load, at one end of each beam. As the tubes were thickest at the
middle and thinnest at the ends, due to them being rolled diagonally, the tube would
have a different strength depending on where the point load is placed.



Introduction

Background research

---Different types of bridge

Our Bridge Design

---Design 1

---Design 2

---Design 3
Experimental work

---Rolling Tubes

---Joints

---Testing the strength of tubes

---Testing Design 1

---Testing Design 2


Testing the prototype
The theoretical maximum load the bridge can take is:
The expected failure point (at a load far below the theoretical load)

Calculating expected breaking load

and faliure point

				
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