Year 12s: what will be the first way you research your future options? >>

Model Bridge Construction

Hello all in TSR!

I am currently undertaking an engineering module as part of a Construction Mgt degree and have been asked to construct a model truss bridge to span a 700mm gap with a point load of 90N. No restriction on height or width or choice of material. It has been requested that the bridge fail when the 90N point load is imposed (or as close as possible)

From initial research online, I intend to make it from popsicle sticks but have concerns about achieving a pinned joint with this material as glue is not preferred by our lecturer. We are also requested to provide our calculations on how we arrived at the chosen design.

I like to ask if anyone can provide any tips as to how to start this, as our only experience to date has been with some simple truss analysis and I seem to hitting dead-ends with my online research.

Thank you in advance for reading this and would welcome any advice/suggestions

Regards
John.

Reply 1

a10

From what you have written in your OP you have identified the main function and the constraints of the bridge structure, now you need to consider what is the best material to meet those particular constraints. What materials are you allowed to use?

Ideally you'll want to build it in a triangular (pin jointed frame) because this provides the greatest stiffness. You will also have to take into account the cross sectional area of whatever material you use because this will also affect the stiffness.

Reply 2

thefish_uk

Original post by Maquel

From initial research online, I intend to make it from popsicle sticks but have concerns about achieving a pinned joint with this material as glue is not preferred by our lecturer. We are also requested to provide our calculations on how we arrived at the chosen design.

Could you not just drill holes through the ends of the sticks and use bolts? This will work for connections of sticks which are in plane. Out of plane connections will require a bit more thought.

Make sure you have considered buckling. Assuming you are planning on carrying out tests to get a strength for the sticks, do separate tension and compression ones (wood is well known for having different properties in each direction), also for your compression one make sure the pieces you test are the actual length of how they will be in your truss so if they fail by buckling this is taken into account. If they are not strong enough then double them up.

Reply 3

pleasedtobeatyou

Original post by Maquel

...

You need to specify the criteria for failure. Would than be when a certain bar in the truss structure reaches a certain proof stress or is it at final fracture?

Reply 4

Maquel

Thanks everyone for the helpful replies.

Our lecturer is recommending that we use a bar in the middle so the 5kg weight can be hooked on to it and when this load is applied the bridge must break.

There are no restriction son the material to use. The popsicle sticks seem to be the most readily available and have been used by past students.
At the moment I am attempting to establish the compressive strength and tensile strength of a popsicle stick.
I will also test for buckling as the design progresses.
Would a cable stay bridge be easier to design?

Reply 5

thefish_uk

Original post by Maquel

A cable stay bridge is probably not the way to go about it, they are really meant for distributed loads over long spans - with your point load in the middle, most of the cables won't be doing anything. They also require the cables to be anchored behind the posts and the anchorages need to provide a horizontal reaction. For something in the lab like this you really want something which can sit on simple supports, i.e. a truss.