Timeline

Here is my initial timeline. It shows what tasks i need to complete and the important deadlines. I hope it will help me to stick on track.

Progress so far

What I know so far:

• Girders are used because they are efficient ways of supporting loads over open spaces.
• The top member is in compression, the bottom member is in tension. Members act differently under forces when in compression or tension.
• Some lattice arrangements are stronger than others: the strongest girders spread the load  most efficiently (already seen in initial trial experiment).
• Assuming pin joints (strains and bending moments belong  to one member only) and assuming the materials are within their elastic limit, the forces in the members can be easily calculated.
• Deflection must be taken into account when designing trusses. Deflection is more likley to be influential on design than the actual failure limit. In real life girders and bridges, the acceptable deflection limit is between 1/1000 and 1/500 of the truss length.
• The strength of an individual member depends on its size, shape and material properties.

Questions to answer through research:

• Background knowledge required to design the trusses:
• can we use plastic (elastic vs plastic behaviour)
• how to calculate deflection in a lattice girder
• behaviour of members in compression
• behaviour of members in tension
• how to fix the beam at each end
• height depth ratio
• effect of having top and bottom beam as a continous member
• Continous member:
• easier to make
• easier to keep strength constant
• distortion of forces
• easier to controll variables
• Multiple members:
• closer to reality
• harder to construct
• connections add to error analysis
• harder to controll variables

To do:

• Drawup potental ideas and theoretically calculate the forces
• Refine/define research question
• Design the definitive experiment; size; shapes; material; rig
• Find equations for deflection
• Use EBSCO to find relevant articles

Things to consider for the experiment:
Variables:
Independant:

• angle between interior members and the straight tom and bottom members
• need to calculate uncertanties
• should test a large range of different angles,
• 5 different angles (=5 different bridges?)
• 6 or 7 different masses (large range without breaking the bridge)
• relationship between deflection and angle (linear?)

Dependant:

• Deflection (in mm)
• Could use a motion detecter and data logging software to measure deflection to a high accuracy

Controlled:

• each individual angle must be the same
• elasticity of the material
• pin joints
• length depth ratio

Not controlled:

•  mass of truss
• amount of material
• single member distortion forces

Other progerss to note:
I collected some library books that were sent from zurich during the week, they are definatley relevant to my topic and are quite specific, giving me precise information. The books are called:

• Advanced analysis and design of steel frames, G-Q Li and J-J Li
• Design of building trusses, J Ambrose
• Simplified truss design, M Melaragno
• Roof truss guide, P Eichenberger

Mr Rawson showed me how to use the EBSCO database during our meeting. This is an extremley usefull tool as it contains many relevant academic articles, including articles on trusses and girders. This will allow me to vary the research media I use.

Testing possibilities

During the week I went to a library in muttenz that is part of an architectural and design college and who have  books relevant to my topic. They are on order form zurich and should arrive within a week. there was only one book that I could not get as it is a reference book in the zurich library, this book sounded quite relevant though, if needs be I may have to go to the zurich library to take a look.

This weekend I bought some materials to start tinkering with and to decide what is possible to perform as an experiment. I decided on using plastic as a material as it is readily available and has a slight flex, meaning it would not require as much force to observe deflection and stresses. I found a solution to making pin joints by pushing nails through holes in the plastic members to secure them (see photo). This seems to work quite well.

 One potential variable to investigate would be the force required for a truss to deflect a specific amount (see photo below for a potential style of experimental rig). Plastic seemed to be well suited to this as the trusses deflected under some weight, yet this was still minimal (I tested the trusses with masses up to 400g), suggesting that for the actual experimant I will have to uses much heavier masses.

In one of my research books I read that in real life, the limit to acceptable flex is between 1/500 and 1/1000 of the trusses length. For the experiment this will not be possible though as the maximal deflection on a 1m long rig would be too small to read. On reflection I will still be able to learn about how the structure of a lattice girder affects it's strength even if I have to surpass the real life acceptability limit.

Today I tried three different types of structure, including a warren truss (which I intend to do my actual research on).

I plan to make a time plan and to design a virtual model of a warren truss on SketchUp this week, as well as continuing the research so that in within the next few weeks I can prepare myself for the actual experimentation.

The Next Step

Today the other 3 books I ordered arrived. The book "invention by design" has a whole chapter on how some of the big bridges of today were built and what influenced their design (including a 1880's lattice railway bridge in new york and several other lattice girder style bridges.

I plan to make a timeline in the near future so that I can keep track of where I am in the overall process of  the project. For this I will ask mrs Round about any intermediate deadlines before the final hand in date.
I hope this will help me keep track.

Tomorrow I am going to a library at the school of architecture and design in Muttenz to see if i can find any more relevant resources.

I also plan to have a google sketchup design of a potential testable structure soon so that I can plan what I will need to obtain in order to perform the experiment.

I wonder if trying to get information form different types of data sources (e.g. articles, reviews...) would help me gain a better understanding of lattice girder theory than just using books and the internet? Perhaps the most usefull part of my preparation could be to do more calculations of the forces acting on each member in a lattice girder to gain a greater understanding in the basics? I will try both of these in order to prepare myself for stating my hypothesis wich i will have to do in the not too distant future.