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.