CALLING engineers/archi's help, on IDEALISATION (1 Viewer)

[xxPanDa]

can someone explain to me what idealistion is. in the context of structures.
i may have gotten the wrong definition of idealisation, cause i dont understand this question

"Make a sketch of support and part of the span of the bridge, explain how the support is IDEALISED"

im doing the gladsville bridge (heres a pic)
it would be great if u could give me an example

due friday(tomrow)

 

[drewbrow1]

Cmon, this is what you do in the first week of statics, it's the first step in every structural problem you'll ever do.

K, you know how arch bridges work? They take some force and transfer it to the supports. The internal actions in the arch are mostly moments, but through the majickal powers of the arch, they are transformed into axial (compression) forces at the supports. So you could say that your supports were pin or roller supports (roller makes analysis easier, cos then you know the direction of the force etc etc).

The strut thingies probably carry most of their load as compression, so you can assume they're pinned at both ends (ie no internal moment). Be careful though, you don't want to draw the pin joints as part of the arch because that would imply that the arch doesn't carry moment.

For bonus marks, find out a bit about how the pretensioning of the concrete in the arch strengthens it in compression.

 

[xxPanDa]

THANX man! i appreciate the drawing,
however i came up with something a bit different:


looking at the pic of the bridge, i see the outer support is fixed onto the grass,

(however, i always see in system diagrams, they put a pin or rollers in the diagram, even though u cant see one in th REAL pic, is that alright?)

I THINK the road on top of the bridge, is made up of two roads
the triangles are my lazy version of rollers

what i really didnt understand was "explain how the support is IDEALISED"?!?!
wth does that mean?

 

[sweet-g]

idealised = an idea that is "lised"

 

[drewbrow1]

mm, both models would be acceptible i guess.

I actually read the question now. Pick any support. Say what type of support you would make it in your model, ie fixed, pinned or roller, and WHY. Then draw the 'real' joint next the the 'ideal' one.

Eg where the struts on the very end plug into the grass.

I say it is pinned, because those struts transfer most of their load in compression, so modelling it as a frame member (ie fixed-end (moment carrying) member) would just be extra work for not much more accuracy in deflections, reactions and internal forces.

You say it is fixed. More work for you in the analysis bit : )

You can spend 20% of the time getting 80% of the answer, then the other 80% of the time getting the last 20% of the answer. I'll stick with getting 80% of the answer : )

 

[xxPanDa]

mm, both models would be acceptible i guess.

I actually read the question now. Pick any support. Say what type of support you would make it in your model, ie fixed, pinned or roller, and WHY. Then draw the 'real' joint next the the 'ideal' one.

Eg where the struts on the very end plug into the grass.

I say it is pinned, because those struts transfer most of their load in compression, so modelling it as a frame member (ie fixed-end (moment carrying) member) would just be extra work for not much more accuracy in deflections, reactions and internal forces.

You say it is fixed. More work for you in the analysis bit : )

You can spend 20% of the time getting 80% of the answer, then the other 80% of the time getting the last 20% of the answer. I'll stick with getting 80% of the answer : )

lol its a PIN than!
ahahah thanx!!