Subject:
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Re: My newest creation, Hell Gate Bridge NY
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Newsgroups:
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lugnet.trains
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Date:
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Fri, 8 Dec 2000 14:09:31 GMT
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Viewed:
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2606 times
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See I learn something new every day. I know geometry but not much about
structural engineering, so I saw an arch and figured it was an arch bridge.
But now I have been inlightened, cool.
jt
In lugnet.trains, Scott Arthur writes:
> In lugnet.trains, Larry Pieniazek writes:
> > In lugnet.trains, James Trobaugh writes:
> > > Looks like the arch is made from 4.5v curved rails, right Arne? I really
> > > like the look of it, very sturdy and even looking. Nice job.
> > >
> > > I'd have to argue with the "two" arch bridges though, I have a technic arch
> > > bridge that I made earlier this year.
> > >
> > > http://www.ngltc.org/GRNBRG2000/image6.htm
> > > http://www.ngltc.org/gats2k/image19.htm
> > >
> > > It not a steep arch, but an arch just the same :)
> >
> > Well, no, I'd have to categorise it as a type of through truss.
>
> I was going to leave this, but now that you started...
>
> I agree that is not a arch, and I expect that is was modelled on a truss.
> However, I'd call it a frame as it is pin jointed (trusses, I think are
> not). However, to be a frame all the members have to be in _pure_ tension or
> compression (ie no bending moments). This is not the case with James’s
> bridge. Additionally, the frame is statically indeterminate as it is not
> "perfect". To be a perfect frame:
>
> m=2j-3
>
> m=the number of members
> j=number of joints
>
> If it were a real structure, I’d expect it to have considerable movement.
>
> Basically, the problem is, it is not constructed from triangles. Despite
> that, it is fit for its intended purpose, and it does look very good.
>
> (This is mostly from memory, structural engineering was part of my UG
> degree, (I did peak at my copy of Durka{1} to get terminology) - I am open
> to correction on this. )
>
>
> >
> > Hell Gate and Sydney Harbor Bridge, for example, are suspension arch
> > bridges. Essentially all members below the arch are in tension and the deck
> > is suspended from the arch. (The arch itself is usually either done as a
> > truss, with some members in tension and some in compression, or as an all
> > compression solid or fabricated structure)
> >
> > Steve's bridge, and the Rainbow Bridge at Niagara Falls, for example, are
> > cantilever arches... Essentially all the members of a cantilever arch are in
> > compression, with the deck carried above the arch.
> >
> > Trusses are characterised by a mixture of compression and tension members.
> > Unless all the members below the arc of the top chord are in tension,
> > (including the deck stringers) it's a truss, not an arch. I believe.
> >
> > I posted a link to a site that gave good bridge information in this group
> > some time in the past, but I'm too lazy to dig it up again.
> >
> > A key point here, though, is that the model that started this discussion is
> > very very nifty.
>
> it is indeed.
>
> Scott A
>
> {1} Durka – almost schoolboy level SE
> http://www.amazon.com/exec/obidos/ASIN/0582251990/qid=976273366/sr=1-5/106-5958974-3391625
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Message is in Reply To:
 | | Re: My newest creation, Hell Gate Bridge NY
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| (...) I was going to leave this, but now that you started... I agree that is not a arch, and I expect that is was modelled on a truss. However, I'd call it a frame as it is pin jointed (trusses, I think are not). However, to be a frame all the (...) (26 years ago, 8-Dec-00, to lugnet.trains)
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