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 11:08:28 GMT
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Viewed:
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957 times
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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 has 3 Replies:
 | | Re: My newest creation, Hell Gate Bridge NY
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| I'm not a civil, thanks for refining my critique. (...) Oh, indeed, it looks nifty. It's just not an arch. I'm not competent to comment on whether trusses can't be pinned, though. While we are identifying arches, though, I thought Ben "April Fool" (...) (24 years ago, 8-Dec-00, to lugnet.trains)
| | | Re: My newest creation, Hell Gate Bridge NY
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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 (...) (24 years ago, 8-Dec-00, to lugnet.trains)
| | | Re: My newest creation, Hell Gate Bridge NY
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| The yellow Technic bridge is a bow truss. It is not an arch. Truss members are either in tension or compression and truss joints are assumed to be pinned or hinged. An arch works in compression. All of the bridges mentioned are cool! Build On! John (...) (24 years ago, 10-Dec-00, to lugnet.trains)
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Message is in Reply To:
| | Re: My newest creation, Hell Gate Bridge NY
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| (...) Well, no, I'd have to categorise it as a type of through truss. 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 (...) (24 years ago, 8-Dec-00, to lugnet.trains)
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