Johnny B Good Wrote in message:
On Wed, 21 Mar 2018 17:48:30 +0000, Jim K wrote:
Johnny B Good Wrote in message:
On Tue, 20 Mar 2018 21:06:43 +0000, Jim K wrote:
Johnny B Good Wrote in message:
On Tue, 20 Mar 2018 16:27:37 +0000, Jim K wrote:
Johnny B Good Wrote in message:
====snip====
Sadly, I've not been able to find an "Earth's curvature
compensation"
figure for the slightly shorter Severn bridge (1 mile span versus
the Humber bridge's 1.4 mile span), just that 388mm[1] 'lean back'
on the towers during construction to compensate for the suspension
cable forces.
It might have been mentioned in the Timewatch/Timeshift documentary
on this record breaking bridge but I've not got the time right now
to spin through the recording just to find out whether or not a
figure was actually mentioned.
Why would any bridge need to be 100% "flat"?
It had nothing to do with making the road deck "100% flat". The
'lean
back' was to compensate for the final cable loading so that the
towers would become perfectly aligned to the vertical downthrust
forces once construction had been completed. I dare say the same
thing applied with the Humber bridge.
--
Johnny B Good
I know.
You said,
"Sadly, I've not been able to find an "Earth's curvature compensation"
figure for the slightly shorter Severn bridge (1 mile span versus the
Humber bridge's 1.4 mile span),..."
Hence my question - why would anyone bother being *that* precise
building a mile long bridge?
The precision will be a consequence of making sure the towers were
precisely aligned to the downthrust forces in both cases. Assuming the
Humber bridge's vital stats were a factor of 1.4 scale up of the Severn
bridge design, my best guess for the divergence at the top of the
Severn bridge's support towers would approximate to a figure of 18mm.
Are you trying to suggest that a mere 30% shortening of bridge length
compared to its successor, the Humber Bridge justifies a disregard for
the same level of precision that was deemed necessary for the 40%
longer bridge?
You have to consider that the contractors working on the shorter
bridge
weren't just 'anyone'. I'd expect they were aiming to achieve a
divergence that matched the theoretical divergence due to the curvature
of the Earth as a consequence of taking the 'lean back' measures to
make sure the towers were in perfect alignment with the down thrust
forces once the cable loading was applied. Whether they were able to
measure it to such accuracy and whether the measurement matched
expectations is, for the time being, an unknown since I couldn't track
down any mention of divergence due to the curvature of the Earth.
--
Johnny B Good
As is obvious, I'm not talking about tower divergences.
True, you seemed to have an obsession with 'flatness' in spite of my
only wondering why I couldn't find a divergence figure to compare with
the one for the Humber Bridge which Tim+ had managed to find.
I'm merely pointing out (again) that your basic assumptions about
bridges needing to be "flat" (and so obsessional adjustments of tower
verticality by mm over miles to adjust for earths curvature) appear
erroneous ...
It was not I who was obsessed with bridges needing to be "flat", it was
you. I think you are conflating Brian's mention about 'flatness' and
curvature of the Earth corrections with my not being able to find a
curvature of the earth divergence figure for the Severn bridge to compare
against the figure that Tim+ had found for the Humber bridge. I made no
mention of 'flatness' until you introduced it into this sub-thread.
--
Johnny B Good
Oh dear. Put very simply, if one wasn't trying to build
something flat, why would one be bothered with calculations to
the mm to allow for curvature of the earth ? !
--
Jim K
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