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