The Humber Bridge

Q. According to various sources the Humber Bridge deviates in distance horizontally by 36mm. There are quotes such as "The bridge towers are 36mm (1.4 inches) further apart at the top than the bottom to take account of the curvature of the earth." How is this explained?

A. The pylons were built vertically in relation to the earth. The pylons were built to be exactly the same height. The difference in horizontal distance at the top of the pylons is only a theoretical figure for what the difference should be if the earth were a globe.

No physical measurement of distance deviation was ever detected on this or any other bridge. Forum user "Niceguybut" once tried to champion the cause that there was a physical, detected difference on the Humber Bridge. Here were his results:

Niceguybut wrote:

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"I once tried to champion this cause, and in the interests of getting a definitive answer, I emailed the Humber Bridge Authority to ask whether the figure was measured or purely theoretical. Here's the reply:

  “ Thank you for your recent email.

The two towers are build vertical to a tangent to the earth, i.e. radial to the centre of the earth, thus, theoretically, the shape between the two towers is an inverted trapesium rather than a rectangle with the length between the bottom of the towers being 36mm less than the length at the top of the towers.

The gap at the base is, of course, the one that was actually "measured" with the apparent increase being a result of building the towers "vertically".

Regards

Peter Hill
General Manager & Bridgemaster ”

So there you have it, straight from the horse's bridgemaster's mouth. I'm man enough to admit I backed a wrong 'un, so can we let this one go now?"

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