Monthly Archives: April 2021

Wild inaccuracies in the plans for the Queensferry Crossing

As I blog tonight, I fear that there are no accurate plans for the Queensferry Crossing, not held by Transport Scotland anyway, maybe not held by anyone.

I’ve gained that impression because the distance figures in the one “AS BUILT” engineering drawing provided to me by Transport Scotland bear no accurate resemblance to a photograph of the bridge.

Here’s part of the “STAY CABLES ARRANGEMENT” drawing

and taking a closer look at the blue box area I’m drawing attention to, I have annotated what the drawing is claiming the stay distance figures were built as.

I’ve used red ink for the identification numbers of the stay cables and for the North Tower Midline and black ink for the figures which the drawing claims are the distances in mm between them.

 

So the drawing claims that the distance between the ends of cables 501 and 502 is “4.75 metres shorter (30% shorter)” than the distance between cables 502 and 503, which isn’t how it looks in this photograph, at all!

The photograph distances are in pixels but regardless it is clear enough that that distance between 501 and 502 (120 pixels) is not in the least “shorter” than the distance between 502 and 503 (117 pixels).

So something’s not right and the drawing has many other issues with it, like this lack of alignment between its figures and the features in the drawing.

There is also a typo “162000” instead of “16200” and the length numbers don’t add up in places. The drawing presents as a shoddy effort with no accuracy and no credibility. If this drawing is typical for the other engineering drawings then Transport Scotland have no plan nor clue as to what accurate size the Queensferry Crossing has been built to.

I’ve written to Transport Scotland suggesting that they might want to hire a surveyor to draw up some accurate plans for the Queensferry Crossing because without accurate plans it is not possible to know for sure if something of safety concern ever changes with the bridge that may need urgent remedial action.

Ideally, Transport Scotland would now commission for the Queensferry Crossing the same 3-D laser scanning survey which was done in 2015-16 for the 3D Forth Bridges Project, but before the Queensferry Crossing was constructed.

I don’t think we should wait until the Queensferry Crossing develops the sort of lean that the Leaning Tower of Pisa has, or some other serious problem that everyone – even without an accurate plan – can notice, before they call in the structural engineers to make the bridge safe, do you?

UPDATE 21st May 2021

I’ve received an email reply from a Transport Scotland official and he claims that regardless of the fact that the wildly inaccurate drawing clearly states “STAY DISTANCE”, as I have highlighted with red ink –

– the distances given do not refer to distances between the stay cables at all but refer rather confusingly to the lengths of the individual deck segments which are not drawn. Whatever. 🙄

Transport Scotland have now provided me with detailed stay distance information extracted from other drawings, which was nice. 🙂

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Queensferry Crossing stay cable strands – insulating timetable

Transport Scotland’s recent release to me 😀 of the details of the number of strands in each stay cable of the Queensferry Crossing allows me now to update my “Restranding Timetable” to allow the strands to be electrically insulated so as to carry a heating current for de-icing purposes.

The number of strands in the bridge 21,186    100%
To be replaced with a new longer strand 440 2%
Replace with an old strand from a longer cable 7,764 37%
To be left as now 12,982 61%

Insulating Timetable South Tower Stay Cables

Click for high resolution

Insulating Timetable Centre Tower Stay Cables

Click for high resolution

Insulating Timetable North Tower Stay Cables

Click for high resolution

Note that the south and north towers stay cables would take longer to insulate / re-strand because the bridge designers apparently specified that those towers’ shortest, most vertical, most efficient cables with the easiest work-load should paradoxically use the strongest of the bridge’s stay cables.

Being shorter those cables are better re-stranded last so that they may re-use strands replaced from longer cables.

The centre tower stay cables seem to be designed more as expected from a structural engineering science point of view. I’m curious as to why the discrepancy?

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Bridge Stay Cable Electrical-Heating Strand Calculator

I’ve written this spreadsheet to calculate the specifications for the electrical power supplies to heat the strands of stay cables of the Queensferry Crossing or of other bridges prone to icing with similar cables of an arbitrary number of strands up to 109 strands.
This is a screenshot – click to view a high-resolution image.

Download QFC Electrically-heated Cables. It’s an Excel .xlsx file but it was developed in Google Sheets (for free!). I don’t have Excel on my computer so it has never been tested in Excel. If it doesn’t work on Excel then try uploading it to Google sheets. That should work but any problems let me know

Email: peterdowaberdeen@gm…….

and if necessary I can let you share my Google sheets version with viewer access which I can authorise to any Gmail account and then you can copy it directly into your Google sheets file space.

Instructions

Select a strands sheet with the closest number of strands to the cable you want to calculate for, choosing between 45, 55, 73, 91 or 109 strands. Best to duplicate the sheet for back-ups.

The strands arrangement is represented by coloured cells where the cell formula calculates that strand’s heating capacity.

To re-arrange and / or change the number of strands, first drag off the strand circles overlay image then add strands by selecting an existing strand cell, Edit Copy (ctrl-C) then select another coloured cell for the new strand and Edit – Paste special – Paste formula only. To remove a strand just delete the formula from its coloured cell.

To use the Strands calculator spreadsheet with data for another bridge, you will have to edit or create a new “Cable Data” sheet with the new data and modify the strands calculator sheets’ (hidden) cell X61 so that it fetches the cable data from the new sheet with the appropriate table range.

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