Tower Building 22 September 2016.
Thanks to Brian Thompson for this © photo of Bridge Construction from Halton Castle.
And this explanation for a member of NWAN Web site.
Yes they are built in situ.
A creeping shutter, mold, is filled with concrete and when set it rises further up, more concrete and rises again so it is a continuous solid beam.
You can see Ron there are holes down the length, like a flute where the suspension cables will come out of.