This is a sidewalk view of the underside of the scaffolding holding up the arch during construction. Note the two short wooden sections in the centre, between the reddish colour long steel beams.
These decorative finials were precast on the Cambridge site, with rebars sticking out of their flat end. These rebars were then inserted up into the future concrete column in the centre of the arch, wired into place, and then the column was filled with cement from the top.
The decorative finials are shown in gold in this photoshop rendering of the arch.Double click to enlarge.
Tomorrow, the lowest six roof sections will be placed on the beam, and their rebars tied into the last bits of beam forms that haven't yet been filled with concrete. The result will be a beam and precast roof sections all locked together with rebar and poured concrete.