3.3.2 Pylons
The north and south pylons feature a hammerhead
which supports the deck on bearings whereas the
central pylon is monolithic with the bridge deck.
The lower parts of the pylons were completed using
standard forms with a plywood face and poured in
four lifts. The hammerheads required shutters
(formwork with the frames that served as a platform)
to exact measurements, with the use of CNC
(computer numerically controlled) machines to
ensure accuracy for putting the formwork together.
Once the central hammerhead was complete, the
formwork was removed and the platform was reused
for construction of the pier tables. About 12 sections
of the pier table were built offsite and transported
onto site, lifted onto the hammerhead by crane and
then had concrete poured into them. Together the
assembly made up the pier table onto which the form
travellers were installed.
A significant challenge for the designers of the pier
table formwork was the requirement to transfer the
high vertical loads into embedded anchors within the
lower pylon. As the lower pylon geometry dictated
the maximum possible number of anchors, a
sequenced construction in layers was adopted to
avoid the accumulation of vertical forces, ensuring
that the loads can be safely supported, making the
process more manageable.
The upper pylons – above road deck level – were
constructed by pouring concrete into the surrounding
formwork, using 5m lifts.
All four formwork platforms were raised those 5m by
way of two hydraulic rams sitting at about the second
level within the formwork. When the concrete had
cured sufficiently – at least 25N/mm² - the vertical
guide rails were first moved upwards 5m and then
secured by huge doorknob-like anchors screwed into
pylon.
The formwork was unbolted from the side of the
pylon and hydraulic rams, one pushing and one
pulling, slide the formwork up 5m on the vertical
guide rails. It was again bolted onto the pylon for
security.
One of the challenges common to all bridge
construction projects is high wind speeds. The
advantage of the hydraulic system for the Mersey
Gateway was that it could withstand higher wind
speeds than standard cranes, meaning windy
conditions caused fewer disruptions.
Additionally, as crane use was restricted on-site, using
this system meant that other areas of the
construction project could fully make use of crane-
time.
To complete an upper pylon, it took around 21 lifts.
Once the pylon was completely poured, only then was
a crane erected to take the formwork off its anchors
and lower the sections back to ground level where it
could be fully dismantled either on site or taken off-
site.
Figures 8 + 9: South pylon
3/2017