Bridge
The steel assemblies have
finger joints that connect
with adjoining sections
Tintagel Bridge: steel material selection
The bridge at Tintagel, designed by Ney and Partners
and William Mathews Architects, is exposed to a
coastal environment so material selection is a key
design step. Richard Henderson of the SCI, with a
review from Matthieu Mallié of Ney and Partners,
discusses some of the issues.
20 NSC
Sept 19
environment with plenty of wind-borne sea
salt around, we chose mild steel for the parts
which can be easily repainted and stainless
steel, which is more resistant to corrosion,
for the areas where painting would be more
difficult,” explains Mr Matthews.
This means the bridge’s main top and
bottom cords are fabricated from mild steel,
while the bracings and cross members are
stainless steel.
The connection points between each
individual steel assemblage two on each
piece are also fabricated from stainless steel
and were welded to the main cord members
during the fabrication stage. The connections
consist of finger joints that interlock with
opposite members on the adjoining section,
similar to a woodwork dovetail joint. Once
the individual sections were lifted and
manoeuvred into place during the erection
programme, the connections were then
bolted up.
Underhill’s fabrication process required
some precise engineering and each section
was trial erected with its neighbouring
piece to ensure the two cantilevers could be
seamlessly erected on-site.
During the erection programme the
permanent handrails, along with temporary
floor boards, were installed to create a
safe working environment. The erection
team took delivery and erected at least
one bridge section every day, and once the
two cantilevers were fully erected, the slate
flooring was installed.
The bridge is part of a larger £5M
programme of works at Tintagel Castle
which also includes improving footpaths
around the site, helping to limit the impact
of visitors on the unique archaeology and
ecology.
19
The Ney and Partners/William Matthews Architects successful
competition entry for the Tintagel bridge involved approximately
33 m long cantilevers reaching out from each cliff face. The design
included delicate balustrades and structural bracing for carrying shear
forces and more substantial tension and compression top and bottom
chords to transfer the bending moments back to the abutments. The fact
that the original land bridge was lost a few hundred years ago underlines
the exposure of the site to the power of the Atlantic Ocean. The choice
of material was affected by durability, maintenance requirements and
visual appearance. In the competition entry, weathering steel was to be
used for the main top and bottom chords and duplex stainless steel for all
other steel elements. SCI was engaged by English Heritage to assess the
suitability of different steels through a programme of corrosion tests at the
site, in collaboration with the design team and Arup Materials.
The table of corrosivity categories in ISO 9223 lists category “C5 very
high” with the example description for a typical external environment
including “… coastal areas with high salinity”. The predicted thickness loss
for carbon steel in the first year of exposure is given as >80 to 200 μm.
Corrosion tests on steel samples were carried out over a one-year period
to determine the rate of corrosion and suitability of steel for the project
before the choice of material was finalised. The C5 corrosivity category
of the site was confirmed by the tests and data on the rate of steel loss
collected. Following the completion of the assessment, weathering
steel was showing a potential 70 to 120 year life span depending on the
prediction models. Nevertheless, the Client could not accept the risk of an
RAILING
BEAD BLASTED
STAINLESS STEEL
HANDRAIL
WOOD
SECURITY GATE
DECK
STAGGERED SLATE
FINISHING
INNER FACE
BEAD BLASTED
STAINLESS STEEL
BOX GIRDERS
PAINTED STEEL
(NCS S6030-Y80R)
DIAGONALS
BEAD BLASTED
STAINLESS STEEL
unexpected corrosion rate and a painted carbon steel solution was chosen
for the tension and compression chords.
The more slender members of the bridge which form the “Telford”
diagonals of the cantilevers, the bridge deck (the trays in which the slates
are placed), its shear bracing in plan view and the balustrades are made
from stainless steel. The stainless steel chosen was duplex grade 1.4462,
which has very good resistance to corrosion combined with high strength.
/Corrosion_of_structural_steel
/Design_for_steel_bridge_construction#Fabricating_the_steelwork
/Steel_construction_products#Stainless_steel_products
/Welding
/Design_for_steel_bridge_construction#Bridge_erection
/Accuracy_of_steel_fabrication#Trial_erection
/Bridges
/Design
/Weathering_steel
/Corrosion_of_structural_steel#Corrosion_rates
/Paint_coatings