When I did the stadium tour I was definitely told that the tension devices outside the stadium are purely 'decorative' and offer no support to the structure or the roof. The roof is, I was told, totally self-supporting.
But I'm not a structural engineer so could be BS of course.
There was an Arup report doing the rounds a while ago - might be something in there?
Edit:
Just found the Arup report and this is the roof text (sorry it's a bit lengthy!):
Roof structure
Concept and structural systems
A central feature of The City of Manchester Stadium is its
distinctive and dynamic form, key to which is the structural
solution for supporting the roof. The roof structure essentially
comprises two separate structural systems, the first providing
primary structural support to the whole roof (the ‘cable-net’),
and the secondary being a more conventional arrangement
of rafters propped from the rear of the concrete bowl and
hung towards their leading edges from the cable-net. The
mast and cable-net roof primary structure uses a ‘grounded
tension ring’ to create a prestress field against uplift wind
loads. 12 cigar-shaped tubular steel masts up to 65m high
support a total of 76 spiral strand forestay cables in fanshaped
groups of five or seven cables per mast. Each
forestay supports an individual rafter. Just above the roof
surface, all the forestay ends are connected by a system of
four spiral strand cables that form the ‘grounded tension
ring’ (also referred to as the ‘catenary’). Prestress to the
catenary and cable-net is provided by four corner-ties
anchored to the ground. The top of each column is tied
back to the ground by pairs of back-stays comprising
groups of Macalloy high tensile steel rods. This mast and
cable-net system not only provides a highly efficient structure
but also is central to the drama of the building’s architecture.
The roof plane structure comprises 300mm wide by
maximum 900mm deep box section ‘radial’ rafters at
approximately 7m centres, supporting UB section purlins
at 4m centres. The rafters are supported at the rear of the
Stadium bowl through integrated V-strut columns on the
concrete bowl. The V-struts allow sufficient headroom
between the rear seating terraces and roof structure as well
as providing for transfer of horizontal thrust from the rafters
to the bowl. Towards the inside of the Stadium the rafters
cantilever by up to 14m beyond the support provided by
the forestays.
Cladding
Most of the roof is clad with an innovative system that typifies
how structure, services, and architecture are integrated in
the Stadium’s design and detailing. 150mm deep aluminium
‘liner trays’ span the 4m between purlins and rest on the
purlin bottom flanges. The liner trays act structurally to
support the aluminium standing seam roof sheeting as well
as creating a hidden zone for acoustic insulation, wiring,
and in-plane roof bracing. The trays also form a visually
clean ceiling to the roof where a ‘normal ceiling’ would not
usually be economical.The inside 10m of the roof on all four
sides is clad in transparent polycarbonate sheeting, allowing
sunlight onto the pitch to assist grass growth and also
ample daylight into the seating bowl. Another advantage of
this form of cladding at the leading edge is its more gradual
transition from full daylight to shadow - particularly beneficial
for television coverage.
The roof’s geometrical form ensures that its surface always
slopes down to the outside edge with an inclination varying
from 1.5° to approximately 17°. Along this outer edge an
aluminium-clad gutter defines the perimeter and carries all
water runoff to two large sculpted downpipes at the northern
and southern ends.
Details
A family of fabricated plate connections with a consistent
architectural language has been adopted for major details
such as the masthead and base, V-strut base, catenary
nodes, and back-stay bases. These were developed in
consultation with the successful first-stage roof steel
tenderer who went on to secure the contract for the roof
construction. Where axially loaded elements are visible, pin
connections are used throughout except for the base of
each mast, which utilizes a series of steel plates and a pot
bearing. This detail enabled greater flexibility for rotation in
any plane during erection whilst maintaining the capacity to
transmit axial compression forces of up to 13 000kN in the
final condition.
Back-stay and corner tie foundations
The back-stay rods and corner tie cables are anchored to
the ground by an innovative foundation system comprising
high strength steel multi-strand ground anchors which
pre-compress concrete piles against the underlying rock
strata. These anchors comprise 8-15 greased and sheathed
15.2mm diameter steel strands bundled and inserted into
plastic ducts. The anchors are installed into bored holes up
to 35m deep, with the lowest 10m of the anchor bonded to
the surrounding bedrock by cement grout. Each anchor
is prestressed so that it is in permanent tension whilst
pre-compressing the concrete piles. This system was
developed to deal with the local ground conditions where
mining had left the underlying bedrock highly fractured. This
bedrock can resist permanent tensile loads, but resistance
to varying tensile loads is less reliable. The advantage of
this anchor system is that the stress on the anchor/bedrock
interface remains more or less constant and fluctuations in
the back-stay or corner tie forces are accounted for by an
increase or decrease in the compression in the concrete piles.
