Concrete Magazine February 2009 Structural strengthening and alterations to Westfield London Courtesy of The Concrete Society

1. CONCRETE FRAME CONSTRUCTION
Structural strengthening –
Westfield, London
“While the London has a dramatic new shopping staff, with previous experience in complex strengthening
changes and leisure complex – Westfield London. procedures. The specialist engineering remedial division
Situated at the heart of the massive of Structural Systems (UK) was asked by the client to pre-
presented pare budget costings and method statements, and was cho-
regeneration of the Shepherd’s Bush
massive and White City area of west London, the
sen to undertake the installation.
problems for The works were split into three main areas, which
scheme – boasting five anchor stores included:
the remedial and over 265 top high street retailers • strengthening of slabs for column punching shear due
team and the – also has nearly 50 eating places. The to increased loads
client alike, the superstructure phase of this prestigious • forming and strengthening for penetrations through
building began in early 2005 and post-tensioned slabs
scheme was • strengthening slabs for increased loads.
Westfield London opened for trading in
completed on November 2008.
time for the Strengthening to overcome punching shear issues
This involved detailed design analysis of the existing
opening of the punching shear capacity of the slabs, accurate site surveys
complex.” RICHARD GASKILL, STRUCTURAL SYSTEMS (UK) and concrete density testing (where required) to optimise
the strengthening design. The solution that was approved
involved drilling approximately 30 holes in the slab per
column and polyester resin fixing M12 and M16 × 200mm
T he structural frame of the complex, which totals
approximately 300,000m2, contains over 105,000m2
of post-tensioned concrete slabs.
high-strength pre-tensioned shear bolts, on set perimeters
around the existing columns. Approximately 100 concrete
With most construction projects, there are always minor columns were strengthened using this method, with around
changes to be expected along the way. On this particular 3000 shear bolts being installed (see Figure 1).
project there was a change in both ownership of the devel-
opment and the main tenants renting space. The change in Forming and strengthening penetrations through
tenants required considerable design modifications to the post-tensioned slabs
structure to accommodate their additional requirements. Due to the change in the main tenants, there were addi-
This included increased loads and many small, and 17 tional requirements put upon the structure to suit their par-
larger, penetrations to various areas of the already com- ticular needs. These, however, were finalised after the con-
pleted structure. struction of the floor slabs. The tenants required additional
The client was looking for a suitable contractor with penetrations through the slabs to allow for the introduction
its own specialist in-house design capabilities, to provide of several large stairs, lifts and escalators, mainly through
a fast but innovative design solution. The remit was for the three to four levels of the multi-level tenancies. Each pen-
contractor to design, supply and install the strengthening etration was typically 1900mm × 4200mm.
measures, using its own design expertise and experienced The majority of the additional service penetrations
Figure 1 right:
(Photos: Structural Systems (UK) Ltd.)
Installation of the shear
bolts, awaiting the nuts
to be added before
final alignment and
tensioning up.
Figure 2 far right:
Exposed tendons
following removal
of the ducting and
cementitious material,
ready to form the
anchorage for the cut
tendons.
Figure 3 right: FRP
strengthening to the
soffit of the slab. The
core holes indicating
the edge of the
penetration are clearly
visible.
Figure 4 far right: Detail
of the strengthening to
the soffit in restricted
access zone using
embedded anchorage.
32 FEBRUARY 2009 CONCRETE

2. CONCRETE FRAME CONSTRUCTION
were easily accommodated in the post-tensioned floors,
due to the lack of complex reinforcement and the typical
spacing of 1800mm for the post-tensioning tendons. The
penetrations required a structural design review and the
accurate pinpointing of the post-tensioned tendons, using
a Ferro-scan initially, and then by drilling 20mm-diameter
holes for exact positioning.
Each tendon had to be fixed into position to lock the
prestress into the strand. To form an anchorage for the
strand, an area 600mm long × 300mm wide was broken out
with the exposed tendon centralised longitudinally. Once
the ducting and cementitious material had been removed,
the strand was fixed using a BASF Masterflow 648CP Plus
epoxy grout. From this point, the tendons could be cut to
form the opening (see Figure 2).
While the penetrations could be formed within the slab,
there was still the issue of how the slab could be strength-
ened around the penetration.
FRP plates were bonded to the top and soffit of the slab,
Figure 5: Example of a FRP plate anchored by using a mechanical anchoring device to
around the perimeter of the openings. A layer of epoxy, provide a permanent fixing.
4mm in the middle of the plate and 2mm at the edges, was
applied to the plate. The plates were then lifted into posi-
tion and pushed into place with a rubber roller to expel the
air from the epoxy, giving an overall thickness of 2mm
(see Figure 3).
FRP requires an anchor beyond the point that is required
to span. Where access was limited due to beams or con-
crete walls, 150mm embedded FRP plates were anchored
into the beams or walls to provide a suitable anchorage
(see Figure 4).
At slab edges or around cores, it was not possible to
provide a suitable anchorage, so a mechanical anchor was
used, bonded to the underside of the FRP. Eight-millime-
tre-thick plates were fixed to the concrete using 125mm
embedded high-performance anchors (see Figure 5).
The problem of strengthening around the faces of the
penetration was efficiently dealt with by using Aramid
FRP A120/420 wrap supplied by S & P, using a wet lay-up
method. The 400gm/m2 sheets were presaturated by pass-
ing them through a trough. The wet sheet was then applied
using rollers. The final sheet was not applied until the first
had sufficiently cured to support the other (see Figure 6).
The use of FRP strips eliminated any problems with
Figure 6: FRP being applied to the face of the opening.
access and services that the more bulky structural steel
strengthening solution would have raised.
Strengthening for increased loads
Significant change in use meant that in other areas, the
original slabs did not have sufficient design load capac-
ity to meet the new demands. The slabs therefore required
a cost-effective strengthening solution, which provided
minimal visual impact. FRP is extremely lightweight with
excellent tensile strength properties and a plate thick-
ness as low as 1.2mm. Having undertaken all the essen-
tial design checks, it was necessary to increase the design
capacity of the slabs to accommodate the additional
load and also to overcome the excessive deflections (see
Figure 7).
After much analysis of suitable products in the market-
place, an FRP solution was found to overcome the aesthetic
limitations, while eliminating the excessive deflections.
Concluding remarks
While the changes presented massive problems for the
remedial team and the client alike, the scheme was com-
pleted on time for the opening of the complex. This was
done using less materials as the design and system chosen
proved to be far cheaper than the alternative options, offer-
ing a saving of approximately 30% in the overall strength-
ening costs. ■ Figure 7: Example of strengthening to the underside of the slab using unstressed FRP.
CONCRETE FEBRUARY 2009 33