appendix g structural report and drawings

Appendix GStructural Report and Drawings

60 SA Ltd60 Sloane AvenueStructural Engineering Report

REP/CMS/APPG1

Issue | 30 September 2019

This report takes into account the particular instructions and requirements of our client.

It is not intended for and should not be relied upon by any third party and no responsibility is undertaken to any third party.

Job number 229690-00

Ove Arup & Partners Ltd13 Fitzroy StreetLondonW1T 4BQUnited Kingdomwww.arup.com

REP/CMS/APPG1 | Issue | 30 September 2019

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60 SA Ltd 60 Sloane AvenueStructural Engineering Report

REP/CMS/APPG1 | Issue | 30 September 2019

Contents

Page

1 Overview of proposed structural scheme 1

2 The Site 12.1 Geotechnical desk study 12.2 Location 12.3 Thames Water storm drain 22.4 London Underground tunnels 22.5 Heritage, listed buildings and archaeology 22.6 Flood Risk 22.7 Preliminary site investigation 32.8 Stratigraphy 32.9 Groundwater 32.10 Geotechnical and Existing Foundation Surveys 32.11 Foundation re-use site investigation 3

3 Existing Structure 43.1 Description based on available information 43.2 Existing Information search 53.3 Key information required 63.4 Use of Existing Information 63.5 Existing Condition 6

4 Proposed superstructure 64.1 Introduction 64.2 Lateral Stability 64.3 Typical floor plates 64.4 Typical Columns 74.5 Roof structure 7

5 Proposed substructure 85.1 Introduction 85.2 Basement wall construction 85.3 Basement area 95.4 Raft foundation 95.5 Basement depth and uplift 9

6 Reuse of Existing Foundations 106.1 Introduction 106.2 Foundation re-use strategy fundamentals 106.3 Summary of proposals and conclusions 10

7 Design Criteria 117.1 Design criteria 117.2 Design philosophy 117.3 Robustness and disproportionate collapse 117.4 Design life 117.5 Existing vertical loading 127.6 Proposed vertical loading 127.7 Vehicular loading 137.8 Lateral loading 137.9 Basement grade 13

8 Movements and Tolerances 148.1 Structural movement 148.2 Movement limits 148.3 Tolerances 14

9 Materials 159.1 Existing structure 159.2 New structure 159.3 Durability 159.4 Corrosion protection 159.5 Fire protection 159.6 Concrete finishes 15

10 Design Standards and References 1610.1 Statutory regulations 1610.2 Codes of Practice 1610.3 Design guidance 16


REP/CMS/APPG1 | Issue | 30 September 2019 Page 1

1 Overview of proposed structural schemeThe development proposal includes demolition behind a retained façade and redevelopment to provide 7 storey (ground plus 6 upper floors) mixed use office, retail and leisure development inclusive of three additional part width basement storeys to provide leisure and retail space, plant and car and cycle parking.

The redevelopment includes:

demolition of the existing three to five storey building, either retaining the Edwardian façade or surgically demolishing and rebuilding it;

deepening of the existing single storey basement over part of the site footprint to form an additional three levels of basement; and

construction of a new seven storey building over the whole site.

Key constraints on the structural grid, and associated transfer structures, are shown in Figure 1 and are described further in the following pages.

2 The Site

2.1 Geotechnical desk studyA Geotechnical Desk Study has been carried out (Arup, Dec 2013) for the consented residential scheme. The desk study remains applicable to the current structural proposals described here since the structural shell, and in particular basement construction, are essentially unchanged. The desk study gathers information relevant to the proposed foundation design and construction, and the impact of the proposed works on neighbouring structures. A summary of this report is presented below.

2.2 LocationThe site is located in the Royal Borough of Kensington and Chelsea (RBKC), at approximate national grid reference TQ 27350 78710; it is shown in Figure 2.

Figure 1: Overview of Scheme Figure 2 Location plan



2.3 Thames Water storm drainA Thames Water storm drain runs beneath the site; its alignment is shown in Figure 3.

Thames Water has confirmed that this is the Smith Street Relief Storm drain, which is understood to be a 100” (2.54m) diameter concrete wedge block storm drain used to store storm overflow. It is part of the Western Deep Storm drain System, constructed in the late 1970s to 1980s. The Western Deep Storm drain System forms the principal storm relief system in London, meaning that this is a critical asset for Thames Water.

Wedge block tunnels of this type rely on the overburden ground pressure to maintain their stability. This means that they are sensitive to any reduction in overburden, as well as to ground movements. The typical construction of this type of tunnel is shown in Figure 4.

The Thames Water storm drain beneath the site presents a major project risk. It is a critical asset in London’s stormwater management system, and is of a construction type which is sensitive to ground movements and the reduction in overburden pressures. It will be necessary to demonstrate to the satisfaction of Thames Water that the proposed development will not adversely affect the integrity or serviceability of the storm drain.

2.4 London Underground tunnelsThe District and Circle Line tunnels run below ground approximately 100m to the north of the site. These tunnels were constructed in 1868, using cut and cover techniques.

The tunnels are far enough away that it is not anticipated they could be affected by any redevelopment of the site. It is likely, however, that the tunnels form a cut off through the shallow aquifer into the London Clay, affecting the groundwater level in the area.

2.5 Heritage, listed buildings and archaeology The site is not located in a conservation area. The closest listed building to the site is the Grade II listed Michelin House; this listing is restricted to the part of Michelin House facing Fulham Road, which is approximately 75m to the west of the site.

The existing 4.5m deep basement covers the entire building footprint and is expected to have removed the vast majority of Made Ground dating from pre 20th Century. It is therefore considered unlikely that any archaeological artefacts remain below the existing basement. There is a possibility that artefacts could be present in ground surrounding the basement. If the proposed redevelopment extends outside the footprint of the existing basement, a full assessment of the archaeological potential of the affected area may need to be considered, although at the time of writing this is not thought to be likely.

2.6 Flood RiskThe Environment Agency flood risk map shows the site to be outside the zone of extreme flooding from the River Thames, being located approximately 1km away from the area vulnerable to flooding. It is, however, shown to susceptible to minor pluvial flooding within a 75 year return period.

Figure 3: Thames Water tunnel below site

Figure 4: Typical wedge block tunnel construction



2.7 Preliminary site investigationA preliminary site investigation comprising a single borehole was undertaken in June 2013, in order to inform the planning submission documents. The stratigraphy encountered by the site specific ground investigation is summarised below.

2.8 StratigraphyMore detailed ground stratigraphy based on British Geological Survey records is:

2.9 GroundwaterA standpipe was installed as part of the Phase 1 site investigation to monitor a response zone within the River Terrace Deposits. The groundwater level recorded at time of drilling (June 2013) was -1.3mOD. Given the top of the London Clay is -1.5mOD, this corresponds to a water column of just 0.2m. This level was consistent over several monitoring visits, and surcharging of the standpipe was carried out to confirm that this groundwater was free draining and representative of the true groundwater level.

This groundwater level is lower than had been initially expected. It is likely that the District Line cut and cover tunnels, running east-west 100m to the north of the site are providing a cut-off to the flow groundwater in the shallow aquifer, which generally flows to the south, draining into the River Thames.

2.10 Geotechnical and Existing Foundation SurveysFurther ground investigation will be required to obtain geotechnical data for detailed design of foundations, basement retaining walls and assessment of the influence of the Thames Water Storm drain on the proposed works. The main ground investigation should also include an assessment of ground contamination present beneath the site, with particular reference to waste acceptance criteria for excavated material. The following particular areas of geotechnical and environmental uncertainty have been identified for further investigation:

extent of Made Ground and the presence and extent of ground contamination across the site;

variation in groundwater levels; and

confirmation of stratigraphy and geotechnical design parameters.

2.11 Foundation re-use site investigationGood quality drawings are available showing the layout and diameters of the existing piles and pile caps, although the length is not known. Drawings also show the location of abandoned piles, indicating that they may represent ‘as built’ information, although this is not confirmed.

In order to give confidence in the reuse of the existing foundations, it is essential that a site investigation is carried out which is targeted to confirm details of the existing foundation location, geometry and existing condition. The pile length is a particularly critical parameter.

An assessment of the allowable load bearing capacity of the existing foundations should consider total and differential settlements, which is influenced by the balance between new and existing loads (particularly existing realistic loads), in addition to the calculated geotechnical capacity of the piles. It should also consider the durability of the existing piles given the proposed design life of the new structure.

Table 2 Stratigraphy based on BGS survey records

Table 1 Stratigraphy based on preliminary SI borehole results



3 Existing Structure

3.1 Description based on available informationThe ‘Clearings III’ building was originally constructed around the time of World War 1, and comprised a steel or wrought iron frame with a loadbearing masonry façade, faced with terracotta. It was used as a warehouse for Harrod’s department store.

In the early 1990s, the internal structure was demolished and reconstructed as concrete frame, whilst retaining the original terracotta façade. The new structure was used as office space with retail on ground floor and basement level. The architectural intent was to ‘complete’ the building by adding a two-storey ‘L’ shape block onto the half of the building that the Victorian’s did not complete. These two interlocking L’s then created a central atrium. The contrast of the retained terracotta and new façade can be seen in Figure 5.

Figure 5: Existing building as viewed on the corner of Sloane Avenue and Ixworth Place

The concrete frame is typically 325mm thick reinforced concrete flat slabs, which taper to 250mm thick at the façade line. Column grids vary between 6m to 9m, and align with the original façade module.

The retained façade is loadbearing and is understood to be supported on a strip footing comprised of steel joists encast in concrete. The 1994 curtain wall is supported on the slab at

each level. As discussed below, there is no information about the strip footing for the terracotta façade.

Lateral stability is provided by concrete core walls around the existing stair and lift shafts. Figure 6 shows the layout of a typical floor level with the reinforced concrete stability walls highlighted in red.

Figure 6: Typical floor plan showing stability cores and grid

A roof extension was added at the south-east of the building circa 2008, shown in Figure 7. This comprises a steel frame with lightweight metal roof. It is understood the original roof screed was removed as part of these works in order to avoid adding load to the existing columns and foundations.

The foundations for the existing building typically consist of 3-pile groups as shown in Figure 8Figure 8. As described in Section 2 there is a 2.5m diameter Thames Water tunnel running beneath the site with the inlet level at -10.8mOD (13.4m below basement B1 level). The discovery or the tunnel during construction led to a rapid redesign affected pile caps to span over the tunnel and transfer load from columns that land directly above to either side. Issues associated with the Thames Water tunnel are described in detail in the Geotechnical Desk Study (Arup, Dec 2013).

The existing retaining structure of the single level basement consists of a pre-existing retaining wall, possibly brickwork, constructed in 1911 and a reinforced concrete lining wall, constructed in 1994. The outline of this wall can be seen in Figure 8.



Figure 7: Cross section through building showing the 1994 structure with 2008 addition

Figure 8: Existing foundations for the building

3.2 Existing Information searchAn existing information search is on-going, and is described further in the Feasibility Report (Mar 2013).

The quantity of information eventually available, in conjunction with the design proposals, will inform the degree of survey work required. Surveys will include geotechnical and structural surveys, and can be expected to continue throughout the design process and during the construction works as the structure is ‘opened up’.

The key parties involved in the buildings lifetime are:

1911 Original Building

Structural Engineer - Hurst Pierce + Malcolm

1994 Reconstruction

Architect - YRM Stanton Williams

Structural Engineer – YRM Associates

2008 4th floor extension

Architect - CBRE

Structural Engineer - Beers Consulting

The following sources have been investigated:

RBKC Building Control

As an initial search point the Royal Borough of Kensington and Chelsea (RBKC) Building Control department was contacted. Arup were informed that they do not hold any records of the building before 1998

Thames Water

Thames Water have provided Arup with information about the form of construction of the storm water relief storm drain that runs beneath the site. This is discussed further in the Geotechnical Desk Study (Arup, Dec 2013).

60 Sloane Avenue building archive

A search through the archive was made by Arup on 16th May 2013, the following drawings and files were found: YRM Stanton Williams architectural drawings (1994 building), Felix drawings (2008 façade), 2008 Health & Safety File (containing CBRE architectural drawings and Beers structural drawings).

The party wall award with the neighbouring school was also made available by the client. This contained information regarding the existing foundations, basement and party wall.



3.3 Key information requiredBased on the current design proposals, information on the construction of the following elements is of most value and should continue to be sought:

The existing piles and their associated pile caps

The existing five-storey historic façade and it’s strip footing foundation;

Reinforced concrete lining wall in B1 constructed in 1994;

Victorian brickwork retaining wall constructed in 1911.

3.4 Use of Existing InformationIn addition to information being incomplete in many areas, that which is available may not reflect the as-built condition. Various modifications are known to been carried out since construction, and information on these is similarly incomplete.

For the purpose of this report the existing building is assumed to be as shown on the available original structural YRM Associates drawings.

Surveys of all existing structure to be retained will be required to confirm the as built arrangement and the nature of any modifications.

3.5 Existing ConditionIt has been assumed for the Stage C design that the existing structure is generally in reasonable condition for its age and adequate for its current purpose. The proposed future uses of the retained elements of the building are encompassed within its current operation, and no evidence has thus far been uncovered that the building is not in adequate condition to continue performing to a similar level. However our visits have been very limited and many areas are not currently accessible for visual surveys to take place since the building is occupied.

4 Proposed superstructure

4.1 IntroductionA reinforced concrete flat slab floor system, stabilised by concrete stability cores is proposed for the superstructure.

Flat slabs provide the benefit of minimising the floor depth, which allows for maximum clear floor to ceiling height. They also allow for easy service distribution beneath the slab and its heavy weight compared to steel will help to counteract the uplift forces arising from the deep basement, described further in Section 5.

At this stage, a conventional reinforced concrete flat slab is proposed. However, a post-tensioned slab solution is also viable. Post-tensioned slabs are likely to be marginally thinner, which resulting benefits from reduction in embodied carbon and load to existing retained foundations. This will be explored further in the next stage, including procurement considerations.

4.2 Lateral StabilityLateral stability will be provided by reinforced concrete cores that are vertically continuous throughout the buildings height and act as vertical cantilevers transferring horizontal wind loads to basement level.

Figure 9Error! Reference source not found. shows which walls will stabilise the building in the two orthogonal wind directions. Core walls directly over the Thames Water tunnel will be in ‘soft core construction’, for example will be non-loadbearing blockwork or plasterboard walls supported on downstand beams. This arrangement is necessary to avoid cores sited over the Thames Water tunnel.

The core walls have been co-ordinated with the building services engineers to ensure that door openings and service penetrations do not affect the overall core stiffness.

4.3 Typical floor platesOn First to Sixth floors, the typical floor plate comprises a 300mm flat slab with a typical grid of ~7.5x7.5m with a central column. Due to the large number of openings in the slab at Ground Floor for retail stairs and ventilation ducts, a two-way slab on beams is proposed. The new basement slabs act as permanent props for the secant wall and are therefore 400mm thick.

The locations of the columns within the new basement are less restricted as they are supported by the raft foundation. This allows the edge columns to be positioned at the slab edge rather than 2.2m inboard of the slab, where they are currently positioned. An edge beam is provided wherever columns are at the slab edge to avoid punching shear issues. Figure 14 shows the relative positioning of the column and edge beam.

The columns sited above the tunnel must remain in their current positions so that the transfer pile caps can be reused. Removal of these pile caps could result in pressure relief on the tunnel, which as explained in Section 2, will not be acceptable. It is also therefore necessary to limit load increases on these columns so that supplementary piling is not required in these areas. This is described further in Section 5.



4.4 Typical ColumnsInternal columns in the building are typically 450x450mm RC increasing to 650x650 in the basement. In order to maximise apartment area the columns have been sized using grade C60 concrete.

4.5 Roof structureThe roof of the building will support lightweight air-handling plant and photovoltaic arrays.

To prevent excessive noise and vibration above the offic an appropriate floor system needs to be provided. The acoustician has advised that the concrete deck should be at least 200mm thick normal-weight concrete to limit noise and vibration transmission from the plant into the offices below.

Additionally, the floor should be as lightweight as possible to minimise loading on the bridging pile caps that will be reused.

Further, internal columns within the sixth floor offices are not desirable for space-planning.

These requirements led to the selection of a steel and concrete composite floor deck on long-span cellular steel beams. Services distribution will be through the beam web, to limit overall floor depth. See Figure 10.

Columns on the 6th floor that support this steel concrete composite deck will be 150x150 RHS.

Transfer beams are required at Sixth Floor where the façade columns are offset from the column grid below.

Figure 9: Typical floor framing

Figure 10: Roof build up



5 Proposed substructure

5.1 IntroductionThe substructure consists of a single existing basement level that extends across the full footprint of the building and an additional three new partial levels of basement at the south-west of the site.. Figure 11 shows a short section and plan view of the proposed new basement. The basement will accommodate leisure areas on levels B2 and B3. The lowest level, B4 will predominantly contain plant.

Figure 11: Section and plan view of new basement

5.2 Basement wall constructionThe new basement will be approximately 18m deep making it relatively deep for London. It is below groundwater level and must therefore resist substantial hydrostatic as well as retained earth pressures.

Figure 12 shows the build-up of the proposed secant pile retaining wall. The piles which make up the walls parallel to the streets (Draycott Avenue, Ixworth Place and Sloane Avenue) are 900mm diameter. The wall parallel to the Thames Water storm drain will need to be 1200m diameter to limit movements of the Thames Water storm drain. They will also be deeper than the typical piles.

It can be seen in Figure 12 that the inside face of the basement wall should be 2500mm from an existing vertical face. This dimension allows for a 1.2m piling rig clearance as well as the piling tolerances and drained cavity wall build up. The piling rig clearance is necessary as the proposal is to remove the ground floor slab and then pile from the B1 level. Vertical obstructions such as the existing RC retaining wall and contiguous wall will need to be avoided.

Parallel to Ixworth Place, extra basement area is required to accommodate the swimming pool on level B3. It is proposed that B1 level will be partially backfilled in order to allow the piling rig to pile from ground level. Doing so will mean no vertical obstruction will exist hence the 1.2m piling rig clearance can be disregarded. Nevertheless, a 400mm wide pile guide wall should be allowed for. This reduces the offset from the existing basement wall to 2150mm. Refer to the Construction Traffic Management Plan (Arup, Sep 2019) for further details.

Figure 12: Form of basement wall and setting out



5.3 Basement areaThe ~100m two-story terracotta façade will be carefully deconstructed and stored offsite, prior to re-erection or replacement following basement construction. This provides greater basement area at three levels, as well as greater flexibility in construction logistics for the contractor.

Currently the decision has been made to proceed with the option of removing the façade. Although this itself will be a slow and expensive process, it is envisaged that the value added from the extra area and lower pile extraction requirement outweighs the construction cost and programme impact. Further benefits include:

Easier site logistics and access without the obstruction of the façade.

Re-construction of extended 5-storey façade can be accommodated without complex transfer structures at 2nd floor. It is unknown at this stage whether the existing 2-storey façade will be able to support the new façade above.

The new façade foundation can be combined with the secant wall.

5.4 Raft foundationIn order to resist heave pressures due to the deep basement excavation, a raft foundation will be adopted as the foundation solution for the new deep basement. The raft will be 1200mm to spread the columns loads sufficiently and to offset the heave pressures.

It is important that the basement remains dry, therefore any water ingress that may occur through the concrete will need to be drained away. This is will be catered for by drained cavity walls and floors, that drain away into sumps.

5.5 Basement depth and upliftThe depth of the basement is limited due to the uplift effect from the hydrostatic head of groundwater and strict tolerances to movement of the adjacent Thames Water tunnel.

To meet the requirements for clear floor to ceiling heights in the new basement and assuming a 1.2m deep raft foundation the underside of the raft will be at -12.43mOD, marginally lower than the Thames Water tunnel invert of -10.8mOD.

The preliminary site investigation found the ground water level to be at -1.5mOD. Allowing for seasonal effects the design ground water level has been taken as a meter higher than the actual ground water level, resulting in 14.9m of hydrostatic ground water head. The buoyant force of water at this level will exert a upwards pressure of ~150kN/m² on the underside of the raft.

To avoid tensions piles, the down force from the permanent weight of the building must result in a greater downward pressure. A calculation has shown an overall downwards pressure of 165kN/ m², which provides an adequate factor of safety against flotation.

Figure 14: Uplift vs. downforce assessmentFigure 13: New basement area achieved if two-storey facade is removed, and replaced



6 Reuse of Existing Foundations

6.1 IntroductionThe north-east half of the building will be supported on piles at basement B1 level. The intention is to reuse the existing piles in this area, however due to the rearrangement of cores and increases in loads certain pile caps will need to be removed and or strengthened with supplementary piles. Additional new pilecaps will also be required in some locations.

Interventions to existing pilecaps spanning over the Thames Water tunnel are to be minimised as modifications in this area could have an adverse effect on the tunnel that Thames Water will not accept. Figure 15 indicates which pile caps can remain in place without strengthening.

A preliminary load run down has determined that the load increase on the pile caps highlighted green in Figure 15 is less than 10% and are therefore likely to be suitable for re-use. However the caps highlighted in red will need to be removed and reconfigured due to the load increases. Pilecaps highlighted in orange will require supplementary piling to withstand the new loads. This is likely to be in the form of mini-piles, and reconstructed pilecaps.

Figure 15: Existing pile reuse

6.2 Foundation re-use strategy fundamentalsInitial load balance appraisal suggest that it should be possible to re-use existing foundations where the new loads do not exceed the realistic existing loads by more than 10%. However, further investigative survey work will be required to determine the geometry and condition of the existing foundations.

The pile caps that bridge over the tunnel provide a confining effect and so removal of these will release overburden pressure on which the tunnel is relying for its integrity. Some pilecaps over the tunnel will need to be removed and reconfigured due to the change in building grid and loads. At this stage it is felt that retaining the caps highlighted green in Figure 15 will be sufficient to confine the tunnel. This will be assessed in further detail in the next design stage.

6.3 Summary of proposals and conclusionsThe proposed works include adding two superstructure levels which results in an increase in load on the foundations. The existing pile cap arrangement at the west end of the tunnel includes three pile caps that support columns with a cantilever & backspan system (sown red in Figure 16 below. This form of support is very sensitive to load increases as the moment induced by the leverarm of the cantilever results in a load increase that is proportional to the distance to the backspan support.

It can be seen in Figure 16 that the secant pile wall (indicated by the blue dashed line) will cut through the existing pile caps. These caps will therefore need to be removed and a new system to transfer the loads onto these piles is required. The proposed solution is to construct a T-shape storey height wall to spread the load onto the piles that surround the tunnel. It was not possible to reconfigure the pile caps in a similar form as the existing arrangement because the load increase on the columns and cantilever spans result in an unacceptable load increase on the piles. By constructing the T-shape storey height wall the load from the columns will be distributed more evenly to the surrounding piles and it is expected that only four supplementary piles will be required.

Figure 16: Intervention required for cantilever & backspan pile caps



7 Design Criteria

7.1 Design criteriaThe following Sections 7 to 10 will form the basis of the Calculation Plan, which will be used as a guide for all subsequent structural design on the project, and will eventually form part of the Building Control submission along with the design calculations.

7.2 Design philosophy

7.2.1 Assessment of existing structure

Existing structural elements that will be retained and/or strengthened are:

Transfer pilecaps bridging over the Thames Water storm drain and all of those to the nort-east of the tunnel;

piles with these pilecaps;

The existing five-storey historic façade and its foundation;

Retaining walls and slab at basement B1 level

Where the proposed structural works result in an increase in load on the above elements, an assessment into their capacity will be required. The assessment will be made using current Codes of Practice and British Standards. Existing material strengths and material safety factors will be based on Codes of Practice in use at the time of original design. Assumed values will be modified as appropriate following insitu testing.

This approach is common for refurbishment projects and will ensure sufficient robustness present within the proposed scheme.

7.2.2 Ultimate limit state designThe design of the building structure will be based upon Limit State design principles. Limit State design utilises Partial Factors of Safety being applied to materials and load combinations, in accordance with the relevant British Standards and Codes of Practice.

Partial Factors of Safety for the maximum (minimum) load effects are to be applied as follows:

(a) Dead Load Adverse 1.4Beneficial (0.9)

(b) Imposed Load Adverse 1.6Beneficial (0.0)

(c) Wind Load Adverse 1.4(e) Earth Pressure Adverse 1.4

Beneficial (0.9)(d) Combined Load Adverse 1.2

Beneficial (0.9)

It should be noted that, although British Standards have been adopted thus far, future design calculations will be in accordance with Eurocodes as these are being phased in to supersede current British Standards.

7.3 Robustness and disproportionate collapseDisproportionate collapse will be considered in accordance with Building Regulations Approved Document A: Dec 2004: Section 5. This document classifies structures based on usage, number of stories and/or floor areas.

Disproportionate collapse rules are used to ensure that if one part of the structure should fail, adjoining parts are sufficiently tied to ensure progressive collapse of the structure does not occur.

7.4 Design lifeThe structure will be designed and constructed in accordance with relevant British Standards to achieve a Design Life of 50 years. In this context, the term Design Life is understood to mean that, provided the structure is appropriately maintained, it will still meet the design criteria set out here at the end of this period.

At this stage it is not possible to make an assessment of the ‘residual design life’ of the existing elements that will be reused. This will be dependent on the current state of repair of these elements and the grade and mix of the concrete used in their construction.

As noted in Section 3, the retained existing structure should be subject to a suite of survey and testing as a means of addressing this risk to the Client.

See Section 6 for further discussion on re-use of existing foundations.

For the purposes of this Stage 2 design, the existing structure to be retained is assumed to be in reasonable condition for its age and adequate for its current purpose.



7.5 Existing vertical loadingThe existing vertical loads presented below have been assumed for the load rundown studies carried out on the building for the foundation re-use assessment. In this assessment, it is important to understand the actual loads to which the foundations have been subject over their life. These assumptions should be verified by site survey in advance of building strip-out in order to validate the assumptions on which the foundation re-use strategy is based.

Existing dead loadsExisting dead loads from the structure have been determined from available YRM Associates drawings and the ‘Integrating the old with the new’ article published in the Architects’ Journal. These were also corroborated with the Beers Consulting Engineers Ltd structural drawings obtained from the Health & Safety file for the 2008 4th floor addition works.

The existing floors typically consist of a 325mm RC slab, which tapers to 200mm at the cantilever tips on the outermost column line. On ground level the floor is a 300mm slab with a grillage of 600mm wide, 650mm deep RC beams on the column grid. The basement level is a 150mm thick ground bearing RC slab.

Existing superimposed dead loadsBased on the use of the building, the superimposed dead loads have been assumed to consist of a 150mm raised floor and 575mm suspended ceiling and services zone.

Existing imposed loads According to a letter from Beers Consulting Engineers Ltd dated 9th July 2007, a report was prepared by CBRE in May2002 for the buildings then freeholder to describe the overall condition of the building . The report included a section on design floor loads and was taken from original information available at that time. The report states that the typical floors have a live load allowance of 5.0kN/m². This load allowance is typical for office buildings designed in the 1990s.

Figure 17: Summary of existing loading assumed for typical floors

Existing cladding loadsThe existing atrium has been assumed to consist of a sandstone and concrete 250mm thick build up with 15% clear glazing area. This results in a loading of 5.1kN/m² on elevation

The curtain walling installed in 1994 is supported by the floor slabs and is not load bearing like the existing historic terracotta façade. Curtina wall load is assumed as 1.0kN/m² on elevation.

7.6 Proposed vertical loading

Proposed superimposed dead loadsThe proposed development will consist of offices and therefore raised floor and suspended ceilings are assumed, per BCO best practice.

Proposed imposed loadsThe imposed loads used in the design are:

Offices 2.5 + 1.0kN/m²

Retail areas 4.0 kN/m²

Leisure areas 5.0 kN/m²

Car park 2.5 kN/m²

Basement plant 7.5 kN/m²

Roof plant 2.5kN/m²

Proposed cladding loadsThe existing historic two-storey façade will be removed and rebuilt to five storeys to match the existing five storey façade. This façade will be loadbearing.

On the perimeter of Fifth and Sixth floors, the curtain wall façade is supported on the new structural slabs. Load allowance is assumed as 1.0kN/m² on elevation

Snow loadsSnow loading will be calculated in accordance with the rules set out in Eurocode 1.

The basic snow load on the flat roof will be taken as 0.4 kN/m². The roof is loaded by lightweight plant so snow loading will be assumed to occur simultaneously with the normal plant live load allowance.



7.7 Vehicular loadingColumns at ground floor and within the basement have not been designed for vehicle-impact loads. Any columns which are in danger of being struck by a vehicle because they are adjacent to a road or loading bay should be protected by means of suitable barriers.

7.8 Lateral loading

Wind loadingWind loading will be calculated in accordance with Eurocode 1.

Notional horizontal loadingNotional horizontal loading will be considered in accordance with Eurocode 2. The building should be capable of safely resisting the notional horizontal design ultimate load applied at each floor or roof level simultaneously. This is equal to 1.5% of the characteristic dead weight of the structure between mid-height of the storey below and either mid-height of the storey above or the roof surface.

The greater effect of notional horizontal loading or wind loading will be used in the design. The effects are not taken to act simultaneously.

7.9 Basement gradeThe basement grades, in accordance with BS8102: Code of Practice for protection of below ground structures against water from the ground, are:

Table 3: Basement grades required

The existing basement to the north-east of the site will remain largely intact, with penetrations for formation of new piles and caps only. The existing basement is above the groundwater table and we are not aware of any reports of water ingress through the retaining walls or groundbearing slab. This will be investigated further by survey and following soft strip.

For basement areas at Level B1 classed as Grade 2, the new pile caps will be carefully detailed to be integral with the existing construction and no additional waterproofing measures should be necessary. For areas required to meet Grade 3 – Habitable, additional measures will be necessary. These could include cavity wall and floor finishes in combination with an active room ventilation system.

New basement levels B2 to B4 are below the existing groundwater level and as such will required full drained cavity wall and floor systems, connected to a system of sump pumps.

Basement Use Grade Description Performance requirement

Plant rooms 2 Better utility No water seepage but some damp patches acceptable

Electrical rooms, store rooms and workshops

3 Habitable No damp patches. Vapour ingress acceptable

Gym 3 Habitable No damp patches. Vapour ingress acceptable

Car park 2 Better utility No water seepage but some damp patches acceptable



8 Movements and TolerancesA detailed Movement and Tolerances Report will be produced during detailed design. This will contain the information for other members of the design team and future contractors/ specialist subcontractors to undertake design work interfacing with the structure.

8.1 Structural movement Structural elements will move vertically and laterally under applied loads. Movements are defined here as those which may occur in a structural element following its construction.

Structural movements are calculated on the basis of certain assumptions about material properties, loads and structural behaviour. Actual material properties and loads will differ when compared to the assumed values used in calculation. Factors such as the increase in overall stiffness provided by the cladding and stiffness in connections will also alter the actual structural behaviour. Real building structures are influenced by the performance of elements not necessarily considered in the modelling of the structural frame, such as the cladding.

8.2 Movement limitsThe structure will be designed with the following movement limits. Generally, these are in accordance with, or more onerous than the current Eurocode.

Sway movementsAs part of the serviceability checks, the overall building drift will be limited to:

δDL + LL + WL (lateral deflection due to all loads) ≤ Height / 500

As there is brittle terracotta cladding around the building perimeter, the sway of one storey relative to a storey below:

δWL (deflections due to lateral loads) ≤ Storey height / 500

Vertical movementsVertical deflections should be limited to:

δLL (deflections due to imposed loads) ≤ Span / 500 and ≤ 20mm

δDL + LL (deflections due to total loads) ≤ Span / 250

In addition to the above, deflection of edge beams supporting the façade, under all superimposed dead and live loads imposed after façade installation, shall be limited to:

δSDL + LL (total deflections post-façade installation) ≤ 15mm

The structure and cladding shall be designed to cater for these imposed movements without visible distress.

8.3 TolerancesTolerances are allowances made in the design detailing to cater for the anticipated accuracy of construction including fabrication and erection. Typically, tolerances relate to the theoretical position of an unloaded structural element at the time of its construction.

Tolerances within the fabrication and erection of the steelwork frame should be such that they do not hinder the erection or induce excessive stresses, deflections or distortions into the structure.

The constructional accuracy shall comply with the more onerous of the following:

1. Concrete National Structural Concrete Specification (NSCS), Current Edition

2. Steelwork National Structural Steelwork Specification (NSSS) Current Edition

3. Plan position ± 15mm (column or beam)

4. Level Accuracy ± 15mm, but including the following limits of flatness:

All concrete floors shall comply with the following flatness requirements:

a) ± 5mm under a 3m straight edge.

b) ± 2mm under a 1m straight edge.

Special tolerancesTo be specified as required as the design progresses.



9 Materials

9.1 Existing structureNo information has been made available of the concrete grade used for the existing structural elements that will be reused (i.e. piles, pile caps, retaining wall). These will be confirmed at the next design stage through insitu testing.

For the existing load rundown comparison the density of reinforced concrete has been assumed to be 24kN/m²

9.2 New structure

Excluded materialsOnly materials in accordance with ‘Good Practice in the Selection of Construction Materials’ by Arup will be specified in the structural works.

Materials standardsAll materials to be incorporated into the works shall be manufactured to the relevant British or European Standard and obtained from a recognized source, carrying the relevant certification of quality control. Suitable records of material quality shall be kept by the Contractor and issued to the client before handover.

All concrete shall be obtained from suppliers carrying a Certificate of Accreditation under the Quality Scheme for Ready-mixed Concrete.

The following minimum standards shall be adopted:

Material Grade/Type Comments

Steel internal S275 / S355 BS EN 10025 : 2004

Steel external S275JO / S355JO – Rolled Open SectionsS275J2H / S355J2H – Hollow Sections

BS EN 10025 : 2004BS EN 10210 : 2006

Stainless steel Grade 316 BS 1449: Part 2: 1983

Concrete general Grade C35/40Sulphate class 2

BS 8500: Part 1: 2002

Concrete for metal deck slabs Grade C28/35 normal weight concrete BS 8500: Part 1: 2002

Concrete Reinforcement Deformed high yield bars (500MPa) BS 4449: 2005

Within the design the following properties shall be assumed for materials:

Materials Specific Weightϒ

Modulus of Elasticity E

Shear Modulus of Elasticity G

Poisson ratio v

Coefficient of thermal expansion α

Kg/m3 kN/mm2 kN/mm2

Mild Steel 7850 205 82.2 0.3 12x10-6 /°C

Concrete typically 2400 14 long term / 28 short term

- 0.15 12x10-6 /°C

9.3 DurabilityAll new elements of the building structure will be designed to be adequately durable under the relevant conditions, to achieve the specified design life.

For concrete elements, this will be achieved by specification of suitable mix and provision of sufficient cover, in accordance with the relevant standard.

9.4 Corrosion protectionSteelwork will be present in the building in the atrium façade and the roof structure. As the atrium façade is external it will require an external corrosion protection specification. This will be paint applied in the form of a suitable primer, barrier and topcoat. Consideration should be given to access for maintenance at the end of paint life, typically 25 years.

9.5 Fire protectionRequired fire resistance periods for structural elements are:

Structure generally 90 mins

Fire-fighting shafts 120mins

UKPN room 240mins

Roof structure Not required

Reinforced concrete elements will have an appropriate level of cover specified.

Applied fire protection to steelwork could be intumescent coating, fire spray or board, and will be specified by the Architect.

Structural elements supporting roofs are not required to be fire resistant unless the supported roof is to be used as a means of escape.

9.6 Concrete finishesConcrete floor slabs will generally have a standard floated finish.

Special concrete finishesRefer to the Architect’s specification for any special finish requirements to visually exposed concrete.



10 Design Standards and References

10.1 Statutory regulationsThe design of the building structure shall comply with the following regulations and bye-laws:

Building Regulations 2000 and the Building Act 1984

Health and Safety at Work Act 1974

Construction (Design and Management) Regulations 2007

10.2 Codes of PracticeThe design of the building structure will be carried out to the relevant British and European Standards, Codes of Practice and Building Regulations, and their various amendments.

Specifications

National Structural Concrete Specification (NSCS) for Building Construction, current Edition, as expanded and amended by the Project Specification

National Structural Steelwork Specification (NSSS) for Building Construction, current Edition, as expanded and amended by the Project Specification

10.3 Design guidanceSupplementary reference documents will be used during the design of the building, in addition to the above Standards and Codes of Practice. These include:

Design Guide on the Vibration of Floors SCI publication 076: The Steel Construction Institute: 1989

Reinforcement Detailing Manual Ove Arup Partnership: 2008

Structural guidance note 1.2: Good Practice Guide – Calculations Ove Arup Partnership: 2000

Appraisal of existing structures (Third edition) IStructE publication: 2010

Refurbishment of concrete buildings: structural and services options BSRIA Guidance Note 8/99:1999

Historical approaches to the design of concrete buildings and structures Concrete Society Technical Report No. 70: 2009

CIRIA Design of reinforced concrete flat slabs to BS 8110: 1994

CIRIA Design of shear wall buildings: 1984

CIRIA The design of deep beams in reinforced concrete: 1977

appendix g structural report and drawings

Documents