View source for Creechurch Place, London

==Article in [https://www.newsteelconstruction.com/wp/wp-content/uploads/digi/2016pdf/NSCApri2016digi.pdf#page=14 NSC April 2016]==
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'''Steel insures City tower'''<br>
A [[Braced_frames|steel-framed]] 19-storey [[Multi-storey_office_buildings|office development]] is the latest high-rise addition to the City of London’s insurance district.<br> 
''By Martin Cooper''<br>

The [[Construction|construction]] of new [[Multi-storey_office_buildings|commercial office]] space in the City of London shows no sign of slowing down. An array of [[Construction#Tower cranes|tower cranes]] on the City skyline is proof that a mini-building boom is in full swing and likely to continue into the near future. Plans are afoot in the square mile for grander schemes, with one 73-storey development recently seeking planning permission. This kind of statement structure is far from the norm in London as the majority of new [[Multi-storey_office_buildings|office developments]] are usually around 20 storeys high.

An example is Creechurch Place, a 19-storey T-shaped [[Multi-storey_office_buildings|office development]] currently being built close to 30 St Mary Axe (The Gherkin) in an area of the City dubbed the insurance district because of the abundance of underwriters. The site occupies an important position in the City of London and the aim is to deliver a new, modern, [[Multi-storey_office_buildings#Flexibility and adaptability|flexible]] and efficient [[Multi-storey_office_buildings|office building]] of the highest quality.

The [[Construction|construction]] will deliver a structure comprising two levels of basement, ground floor and 17 upper storeys, plus rooftop plant, providing 25,350m<sup>2</sup> of flexible, Grade A [[Multi-storey_office_buildings#Flexibility and adaptability|flexible]] and efficient [[Multi-storey_office_buildings|commercial office]] space and 284m<sup>2</sup> of retail/café space. Below ground floor the structure’s basement is formed with a concrete frame within a perimeter secant piled wall. From ground floor slab upwards the building is [[Braced_frames|steel-framed]].
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“The [[Design|design]] for the majority of the tower was always a steel building as the material enables the structure to have [[Multi-storey_office_buildings#Long span systems|long clear spans]], which would not have been possible with concrete,” says Ramboll Project Engineer Rebecca Archer. “The first floor along the main elevation is set-back and hung from the floor above via [[Steel_construction_products#Structural hollow sections|CHS sections]], creating a column-free entrance area. This important [[Design|design]] feature would also not have been possible in any other material than steel.”

Ground floor has a higher floor-to-ceiling height throughout, and as well as accommodating the building’s frontage and main entrance lobby along the top of the T-shape, the other part of this level has back-of-house and refuse collection areas. Interestingly, the ground floor is the only part of the scheme that does not extend over the site’s entire footprint, as the top part of the T is split from the upright stem section because the building spans over a pedestrian thoroughfare. This walkway has been temporarily closed during the [[Construction|construction]] programme, but has to be reinstated as part of the scheme’s planning conditions.
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Main contractor Skanska started on site in August 2014, following the completion of the demolition programme. “One of our initial tasks was to deepen the existing basement by 12m to give us the required two underground levels,” says Skanska Project Director Paul Davies. “The concrete basement floors, ground floor slab and two slip-formed [[Concept_design#Concrete or steel cores|cores]] were then completed in readiness for the steelwork [[Construction#Steel erection|erection]] to begin last August.”

Steelwork contractor William Hare was involved in the [[Design|design]] of the project early in the process and this helped improve efficiency. “Not only was a value engineering process undertaken, but we also had the majority of the steelwork [[Fabrication|fabricated]] and [[Paint_coatings| painted]] ahead of programme, which eliminated the possibility of delays later in the [[Construction|construction]] sequence,” says Mr Davies.
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The steel frame is based around a regular [[Concept_design#Floor grids|grid]] offering [[Multi-storey_office_buildings#Flexibility and adaptability|open-plan office space]] with column-free spans of up to 16.5m-long. Only two internal columns are present throughout the entire structure. William Hare has installed Fabsec [[Long-span_beams#Composite beams with web openings|cellular beams]] throughout the majority of the structural frame to allow the building’s [[Service_integration|services to be accommodated]] within the structural void.

Despite the long spans, the use of [[Long-span_beams#Composite beams with web openings|cellular beams]] was not an issue for the [[Floor_vibrations|floor vibrations]]. Ramboll used its experience of [[Floor_vibrations#Dynamic testing of floors| in situ testing]] to demonstrate that through careful consideration of the layout and interactions between the [[Long-span_beams#Composite beams with web openings|cellular beams]], they could still be used efficiently, without having to adopt the more traditional method of using heavier beams to increase the stiffness. “This would generally add weight and cost to a project,” says Ms Archer.
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For the steel [[Construction#Steel erection|erection]] William Hare split the structure into four zones for its [[Construction#Steel erection|erection]] programme. This allowed two gangs to work their way up the building, two floors at a time, in a staggered methodology along with the [[Construction#Installation of metal decking| metal decking installers]]. “All of the steelwork has been [[Construction#Steel erection|erected]] by the site’s two [[Construction#Tower cranes|tower cranes]],” says William Hare Project Manager Ivo Garcia. “It’s all been within the [[Construction#Tower cranes|tower crane’s]] lifting capacity, including the project’s heaviest members – two ground floor column sections each weighing 14t.” The members are 1,350mm × 350mm box sections and they are larger than the other columns along the front elevation because they are unrestrained, positioned on corners and have been designed to absorb vehicle impact loadings. From second floor upwards the [[Steel_section_sizes|section sizes]] decrease.

The principal [[Facades_and_interfaces#Types of façade systems|cladding system]] is a unitised, interactive double skin - a double-glazed inner layer, single-glazed outer layer and an operable blind in the cavity between. This is said to allow control of [[Facades_and_interfaces#Solar gain, light levels and views out|solar gain]] and optimises natural daylight within the offices. Anodised aluminium vertical fins of varying colours and spacing, improving the [[Facades_and_interfaces#Types of façade systems|façade]] performance and adding vertical emphasis, enhance this system.

The building will be completed in September and is aiming for a [[Sustainability#Understanding BREEAM| BREEAM ‘Excellent’ rating]] and a B-rated Energy Performance Certificate.

==City logistics==
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City centre [[Construction|construction]] projects always throw up logistical challenges and Creechurch Place is no exception. As the building occupies the entire site footprint there is little or no room for material storage, so steelwork is [[Fabrication#Handling and transportation|delivered to site]] on a just-in-time basis.

Along the main Dukes Place elevation the project has a pit lane that can accommodate two trucks for unloading. None of the other elevations of the building are suitable for deliveries as they are either party walls, narrow roads or pedestrianized zones such as Mitre Square. However this means the neighbouring buildings, including the John Cass School are very close

“Keeping the neighbours happy is very important and we have a number of quiet times on site that corresponds with the school’s playtimes,” explains Skanska Project Director Paul Davies.

==Dynamic performance of the Creechurch Place office floors==
''By Dr Richard Henderson, SCI''<br>
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The office floors at Creechurch Place are largely column-free spaces because the narrow floor plates allow secondary beams to span across the full width of the floor. The [[Long-span_beams#Composite beams with web openings| composite Fabsec cellform beams]] at centres varying between 3m and 3.25m are 725mm deep and span up to 16.5m. Perimeter columns support primary beams spanning up to 10.5m that carry as many as three secondary beams. In some parts of the floors the secondary beams cantilever beyond the primaries.

The [[Floor_systems#Composite slabs|floor slabs]] are nominally 130mm thick normal weight concrete and consequently give better [[Floor_vibrations|dynamic performance]] than would have been achieved with lightweight aggregate concrete. The higher mass of the [[Floor_systems#Composite slabs|floor slab]] results in lower accelerations for a given input force (such as [[Floor_vibrations#Human induced vibration|footfall excitation]]) and therefore lower [[Floor_vibrations#Design criteria for vibrations|response factors]]. The effect on dynamic performance of the higher stiffness of the normal weight concrete is not significant.

[[Floor_vibrations#Vibration analysis|Dynamic analysis]] of the floors was carried out using Robot structural analysis software to perform a modal analysis followed by a time history analysis with moving footfall input. Finally the [[Floor_vibrations#Design criteria for vibrations|response factors]] of the floors were determined and compared with [[Floor_vibrations#Regulations and design rules|acceptance criteria]]. Ramboll have found it an effective arrangement to introduce a line of tertiary beams spanning between the secondaries to reduce the tendency of secondary beams to vibrate on their own. The size and position of the tertiary beams along the span are adjusted iteratively to determine their most effective arrangement. This approach has been found to be 15% more efficient than achieving a satisfactory result merely by increasing the size of the secondary beams.

The [[Floor_vibrations#Damping|damping]] ratios normally assumed in [[Composite_construction|composite floor construction]] are 1% of critical damping for an as-constructed floor with no fit out, and 3% of critical for a fitted out office floor. (These values are recommended in [[Floor_vibrations#Resources|SCI publication P354]] Design of floors for vibration: a new approach). A lower [[Floor_vibrations#Damping|damping]] ratio was used in the analysis of the Creechurch Place floors (with potentially higher responses) because in-service measurements by Ramboll have shown that for a number of lightweight fit out arrangements the actual [[Floor_vibrations#Damping|damping]] ratio achieved was between 2% and 2.5% of critical, i.e. less than the value of 3% often used in [[Floor_vibrations#Vibration analysis|analysis]].

Full height internal partitions are known to affect the [[Floor_vibrations#Damping|damping]] ratio in floors so where they are present in the fit out, 3% [[Floor_vibrations#Damping|damping]] remains a reasonable upper-bound value.
 
{|class="wikitable" width="400"
|-
|'''Architect'''||Sheppard Robson
|-
|'''Structural Engineer'''||Ramboll
|-
|'''Steelwork Contractor'''||William Hare
|-
|'''Main Contractor'''||Skanska
|-
|'''Main Client'''||Helical Bar / HOOPP
|}

[[Category:Case studies - Multi-storey office buildings]]