Principal Tower

WSP has utilised Bentley's technology to overcome complex challenges in delivering a new high-rise tower in London, subsequently winning the Structural Engineering category at the Year in Infrastructure 2020 Awards

Bentley's Year in Infrastructure conference was held online last November, for obvious reasons, but rather than limiting access and content, it gave an opportunity for many more Bentley users across the globe to tune into a very impressive line up of keynote sessions, presentations and tuition videos. Neither were entries to the Year in Infrastructure Awards affected, as all categories were well represented and available online for full scrutiny by interested architects and engineers in each technology.

We are always especially interested in UK-based winners, and this year we were delighted to see entries from Network Rail, winners of a special recognition award for Advancing Virtualisation through Digital Twins for its work in overcoming challenges in the Covid-19 lockdown, and SWECO, winners of the Advancements in Project Delivery Award with its Digitalisation with BIM.

It was the Structural Engineering award given to WSP for its innovative approach to the design of a new addition to London's skyline, Principal Tower, for its client Brookfield Properties & Concord Pacific, that particularly interested us. Building anything of substance in the centre of London is always going to be a challenge, not only because of the wealth of historical architecture, but also because of the concentration of adjacent construction above and below ground and the supporting substructure that accommodates services and a complex underground transport system.

The main challenges that WSP had to overcome were the very complex site constraints. Due to the presence of a height restricted viewing cone to the west of the site and a 6-track Victorian aged railway infrastructure to the east, the 50-storey tower had to be located above a 2-track protected railway corridor (for future expansion) located next to the existing 6-tracks, therefore creating complex transfer structures at ground level as well as construction challenges.

The challenge at feasibility stage, and where Bentley products were used effectively, was to persuade the local railway authorities that constructing piles 50mm away from the Victorian railway infrastructure would not have any detrimental impacts. WSP used Bentley's Plaxis and Ram Concept in unison to design the top down approach.

Multiple design cases were analysed in Plaxis to understand the movement of the adjacent tunnel during the excavation, construction and long-term periods. The predicted movements in each case indicated that a horizontal prop would be required to reduce any detrimental impacts on the tunnels. Top down construction fitted this challenge perfectly, allowing the ground floor slab to be built first, which acted as this permanent prop, hence ensuring tunnel integrity. The predicted lateral forces exuded by the arches were also extracted from the Plaxis model and applied to the ground floor slab, which was designed in Ram Concept. This cross-platform analysis gave WSP confidence to present the solution to the railway authorities and for them to give permission to proceed with the development.

WSP effectively used Bentley products on other parts of the building. One being the use of RAM concept in an innovative approach to the design of the junction between two critical supporting elements, the 3m deep piled raft and the surrounding 500mm deep ground bearing raft at basement 2 level. A series of RAM Concept models were created with varying dead loads to replicate the Tower construction sequence.

This allowed WSP to identify the areas of high stress and specify a pour strip to be infilled when the superstructure was near completion, as at this stage most of the vertical movement had occurred under the self-weight of the structure. This solution actually resulted in a 60% reduction in reinforcement requirement, which was equivalent to 17 tonnes. Although a relatively modest material cost saving on paper, there are other factors to include such as labour cost and health and safety to on-site personnel.

The refined design methodology resulted in the reduction of two layers of large diameter reinforcement bars over a considerable area. There is a significant cost in labour relating to the construction of these large rebar (40mm diameter) which sometimes require multiple operatives to install just one of the bars in some locations. It also reduced the risk relating to the health and safety of construction workers who have to handle such heavy materials, a perfect example of using Ram Concept to show how good design practices can improve the health and safety risks of the on-site operatives.

It is interesting to note how WSP addressed the changing forces that large scale construction projects are subject to during the whole of the construction phase. RAM Concept analysis was used to give advice to the contractor about the impact of temporary works to the substructure slab designs, particularly as the analyses also included the assessment of tower crane bases, hoists support, mobile crane loads amongst other things.

Even more interesting, the contractors were able to dispense with the customary temporary propping that is usually required from ground floor slab down to the foundations to accommodate higher construction loadings associated with dissembling tower cranes. Due to the confidence in the optimised design, WSP using analysis in Ram Concept could demonstrate the ability to share extra capacity out of the slab. Being able to validate to the contractor that propping wouldn't be required saved them money and time.

WSP developed their innovative engineering solutions to unlock this challenging site using RAM Concept and Plaxis. From modelling 3m deep rafts, designing slabs on elastomeric bearings, hanging swimming pools and indirect outriggers to long term ground movement predictions Bentley's tools have been critical for the viability and timely delivery of the Principle Tower project.

Explaining the rationale behind using WSP's innovative use of Plaxis and RAM Concept, Brandon Eastwood, Senior Structural Engineer at WSP said, ''Because construction in London is heavily constrained by the presence of tube tunnels and railway infrastructure, our project data did double duty. Not only did it guide our design, but it helped get the project off the ground by proving the structural integrity up front to the local railway authorities, therefore allowing the clients vision to be realised.''

He expounded further on this, saying that the data they were able to provide in such a constrained site proved WSP's ability to maximise the footprint and comply with the spirit of local codes. Although there is no set rule on how close construction can occur to these assets, an exclusion zone is generally set of at least 1m, not 50m, which was only achieved by instilling confidence in the local rail authorities following the Plaxis analysis.

If this 1m guideline would have been required at Principal Place, the entire development (including the 50-storey tower) would not have been feasible, as the building would have encroached into the viewing cone to the west. The combination of RAM Concept & Plaxis was instrumental, therefore, in delivering a design that provided the ROI to the client which proved that the project was economically viable.

Whilst the innovative practices outlined above won WSP the Structural Engineering category at the Year in Infrastructure Awards, the project had worthy results in other areas too. It was delivered in a BIM environment, providing an excellent example of an embedded sustainable design from a project's inception.

It was also developed with a bespoke Sustainability Implementation Plan (SIP) which was used extensively to monitor the design process. This included features such as a fa├žade designed to maximise daylight while minimising summertime solar gain, and office floors (in the 11-storey commercial building adjacent to the tower) designed to accommodate traditional HVAC strategies, along with ultra-low energy mixed mode ventilation, extensive energy metering, and the specification of high-performance plant and equipment. The building achieved an improvement over Part L by up to 30%, an EPC B rating and a BREEAM rating of Excellent.

Nello Petrioli, WSP Associate Director and Project Engineer on the Principle Tower project, said "Much like fine craftwork relies on excellent tools, engineers rely on software to produce fine engineering solutions. Bentley's applications have proven to be an indispensable tool in providing the fine engineering solutions necessary for a highly complex project like Principal Tower."