Structural solutions for the Twickenham Stadium roof

22 June 2023

In 2019, the Ridge Structural Engineering team was commissioned to strengthen the roof of the internationally famous Twickenham Stadium in London so it could safely bear the load of two large new PVC advertising banners for British Airways. Here, we talk to Winchester Structural Engineering Partner James McCulloch about the pioneering project and its challenges.

Proposed roof banners 3D view (Image courtesy of Pearce Signs)

What was the background to the project?
Twickenham is on the flight path to Heathrow Airport and can be seen clearly by thousands of passengers a day as planes descend towards the runway. The Rugby Football Union (RFU), which owns the stadium, took advantage of this fact by leasing roof space for two very large advertising banners to British Airways as part of a £30 million sponsorship deal.

The banners would essentially become a second roof structure, in terms of potential wind and snow loads, with the load effectively doubled because of the existence of the banner structure on top of the existing roof structure.

Ridge was asked by CBRE, which is involved in the day-to-day operation of Twickenham Stadium on behalf of the RFU, to check the capability of the structure and the proposed structure design to accommodate the two banners, one each for the East and West stand roofs (98m x 28.55m). Ultimately, we re-engineered the entire design to ensure the banners were structurally secure and safe.

What were the key challenges?
The stadium had been built in stages over several decades, with each quadrant of the stadium separated from others by movement joints. Modern construction works require detailed recording of information but, as much of the development at Twickenham is more than 20 years old, often much older, we were confronted with large gaps in information – perhaps the biggest problem with which we had to deal.

We had to physically go up into the roof and find out what we needed to know by inspection (including crawling into confined roof spaces to examine how elements worked and connected with each other). There were a few occasions where we went to site expecting an element to be in place which wasn’t there at all!

The strengthening works, including the design, fabrication and installation of new members and the strengthening of existing members and connections needed to be completed within a six-month timeframe.

What solutions did you employ?
The new banners were be fitted to the top chords of the roof trusses; we knew that, if wind were to travel over the banner, it would create uplift, effectively compressing the top chord and increasing the buckling load. It was clear that the ties between the trusses needed to be upgraded, with additional tie members to reduce the effective length of the top chord. Due to the nature of the tensile fabric banner and the offset bracketry, the supporting members needed to be designed for combined axial, bending, shear and torsion loads. Analysis of the banner was carried out by a specialist designer, Tensys, and the loads identified were applied to our global analysis model.

We identified significant overstress in some of the members, so we knew we had to reduce the compressive lengths and re-engineer the whole bracing stability system in a revised 3D BIM model. We modelled up to three-quarters of the entire roof space in BIM (everything except the two ‘ends’ of the bowl), including smaller details (such as that shown in the images below, before and after the new load is applied).

Truss node connection load input and wireframe model

Stresses in truss bottom node connection

Photo of existing connection

We undertook detailed wind calculations from 12 directions around the stadium, calculating worst case uplift and downward pressures on the roof. The approach direction of the wind was important, due to the possibility of wind travelling over one stand and gusting up underneath the opposite stand. The possibility of wind funnelling between the banner and the sheeting was also considered. We always remained within Eurocode parameters, introducing load reductions where possible and, where we found excessive stress, providing cost-effective solutions.

Load cases were set up to check for combined snow and wind loads, scaffolding and construction loads, accidental conditions (in case the banner were ever to tear) and drag created by wind travelling perpendicular to the roof sheeting.

We took out sections of the steel structure in areas that weren’t very highly stressed and had them lab-tested to ensure that we understood how well the steel performed. The existing truss members were surveyed with ultrasonic thickness gauges to establish steel wall thicknesses and suitable members were identified for tensile testing to determine the steel grade used, and a sampling, testing and reinstatement procedure was provided to the contractor.

The connection designs by the fabricator were analysed using FEA modelling and adjusted, where necessary, to prevent overstress of the existing circular hollow section (CHS) walls. This involved plastic analysis to simulate the accurate distribution of stresses and behaviour of the connections under load, using Von Mises criterion and strain hardening. Where stiffeners and other alterations were required to the connections, these suggestions were passed on to the steel fabricator to update their designs. We re-engineered all the roof connections, which involved a lot of non-linear structural analysis.

Ridge engineers were involved in every stage of the sub-contractor design process, including reviewing scaffolding designs and checking the structure for construction loads due to temporary storage of materials as well advising on the build sequence to ensure the structural integrity and stability of the roof structure at all times. We had to consider the construction sequence and scaffolding loads to ensure the stability of the roof and prevent undue sway of the trusses.

How did Ridge add value for CBRE and the RFU?
The rooftop advertising banners project has been planned by the RFU for many years but had never been implemented due to concerns about the strength capacity of the trusses. Ridge structural engineers successfully made the case for, and designed, the strengthening works which finally made the initiative viable.

We were able to reduce costs for the client by using an effective, cheaper alternative to the initially proposed and very expensive Macalloy tie rods – the latter are aesthetically pleasing and architectural, but unnecessary for a place like Twickenham Stadium, which has a more functional, industrial feel. Instead we used steel CHS ties, which cost a quarter of the price, have much shorter lead-in times and do the same job just as well.

We also liaised effectively with the construction contractors who were re-roofing the stadium at the same time, ensuring that what we proposed to facilitate the banners did not interfere with the aim of allowing more light into Twickenham through the re-roofing work. Roof sheeting on the underside of the roof was replaced with lightweight and transparent plastic sheeting to allow the light in, as shown in the below image.

Trusses with new roof sheeting installed

Can you give us examples of some of the re-engineered designs?
The images below show one of the non-linear tensile connection points we analysed and strengthened. The stadium roof exerted an existing load on these connections, but we had to ensure it could cope with extra load. We squashed the tube enough so that it had the right level of force going through it and then applied the projected new loads to that, assessing what would happen. The roof would have failed, for example, if we had attached the four elements without the vertical disk at the central joint, which prevents the tubes from pulling apart.

Plastic analysis of stresses in connection

Plastic strain analysis of connection

Photo of installed connection

This is one example of many micro-models we created to analyse the roof structure and test resilience, applying the information we had from the macro-scale roof models (such as the East Stand model below). This detailed level of analysis will help ensure that there will be no need for significant roof repairs for a long time.

East Stand structural model

The below images show details from a rear section of the roof during construction, including the scaffold gantries. The detail model is an example of where we collaborated with the steel fabricator to ensure the product they created was appropriate, here ensuring that a load-bearing junction was strong enough (the blue of the collar indicates its resilience). We maximised the efficiency of the collaboration process, both with the steel fabricator and the contractor, by running plastic analysis of the connection designs in alignment with the contractor’s connection installation sequence to ensure that the connections could be installed immediately after the checks had been completed.

Stress distribution in collar detail for ties at back of East Stand

Photo of installed connection

Scaffolding installed on trusses

Was it a challenging programme?
The project schedule was quite tight, but we worked hard to meet it. Construction continued even while international games were being played (although not on match days) – we achieved as much as we could during the gap between the Autumn Internationals and the 2020 Six Nations Championship, but we still had to continue our work during the opening stages of the latter to meet the desired programme. The programme was slightly complicated by the arrival of the Covid-19 pandemic and lockdown during the Six Nations tournament (March 2020), but we had, by then, largely completed our work (analysis, construction-stage loadings, etc.).

CBRE and the RFU are very happy with the resilience of the structural design solutions we delivered to enable the advertising banners to remain in place safely and securely for a long time. Should the space need to be used for someone else’s advertising banners, swapping over will be easy so long as the dimensions remain the same.

The project was a demanding one, but it was immensely rewarding to see our structural solutions, at both micro and macro levels, overcome the problems we faced initially and enable the installation of what is becoming a very well-known sight on the flightpath towards one of the world’s busiest airports. It was a bonus that we were also able to value engineer the designs without in anyway compromising their resilience. The lengths to which we and the wider project team went to ensure that the roof is robust enough to bear the new loads will ensure the safety of those within and near the stadium for the many years to come in which the banners, or their replacements, will be in place.

We have also recently provided roof inspection services for CBRE in relation to the Emirates Stadium, home of Arsenal Football Club, although without the advertising banners – checking the structural resilience of the roof in detail as loads are added in some areas and removed in others. There was only 2D information for this structure, so we are currently creating a detailed 3D BIM model to enable comprehensive analysis and recommendations.

The Ridge Structural Engineering team is highly experienced in producing advanced analysis for stadium roof structures to ensure that they can bear varying loads, with the capability to engineer cost-effective solutions while putting safety and resilience at the heart of the design.

For more information, please contact:

James McCulloch
Structural Engineering Partner