Next came the designs of the steel framework, known as a diagrid because it’s made of diagonally intersecting beams, and the lightweight facade. These were chosen over a steel or concrete shell as the most practicable solution for a building of this shape and complexity.
With endless possible permutations for the diagrid, BuroHappold wrote its own growth algorithm to arrive at the optimal arrangement.
Creating the parametric scripting was a lengthy process, but it allowed BuroHappold to dictate outcomes – such as having the steel members be of a single diameter size and orderable from the market. “It was quite new for what we do,” says Bauly.
The result was a diagrid that was optimised in terms of the number of connection points, the diameter of the steel tubes and even the total weight of steel used – which, according to BuroHappold, translated into cost and time savings for the client, contractors and fabricators. Contractor Bam Higgs & Hill and structural steel fabricator Eversendai Engineering then conducted the construction sequencing analysis and connection design for the structure, taking into account the shifting stresses and loads during the erection process, before moving ahead with construction.
“The construction sequencing analysis is always a big risk to a project and you usually get a lot of member changes because of it,” explains Bauly. But on this project, he says, none of the steel members were altered due to the effort that had earlier gone into optimising and modelling the diagrid. “We applied the lessons from previous projects and using new technologies, we developed tools to solve problems that we know exist so we didn’t have to face them again.”
Having built the reinforced concrete ring beam and tower that support the diagrid, it took 14 months to complete the steel work. Throughout the construction process, laser scanning was used to compare the as-built positions with the 3D model.
The exhaustive 3D modelling also removed the typical problems associated with the mechanical, electrical and plumbing (MEP) installation. “The MEP usually trips up a project in the later stages of the construction phase. We thought it would be one of the biggest risks, but it has just not happened. It has all gone in so well,” says Bauly. “You have gone through all of the heartache in the digital model, so as long as you stick to what you draw, you end up with what you had in the model. It almost becomes not very exciting because of how much work is put into the modelling side of it.”
The real game changer in the switch from working with 2D drawings to 3D models has been that coordination of the MEP engineering began much earlier in the project and so problems were resolved long before work moved onsite. Laser projections of the Bim models also helped.