[ Page 1 of 2 ]

In an engineering tour de force, Buro Happold draped a delicate canopy over Foster’s restoration and redesign of the great court, creating europe’s largest covered courtyard.
By Sara Hart

Continuing Education Use the following learning objectives to focus your study while reading this month’s ARCHITECTURAL RECORD / AIA Continuing Education article. Learning Objective: After reading this article, you will be able to: 1. Describe the structural support for the glass courtyard roof. 2. Explain the structural composition of the existing dome and gallery facades. 3. Describe the problems that can occur when supporting a roof on the existing perimeter facades. 4. Describe how the glass was designed to block out solar radiation. 5. Explain the structural composition of the roof.

Sitting in the legendary Reading Room of the newly opened Queen Elizabeth II Great Court at the British Museum, Buro Happold engineer Stephen Brown demonstrates the structural forces at work on the massive glass-and-steel roof, which covers the two-acre courtyard. He takes a business card and bends it into a segmented arch. With his index finger he secures one end of the arch, and with the other index finger, he presses down on the apex. The unsecured end pushes outward as the arch deflects. The secured end resists the load. Such a simple demonstration, and yet it reveals the mechanics that governs what appears to be a dazzling magic trick.

 

In April 2000, workers installed glass panels on the undulating, spiraling steel grid. The roof spans the Great Court without disturbing Sydney Smirke's famed domed Reading Room.

Buro Happold worked with Foster & Partners on the winning competition entry to develop the Great Court (see page 114). Its role was then expanded from the roof to include all structural, fire, and building services, as well as engineering and planning supervision for the multilayered project. Brown led the engineering team for the Great Court restoration and the expansion below it for a new educational center.

Norman Foster envisioned a lightweight, transparent shell springing from the drum of the domed Reading Room and resting on the walls of the existing quadrangle. Designing a canopy to span between a circle and a rectangle over a 230-by-328-foot courtyard was a computational and geometric feat in itself. The project was made more difficult by existing conditions and height restrictions. The famous circular Reading Room, designed by Sir Sydney Smirke, is 140 feet in diameter and rises 102 feet from the floor to the lantern of the dome. Built of cast and wrought iron, it was considered an adventurous design in the 1850s. A frame of 20 iron ribs was clad externally with a brick drum pierced by large arched windows between the ribs. According to Brown, “In overall structural terms, the Reading Room is a braced shell in which the iron framing and the brickwork provide mutual restraint and support.” Endoscopic tests showed that the structure of the Reading Room was too brittle to endure any movement from lateral loads applied. As a matter of fact, Brown judged the tolerance to be no more than half an inch.

A heavy load
Demolition of the courtyard to create basement levels and new foundations required surgical precision in order to avoid any displacement of the fragile Reading Room. Tight site conditions made the use of standard piling rigs impossible. Instead, a jet grouting system was employed. Constant real-time monitoring of the vibrations allowed the contractor to make adjustments throughout the process. In the end, the final displacement was less than 1¼8 inch.

A similar problem regarding lateral loading occurred at the quadrangle facades, which enclose large open galleries with few load-bearing walls to brace the exterior walls. Therefore, these massive facades essentially have no lateral support. They support only the vertical load of their own weight and the static load of the roof. Brown had to consider how to support the perimeter of the glass canopy on the walls without creating lateral loading. “Any lateral loads [from the roof] would have caused racking and collapse,” he explains.

 

Sightlines of the famous Reading Room dome were manufactured after the Great Court canopy was erected. During demolition, the exterior of the Reading Room was revealed for the first time in 150 years.

Creating the grid
Happold’s solution is ingenious. The finished shell is a continuous torus, which many have described as a square doughnut cut horizontally. The shape of the dome is due in part to height restrictions. Because the copper-clad dome is one of London’s classic landmarks, the city required existing sightlines to be maintained. This limitation resulted in a toroidal shape instead of a more conventional arch.

The roof’s shell structure spans in three directions from the four sides of the quadrangle on to a new seven-foot-wide reinforced concrete slab, identified as the snow gallery, topping the drum of the dome. The gallery is supported on sliding bearings so that it floats above the Reading Room and is supported by 20 new concrete-filled tubular steel columns, which circle the Reading Room and carry the roof load to the new foundation. Therefore, no additional load is applied to the Reading Room, and the columns are hidden behind the Spanish stone cladding of Foster’s elliptical facade.

Happold used sliding bearings as well on the quadrangle facades. They allow the roof to spread laterally under load, perpendicular to the relevant facade, and independent of the facades. “This freedom means that for the roof to hold its form, the outer radial members near the perimeter quadrangle must work in bending and compression,“he explains. “The structure is further stiffened with a tension cable across each corner.”

Cross bracing occurs behind each of the four porticoes working parallel to the relevant facade. “At the center of each side of the roof, behind the porticoes, the lateral spreading movement of the roof is one-directional, normal to the line of the facade,” says Brown.

Happold initially calculated the geometry of the roof using standard static, or linear, computer programming. Such programming considers the structural integrity by examining the gravity effects alone. To study the deformation, a custom dynamic, or nonlinear, program was developed by Chris Williams, a consultant to Happold and a mathematician at the University of Bath. Using a program that described the overall shell mathematically, engineers could modify it and study the consequences of each change.