Understanding long span roof design

Author: Jon Leach

Date published

9 April 2021

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Understanding long span roof design

Date published

Jon Leach

Date published

9 April 2021


Jon Leach

What do we mean by “long-span”?

Long-span roofs are often associated with iconic projects, be they stadia, airports or other major public buildings. For us as building designers they are often quite unique in the level of integration and honest expression of their architecture and engineering, from the visual drama to the creation of adaptable, multi-functional structures. But what do we mean by “long-span” in this context? 
Many structural forms may fall into this category, from simple planar trusses to complex space frames, arches and shells to catenary and tensegrity structures. Some may define a “long-span” as being greater than an arbitrary distance, say 50m, but from a structural engineering perspective it is best to categorise these structures as those where many of the simplified modelling assumptions that might be justifiable for more general structures cannot be applied. For example, long span structures are typically designed considering the global stiffness of the structure as a whole and not just the stiffness of individual members. This can lead to situations where deformations can be much greater than the cross-sectional size of the structural elements within, requiring complex non-linear analysis.
Other examples that require us to move beyond the norms of design codes, and towards more experience and performance-based design, include the management of thermal effects and movements, global dynamic behaviour and excitation, methodical consideration of robustness and the avoidance of single-point failure, the impact of load reversal in light-weight structures, and the need for CFD analysis and physical testing to accurately predict wind and snow loads.
Constructability is another major consideration for free-standing roofs, with up to 40-50% of the total cost commonly associated with their erection. The extent of temporary works and the logistical constraints of transportation, lay-out space and craneage should be an early consideration, often informing the choice of structural concept as well as the detailed design. Furthermore, the effects on the design of locked in forces and sequential erection can have very significant effects on the design compared with a simple static analysis of the completed structure, particularly for indeterminate structural forms subjected to load reversal under different support conditions.

Many of the issues associated with the delivery of large roofs are therefore holistic challenges that impact on all aspects of their design and construction, whether applied to distinctive architectural statements or to more conventional utilitarian buildings. 

A sustainable form and function?

Stadia, as one example, have often been conceived as dramatic in scale.  Many modern venues rely equally on the ability to create intimacy and social interaction at a human scale through beautiful and elegant design detailing.
But in a world where the environmental impact of the built environment, particularly in terms of embodied and operational carbon, is rightly a key focus for practising engineers, is there still a place for such iconic structures?  Some can perhaps appear as wilful and extravagant, but many of the best designs have evolved to be much more than just a simple canopy or an architectural skin.
For example, the theory that many of the best stadia are designed from the inside often rings true, and the integration of the bowl and roof design is essential in generating a holistic design. 
The roof design itself can be a complex interaction of architectural form, shelter, insulation, pitch conditioning (sunlight and ventilation of natural turf), acoustic atmosphere and sound quality, ventilation, the positioning of lighting, speakers, gantries, audio-visual and communications technology, camera and broadcasting platforms, rigging systems, drainage and fire protection.  
Similar can be said for the roofs of other major public spaces, for example airports and ground transport hubs, or even shopping malls and public squares.  Although frequently perceived as “light-weight” envelopes, they often incorporate complex back-of-house functionality that requires the seamless integration of an abundance of building services with associated access and maintenance requirements that avoid impacting on the activities below. 
Therefore the inter-disciplinary coordination of a well-executed roof can be as complex as the internal design of any other building, which makes the engineering of such structures a challenging and inspiring proposition.
As an example, for the design of Al Janoub Stadium for the Qatar 2022 FIFA World Cup, AECOM’s parametric modelling of the steel roof structure, which spans over 200m, was used to optimise between structural efficiency, envelope surface area, pitch condition, the performance of the bowl and pitch cooling systems, and the incorporation of state-of-the-art lighting and audio facilities.  Fine-tuning of the cooling systems involved a highly innovative and research-led approach, including transient energy modelling, sun path analysis, aerodynamic optimisation of the roof and bowl shape, and validation with physical wind tunnel testing.  The structural analysis was just one of a multitude of input variables in this process.

Innovation in the form of new high-performance materials, computational design and modern fabrication methods allow us as engineers to continue pushing the bounds and the art of the possible.  But innovation comes in many guises, and this can still include the use of simple, low-technology solutions to suit buildability and budget constraints.  Each project is different, governed by different drivers and performance requirements, and best-practice design requires a considered, appropriate response to the needs of any given client and project.

And as well as the complex technical aspects the importance of such structures in being socio-economic catalysts to inspire people and attract investment cannot be under-estimated.  So, in my view, yes there is still a place for these iconic structures.  But those functional and socio-economic benefits, as well as the pure structural efficiency, must be carefully weighed up as we strive to achieve our net-zero carbon goals as an industry.

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