Reviving Historic Structural Forms: The Belfast Roof
Introduction
The Belfast roof, originating in the industrial city of Belfast during the late nineteenth century, is a testament to innovative structural engineering. This wooden roof truss system provided expansive column-free spaces and boasted lightweight construction and ease of assembly.
In a seminal article in The Structural Engineer, Professor M. H. Gould of Queen's University, Belfast, delved into the historical significance of the Belfast roof, presenting four distinct forms of this architectural marvel.
Change from wood to structural steel
It is proposed to use structural steel in place of wood and use Belfast roof configuration.
Utilizing Maxwell's Shortest Load Path Theorem, we evaluated four configurations to identify the most optimal design for modern construction needs.
GENERAL ARRANGEMENT
McTear's Design
In this configuration, the upper/top chord (arc) is equally divided ( purlin spacing). The two support points are copied down. The offset distance is kept at one-third of the span. The lines joining the new points below support and top chord nodes give bracing members.
Anderson's Design
Here geometry is developed by drawing tanget to the arc at upper chord equidistant nodes. The bracing member is drawn from the upper chord node at a 45-degree angle from the tangent line ( refer to Figure 6). This procedure is repeated for all upper chord nodes.
Modern Belfast Roof
In this form, the bracing members are drawn from equidistant upper chord nodes making 45 degrees with the horizontal (refer to Figure 8)
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Bow-string Belfast roof
The lower/bottom chord member is divided into equal parts. The bracing member is drawn at 45 degrees from the horizontal. This process is repeated for all lower chord equidistant nodes
Shortest Load Path Theorem
Maxwell's shortest load path theorem requires axial force in the truss member and the length of the member.
Four configurations are modelled in general purpose structural analysis software SAP2000 with the following critical wind load case ( wind perpendicular to span of roof)
A summary of work done in four configurations is presented in Table 1 below
The work done in three configurations Anderson design, Modern Belfast and Bow-string are close. The total length of the bracing member is lesser in Anderson's design compared to the Bow-string truss. Anderson's design is a favourable configuration for the Belfast roof.
Conclusion
By incorporating structural steel, we aim to preserve the essence of the Belfast roof while enhancing its structural performance and durability. The structural form also has application in pedestrian crossing bridges (in a bowstring configuration). Anderson's design is the optimum configuration among four roof trusses.
The inherent strengths of this structural form, including its column-free spaces and lightweight construction, make it a compelling choice for modern engineering challenges. In conclusion, by drawing inspiration from historic structural forms like the Belfast roof and integrating them with advancements in materials and construction techniques, we can reinforce our understanding of structural engineering principles while pushing the boundaries of innovation in the built environment.
References
3. Roof, A guide to the repair of historic roofs https://www.dlrcoco.ie/sites/default/files/atoms/files/roofs.pdf