Force lines in semicircular and segmental arch bridges

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The  semicircular arch (left) has a voussoir arch thickness about one fourteenth of the span.  In the 15th century, Alberti recommended a ratio of less than 1: 15.  It can be seen that the best fit parabola is only just contained within the voussoirs, yet Alberti knew nothing of lines of thrust and his recommendation was empirical. Modern investigations by Couplet, Milankovitch and more recently Heyman, all  demonstrated, in fact,  that true lines of thrust in such arches required a minimum ratio of around 1:18.(width/span),  for all stress to be compressive.  This underpins the application of  Heyman’s Safe Theory.   In fact, many semicircular bridges with a greater ratio than this appear to be stable.  For example, in Fife, Guard Bridge's semicircular arch has a ratio of almost 1:30, and yet it has been standing since the 16th century.   

As an explanation, a wider flatter parabola might might be considered, to get round these limitations.    Below left is a theoretic force line in which the upper part lies comfortably in the middle third of the arch.   However the lower parts of the force line will not do at all. 

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The solution lies in moderately high abutments which will then contain the forces. (This whole proposition is really the wrong way round: abutments  permit the masonry to contain a wider range of parabolas/ catenaries).   There are further advantages in this. The arch could be very slim but still contain at least one suitable force line.  Furthermore, the angle that the force line makes with each voussoir joint is much more right-angular which confers additional strength and stability.   

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In fact, the implication is that the parts of the voussoir arch  below the top level of the abutments are really redundant and could be filled entirely by thicker abutments.    The result  is a raised (or stilted) segmental arch as in the above right railway bridge.  The equilibrium of forces is effectively the same. The difference is cosmetic. Curiously, because of the improved relationship between the line of force  and the voussoir joints, it may be a more stable option than smaller semicircular arches which have less abutment support.  

Why do we see so few catenary arches on buildings and bridges? Since the early 20th century, when the Serbian engineer Milutin Milankovitch fully described the geometry of force lines,  it was quickly acknowledged that catenary or parabolic architecture was expensive compared with rounded or segmental arches. The wooden centering was difficult to optimise, as the radius of curvature varied through the arc. If ashlar was used,  each voussoir needed a tailored template. It was cheaper to use loading and buttressing to manoeuvre the force lines to match the geometry.   Furthermore, masonry had ceased to be the only option: new reinforced materials with tensile stregth had arrived, hugely reducing the weakness of non-optimal shapes.    Some would also say that pure structural integrity cannot be the only determinant of architectural design.  

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Dec. 2012                                      Site last updated  March 2018