Arch Themes and Variations

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How many different forms of arch can you imagine?  As an example, let us consider steel arches, such as Hell Gate bridge or Sydney Harbour bridge.  Both bridge have four chords, only the lower two reaching the abutments to take the thrust.  How can we vary the design?

Firstly, we could have any number of chords from one to four.  With two arches, they could be in the same vertical plane, like one half of Sydney Harbour bridge and Hell Gate bridge.  They could also be parallel horizontally.

Two or more chords can be more or less parallel, or they can taper towards the pins at the springing.  With three or more chords, the taper can be horizontal or vertical, or both.  The chords can taper the other way, towards the crown.

With any number of chords, we can have no pins, pins at the springing, and also a pin at the crown.

With vertically displaced chords, both chords can reach the abutments, or only the lower chord can reach the abutments, as in Sydney Harbour bridge and Hell Gate bridge.

The chords can be box-girders, I-beams, plate girders, tubes, etc. Isambard K Brunel even built an arch with a triangular cross-section, not often seen in bridges, though there is a beam of triangular cross-section between the services at Wancourt on the French A1 autoroute.

If we try all the combinations of all these variables, we find a large number of possibilities.  A few of these are shown in the next panel.  In each case, a small circle represents the pin of an arch.  Arches with no circle are not supported at the ends.  Note that in examples with two or more chords, stiffening truss members have been omitted for clarity.  They may be assumed to be included in the actual design.  In each case, what are the advantages and disadvantages of the design, and what requirements does the design place on the components?  Which of these designs are impractical or plain silly?

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What other designs can you imagine?  All these designs have the arches mainly above the deck.  There are many designs with the arch entirely below the deck, and many in which the deck cuts through the arch.  Then there are concrete arches, some reinforced and some pre-stressed, masonry ones, and even wooden ones.  There are even bridges in which the arch is not in a vertical plane and the deck is not straight.

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Arches One

Arches Two

Arches Four

Arches Five

Arches Six

Arches Seven

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