Which Arch Shape to Use?

There are many arch shapes to choose from when building a stone arch bridge. In this post we look at some of the options, and examine the advantages and disadvantages of each shape.

We have discussed many different types of arch shapes, ranging from the ubiquitous Roman arch all the way to pointed Gothic-style arches. Each of these shapes has distinct advantages and disadvantages. The question for the builder then becomes: What shape is the best?

A Variety of Options

Like many design choices, the choice of arch shape is a choice of compromises. There are a variety of traits that recommend some arches over others. Perhaps the best way to sum up the traits is to show them in a table.

Arch Type	Approximate Rise/Run	Relative Ease of Build	Advantages	Disadvantages
Roman Arch	.5	Easy.	Simple, essentially has no outward thrust.	Must be made thick in order to free-stand without collapse. The rise may also be excessive for some applications.
130-Degree Segmental Arch	.3	Easy.	Simple to build, limited outward thrust, yet has lower rise than a Roman arch. Can be made freestanding.	Less forgiving to settlement than a Roman arch, needs small abutments to resist thrust.
90-Degree Segmental Arch	.2	Moderate.	The low rise is useful in many applications.	Somewhat vulnerable to settlement, must have good abutments with enough masonry to resist thrust.
Low-Rise Segmental Arch	<.2	Difficult.	The low rise is useful in many applications.	Heavy outward thrust and vulnerability to settlement makes construction difficult and requires substantial abutments. Unless the arch is made extra thick, this is a weak arch.
Flat Arch	Has little to no rise.	Very difficult.	Has little to no rise, which can be advantageous in some applications.	This is an incredibly weak arch, has very limited applications for bridges; mostly seen in buildings spanning modest openings.
Gothic Arch	>.5	Very easy.	Super easy to build, very forgiving to settlement, can easily be made freestanding.	The rise on this arch is rather high. Also is best suited for small-span bridges or bridges with large amounts of fill over the arch due to the way the forces travel through structure.
Pointed Segmental Arch	Typically <.5.	Easy.	Same advantages as Gothic arch above, but with less rise.	Flatter pointed segmental arches are vulnerable to the same disadvantages of segmental arches. Sections of the arch tend to be fairly flat, possibly making the structure weak under load applied to the sides.
Basket-Handle Arch	Variable.	Moderate to difficult.	Has lower rise than Roman arch, but without the need for potentially massive abutments to resist the thrust at the ends of the arch. Also tends to approximate the line of thrust naturally found in the arch of a completed stone bridge.	Does not lend itself well to free-standing. Will need to resist outward thrust located where the arch starts to curve heavily down to the abutments; extra arch thickness can be used to accomplish this. Low-rise basket-handle arches have the same disadvantages of a low-rise segmental arch, and tend to be weaker in some aspects.
Tudor Arch	Variable.	Variable.	Cross between pointed-segmental and basket-handle arches, tends to have the advantages of both.	Cross between pointed-segmental and basket-handle arches, tends to have the disadvantages of both.
Catenary Arch	Typically >.3; note that segmental arches of about 130 degrees and flatter follow essentially the same curve as a catenary arch, so please refer to these segmental arches for information on low-rise catenary arches.	Moderate.	This arch follows the natural line of thrust for a freestanding arch, so, obviously, can freestand readily.	A catenary is not the shape of the line of thrust for a finished stone bridge. The relatively straight sides of the catenary can prove a weakness for large spans.

Augusta/Towanda Township Line Culvert — Gothic Arch

So What Arch is Best?

The choice of arch seems to be primarily dependent on how much rise is tolerable coupled with what the builder feels up to building. The material required for any bridge can depend heavily on the abutments; thus thinner abutments mean less material required. In the end, it is up to the designer to choose what makes sense. Roman arches are a good choice for many small structures, but, again, conditions will dictate what is needed. In general deep gorges are easier to span with a stone arch bridge than shallow swampy areas. Another option for swampy areas, of course, is to use multiple smaller arches, as this keeps the arch rise low.