In our last post, we described the class of so-called “basket-handle” arches: The basket-handle arch basically forms a compromise between a Roman arch and a segmental arch. The basic form is an ellipse, but most bridges using a basket-handle arch use a so-called “three-centered” arch — it is drawn using three separate radii.
The Primary Purpose of the Basket-Handle Arch
Why do we need a basket-handle arch?
The primary advantage of this arch form is that it allows for a long span with a low rise without needing massive abutments to resist the horizontal thrust inherent to low-rise segmental arches. This is huge — the amount of material saved is significant.
The upshot is that an arch bridge with a reasonable grade can be built more simply. But there is another advantage to the basket-handle arch.
An Advantage of the Basket-Handle Arch: Increased Waterway
Due to the shape of the basket-handle arch, there is greater waterway available than with a more conventional segmental arch of the same rise and span.
Keeping the waterway open is always beneficial. For streams that for one reason or other cannot have any obstruction in them, a basket-handle arch may very well be the best solution.
One way the basket handle arch can be used to span a stream that cannot be obstructed, is to determine the acceptable bridge rise over the stream banks and draw a basket-handle arch with the shallow main arc starting at the stream bank level and rising to this maximum predetermined rise over the stream. Then all that is left is to round out the ends of the arch to form a basket-handle shape. These rounded ends cut into the bank, landing at about streambed level.
A three-centered basket-handle arch drawn so as to avoid obstructing a stream. The dots mark the stream banks and the flat line that the arch ends on represents the streambed level. While in this example there is still significant rise over the stream, the arch could be drawn even flatter, though it would be weaker. Nevertheless, it can be seen that the basket-handle arch lends itself readily to this use.
The Weak Point of the Basket-Handle Arch: The End Curves
One of the primary weaknesses of basket-handle arches is that the basket-handle shape does not bear much resemblance to the parabolic lines of thrust you see in an arch. The result is that the basket-handle arch needs some help to remain stable.
The trouble is best shown in the case of the three-centered arch. The weakness is near the point where the main low-rise segmental arc transitions to the rounded arcs that take the arch to a horizontal plane. Below this point of transition, lines of thrust tend to escape the arch — the top of the arch wants to collapse, bulging out the rounded end arcs to do so.
The typical solution to this problem is to pile weight against these end arcs to resist this thrust. This is comparable to the abutments required to resist the thrust of a segmental arch; however, much less material is needed to do this with a basket-handle arch — the lower arcs do still tend to transition the force downwards.
An alternative solution is to build the arch ring thicker at these lower curves. This method adds more arch where it is needed most, and is better than adding weight at the ends of the arch alone.
While you still may need to add more backing against the ends of the arch for stability, thickening these lower arcs is a great solution. This extra thickness at the lower points of the arch ring should not add to the grade of the bridge as it is lower down on the structure.
A close look at this bridge reveals that the arch becomes thicker at the ends where it starts making its sharp turn downwards to a horizontal plane. This is to prevent these end arcs from bulging out due to the thrust of the top, main arc.
