In this blog post, we’ll examine the principles behind how arch structures and suspension structures support loads, explore the characteristics of each, and discover why suspension structures are used more frequently than arch structures today.
Why are arch structures so strong?
In architectural engineering, there are various methods for supporting loads. In ancient and medieval architecture, the most common method was load transfer via arches. When traveling through Europe and the Islamic world, it is not difficult to find buildings that utilize arch structures. In buildings constructed today, however, other methods besides arches are used. Suspension structures are one such method. In this post, we will examine how these two structures transfer loads and why suspension structures are used more frequently than arch structures today.
Rectilinear Structures and Arch Structures
An arch refers to a shape that is high in the middle and curves along its length. In architectural structures, an arch functions as a curved column; due to its structural rigidity, it has been used since ancient times in various applications, such as windows, doors, and bridge piers. Let’s examine why arch structures are structurally rigid by comparing them to rectilinear structures. Fundamentally, an arch is a masonry structure built by stacking small components. In a linear structure, gravitational forces can be divided into forces acting along the column and forces perpendicular to the column. While the forces along the column can cancel each other out through the action-reaction between the masonry units, the forces perpendicular to the column cannot be canceled out, causing the column to bend. In contrast, in an arch structure, the resultant force of the forces acting tangentially to the arch’s curve acts in the opposite direction of the force perpendicular to the arch’s normal. This counteracts the force that causes the column to bend, thereby resulting in a more stable structure. An important consideration when constructing an arch is that it is a structure designed to transfer compressive forces to the ground. Therefore, it must be built using masonry materials that can withstand compressive forces well. In modern architecture, where technology has advanced significantly compared to the past, arch structures are not as common as they once were. This is because masonry structures in modern architecture are significantly less efficient to construct. Furthermore, even if masonry materials with excellent compressive strength are used, they are still insufficient to support the heavy loads generated by modern buildings.
A suspension structure refers to a structure that supports loads via cable members. A prime example is a suspension bridge, which is constructed when the water depth is too great to allow for the installation of piers. Unlike other bridges, a suspension bridge must support loads without piers; this role is fulfilled by the cables. The principle behind suspension structures can be explained as the opposite of that of arch structures. Let’s look at a clothesline—a common everyday object—as an example illustrating the principle of a suspension structure. As long as a clothesline is thick enough, it won’t break easily, no matter how heavy the laundry hanging on it is. This is because the gravitational load on the clothesline is transferred to the ground through the clothesline pole.
The Principle of a Clothesline
If you think of a clothesline and the cables of a suspension bridge as inverted arches, it’s easy to understand why they don’t collapse under heavy loads. Unlike masonry, cables and clotheslines cannot withstand compressive forces, but they can withstand tensile forces—which act in the opposite direction of compression—very well. Unlike an arch structure, where the combined compressive forces between masonry elements counteract the bending of the columns, in a suspension bridge, the cables—which were originally straight—take on a sagging shape in the middle when subjected to gravitational loads. The resulting tensile force then transfers the gravitational load to the towers to which the cables are attached. Consequently, clotheslines and cables are subjected only to tensile forces and bear almost no direct gravitational load. Therefore, when constructing a suspension structure, it is essential to use materials capable of withstanding large tensile forces for the cable members. In the case of suspension bridges, the cables serve this purpose. Additionally, the towers from which the cables are suspended must be securely anchored to the ground to prevent collapse when the load is transmitted. Today, thanks to advances in construction technology, it is possible to construct piers using materials such as reinforced concrete that can adequately support these loads. Furthermore, the suspension cables—which must withstand tensile forces—are made of steel in the case of suspension bridges; they are constructed by twisting multiple strands of steel rope together, ensuring they can withstand even heavy loads. Structural engineering has evolved in response to the question of how to support heavier loads. In the past, arch structures were considered highly innovative, but as the nature of applied loads has become more complex and construction techniques have advanced, they have now become less efficient than other methods. However, without arch structures, the development of suspension bridges—which operate on the exact opposite mechanical principle—would not have been possible, and the fact that arch structures contributed to the advancement of other modern architectural technologies remains unchanged.
