When constructing an arched structure, especially a very large one, choosing the right materials is critical. As the blocks of material are piled up, their weight adds up quickly and each block gets squeezed, or compressed, under this weight of the blocks above. Not all materials can equally withstand this compression force, which squeezes a material from opposite sides. But this is the most important force to consider when building an arch. The Romans chose stone for their arch bridges, aqueducts, buildings, and more, with good reason: very few materials can withstand greater compressive forces than cut stone. Contemporary builders often use concrete or steel for arched structures because these materials can be readily molded into a variety of shapes and are nearly as strong as, and much less expensive than, stone.

The signature Gateway Arch in St. Louis, Missouri, is an elegant example of what builders can do with modern materials. When the 630-foot-tall arch was constructed in 1967, builders chose to make the walls of carbon steel. The steel surrounds reinforced concrete from ground level to 300 feet, and steel reinforcing bars from 300 feet to the peak. Wrapped around the walls is a stainless steel skin. While modern materials allowed builders to create this magnificent structure, the materials also caused some trouble. The legs were built simultaneously from the ground up, and when it was time to connect them at the apex, the steel of the south leg, warmed by the sun, had expanded so much that it no longer aligned with the north leg. This unforeseen problem called for a creative solution: the St. Louis Fire Department sprayed the south leg with cool water until the legs lined up.

A more modern arch can be seen in the soaring Hoover Dam Bypass Bridge. Opened in 2010, the bridge connects Arizona and Nevada highways nearly 900 feet above the Colorado River. When designing the bridge, engineers and architects wanted a structure as elegant and striking as the nearby Hoover Dam. They chose a concrete arch topped by a steel deck. The design draws on the strengths of both materials, using strong, relatively inexpensive concrete for the huge arch at the base of the bridge, and lighter steel for the upper parts of the structure. Yet, as with the St. Louis Arch, these materials also led to some construction challenges. When construction crews pour concrete in the desert, it can overheat, leading to cracks when it dries and shrinks. To prevent this problem at the Bypass Bridge, crews mixed and poured concrete only at night. This worked well most of the year, but not during hot summer nights. The solution? In summer, crews cooled the concrete with liquid nitrogen.

How do architects and designers know that these new materials will work when they build these structures? Newer technologies enable them use computer simulations to design arches, bridges, and buildings. This allows them to test their designs and make sure the final products will be strong, safe, and durable before the first block is laid or concrete is poured.

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