Seeking Balance

All the elements of a structure seek a balance between the pressures of compression and tension. When this balance doesn’t happen, a structure can become too stressed out, forcing it to buckle or break down. Equilibrium between stresses in a building – or stasis – can be achieved by shaping and sizing materials into stable systems of elements. The analysis of forces acting upon building elements to achieve this balance is called statics.

The science of statics came about through trial and error. In the past, architect and builders would experiment with risky structures until, faced with failure, they would have try something else. The daring brick dome of Hagia Sophia in Istanbul, one of the largest domes ever built, fell twice before it was finally able to stand freely.

Trabeated Systems

The simplest type of structure is formed by balancing horizontal elements called lintels or beams between two upright posts or columns – a system referred to as trabeated, or post-and-lintel. Since ancient times, posts and lintels have been used to construct wooden huts as well as huge stone buildings.

S tonehenge and the Empire State Building, for example, are designed according to the same trabeated structural system. But while Stonehenge is built of huge rocks and manages simple loads, the Empire State Building is built with a more complicated system of steel so that it can span greater distances and heights and resist greater forces.

The post-and-lintel unit is not entirely sturdy. The weight of the lintel pushes on the posts to create compression in each of them. At the same time, the beam wants to curve upward and bend. So, the top of the beam shortens under compression, while the bottom pulls in tension. This bending action causes cracks to appear in beams made of low-tensile materials, such as concrete or stone. Steel rods are often set into concrete beams to prevent cracks from forming.

Footings and Foundations

Footings spread the load over a larger area than a foundation wall to reduce the pounds per square inch. They are typically constructed of stone or concrete. The loads on the footings can be extremely heavy for skyscraper – thousands of tons. Only solid soil can carry such loads. When this solid soil is buried deep below the ground, heavy, hollow steel cylinders called caissons are sunken into the earth and filled with concrete to support the weight. When the soil is sandy and weak or filled with rocks, long round poles called piles are driven into the ground to support the footings or the foundation mat.

Cantilevers

Post-and lintel structures can withstand a lot of loads, but they can also be boxy and boring. One way to transform them into more dynamic, open spaces is to extend a beam from one of the posts as you remove the other post from under the beam. This type of unencumbered beam is called a cantilever. A cantilever is a beam anchored at only one end. Cantilevers are widely found in construction, notably in cantilever bridges and balconies. Architect Frank Lloyd Wright especially liked using Cantilevers so that the roofs and terraces of his houses would appear to float in space.