Girder

A girder is a horizontal beam or member that supports the weight of a structure, typically floors or roofs. It is often made of steel, concrete, or wood and is designed to span large distances without sagging or collapsing. 

The use of girders can be traced back to ancient civilizations, who used wooden beams to support their structures. As technology advanced, steel and concrete became more common materials for girders, allowing for the construction of taller and more complex buildings. 

Girders are essential components of many types of buildings, including skyscrapers, bridges, and industrial and residential buildings.  

EXAMPLES IN CHICAGO:   

1. The Willis Tower: This skyscraper, formerly known as the Sears Tower, is supported by a complex network of steel girders. 
2. 875 N. Michigan Avenue (formerly the John Hancock Building): The "X" bracing system used in the John Hancock Center incorporates steel girders to provide structural support. 
3. The Merchandise Mart: This massive building, once the largest office building in the world, relies on a grid of steel girders to support its weight.