Engineers at Washington University have developed a design for the framework of columns and beams that support reinforced concrete bridges so they will better resist earthquakes and be erected faster and less expensively.
The design relies on prefabricated components made off-site and shipped to construction sites when crews are ready to erect them. This lets the structural parts arrive already cured, so there’s no waiting for concrete to set up before going on to the next step. This saves time and money, and reduces traffic delays.
To ensure the columns resist earthquakes, pretensioned cables are embedded in the column’s concrete and stressed. This is done off-site when the columns are built. The columns also contain conventional rebar. When the column is shaken, it can slip and sway, but the cables hold it in one piece and let it retain much of its structural integrity. This technique of pretensioning concrete columns has been used in buildings since the 1990s, but this is the first time it’s been applied to bridges.
A weak point is where the base of the column goes into the footing. In traditional concrete-bridge columns, earthquakes generate high local stresses where the column rises above the ground and the protective cushioning of the footing. The stresses crush the concrete and leave the column no firm base, so it falls. To prevent this in the new design, researchers protect the bottom of each column with a short, steel jacket, much like metal hoops keep barrels in shape and structurally sound.