Building Model Bridges Following the Engineering Process
Joe Lewis
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Arch Bridges
An arch bridge is composed of a curved structure with abutments on each end. Instead of the weight pushing straight downward like on the beam bridge, the weight is carried outward along the curve of the arch to the abutments at each end of the arch. The abutments also keep the end of the bridge from spreading outward. Therefore, the arch bridge is always under compression because the weight of the deck is pushed outward along the curve of the arch towards the abutments. The rise in the form of the curved arch causes the vertical load to have a horizontal thrust. The amount of tension placed on the arch is determined by the degree of curvature of the arch. The greater the degree of curvature of the arch, the greater the amount of tension on the bottom if the arch itself.
There are three basic types of arches, the false arch, the ribbed arch and the true arch. A true arch is made up of wedged shaped bricks called voissoirs that are fitted together between vertical supports. True arches are built in from the end, towards the middle. The final wedge called the keystone is set in place at the top of the arch. Sidewalls called spandrels are built up between the arches and filled with rubble. Ribbed arches are actually several rows of arches built next to each other. Long, flat stones are laid across the ribs for support. This technique reduces the weight of the arch and cuts construction time and the amount of materials used. The false arch resembles the corbelling of bricks in order to span an opening. The composite form and behavior resembles the arch but it needs heavy vertical loads to secure its shape and structure.
