What Is Cantilever Deflection?

Structural beams are used in construction of bridges, commercial buildings, and homes. A cantilevered beam is typically a horizontal piece supported only at one end, or supported at both ends and without support in the middle. Cantilever deflection is the amount of vertical movement that will occur when additional loads are placed on the beam.

Bridges use structural beams to support the loads of vehicles and pedestrians. Design or land restrictions may require the beam supports to be placed at one end of the beam, or in bridges only at the ends to permit an open area beneath the bridge. These cantilevered beams may be supported on one end for aesthetic or artistic design reasons, or because the land may be unsuitable for bridge supports at the other end. Draw bridges, which can open and close to permit ships to pass, often rely on cantilevered beam systems to allow them to tilt or swing open.

Buildings use beams to support floors and transfer the loads to vertical supports called columns. Cantilevered beams in buildings create open floor plans with minimal support columns. Single-support cantilevered beams are often used for balconies, pedestrian walkways or extended rooflines.

When a beam is placed in a structure, the loads pass horizontally to both ends if supported there, but the beam moves down in the center under load. A cantilevered beam supported at one end will bend at the free end when loads are added, and from the load of the structure itself. This is cantilever deflection, and it is important to determine the effect of loads before construction. Adding structural support from cables or other beams can help reduce deflection and permit higher acceptable load limits.

The amount of cantilever deflection depends on several factors. Wood, steel or plastic all have different amounts of deflection under load. Beam length will create different static loads, or the loads due to the weight, and dynamic loads, which are loads caused by movement of people or vehicles. Position of the load is very important to understanding cantilever deflection, because a load at one point will create different bending characteristics than a load spread across the beam length.

Designers normally design beams for maximum expected loads and a safety factor. A bridge will carry different loads depending on traffic and pedestrian factors, such as time of day. Studies are normally performed for a proposed bridge design to determine maximum traffic density.

Building design loads are affected by the furnishings, occupancy, and wind conditions. Structural engineers have tables of data available for different building floor designs, beam lengths and materials. Wind data can be determined from historic weather conditions for the area. As the building experiences different people or wind loads, the amount of cantilever deflection changes regularly. Design data will include these changes and additional safety allowances.