What is Deflection?
Deflection refers to the distance moved in the direction of the applied load before and after deformation when a force acts on a structural member.
Why is Deflection check important?
Deflection is considered in usability assessments, along with factors such as cracking, vibration, and fatigue. Assessing the limit state involves determining the condition in which the structure does not collapse under specific external forces. While direct failure of the member may not occur, reviewing excessive deflection and abnormal vibrations helps prevent indirect consequences that may lead to failure.
Deflection is broadly categorized into long-term deflection and instantaneous deflection. Long-term deflection is caused by the self-weight and additional loads on the structure, which can be offset by providing pre-deformation (camber). Instantaneous deflection occurs due to live loads, wind loads, earthquakes, etc., and its maximum limit is established to assess safety.
Origin of Deflection Criteria
Research on the correlation between bridge deflection and vibration discomfort for pedestrians began in the early 1930s. Studies concluded that deflection exceeding L/800 induced noticeable vibration discomfort. (ASCE, 1958; Oehler, 1970; Wright and Walker, 1971; Fountain and Thunman, 1987). This aligns with the deflection limits proposed by AASHTO today. Unfortunately, detailed evidence (bridge names, types, materials, span lengths, etc.) from these studies is lost, making the exact origin unclear.
In the 1960s, deflection limits were established when considering pedestrian use. Interestingly, a more stringent standard of L/1000 was set, thanks to a real-life incident where a baby waking up on a bridge was wrongly attributed to vibrations caused by the bridge (Fountain and Thunman, 1987).
The following video demonstrates an example of applying camber to account for the deflection of horizontal members under load.
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