Bridge Deck Overhang (Cantilever Slab) and Barriers (Railings)

The cantilever portion of the deck slab (deck overhang) must be designed for dead and live load moments for the strength and service limit states, where the moments are based on traditional beam theory. However, the deck overhang design must also consider a vehicular collision load with the railing system at the extreme event limit state. Article A13.4 of the AASHTO LRFD (7th Edition, 2014) provides the design procedures associated with the vehicular collision load. In accordance with Article A13.4.1, bridge deck overhangs should be designed for the following three design cases:

• Design Case 1: Transverse and longitudinal forces specified in Article A13.2 for the Extreme Event Load Combination II limit state;
• Design Case 2: Vertical forces specified in Article A13.2 for the Extreme Event Load Combination II limit state;
• Design Case 3: The loads specified in Article 3.6.1 that occupy the overhang for the Load Combination Strength I limit state.

Although not explicitly stated in the above three design cases, the design of the bridge deck overhang should also consider serviceability requirements with regard to crack control and minimum steel reinforcement required for shrinkage and temperature effects.

Additionally, the bridge barriers (or railings) must be designed to withstand a predetermined level of crashworthiness, typically specified by the bridge owner or governing agency. The combination of the deck overhang and the bridge barrier must be capable of resisting a horizontal vehicular collision force. Bridge barriers (or railings) used on the National Highway System must be crash tested, and the crash test specimen should include the barrier and deck overhang. In accordance with Article 13.7.3.1 of the AASHTO LRFD (7th Edition, 2014), “a railing system and its connection to the deck shall be approved only after they have been shown through crash testing to be satisfactory for the desired test level.” Most States have typical barrier designs that have been tested and meet the specified levels of crashworthiness. 

To develop a preliminary design of a barrier and overhang the engineer should reference Section 13 of the AASHTO LRFD (7th Edition, 2014), which provides design guidelines and specifications. Most often, concrete barriers (railings) are employed, and Article A13.3.1 provides a methodology for the design of the barrier based on an application of the yield line theory. For further information, design examples demonstrating the application of the yield line theory, in accordance with Article A13.3.1 have been previously published by the Federal Highway Administration (FHWA), see references (2) and (3).