Haunches

The haunch in a steel girder bridge is typically referred to as the distance from the top of the steel girder web to the bottom of the concrete deck slab. Alternatively, some agencies refer to this area as the fillet, but where the fillet is the distance from the top of the top flange to the bottom of the concrete deck slab. The main difference between the haunch and the fillet is that the haunch will theoretically remain constant along the length of the girder, while the fillet dimension will vary due to top flange thickness changes along the length of the girder. However, both dimensions may need to be adjusted in the field due to girder fabrication and erection tolerances. The detailing of this concrete haunch (or fillet) needs to be considered by the bridge designer.

The haunch width is typically set as the same width as the top flange. The deck forming method will affect the haunch width. For example, where steel stay-in-place deck forms are used, they typically employ clip angles which are attached to the top flange, requiring the haunch to be the same width as the flange.

The haunch depth is usually set to accommodate all variations in top flange thickness, along with consideration of the deck cross slope and deck forming method. The thickness of splice plates must also be considered. Typically, the thickness of concrete above the flange (the fillet), is allowed to be no less than 0.75 inches at any point across the top flange width, and considering the thickness of the top flange splice plates. The haunch depth at the centerline of the girder web is typically shown on the contract plans, and depth must consider the cross slope and superelevation of the deck slab.

Shear reinforcement in the concrete haunch is often required when the depth of concrete measured from the top of the top flange to the bottom of the concrete deck exceeds a certain thickness (agencies specify the minimum thickness anywhere from 3.0 to 6.0 inches). Shear reinforcement is also typically required if the shear connectors (studs) do not penetrate a minimum of 2.0 inches into the deck slab. In this case the haunch must be reinforced to contain the shear stud connector and develop its load in the deck (Article 6.10.10.1.4).