Skewed Bridges

Support skew can affect the detailing of the reinforcement in the concrete deck slab. For support skew, it may be acceptable to orient the deck transverse reinforcement parallel to the skew. However, the deck design calculations must account for a reduction in the effective steel area due to the skewed reinforcement and thus a reduction in the concrete deck slabs flexural resistance. Per Article 9.7.1.3 of the AASHTO LRFD (7th Edition, 2014), the primary reinforcement may be orientated along the skew for skew angles that do not exceed 25 degrees, where the skew angle is measured from a line that is perpendicular to the centerline of the bridge to the centerline of the support.

The acute corners of a skewed concrete deck slab are often difficult to adequately reinforce (8). As the angle of skew increases, large portions of the deck can be unreinforced and therefore subject to spalling and chipping, as shown in Figure 8. Because the orthogonal bars are too short to develop, it is typically necessary to detail diagonal bars that extend into the deck over the girders, to carry the deck overhang loads. Similarly, acute corners in concrete barriers are also difficult to reinforce, and require special consideration.

A possible technique to combat the skew effects related to the detailing of the reinforcement in the concrete deck and barriers is a detailing method commonly referred to as breakback detailing. Breakback detailing is where the ends of the skewed deck are turned so that the end is normal to the longitudinal edge of the deck, as shown in Figure 9. This breakback detailing effectively eliminates the acute and obtuse corners of the concrete deck and barriers. Various owner agencies may include guidelines for breakback detailing in their design manuals, policy memos, or standard drawing details (8). These guidelines typically address the degree of skew at
which breakback options should be considered.