A bridge is “skewed” when its deck crosses the obstacle below (a river, highway, or railway) at an angle other than 90 degrees. This skew is usually an intentional design choice forced by the geometry of the site, but it introduces a range of structural challenges that can cause the bridge to shift, rotate, or deteriorate faster than a straight crossing. The causes fall into two broad categories: the reasons a bridge must be skewed in the first place, and the forces that act on a skewed bridge once it’s built.
Why Bridges Are Built With Skew
The most common reason for skew is simple geometry. When a road or railway needs to cross a river, canal, or another road that doesn’t run perpendicular to it, the bridge has to span the gap at whatever angle the two paths meet. Engineers measure this as the skew angle: the difference between the direction of flow (or the path being crossed) and a line drawn perpendicular to the bridge’s deck cross-sections.
Tight urban layouts, curved highway ramps, and winding river channels all create situations where a perpendicular crossing is physically impossible or prohibitively expensive. A bridge might also be skewed to preserve a floodplain’s natural drainage pattern or to align with an existing interchange. In every case, skew is a compromise: the bridge fits the site, but the angled geometry creates uneven load paths that engineers must account for throughout the structure’s life.
Thermal Expansion and Lateral Movement
Temperature changes cause all bridges to expand and contract, but skewed bridges don’t move in a straight line. Research from the Ohio Department of Transportation found that skewed semi-integral bridges tend to both expand and rotate as ambient air temperatures rise through the seasons. Instead of pushing straight outward, the deck slides at an angle that follows the skew, generating forces in directions the supports weren’t primarily designed to resist.
This movement is transferred into the wingwalls, the short retaining walls at each end of the bridge. Wingwalls aligned parallel to the bridge’s end diaphragm experience a pushing (axial) force. Wingwalls angled back from the diaphragm get hit with bending forces on top of that push. The resulting behavior is complex: the joint between the wingwall and diaphragm shows a combination of longitudinal sliding, movement along the skew direction into the wingwall, and rotation, all driven by temperatures that vary in intensity, duration, and location across the structure. Over many seasonal cycles, this repeated off-axis movement can wear out bearings, crack concrete at the joints, and gradually shift the deck out of its original alignment.
Seismic Forces and Deck Rotation
Earthquakes pose a heightened risk to skewed bridges. During ground shaking, a skewed deck tends to rotate in its own plane around a vertical axis, like a rectangle being spun on a tabletop. As it rotates, the superstructure separates from the abutment at the acute (sharp) corner while jamming into the abutment at the obtuse (wide) corner. This creates an uneven wedge of soil pressure behind the abutment backfill, which amplifies the rotation further.
The 1994 Northridge earthquake demonstrated this clearly. Skewed bridges suffered abutment and column failures driven by intensified demand for deck rotation, particularly at sites close to the fault rupture. More recent analysis has confirmed that the skew angle’s effect is most dangerous in near-fault zones, where the sharp, pulse-like ground motions are strongest. In areas far from the fault, the skew angle has much less influence on how the bridge responds. For ramp bridges with curved decks, the torsional (twisting) force in the columns nearest the abutments increases significantly with skew, making those columns especially vulnerable.
Uneven Load Distribution and Torsion
In a straight bridge, loads travel through the deck and into the supports in a relatively symmetrical pattern. Skew disrupts that symmetry. The deck’s geometry means that forces don’t travel perpendicular to the supports. Instead, they follow a diagonal path, which creates twisting (torsional) forces in the deck and unequal reactions at the bearings. The acute corners of a skewed deck carry different loads than the obtuse corners, and this imbalance is present under everyday traffic, not just extreme events.
This constant asymmetry is one reason skewed bridges wear out faster. A Federal Highway Administration study examining New Jersey bridge data found that the skew angle was a significant predictor of deterioration rate. Bridges with skew angles greater than 20 degrees experienced substantially more bearing damage than those with skew angles under 20 degrees. The sharper the skew, the more the load path deviates from the ideal, and the harder the bearings, joints, and deck edges have to work.
Cracking at Acute Corners
The acute corners of a skewed bridge deck are a well-documented weak point. These sharp-angled corners concentrate stress in a small area of concrete, making them prone to cracking. Research has identified skew-related deck cracks as a recurring problem in girder bridges, with cracks forming early in the bridge’s life and worsening over time as traffic loads and thermal cycles compound the initial damage.
The obtuse corners, while less crack-prone, face their own issue: during seismic rotation or thermal movement, they bear the brunt of the deck pressing into the abutment. This can damage the backwall and deform the bearings at those locations. Together, the stress concentrations at acute corners and the impact forces at obtuse corners mean that the entire perimeter of a skewed bridge demands more frequent inspection and maintenance than a comparable straight span.
Construction-Related Causes
The building process itself can introduce or worsen skew-related problems. Skewed bridges are harder to fabricate and assemble because the girders, diaphragms, and deck reinforcement all meet at non-right angles. Small errors in placement or alignment become magnified by the geometry. Studies have documented cracks appearing in skewed bridge decks that were constructed using self-propelled modular transporters, large platforms used to move prefabricated bridge sections into place. The complex geometry makes it difficult to achieve perfectly uniform support during and after the move, leaving residual stresses that eventually surface as cracks.
Bearing alignment is another construction-sensitive factor. Bearings on a skewed bridge must accommodate movement along the skew direction, not just longitudinally. If they’re installed at the wrong orientation or without enough freedom of movement, they resist the bridge’s natural thermal path and accelerate wear on themselves and the surrounding concrete.
How Skew Angle Affects Severity
Not all skewed bridges are equally at risk. The 20-degree threshold identified in the New Jersey data serves as a practical dividing line. Below 20 degrees, the effects of skew are present but manageable with standard design practices. Above 20 degrees, the load path distortion, thermal rotation, and seismic vulnerability all increase sharply. Many transportation agencies flag bridges above 30 degrees of skew for special design provisions, additional bearings, or modified joint details to handle the extra demands.
The relationship between skew angle and structural behavior isn’t linear. Doubling the skew angle more than doubles the torsional forces and the tendency for deck rotation. This is why engineers aim to reduce skew wherever site conditions allow, even if it means a slightly longer or more expensive bridge alignment. Every degree of skew removed from the design pays dividends in reduced maintenance costs and improved long-term performance.