ENGINEERING LESSONS - EPISODE 9 - EFFECT OF SKEW ANGLES IN BRIDGES
This Article is a compilation of the below Technical Papers, only the Essence of the Papers are provided here for quick grab in this section, detail report is attached below.
A Bridge built obliquely between the Supporting Spans are called as Skew Bridge. The angle between the normal to the center line of the bridge and the center line of the abutment is known as Skew Angle. Skew bridges are built where geometry cannot accommodate straight bridge. Alignment should be straight to facilitate high speed and safety requirements of the traffic, this resulted in increase in number of skew bridges.
Study 1:
This study comprises of Seismic analysis of Box & T Girder Bridges using Response Spectrum Method performed using SAP2000 varying skew angles of the Bridges. Responses of the Substructure, Girders and Deck are studied.
An attempt was made to determine skew angle up to which skew bridges have comparable characteristics with straight bridges and also to establish best possible alternative among both the girder types with respect to seismic response.
It was concluded that up to skew angle of 15 degrees skew bridges have comparable responses with straight bridges and seismic performance of skewed box girder bridge is better than T girder bridge for the considered span.
Skew Bridge differs from Straight bridges with respect to its following characteristics such as significant Torsional Moments in the deck slab, decrease in Longitudinal Moment, increase in Transverse moment, concentration of Reaction forces and Negative Moments at the obtuse corners and uplift reaction forces at the acute corners.
Global seismic behavior of skewed bridges is affected by a number of factors, which includes bridge Skew Angle, Deck width, Deck flexibility, Number of Spans, Number of columns per bent, Column ductility, Soil-Abutment-Superstructure interaction, Abutment shear keys, Soil-bent Foundation-Structure interaction, abutment bearing pads, and characteristics of the Seismic source.
Comparing the Box girder and T girder bridge results, inferences can be summarized as follows:
- Modal frequency increases with increase of skew angle for Box girder bridge whereas it decreases for T Girder Bridge. Frequency obtained for skewed Box Girder Bridge, for all skew angles, is more than skewed T Girder bridge having a maximum of 59% difference for 60° skew.
- Large skewness induces vibratory modes such as Torsion and Lateral Flexure, which causes an increase in axial forces, shears, moments and Torques in the supporting columns.
- Torsion induced in columns of skewed T Girder Bridges is more than that of skewed Box Girder Bridge. Skewed Box Girder Bridge resists the Shear Torsion combined effect more efficiently than skewed T Girder Bridge.
- Skew supports act as eccentric loads on adjacent boxes thus accentuating the distortional stresses in Box Girders and Transverse Moments in the Slabs.
- Skewness induces both Translation & Rotator deformations in the bridge bearings. It can be noted that box girder bearings have less deformations than T girder bearings.
- As skew angle increases displacement of the deck also increases since the longitudinal stiffness of bridge reduces. As it can be noted that for 60° skew maximum displacement in box girder deck is 77% less than T girder deck. Because of Torsional Resistance of box girder it accommodates skew effects better.
- Up to skew angle of 15° the responses of skew bridge are comparable to straight bridge. Overall Seismic performance of skewed Box Girder Bridge is better than skewed T Girder Bridges.
- Deflection decreases with increase in skew angle in 2 or 3 span skew slab, single span Deflection increases with increase in skew angle. This shows that the effect of deflection is more in single span skew deck slabs as the stiffness of slab is less.
- Bending Moment has reduced with increase in skew angle under dead load in 1, 2 and 3 spans deck. But under moving load there is slight reduction in Bending Moment up to 20° and then increased for 30° and further reduced for 40° skew angle only on single span deck.
- When compared with all the three spans, the magnitude of Bending Moment has reduced its maximum value in single span deck.
- The magnitude of Shear Force has slightly reduced with increase in skew angle under Dead load in 2 and 3 span deck, it was observed that the magnitude had increased under moving load.
- In 1 span, the Shear Force remained same in all the models (skewed bridge) compared with normal bridge under Dead Load but there is increase in Shear Force with increase in skew angle under moving load.
- For IRC AA Tracked and 70R loadings, the values of Bending Moment increases by about 10% when IRC 70R is replaced by IRC AA Tracked loading.
- For IRC AA Tracked loading, there is increase in Bending Moment by about 12% as the skew angle increases from 0 to 45°.
- There is an increase of about 3 to 16% in the Shear Force b/w IRC AA Tracked & 70R loading as the skew increases from 0 to 45°.
- There is an increase in Torsion of about 20% when IRC AA tracked loading is replaced by 70R loading.
- However it is observed that there is decrease in the value of Torsion for both IRC AA Tracked & 70R loading with skew angle ranging from 0 to 45°.
- Comparison of IRC AA Tracked & Wheeled Loadings show a very large variation in the values of all 3 parameters (BM, SF & Torsion). The variations are as large as 40% when IRC AA wheeled is replaced by Tracked loading. However with variation in the skew, both IRC Tracked and Wheeled shows similar curves.
- IRC A & B loading show an increase of about 10-15% in the values of Bending Moment with increase in skew from 0 to 45°.
- IRC B is replaced by IRC A loadings, there may be increase in about 40% in the Bending Moment without considering the skew.
- IRC AA Tracked & Wheeled loadings does not show any significant change in the values of Shear Force with the skew angle however there is a slight increase in Shear Force in Tracked loading.
- Comparison of 4 loadings (IRC-A, IRC-AA Tracked & Wheeled & 70R) show increase in the values of Bending Moment and Shear Force and decrease in the value to Torsion with skew.
- Deflection occurs for Live L/C Case II of various Skew angles result is increase by (1.750%) with increase in Skew angle are compared. Bending Moment occurs for Live L/C Case II of various Skew angles result is increase by (1.525%) with increase in Skew angle are compared. Shear force occurs for Live L/C Case II of various Skew angles result is increase by (1.376%) with increase in Skew angle are compared.
- Torsional Moment occurs for Live L/C Case II of various Skew angles result is increase by (135.36%) with increase in Skew angle are compared. Support Reaction occurs for Live L/C Case II of various Skew angles result is increase by (0.001%) with increase in Skew angle are compared.
- The Deformation increase as the skew angle increases. As skew angle increases the deformation is shifted from the center of the skewed corner.
- Maximum deformation is showed by the 60° skew angled bridge and the maximum deformation is found at the corner position. Up to 45ยบ the deformation is uniform in the structure. Further increase in skew angle leads to the formation of maximum deformation to the skewed corners.
- Increase in skew angle reduce the stability of the structure. Up to 45° skew angle the structure can withstand the changes. But the increase in the skew angle above 45° will leads to collapse of the structure.
- For 2 or 3 spans under Dead Load the Bending Moment reduces as rise in skew angle & Live Load BM raises with rise in skew angle.
- SF under DL reduces with rise in Skew Angle & Live Load SF raises with increase in skew angle.
- The effect of skew angle is witnessed on Max BM, Max SF, Max Torsional Moment, Max Deflection due to DL & LL at acute locations. Live load “IRC Class AA Tracked Vehicle” is applied as per IRC: 6 2000 guidelines.
- For Sagging Moment under Dead & Live loads the BM increases with increase in skew angle.
- For Hogging Moment under DL the BM reduces with increase in skew angle but it increases under moving load with increase in skew angle.
- under Dead Load the SF reduces with raise in skew angle & under moving load the SF increases with rise in skew angle.
- under DL & Live Load Torsional Moment increases with increase in skew angle.
Comparison
of Skew Bridges with Different Skew Angles
Comparative
Study of Normal & Skew PSC Box Girder Bridge
Effect
of Skew for Various IRC Loadings on T Beam Bridge
Effect
of Skew Angle in U Girder Bridges
Parametric
Study of Skew Angle on Box Girder Bridge
