Effects of skewness angle on the behavior of prestressed reinforced concrete bridge

Skewed bridges are designed with an angled orientation between the roadway and supporting piers, often due to terrain, road alignment, or obstacle constraints. This unique design affects load distribution, requiring specialized engineering to maintain structural integrity and optimize safety. Skewed bridges help navigate difficult landscapes and improve transportation efficiency. The structural behavior of skewed bridges has a significant impact on their design and overall performance. The skewness of the superstructure in a bridge plays a critical role in how the bridge responds to various loads, as the angle of skew affects the distribution of forces and the overall stability of the structure. Therefore, in this thesis, the effects of skewness angle on the behavior of prestressed concrete superstructure bridge are investigated by analyzing four model with continuous three spans. In this study, dead and live loads were applied. Dynamic analysis (earthquake loads) were not applied. Four models, each with a different skew angle, were analyzed. The skew angles considered in this research were 0°, 21°, 37.6°, and 49°. These models were analyzed using the Finite Element Method (FEM), a highly effective computational technique that allows for detailed and accurate structural analysis. The analysis was conducted using Midas Civil, a specialized software designed for analyzing and designing all bridge components and structures using Finite Element Method. The findings of the study revealed that bending stresses, shear forces, bending moments, torsional moments, and displacements in the bridge structure are significantly affected by the increase in the skew angle. The impact is particularly noticeable when the skew angle exceeds 21°. As the skew angle increases, the distribution of internal forces, such as bending and shear forces, becomes more uneven, which can influence the overall stability of the bridge. Designing skewed bridges involves adjusting angles for load distribution, ensuring structural stability, and accommodating terrain or alignment constraints effectively. Therefore, proper consideration of the skew angle is essential during the design phase of skewed bridges. This consideration will ensure that the bridge is both safe and efficient in its performance, accounting for factors that could otherwise lead to inaccuracies or potential structural issues under load. This research highlights the importance of precise modeling and accurate design adjustments for skewed bridges to optimize their functionality and longevity. In this study, the stresses in four I-beams with continuous spans of three spans of 24 m each were investigated. From this study , general behaviour can be observed in the bending stress diagrams for the first, second, third, and fourth beams. Initially, the stress distributions follow a similar pattern, with negative values extending between the supports and positive values occurring near them. As the skew angle increases, the stress values increase, especially in the middle of the span. For the first beam, the stresses in the central span increase slightly with a larger skew angle, while the second beam maintains almost the same stress distributions despite the change in angle. In the third beam, significant changes begin around 37.6°, with stresses increasing significantly between 0 m and 12 m, and an unsymmetrical stress pattern is observed. Similarly, in the fourth beam, the stress values remain close between 0° and 21°, but increase significantly in the middle span (18 m to 36 m) after 21°. Asymmetry is evident in all models, and in the first and second I-beams, the bending stress values in the early spans tend to be higher than those in the later spans, and in the third and fourth beams, the opposite is true. The shear force diagrams for the first, second, third, and fourth I-beams reveal a general behavior regarding the effect of increasing skew angles. In the first beam, the shear forces remained largely unchanged in the initial span but changed in the middle and third spans, decreasing in the first half of the spans as the skew angle increased to 49°. The second beam showed little change with increasing skew angles, but beyond 37.6° the values returned to the values at 0°, while the third beam showed little change until it reached 49°, with a significant decrease in shear forces in the second half of the middle span. The fourth beam showed a gradual increase in shear forces at 37.6°, followed by a decrease at 49°, with significant decreases (approximately 30%) in the second half of the middle span, and a significant 40% increase in the final part of the third span. In general, the shear force behavior varied between spans and beams, particularly in response to higher skew angles. Bending moments about the Y axis were also investigated. A general pattern emerges from the moment diagrams for the first, second, third, and fourth I-beams regarding the effect of the skew angles on the moment response. In the first beam, moment changes were minimal across the spans, with a slight increase near the first support at 49° skew, while the middle and third spans maintained similar values despite the skew changes. The second beam exhibited a convex moment line across the first span, increasing in the middle but shifting 25% at 37.6° before returning to its original shape at 49°. The third beam maintained consistent moment values across the spans with 25% increases at 37.6° and 49° skew angles, causing the diagram to change shape, but there was no significant change in the third span. The fourth beam exhibited mostly constant moment values across spans and skew angles, with values increasing by approximately 25% at 49° between 18 m and 30 m, with a slight increase as the skew angle increased in the third span. In general, the moment behavior varied between the beams, with the most pronounced changes occurring at higher skew angles, particularly around 37.6° and 49°. The displacement diagrams for the first, second, third, and fourth I-beams reveal a clear trend in response to increasing skew angles. In the first beam, the displacement values remained consistent for skew angles of 0° and 21°, but increased by approximately 35% at 37.6° and 75% at 49°, while the shape remained unchanged. The second beam exhibited a similar pattern, with displacement values increasing by 20% at 37.6° and 45% at 49°, and maintaining the same shape. The third beam followed this trend, showing a significant increase of 50% at 37.6° and 100% at 49°, but the signs of the displacements varied between spans. In the fourth beam, the displacement values increased by 60% at 37.6° and 70% at 49°, changing both shape and sign. In particular, the displacement values in the first span were generally higher than in the subsequent spans, and no symmetry was observed in the diagrams. The increases in displacement closely followed the increase in the skewness angle beyond 21°, highlighting the significant impact of geometric adjustments on the structural response. From the above, it can be concluded that with an increase in the skew angle, the stresses in the skew ed bridge deck differ significantly compared to the stresses in a flat bridge deck. The critical value of the skew angle, which changes the most in the behavior of the superstructure, occurs at the skew angle of about 37.6°. Therefore, the prestressed continuous superstructure of the bridge should be designed according to these behaviors. In addition, the superstructure behavior should be examined with other close skew angle values between 30° and 45° to determine the maximum values of moments, stresses and shear forces to be produced by using different skew angles. The torsional moment was also investigated. It was noticed that the behavior of the superstructure changed in different ways after the skew angle was increased beyond 21°. The results of this study largely coincide with the AASHTO standard specifications, which recommend that bridges with a skew angle of 20° or less should be designed as straight bridges. Anagha et al. (2016), studied the effect of skew angle on the performance of highway bridges. The objective of that study was to investigate the effect of skew angle on varying carriageway width and span length. The parameters considered are: Variation of skew angles as 0°, 20°, 40° and 60°, Variation of carriageway width as 4.5 m, 7.5 m and 10.5 m, Variation of span length as 10 m, 15 m and 20 m. The results are obtained based on the bending moments, shear forces and torsional moments. Thecritical structural responses are represented in various graphs. First, bending moment for skewed slabs in Knife Edge Load condition and Concentrated Load condition compared to that of straight deck slab decreases with the increase in skew angle for all carriageway widths. The load is transferred through the strip area at the ends i.e., at the obtuse corners. Second, torsional moment, the maximum torsion in beam bridge decks for skewed bridges compared to that of straight bridges increases with increase of skew angle up to 40° for considered span lengths 10 m, 15 m and 20 m. Beyond 40°, a decrement occurs as skew increases. The torsional values are considered based on the aspect ratio (span: width). Around 60 % increment in the torsional moments. In the case of concentrated load at the center, when skew angle increases from 20° to 60°, the increments of torsional moment for aspect ratios 2.222, 1.333 and 0.952 are 70.66%, 70.71% and 71.18% respectively. Around 70% increment in torsional moment as skew angle reaches to 60°. According to the analytical investigation on skew bridge with varying span lengths and skew angles and also with varying carriageway widths, the following conclusions are arrived. The shear force in the knife edge loading condition is increasing gradually, and the increment is nearly 30%. The bending moments in the concentrated load condition and knife edge load condition are decreased up to 65% and 75% respectively as the skew angle increases to 60°. The increment of torsional moments is 60% in both conditions, i.e., in concentrated loading condition and knife edge loading condition. From the present study, it is evident that most effective skew angle is 20° skew angle, the increment of torsional moment is larger and so that the failure of bridge will be greater compared to the other skew angles. In our study, the superstructure of prestressed concrete continuous skewed I-beam bridge was analyzed with FEM using Midas Civil program. As a result, it was seen that the results showing that the change in skewness angle affects the behavior of the superstructure in terms of stress, moment, and shear forces were clear. A good agreement was found between AASHTO and theoretical results. The results of this study largely coincide with the AASHTO standard specifications, which recommend that bridges with an skewness angle of 20° or less should be designed as straight bridges. If the geography of the location where the bridge will be constructed forces us to use an skewness angle greater than 20°, some precautions should be taken such as: using the span of the bridge as short as possible, using simply supported structure instead of using continuous bridges, using large sections in the superstructure for Ibeam (using AASHTO code), design of column heads should be done carefully by considering the bending moments with suitable sections, using diaphragms between beams, and make investigation of losses in the prestressed concrete system. Keywords: Skewed Bridge, Prestressed Superstructure, I-Girder, Stresses, Shear Forces, Torsion, Displacement