We have been having a discussion in the office as to how to best lay out the top of road and bounding cross-sections to make our work proceed as quickly and accurately as possible. Not quite sure I understand what you mean by "0" stations being parallel to the stream centerline or perpendicular to the road.
In general, your bounding cross sections sections 2 and 3 should be parallel to the bridge and roadway approaches, even if they are skewed. Then you skew those bounding cross sections as well as the bridge deck.
All other cross sections should be perpendicular to flowlines. Not sure if this answered your question…. For a pre and post bridge comparison, as I understand it, the cross sections would need to be the same in both models. I have a skewed bridge and have the modified locations of the cross sections upstream and downstream of the bridge to be as close to parallel as possible given my the bridge has a slight s curve. Please tell me if my thought process is correct.
I would need to apply a skew to these cross sections in both the pre and the post bridge models and the bridge in the post bridge model. Also, to verify I am understanding, after the skew, the terrain I originally cut my cross sections from will no longer match. Lastly, I know my piers are going to be placed parallel to the flow and are cylindrical, so I am wondering if I would need to skew them as well.
I am also realigning the channel. Thank you for any help you can provide. Hi Chris! The situation is like there is existing road crossing the river and now new road joins the existing road just before the bridge. The new road after some distance runs parallel to the river. So I want to take new cross section perpendicular to the river till the road and along the road then after. If I just take the stations and elevations of my proposed cross section, will HEC-RAS take it straight line or it will be along the road?
If it takes straight, then is there any way to bend the the line? In order to include proposed roadway, some station and elevation are added in revised model. In addition, levees option is used at top of pavement to make the roadway act as barrier.
So, I want to know the functionality of culvert if it is provided across the barrier roadway. Will barrier allow water to flow at defined culvert location through it? If yes, can we see that in "View Cross sections" tab? You can only remove water from a river at a cross section by using a lateral inflow outflow with negative values boundary condition.
If you want to use a culvert, then you'd have to simulate the barrier with a lateral structure. Is the skew angle that is entered for the bridge cross sections and the US and DS sections the same skew angle entered for the angle of attack of the flow on the abutments on the abutment scour tab under the Hydraulic Design Function?
The manual states that 90 degrees should be entered under angle of attack for abutment that are perpendicular to the flow normal condition , but I don't see how that would agree with the figure presented here.
Can you please explain? Great Blog! Do we have to adjust them manually or it does not matter? Let's say the bridge in Figure 1 above runs east to west. If you were to orientate the bridge to run north to south, would your skew angle then exceed 45 degrees? The skew angle is always the angle between the bridge alignment and a line perpendicular to the flow lines at the bridge.
HEC recommends not exceeding 45 degrees because typically the flow patterns will adjust to flow more directly through the bridge opening for overly-skewed bridges. I am creating a Corrected Effective Model for a bridge on skew of 17 degrees. The bridge is supported by 4 sets of beams supported by 4 separated piles at a skew to the flow. The bridge is to be replaced by a bridge supported by two sets of beams supported by two piers.
I am wondering how to deal with non-continuous piers on a skew. Piers are continuous. You might experiment with less skew on the piers or a smaller drag coefficient to see if that can help correct the error.
I have a bridge which is skewed to stream about 40 degree. When entering bridge data, is it advised to enter bridge data distance between piers and slop walls, total bridge length etc.
Hi Peter. Your email address will not be published. As a result of the skew geometry, main girders will twist about their longitudinal axes during construction and it can be difficult to predict the exact rotation at the end of a sequential construction sequence for a composite bridge. The girder webs may not be truly vertical at the end of construction. Advice on this aspect is given in Guidance Note 7. General guidance on detailing, including the orientation of the slab reinforcement and the need to avoid conflict between stud connectors and reinforcing bars, is given in Guidance Note 1.
Navigation menu Home. Share Tweet. Tools Printable version. From SteelConstruction. This article gives an overview of the design consequences for skew alignment of bridges. Plan of typical skew bridge. Girder arrangement for a single-span deck. Layout of main girders and bracing for skewed multi-girder decks — small skew. Layout of main girders and bracing for skewed multi-girder decks — large skew. Arrangement of girders for skew half through railway bridges.
Cross girder to trimmer girder connection in a skew bridge inner bolts could only be inserted with head on open face. Longitudinal reinforcing bars in a skew ladder deck bridge. This amounts to a lot of strain for the ft-long piers supporting the I over the HAST bridges, especially at the ends of the pier where the movement is the greatest.
These temperature strains can create uplift forces in the supporting pile foundations. Whether pile uplift is a problem or not depends on the soil. The soil conditions at the HAST crossing did not provide much uplift capacity due to a hard gravel layer with potential boulders about 30 ft down, making it impractical to drive piles deep enough into that hard layer to provide sufficient uplift resistance.
Additionally, many piles had to be pre-bored to avoid damage to various utilities, including a in. The pre-boring effectively removes all uplift capacity, so the piers had to be designed for little to no uplift. It was clear that a ft-long pier would move too much along its length due to internal thermal forces and shrinkage, so each pier cap was designed with an expansion joint near its middle, effectively turning each pier into two piers.
This allows each pier section to function independently and drastically reduces the pier movements and forces. However, the bearing types and locations had to be carefully chosen to allow the piers to move independently. Typical modern three-span prestressed precast bridges in Wisconsin have fixed diaphragms at both piers that lock the superstructure to the pier, which would have negated the benefits of the pier cap expansion joint on the HAST bridges.
Using typical fixed diaphragms or bearings at the pier would have caused another problem common to many skewed bridges. The superstructure would tend to rotate in plan view as the bridge expands and contracts.
The reason for this rotation is the bridge skew and the fact that the piers are very stiff along their length, but much more flexible in their other direction. A hammerhead pier with a single square or round column does not have this problem, but a ft-long pier with many columns has it in abundance. As the bridge expands or contracts, the piers are so rigid along their length that they restrict any movement in that direction.
So instead the movement happens in the direction of least resistance, causing bridge rotation. This wreaks havoc with the expansion joints. One end of the joint would quickly close up, and the concrete traffic barrier plates would be pulled out of alignment and damaged. The solution to these challenges was a mixed bearing arrangement that used fixed bearings over half of each pier and expansion bearings everywhere else. The sections of fixed bearings, on the right half of the first pier and the left half of the second pier, face each other perpendicularly across the HAST.
A mixed bearing arrangement is sometimes used for steel bridges but rarely for prestressed concrete. This arrangement allows the pier cap expansion joint to function and the bridge to breathe in a uniform way without significant rotation.
The piers can breathe along with the superstructure since they are not dragged along in their strong direction. This reduces the pier forces and minimizes pile uplift, allowing for economical piers. The superstructure movement is reduced and controlled to the point that a standard strip seal expansion joint is feasible instead of having to explore more costly joint types.
The main purpose of this research was to recommend design details and practices that might mitigate certain negative effects on deck and bearing elements. The study investigated the typical performance of highly skewed bridges, identified limits for simplified analysis methods, and analyzed different factors which influence the effectiveness of the mixed bearing arrangement. The bridge was one of two structures instrumented through this study, the other was a steel girder bridge, under short-term live load conditions and long-term month temperature loading conditions.
Strain gauges were cast into the reinforced concrete deck at an acute corner to measure strains from shrinkage and temperature changes. Displacement sensors and strain gauges were placed on prestressed concrete girders to measure shear and flexural strain for a variety of live load configurations.
Displacement sensors were also placed at laminated bearings to measure both the live load displacement and the long-term temperature displacements. The most relevant field measurements, which show support for the effectiveness of the mixed bearing arrangement, are the transverse bearing movements. The maximum transverse bearing movement at an acute corner worst case was limited to 0.
This is less than the computer model, which may indicate some other restraining forces are limiting this movement. With such small transverse bearing movements, the data did not correlate well to the temperature readings. The researcher was not able to perform additional substructure modeling within the confines of this study.
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