geoman77
Geotechnical
- Feb 11, 2011
- 10
I am modeling the performance of the foundations of a bridge that suffered damage due to foundation movement during an earthquake and need some assistance in properly describing the bridge structural systems. My background is geotechnical and I am primarily concerned with modeling the foundation behavior, but I want to make sure I describe the rest of the bridge accurately. The following are my interpretations from the plans:
-The bridge has 10 spans, each 20m in length.
-Each span consists of 7 precast-prestressed I-shaped girders which are supported on laminated neoprene bearings on a bent cap; the bent cap is supported on four columns.
-There is a 4cm gap between the ends of the girders in the longitudinal direction at every pier.
-Precast slab panels about 1m-x-1m span between the girders over the entire 20m span
-A cast-in-place deck slab on top of the precast slab panels is continuous over the first three spans, then the center four spans, and finally over the remaining three spans. In other words, the deck slab was constructed in three pours.
-There are vertical diaphragms between the ends of the girders in the transverse direction. At each pier, there are two of these transverse diaphragms, one connecting the ends of each of the two sets of girders that meet at that pier. The diaphragms are prestressed in the transverse direction across the entire 11m width of the bridge; the prestressing cables pass through ducts cast into the ends of the girders.
-At the piers where the deck slab is continuous over the joint, the diaphragms have a "shear key" (not sure if that's the right term, please advise-- the detail on the plans is called "diaphragm anchorage") that rests in a slot in the bent cap. The slot is rectangular such that it restricts movement of the shear key in both the transverse and longitudinal directions. The shear key concrete is separated from the bent cap concrete by a 1cm thick asphalt pad and no reinforcing bars are continuous across the joint. This type of joint is denoted on the plans as a "fixed support."
-At the joints where the deck slab is not continuous, the diaphragms do not have the "shear key" feature and there is a gap of 4cm between both the girders and the slabs. This is denoted as a "free support."
My main question is whether or not all of the spans have a simply-supported end condition in the longitudinal direction, i.e. is it appropriate to describe the entire bridge as simply-supported? Given that the "shear keys" at the "fixed" supports do not have reinforcement that is continuous into the bent cap, I don't see how moment could be transferred and thus I interpret this as a "pinned" connection (translation restricted; rotation allowed), whereas the joints where the diaphragms do not have a "shear key" are like "roller" connections (translation allowed but restricted by the stiffness of the neoprene bearings; rotation allowed). Therefore I interpret every span as being simply-supported. Does this sound right? And if I am interpreting this correctly, what's the reason for having transverse and longitudinal restraint only at some of the piers; why not have "shear keys" at all the piers? If you're just worried about thermal expansion of the deck slab, couldn't you simply put an expansion joint through the slab only at each pier? And why do the girders rest on neoprene bearings at the "fixed" supports if translation is restricted by the shear key?
Making this more complicated is the fact the plans are in Spanish and the site is in Mexico, and I haven't had a chance to make it down there yet and don't have clear photos showing any of these details. I can email portions of the plans if that would be helpful.
Thanks for the help!
-The bridge has 10 spans, each 20m in length.
-Each span consists of 7 precast-prestressed I-shaped girders which are supported on laminated neoprene bearings on a bent cap; the bent cap is supported on four columns.
-There is a 4cm gap between the ends of the girders in the longitudinal direction at every pier.
-Precast slab panels about 1m-x-1m span between the girders over the entire 20m span
-A cast-in-place deck slab on top of the precast slab panels is continuous over the first three spans, then the center four spans, and finally over the remaining three spans. In other words, the deck slab was constructed in three pours.
-There are vertical diaphragms between the ends of the girders in the transverse direction. At each pier, there are two of these transverse diaphragms, one connecting the ends of each of the two sets of girders that meet at that pier. The diaphragms are prestressed in the transverse direction across the entire 11m width of the bridge; the prestressing cables pass through ducts cast into the ends of the girders.
-At the piers where the deck slab is continuous over the joint, the diaphragms have a "shear key" (not sure if that's the right term, please advise-- the detail on the plans is called "diaphragm anchorage") that rests in a slot in the bent cap. The slot is rectangular such that it restricts movement of the shear key in both the transverse and longitudinal directions. The shear key concrete is separated from the bent cap concrete by a 1cm thick asphalt pad and no reinforcing bars are continuous across the joint. This type of joint is denoted on the plans as a "fixed support."
-At the joints where the deck slab is not continuous, the diaphragms do not have the "shear key" feature and there is a gap of 4cm between both the girders and the slabs. This is denoted as a "free support."
My main question is whether or not all of the spans have a simply-supported end condition in the longitudinal direction, i.e. is it appropriate to describe the entire bridge as simply-supported? Given that the "shear keys" at the "fixed" supports do not have reinforcement that is continuous into the bent cap, I don't see how moment could be transferred and thus I interpret this as a "pinned" connection (translation restricted; rotation allowed), whereas the joints where the diaphragms do not have a "shear key" are like "roller" connections (translation allowed but restricted by the stiffness of the neoprene bearings; rotation allowed). Therefore I interpret every span as being simply-supported. Does this sound right? And if I am interpreting this correctly, what's the reason for having transverse and longitudinal restraint only at some of the piers; why not have "shear keys" at all the piers? If you're just worried about thermal expansion of the deck slab, couldn't you simply put an expansion joint through the slab only at each pier? And why do the girders rest on neoprene bearings at the "fixed" supports if translation is restricted by the shear key?
Making this more complicated is the fact the plans are in Spanish and the site is in Mexico, and I haven't had a chance to make it down there yet and don't have clear photos showing any of these details. I can email portions of the plans if that would be helpful.
Thanks for the help!
