Span configuration for tall bridge in horizontal curvature

[Imran Structural Engineer]

I am planning a bridge over a waterbody. The high flood level is 15meter. I've attached images with two possible configuration, one with pier in middle with height of 28.4 meter while other configuration avoids the deepest part by 3.4m in height. There is an existing older bridge at downstream with pier in the middle. Superstructure is discontinuous with 35m box girders and the radius of curvature is 60m. Can anyone give suggestion regarding pros and cons that would help me finalizing the configuration.
Any sort of help would be highly appreciated.
Regards

 

[BridgeSmith]

I don't know what the horizontal to vertical scale is, but if it were me doing the layout, I would consider longer spans to avoid having a pier in the deep areas. I would look at realigning the approach roadway to decrease the curvature and avoid crossing the widest part of the body of water.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Imran Structural Engineer]

I don't know what the horizontal to vertical scale is

Vertical scale is set at 5xhorizontal. But I've edited the heights and mentioned actual heights i.e. 28meters without scale factor.

I would look at realigning the approach roadway to decrease the curvature and avoid crossing the widest part of the body of water

The terrain is mountainous. To reduce curvature, I'd have to get alignment relocated to downstream but it increases height substantially. Previously, the bridge was straight but height was 60meters. So to reduce height and find narrower opening, the alignment was shifted upstream. Mountains are very steep to go further upwards to find narrower and shallow path, we'll have to cut through mountain which is another issue. On this alignment, I was unable to convince transportation people to provide straight bridge as it breached their geometric constraint due to design speed(30kph min). I'm kind of stuck with this scenario.

 

[BridgeSmith]

What is the out-to-out length of the bridge?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Imran Structural Engineer]

@BridgeSmith
125 meters. 3x35m span+1x20m

 

[BridgeSmith]

Given the size of the piers required, I would be strongly considering a 3 span option, maybe a 35m-55m-35m configuration, or even 30m-65m-30m spans.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Imran Structural Engineer]

Yes, but is 55 or 65 meter span with a radius of 60m going to be feasible? I mean given that the amount of torsional moment it will generate?

 

[BridgeSmith]

The torsional load will definitely be more significant with the longer spans, but it will be high for the 35m spans, also. I think either way, you'll want to make the superstructure continuous. the hold-down forces at the inside edge would be very large and difficult to successfully transfer into the superstructure. I'm out of my expertise on this, but I think your best option for the superstructure would be segmental post-tensioned, made continuous from end to end.

Btw, the unsymmetrical span configuration of your original proposal will make the structural analysis much more complex than it's already going to be. If you stay with your 4 span configuration, I suggest a symmetrical configuration with 27.5m end spans and 35m middle spans.

There's also the 2 span option. The piers are going to be massive, anyway, controlled by slenderness. You'll find that a pier big enough to be stable at that height will be able to carry the superstructure loads easily. You may find that overall that's the most economical option.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Imran Structural Engineer]

I think either way, you'll want to make the superstructure continuous.

Yes, we are working continuous structure now. Thank you.

There's also the 2 span option.

Can you expand a little bit on this option? you are suggesting pier in middle with length of each span to be 50m approximately?

 

[BridgeSmith]

Can you expand a little bit on this option? you are suggesting pier in middle with length of each span to be 50m approximately?

Yes; 2 equal spans of whatever length they would need to be, with a single pier.

The superstructure would need to be deeper for the longer spans, but that additional depth will also help with the torsional capacity, as long as it's a closed shape, such as a box or tub girder.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Imran Structural Engineer]

@BridgeSmith
Thank you. Currently I am working of model of 35x4m continuous box structure connected rigidly to substructure. After that I'll work on 2 span option as well.

 

[BridgeSmith]

For the 4-span option, you'll get better balancing of the vertical loads, and I suspect also the loads due to the curvature with something closer to a span ratio of 1 to 1.3 for the end spans to interior spans (30m-40m-40m-30m). The 4-span superstructure may be more economical at a 1 to 1.5 span ratio, due to the top being larger (larger span ratios = relatively larger negative moments). It seems moving P2 & P4 farther out, towards the ends of the bridge, would decrease the unbraced height of them, also.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[MIKE_311]

In water work can be expensive plus the impacts of scour and H&H on the design, not to mention any permitting and approvals. If you can keep piers out of the water, that's where I would start.

 

[Imran Structural Engineer]

For the 4-span option, you'll get better balancing of the vertical loads, and I suspect also the loads due to the curvature with something closer to a span ratio of 1 to 1.3 for the end spans to interior spans (30m-40m-40m-30m).

So far i've worked on the same configuration that you mentioned. Kept length of interior spans to 35 meters. however I've designed section without prestressing. I'm afraid pull-out forces due to sharp curvature might result in considerable thickening of webs which i haven't calculated as of now.

Any suggestion that at what height should I start considering a hollow pier?
Regards

 

[BridgeSmith]

I think you're definitely tall enough at the center pier to warrant serious consideration of a hollow post-tensioned pier. The others are probably borderline, but you probably wouldn't want to mix types.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[spsalso]

A flood level of 50 feet. And you're considering putting a pier in the middle of that.

Yikes. Quite the challenge!


spsalso

 

[BridgeSmith]

With that degree of curvature, I think at least one pier is going to be a necessity. For a single span, the torsion at the abutments would be very difficult to manage. With the flood level that high, it may not be possible to avoid substantial stream forces on the piers. However, stream forces, even from the high flood level shouldn't be a critical loading. If there is ice potential at higher flows, that could be a considerable design loading, though.

Rod Smith, P.E., The artist formerly known as HotRod10