Hydrodynamic Force on Inundated Bridge 1

[vandede427]

I don't usually design bridges so I'm asking for as much imput as possible on this one. I've been asked to design the abutments for a 90' span to go on private land over a creek. The owner wants to use a flat rail way car as the superstructure. The traffic on the bridge will be very light: atv's and pick-up trucks.

Because it's a private bridge on private land, I'm not sure what flood interval to use for the water height. If it were a public structure, I'm sure I'd be under AASHTO code. Determining the elevation of the bridge is something I need to do.

ASCE 7 flood hydrodynamic loads are based on the DFE, design flood elevation. This is based on the "community's flood hazard map." This is a very rural area and I can't find this information. What percentile and year interval are flood maps usually based on?

I recreationally kayak so I'm pretty familiar with the USGS water gages. This particular creek doesn't have a gage, but a similar one only a few miles away does, so I'm using it's numbers. The gage for the similar creek has velocity and height data dating back to 1980. I'm thinking about using the 95% percentile numbers over this time period (setting the bridge elevation to be NON inundated 95% of the time over a 29-year span).

I'm going to design the bridge for the inundated flood loads, but I'd like to have some justification for the expected interval.

 

[cntw1953]

Other than local code officials, I think the owner has the most say on how much chances he is willing to take, and how much he wants to pay.

Remember that an area can experience any kind of flood (if there is such potential) at anytime that is outside of the return period suggested by statistics.

 

[Compositepro]

A neighbors driveway bridge over a creek was washed-out earlier this year. It was two 18" I-beams with wood timber planking. A week later the county highway department dumped a few truck loads of 12"+ rock fill, put the I-beams and timbers back in place. In the country you often just do what works. If a flat bed car washes away what have you lost? You just have to put it back. In most rural areas the roads department are pretty helpful to residents.

 

[ishvaaag]

Even acknowledging that there's economical rationality to your post, they can lose access, and that can be critical sometimes.

 

[vandede427]

"if it washes away, just put it back" is about the silliest thing I've ever seen typed on this forum. It's really not that easy to retrieve a 90' long rail car that's been swept downstream on land with nothing but a few dirt roads. I'm pretty sure the sole reason the owner is employing an engineer (me) is to keep the thing from washing away.

I don't think losing access to the bridge is a concern if it were to be inundated. It won't be used for any critical purpose. Plus, flood level waters don't stay at that elevation for long.

 

[MiketheEngineer]

In many (most) areas the local Fire Department or Fire Marshall wants to be involved to verify they can get over the bridge - even on private property.

You might want to check that out..

Just remember - Mother Nature is a bitch and she will always win - we just try to put off the inevitable.

 

[cvg]

you are asking questions that should be answered by a flood control engineer. You say the owner

"is employing an engineer (me) is to keep the thing from washing away"

. If that is true than you should seek the help of a experienced hydrologist / flood control engineer to help with your design. This is not something that is typically done by a structural engineer.

 

[cntw1953]

I think the most critical stage for the bridge deck is when it subjects to imminent overtopping. At that moment, there are going to be several loadings involved - hydrostatic, hydrodynamic, uplift, wind, wave impact, drag force, suction force, and debris impact with increased velocity.
If I am you, I would set at least a few feet clearance in between the anticipated maximum flood level and the deck.

 

[DWHA]

I would use the states local roads manual. Considering it a low volume road, the local roads manual will give you a design flood frequency. You can later justify your design based on a relivant code. I would also rate the bridge structure using the AASHTO HS rating format. That way if there is ever any question, it is rated the same way all other bridges are rated.

 

[Qshake]

DHWA is right on the money. Use the Local Public Agency Manual (LPAM) for guidance to the work. Also, be mindful that water under the bridge (and against it) is more than a structural issue. If you don't have the correct opening, the water will back up. That might be fine so long as the bridge owner owns the land for quite a distance above the bridge. However if he doesn't then he stands the risk of impacting a neighbors land and that can be critical.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[cvg]

the road manual may give you the desired flood frequency for design, but you will still need to do the hydrology and hydraulic analysis of the watershed and the river in conjunction with your proposed bridge abutments to determine length of the bridge, height of the superstructure and depth of scour. This can't be done by using a gage on a nearby river. Ask your client to have a H&H study done to support your design.

By the way, if you are in the US, you may want to check into FEMA criteria.

 

[vandede427]

I can't find any info titled LPAM or anything even close to being a "local roads manual."

The length of the bridge is already set. The owner is buying a 90' long flat rail car. I have no control over that. Therefore I can't set the length of the bridge, just the elevation. To set the rail car high enough to clear the 100-year FEMA flood mark would be very unreasonable in this case. I've determined what I believe to be the 10-year flood mark and have set it just above and am designing it for the hydrodynamic loads from inundation. Locks of concrete and dywidag anchors into the bedrock should keep it in place.

I appreciate all the help and comments, but some of the suggestions have gone over and above the nature of this project. It's a small creek on private land (100's of acres) that the owner just wants to drive his atv's over and go hunting on. Hydrologic studies and AASHTO HS ratings are beyond the scope of this bridge.

 

[Qshake]

Then probably won't want to hear about the scour critiera either or the impact on the soil in front of and under your abutment, which you'll likely use a slab on grade, I suppose.

We all know about these types of projects very well and most of us won't touch them which is how they wind up in your hands..."I don't usually design bridges" whereas we're concerned with codes, legal matters and ethics.

You asked for our help "asking for as much 'imput' as possible" and then you have the audacity to criticize it.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[DWHA]

Vandede
Contact the local DOT and ask them where to obtain a copy of the local roads manual.

Also, you cannot look a 1 stream and expect another steam to act the same way. The have completely different watersheds. That is like saying all W12's have the same capacity just because they are both the same shape and are 12" deep.

Here is a suggestion, follow the stream downstream and take a look at the geometry of the nearest downstream structure. Then contact the landowner of the ground on the upstream side of this structure and ask him if he has ever seen any major backup from the structure. If your bridge opening is as large or larger then you have something to base your design on.

 

[vandede427]

DHWA and cvg, thanks for the help. I will contact the DOT for a copy of the roads manual. I understand that different creeks will have different watersheds. I was using the numbers from a gage on the creek that this particular one feeds into, so in do you think it's conservative to assume those numbers are worse than the feeder creek I'm spannin? I know it's not perfect, but it's all I've got to go on. I've attached a sketch of the proposed elevations. Please take a look.


Qshake, you should probably give your high horse a rest. He's tired, hungry, and needs some water from carrying you so long. Just as I assume you are, I am a professional licensed in multiple states with many years experience. I'm not throwing codes and legal matters to the wind and designing something that's unsafe to the owner (the performance of the bridge) or the rest of the public (it's affects on the environment downstream). However, I was just reminding this forum that this scale of this work is small and private. We has engineers tend to go overboard sometimes with "hire a geotech", "hire a hydrologist", etc. I'm trying to provide the ownwer with an economical bridge that will not wash away during a flood event, will not adversely affect upstream or downstream environment, but will also not be 30' above grade so that he has to drive his atv up a spiral ramp to get to it.

 

[Stillerz]

Have you considered a draw bridge?
A draw bridge could be raised after it was used and would help to reduce the effects of flooding.

 

[cntw1953]

As long as the owner understand and accept the risk and consequence, your hands are free, no need to get excited over dissicusions and personal opinions - good or not.

 

[Hemifun]

You really should check your state's Dept. of Environmental Protection/Conservation regulations. In most cases you will need a permit to construct the bridge and as part of that process the regs. will most likely have requirements regarding the frequency of the design storm you will need to design for. In addition, there are likely erosion and sedimentation requirements as well. You really need to do a hydrologic study to determine the peaks flows for whatever design storm you choose. If you are not comfortable doing these tasks you should find someone to help you.

 

[ishvaaag]

Up the airy mountain,
Down the rushy glen,
We daren't go a-hunting
For fear of little men

Hmmm, may be a X year flood where X the number of years the owner is expected to still go hunting will fare better. Also, from your scheme, a bit of encasing on the hard ground will enhance higly the survability of the bridge on its enhanced capacity to sustain moment from hydraulic forces and less heavy scour. Sometimes a small difference gives noticeable advantage.