# Calculating the force on a submerged bridge. **URL:** \ **Category:** Factual Questions\ **Created:** [June 6, 2019, 11:43pm UTC](https://boards.straightdope.com/t/calculating-the-force-on-a-submerged-bridge/835146 "2019-06-06T23:43:32Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [June 6, 2019, 11:43pm UTC](https://boards.straightdope.com/t/calculating-the-force-on-a-submerged-bridge/835146/1 "2019-06-06T23:43:32Z")
The Arkansas river is almost at the top of the rail road bridge piers in Little Rock. I wondered how much force it could take if the bridge became submerged. I know that the force on a vertical plane under water is ro_G_h, but I have no idea how to calculate the force of the current on a bridge under water. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [June 6, 2019, 11:53pm UTC](https://boards.straightdope.com/t/calculating-the-force-on-a-submerged-bridge/835146/2 "2019-06-06T23:53:34Z")
> [@carnivorousplant](#): > > The Arkansas river is almost at the top of the rail road bridge piers in Little Rock. I wondered how much force it could take if the bridge became submerged. I know that the force on a vertical plane under water is ro_G_h, but I have no idea how to calculate the force of the current on a bridge under water. Really too many unknowns. The real problem is trees and other debris that get caught up on the bridge making the force of the water many times stronger as it tries to go over, around or underneath. Or what happens, through. Floods bring a lot of crap with them. ---
**Author:** ![Gray\_Ghost](https://avatars.discourse-cdn.com/v4/letter/g/c0e974/32.png) [@Gray\_Ghost](https://boards.straightdope.com/u/Gray_Ghost)\ **Post date:** [June 7, 2019, 12:37am UTC](https://boards.straightdope.com/t/calculating-the-force-on-a-submerged-bridge/835146/3 "2019-06-07T00:37:08Z")
> [@carnivorousplant](#): > > The Arkansas river is almost at the top of the rail road bridge piers in Little Rock. I wondered how much force it could take if the bridge became submerged. I know that the force on a vertical plane under water is ro_G_h, but I have no idea how to calculate the force of the current on a bridge under water. What anchors the bridge to the piers? Just gravity? I doubt it’s strapped to the piers, like is normal in earthquake zones. How much buoyancy force would it take to lift the bridge and what depth of water and velocity of current would be enough to obtain that buoyancy force? ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [June 7, 2019, 12:43am UTC](https://boards.straightdope.com/t/calculating-the-force-on-a-submerged-bridge/835146/4 "2019-06-07T00:43:24Z")
> [@Gray\_Ghost](#): > > What anchors the bridge to the piers? Just gravity? I doubt it’s strapped to the piers, like is normal in earthquake zones. > > How much buoyancy force would it take to lift the bridge and what depth of water and velocity of current would be enough to obtain that buoyancy force? There is the New Madrid fault not far away, that may have figured into the design. Buoyancy is a different question, but may be more applicable than the force of the flowing river. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [June 7, 2019, 1:33am UTC](https://boards.straightdope.com/t/calculating-the-force-on-a-submerged-bridge/835146/5 "2019-06-07T01:33:23Z")
Doubt buoyancy of the bridge has much, if anything to do with it (unless it’s a wood bridge). Sure, it matters a little bit, but I think people are WAY underestimating the force of water. Especially if it’s direction of flow becomes blocked. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [June 7, 2019, 1:43am UTC](https://boards.straightdope.com/t/calculating-the-force-on-a-submerged-bridge/835146/6 "2019-06-07T01:43:01Z")
We just had a flash flood warning. It has been raining like hell this afternoon. I may see a practical application. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [June 7, 2019, 2:18am UTC](https://boards.straightdope.com/t/calculating-the-force-on-a-submerged-bridge/835146/7 "2019-06-07T02:18:50Z")
I am not a Civil Engineer but some of my colleagues are. Based on previous discussions, the hydrodynamic forces or loads during a flooding event are : Hydrodynamic pressure forces, Drag forces, lift forces, debris/log impact forces in addition to the hydrostatic forces. The American Bridge Standard (AASHTO) ([https://www.transportation.org/](https://www.transportation.org/)) has detailed formulas and constants for estimating these loads. For example, the impact force is typically estimated using a log of 2 metric tons. You can see the results of such a calculation on page 7 (Page 18 of 53) for different types of bridges. This link is from the army corps of engineers - [https://apps.dtic.mil/dtic/tr/fulltext/u2/a398916.pdf](https://apps.dtic.mil/dtic/tr/fulltext/u2/a398916.pdf) ---
**Author:** ![rbroome](https://avatars.discourse-cdn.com/v4/letter/r/838e76/32.png) [@rbroome](https://boards.straightdope.com/u/rbroome)\ **Post date:** [June 7, 2019, 2:40am UTC](https://boards.straightdope.com/t/calculating-the-force-on-a-submerged-bridge/835146/8 "2019-06-07T02:40:56Z")
> [@enipla](#): > > Doubt buoyancy of the bridge has much, if anything to do with it (unless it’s a wood bridge). Sure, it matters a little bit, but I think people are WAY underestimating the force of water. Especially if it’s direction of flow becomes blocked. The kind of material has relatively little to do with whether the spans will stay on the pilings. In Katrina something like ⅓ of the spans between the pilings lifted off and went into the water when the storm surge hit the 5 mile Interstate 10 bridge across Lake Ponchartrain. Those spans are made of steel and concrete (which admittedly doesn’t weigh much underwater). The force of moving water is unbelievable. Think tons of force where one would expect lbs of force.