Scour and Geotechnical Parameters - What are the essential info bridge engineers require? 1

[Eugene Lim]

Hi guys. My question relates specifically to scour in bridges over water bodies. I'm wondering what are the essential information that bridge engineers require from geotechnical engineers apart from particle size distribution and atterberg limits testing.

Input much appreciated!

 

[BridgeSmith]

As a bridge structural engineer, particle size distribution and atterberg limits mean nothing to me. They would probably be important to our hydraulics engineers, who would use that, the flow velocity, and I'm not sure what else, to determine the depth of scour, which they put in a report for us.

Of course, that's just how we do things at the DOT; not sure if bridge designers other places have to do their own hydraulic analysis. (If they do, I pity and admire them at the same time)

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

 

[Eugene Lim]

BridgeSmith Thanks for your input! Yes, you are right. The hydraulics engineer is the key person, who takes the geotech input and converts it to a design scour depth which bridge engineers like yourself will use. I have discovered that the geotech parameters needed are PSD and Atterberg limits test.

 

[STrctPono]

RQD (Rock Quality Designation) is another parameter that is important to know. Basically tells you how susceptible the rock is to scour.

Agree with BridgeSmith. Around here, Structural Bridge Engineers do nothing when it comes to establishing the scour parameters and depths of scour but then look at that information provided (roll their eyes when they see 20ft scour depths) and then proceed to run the structural analysis with the unrealistic.... erm, cough... given parameters because the hydraulic Engineers with their HEC-RAS black box program are never wrong.

 

[BridgeSmith]

STrctPono, the case in our office is that the eye-rolling usually is because they give us 4 different scour depths - Einstein's, Laursen's, etc. and seem to expect us to choose the appropriate one.

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

 

[STrctPono]

the case in our office is that the eye-rolling usually is because they give us 4 different scour depths - Einstein's, Laursen's, etc. and seem to expect us to choose the appropriate one.

In all seriousness, we abide by the following protocol... Not sure how this stacks up to your office's practice.

If bridge is within FEMA floodplain:
100 year flood - Hydraulics and freeboard elevation requirements of superstructure need to satisfy
200 year flood - Structural Strength and Extreme level design at full scour
500 year flood - Check structural stability of bridge due to scour for DL only.

If bridge is NOT within FEMA floodplain:
50 year flood - Hydraulics and freeboard elevation requirements of superstructure need to satisfy
100 year flood - Structural Strength and Extreme level design at full scour
200 year flood - Check structural stability of bridge due to scour for DL only.

 

[dik]

Thanks STRct... I wasn't aware that was done... my sheltered life, I guess...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BridgeSmith]

We typically do 100 year scour for design, with other loads per the LRFD load combinations, and then check structural stability with just dead load for the 500 year scour. For some non-essential and low traffic bridges, we drop to 25 year for design and 100 year review. Our Hydraulics people determine the design and review return periods and specify the scour depths for them, usually for a range of channel widths.

Not sure if the design or review parameters change for USACE flood control zone, but we've done a few of those. The Corps of Engineers know their stuff, and they are sticklers for how things are done in those corridors.

Don't know what they do for FEMA floodplains. I know they do a property hazard review for all of our bridges to make sure we're not increasing the flood hazard to developed property with the bridge or culvert we're putting in.

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

 

[Imran Structural Engineer]

We typically do 100 year scour for design, with other loads per the LRFD load combinations, and then check structural stability with just dead load for the 500 year scour. For some non-essential and low traffic bridges, we drop to 25 year for design and 100 year review. Our Hydraulics people determine the design and review return periods and specify the scour depths for them, usually for a range of channel widths.

AASHTO 3.7.5 states that the changes in foundation due to scour resulting from check flood for bridge scour shall be considered at Extreme event states. Keeping that in mind, for EE-I, will we not take EQ loading?
And also what does it mean by 100 year scour for design and 500 year for stability? Aren't we going to design at whatever condition will govern i.e. strength, Service or Extreme event, and then taking the respective parameters?

TrctPono, the case in our office is that the eye-rolling usually is because they give us 4 different scour depths - Einstein's, Laursen's, etc. and seem to expect us to choose the appropriate one.

I'm relatively new in Bridge design and this issue is also bugging me a lot. Can you suggest me which one of the scour depths do we prefer or is it the job of hydraulic engineer to suggest one?
Thanks

 

[BridgeSmith]

AASHTO 3.7.5 states that the changes in foundation due to scour resulting from check flood for bridge scour shall be considered at Extreme event states. Keeping that in mind, for EE-I, will we not take EQ loading?

The commentary for the EE load combinations in 3.4.1 states the following:

Although these limit states include water loads, WA, the effects due to WA are considerably less significant than the effects on the structure stability due to scour. Therefore, unless specific site conditions dictate otherwise, local pier scour and contraction scour depths should not be combined with BL, EQ, CT, CV, or IC. However, the effects due to degradation of the channel should be considered. Alternatively, one-half of the total scour may be considered in combination with BL, EQ, CT, CV, or IC.

And also what does it mean by 100 year scour for design and 500 year for stability? Aren't we going to design at whatever condition will govern i.e. strength, Service or Extreme event, and then taking the respective parameters?

It means that for the 100-year event combined with other applicable loads it must survive with minimal damage and still be serviceable (strength and service limit states). For the check flood (typically the 500 year event), the structure has to be stable, i.e. not fall down (Extreme Event limit state).

I'm relatively new in Bridge design and this issue is also bugging me a lot. Can you suggest me which one of the scour depths do we prefer or is it the job of hydraulic engineer to suggest one?

Our hydraulics people have tried to explain which is more applicable to what conditions, but if it's not in the report, I walk over and ask them, because I still have no clue. apparently it varies with the type of channel material.

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

 

[Guest090822]

STrctPono, the case in our office is that the eye-rolling usually is because they give us 4 different scour depths - Einstein's, Laursen's, etc. and seem to expect us to choose the appropriate one.

Or my favorite scenario - Geotechnical Engineer writes a report that rock is unscourable, Hydraulics Engineer calculates 20 feet of scour!

 

[BridgeSmith]

Or my favorite scenario - Geotechnical Engineer writes a report that rock is unscourable, Hydraulics Engineer calculates 20 feet of scour!

There's a standard disclaimer in our hydraulics reports that the presence of bedrock may limit scour. Of course, they're usually writing the hydraulics report before the geology guys do their thing. Kinda has to be that way, I suppose. We need the the hydraulics report to determine he bridge length, and Geology needs the bridge layout to know where to drill for the foundation investigation.

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