Drilled Shaft Pier Modeling 3

[Hunt1999]

After performing our pier analysis, we send the loads to the geotech. to perform the L-Pile analysis. On a standard continuous prestress I-beam bridge with concrete diaphragms on a pier cap, I am trying to decide what my top fixity condition is to give to the geotech for their analysis. Right now they are analyzing the shaft as a "flag-pole" column (rotation-free and translation-free). But I think that due to the stiffness of the superstructure, the keyways between the cap and the diaphragm, and with integral end bents, the top of the pier is not very free to rotate or translate. So I think that designing the shafts for this (for deflection-servicability) is too conservative. Anyone have a thought on how to model this?

 

[yakpol]

Normally I model drilled shafts as a vertical element with properties defined with 6x6 matrix. LPILE has an option to generate the stiffness coefficients for 20 different loads. If the structural program you are using is not capable to model such element it can be replaced with the normal beam element (EI, L) and additional translational spring at the mudline.
If you design for seismic the upperbound forces from your approach (fixed at mudline) will be tooo large indeed.

Hope it helps!

 

[BridgeEI]

My first impression is that it would be rotation/translation free since the pier would deform under the temperature loading. However depending on the span arrangements and fixity of the bearings I may choose different conditions.

 

[BridgeEI]

Also, you can check out COM624P from FHWA and look through its user manual. It has several design examples specifically covering bridges and it's pretty helpful.

 

[VoyageofDiscovery]

This is just an observation from Canada.

I was speaking to a colleague within my company who worked in the US and I was suprised to learn that in the US designers give design loads to the geotechs for them to design the deep foundations.

First I don't particularly agree with this approach as I can't see how one would reconcile Limit State Design of a bridge with the p-y curves that give you results like a black box, for example, I understand that each type of pile leads to differing p-y curves. Honestly, does it include prediction of Euler buckling at limit state within the soil mass. If it does perhaps it has merit. Then why are there restrictions on deflections, as it is allowed to reach some percentage of the Euler buckling load?

Here in Canada, I am a bridge engineer and I design my own deep foundations without LPILE and p-y curves. Based on my experience, not to denigrate the work geotechs do, and the practice in Canada, I believe the structural engineer is most probably the best person to understand the structure and its intended behaviour under LSD.

Just my ignorance talking!

 

[tumbleleaves]

I "manage" bridge projects here in the U.S. This is true about geotechs independently designing foundations. IT IS A BIG PROBLEM, AND GETS WORSE EVERY YEAR.

The consulting firms have gone into a specialty mode, where everyone is a specialist, geotechnical, structural, hydraulics, traffic, surveyor, etc. It is a mess, each working individually and separate.

The worst part is the contact man is nearly always a specialist in making work breakdown structures, and critical path method charts that look like circuit diagrams--with little design experience. I suspect that these scheduling tools and planning have reached the point of diminishing returns, a lot of it is just smoke and mirrors.

Management in government is easily impressed by PhD's and snazzy looking proposals. I'd trade them all for one (yes one) single bridge engineer.

One of the big problems in America (IMHO) is that design work doesn't pay as much as management. The design job's wages are usually undercut, by visa holders. Yes, we build over-priced poorly designed projects (IMHO) here in America (IMHO).

 

[tumbleleaves]

I should also mention that (IMHO)a "structural engineer" is far from best, when it comes to bridges.

In my state (CA) there are generally two types of structural engineers. Those that specialize in buildings and those that went to big city schools. "Structural engineers" are few and far between, and the State has made it into a "click" with their licensing requirements: the licensing is more about who you know then whether you're a qualified designer.

STRUCTURAL ENGINEERS (in CA, IMHO) ARE A PART OF THE PROBLEM. The worst plans I have seen were prepared by structural engineers, either they were specialists in buildings that did not understand bridges, or they completely neglected the route geometrics or hydraulic considerations. That "structural" stamp scares me.

 

[TDAA]

Thanks for the follow up post tumbleleaves. As a lowly geotech, I was about to get offended.

I find it interesting that the general belief of those above (from the way I read it) is that a qualified geotechnical engineer is incapable of designing a foundation.

It really takes a combined effort to get it right, and this goes beyond a structural reading a geotech report and doing their design. Hate to burst your bubble, but you may design something that will not work with the recommendations given in the report. This is because we often do not have final information to base these recommendations off of, and there is an assumption that you can draw your own conclusions based on the data we provide.

I have had structurals on buildings think they could shorten some shafts due to skin friction, when the report was based on end-bearing the shafts in a particular strata (very large loads on single shafts).

Heck, on the last bridge I did, the "bridge engineer" was whining that he did not get the required inputs for one of the materials. What he missed was the line in the table telling him what the recommended p-y curve was.

All I am saying is that unless you have someone on the team that fully understands soils and structures, neither should run away with the design on their own. It will probably be screwed up from the others perspective!

 

[tumbleleaves]

TDAA please re-read my post: The point is that specialization has reached a point of discord and results in poor design work. The original post is about a "structural" specialist preparing a black-box to be handed off to another specialist called a "geotech". Instead of craftsmen the industry has moved to assemblymen, each assembling his own little part independently.

The examples you cited, supported my proposition about the blunders and discord that occur. Anyone that claims to "fully" understand even a small avenue of soils or structures is full of it.

 

[DRC1]

Having been a geotech engineer for a consultant and having done structural engineering and now a contractor (who does a lot of engineering) I do agree with both tumbleleaves and TDAA.
I do agree that we have become too specialized. But the other problem is that we have become too compartmentalized. Structural engineers can design a frame with out ever seeing the steel or concrete, and assume foundation engineers, given a packet of boring logs and some applied loads can design a foundation. In actuality a lot of factors effect the design of the foundation, such as water levels construction sequence or duration of loads. Geotechs need to be involved as a team member, not just hired to do a few borings, then six months later run an L-pile analysis and then come out and do a compaction test, and most importantly, furnish E&O insurance so if the thing settles, or the contractor has a claim, we have some one to go after.
ASCE has been crying for years about how engineering is viewed by the outside world as commodities, yet now engineers hire other engineers like they are commodities. How do you get past that?

 

[minorchord2000]

Getting back to the original question: For piers in the transverse direction I allow top of pier translation but no rotation since the pier is firmly attached to the pier cap. In the longitudinal direction, the top of the pier acts in a similar fashion. It can translate but not rotate. I model the piers using soil springs and run the whole mess on STAAD Pro.

 

[Hunt1999]

Thanks minorchord, apparently somehow I touched a nerve with this question. Your recommendation seems to be the consensus that I have reached from talking to other experts (both structural and geotech.).

 

[minorchord2000]

Good luck, Hunt. Glad I could help.