Until a few years ago, I experienced what Stephanie described in terms of Geotechnical - Structural separation.
The only link we, the Structural engineers, had with The Geotechnical engineers was to receive their Geotechnical report, and use it; often without even meeting its author.
That changed as we faced greater challenges in deep underground structures where Soil-Structure integrated design becomes inevitable, especially with the new computing capabilities.
The first such a challenge happened in 1996 when an existing LRT tunnel, built of slurry walls was to be verified for capacity under imbalanced lateral loads due to new construction. The structure had very little bending moment continuity between slabs and slurry walls (virtually pins). There was only continuity with the CIP middle wall that was too flimsy to offer sufficient continuity. There was no way lateral resistance to imbalanced loads can be achieved without calling on the lateral capacity of soil. That was heresy at the time. But under the pressure of necessity, the Geotechnical provided us with lateral Subgrade Reaction spring modulus. They were so reluctant to provide it to us, that they gave us the softest springs possible (as conservative as they could). Even with that, it made the design possible; yet with a lot of struggle:
- Placed lateral springs in the computer model. Applied imbalanced LOAD COMBINATION (not load cases, since this will not be linear analysis)
- After the first analysis run:
- The springs with tensile forces were eliminated (since soil can't take tension).
- Springs that generated reaction exceeding the passive pressure envelope, were softened by prorating the passive pressure allowed at that point/ the pressure resulting from analysis.
- A second computer analysis run was done based on the above changes. Same steps were repeated: new tensile springs removed; those that exceed passive pressure envelope are softened by proration.
- After several tedious cycles, the analysis converged to a stable result.
Hallelujah, it worked; that existing structure was finally justified.
A precedent was set.
It came handy in 2012 when we were initiating 10 underground LRT stations to a country oversees. Those stations were deeper than usual, and the lateral imbalance was the governing factor. At the same time, their soil was exceptionally firm; offering great help if Soil-Structure integration was used. But there National Transit Authority of that country has never seen it before, and was reluctant to use it. However, with the help of our Geotechnical partner, a reputable European firm, we demonstrated that there will be major savings, running in hundreds of millions of dollars, if we tapped into the hidden capacity of soil. Our proposed method was approved, and it was incorporated in the design criteria of the transit authority of that country.
In 2014, we used the above two precedents to introduce it to Canada's largest transit project of 15 underground stations. This time, with the help of more advanced computer programs that perform the above-mentioned iterations automatically. For that reason, the Geotechnical Engineer was a member of our Structural Focus Group. Perfect Geotechnical-Structural integration resulted in the greatest Value Engineering contribution to Canada's largest project. An added advantage, of thinning the external walls, was reducing the footprint of underground stations; therefore reducing interference with existing utility lines, a winning proposal.
It has been a long journey, from no communication (pre 1996), to bare communication (1996), to cooperation (2012) to integration (2014-…). It's the way of the future. Don't be left behind in the dust.
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Neil Kazen, M.Eng., M.Sc., P.Eng.
FASCE, FCPCI, FEC,
Retired Structural Engineering Manager, Transportation Division, SNC-Lavalin
Toronto, Ontario, Canada
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Original Message:
Sent: 11-07-2018 09:54
From: Stephanie Slocum
Subject: Geo and Structural Engineers - Collaboration Reality Check
Last night, I gave a presentation to the SEI-MD chapter about how geotechnical engineers and structural engineers work together (to a room with an even split of both engineers). The lively Q&A and discussion afterward indicated that although we know we need to work together closely for the best project outcomes, it's currently very hit and miss. This seems to be partially for contract/liability reasons. For example, in building structures with an architect, the <g class="gr_ gr_1212 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling" id="1212" data-gr-id="1212">geotech</g> is often not part of the design team to the extent of other team members. Instead, the geotechnical report is "owner furnished" and under a separate contract.
There was also a consensus that during the RFP process, neither the structural engineer nor the geotechnical engineer is getting the information they need from the other. We had one geotechnical engineer comment that RFP's are missing information. Another commented that trying to get accurate loads from the structural engineers was met with pushback. From the <g class="gr_ gr_3006 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del" id="3006" data-gr-id="3006">structurals</g>, we heard that some reports lack clear, unambiguous foundation recommendations. Or, that the geotechnical engineer disappeared from the project once the report was delivered.
Although we as engineers may not always have control over the contractual part, we do have control over our levels of collaboration and how we interact with one another. Why do you think we don't collaborate as much as we should? And, can you give any specific examples of things you have done on projects to improve this process/work better with your structural or geotechnical colleagues?
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Stephanie Slocum P.E., M.ASCE
Founder
Engineers Rising LLC
www.engineersrising.com
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