Greatone76
Structural
- Feb 2, 2006
- 64
We have been having a discussion at our office about who's responsible for the differential settlement recommendations for a building project. I have always been under the assumption (I hope not incorrectly) that the geotechnical report will take into account some reasonable information and a judgement on the rough differential settlement and if it will come into play. We have had (2) recent projects where we have gotten into situations were we have been asked to provide the recommendation for differential settlement. We ended up with 2 odd situations where the assumptions used in the original soils report for maximum column loadings were exceed. We reported the issues to the geotechnical engineer and we have been asking for a complete revised recommendation for foundations and allowable pressures.
The first time construction was in progress so we worked with the geotechnical firm to get a solution as quickly as possible. The basics were that if they would have been given the actual max loads to begin with, most likely the recommendation would have been to use "Deep" foundations. But since we already were designed and in the process of construction we worked with the geotechnical firm directly and provided a "shallow" foundation designed to settlement as opposed to bearing pressure. This geotechnical firm was reasonable in helping us determine reasonable settlement recommendations.
With the second time, having had the previous experience, we noted this at the 50% design progress set and did not design the foundation fully to await for the revised recommendation. We made the request to this geotechnical firm to provide a new recommendation as if the actual loads were provided at the beginning of the work and create the report based on those loads. This geotechnical firm refuse to do this. They continue to provide us numbers of settlement for any footing size and load we provide and keep leaving it in our court to figure out if it works. I've been pushing and fully believe it is their place to make the recommendation for foundation and allows which includes their evaluation of differential settlement concerns.
To say the least, the people above me, are now trying to get recommendation of allowable settlements. I feel that we can dig out numbers from somewhere and make a guess at an allowable differential settlement, but my issue is I don't feel like we have enough information and expertise to really determine what is acceptable. It would be my opinion that the soil variability over the site would be the largest concern for detrimental differential settlement. We are getting "settlement" based on a load and footing size. We aren't getting differential settlement which is what the book values you find care about. So I don't feel that it is fair to take full load numbers for settlement to look at detrimental differential settlement. Having the dead loads there and variability of the live load and knowing the likelihood of full live load on the footing all doesn't seem like I would be getting the correct info to make a decision.
In these case it appears that the real issue that there is a self layer 3 to 5 feet below proposed bearing that is dramatically affecting the settlement at slightly higher loads. Does anyone ever have to make these decisions as the structural? Does anyone ever provide footing sizes and loads with the geotechnical firm to get actually settlement numbers back and processes those in any way? Am I wrong in using my past situations where I'm given the soils report and just use the allowable to design the foundation? Previous to these projects in all my buildings I have never been back and forth with a geotechnical firm. In this case I think a large part of the issue is we as the structural firm highbred for the owner the geotechnical firm where as in the past the owner typically hires the geotechnical firm and in general the reports are produced long before design begins. Any input would be help on this topic. Thanks in advance to all.
The first time construction was in progress so we worked with the geotechnical firm to get a solution as quickly as possible. The basics were that if they would have been given the actual max loads to begin with, most likely the recommendation would have been to use "Deep" foundations. But since we already were designed and in the process of construction we worked with the geotechnical firm directly and provided a "shallow" foundation designed to settlement as opposed to bearing pressure. This geotechnical firm was reasonable in helping us determine reasonable settlement recommendations.
With the second time, having had the previous experience, we noted this at the 50% design progress set and did not design the foundation fully to await for the revised recommendation. We made the request to this geotechnical firm to provide a new recommendation as if the actual loads were provided at the beginning of the work and create the report based on those loads. This geotechnical firm refuse to do this. They continue to provide us numbers of settlement for any footing size and load we provide and keep leaving it in our court to figure out if it works. I've been pushing and fully believe it is their place to make the recommendation for foundation and allows which includes their evaluation of differential settlement concerns.
To say the least, the people above me, are now trying to get recommendation of allowable settlements. I feel that we can dig out numbers from somewhere and make a guess at an allowable differential settlement, but my issue is I don't feel like we have enough information and expertise to really determine what is acceptable. It would be my opinion that the soil variability over the site would be the largest concern for detrimental differential settlement. We are getting "settlement" based on a load and footing size. We aren't getting differential settlement which is what the book values you find care about. So I don't feel that it is fair to take full load numbers for settlement to look at detrimental differential settlement. Having the dead loads there and variability of the live load and knowing the likelihood of full live load on the footing all doesn't seem like I would be getting the correct info to make a decision.
In these case it appears that the real issue that there is a self layer 3 to 5 feet below proposed bearing that is dramatically affecting the settlement at slightly higher loads. Does anyone ever have to make these decisions as the structural? Does anyone ever provide footing sizes and loads with the geotechnical firm to get actually settlement numbers back and processes those in any way? Am I wrong in using my past situations where I'm given the soils report and just use the allowable to design the foundation? Previous to these projects in all my buildings I have never been back and forth with a geotechnical firm. In this case I think a large part of the issue is we as the structural firm highbred for the owner the geotechnical firm where as in the past the owner typically hires the geotechnical firm and in general the reports are produced long before design begins. Any input would be help on this topic. Thanks in advance to all.
