Differential Settlement over a Long Span

[GGGGeo]

Dear Professional, may I ask what is the differential settlement deflection value with an associated span such as 1/2" over a 40-50 linear-foot span.

Is this derived from calculation fro lab data? Our office derive it from the NAVFAC figure...

I was told that this is also based on building angular distortion chart; if this is the case, then can anyone helping me that why a soil engineer needs to recommend this differential settlement to determine column span?

Kind regards,

 

[140lbHammer]

Our company did the same. We provide bearing values for both continuous and square footing. Then we provide both settlement and angular distoration for both. So far never receive comments from structural engineer. Check with my structural friend he says he never need the settlement over a long span. He simple total ignore this recommendation and design his stripe footing as grade beam.

Hope this help!

 

[BigH]

It has been normal practice of which I am familiar to give the total estimated settlement and the estimated differential settlement that could be expected. The structural engineer would determine, based on the differential settlement of adjacent footings, to ascertain the likely distortion and how this effects his structure - at the time of most geotechnical investigations, the footing spacings are not well established. Tomlinson's Foundation and Construction Practice book summarizes a number of distortion limits and effects on the structure.

 

[Bluefoot]

If the loading condition is not defined (in most cases at the time of investigations, as mentioned by BigH), geotechnical engineers usually calculate (back) the limiting load for serviceability, based on the site differential subsurface conditions, assumed footing types and configurations.

The serviceability limits are (or should be) specified by the different design codes, industry practice or by structural engineers (newly implemented Euro Code leave this portion to different National Annex; North American States and provinces may have their own limits for buildings, bridges, dams, structures, etc.).

Allowable settlements and limiting distortions are reviewed and discussed by Robert Day in Foundation Engineering Handbook Section 7.6. I remember seeing a similar discussion in one of the Das foundation books.

In the original post, the NAVFAC figure and the building distortion chart are used to define the design limits. The calculation for settlement using field or laboratory data is performed to verify that these are within the limits.

It is necessary to determine the potential total and differential settlements or allowable distortion. It is more so when the structural engineers wonder whether he should implement structural solutions (say, strengthen the beam or footings at high load places).

 

[GGGGeo]

Thank you all for pointing out the direction! Much appreciated!