How to take settlement effect into consideration when

[GalileoG]

How can one take the effects of settlement on a building when one designs for buildings? I'm not talking about the design of the foundations, but rather, the building itself. Thanks!

 

[msquared48]

Are you talking settlement due to differential settlement in the soil structure, or settlement due to creep/shrinkage as in wood members or concrete?

Mike McCann
MMC Engineering

 

[GalileoG]

Due to differential settlement in the soil structure.

 

[msquared48]

I try to take this into account at the foundation level so any settlement is not transmitted to the structure above. This can be done several ways:

1. Try to equalize, as best as possible, the long term dead loads to all the footings, spread and strip.

2. Design the concrete stem walls to span at least 10 feet should there be any localized settlement.

3. For isolated pad (spread) footings, use grade beams to transmit any settlement to other footings.

4. Make sure that the geotech, in his soils report, outlines the anticipated settlement amount and makes any recommendations he thinks should be undertaken to minimize the effects.

5. Over-excavate and place structural backfill for better bearing, possibly less settlement.

6. As a last resort - piling.

I'm sure others in this forum have more ways, but these are the most typical I know.

Mike McCann
MMC Engineering

 

[Teguci]

Saw a project once where the foundation system was selected based on differential settlement. The deflections were determined for spread footings under each column and the differential settlements were than mapped. Based on this information the client decided to go to a deep foundation system.

Regarding the building itself, I believe there are rules of thumbs regarding allowable differential settlement for different building systems.

Strategies for the building include:
- Steel - keep connections loose during construction per standard erection procedures (simplification).
- Concrete and P/T - Use a stiffer foundation or have sand pockets at the columns (expensive).
- Precast - Everything is pinned - shouldn't be a problem.

 

[Doc08]

The geotech engineer should be able to give the anticipated settlement based on a footing size and preesure applied by that footing which natuarlly comes from the column load. The next step should be a joint effort betwen the structural and geotechnical engineer with the first supplying the column loads and the second calculating the settlement associated with each which will involve dimensioning the footings and when necessary switch to a mat foundation, deep foundations (e.g. piles) or perhaps soil improvement etc...

from a structural and architectural point of view, there are set limits for differential settlement / rotation etc.. which will vary from one structure type to another. You may want to check the chart presented in the NAVFAC manual, Bowles also has a table for the same. From a structural point of view I believe that structural engineers will agree with me that differential settlement could add significant loads to the frame elements which must be accounted for in the design.