Differential Settlement: How to evaluate? 1

[SmithJ]

Hello All,

How does one determine if differential settlement in a high rise building is dangerous or not? What code and what section of the code addresses this issue of differential settlement and gives guidelines as to what is acceptable or not?

Thanks for your help.

JS

 

[Ron]

Differential settlement distress can take on several forms. The obvious is cracking...that depends on the materials, but for instance, a brick facade will usually crack if differential settlement exceeds about 1/4-inch in 20 feet.

The not-so-obvious is the induced stresses in structural members. As an example, suppose you have a reinforced concrete frame with columns spaced at 40 feet. Now you get one inch of settlement in one footing, no settlement in the next footing. How much moment gets induced in that beam-column connection? How much was it designed for?

As a structural engineer, you decide how much settlement you can tolerate. You can design your building to handle as much differential settlement as you want...if you have enough money and you don't care about cracks all over the place.

Codes don't typically limit differential settlement as a parameter...they limit the effects of differential settlement by requiring consideration of settlement in the loading, and by putting limits on stresses and structural stability.

 

[BigH]

There are a number of textbooks (good ones) out there that identify permissible "rotations" to prevent distress. Tomlinson's Foundation book comes to mind; I think that Bowles also presents the data.

 

[SlideRuleEra]

SmithJ - Your question sounds like you have an existing high rise to evaluate. If it is steel framed, one way would be to work backward from the AISC "Code of Standard Practice for Steel Buildings & Bridges".

See the section on Columns (paragraph 7.11.3.1 in my 9th Ed ASD Manual). Have surveyors determine if the columns are displaced from the theoretical (plumb) column lines and if they meet the allowable deviations. This may not be easy to do, but it is probably possible.

If the deviations are within spec, differential settlement may not be a problems (yet). If the columns are out of spec, there could be many reasons (including poor original erection). But you now have a basis to justify further investigation to determine the cause.

Also, having the building resurveyed at a later date and comparing the results with the initial survey should tell you if the problem is getting worse (regardless of the cause).

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