Grade Beam Allowable Settlement

[hoshang]

Hi
Please find the attached link:
For the given grade beam material and geometry, is the allowable differential settlement calculated correctly?

 

[JoelTXCive]

I'm confused.....Won't the allowable deflection be L/300 or L/200, or whatever your project parameters are?

I think you are attempting to calculate the ACTUAL service level deflection. I'm not familiar with the formula that you have posted, but I can tell that it does not have any variables in it for the soil spring reaction.

A true deflection amount will account for the deflection due to the load; and the resistance provided by the soil springs.

 

[Greenalleycat]

These calcs are really lacking a lot of context. There are several notable omissions though
- Soil-structure interaction, as noted by Joel. This is the critical one as your soil will almost certainly govern your actual soil characteristics
- A reduction of the concrete Ix value to account for cracking etc
- Consideration of creep of the concrete beam

The lack of soil spring consideration is the major one though, these calcs are pretty worthless for estimating deflection
You need to build a beam-spring model in analysis software for this

 

[phamENG]

Keep in mind in some places, grade beam is only used for deep foundation systems where all support comes from piles and soil interaction with the beam is ignored. (Saw one building here that had the entire site drop 6 inches within 2 years of completing the building. Had to come back and put the sidewalks and fences on piles, too.)

OP should clarify what is meant by grade beam as it is not a universal term.

 

[hoshang]

OP should clarify what is meant by grade beam as it is not a universal term.

A grade beam is a beam between column spread footings to account for differential settlement.

 

[Brad805]

A grade beam to us means a concrete beam between piles. There is a void form between the grade beam and ground that isolates interaction between the two. Our geotechs do not recommend mixing foundation types since we are dealing with CH or CM clay soils.

You need to add a sketch. Your calc sheet seems to be missing many terms and is not overly helpful in defining your actual question.

 

[hoshang]

Hi
I edited the original post.
some notes:
fc=1/2 f'c
so the concrete can behave elastically.
I meant the grade beam having these properties can accommodate a calculated differential settlement of 2.497mm. Is it understandable? Am I correct?
In other words how much differential settlement a grade beam with stated properties can accept?