Differential Settlement in PEMBs 4

[structuralengr89]

I contacted a local "PEMB Engineer" today to ask if they include differential settlement in their design calculations. I practice in an area with highly expansive soils and shallow foundations frequently exhibit differential movement vertically between the columns.

He stated that horizontal movement of the foundation would cause increases in Moments (I agree), but vertical movement would not. I then asked him, so if you have a portal frame spanning 20' and then one end drops 6" due to the soil, this would not cause changes in Moment to the frame? He said it would not. He said that if there was an interior post, this could cause increases in stresses/moment...but for clearspans there would be no change in the Moment.

What am I missing here??? Do PEMB suppliers assume that the beam/columns are not fully fixed or something?

When I've gone back and performed calculations of old PEMB in our state (using Ram Elements), they rarely have the capacity to handle code gravity loads, much less any lateral load. Additional stresses due to vertical differential settlements would push them past what is allowable.

Then again, I've seen many PEMB that are on foundations that have moved substantially...and they are still standing!!!

 

[MotorCity]

Theoretically he is correct if it is a "pinned" connection as it should be able to rotate freely with no change in moment. However, practically speaking, the beam connections have SOME moment fixity as long as its not a single bolt connection. That fixity would become apparent in the form of a mangled looking connection (broken bolts, twisted clip angles, or shear tabs....). What a nice world it must be where all "pinned" connections behave as the ideal pin!!

 

[canwesteng]

You're going to have huge issues no matter the building system if you have 6" of settlement. I think for more reasonable numbers, say 1/4", the effect on the structure is negligible, as it will act like a pin.

 

[ajh1]

It is going to depend upon the horizontal restraints on other parts of the frame. Looking at a typical clearspan rigid frame with the knees free to move laterally as the bases settle, as long as the actual horizontal distance between the two column bases is unchanged, there should be minimal stress changes if one side settles. The whole frame would simply pivot about one end. Note that most PEMB frames are designed as pinned base connections (and yes, there will be some fixity inherent even in those connections) so there would be minimal impact from the foundation movement as long as the relative dimensions are maintained. If there is something significant that is forcing the columns to remain vertical then you will see stress increases. A portal frame being used as bracing that is anchored between two primary frames would probably be more problematic than a basic primary frame where the only restraints are secondary members and paneling.
A modular frame would have more significant problems unless by some miracle all the bases remain in a single plane, albeit tilted.
Realistically any building would have issues with 6" of unregulated settlement, if not the structure, then things like doors and windows, plumbing, interior walls, etc.
In some cases on shallow foundations, the soil settles and the point in question on the building simply pulls free from the soil and is hanging in mid-air.
Note that while the main frame may be okay in some circumstances, components such as continuous purlins or girts are going to see a lot of stress increase for the same reason. Simple span secondaries will be less likely to be impacted, although the initial sizing will be larger than the equivalent continuous design.

 

[hokie66]

Good summary, ajh1.

 

[structuralengr89]

star for you ajh1....You cleared it up for me...Thank you!