Differential Settlement in PEMBs
Differential Settlement in PEMBs
(OP)
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!!!
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!!!






RE: Differential Settlement in PEMBs
RE: Differential Settlement in PEMBs
RE: Differential Settlement in PEMBs
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.
RE: Differential Settlement in PEMBs
RE: Differential Settlement in PEMBs