Accounting for Differential Settlement in Structures
Accounting for Differential Settlement in Structures
(OP)
I have been asked to determine "how much settlement" a structure can withstand before members in the structure become overstressed.
I can rather easily model the structure and force the movement of hte supports to represent differential settlements and thereby capture the effects on the structure.
The issue I am having is whether to include the settlements along with the effects of other loads.
Have others run into this?
I can rather easily model the structure and force the movement of hte supports to represent differential settlements and thereby capture the effects on the structure.
The issue I am having is whether to include the settlements along with the effects of other loads.
Have others run into this?






RE: Accounting for Differential Settlement in Structures
1. Determine what stresses will be put onto the foundation structure by the differential settlement mentioned in the Geotechnical report you are supposed to have for the project, and then see how much other dead, then live load the system can tolerate without being overstressed, or
2. Determine the stresses seen by the
lovelive loads and see how much settlement, with the different levels of induced stresses assiociated with varying degress of settlement, that you can tolerate.Years ago had to analyze a fire station slab and beam system that had differential settlement problems - where the vehicles parked. The damage had already occured, so the settlements were known. We just had to figure out what to do to fix it.
Superposition works great for this.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Accounting for Differential Settlement in Structures
Not sure I am following you 100%.
For a hypothetical, say you designed a frame with no consideration for differential settlements and took the design ratios of the members up to 1.0.
Now you analyze the frame with one of the foundations displaced along with your normal load combinations. You're design would likely not be suitable any longer.
RE: Accounting for Differential Settlement in Structures
Say you had a two span continous beam where the middle support settles one inch. To see the effect of the settlement, analyze the tewo span structure as a one span beam for the settlement, findint=g out what load will induce the settlen=ment, or deflection at the support, then backfigure to see what moments and shears that generates, adding those to the oned generated for the loce and dead load conditions of the two span condition.
I can do it easier than I can explain it. Sorry for any confusion here.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Accounting for Differential Settlement in Structures
One way to do it would be to apply a downward load at each support in turn, noting the stresses caused at every point in the structure and the deflection caused at the support with the added load. Then consider live load for each span in turn and finally consider dead load throughout.
Finally, you could combine them in a way which produced the worst effects at every location. A lot of work for a nebulous purpose.
BA
RE: Accounting for Differential Settlement in Structures
RE: Accounting for Differential Settlement in Structures
However, if you are given a spring stiffness at the foundation level, then the program will automatically account for the additional loads coming for differential spring settlements. The only issue with this is that the it will use load factors to calculate the settlements from factored loads, I think this means you will be conservative while designing it this way.
RE: Accounting for Differential Settlement in Structures
RE: Accounting for Differential Settlement in Structures
RE: Accounting for Differential Settlement in Structures
As for the liability of the geotech for not including such potential....well.....
RE: Accounting for Differential Settlement in Structures
RE: Accounting for Differential Settlement in Structures
RE: Accounting for Differential Settlement in Structures
Oh Lord I hope the codie's aren't reading this...there is going to be another chapter in ASCE 7 in '15
RE: Accounting for Differential Settlement in Structures
SECTION 1624 HIGH-VELOCITY HURRICANE ZONES-FOUNDATION DESIGN
1624.1 Design procedure.
The minimum area of a footing or number of piles under a foundation shall be determined in the following manner:
1624.1.1
The total load of the column that has the largest percentage of the live load to the total load shall be divided by the allowable soil pressure or pile capacity.
1624.1.2
The balance soil pressure or pile capacity shall be determined by dividing the total dead load by the area of the footing or the number of piles.
1624.1.3
The minimum area of other footings or number of piles shall be designed on the basis of their respective dead loads only.
1624.1.4
In no case shall the total load of the combined dead, live, wind and any other loads exceed the allowable bearing pressure of the soil for capacity of any pile upon which the foundation is supported.
1624.1.5
The live load used in the above calculations may be the total reduced live load in the member immediately above the foundation.
The minimum area of other footings or number of piles shall be designed on the basis of their respective dead loads only.
1624.1.4
In no case shall the total load of the combined dead, live, wind and any other loads exceed the allowable bearing pressure of the soil for capacity of any pile upon which the foundation is supported.
1624.1.5
The live load used in the above calculations may be the total reduced live load in the member immediately above the foundation.
RE: Accounting for Differential Settlement in Structures
Yeah, I see your point. Especially if asked I would definitely make up a model that offered some idea of diff. settlement on the structure. I am just saying that buildings are built to last (barring some economic design), and planning for settlement isn't a solution, unless you have special reason to. Why not put your mental energy into designing a foundation system that works? Account for that discrepancy and increase compation or FS. Utopian I know. I must be just in that mode at the moment.
RE: Accounting for Differential Settlement in Structures
Slickdeals....you've given another excellent example of how ridiculous some of our code provisions are. Some provision such as this only show up in the "HVHZ" in the Florida Building Code (High Velocity Hurricane Zone), which by definition is two counties...Dade and Broward; however, they should also apply to all other areas of the state, since the criteria go well beyond wind loading alone. Makes my life on the forensic side a bit difficult at times....no code backup for anything other than HVHZ. Same difficulty for requiring PT wood in contact with masonry or concrete....only shows up in the HVHZ, so opposing attorneys argue that it doesn't apply to the rest of the state. Absurd argument, but effective in reducing settlement costs for their clients.