Ftg settlement and loads
Ftg settlement and loads
(OP)
When you size a foundation for settlement, do you use all or a percentage of variable and or transitory loads such as wind, snow, and even live load? Do you do this based on cohesionless soils, almost immediate settlement vs. cohesive soils, settlements taking numbers of years with creep occurring over a decade type time scale never minding changes in pore water pressure. So this question is the same whether LFRD or WSD...settlement. What would be your opinion of this combined with also the use of live load reduction based tributary area? Might have been better to have posted in a different forum but it deals with the demand side of the equation so there you go. Thanks.






RE: Ftg settlement and loads
It is at the purview of the soils engineer. Actually it is very involved since you have to run consolidation test results, evaluate if you have normally consolidated soils or overconsolidated soils, avoid deep seated settlement & on and on. Why not have the soils engineer of record do this analysis for you?
RE: Ftg settlement and loads
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Ftg settlement and loads
For clay soils, controlling limits of settlements govern very early in the size of the footing; i.e., after say 3 to 4 ft (as an example). Carry out bearing capacity computations for various size footings and then determine the settlements that would occur. Then take, as an example, a permitted settlement of 1" (25 mm) and determine the bearing pressure for that size of footing that would cause this to happen. Graph and see what happens. I take into account elastic (immediate) settlements in the computations - for footings this may be important; for embankments probably less so as you would "make up" the immediate settlements with succeeding embankment layers.
In sands - take the traditional qall vs N charts that have been developed and you will also see that the allowable bearing capacity (say with SF=3) will be higher than by the chart approach - of course, this is a guide.
As for the poster's original question - settlements in sands and that should take into account the loadings of a transient nature - wind loading (sustained) or seismic (depending, of course, on the level of seismic). This is because the pore-water pressures that develop under the "quick" loadings will dissipate quickly and the transient loadings can have an effect on the total settlement. For clay soils where the time of the loading is very long to cause settlement, one would consider dead and sustained live loading. The OP has correctly identified that all soils, including sands, creep; however, for the most part, the settlements will be fairly uniform in the creep range and occur gradually (see Schmertman's factors). So these should not be a problem. For clay soils, creep (secondary consolidation) will occur many years after loading - and usually is quite small compared to the primary consolidation.
The above is how I look at things. Like Bama losing - Go Ohio State!
RE: Ftg settlement and loads
That would be it exactly. Cohesive, cohesionless soils and their rate of settlement contrasted with loads and their rate of application and expected duration. Clay soils and transient loads such as live, wind, snow, earthquake as an example.
Question is, if you have a clay type soils and you, in consultation with the Geotechnical Consulutant, determine the use load factors for the transient loads can be less than 1.0 to determine settlement, would you also use Live Load (Use and Occupancy) reduction factors based on tributary areas?
But other questions come to mind out of this as always. How do you determine the 'sustained' portion of the Live Load? And also what of the the case of a Partition Allowance, in Canada it is defined as a Dead Load, yet in the US it is defined as a Live Load. This results in two completely different load types with two completely different loads to be considered. Obiviously what applies in a particular jurisdiction governs, but why the apparent disconnect between these two views?
RE: Ftg settlement and loads
The Florida Building Code in the High Velocity Hurricane Zone (HVHZ) has a requirement of designing for dead load pressures. I think the idea behind it is to have a footing that is not considerably oversized compared to the adjacent footing due to live loads and thereby causing differential settlement issues.
See Section 1624 http://eco
RE: Ftg settlement and loads