Ftg settlement and loads

[connect2]

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.

 

[FixedEarth]

Usually, the service loads along with the foundation plan is given to the geotechnical engineer. Then you get a report containing the net allowable soil bearing capacity, total settlement & differential settlement over a fixed length.

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?

 

[msquared48]

The footing is designed to a certain bearing capacity based on the combination of loads applicable. Just make sure that the footing can span the settlement distance specified in the geotech report for the bearing area provided for the footing and you will be fine.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BigH]

Sorry - I do not agree that, for most cases, the footing is designed to a certain bearing capacity. It is well known that the serviceability governs, again, in most cases, the strength states (i.e., bearing capacity).

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!

 

[connect2]

Yes Big H,

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?

 

[slickdeals]

@Connect2:
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://ecodes.citation.com/cgi-exe/...ef=/nonindx/ST/fl/st/b200v07/index.htm#b=1624

 

[BigH]

Sorry - but I still can't get my head around the use of the LRFD, etc. - especially since they "calibrate" it to the normal way we have always done it. For sustained live loading, I would take that as, say for normal occupancy, at least partial of the design live load. (is this the "partition load" you speak of?) It really is a live load - but I would gather that the US doesn't have "sustained" live loads that they would lump with the dead load. In windy areas where there is sustained wind, you might take a small wind load and apply it. Subjective.