relationship between k values and allowable bearing pressure 2

[oneintheeye]

any one know of a relationship between the allowable bearing pressure and the modulus of subgrade reaction (k) value? I have one reference but I theres a typo on the page as none of the units tie up or make sense.

 

[GeoPaveTraffic]

Subgrade reaction and allowable bearing pressures are two different animals. I have not seen any relationships and would not expect to see any. Subgrade reaction would only acount for the near elastic portion of the soils reaction to load, whereas, the allowable bearing pressure must also acount for consolidation and other non-elastic compression of the soil.

If you have found a relastionship, I would expect it to be imperical with the input and output units specified and a "factor" making the units come out correct.

 

[oneintheeye]

i have seen one yes, 120 x pressure. although pressure is shown in kn/m3??? and then in sentence underneath in kpa. I have tried multiplying out using published typical figures and get no where near. ( bowles - foundation analysis and design and terzaghi and peck - soil mechanics in engineering practice)

 

[himat42]

Herewegothen,

I am not sure about the relationship between "k" and bearing pressure. I did find the relationship between passive pressure and "k" for pile design.
If you are looking for this information, please look at the article in ASCE CIvil Engineering, May 1969, pp.65-66.
("Side Bearing Pier foundations": By: B.L.McCorkle)from his His table you could interpolate the bearing capacity of shallow foundation by using "N" value (Bearing capacity of soil versus "N").

 

[DocDirt]

In some cases, for footings on sand (when the width of footing is greater than 4 ft), settlement influences bearing capacity. Since K is the ratio of settlement to applied pressure, there can be a correlation in that case between allowable bearing pressure (based on a settlement of usually 1 in) with K.

 

[BigH]

DocDirt - do you mean "bearing capacity" or "bearing pressure"? Not sure that settlement influences bearing capacity (don't see anything in the bearing capacity equations that take into account soil strain - other than, I suppose, with some settlement, the relative density would be slightly more with a higher friction angle . . . I thin that you are talking about decreasing of bearing pressures due to the greater influence of bigger footings.

 

[Mccoy]

I would tend to agree with BigH.

I believe we should be careful not to mix apples with oranges.

Such would be the case if we mix problems of plasticity (bearing capacity)with problems of elasticity (settlements/distortions).

Bowles does indeed provide a formula relating Kw to beearing capacity, in this case mixing, strictly speaking, apples & oranges.

He also provides more sensible formuals relating edometric modulus, or Young modulus to Kw. Oranges stay with oranges.

When I'm asked the k values (wothout in situ tests) I always apply the definition,that is figure out the settlements, my final value being the ratio Applied Load/settlements

 

[Myoho]

Ciria Guide 103 Design of laterally loaded piles, will give you impirical equations to work out K value.

 

[DocDirt]

Folks
Sorry I have been dormant all this time. What I meant was that the design bearing pressure could have some connection with the subgrade modulus. The design bearing pressure for shallow foundations on sand, if the footing width is greater than 4 ft, is calculated based on allowable settlement (in most cases it could be 1 inch, once again this value could change based on the situation). The calculated design pressure divided by allowable settlement could then be related to the subgrade modulus. Concept of CBR can be explained in the same vein. But I do not want to start another issue for now. That is probably for another post in this forum. The definition of subgrade modulus is pressure/settlement. This is my take on this matter. All the correlations have limitations and one has to understand these limitations before using them.

 

[BigH]

Check out SlideRuleEra's web site - he has posted several "correlations" of the various "measurements".

 

[chandanaedirisooriya]

I am analyzing a building using sap2000. I have to analysis an Inverted T-type foundation. I want to analysis a beam part of inverted T –type foundation..

Manual method I followed is
1) Assumed beam is a line element
2) Calculated column loads
3) find out upward UDL coming from soil according to its bearing capacity (soil udl has found equaling the vertical loads of columns & soil load)
4) Considering udl upward, column loads downwards
5) Then calculated the BM & SF along the beam

But I want to do this via SAB2000 (or SAFE or ETABS).

I have done below two methods
? Analysis with both soil udl, column point loads & put column places as pin supports
? Analysis with soil udl, neglecting column point loads , but keep column places as pin supports


My problem is

If I use 2nd method then the reactions coming from the supports should be equal to the column loads? But it’s not happened?

Bending moment diagrams are differing. That means manual BM diagrams are not same with computer analysis methods..?

Thanks you very much,
Chandana

 

[oneintheeye]

I have spotted this using your second method as well. Curiosly this is shown in most text books as a valid way to analyse. I do depending on the circumstance. In your case it sounds like an instance when i would. Personnally i dont believe that the method gives a accurate description of the forces. For instance if it was founded onto a concrete slab (or very stiff ground). It is however a recognised technique. You could analysise by soil interaction methods.
Incidently no one i have ever asked has given me an adequate explanation of how the method is ok giving the difference in column loads you describe, other than its good enough for 'insert well known authers name' its good enough for me.

 

[howardoark]

Modulus of subgrade reaction is based on tests using a 1 foot square plate. It won't pick up soft strata three feet below the plate - say you have a dessicated crust overlying 50 feet of soft bay mud. If you then build a four foot wide footing on the dessicated crust, you'll end up with unacceptable settlements.