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Modeling Raft

Foundation usingArea element (II)

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Modulus of Subgrade Reaction

When a foundation settles into the soil (the subgrade), thenthe reaction of the soil on the foundation is reverselyproportioned to the amount of settlement.

This is very much similar to having springs underneath thefoundation.

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Modulus of Subgrade Reaction; ks

>48000qa > 800 kPa24000 48000200 < qa 800 kPa

12000 24000qa 200 kPa

Clayey Soil:

24000 48000Silty Medium Dense Sand

32000 - 80000Clayey Medium Dense Sand

64000 128000Dense Sand

9600 80000Medium Dense Sand

4800 16000Loose Sand

ks; kN/m/m2Type of Soil

Reference: Foundation Analysis & Design, 5th Edition, Joseph E. Bowles

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Modulus of Subgrade Reaction; ks

In general:

ks = 40 (F.O.S) qa (kN/m/m2)

qa; the allowable bearing stress (kPa)

This equation is based on having maximumsettlement equal to 1 or 0.0254m.

Reference: Foundation Analysis & Design, 5th Edition, Joseph E. Bowles

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Modeling Raft Foundation using

SAP2000

Column loads could be assigned to areaelements within the column perimeter as arealoads.

Subgrade reaction could be assigned asarea springs with k = ks

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Springs to model Foundation Reaction

Coefficient of subgrade reaction is in units offorce/volume.

In other words, it is in units of force/unit areafor each unit displacement or settlement in thesoil.

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Assigning Area Springs

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Assigning Area Springs

For foundationchoosecompressiononly

Use ks directly

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Assigning Automatic Meshing

Maximum

size ofelements

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Assigning Area Edge Constraints

Defaultoption

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Example2