You want to have grid intersections at pile locations, load locations, geometry and thickness changes and for changesin section properties. Below is a really basic grid configuration for a block foundation on soil

Use Quick-draw area to draw areas at Z = 4ft so that we can extrude downward to add/model solid elements. The sectionproperty of the area does not matter since were only using the shell areas to help us model the solids . The quick-draw area toolwill divide/mesh at each grid intersection.

Select areas then use Edit menu to Extrude areas to Solids. In this example, we extrude only in the downward Z direction byspecifying 4ft. in the -3 local direction of the selected areas dividing the 4ft. extruded section into 2 slices using the number field.

Extruded solid elements have been added

Define menu>Section properties>Solid properties and change material to concrete

Now select the middle area (not solid) and extrude it upward (+3 local, +Z global) 4 ft., dividing it into 2 slices.

Use select menu to select by properties area sections, then select the area shell section that we originally used in order to deletethem. If you dont delete them, it will double-count the shell area stiffness and mass with the solid elements. After selecting thearea elements, press the Delete key on your keyboard to delete them.

Use Edit menu>Divide solids to further mesh the model if needed

Select the point in space at the centroid of the unbalanced load and also select joints on the foundation where the equipment isbolted/connected to the foundation. We are going to assign a rigid body constraint in order to link these joints together,automatically considering any moment differentials from the offset distance

Assign menu>Joint>Constraint, Body type. Click Add new constraint button and press OK to accept the default Body1constraint in all 6 DOF, then press OK once more to apply the constraint.

Assign max unbalanced load in Y and Z directions using separate load patterns. In this example, 1 Kip is the max unbalanced load

In plan view, go to the bottom of the base and window select. Thats usually the easiest way to select the bottom face ofsolid elements. Before you select, use Options>Tolerances and change + - 2D cutting view plane tolerance to .1 orsomething similarly small to avoid selecting elements outside the plane in 2D view

Next, assign area unit springs to the selected faces of solid elements based on subgrade modulus. In this example, we assign .15Kip-in. If you are unsure which is the solid face, use Display menu>Show misc. assigns>Solid and checkbox color-coded faces toview graphically. In this example, face 5 is at the bottom. Inward direction is always toward the center

In elevation view, window select one side of the bottom portion as shown in order to assign lateral soil springs, which are usually lessstiff than the compacted soil beneath the base. We select only one side because the opposite side outer face of the bottom foundationis different, and with solid element area spring assignments, you must specify a face for assignment of area springs

In this assignment, because the soil on the sides is less stiff than at the bottom, we assign .1 Kip-in area springs to the selected face,and we make sure that we change to Add to Existing springs so as not to delete previous spring assignments. Repeat thisprocedure on the remaining 3 faces of the bottom portion of the block foundation, assigning .1 Kip-in load to those faces.

Change face for each springassignment

Use Define menu to Define/add new time history functions. Period of .01667 is equivalent to 3600 rpm.

Period of .0333 is equal to 1800 rpm

Add new load case for linear time history periodic. The periodic time history option will eliminate the transient start-up effects. Note how we add the Unbalanced Y load to the sine function and the Z load to the cosine function

We forgot to assign the machine weightwhich is required for an accurate massmodel. In this example we assign a 10 Kipload in the gravity direction to the same jointas the unbalanced load, assuming that jointis also the center of mass for this piece ofequipment. In cases where the center ofmass is offset from the centroid ofunbalanced load, you would add a joint foreach center of mass and constrain it to theunbalanced load joint, and also to joints onthe foundation

Only 1 load pattern should have a self weightmultiplier. Otherwise, you can double or triple theselfweight in the analysis.

SAP2000 also offers the possibility of using Frequency-dependent springs (K) or external damping (C), since some geotechnicalreports provide these values as a function of frequency.Run the analysis and you will see some warnings. If you use the standard solver to examine the warning messages you will see thatthe warnings are related to the rotational DOF RX,RY and RZ. Please note that solid elements only have translational DOF, thereforeyou can disregard these warnings.

Use Display menu>Show tables where you can select load case to minimize output. In addition, you can select joints todisplay only the selected joints or elements to further minimize output. For this example we choose joint displacements,relative velocities and relative accelerations. This generates tables which can be sorted and filtered to quickly obtain maxand/or min values.

You can select a joint and use Display menu>Plot functions to plot displacements, velocities or accelerations