You want to have grid intersections at pile locations, load locations, geometry and thickness changes and for changes in 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 section property of the area does not matter since we’re only using the shell areas to help us model the solids . The quick-draw area tool will 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 by specifying 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 delete them. If you don’t delete them, it will double-count the shell area stiffness and mass with the solid elements. After selecting the area 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 is bolted/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 Body1 constraint 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. That’s usually the easiest way to select the bottom face of solid elements. Before you select, use Options>Tolerances and change + - 2D cutting view plane tolerance to .1’ or something 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 .15 Kip-in. If you are unsure which is the solid face, use Display menu>Show misc. assigns>Solid and checkbox color-coded faces to view 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 less stiff than the compacted soil beneath the base. We select only one side because the opposite side outer face of the bottom foundation is 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 this procedure 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 spring assignment
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 startup 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 weight which is required for an accurate mass model. In this example we assign a 10 Kip load in the gravity direction to the same joint as the unbalanced load, assuming that joint is also the center of mass for this piece of equipment. In cases where the center of mass is offset from the centroid of unbalanced load, you would add a joint for each center of mass and constrain it to the unbalanced load joint, and also to joints on the foundation Only 1 load pattern should have a self weight multiplier. Otherwise, you can double or triple the selfweight in the analysis.
SAP2000 also offers the possibility of using Frequency-dependent springs (K) or external damping (C), since some geotechnical reports 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 that the warnings are related to the rotational DOF RX,RY and RZ. Please note that solid elements only have translational DOF, therefore you can disregard these warnings.
Use Display menu>Show tables where you can select load case to minimize output. In addition, you can select joints to display 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 max and/or min values.
You can select a joint and use Display menu>Plot functions to plot displacements, velocities or accelerations
