You can reduce the tension by iteration. The tension gets closer to zero with an increase in the number of iterations. See “Zero tension iteration options” in the program manual for more information.
The geotechnical engineer commonly provides a value called the “subgrade modulus” or “modulus of subgrade reaction”. As a guide only, realistic values vary from 100 pci (approx. 25 MN/m3) for soft clay to 750 pci (approx. 200 MN/m3) for very dense gravel.
RAM Concept automatically multiples the area spring constant (units of force per cubic length) by the tributary area of each finite node (determined from mesh size) to determine the spring constant (units of force per length) at each node.
The subgrade modulus is determined in the field using a plate load test, typically using a 1ft x 1ft square plate. The actual modulus value is dependent on the size of the loading area: for a given displacement, the modulus value decreases as the plate size is increased. RAM Concept does not automatically adjust the input area spring constant to account for this size effect. If you are not sure what area spring constant to use, consult with a geotechnical engineer. Some engineers choose to run the analysis with a spring constant varying from 0.5 to 5-10 times the provided value and design for the worst case (see ACI 336.2R-88). In many cases, the stiffness of the mat is much larger than the soil stiffness. In these cases, the forces in the slab and the computed soil pressure may not be sensitive to the area spring constant.
Note: Area springs are always assumed to be compression only springs in the Z direction, but they behave elastically in the R and S axes. Line and point springs are also linear elastic supports resisting tension or compression.
Yes, soil area spring stiffness values are cumulative. If you model two area springs with 100 pci stiffness you will get 200 pci stiffness in the intersection.
Not directly. You could draw spring supports that approximate varying soil support.
No, but it is a good idea. It ensures a node is placed at that location where there is likely to be a heavy point load. The columns also provide convenient snap points for tendons or design strips.
Yes. Use either (flexible) columns under, or point springs. Skin friction is not considered.
Yes, but the results could be very susceptible to variations in geotechnical parameters. For example, if the soil’s stiffness is overestimated, the actual pile reactions could be significantly underestimated. Use caution.
No. An oversized area spring is fine. The program applies the individual nodal springs based on the mesh tributary area.
Yes. You can vary the stiffness in two directions. See “Area spring properties” on page 55.
This is not an input parameter. You need to look at soil bearing pressure plans (which have a maxima / minima legend) to assess the maximum pressures.
Instability in Mat Foundation Analysis