Passive Force in Risa Foundation

[Stazz]

In Risa foundation when you want to consider passive pressure to resist sliding you have to input a force (in kips) for each footing. Why can't this be a pressure (in ksf) since the passive force on the footing is a function of the dimensions of the footing. There must be some reasoning behind this decision. Defining a passive force requires the user to change the force everytime they change the footing dimensions, and not to mention they must manually calculate this force heaven forbid.

 

[JoshPlumSE]

In RISAFoot, you have the option of considering two methods of sliding resistance Friction or Passive.

Friction is easy because it is based on the normal force in the footing and the coefficient of friction.

Passive is more difficult becasue it is based on passive pressure and the depth of compacted soil. We considered added in two entries for calculating passivle pressure resistance (depth of soil and passive pressure). We could even have used the soil depth to partially elimate the need for entering "soil overburden pressure".

There were a number of issues that we realized would not be addressed with this implementation.
1) How to handle a shear key that extends below the footing to increase passive resistance.
2) For footings with HIGH lateral loads, we would never use the full friction and the full passive. At least not in my past design life. Instead we would use 100% of the higher and 50% of the lower value. The rationale behind this is that the friction resistance develops BEFORE any movement occurs and is reduced when movement happents. The passive resistance, on the other hand, REQUIRES movement to occur in order to develop that pressure resistance.
3) I imagine that many engineers will not completely agree with the reasoning in method 2. Therefore, I would expect a wide variation in the procedures used for combining these two resistances.

We quickly realized that, because of the above, it would be difficult to come up with a method of sliding resistance that was acceptable to everyone. Therefore, we implemented a scheme that required more work for the user, but which could be easily manipulated to whatever procedure the user desired.

 

[Stazz]

I never thought about the interaction between friction and passive forces but it does make sense. It's exactly like the baseplate issue where you should only consider grout friction resistance and anchor rod shear resistance separately but never together.