RISA 2-D compression only springs

[carnahanad]

Hello,

I'm basically trying to model a strip footing that is about 80 feet long and 16 feet wide and 6 feet deep. I have 5 point loads from columns loads from a different model. I figured the best way to model my footing is to add nodes at 1 foot intervals and then add a compression only spring constant of 4.5ksf (bearing pressure) times 16ft (strip width) to get 72kips/ft. Now the tricky part is that the spring is in k/in, but the inches are the vertical deformation of the spring. Can I get an opinion on if assuming my 72kips is good for 1 inch. Basically, I'm saying it will take 72 kips to depress the footing 1 inch. I don't know if this has any effect on your opinions, but this is for a high seismic area.

Looking forward to the discussion.

 

[JoshPlumSE]

The value of the compression only spring depends entirely on what you are connected to. Some thoughts:

1) Soils do not, for the most part, exhibit linear elastic behavior. So, representing this as a linear spring is an approximation. It's not uncommmon for us structural guys to do this. But, the geotech guys out probably roll their eyes at us whenever we do this.

2) If you've got a soils report then look for a modulus of subgrade reaction. You can use this as the basis for coming up with an estimate of the elastic stiffness of the soil. Though, in my experience, the soils guys won't usually put this in the report unless you make it part of their contract.

 

[weab]

Josh is right. The required approximate spring constant is a function of the compacted soil properties, that is, how much does it compress for a given soil pressure. Soil can theoretically have the same allowable pressure which creates different deflections.

Note that the modulus of subgrade reaction is often in load per cubic inch such as pcf. This can be interpreted as pounds per square inch per inch of deflection.

 

[carnahanad]

Thanks for the tips. I did look through the Geotech report and was not able to to find the subgrade modulus value you refered to. I appreciate the input!