Foundation Analysis and Design Software Recommendations 1

[Delmarva_Struct]

I am trying to design foundations for a building addition for an exiting building. The columns that are near the existing building experience high seismic loads as they form a part of the SFRS of the new building structure. The footings for these columns will have highly eccentric (in both directions) large column loading wherein the column is placed at the corner of a footing. Also we have irregular shaped footings to accommodate the existing footings, utilities etc. I have tried to used strap footings, combined footings etc up to the extent that software's like ENERCALC, ASDIP etc. will allow but they do not work..Does anyone have any recommendations for an FEM software for the analysis and design of highly irregular shaped footings with multiple eccentric loadings?

 

[JoshPlumSE]

Yes, Celt83 has cited the perfect page from the reference manual. One thing to keep in mind, is that the accuracy of the soil springs (rotational and vertical) is dependent on the design size vs analysis size of the footings. As such, in these cases, it is best to NOT have the program optimize the size of the footing for combined footings like shown in Celt's pic from the manual.

 

[Delmarva_Struct]

@Celt83

I am aware of this page. This was my initial approach to model these foundations. The beams as modelled are connected to the nodes of the pedestal and not necessarily to the discretized nodes of the mat foundation. Also, all parts of beam and foundations are considered to be rigid bodies. Whether the connection between the strap beam and foundation is made to be fixed-fixed or pinned=pinned, there is lesser redistribution of forces. Also I do not get a "stress distribution diagram" for the bearing pressures which helps me see how much stress is being distributed over the strap beams.

I ended up calling RISA tech support and they suggested I use mat foundations to model strap beams and the individual foundations.
Please see below.

 

[Celt83]

"...bearing pressures which helps me see how much stress is being distributed over the strap beams"
There should be no bearing stress distributed over the strap beam, the strap needs to be isolated during construction from the soil below via a compressible material so that it is free to deform so that it ends up "spanning" between the two foundations and engages it's flexural stiffness to redistribute the eccentric column reaction.

see this thread: Link

 

[Delmarva_Struct]

oh! I did not know that.. I figured the strap beam would have soil pressures as well. I guess that is the fundamental difference between the two types of models i.e modelling this as mat slab vs modelling it as two individual foundations connected by a strap beam.

The mat slab approach seems to work better with the stress redistribution and with the number of flexural rebar required to resist the negative moment on the strap beam/Mat slab.

That's a very informative discussion you all had in that post. Thank you!

 

[bones206]

Also I do not get a "stress distribution diagram" for the bearing pressures which helps me see how much stress is being distributed over the strap beams.

You should be able to get this visual, but you have to play around with the view settings. See screenshot below:

There should be no bearing stress distributed over the strap beam, the strap needs to be isolated during construction from the soil below via a compressible material so that it is free to deform so that it ends up "spanning" between the two foundations and engages it's flexural stiffness to redistribute the eccentric column reaction.

Risa allows you to model discrete soil regions, so you can create a soft spring soil to represent a compressible material, then create a region just below the beam. I've tried it before and the results have been a little inconsistent, but it is a tool that's available in the software.