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?

 

[dold]

RISA Foundation would do the trick. But it's a bit pricey for just a one-time project.

 

[Delmarva_Struct]

Can RISA Foundation be used without the other RISA modules? We might consider it as we do have a few similar projects..

 

[dold]

Yes. You can use RISA Floor to build an overall building/gravity model. Then it has functionality to import that model into RISA 3D where you perform lateral design. Then import the reactions from the R3D model into RISA foundation to design your footings (and other foundations).

 

[WinelandV]

RISA Foundation also works without 3D, Floor, etc. It's fairly powerful w/lots of options, but could be a bit overwhelming at first.

Please note that is a "v" (as in Violin) not a "y".

 

[MotorCity]

I like Risa Foundation and Risa Floor, but they can be a little klunky to use at times.

My biggest gripe with those Risa programs is that the user interface looks like it has not been updated since the '80s.
Clearly, Risa has devoted more time into developing Risa 3D than those programs.

Maybe one day all of those packages can be combined into a single program that does everything.....er, wait...nevermind, there goes the revenue stream.

 

[JoelTXCive]

Your situation sounds pretty complex, but IES's Visual foundation might work.

I use their software all the time because it's so user friendly.

They have a 30 day free trial and you could probably figure out if the software is suitable for your needs in less than an hour.

Company Website:

https://www.iesweb.com/

Their Youtube channel for Visual Foundation:

https://www.youtube.com/playlist?list=PLCA1GXGZo1FeOlx2Fs5TGZc-NpEhhx7ZT

 

[milkshakelake]

For eccentricity, I connect a grade beam and assume uniform bearing pressure. This case isn't covered in typical software, which assumes a triangular load distribution with eccentricity. So I made spreadsheets to deal with this case, and for the grade beams as well.

For software, I recommend SAFE. But I try to avoid it because a bunch of spreadsheets is quicker than modeling and troubleshooting FEM. And getting the correct subgrade modulus of soil is a bit tricky, though it can be estimated.

For eccentricity in two directions, use grade beams in two directions. I've done this for connecting new and existing footings. Just standard spreadsheets can be very versatile with a few modifications.

 

[Delmarva_Struct]

Thank you all for your help! I will try visualfoundation and RISA and let you all know how it goes....

 

[JoshPlumSE]

My biggest gripe with those Risa programs is that the user interface looks like it has not been updated since the '80s.
Clearly, Risa has devoted more time into developing Risa 3D than those programs.

That's definitely true. RISA-3D is there cash cow. Every other program (IMO) is just a way to leverage there RISA-3D market share for profitability. You want to do Slab Foundations, buy RISAFoundation. Oh, you want to do elevated slabs, now you need to buy another slab program RISAFloor ES (on top of buying RISAFloor). Post Tensioned slabs, that's probably going to have to come from Adapt. So, you've got to buy 3 different programs just to do a variety of common slab analysis / design.

Caveat:
I worked for RISA for 16 years ending in November of 2017. Before I left I was essentially the VP of Engineering, though my title was like VP or Technical Applications and support. New management cleaned house when the original owner (Bruce Bates) left and I was out.... with some hard feelings and frustration on my part. Therefore, I hold a bit of a grudge against current management and many of their decisions that are less "customer friendly" that I would have liked. Therefore, take my response above with a grain of salt.... Understand that I have bias in this matter and that you might get a different story if you talked to the RISA folks about how everything they seem to be less customer friendly than they used to be.

This attitude of mine is probably one of the reasons why the Nemetchuk management and I were doomed to part ways. I kept looking at the similarities in programs and suggesting that we expand the features included in the existing programs to include some of what they were looking for. The new management looked at each new feature as a way to bleed more money out of their users for relatively little development effort.

The attitude of the current RISA management (which, BTW, came over from STAAD), reminds me when STAAD decided that STAAD-Pro was a totally different program from STAAD-III and pissed off their users by forcing them to pay full price for their NEW program rather than just getting it for free because they'd been paying STAAD-III subscription / maintenance for years.

 

[JoshPlumSE]

Oh, almost forgot. Another reason that my current opinions are biased is that I currently work for CSI, the company which produces SAP, ETABS, Perform, CSiBridge and such. Probably RISA's biggest competitor. I had all these opinions about RISA's current management BEFORE coming to CSI. But, it's another source of potential bias that could affect what I write in my posts....

 

[Delmarva_Struct]

JoshPLumSE

I have used SAP2000 extensively for finite element modeling and seismic analysis such as non-linear time history analysis. I have yet to find a software that is better than SAP2000.

But a lot of companies either do not know about it or don't want to use it. Not sure why...

 

[JoshPlumSE]

I have yet to find a software that is better than SAP2000.

But a lot of companies either do not know about it or don't want to use it. Not sure why...

If you grew up as a young engineer using a particular software, then you tend to "think" like that software when you're building your models and such. It takes a lot for those of us who are set in their ways to switch to a totally new / different interface. The frustration (and time) it takes to become proficient just isn't worth it for most people. Not unless the software they are currently using really messes up their customer relations and makes their users angry enough to switch.... Like STAAD did that one time in the transition between STAAD-III and STAAD-Pro. That was a huge windfall for RISA at the time. So many STAAD users who decided to give up on that company and switch to RISA.... entirely because their management made it clear that they didn't care about their customers concerns.

 

[Delmarva_Struct]

Guys I need you help on this again...

In RISA Foundation, when modifying the subgrade modulus I find that a higher subgrade modulus produces a higher soil bearing pressure. However, in reality a higher subgrade modulus should reduce bearing pressure under the footing. I understand this is happening because a higher subgrade modulus results in a spring with higher stiffness being used under each plate element node in the RISA model. The higher stiffness spring attracts more of the column load over a smaller area of the slab, i.e. there is lower/lesser load transfer to the other areas of the mat slab. This results in a high soil bearing pressure over this area of the slab.

I'm trying to design foundations with large column loads that are to be situated next to an existing building. I have large eccentricities for the column loading. My allowable soil bearing pressure is 3 ksf. No matter how large I make the foundation I am unable to resolve the higher bearing pressure at the eccentric column loading location. I've even tried connecting the eccentrically loading footings to adjacent footings (to make it work like a strap foundation) but still doesn't work. I tried making the slab 3 ft. thick, this helped with distributing some of the load but I'm still over the allowable. Making a slab 4ft thick seems unrealistic.

Does anyone have any experience on how to resolve this ? Should I just make an engineering judgement that in reality the load will be more evenly distributed?

I did read that I would have to calculate coupled spring stiffness values (based on Winkler foundation analogy maybe?) and enter them manually. Seems like a lot of extra effort. What is the point of using this software if I have to do all the tedious work? I will do it if I have to but hoping there's another solution...

Please see attached picture for the soil bearing pressures and footing layout...

 

[TLHS]

What does the footing look like? If it's eccentric right at the edge of a footing and you're increasing the size away from it you're not necessarily going to get a lot better bearing pressures.

That being said, most shallow footing related allowable bearing pressures are really deflection or settlement based. If you're exceeding your bearing pressure locally it might mean that you're just going to further soften. Allowable bearing pressure may not be the right way to think about a spring based FEA analysis. You'll have to talk to your geotech, though. For odd foundations, it can sometimes make more sense to try to get a realistic sense of stiffness, then set a deflection criteria. You then might need to look at an ultimate bearing criteria.

Depends on what you're doing, though. That's more commonly seen with rafts. Smaller foundations you might just need to make a judgement call and then confirm with the geotech.

 

[centondollar]

In RISA Foundation, when modifying the subgrade modulus I find that a higher subgrade modulus produces a higher soil bearing pressure. However, in reality a higher subgrade modulus should reduce bearing pressure under the footing.

No, what you are seeing is what you should expect. The pressure in the spring model you use is just displacement multiplied by the modulus of the subgrade; when you increase the modulus, the constant multiplier increases. At first glance this may seem unintuitive, but if you have concentrated loads (column) on a slab supported on springs, using a constant modulus of subgrade and increasing it will not reduce deflection linearly at all points near the loaded area.

I'm trying to design foundations with large column loads that are to be situated next to an existing building. I have large eccentricities for the column loading. My allowable soil bearing pressure is 3 ksf. No matter how large I make the foundation I am unable to resolve the higher bearing pressure at the eccentric column loading location. I've even tried connecting the eccentrically loading footings to adjacent footings (to make it work like a strap foundation) but still doesn't work. I tried making the slab 3 ft. thick, this helped with distributing some of the load but I'm still over the allowable. Making a slab 4ft thick seems unrealistic.

Are you modelling columns as beams connected to a plate? This will cause unrealistic results; consider adding some type of coupling that spreads out the column load and stiffness to an area on the slab.

A 4ft thick slab is not necessarily unrealistic. Much thicker slabs are used for many types of structures.

Does anyone have any experience on how to resolve this ? Should I just make an engineering judgement that in reality the load will be more evenly distributed?

Engineering judgement is based on calculations and empirical evidence, not guessing, which your proposed argument amounts to. In this case, the result is also not a singularity in the classical sense (hard boundary conditions, corners etc.), so what you see on the FEA graphical output screen is what you get. I suggest making the model more realistic.

I did read that I would have to calculate coupled spring stiffness values (based on Winkler foundation analogy maybe?) and enter them manually. Seems like a lot of extra effort. What is the point of using this software if I have to do all the tedious work? I will do it if I have to but hoping there's another solution...

There is no coupling in Winkler springs. However, you may receive more realistic results (see e.g., the literature) by using a higher modulus near the edges of the slab and a smaller one near its middle. Maybe a colleague can show you some publications where advanced 3D models are compared to Winkler models with stepped change in modulus of subgrade.

Regarding software, it is seldom a magic wand that solves all your problems.

 

[Delmarva_Struct]

Please see attached for foundation model

 

[WinelandV]

Delmarva_Struct,

Unfortunately, that pressure distribution looks as expected, based on the geometry.

To your subgrade modulus question, soil is, well, kinda tricky. GERNALLY SPEAKING (w/ any and all required caveats that a passing geotech might demand), soils with higher allowable bearing pressures tend to be stiffer (i.e. higher subgrade modulus). But a stiffer soil can mean that the "foundation concrete is essentially rigid compared to the soil" assumption is no longer valid (this being the assumption the we use for uniform pressure under a column footing). In order to spread the column load to more area, you need a stiffer foundation. Which is usually means a thicker foundation. How this affects your constraints, I don't know, but it's the give and take of plate-modeled foundations.

In your model, it appears you have just modeled a slab. Perhaps you can add some beam elements to directly connect your top right column to your top left column, and that may help transfer the load over more area.

As for your bottom right column, you might propose a deep foundation as an option. Otherwise, you're left with the highly inefficient (b/c of how eccentric the footing is) "make the slab thicker" option.

My 2 cents. Consulting with the geotechnical engineer on the project is recommended.

Please note that is a "v" (as in Violin) not a "y".

 

[Delmarva_Struct]

WinelandV - Unfortunately, in RISA foundation, this is the only way to model strap beams/grade beams between columns. Just generally drawing in a beam element between columns does not redistribute the column forces. I'm starting to think deep foundations may be the only option for this building.

 

[Celt83]

@Delmarva_Struct:
"Unfortunately, in RISA foundation, this is the only way to model strap beams/grade beams between columns." I don't believe this is an accurate statement see below image taken from the manual:

For strap beams you need to make sure the beam is stiff enough to redistribute the pressure and the beam should not be supported along it's length by soil springs.