Prokon And Case Of The Flying Footings

by yours truly

Anyone here who uses Prokon’s column base design module? The one that deals with good old conventional isolated pad footings design?

Anyone?

Despite the new pad-to-soil interaction analogous to strength design of concrete (see Joseph Bowle’s Foundation Analysis and Design), the traditional way of computing isolated pad footings (e.g. computing the moment of inertia, the all too familiar combination of P/A and My/I, etc) won’t go away that quick. In fact, it’s still a staple in universities and even state licensure exams.

So if you’re still not following the latest soil analogy, you should be fine. And no, you’re design won’t fail if you don’t. It’s still relevant as fact because even software just like Prokon follows this algorithm.

If you don’t have a finite element analysis software and you don’t have the luxury of time to create a spreadsheet from scratch, Prokon is one great way to get answers quick if you don’t want to sweat the small stuff.

And just in case you’re wondering whether Prokon is a sponsor of this site or post, then you’re wrong. I just got hyped when I was able to prove that it is capable of considering the case of flying footings. Flying footings if you remember are footings where because of significant uplift due to large bending moment to axial load ratios, is experiencing zero soil bearing pressure on certain parts of the footing area.

But before you can say that a footing is flying, you have to find out whether an end is in tension.

Referring to the image above, simple statics will tell you that the other end of the footing experiences soil in tension which is of course invalid. A more correct calculation will show that only a portion of the footing area will bear on soil.

Still referring to the image above, the induced bearing reaches 164.43 kPa. Unfortunately, the software offers no additional calculations on how this came to be except for the hint given by the plan view above where only a certain portion of the foundation is bearing on soil while the rest already took off. To aid our understanding, we will need something to bridge the gap to get to the diagram shown in Prokon’s calculations.

That proof is in this manual calculation (the “this” word, in case you haven’t noticed, is a link which will direct you to my manual calculations.)

As you can see, only 16 percent of the footing area is engaged in bearing while the remaining 84 percent is on air. So, software results = manual calculations. The mystery had been demystified.

Is that stable, you ask?

Well that’s a different question altogether. Numerically yes it’s still fighting hard gripping the soil for barely a hundred millimeters. But if you’re ultra conservative you might want to look into ways of beefing things up a bit if you still have the option to alter things. But if you’re trying to make it work, well, no code tells you that it isn’t working.