The Bearing Capacity Dilemma…Solved ?

Quick question… ??

When solving bearing capacity problems, do you use the ultimate or the net bearing capacity to find the allowable bearing capacity? ??

That's a question that pops up quite often in our private communities for students of both our civil FE course and civil PE courses — just take a look for yourself. ??

So, as it keeps popping up over and over again, we decided to break it down for you so you know exactly what to do to solve bearing capacity problems. ??

And the best part?

There’s a little-known hint the problem statement itself will give you, telling you whether you should use the ultimate or the net bearing capacity. ??

What’s Ultimate Bearing Capacity?

Bearing capacity of soil is the soil’s ability to support the pressure created by the total load coming from the foundation. It’s the load-carrying ability of the soil.

Ultimate Bearing Capacity, then, is the maximum pressure the soil can handle before it shears (aka, fails).

That is, the load per unit area at the base of the foundation that causes the soil to “break” and move. This causes the foundation to move with it, tilting the structure…if not collapsing altogether. ??

The Ultimate Bearing Capacity value can be found by Terzaghi’s equation, as shown below. But as you can imagine, this is one load value we engineers stay away from.

Reaching this load value is catastrophic, and that’s why we add the factor of safety to make sure we do not get even close to it. ??

What’s Net Bearing Capacity?

That’s when things start getting a little bit confusing for those taking the FE or PE exam. Why?

Because the Civil Engineering Reference Manual (CERM) and other Geotech textbooks touch on what’s called Net Bearing Capacity…while the FE Handbook and the PE Handbook don’t.

But let’s get into it now. ??

You find the Net Bearing Capacity by subtracting the pressure created by the weight of the soil located above the footing from the Ultimate Bearing Capacity value you found before.

The Ultimate Bearing Capacity is the maximum pressure the soil can take on before it fails, right? ??

The problem?

Not all of it this total pressure comes from the building or structure itself via the foundation!

Why? Because the soil is already subjected to pressure from upper layers of soil…in addition to the loads of the building. ??

That’s why you find the Net Bearing Capacity. It’s the “real" maximum pressure (which is less than the Ultimate Bearing Capacity) the structure (alone) can apply to the soil via the foundation. ??

What’s Allowable Bearing Capacity?

Allowable Bearing Capacity is what you get after adding the factor of safety to the Ultimate or Net Bearing Capacity value you’ve already found.

This guarantees that the pressure applied to the soil via de foundation is not even close to the failure load that will cause the soil to shear — and the building to collapse.

A good way to understand this is to think of it in terms of supply and demand. ??

The Allowable Bearing Capacity is how much pressure you can apply to the soil — your “supply.” The “pressure demand” made on the soil by the foundation must then be lower or at most equal to this supply value for the foundation to be safe.

Now comes the confusing part… ??

You can calculate the Allowable Bearing Capacity in two ways:

• Applying the factor of safety to the Ultimate Bearing Capacity value; or
• Applying the factor of safety to the Net Bearing Capacity value.

“Which one should I use then,” you ask? ??

That’s hands down the most asked question about bearing capacity ever! But as promised, let’s put an end to this dilemma once and for all.

The Little-Known Bearing Capacity Secret

Here’s the one little-known hint the problem statement will give you, explicitly telling you which one to use. ??

You’ll only use the Net Bearing Capacity to find the Allowable Bearing capacity if, and only if, the problem statement includes things such as “consider the weight of the soil above the footing” or “consider correction for overburden.”

Whether it’s a practice problem from our YouTube playlists, a question you run into while taking a practice exam, or even an exam question during the real deal, you should be on the lookout for this hint whenever you get a Bearing Capacity question!

If you see anything like that in the problem statement, find the Ultimate Bearing Capacity, use it to find the Net Bearing capacity, and then apply the factor of safety to the latter to find the Allowable Bearing Capacity.

If nothing like “weight of the soil above the footing” or “overburden” is even mentioned in the problem statement, then find the Ultimate Bearing Capacity and use it to find the Allowable Bearing Capacity directly.

Easy, right? ??

What to Do If You Have Any Other Questions?

Now that you know the secret to solving the “Ultimate vs Net Bearing Capacity” dilemma, who do you reach out to if you struggle with something else? ??

That’s a real thing if you are preparing for the FE or PE exam. Having someone to turn to when you simply can’t solve a problem or wrap your head around a topic is invalid!

That’s why we have private communities for each one of our courses, and it’s one of the things all past CEA students rank really high when they finish the course and get their passing score.

So, this is an invaluable add-on that you get as a student of both The Ultimate Civil FE Review Course, The Ultimate Civil PE Review Courses, and The Ultimate Floodplain Manager (CFM?) Review Course.

We're looking forward to be able to help you from the inside! ??

If you're on the journey to become a licensed PE or CFM?, then don't forget to follow Civil Engineering Academy for more exam prep tips, tools, resources, and insights...just like this one. ??

And as always, feel free to email us at [email protected] with any questions you may have or topics you may be struggling with. We're here to help!