Practical Maximum Spread Footing Size 1

[pyseng]

As the title suggests, I am curious to know what others use as a "rule of thumb" for the maximum size (square side length and/or thickness) of an individual spread footing before they move on to a different foundation system.

Thanks!

 

[JoshPlumSE]

I'd say if you have to splice the reinforcement in your footing then you're probably dealing with something more like a mat foundation.

 

[pyseng]

I'm sorry, was this an unreasonable question?

The reason I ask is that I often find I am asked to design an isolated spread footing for something like a braced frame foundation. My designs always come out rather large because I need a lot of dead load for the overturning load case. As such, my designs are continually scrutinized for being too conservative. I know there is no "right" answer here, I'm just trying to get a ballpark estimate of what other engineers deem as a reasonably sized spread footing. This information will be very helpful for me to go to my boss and have a discussion about that fact that either A) my sizes are not that unreasonable or B) we should really be looking at alternative foundation systems when we run into spread footings this large.

 

[Celt83]

rule of thumb upper bound I hold to 20' square foundations, which even at that size your probably deviating from the rigid plate assumption used for the design of isolated spread foundations

For your braced frame condition I usually go with a combined foundation to help counteract the uplift rather than putting an individual foundation under each column.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[dold]

Not an unreasonable question, but a difficult question to answer without some more context. Roughly what is your overturning moment, resisting moment, and overburden? And what size footings are you coming up with? Are you using spread footings under each column or are you looking at a combined footing that captures the whole frame. Are you using software? If so, what software. A sketch would go a long way.

Another thing to keep an eye on is your safety factor and load combinations. And to which components of the system the 0.6DL factor applies to. There are many threads that discuss the old 1.5 overturning/uplift safety factors. For example, if youre looking at a footing experiencing pure uplift from wind, for example, does the self weight of the footing get penalized?

 

[HTURKAK]

There is no "rule of thumb" for the maximum size of an individual spread footing. Consider the size of wind turbine tower ftg.. Let me explain my personal preferences;

-For building type structures if the total area of individual spread footings is less than around 30 % of foot print area of the structure, the selection of individual spread footing type is OK,

- If the footing area around 40% of bldg area, a combination continuous + combined ftg could be feasible..

- If the footing area more than 60 % of bldg area, raft foundation will be more feasible..

For industrial bldgs with large span, if distance between individual ftgs less than ftg width, continuous ftg will be more feasible..

I remember one project ( coal power plant), we preferred box type mat foundation to reduce the effective bearing stress.

finally,if you tell me your superstructure, the design loads, soil conds .. i may tell you the applicable foundation type..

 

[JoshPlumSE]

To some extent there is some "semantics" going on with this question.

I've worked on some pretty large mat foundations that could have been referred to as a spread footing. These were for large vertical vessels. About 2-3 meters thick, if I remember correctly. 20 meters square.

We called it a mat foundation and designed it as such because we had other pedestals on it to support misc columns and such. But, the basis of the design was really a spread footing for the vessel pedestal. Thickness governed by punching shear. Width governed by overturning stability ratio. That gave us most of the final design. So, even when you have a foundation of another type, you can still use the basics of spread footing design to give yourself a reality check on the design.

 

[BridgeSmith]

As others have said, you really can't put a 'rule of thumb' on it, since it's it's a matter of the cost of the other options. Depending on the foundation materials and the pattern of the load or loads, even a large spread footing may be the most economical option, or a small one may be more expensive than a deep foundation.

For bridge foundations, we often do a cursory exploration of several options and do cost comparisons. After almost 20 years in bridge design, I still get surprised sometimes by what the most economical option turns out to be.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

If it's for overturning, can you use a different foundation type?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[skeletron]

I'm onboard with @Josh's semantics here. Max size for a reasonable spread footing is probably in the 20ft range and probably anything under 2'-6" or 3'-0" thick. Anything beyond that, you are probably dealing with a different foundation method.

 

[pyseng]

We typically do relatively low-rise steel buildings. The AISC System Design Example (v15) is actually very nearly what we typically do. In fact, I'm planning on using that example as a demo for a future discussion. In terms of context, I would say that I've gotten funny looks from both my colleagues and contractors in the past for providing a 16'X16'X3' spread footing before.

Side note: I am aware of the discussion surrounding overturning. It is my understanding that the intent of the 0.6 factor on DEAD is primarily to achieve a (roughly) 1.5 overturning factor of safety, NOT to account for uncertainty in the dead load (as many people believe)... although, I suppose the intent of the 1.5 factor of safety is to account for uncertainty in the dead load, so tomato tomato I guess. Regardless, I DO apply that 0.6 factor to the foundation weight because there are no provisions (that I am aware of) that explicitly exclude foundation weight from the 0.6 factor. I figure, if I have to apply it to my super structure, I have to apply to my substructure.

 

[pyseng]

Thank you for the responses, looking back at my initial post, I definitely made a lot of assumptions regarding the particular situation I'm in. That's my bad.

 

[azcats]

You'll find plenty of threads in here about that 0.6D factor and everyone's opinions whether or not it should apply to soil bearing pressure calculations. I have a hard time swallowing that factor when bearing pressure is controlling.

 

[KootK]

1) In a low rise steel building in a low seismic region, many project stakeholders will expect the building lateral system to be more or less "free". Free = comes with a nominal cost penalty relative to the bare gravity design. So, in the roughest of terms, if the size of your brace hold down footing is more than, say, 50% larger than a typical gravity footing for the building, you can expect questions.

2) If there's an "answer" to the problem of stupidly large hold down footings, it's surely a thoughtful framing layout that:

a) engages meaningful dead load where the overturning demand exists.

b) deals with the lateral demand in several locations about the building rather than aggregating it into just a few.

Unfortunately, this seems to be more and more difficult to bring about with each passing year in the evolution of architecture.

3) I like a stiff grade beam running where it needs to to engage additional dead load. For obvious reasons, that tends to be be more feasible with perimeter frames.

4) It sounds to me like your boss ought to put pen to paper and show you how it's done if she's not loving your output.

 

[pyseng]

KootK, thank you. As you seem to note, Points 1, 2 and 3 require either A) an architect with above average understanding of structural engineering or B) a very productive pre-design meeting. Both of which are unicorns in my limited experience.

As for Point 4, that's basically the direction I'm heading, I just wanted to make sure that my designs are reasonable before raising the issue.