Continuous Footing Design/Terminology 1

[JJStructural]

Hi Everyone,

I'd like to get some clarification on nomenclature for foundation elements. I have a little over 1 yr of experience and so far, it seems that everyone has their own definition for grade beams, continuous footings, trench footings, etc. In a past project, I designed stem/strip footings at exterior walls and the contractor asked that we use grade beams instead, where the footing goes from just below grade to below frost depth. I understand their objective was to avoid the formwork with the strip footing. When I asked the geotechnical engineer if it was okay to use grade beams, he was confused because he called it trench footings.

My understanding now of grade beams is that they span between two elements such as piers and this is often done when the soil quality is low and cannot support a continuous footing. However, at my job and from other drawings I have seen, engineers still call out "trench footings" as "grade beams" even though they are not technically spanning between piers and the soil is relied on for support. Is this acceptable? If not, what would be the proper way to note this?

Now for the design, we show rebar running longitudinally with stirrups every so often. This is typically how I have seen deep continuous footings designed in school, for work, etc. However, if the soil is being used for support and there are no piers, should the rebar span the short direction, or the width of the continuous footing, rather than longitudinally? I am currently designing the foundation for a free-standing wall and planned to use a deep continuous footing to be consistent with what I have on the rest of the project. There is significant moment at the base of the CMU wall so I want to ensure that I am designing this correctly. Thanks!

 

[r13]

I believe trench footing is the same as strip footing, for which length > Width > thickness, and is directly placed on prepared subgrade/soil. Sometimes it may have stem above, make it looks like the invert tee to have better strength for relatively large load, and if differential settlement is a major concern. The strip footing itself is usually reinforced with a single layer of rebars in each direction, while the stem is reinforced similar to the regular beam. Note that the strip footing may not require reinforcement for sound subgrade with relatively light loading, such as in low-raise residential construction, especially in the past.

A grade beam, as the name suggest, is a beam spanning between supports, such as pile, pile cap, or spread footing. The grade beam may in contact with the subgrade/soil, but should not relied on the soil for support. It is analyzed as continuous beam, and shall be reinforced as thus per code.

For CMU wall with significant moment at the base, depends on the intensity, you might need a continuous strip footing with stem, or spread footing with grade beam, or deep foundation such as pile/pier with pile cap and grade beam. Which one to use is largely depends on the bearing capacity of the subgrade, and the type of framing to be supported. For wood and masonry with pilasters, strip footing with stem may work, for all others, spread footing, or piling is in the order.

 

[BAretired]

People may use different terms to describe a particular type of footing, but the terms you are asking about, grade beam, continuous footing (or strip footing) and trench footing are clearly defined on the internet.

I designed stem/strip footings at exterior walls and the contractor asked that we use grade beams instead, where the footing goes from just below grade to below frost depth. I understand their objective was to avoid the formwork with the strip footing. When I asked the geotechnical engineer if it was okay to use grade beams, he was confused because he called it trench footings.

You should have asked the contractor why he wanted to use grade beams instead of strip footings. I doubt that it was to avoid formwork for the strip footing. More likely, he wanted to avoid carrying the foundation below frost depth (my guess). In my neck of the woods, frost depth is five feet adjacent to a heated building, so a strip footing would not be used for a building with no basement; we would typically use a grade beam supported on drilled piles or screw piles. For a building with basement, it would be normal to use a strip footing because excavation will be going below frost depth anyway.

I am currently designing the foundation for a free-standing wall and planned to use a deep continuous footing to be consistent with what I have on the rest of the project. There is significant moment at the base of the CMU wall so I want to ensure that I am designing this correctly.

Sounds like a cantilever retaining wall. It is important that you design it correctly. You may need a fairly substantial footing depending on the magnitude of the significant moment.

BA

 

[JJStructural]

Thank you both for your feedback. We have a frost depth of about 3' here but stem/strip footing may be the best option for my wall foundation in this case. BAretired, it's not a retaining wall, just a wall to hide some mechanical equipment so the moment from wind loading is much less than it would be from retaining soil.

I am still a little lost with typical trench footing design. Let's say I have a continuous trench footing that is 4' deep and 2' wide and it is bearing on the soil. Why is it designed with longitudinal reinforcement and stirrups? This is how I would reinforce a grade beam, but the trench footing is not spanning between piers. Not saying that this is correct, but it's typically how I see trench footings detailed. I would think that you would want your main reinforcement in the transverse direction so that it is spanning the shorter distance.

 

[JAE]

In my neck of the woods:
Strip Footing - otherwise also known as a continuous spread footing, is usually a relatively thin (8" to 18") continuous footing set below frost and is almost always used to support small vertical stem walls (either concrete or masonry) and is usually reinforced with continuous longitudinal bars (0.0018 x A). Cross, or transverse, bars are sometimes required if the footing gets fairly wide and bending moments need to be resisted and plain concrete provisions don't provide enough strength.

Trench Footings - These do exactly the same thing as strip footings except instead of a thin continuous strip of footing, the whole width of the footing width is "trenched" down to below frost line and the entire trench is filled with concrete, negating the need for a formed stem wall and thus saving money. The question then arises as to whether you still need to place 0.0018 x A of rebar in an obviously oversized section of concrete. Per a response we got back from ACI a few months ago, you can, in effect, reduce the required minimum longitudinal steel as long as you don't otherwise need it for flexure.

Trenched footings "look like" grade beams but are fully earth supported and function just like strip footings.
For stirrups in these trenched footings, we include them at 48" o.c. only to hole the longitudinal steel in place, not for shear.
If your geotechnical engineer suggests that the foundation should be able to span over "soft" areas or other such design criteria, then the design may include some bending/shear attention.

Grade Beams - Per what others have said above - spans between points of support (either piers, piling or other spread footings). Design per ACI as a spanning element
Note that we tend to use the term Grade Beam loosely sometimes when we are talking about Trenched Footings....just a bad habit but it happens a lot.

 

[msquared48]

Grade beams are also utilized to restrain spread footings and pile caps from “walking” laterally during a seismic event.

Mike McCann, PE, SE (WA, HI)

 

[r13]

JJ,

You have a beam on elastic foundation. The soil acting as spring supports, with external load, there comes internal stresses along the longitudinal axis that require longitudinal and shear reinforcement. Also, for good practice, provide longitudinal steel to strengthen the beam to bridge over weak soils.

You have mentioned wind load, is the wall form an enclosure, or is an independent/isolated wall? For the latter case, I think you at least need a wider beam/footing, or an invert tee, that will increases soil-footing contact area to reduce soil bearing pressure, and resist the horizontal shear and overturning moment. Don't be surprised that you might need a width more than the depth.

 

[JJStructural]

Thanks everyone! Glad to know about the reinforcement. I always felt that my trench footings were over-designed when using 0.0018xA. I was thinking my understanding of foundations was off because trench footings are called grade beams so often but I guess that's just how it is.

The wall is extending out from a building so it is tied into another wall on one side but the other side is free. If I had a wide trench footing with longitudinal and shear reinforcement, I'm assuming I would be designing for flexure the same way I would design a typical concrete beam. What would you consider your span to be if there are no piers?

 

[r13]

Beam on elastic foundation, or a Winkler foundation, the span is the entire length in touch with soil. You can find design equations from Roark's Formula.

 

[Hokie93]

With a free-standing (or cantilever) exterior wall, the proportions of the continuous wall footing must be sized to keep the maximum bearing pressure within the allowable limit and provide an adequate factor of safety against overturning and sliding. With wind load on the wall, you will have a non-uniform (trapezoidal or triangular) bearing pressure across the width of the footing, creating internal shear and moment in the footing. The reinforcing steel perpendicular to the wall (across the width of the footing) are the ones of greatest concern for a cantilever wall condition, not the longitudinal steel. You can investigate the longitudinal footing as a beam on elastic foundation but minimum reinforcement will likely be adequate unless you have some known 'span' condition (such as a known pocket of poor soil). Do you have a good foundations and/or reinforced concrete textbook? Several of the good reinforced concrete textbooks have a chapter on foundations and give cantilever wall footings decent coverage.

 

[JJStructural]

Thanks Hokie93! I totally agree with you. I do have a couple of textbooks to reference and I am familiar with designing the transverse reinforcement for shear and moment from designing retaining wall foundations. Nonetheless, I am very interested in studying Winkler's foundation and Roark's equations more in depth for typical foundations under my exterior building walls. I don't know about you all, but I cannot remember that being covered in school and my foundations course really wasn't that long ago. The only part that is a bit confusing is the foundation modulus but it seems that there is a lot of information out there already about that subject. Thank for your help

 

[BAretired]

Thanks everyone! Glad to know about the reinforcement. I always felt that my trench footings were over-designed when using 0.0018xA. I was thinking my understanding of foundations was off because trench footings are called grade beams so often but I guess that's just how it is.

It is not always wrong to call a trench footing a grade beam. If the trench footing supports concentrated loads (columns) spaced along the length of the footing, it acts like a beam in every sense of the word. It is a beam on an elastic soil mass. If the load is perfectly uniform along the foundation, it has no longitudinal bending and is a strip footing.

The wall is extending out from a building so it is tied into another wall on one side but the other side is free. If I had a wide trench footing with longitudinal and shear reinforcement, I'm assuming I would be designing for flexure the same way I would design a typical concrete beam. What would you consider your span to be if there are no piers?

Frost depth is greater away from a heated building. In my location, frost depth is said to be 5' adjacent to a heated building and 8' in an open field. To avoid differential frost heave, it is not a good idea to tie the foundation to the building. Let it heave as nature dictates.

The span has no relevance. If the foundation is continuous, call it the entire length of the foundation; otherwise the length of each segment between construction joints.

BA