Different Types of Foundations?

MeyerFire University | FX108.15

By Joe Meyer, PE

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TRANSCRIPT

What is a building foundation? 

In this series, we're breaking out the different components that go into a building structure. In the last video, we introduced different topics of  the building structure. Well today, we're breaking out building foundations in more detail. 

Vitruvius in the 1st century BC said that durability will only “be assured when foundations are carried down to solid ground.” Well, the longevity of many ancient Roman structures is a testimony to this most important function of a foundation, which is transferring the weight of a building down into stable ground. 

Structurally, a foundation is mostly responsible for this load transfer to the ground, but it also anchors the building against wind, against earthquakes, isolates the building from frost heaving (which is the upward force from when water expands into ice), isolates the building from expansive soil, provides moisture separation, insulates between interior and exterior, and can provide storage, living or mechanical space for a building. 

In northern US climates, the foundation is sometimes synonymous with the basement. Digging into the earth where there is a low water table can double up as fairly easily or fairly attainable usable floor area throughout a basement of a building. 

In southern US climates, the level of the water table and the soil type makes basements more prohibitive or at least fairly difficult. 

There are many different combinations of foundation elements that can be used structurally to achieve as somewhat similar purpose. 

The most common setup we see in commercial construction is a continuous footing along the base of a foundation wall, or a spread footing at the base of a column.  

Principally, we have three components of a continuous foundation, that’s the footing, the foundation wall, and the floor slab.  

The footing is the horizontal component, that rectangular component, that goes along the base of the wall and is the actual point of contact between the soil and the weight of the building. 

The foundation wall is the vertical segment that brings loads down from the building above all the way down into the footing. The foundation wall could be made of masonry with concrete blocks or could even be made of stone like my old house, but it could also be poured concrete which is probably the most common for commercial US projects today. 

And then the floor slab. The floor slab makes up the floor of the building. This could be slab on grade where concrete is poured over large gravel which is in direct contact with the earth, or this could be made up of floor joists which bear weight onto the foundation wall. If there is a basement, the basement floor slab would be on grade and there would be joist supporting the floor above it. If there is a crawl space only, then maybe we don’t have a floor slab that’s in contact with the earth, there would just be dirt across space joist and the floor above. 

Let's start talking about a continuous footing along the base of a wall. The width of this footing depends on the vertical load, like how much vertical load is coming down from above, and the bearing capacity of the soil to provide the opposite force upwards at the bottom of the footing. If the soil has a very low bearing capacity, meaning it can’t handle much pressure which would be the case with something like sand or silt, then our footing may need to become significantly wider in order to spread out that force into the ground at the pressure that soil can handle. On the contrary, where we have vertical forces that are significantly less, or there is a high bearing capacity of the soil below, let’s say that's rock, well then our footing may not need to be very wide at all. 

One of the other purposes of a foundation is to isolate the building from frost heaving. In order to achieve this, the footing has to be lower than the frost line, or at the depth where the ground will not freeze during the winter. Now in warmer climates, this frostline could be pretty shallow, so a footing only has to be slightly deeper than that line. Well in colder climates where we get frost that goes deeper down into the soil because we have colder temperatures or colder duration, longer durations of cold temperatures, this frost line could be significantly deeper. Well, as a result, our footing also has to become significantly deeper. In cold climates, this could be a significant depth, which again encourages making use of that space on the inside by moving the footing even lower and just creating a basement. Now we have additional usable square footage. 

A spread footing is used to take a point load, which is a vertical load that happens at a single point instead of along a linear path. A spread footing will take that vertical force and spread it out into the ground, that’s why we call it spread footing, at a pressure that the soil can handle. If we have sandy or silt soil type, then this spread footing could become very large, it could become tiered, or could become combined with other footings in a host of different structural configurations. 

If we have a major column near our water service entry, that’s a red flag, these spread footings can present obstacles. We can't route through the footing because it's a critical component to the structure, and that will include running horizontally or vertically through that footing. So, if our water service entry is planned to stub-in next to a column, look out for these spread footings. They can be wide, they can be deep, and they can be in our way. 

Foundations also become more complex with heavier building weights or tricky soil conditions. High-rise structures for instance puts a significant weight on the area beneath the building. Because we have high rises often in urban centers, there's not a lot of room for that force to be spread out along the ground, so it mostly has to travel vertically straight down right below the building. 

In these scenarios, the structural design can't get significantly more complex, which might include piles or other deep earth digging methods where we drive down and help distribute that force down far into the soil. 

In terms of Fire Protection, foundations affect our work when we are trying to bring the service entry into the building. I’ll elaborate on this from a fire sprinkler perspective because that’s our most common run-in with building foundations. 

There’s a few things here that we need to consider. 

The first is that our pipe needs to be below frost depth. In my personal experience in talking with structural engineers, they’re okay with and may even prefer that our pipe routes directly horizontally through the foundation wall. This is the wall that is above a footing, and we can provide a sleeve that can accommodate a pipe, and the impact to the structure is not particularly significant. I’m not a structural engineer, that’s just what I’ve been told. When there is a sleeve through the wall, it also allows the pipe to be routed through that wall without bearing any pressure from the building weight. it's kind of a win-win scenario, but it requires some coordination and that’s not always possible to achieve. 

If we have a basement, well, routing horizontally through the foundation wall is easy and ideal. We are below the frost depth on the outside of the building, but we route right into a tenable space on the inside. The aspect here that we need to coordinate is the height where that service entry comes into the building. If we comes in high, we may actually need to pipe down towards the basement floor before turning horizontally or going back up vertically in order to give ourselves enough room to locate a backflow preventer or set up our riser equipment. 

If a footing is very shallow, or just below the frost depth, there may not be enough room to run a pipe below the frost depth line but above the top of the footing. 

This is probably our most common scenario for fire service entries which means we have to run our pipe underneath the footing before we turn up and stab into the building. 

Under this arrangement, we keep the pipe below the frost depth line and below the footing. Running underneath the footing, however, means that that pipe is going to be subject to the bearing weight of the structure above or routing through that bearing zone or that bearing area. 

Depending on the soil type and how the soil reacts to shear forces, this bearing area could be very narrow vertically underneath a footing or could be spread out and taper out the lower you go. Either way, if we are going to route underneath the footing, we don't want any fittings or couplings to be located in this bearing area. The connection between a pipe and a fitting is the weakest spot in that assembly and we don’t want any weak points to rupture overtime. In general, we like to have our stub-in as close to the exterior wall as possible just to save space inside the building, but we also want to recognize that the closer we get to the exterior wall, the more likely we are to have a fitting that would be in that bearing area. 

One way we help combat this is to make sure that all the fittings around this area are mechanically restrained. That means that the fitting is braced against compression and tension, but it doesn’t allow the pipe to easily slip out of that connection at the fitting. If we're looking for more detail here, check the requirements of NFPA 13 and also look at the product listings around the fittings and the pipe. 

If we have to run underneath a building, we would ideally want a continuous piece of pipe from one side of the footing that’s outside of our bearing area go through the bearing area and then have another fitting that’s clear of the bearing area on the interior side of the building. 

One relatively recent and common solution for this is a single welded, prefabricated piece known as in-building riser. This is a continuous stainless steel welded piece that allows contractors to have it held in place from the outside of the building before the foundation is even poured, and it helps us prevent locating any fittings right into this bearing area. 

So, for building foundations, that's a more detailed look at why they exist, what goes into the design behind them, and some of the issues and things we have to think about when we’re routing services around them. 

I'm Joe Meyer, this is MeyerFire University.