Allowable Soil Bearing Question 1

[SteelPE]

I have gotten into a debate with a colleague recently. He mostly works on residential projects, I mostly work on commercial projects.

With regards to soil bearing, I am typically provided with a geotechnical report, however, when I am not, I typically look at table 1806.2 of IBC 2015. He typically has to reference table R401.4.1 of the IRC. Our debate centers around deck footings. He likes to use sonotubes with no base (12", 16", 18") I insist on using an enlarged base at the bottom of the footing, typically something called a Bigfoot. His argument is that the load are usually small, and if you are using a 18" dia sonotube, with an allowable soil bearing pressure in accordance with table R401.4.1 then the capacity of the pier is 5,300 lbs.

My beef with this method revolves around every soils report I have ever read has the following statement (or something very similar):

"The soil bearing capacity is based on a minimum footing width of 3' and must be reduced proportionately for narrower footings. Footings should be designed in accordance with SBC section 1806. For footings smaller than 3 feet in least lateral dimension, the allowable bearing pressure should be reduced to one-third of the above value multiplied by the least lateral footing dimension in feet. Also, we recommend that continuous wall footings be at least 18" wide and isolated footings at least 24" wide."

We are not talking large allowable bearing pressures in the case of the given soils report (typically 3,000-4,000 psf sometimes more).

I can't seem to find any minimum footing with in the IBC or IRC, however, every geotechnical report gives the same outline of parameters. So the question is:

-Are there any provisions in the IBC or IRC to reduce bearing capacity for footing size?
-If there are not provisions, would you use the full bearing capacities as outlined in table 1806.2 of the IBC or R401.4.1 of the IRC?

 

[North2South]

I believe there is Minimum Footing Width table in the IRC. I am not aware of any provisions to reduce bearing capacity. However, like you, every geotech report I have seen recommends limiting the footing width to no less than 18”/24”. While what you’re friend is designing for may be code compliant, I would not consider it best practices. I would not go less in size than what you are seeing in geotech reports but I would design for full bearing capacity provided in the tables.

 

[kissymoose]

Much of my work is residential and I have always taken the area of the footing * 1500psf for my bearing capacity.
-Edit: For exterior decks. Also, I don't go less than 12"Ø

 

[dauwerda]

The minimum width of footings given in the IBC is 12" (Section 1809.4) So I would say the presumptive values given are at least good down to that size for a continuous footing. The prescriptive footing table (Table 1809.7, footnote c) states that isolated footings should have a length and width twice of that which is shown for strip footings - so at least 24"x24"

The 2018 IRC actually has a specific section for deck footings now, section 507.3. This section states that you can use the presumptive bearing values given in 401.4.1 and has minimum sizes given that go down to 12" square and 14" diameter for round

 

[r13]

The reduction is for shape effect - spread footing vs strip footing, not size.

 

[SteelPE]

The 2018 IRC actually has a specific section for deck footings now, section 507.3.

I suppose that table R507.3.1 of the 2018 IRC puts and end to the argument as they give the required footing size based upon supported area/loading/allowable bearing.

We don't follow the 2018 ICC codes... yet, but I suppose if you are ever in a pinch with regards to absence of information and point to that table to back up your reasoning.

 

[phamENG]

SteelPE - I would be okay with what your colleague is doing for most residential decks. That is, single level decks near grade. When you start getting into complex, multilevel decks or decks that are actually balconies, I would go with a more robust footing.

Your average, simple residential deck has relatively low self weight that is pretty evenly spread out, and the long term component of the live load is very small (a table, a few chairs, an umbrella, and a grill in most cases). So settlement isn't going to be much of a concern. These are also short lived structures. Most don't make it to 30 years. A lot of them look awful after 10. I'm willing to rely on the "tried and true" approach for this sort of thing.

 

[SteelPE]

pham,

I understand what you are saying, but if the deck looks like a POS and needs to be replaced, how often are the piers replaced as well? Not often. My neighbor just had his deck replaced..... they used the same piers (under construction now).

I understand the logic but don't fully agree with what you are saying.

 

[dauwerda]

The other thing to consider - is it going to negatively affect the structure if it does settle 1" or more - for most decks I would argue that this really wouldn't be an issue. In the case of rebuilding with the existing foundations - even better, now you are building on foundations that have already been pre-loaded and settled from seeing the typical loads so you should expect much less movement the second time around.

 

[kissymoose]

SteelPE,

Why would the piers need to be replaced? If you are simply replacing a deteriorating deck and the piers have performed adequately/there are no current defects, then it is perfectly reasonable to continue using them. However, if you are adding a roof cover or expanding the size, then new piers should be specified or the existing pier sizes determined and worked with in the new design. Removing existing piers for the construction of a large deck/cover project is an appropriate expectation for designer and contractor.

 

[SteelPE]

Ok, so I think you guys are getting confused.

pham made the argument that these deck structures are short lived and temporary and that it would be reconstructed in as little as 10 years but more than likely under 30 years. My response to the argument was that while the deck may be rebuilt, they rarely rebuild the piers.

The reference is in no way related to my neighbors deck.

 

[kissymoose]

I see. Yes, I agree with you, phamENG's point on a residential deck often being a short-lived structure really only applies to the decking/maybe joist and columns, whatever wood seems to be in bad shape. In my experience, the squeaky wheel that gets addressed is whatever component "looks bad", and the piers really only make the list if installed on a slope with a significant portion above grade that visibly leans. Then you can actually get the home owner to care. Much of residential design is rooted in historical practices and what's worked, and this is reflected in the IRC. For example, the shearwalls in the IRC per the prescriptive method don't require holddowns at either end.

 

[jayrod12]

Pham made the argument that these are short term structures and therefore long-term settlement (which is the basis for end bearing design of foundations in most cases) is less of a major concern. The reason that re-using the "undersized" piers is acceptable in a situation like that is a combination of what Pham indicated and dauwerda said. You're going to re-built the deck relatively quickly just due to standard wood deck deterioration, and therefore whatever settlement has occurred to date can be corrected by providing a longer post, and the piers are likely to perform better the second time around as much of the original settlement and a portion of the long term settlement will already have taken place.

 

[RPMG]

Where I live, frost depth is 3'-6" and people drop decks on Home Depot precast deck pads, similar to an 18" sonotube. They don't even bother to remove the organic layer of soil. Frost heave and settlement aren't really an issue because it's a deck that his hinged to the house. Everyone is fine with this practice.

Now, I did get into this discussion with a residential contractor over whether or not that would be appropriate for a covered porch that tied into the roof. I said it required a foundation to frost depth, and he never spoke to me again... or paid me.

 

[r13]

Note that values listed in the code presumptive bearing table are very much rough, for that ignores the following:

(i) Effect of depth, width, shape and roughness of foundation have not been considered.
(ii) Effect Of angle of friction, cohesion, water-table, density etc., have not been considered.
(iii) Effect of eccentricity and indication of loads has not been considered.

For reason above, the code tends to adopt the lowest ultimate bearing strength for certain type of soil, and derive the universal allowable bearing strength with a large safety factor, so to be conservative.

Also, from a technical/practical point of view, the formula for bearing capacity is qu = 1.2 CNc + AγBNγ + γDNq, in which B is footing width, or diameter of round footing, D is depth, and A = 0.4 for square footing, 0.3 for circular footing. It is to be noted that while reduces the width decreases the capacity, but at the main time, the capacity increases with depth - that can be a wash, provides the large safety factor to be applied.

 

[Phil1934]

AWC deck guide gives minimums based on joist and beam spans

 

[Settingsun]

What's the reason for reducing allowable pressure for small footings? In case there's a (very) local bad spot? A smaller footing would give less settlement for the same pressure, all else equal.

 

[SteelPE]

Steveh49,

No particular reason other than every soils report limits bearing pressure to a minimum size footing. You go below those requirement and you need to take bearing reductions. I just find it odd that a all soils report have this footnote, but the code doesn't.

 

[r13]

That's why the presumptive bearing pressure is usually on the low side compared to the actual.

 

[phamENG]

I think jayrod summed up my point pretty well. Thanks.

As for the reduction for smaller footings, I've understood there to be a sort of balance point where smaller footprint puts you into a shear/strength failure in the soil and larger puts you into wider influence and greater settlement.

I would suspect that the reason the code doesn't really address this is because most buildings that rely on code minimums aren't all that big or they have lower performance expectations. At least at the professional level. Lots of homeowners expect their houses to be as solid as an immovable layer of - something immovable. Sadly most homes are built with the absolute minimums and, so, anything less than the "average" condition the code depends on and you have problems. They don't endanger the structure or cause life safety concerns, but cracked drywall is pretty annoying.