jar546
CBO
I am revisiting this as it gets a lot of views.
Does anyone have anything to add to this?
Does anyone have anything to add to this?
-
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Deck Pier/Footing Size and Tributary Loads
- Thread starter jar546
- Start date
georgia plans exam
Silver Member
http://www.dca.ga.gov/development/constructioncodes/programs/documents/2012IRC-DeckManual2014Amendments_001.pdf
This is an optional method in Georgia.
GPE
This is an optional method in Georgia.
GPE
Last edited by a moderator:
georgia plans exam
Silver Member
Oops! I don't think that link worked. Will try again tomorrow
GPE
GPE
This subject is being discussed by the group I am working with on deck codes. We did not propose a deck foundation table for the 2015 because we could not agree. It is the only part of a deck vertical load path that won't have a prescriptive design method in the 2015 IRC.
There are many areas where 10 and 12 inch round piers, 36 to 48 inches deep have been performing just fine, though undersized according to IRC methods. It is hard to suddenly begin requiring 16 inch round piers on backyard decks when 10's have been there for 20 years doing just fine. This is on areas with 1500 to 2000 PSF soils (soils reports)
On the contrary, regions without frost depth have no problem with wide footings, as they are only 12 inches deep.
Regarding your average deck, here's the deal...
Everyone shoots for a 2 to 3 bag hole, whether wide and shallow or narrow and deep. That's my observation.
Though skin friction is disregarded in the upper 10 or so feet for deep piers, perhaps shallow piers for decks can still generate side shear. From what I have studied on this topic, the lateral forces of a building impose the most lateral force at the top of the piers, which reduces the friction at the sides of the piers at the top...thus the discredit. On a typical upper level deck, there is no lateral system that extends to the top of the piers. Therefore loads on a deck may move the deck, but only rotate the top of the post about the pier below. For this reason, perhaps a shallow deck pier maintains friction against the adjacent soil and thus has some side shear value that helps support the pier. I've also read that the side shear will engage first, before the end bearing, thus taking some load off of the soil below.
But there's more. There is also a wonder if the tabular compressive strengths for spread footings in the IRC, designed for rigid structures like a home, are appropriate for more flexible structure like a deck. For example. A 40 foot long spread footer supporting a house cannot tolerate much differential movement across it's length. The slightest will result in cracking and noticeable serviceability issues in the house. A deck, however, is on isolated piers or footing pads. Slight differential movement between these do not create near the damage or issues as the former. There are safety factors included in the derivation for the compressive values. It has been questioned whether different, more liberal, safety factors could be used in decks.
8" diameter piers are even common for decks around here. Here's the deck handout from the Colorado Chapter ICC... "8" minimum" Why is that? Because they seem to have been working for decades.
This topic is definitely being discussed amongst the deck industry. I don't know what the best answer is yet. This is something still to be discovered.
Hey Jeff... You asked for it.
There are many areas where 10 and 12 inch round piers, 36 to 48 inches deep have been performing just fine, though undersized according to IRC methods. It is hard to suddenly begin requiring 16 inch round piers on backyard decks when 10's have been there for 20 years doing just fine. This is on areas with 1500 to 2000 PSF soils (soils reports)
On the contrary, regions without frost depth have no problem with wide footings, as they are only 12 inches deep.
Regarding your average deck, here's the deal...
Everyone shoots for a 2 to 3 bag hole, whether wide and shallow or narrow and deep. That's my observation.
Though skin friction is disregarded in the upper 10 or so feet for deep piers, perhaps shallow piers for decks can still generate side shear. From what I have studied on this topic, the lateral forces of a building impose the most lateral force at the top of the piers, which reduces the friction at the sides of the piers at the top...thus the discredit. On a typical upper level deck, there is no lateral system that extends to the top of the piers. Therefore loads on a deck may move the deck, but only rotate the top of the post about the pier below. For this reason, perhaps a shallow deck pier maintains friction against the adjacent soil and thus has some side shear value that helps support the pier. I've also read that the side shear will engage first, before the end bearing, thus taking some load off of the soil below.
But there's more. There is also a wonder if the tabular compressive strengths for spread footings in the IRC, designed for rigid structures like a home, are appropriate for more flexible structure like a deck. For example. A 40 foot long spread footer supporting a house cannot tolerate much differential movement across it's length. The slightest will result in cracking and noticeable serviceability issues in the house. A deck, however, is on isolated piers or footing pads. Slight differential movement between these do not create near the damage or issues as the former. There are safety factors included in the derivation for the compressive values. It has been questioned whether different, more liberal, safety factors could be used in decks.
8" diameter piers are even common for decks around here. Here's the deck handout from the Colorado Chapter ICC... "8" minimum" Why is that? Because they seem to have been working for decades.
This topic is definitely being discussed amongst the deck industry. I don't know what the best answer is yet. This is something still to be discovered.
Hey Jeff... You asked for it.
JBI
REGISTERED
I agree with Glenn (I think... :? ). The Residential Code's floors are rigid diaphragms and intended to be a part of a much larger system of components that resist a variety of lateral forces while supporting various loads.
Decks are not. They are more like large kites bolted to the exterior and subject to much different forces.
Every cycle the IRC moves closer to prescriptive deck design, be that good, bad or somewhere in between.
Though not generally a fan of big box retailers, the plans I've seen from their software seem to be conservative in design - 6 x 6 columns and 2 x 10 joists for a 4' deep, 6' foot wide entery deck for example. In my NYS jurisdiction that was a tad bit of an overdesign.
Decks are not. They are more like large kites bolted to the exterior and subject to much different forces.
Every cycle the IRC moves closer to prescriptive deck design, be that good, bad or somewhere in between.
Though not generally a fan of big box retailers, the plans I've seen from their software seem to be conservative in design - 6 x 6 columns and 2 x 10 joists for a 4' deep, 6' foot wide entery deck for example. In my NYS jurisdiction that was a tad bit of an overdesign.
jar546
CBO
The issue with soil bearing can never be a one or two size fits all due to the complexity and variation of soil types beyond bearing capacity and frost or no frost. I for one do not see any massive simplification changes of the codes in this area. The amount of solid data from all inclusive testing would be extremely expensive to attain. The situational variables are too numerous to even attempt to dumb this down for the profit of a niche industry. Engineers and architects for the most part seem to get it as their designs routinely exceed the prescriptive code requirements by a pretty good margin. It is the contractors who push the envelope in the opposite direction due to their ignorance of the codes and simple math.
I see this as really simple math based on soil conditions and soil bearing capacity. What has "always worked" in the past is nonsense because it is not scientific. Much of this is based on the fact that the structure is still standing. No deflection or elevation data during, and post construction was ever recorded over a period of years. In many areas of our coverage covered decks have obvious defects 5, 10 and 15 years later with stretched roof flashing, open "v" gaps at the point of attachment and deck floors that are wavy due to sinking of piers. In all of those situations, improperly sized footings and piers were to blame. The soils in many areas of coal country is loaded with old coal ashes that were dumped in back yards for over a hundred years and worked their way into the soil. We saw this same issue outside of the coal region too in better soil conditions.
Until a micro-niche, for profit industry is successful is changing basic mathematical, time proven physics in order to change the code, we will continue to use common sense and prescriptive methods as enforcement. Life will go on for all of the successful, profitable contractors who have a grasp and understanding of soils and bearing capacity and have been designing and installing deck piers this way for years.
I see this as really simple math based on soil conditions and soil bearing capacity. What has "always worked" in the past is nonsense because it is not scientific. Much of this is based on the fact that the structure is still standing. No deflection or elevation data during, and post construction was ever recorded over a period of years. In many areas of our coverage covered decks have obvious defects 5, 10 and 15 years later with stretched roof flashing, open "v" gaps at the point of attachment and deck floors that are wavy due to sinking of piers. In all of those situations, improperly sized footings and piers were to blame. The soils in many areas of coal country is loaded with old coal ashes that were dumped in back yards for over a hundred years and worked their way into the soil. We saw this same issue outside of the coal region too in better soil conditions.
Until a micro-niche, for profit industry is successful is changing basic mathematical, time proven physics in order to change the code, we will continue to use common sense and prescriptive methods as enforcement. Life will go on for all of the successful, profitable contractors who have a grasp and understanding of soils and bearing capacity and have been designing and installing deck piers this way for years.
JBI
REGISTERED
"What has "always worked" in the past is nonsense because it is not scientific."
That is actually the basis for most of the masonry provisions in the code... 'Empirical' design.
Other than that I agree with you Jeff.
That is actually the basis for most of the masonry provisions in the code... 'Empirical' design.
Other than that I agree with you Jeff.
mjesse
REGISTERED
I have to disagree with you here. Empirical methods of design DO work and are often retroactively verified.jar546 said:
Joist spacing, joist spans, rafter tables, etc. have all "evolved" from the tried and true of our great-great-grandfathers.
Just because grand-dad couldn't multiply the modulus of elasticity by the moment of inertia doesn't mean his knowledge was nonsense.
{JBI beat me to the punch}
jar546
CBO
JBI said:
Both of you are taking one part of a statement without taking the explanation into consideration for reasoning. That is not a blanket statement, it was specific to deck piers and came with an explanation. Just because it is still standing does not mean there are not defects and movement, deflection. Taking this to lumber and grand-dads is going out of context to the initial intention.mjesse said:
mjesse
REGISTERED
Fair enough.jar546 said:
Engineering and science are key to good design, no dispute.
There are folks with local experience can look at an excavation and tell you if the soils are good for 1500 or 3000 psi no test required. They've been digging the same dirt for a long time.
As Glenn says, hard soils technically require a larger footing than history proves is necessary. The first time I had an architect spec 24" diameter piers for a ground level deck, I was floored! I and others had built untold numbers of decks with 8" or 10" piers in same area over many years. Why the giant increase? Someone finally took the time to do the math.
Should this cause immediate concern that all the decks on 8" piers are danger? Probably not. Are they subject to undesirable movement? Maybe. Should they all be discounted as defective? No.
As you've said, this niche industry shall continue to use common sense and prescriptive design, and life will go on.
Jeff you seem to be going off about the evil contractor just wanting to make an easy buck. I said nothing about that. I spoke quite a lot of science and research in my post, but you state I am trying change physics? Uh...no, I was quite exactly talking about science and physics. You are also speaking for roof assemblies, which is a different animal entirely from a "deck" (the subject of the thread).jar546 said:
I simply share with you the facts about my region. The decks aren't settling. I would prefer no one takes that as an attack at how things are in their region, nor attack mine. The link I provided with 8" piers is not from a group of greedy contractors, it's from the state's very recognized ICC chapter of building officials. The owners of the properties are the one's that don't want a 24 x 24 block of concrete in their planter boxes when for the last 20 years, they've had a 10" round pier. Sensitivity to that perception must be recognized, especially when you are trying to explain "why" it has to change suddenly...and can't.
Again... I was very much speaking about science and engineering, and yet you did not comment on any of it. (skin friction, safety factors, when compressive strength engages....all science...all important). Please stop thinking of me as some enemy just out to help sloppy contractors make a quick buck. I think the nature of my post deserves a little more credit than that.
I love the discussion, though.
This is not true for my region by any stretch. I regularly see engineered plans for decks come across my desk with piers undersized when using end bearing calcs only. No evil contractors or profit driven devils anywhere to be seen.jar546 said:
Here is a photo I took this week and recently shared with the deck study group. I will share it with you all too. I would love some scientific talk.
In this photo, the above document is the original soils report from the 90's. A side shear value of 500 psf is provided for deep drilled piers, but the upper portion is discredited.
The document below is the design criteria for the newly engineered deck. The current engineer provides end bearing of 1500 psf, but a side shear of 2,000 psf for a 36" deep pier. The end bearing alone did not calc out when I checked it. This is from a licensed professional engineer, and this is not the first time I have seen this. 36" deep piers that engineers are accounting for side shear. These piers are 12" diameter (undersized when only looking at end bearing).
The comfort level with narrow shallow piers is across all kinds of professionals in my region. I suppose it's because there is no failure of them.
jar546
CBO
My apologies if I seemed a bit strong. I too am being factual. There are simply too many variables in this situation and only the local officials and contractors know their soils. If it is working in Colorado then great, just don't think this needs to be a future code change on a global scale. As far as the issue with building a roof over a deck, it happens all the time and is certainly an issue, especially when 9 times out of 10 they think the existing deck piers are adequate for the new roof and snow load. Concerning skin friction, safety factors, etc…… I can't comment on something that I need to do research on. Yet.Glenn said:Jeff you seem to be going off about the evil contractor just wanting to make an easy buck. I said nothing about that. I spoke quite a lot of science and research in my post, but you state I am trying change physics? Uh...no, I was quite exactly talking about science and physics. You are also speaking for roof assemblies, which is a different animal entirely from a "deck" (the subject of the thread).I simply share with you the facts about my region. The decks aren't settling. I would prefer no one takes that as an attack at how things are in their region, nor attack mine. The link I provided with 8" piers is not from a group of greedy contractors, it's from the state's very recognized ICC chapter of building officials. The owners of the properties are the one's that don't want a 24 x 24 block of concrete in their planter boxes when for the last 20 years, they've had a 10" round pier. Sensitivity to that perception must be recognized, especially when you are trying to explain "why" it has to change suddenly...and can't.
Again... I was very much speaking about science and engineering, and yet you did not comment on any of it. (skin friction, safety factors, when compressive strength engages....all science...all important). Please stop thinking of me as some enemy just out to help sloppy contractors make a quick buck. I think the nature of my post deserves a little more credit than that.
I love the discussion, though.
I completely agree...folks get real surprised when they realize that changing their deck into a covered deck or sun room changes things big time.jar546 said:
I may have not been clear in my intentions. I DO NOT think there is enough data to make a prescriptive table specifically for deck foundation in the IRC. I DO NOT think the DCA 6 table is appropriate for all areas and all foundation types. I was one of the leading voices to STOP the table proposed for the 2015 IRC. It proposed to make all decks have a minimum 15 inch diameter pier/footing. My involvement in the deck group and my comments in this post are the argument for why it should NOT be made prescriptive and apply to decks following the current IRC methodology for IRC footings. In absence of a specific deck foundation table, jurisdictions can continue to use empirical experience and judgement and curious minds can continue to find more information and research.
That said, forces continue to work to put the DCA6 type of table with large diameter piers into the 2018 IRC. That is exactly why I ask "why" is it that narrow, shallow piers seem to work in many regions.
We're all good. I don't doubt your local experiences at all. I found the coal ash comments to be quite interesting.
jar546
CBO
Concerning the coal ash. Along with garbage pickup was coal ash pickup but then it went away and people needed somewhere to put it so gardens, driveways, yards and just about everywhere you can think became a dumping ground. (the tomatoes are excellent from this area, best in the world). On many projects soils engineers had to be called in due to the amount of coal ash uncovered, even in residential settings. In our area, even with pre construction boring and testing, the soils eng. companies follow the excavation and do testing as the footer is being prepped. More often than not, the values change and the engineers have to modify the original design due to changing conditions.
When you get up into the mountain areas where there is more rock and shale, we have less issues. Just an FYI
When you get up into the mountain areas where there is more rock and shale, we have less issues. Just an FYI