Deck Piers, yet again

[jar546]

OK, another thread hit my hot button so I wanted to expand on it and bring it to the surface again.

The sizing of deck piers and adding a roof on it. This is a major pet peeve because I use to get grief all the time because we would reject plans based on lack of adequate footing of the existing deck and would always hear: "I've built them all over the county like that and you are the first one to deny me"

Let us dive in.

Let's say for simple math we have a 10'x10' deck attached to a house with two piers, one on each corner. A very simple setup. Break it down:

10x10=100 square feet, 50% of the load is carried by the house and the other half is split up between the two posts (assuming it is framed that way). We have to charge a 10# sq/ft for dead load plus a design load of 40# sq/ft. That sounds like 50# sq ft to me. 50lbs x 50sq' is 2500 pounds.

Without soil testing or confirmed that we are on solid rock, the code has us assume a 1500psf soil bearing capacity. Here is why I love math.

Each of the posts much be capable of supporting 1,250 pounds by design. A 12" sonotube is only 113 sq/inches so on a 1500 soil, that is only 1,170 pounds of bearing capacity. Even the DCA6 starts with an 18" sonotube or 16x16 x 7" thick footer.

My point is this. Most decks have barely enough bearing capacity to support their own design load, let alone adding a roof on it with a snow load and dead load.

 

[steveray]

Agreed....We typically use 2000#...but it is still tough....Same thing with 24x24 piers for the house main beam these days....

 

[classicT]

This is why I run beam calculations on 90% of the decks that I review. We use the beam reaction to size footing.

(Beam reaction) / 1500psf = minimum area
Sq root of (Minimum area) x 12 = minimum dimension in inches for footing width

Footing depth for plain concrete = [ (footing width) - (post width) ] / 2

 

[Glenn]

This link has the proposals for the 2021 IRC. Proposal RB184-19 includes expanded tables for all deck framing components up to 50, 60, 70 psf snow loads, including footings. The footings and posts tables are based directly on tributary area supported by each post or footing/pier. Sizes accommodate a minimum 8" diameter pier for something as small as the corner of an intermediate stair landing, all the way up to enormous piers for large areas. You could use these easily for decks with porch roofs because the table goes up so large in trib area. Simply add the area of the roof supported with the area of the deck supported by each post and pier. Then use the correct snow load table.

I will be at the hearings speaking for this proposal and other deck related proposals. Ask me any questions you have. Thanks.

http://media.iccsafe.org/code-development/group-b/IRC-B-compressed.pdf

 

[TheCommish]

Decks are one the most dangerous structures built, on Saturday or Sunday by people
armed with little knowledge, armed with advice from the Big Box Store that generate
pretty pictures and a stock list, or firmed by the experts in a can, ..Bud and Wiser.

Decks are used daily by a small number of people at low load condition, collapse
when the wedding, birthday or graduation party shows up, the participants do the hoky
poky and edged on by their friends Bud and Wiser, and the deck falls down.

 

[rktect 1]

You forgot the weight of the deck or maybe the weight of the roof. But you forgot one of them.

Deck is 10# plus 40# = 50#/sf
Roof is 10#/15# plus snow load around here is 30# = 40#/sf

You need an architect for this. It is not in the prescriptive code.

 

[classicT]

You forgot the weight of the deck or maybe the weight of the roof. But you forgot one of them.

Deck is 10# plus 40# = 50#/sf
Roof is 10#/15# plus snow load around here is 30# = 40#/sf

You need an architect for this. It is not in the prescriptive code.

Who forgot the dead load?

And no architect is necessary. AHJ's arbitrarily requiring design professionals as a scapegoat for their own lack of knowledge is one of the biggest plagues across building departments. The basic math provided above can be done by a grade school student, so why not a homeowner; why not a plans examiner?

 

[jar546]

You forgot the weight of the deck or maybe the weight of the roof. But you forgot one of them.

Deck is 10# plus 40# = 50#/sf
Roof is 10#/15# plus snow load around here is 30# = 40#/sf

You need an architect for this. It is not in the prescriptive code.

Who forgot?

 

[Glenn]

Who forgot the dead load?

And no architect is necessary. AHJ's arbitrarily requiring design professionals as a scapegoat for their own lack of knowledge is one of the biggest plagues across building departments. The basic math provided above can be done by a grade school student, so why not a homeowner; why not a plans examiner?

This is why I am very happy to see "tributary area" being used in the IRC for the first time for prescriptive design. (2018 IRC deck footing table). The proposal I linked to above provides more tables for deck design that use tributary area. Using that variable, prescriptive design tables, such as for decks, can be much more flexible.

 

[Mr. Inspector]

I always used the AWC Prescriptive Residential Wood Deck Construction Guide footing table. It assumes a 1,500 psf soil bearing and where I work is a rocky area where I the soil bearing capacity is much higher so I always let them build a roof on this size pier.

(Beam reaction) / 1500psf = minimum area
Sq root of (Minimum area) x 12 = minimum dimension in inches for footing width

Footing depth for plain concrete = [ (footing width) - (post width) ] / 2

And no architect is necessary. AHJ's arbitrarily requiring design professionals as a scapegoat for their own lack of knowledge is one of the biggest plagues across building departments.

Where does this calculation come from? I am a plan reviewer but never saw it before. I always required the designer to do the calculations.

 

[classicT]

I always used the AWC Prescriptive Residential Wood Deck Construction Guide footing table. It assumes a 1,500 psf soil bearing and where I work is a rocky area where I the soil bearing capacity is much higher so I always let them build a roof on this size pier.




Where does this calculation come from? I am a plan reviewer but never saw it before. I always required the designer to do the calculations.

I suppose you wont find it in the Codes, but it is the basic math for taking applied load from beam and sizing the footing for 1500psf bearing capacity.

For those that are unfamiliar with these calculations, are you reviewing engineered designs? Or do they get the rubber stamp automatically?

In all seriousness, I reject probably 1/3 of the engineered plans I see for some mistake or another. Engineers/architects are not saints that are immune from mistakes.

 

[rktect 1]

Who forgot the dead load?

And no architect is necessary. AHJ's arbitrarily requiring design professionals as a scapegoat for their own lack of knowledge is one of the biggest plagues across building departments. The basic math provided above can be done by a grade school student, so why not a homeowner; why not a plans examiner?

Because, we at the city level, are not here to do your work for you nor take the responsibility or liability for possible mistakes.

Hire a design professional for the work you can not do legally per your own states regulations and/or the prescriptive code.

 

[rktect 1]

In all seriousness, I reject probably 1/3 of the engineered plans I see for some mistake or another. Engineers/architects are not saints that are immune from mistakes.

You have no idea how many architects I have corrected for beam and column sizes in my 11 years at this. But I don't design the beam or column for them just because I can. I explain to them that something isn't correct with the loads and point them into the correct direction. A couple fo times I have had them call me and thank me for it but usually not.

DIY people, I point them into the correct direction as well. Licensed design professionals.

 

[classicT]

Because, we at the city level, are not here to do your work for you nor take the responsibility or liability for possible mistakes.

Hire a design professional for the work you can not do legally per your own states regulations and/or the prescriptive code.

You must work in a department where customer service is not a desire of your City Management/Council.

For me, I have the knowledge and am willing to help folks figure something out without blanket requiring a design professional. I'm willing to bet I can get better code compliance with this method than you do with blanket design professional requirements. You catch more flies with honey than vinegar.

BTW, still interested in who missed the dead load.

 

[SCBO1]

I don't have any issues with the big box stores pretty pictures, material list, cut list, lay-outs and elevations. I still pull out the AWC and review the plans and check my little beam buddy calculations.

I do have issues with ink pen drawings and partially filled out applications and the dude wanting a pier inspection because its going to rain and he just submitted the half finished drawing on notebook paper.

At the front counter last week:

Me behind the bullet proof glass: Aw... "What size floor joist?" What do you mean? "What's the width?", "it's not on your notebook paper, 2x8?" Are you using joist hangers? Ya... I always use joist hangers!... can I use screws in the hangers? Man, your being hard to get along with, why so many Q's dude! ...I build decks all over and nobody asked all these Q's. Just wanting to make sure it's safe! You might want to have a big box store help you with your project or the local lumberyard? So I'm not going to get a permit? I can't design it for ya!..oh.. I was just told you need a Business License and proof of workers comp... and the Police Department want's to know if that's your rusty red Ford parked in the handicap spot?I gotta go....

 

[ADAguy]

You could do a TV show with comments like that (smiling). Sounds like a Bob Villa or Tim Allen monologue.

 

[jar546]

I always used the AWC Prescriptive Residential Wood Deck Construction Guide footing table. It assumes a 1,500 psf soil bearing and where I work is a rocky area where I the soil bearing capacity is much higher so I always let them build a roof on this size pier.




Where does this calculation come from? I am a plan reviewer but never saw it before. I always required the designer to do the calculations.

Without soil analysis, you have to assume the 1500psf by code. I am familiar with most of NEPA and have reviewed soil analysis from many areas considered "rocky" whatever that means and in many instances they were required to bring in and compact engineered soil just to meet a minimum design criteria of 2000psf.

 

[steveray]

Without soil analysis, you have to assume the 1500psf by code. I am familiar with most of NEPA and have reviewed soil analysis from many areas considered "rocky" whatever that means and in many instances they were required to bring in and compact engineered soil just to meet a minimum design criteria of 2000psf.

No....
R401.4.1 Geotechnical evaluation. In lieu of a complete
geotechnical evaluation, the load-bearing values in Table
R401.4.1 shall be assumed.

It does not say that you have to use the minimum.....Just the table...

 

[classicT]

No....
R401.4.1 Geotechnical evaluation. In lieu of a complete
geotechnical evaluation, the load-bearing values in Table
R401.4.1 shall be assumed.

It does not say that you have to use the minimum.....Just the table...

I think what Jeff was getting towards is that most jurisdictions have variable soils and are unwilling to take the liability of determining or even approving a soil type without some documentation. Minimum documentation would be a soils analysis including a sieve analysis report (particle grain size analysis).

 

[Keystone]

We assume 1500psf for all, despite 1500psf being complete garbage soil, soils report accepted but we have yet to see one provided for deck piers. Although with the 2015 IRC being newly adopted we are seeing them for house footings.

The vast majority of our plan rejections for decks, inadequate footing size.
The second most common issue is beam size.

After all this time I’ve had my first techno post installation and engineer report this year, such a simple system if you do not live in a new development that requires all types of underground drainage systems.

Decks with roofs, at first we required engineering or the deck built with its pier system and the roof built with its own pier system, skeleton. The skeleton system works great for applicants wanting to place a roof over an existing deck. Now we will run the calls but it’s up to the applicant to provide accurate details.

Also so other don’t misunderstand, A soil that sieves at a particulate size does not mean it achieves a particular PSF. Can it be used as an indicator toward that likelihood yes but either way this is outside most inspectors knowledge and even still would require a PE to provide the final details of its acceptance to achieve that PSF.

 

[tmurray]

We started with deck blocks. The decks heaved with the frost, then fell off the houses when the fasteners withdrew from the ledger boards.

Then we moved onto sonotubes. 8" diameter concrete piles that went down below the frost. The decks heaved again, but this time is was frost jacking against the pile. Then they dropped. Dropped further than they installed it because the bearing pressure of the soil was exceeded. Again, the decks fell off the houses when the fasteners withdrew from the ledger boards.

Now, we use footings. Sized the same as if it were a part of the house. The enlarged footing area prevents frost jacking of the sonotube (10" diameter now) and provides sufficient area so that the bearing pressure of the soil is not exceeded. You want to put a roof on it? Great. It is already sized for it.

 

[steveray]

We use 2000# as a default typically....Nothing wrong with a reasonably sized deck on 10" piers....Never heard of a failure around here for this non-issue....Rooms on piers are an entirely different animal, but we already have a code section for that...

R602.10.9 Braced wall panel support. Braced wall panel
support shall be provided as follows:
1. Cantilevered floor joists complying with Section
R502.3.3 shall be permitted to support braced wall
panels.
2. Raised floor system post or pier foundations supporting
braced wall panels shall be designed in
accordance with accepted engineering practice.

 

[Mr. Inspector]

I see deck blocks around here but I don't inspect them. In PA residential decks less than 30" above grade are except from a permit (except if part of a means of egress, which is the front door) even if attached to a house and the manufacturers instructions for the blocks are that they can only be used for a deck less than 30" above grade.

 

[SCBO1]

The vast majority of our plan rejections for decks, inadequate footing size.
The second most common issue is beam size.

I would add to your list here:
3) No handrail
4) No flashing
5) No risers to prevent a 4-inch sphere from passing through
6) Stair issues, over cut stringer, rise and step width measurement issues
7) Wrong brand of beer at the job site.