Is there a formal definition of cantilever

[cyberwombat]

I am working on a deck design and code prohibits attaching deck ledgers to the rim joists of cantilevered floors. The house is a pier and beam with the piers center 12" from edge meaning roughly 10" of the house stick out past the bracket on top of pier.

Is there somewhere that defines a distance at which the house is cantilevered and therefore I cannot use a deck ledger board? I am on IRC 2018

 

[Yankee Chronicler]

What code section are you looking at?

 

[cyberwombat]

Mostly articles:
- https://www.decks.com/how-to/articles/deck-joist-cantilever-rules-and-limits (under "deck ledger attachment")
- https://www.jlconline.com/deck-builder/attaching-a-deck-to-cantilevered-joists_o
The latter says "Here’s what the 2015 IRC (R507.2.2) says: “Band joists attached by a ledger in accordance with Section R507.2 shall be fully supported by a wall or sill plate below.” Sure, it doesn’t specifically say you can’t connect a ledger to a cantilevered rim joist, but that’s what it means, since a cantilevered rim joist by nature is not supported by a wall or sill plate. And this isn’t something that was added to the building code for no reason."

So it's a loose description and I dont consider this house to be cantilevered though I suppose it is if it sticks out a few inches. It can clearly hold the deck. Not sure if this is going to be something I just need to get approved or if there is actually a definition

 

[Yankee Chronicler]

Okay, in the 2021 IRC that section R507.9.1.2:

R507.9.1.2 Band joist details. Band joists supporting a ledger
shall be a minimum 2-inch-nominal (51 mm), solid-sawn,
spruce-pine-fir or better lumber or a minimum 1-inch (25 mm)
nominal engineered wood rim boards in accordance with
Section R502.1.7. Band joists shall bear fully on the primary
structure capable of supporting all required loads.

I think it means exactly what it says: the band (rim) joist must bear firmly on the primary structure (the foundation).

Why? Because what typically attaches a rim joist to the ends of the floor joists? A couple of nails. The wall above is either 2x4s or 2x6s, so the weight is carried b not only by the band joist but also by the floor joists. But when you attach a deck ledger to the band joist, NONE of that vertical load is transferred to the floor joists, it's all vertical and it puts the nails holding the band joist to the floor joists into sheer -- far more than they were ever designed to carry.

If you need to attach a deck to a cantilevered floor, the code always allows for an engineered solution. That means hiring a structural engineer to design a system that will transfer the vertical loads from the deck ledger back through the cantilever floor structure to the foundation wall.

 

[Yankee Chronicler]

Time out -- I just took a closer look at your image. You don't have a cantilevered band joist. A band joist rests on a continuous foundation wall. You have three girders that are cantivered out beyond their support piers. I doubt that you have a single 2x band joist -- either the floor joists run perpendicular to the three main carrying girders, or there's a 2- or 3-ply header beam under the wall with the patio door in it.

What you have is either a load-carrying header, or a single- or multi-ply end joist.

 

[cyberwombat]

Yes you are right. All makes sense now. yes that's a 4 ply beam. I'll go ahead and slap my deck against it. Thanks for clarifying!

 

[Mark K]

This is a non issue for an engineer who can show how the loads are transferred

 

[bill1952]

This is a non issue for an engineer who can show how the loads are transferred

Certainly true. How many folks here not employed in design and construction have tried to hire an engineer for small residential work? Just about impossible.

 

[steveray]

Yes you are right. All makes sense now. yes that's a 4 ply beam. I'll go ahead and slap my deck against it. Thanks for clarifying!

Still not really covered by the IRC......But not prohibited necessarily either....

 

[Yankee Chronicler]

Still not really covered by the IRC......But not prohibited necessarily either....

I submit that it is covered:

R301.1.3 Engineered design. Where a building of otherwise
conventional construction contains structural elements exceeding
the limits of Section R301 or otherwise not conforming to
this code, these elements shall be designed in accordance with
accepted engineering practice. The extent of such design need
only demonstrate compliance of nonconventional elements
with other applicable provisions and shall be compatible with
the performance of the conventional framed system. Engineered
design in accordance with the International Building
Code is permitted for buildings and structures, and parts
thereof, included in the scope of this code.

This is from the ICC 2021 IRC. My state tinkered with that section when we adopted it, but it's still there. The actual ICC language does not require a licensed PE, it just says the design must be performed according to accepted engineering practice. For a house, the owner or any architect or building who can look up how to add up loads and calculate a beam can do the design. The BO is then faced with how to review it. I would ask for the calculations to be submitted, and just check to verify that the calculations appear to be reasonable.

I take note that the IRC Commentary says these elements should be designed by a design professional -- but the code doesn't say that, and the Commentary is not enforceable.

 

[bill1952]

accepted engineering practice

I thought most interpreted that as requiring a registered design professional? I didn't know a non- registered design pro could design to accepted engineering practice.

 

[Mark K]

Certainly true. How many folks here not employed in design and construction have tried to hire an engineer for small residential work? Just about impossible.

Is not impossible. Many years of experience has shown that there are inevitably engineers who will undertake these projects. In addition architects have some training about engineering and should be able to undertake such a task.

 

[bill1952]

Is not impossible. Many years of experience has shown that there are inevitably engineers who will undertake these projects. In addition architects have some training about engineering and should be able to undertake such a task.

I contacted every one in county - no interest. Even at very high rates, there's no profit in little projects, plus everyone I know on building design is busy as hell.

 

[Mr. Inspector]

We get a lot of registered design professional designing decks where they are putting a hot tub or sunroom on the deck or 2 story decks all the time. Just got one for a deck supported by a house on piers.

 

[Yankee Chronicler]

I thought most interpreted that as requiring a registered design professional? I didn't know a non- registered design pro could design to accepted engineering practice.

It depends on the jurisdiction. In my state, anything requiring engineering MUST be performed by a professional engineer licensed by this state AND practicing within the realm of his training and experience. That means technically a licensed mechanical engineer can't do electrical engineering, and a chemical engineer can't design building structures. (In practice, there are a LOT of PEs in this state who practice outside of their specialty, and many who sell their seal to anyone who's willing to pay for it.)

So a structural engineer licensed in California could certainly design a structure for my state "in accordance with accepted engineering practice," but as far as my state is concerned he's not licensed here so he's not an engineer. But in my state single-family residences of any size do not require a licensed design professional, so if someone submits a sheaf of calculations that check all the boxes for "accepted engineering practice," maybe we'll accept it even if the person isn't a licensed structural engineer.