Because it has never been proven incorrect. A Rule of Thumb is much like a theory. Theories are only proven correct to the extent that they have yet to be shown to be incorrect. Maybe google the definition of “Rule of Thumb” and “Can theories be proven correct?”
Why should we “forget” an equation we are told to use?
We are not assuming a sub deck…Only a standard deck
Why are you stating we should ignore an equation that can guide us towards a safer deck??
From Wikipedia:
This rule of thumb determines a “safe” spacing…
So please show “why” it is OK?
Yes…very nice.
But this equation is used to determine a “safe” spacing…
Here is InterNACHI’s Rule of Thumb again:
Yes, I saw that before…
Also:
A deck with substantially fewer ledger fasteners than that recommended by InterNACHI’s formula may be unsafe.
Not exactly. It is determining a spacing in inches between ledger fasteners such that a deck with substantially fewer fasteners (substantially larger average spacing between fasteners) may be unsafe.
The Rule of Thumb does not determine how many ledger fasteners a deck should have. It determines a point where, if substantially crossed, could be cause for concern.
Nick, the rule of thumb at some point had to have been based on some assumed load and lateral thrust. Do you know what that was?
The challenge is the equivalent our strength of materials final exams which were eight pages of calculations for one problem. No offense but if you have an issue with the rule of thumb then just state it. The challenge is absurd and would tax the abilities of an engineer. It is so far out of the SOP for any inspector that there HAS to be a rule of thumb if you are going to include this in your inspection.
What point is that?
The point where the spacing between the actual ledger fasteners exceeds the spacing determined by the Rule of Thumb.
Yes what was that load?
The equation states “A deck with substantially fewer ledger fasteners than that recommended by InterNACHI’s formula may be unsafe”.
Well this does not take 8 pages…Can you show how this is true?
Why would this tax a PE? That doesn’t make sense.
You were not told to use that equation on a regular deck with a hot tub on it.
I told you to ignore the equation because you have changed the subject to “decks with hot tubs”.
Why are you not understanding the simple fact that hot tubs on decks are more involved than decks with no hot tubs?
Are you an inspector?
What is your background?
My advice to you is to always recommend a detailed inspection if you see a hot tub on a deck because you are not getting the basics. 
The answer is it doesn’t. It only addresses a minimum requirement. If you want to put a hot tub on a deck hire a SE to design it. If I ever saw a hot tub on a ledgered deck it would be called up for further evaluation. Ledgered decks are not built for that kind of load. I would be more worried about the hangers then the ledger!!
Agreed. I see little here that makes sense.
You didn’t just say that.
:D:D:lol::lol::?:](*,)
That was not the original question/challenge you posed.
The challenge you posed was to work backwards from the rule of thumb NACHI equation and prove it is correct using a different method than the equation itself.
To properly do that you would first have to know the materials being used:
- lumber sizes
- species of lumber
- rated deflection, shearing and tearout tolerances of the lumber
- type, size and grade of all fasteners
- rated strength of all fasteners in their different applications
- the static weight of all the materials needed to build the deck
Then you would have to decide on the:
- methods of construction of the deck
- the width, height and length of the deck used in the proof
- the assumed live and static loads the deck would have on average.
- the percentage of weight beyond the actual weight you want to build the deck to withstand
Once you had this information you could begin calculating how the loads would transfer through the example structure and this would give you weight which the ledger would actually carry. This number could be used to determine how many bolts needed and then comparisons could be made to the NACHI equation.
The point is, your challenge was already done by someone decades ago and they determined some general standards of deck construction which were used to make the NACHI equation. You would be reinventing the wheel with your challenge. All you really need to know is where did NACHI “borrow” the rule of thumb from, or what standards of construction were used to figure it.
I have not changed anything. It is a fact that Hot Tubs are placed on decks.
The facts are that an equation is being used to determine if the # of bolts (or bolt spacing) is SAFE. The notes indicate that anything less than what the equation specifies may be unsafe, therefore a “red flag” for the inspector. This is the point of the challenge…
If you did see one. Would you use the equation to check the bolt spacing (assuming it is bolted)? If so, can you explain “why” the equation results would be correct. That is the challenge. Nothing else is of concern for this challenge…