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Amount of wood beneath the joist pocket

Started by Planeiron, October 02, 2024, 04:19:12 PM

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Planeiron

Can anyone point to a calculation to make for the amount of wood needed beneath the joist pocket. I've calculated joist sizes based on span, dead plus live load and worked out deflection, bending and shear. Also used the rules of thumb for sizing the joist that Jim posted before. Just would like to know how to work out what that depth of wood beneath the bearing face of joist pocket should be or more precisely, how to work out the minimum safe amount required (D in my drawing showing a long sill and 2 of 3 brick bearing piers). Do the proportional rules of thumb only work down to a certain size? I suppose in the vast majority of real world cases this works out but just wondered. Thanks

Jim_Rogers

When calculating the size of a floor joist you need to understand the load it will be carrying.
This is done by measuring the unsupported span from timber to timber. (In this case it looks like these timbers are sills.)
Once you know the span you have to decide on the spacing from joist to joist.
Now that you know the span, the spacing and the load you can use the online calculator in the red toolbox here on this forum created by DonP. You'll also have to enter the type of wood that you want to use. If the size of joist you have selected based on the rules of thumb and size of the sill, will not pass, then you'll need to change something. The easiest to change is the spacing. Move the joists closer together to carry less load. Until the timber passes.
Another way to get the size or spacing to work is to change the type of wood to a stronger one.
Now that you have the size of the joist calculated, you can use the rule of thumb to shape the end of the joist, to eliminate the point of fracture by cutting the scoop out of the bottom of the end of the joist.
This tells you the depth of the drop in joist pocket in the sill, vertically. Next you need to figure the depth into the sill for the pocket to support the load.
To calculate the amount of surface the joist end needs to support the load you need to understand the strength of the wood perpendicular to the grain, per square inch, of the joist. And if that wood is different than the sill, you'll need to know both the sill's strength and the joist's strength. And use the smaller of the two values to calculate the size.
To calculate the depth of the pocket into the timber, I have used a spreadsheet calculator.
Here is how it works:

Width of Tributary Area (in) =132.0
Unsupported span in inches136.0
SQUARE INCHES OF AREA17952
SQUARE FEET OF AREA124.67
LOAD PER SQUARE FOOT50.0
LOAD ON TIMBER IN LBS6233.33
LOAD ON ONE END IN LBS3116.67
COMPRESSION PER TO GRAIN350
SQ INCHES NEED TO SUPPORT LOAD8.90
WIDTH OF JOIST4
DEPTH OF POCKET NEEDED2.5
SQ INCHES OF SUPPORT10
TIMES COMPRESSION PER3500

Here we see that the last line gives us the total number of pounds the drop in joist pocket will support. Which is higher than the "load on one end in pounds".
This proves that the 2 1/2" deep (into the sill) pocket 4" wide will support the load.

Next to answer your question. How much wood do you need under the pocket to be sure that there is enough?

All I can say is that years ago, at a timber framing national conference the engineering council did some live testing of this exact question.
They asked members to bring in some "sample sills" and floor joists. The sills and floor joists were put into a testing machine. The sills were only about 4' apart. And they tested all kinds of woods, joists, and sizes of drop in pockets.
In every test that I watched, the joist failed, as they pulled down on the joist long before the sills every did. Some sills cracked but no wood every blew out below the drop in pocket.
I would have to contact the council to see if they ever published a conclusion on these tests, as I just don't remember.
Personally, I just use the rules of thumb and size my joists and ends based on that.
If you have further questions, you may need to contact an experienced timber frame engineer to review your design and help you.
None of my frame designs that have been reviewed by my engineer have ever had these drop in pockets questioned or altered.
Good luck with your frame design.
Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Planeiron

Thanks for running through those steps. I'm sure that my joists will be fine and the sills too but I always wondered about that actual depth of wood beneath the pocket.

Your spreadsheet is helpful but could you clarify what " COMPRESSION PER TO GRAIN" is? Is that  your compression perpendicular to grain value of the wood you've used in the calculation?

It's definitely my lack of understanding but it's that the unit is quoted as a force per square inch i.e. an area rather than a volume, that I could never quite understand. For example it seems you could have a sufficient area that would on paper be fine to cover the force required but it could be wafer thin. Obviously that's daft and no-one would do that. I just always wondered about the lack of a value in force per cubic area or volume. Thanks 

Sod saw

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Jim,    That's a nice explanation.  Thanks for the time to put it together.

I have often wondered how those old (and new) barns stayed up.


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LT 40 hyd.          Solar Kiln.          Misc necessary toys.
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It's extremely easy to make things complicated, but very difficult to keep things simple.
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Jim_Rogers

In the NDS (National Design Specification) book there are strength values for normally used species of wood used in building.
Here is an example from the book:


For use in timber framing, we search the book for chart 4D which as values for timbers 5"x5" and larger.

Next, we have to understand that the which values to use.
They (the authors of the book) have created two categories. You see that in the picture. One is "Beams and Stringers" and the other is "Posts and Timbers".

So, how do you know which ones to use? The difference is that they assume that if a timber is more than 2" deeper than it is wide it is going to be use as a horizonal timber. And in case no one every told you so, the word beam means horizontal timber. There is no such thing as a vertical beam.

The spreadsheet I use has an "IF" calculation in it. And if the dimension is more than 2" greater than the width it automatically selects the "Beams and Stringers" values from the chart in the second page workbook of the spreadsheet.

I got this spreadsheet beam sizing calculator from the school when I took an engineering course there.
And in order to use this spreadsheet, I had to buy a copy of the NDS to look up the values and enter them by hand into the spreadsheet.

A fellow student of that class went home and entered the entire chart "4D" into the spreadsheet and put it a function to look up the values and automatically insert them into the formulas in the spreadsheet. Thus, saving this extra step. A great time saver for sure.

DonP's calculator here in the red toolbox does the same thing using the pull-down arrow to select the type of wood and its grade.

If your drop in joist pocket is not large enough to support the load in pounds vertically, then the fibers in the timbers will crush until they support the load. If they can't support the load the joist may break. Or deflect beyond acceptable levels.
You wouldn't want to put in a floor joist knowing that it's going to sag an inch under the standard intended load. So, we also check for the amount of "deflection" it will have. And try to keep it a minimum. 

I'm not sure if I have answered your question or not.
Again, keep asking questions, and I'll try my best to answer them.
Good luck with your design.
Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

doc henderson

You are a wealth of information Jim. :thumbsup:
Timber king 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.  2007 Chevy 3500HD dually, home built log splitter 18 horse 28 gpm with 5 inch cylinder and 32 inch split range with conveyor powered by a 12 volt tarp motor

Jim_Rogers

Recently, while at the national conference of the Timber Framers Guild, I had two opportunities to have dinner with an engineer. 
During these conversations, I questioned the engineer about the amount of wood left below the drop in joist pocket.
And I asked if the Timber Framers Engineering Council ever published the results of the "joint busting" tests we did when we pulled down a floor joist to see which would fail first.
And they said that there were not enough tests done to get a good "rule of thumb" from the results. 
Both stated at the current "rule of thumb" for the depth below the drop in joist pocket should be at least 3".
I hope this helps.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

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