Anyone familiar with the 4/7 Rule to support objects?

[ri702bill 8,322]

All:

I thought about this earlier placing deck boards off my garage floor as I unloaded the truck today.....

 

While working back in the 1980's as a Machine Builder, it was a common occurance to stage or store large components on the concrete floor prior to assembly.

My Supervisor worked for years in the trade, and required us to do the 4/7ths calculation when placing large flat objects on short 4x4's covered with protective paper. He stated that you needed to take the length of the object, divide it by 7 - take that number and multiply it by 4. That resultant number would be the "correct" centering  for the optimum wooden block spacing so as to negate the sag in the middle and the cantilever tendencies at the ends.... They would supposedly cancel each other out.... He would come by and God help you if they were placed wrong! Same goes for dimensional lumber I suppose....

Have any of you folks heard of this and used it?? @peter lena, @Pullstart, @Handy Don  @formariz??

Edited May 19, 2023 by ri702bill

[ebinmaine 67,506]

I've heard of that somewhere way back in the past. 

 

Far as using it though... I've always worked for places that believed in doing many things a little overboard and hopefully to a good cause.  

 

We'd have just placed 2 or 3 extra planks instead of two. 

 

 

[Ed Kennell 38,178]

    I had to use my pizza to figure this out.

 

[Handy Don 12,232]

New one to me as a formal calculation.

In the factory where I worked summers and when placing support for large flat or long symmetrical items with evenly distributed weight, we’d follow @ebinmaine’s system -- LOTS of blocking.

Of note, however, someone had transit-checked the storage building floor and there were a couple of high/low spots that had yellow paint stripes. Could only put palleted materials in those spots!

 

But you got me curious, Bill, so I went to my favorite beam calculator to test this out.

I put an evenly distributed weight over a constant cross section beam and used 29,000 ksi for the Young's Modulus (i.e. medium steel)  and put supports at 3’ and 11’ from one end. Pretty even deflection.

Tried it at 2’ and 12’ (more middle sag) and then again at 4’ and 10’ (more end sag).

Tried it again with a lower modulus 1,300 ksi (standard plywood) and the sag was greater (of course) but still evenly distributed.

 

I don’t know that 4/7’s is a precise determinant since I didn’t feel like factoring all the equations but it does seem like a pretty darned good rule of thumb!

[ri702bill 8,322]

Not exactly the same there, Ed! 

[wh500special 2,184]

That's an interesting rule of thumb in that no matter the length or stiffness of the thing being supported you'd always use just two pieces of support.  I would suppose for very stiff and strong structures that two supports could be enough but it seems like there would be a lot of cases where that's not enough to do the job.

 

I did a quick scan of bending moments and didn't see 4/7 (the fraction) show up anywhere so I wonder what math this might follow.

 

This is interesting.

 

Steve

 

@Handy Don replied while I was typing.  That's a nice calculator.

Edited May 19, 2023 by wh500special

[ri702bill 8,322]

7 minutes ago, wh500special said:

That's an interesting rule of thumb in that no matter the length or stiffness of the thing being supported you'd always use just two pieces of support.  I would suppose for very stiff and strong structures that two supports could be enough but it seems like there would be a lot of cases where that's not enough to do the job.

 

I did a quick scan of bending moments and didn't see 4/7 (the fraction) show up anywhere so I wonder what math this might follow.

 

This is interesting.

 

Steve

 

@Handy Don replied while I was typing.  That's a nice calculator.

Yup - just two supports, to allow placement and removal with either a pallet jack or fork lift...

Edited May 19, 2023 by ri702bill

[ri702bill 8,322]

I figured I'd toss this one on the table for us fellow "Techno-Dweebs" to digest. This probably goes back to the Egyptians & the pyramids...??

[peter lena 8,628]

@ri702bill  never did that , but having installed a lot of floor based mechanical units , would often make up an I BEAM FRAME TO SET IT ON . power house floor was open reinforced grating that was also the 1st floor venting for the turbines to the roof vents , 3 floors up . usually the refractory  set up team set that up . between the heat / noise and the 3 floor see thru that you were standing on . it was an interesting place to work , crew was also squared away , so it made work better , pete 

[formariz 11,987]

I never heard of it. However playing with it now if I understand it correctly, it would not work for any of my purposes such as stacking lumber. For example one 8’ board would require stickers every 4’6-7/8” to be supported. Essentially two stickers only that regardless where one would place them on the board would be incorrect. Again for my purposes many other factors would determine location if supports such as material thickness and height of pile. 

[ebinmaine 67,506]

31 minutes ago, formariz said:

I never heard of it. However playing with it now if I understand it correctly, it would not work for any of my purposes such as stacking lumber. For example one 8’ board would require stickers every 4’6-7/8” to be supported. Essentially two stickers only that regardless where one would place them on the board would be incorrect. Again for my purposes many other factors would determine location if supports such as material thickness and height of pile. 

In the lumber industry up here they used predominantly just an even number scale.  

Every couple feet there's a "sticker/dunnage".  Or every 4 feet.   

 

 

[Pullstart 62,884]

Either way, that’s a great bit of knowledge to bury with….

 

has anyone seen my 10mm sockets?

[ri702bill 8,322]

Kevin - I have 3  - a 6 pt and a shallow and deep 12 pt - 'cause the little buggers DO like to run off!!!  

 

[SylvanLakeWH 25,560]

4 hours ago, Ed Kennell said:

    I had to use my pizza to figure this out.

 

 

I like pizza. 

[ebinmaine 67,506]

1 hour ago, SylvanLakeWH said:

 

I like pizza. 

Yeah buddy me too.  

 

 

4 hours ago, Pullstart said:

10mm sockets?

 

1 hour ago, ri702bill said:

I have 3  - a 6 pt and a shallow and deep 12 pt

 

I'm telling you boys I just don't even know when was the last time we used a 10 mil socket around here.  

 

[wallfish 17,000]

4 hours ago, Pullstart said:

has anyone seen my 10mm sockets?

Yeah. they're with the 1/8" hex keys (allen wrenches)

[SylvanLakeWH 25,560]

[Pullstart 62,884]

If you could stick an A-60 in your fingers @SylvanLakeWH, that would be what I saw in my back barn today!  Maybe with a shirt on!

[Wheel-N-It 2,969]

I wish I had know about this calculation last month when I brought home a 396 Chevy engine for a future project that I'm collecting parts for. The engine is in a wooden crate, sitting on plywood, sitting on two dolly's. It is safe but it does have a slight "Gangster Lean"  to it LOL 

[rmaynard 15,461]

17 hours ago, Handy Don said:

New one to me as a formal calculation.

In the factory where I worked summers and when placing support for large flat or long symmetrical items with evenly distributed weight, we’d follow @ebinmaine’s system -- LOTS of blocking.

Of note, however, someone had transit-checked the storage building floor and there were a couple of high/low spots that had yellow paint stripes. Could only put palleted materials in those spots!

 

But you got me curious, Bill, so I went to my favorite beam calculator to test this out.

I put an evenly distributed weight over a constant cross section beam and used 29,000 ksi for the Young's Modulus (i.e. medium steel)  and put supports at 3’ and 11’ from one end. Pretty even deflection.

Tried it at 2’ and 12’ (more middle sag) and then again at 4’ and 10’ (more end sag).

Tried it again with a lower modulus 1,300 ksi (standard plywood) and the sag was greater (of course) but still evenly distributed.

 

I don’t know that 4/7’s is a precise determinant since I didn’t feel like factoring all the equations but it does seem like a pretty darned good rule of thumb!

[squonk 41,118]

16 hours ago, Pullstart said:

Either way, that’s a great bit of knowledge to bury with….

 

has anyone seen my 10mm sockets?

Lost a shallow 18mm 3/8 Dr. impact socket a couple of weeks ago. It was stuck on a bolt so I put the bolt in a vise and knocked it loose. Socket hit the floor and vanished. I may have to fall down to get a good look under the work bench.

[953 nut 55,231]

2 hours ago, squonk said:

. I may have to fall down to get a good look under the work bench

Mike,                  have your phone close by so you can call for help getting back up!             

[squonk 41,118]

1 hour ago, 953 nut said:

Mike,                  have your phone close by so you can call for help getting back up!             

I have a chain hoist for such of an emergency! 

[Sailman 1,291]

4 hours ago, squonk said:

Socket hit the floor and vanished.

 

I thought I was the only one who can NEVER find that small screw, nut, etc. that manages to fall onto what appears to be a perfectly clear surface....and disappear!

 

[wallfish 17,000]

34 minutes ago, Sailman said:

 

I thought I was the only one who can NEVER find that small screw, nut, etc. that manages to fall onto what appears to be a perfectly clear surface....and disappear!

 

They fall inside the missing socks from the dryer and become invisible!

Someday when my kids have to clean out the shed, there will be a big pile of little springs somewhere.

"What the hec was he hiding these for?"