'Dadio throws a big top'. A reply to unjust criticisms

[Pete Bannister]

This is a belated response to the closed thread ‘Dadio throws a big top’ posted 05-09-11. I‘ve considered the posted criticisms of the techniques used in the movie from an engineering perspective, using structural mechanics as a guide. Seems to me that several of the hurried criticisms don’t stand up to much scrutiny at all. Whereas, if one takes the time to absorb the content of the movie and all information the maker clearly provided, the mechanics can be analysed using basic structural mechanics and a basic knowledge of rigging

The geometry of the stem and the cuts (the hinge and the step) are all clearly there at the end of the movie and the text. Looking at the images, I have estimated the position of the hinge and I’ve also estimated the distance between the hinge and the pull line anchor point as maybe 6m (20’) from the fact that the stem above the cut is stated as 40’

I’ve estimated the offset (step cut) will provide a theoretical resistance of about 2kN (cleavage resistance) before the hinge starts to load. That’s the ‘safety valve’ preventing a barber’s chair (that’s my assumption anyway). This was a major issue for some Arbtalkers

The moment of resistance of the hinge wood can be calculated given the information in the movie. Admittedly, some engineering knowledge is a grasp of maths is required here. A basic cantilever calculation can then be applied to find the approximate load required to make the hinge yield. That’s the maximum force that could develop in the pull line because above that, the hinge will fail. Do the calcs as I have and you will find the hinge provides a moment of resistance (to rupture in bending ) of not more than 30kNm. So, with a lever arm of 6 m it will take a force of 5kN to break the hinge. That’s the theoretical max possible force in the pull line and I’ve probably conservatively estimated the moment of the hinge wood (I’ve used 80 N/mm 2 as the modulus of rupture which is probably high). That’s well within the permissible stress for the line. Dadio also gives information about the pull line ‘True Blue’ and clearly knows something of its properties. Its manufacturer states a breaking strength of about 32.3kN (I’ve assumed it was 12mm)

There has been a statement that this line could have been dangerous if it had snapped: well if it were steel then maybe. But it’s not; look at the rope manufacturer’s published data on its elastic properties. There might have been a danger to the cameraman from the block catapulting towards him but is it not possible that the cameraman was out of range?

Dadio discusses the size of the notch and the vectors evolved as the stem is pulled. No calculations needed really; you can see his assumptions were broadly correct: the stem landed where he engineered it to land!

So what have I learned from this exercise?

1. Absorb all of the information available before proceeding to criticise. I know its a hazard of instantaneous communication, but ignorance is still no excuse

2. Just because a technique is unfamiliar does not necessarily make it unsound

3. Conversely, those who may believe that because they have ‘always done it like this’ they are necessarily always correct, should perhaps reflect on the principal at 2 above

4. Having raised my head above the parapet I should now expect to receive incoming fire!

[PhilBeech]

Sorry, couldn't be bothered reading all that tosh!

[tommer9]

This is a belated response to the closed thread ‘Dadio throws a big top’ posted 05-09-11. I‘ve considered the posted criticisms of the techniques used in the movie from an engineering perspective, using structural mechanics as a guide. Seems to me that several of the hurried criticisms don’t stand up to much scrutiny at all. Whereas, if one takes the time to absorb the content of the movie and all information the maker clearly provided, the mechanics can be analysed using basic structural mechanics and a basic knowledge of rigging

The geometry of the stem and the cuts (the hinge and the step) are all clearly there at the end of the movie and the text. Looking at the images, I have estimated the position of the hinge and I’ve also estimated the distance between the hinge and the pull line anchor point as maybe 6m (20’) from the fact that the stem above the cut is stated as 40’

I’ve estimated the offset (step cut) will provide a theoretical resistance of about 2kN (cleavage resistance) before the hinge starts to load. That’s the ‘safety valve’ preventing a barber’s chair (that’s my assumption anyway). This was a major issue for some Arbtalkers

The moment of resistance of the hinge wood can be calculated given the information in the movie. Admittedly, some engineering knowledge is a grasp of maths is required here. A basic cantilever calculation can then be applied to find the approximate load required to make the hinge yield. That’s the maximum force that could develop in the pull line because above that, the hinge will fail. Do the calcs as I have and you will find the hinge provides a moment of resistance (to rupture in bending ) of not more than 30kNm. So, with a lever arm of 6 m it will take a force of 5kN to break the hinge. That’s the theoretical max possible force in the pull line and I’ve probably conservatively estimated the moment of the hinge wood (I’ve used 80 N/mm 2 as the modulus of rupture which is probably high). That’s well within the permissible stress for the line. Dadio also gives information about the pull line ‘True Blue’ and clearly knows something of its properties. Its manufacturer states a breaking strength of about 32.3kN (I’ve assumed it was 12mm)

There has been a statement that this line could have been dangerous if it had snapped: well if it were steel then maybe. But it’s not; look at the rope manufacturer’s published data on its elastic properties. There might have been a danger to the cameraman from the block catapulting towards him but is it not possible that the cameraman was out of range?

Dadio discusses the size of the notch and the vectors evolved as the stem is pulled. No calculations needed really; you can see his assumptions were broadly correct: the stem landed where he engineered it to land!

So what have I learned from this exercise?

1. Absorb all of the information available before proceeding to criticise. I know its a hazard of instantaneous communication, but ignorance is still no excuse

2. Just because a technique is unfamiliar does not necessarily make it unsound

3. Conversely, those who may believe that because they have ‘always done it like this’ they are necessarily always correct, should perhaps reflect on the principal at 2 above

4. Having raised my head above the parapet I should now expect to receive incoming fire!

 

Yes, you probably should...

[Andy Collins]

I love the term "cleavage resistance" I shall be looking to use this term as often as possible from now on

[Stephen Blair]

Personally resisting cleavage goes against my nature, so for that reason, i am out:biggrin:

[Andy Collins]

However, if the cleavage does indeed resist gravity, then I'm all for it, so on that basis.....I'm in

[MattyF]

Oh my days......

[Ian C]

yawn.............ZZzzzzzzzzzz

[White Noise]

Sorry, couldn't be bothered reading all that tosh!

 

[Steve Bullman]

Thanks for taking the time to post Pete, dont pay this lot no mind