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A-Frame Collapses

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Hynnimac
Deckhand

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USA
10 Posts

Initially Posted - 03/28/2004 : 23:18:45

I constructed an A-frame out of 3/4 in conduit. THe nipples were 3" cast iron pipe. Upon using the frame to raise the mast it simply folded creating a very hazardous situation. What material should have been used to prevent this from happening?

Cliff Hynniman

Edited by - on

Buzz Maring
Master Marine Consultant

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USA
1772 Posts

Response Posted - 03/29/2004 : 00:26:22

Hi Cliff,

'Sorry to hear about the collapse ... I'm glad no one got hurt.

It sounds like you might have used "regular" conduit instead of "rigid" conduit. 'Sorry ... I don't know enough about construction materials to explain the difference, but I DO know that I asked for rigid conduit when I bought the materials for my A-frame. BTW, my A-frame is at my boat right now so I can't confirm this, but I also think I used 1-inch diameter.

'Hope you get it working OK ... good luck!

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 03/29/2004 : 09:41:07

As you discovered, 3/4" electrical conduit is fairly whimpy stuff, it was never designed to carry a load other than supporting it's own weight in a 16' span (or so). I think even 3/4" schedule 40 water pipe might be too weak in a 10' section under a load like the A-Frame.

If I was fabbing one (might do in the future) I think I'd go for 1-1/2" heavy-wall aluminum tubing... or maybe try 1-1/2" aluminum conduit. You pick up quite a bit of rigidity when you increase diameters in tubing.

Edited by - on

Gloss
Master Marine Consultant

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USA
1916 Posts

Response Posted - 03/29/2004 : 09:43:47

It has been a while since I took some engineering coursework as part of my original construction management degree. (I now do anesthesia)

When you design a column for a building, or any other structural component which in compression, one of the criteria you must satisfy is the L/R ratio, which is Length divided by radius of gyration. (this has nothing to do with nudie bars) The greater the cross sectional area of a structural component, as well as the thickness, and shape of the component influences its Radius of gyration. For example a 1" conduit would have a greater R than a 3/4" conduit, or a thicker wall would be greater than a thinwall. Get my drift?
So the longer your unsupported (no side bracing) column, the higher your R must be to satisfy the criteria.
Confused?
The longer your column, the thicker, or bigger your piece must be to avoid collapse.
If there are any PE's out there, feel free to comment on my explanation. I'm out of practice on engineering, now I just pass gas for a living.

Edited by - on

frich
Captain

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USA
418 Posts

Response Posted - 03/29/2004 : 12:06:11

Regarding the collapse, thank goodness no one or the boat was hurt. I used 1" diameter and had no problem. Also dont get any of that thinwall stuff, use rigid galvanized conduit available at Lowes.

Frank R
C25 SK

Edited by - on

LeighMarie
Navigator

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USA
188 Posts

Response Posted - 03/29/2004 : 12:44:54

What timing! I am going to purchase my materials this weekend to build my A-frame and I didn't know what type of conduit or pipe to get. I looked at a 1/2 inch pipe and I just don't know how I would ever begin to bend it! I will head back to Lowe's tonight and see what else they have. I printed this topic out and will bring it with me. I want it to be rigid, but I also want to be able to bend it - I'm not known for my brute strength!

Edited by - on

Scotd
Navigator

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USA
136 Posts

Response Posted - 03/29/2004 : 13:23:16

EMT- Electrical Metallic Tubing Steel

Size 3/4 Weight 4.6 lbs Wall Thickness .049 Inches

Rigid Metal Conduit Steel

Size 3/4 Weight 10.9 lbs Wall Thickness .107 Inches

Edited by - on

CGood
Deckhand

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10 Posts

Response Posted - 03/29/2004 : 17:20:31

Using the Thin Annulus method to calculate Moment of Inertia for your 3/4" tubes, you get I=.065 and .142 in4 respectively. Using a steel modulus of 28E3 ksi and a length of ~10 ft, the theoretical critical force for buckling is around 1130 and 2468 pounds respectively. Of course, this is for uniform force (which in this case is not a bad assumption) and assumes no moments on the tube. It is probably this last assumption that caused the problems.

I could not find any good equations that took moment into account. If anyone has such a reference, I would find it very interesting.

Btw: according to my analysis, I estimate approximately 96 pounds compression for the A-frame - or about 51 pounds axial force in each tube when the mast is at ~ 24 deg (6' above rails - about where someone in the cockpit standing on the seats could catch it). To eliminate all moments, the deck would then have to cary about 15 pounds lateral force (port-starbord). With no lateral force at the deck, a moment WILL be put into the rods of up to 155 ft-pounds!!! This is probably the culprit. The easiest way to ensure no moment is to pin (allow free rotation relative to one-another) where the two rods come together so they can't carry a moment.

At this mast angle, the cable from the mast top to the A-frame appears to bear about 125 pounds and the rope from A-frame to deck 130 pounds (at an angle where it is ran through the jib tack cleat). This assumes an ~80 pound mast.

Edited by - on

gnorgan
Admiral

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USA
563 Posts

Response Posted - 03/29/2004 : 21:35:03

Whoa! Let's see...I have raised and lowered my tall rig mast (feels like it weighs a ton) how many times? I think my A-frame uses two 10 foot sections of 3/4 inch regular electrical conduit and hasn't looked like it was going to collapse. But I haven't even considered that it might!

Looks like another "heads-up" situation that will entail another boat project. I better go measure the conduit right now and see what I'm using.....
Thanks again to this website. What a place

Edited by - on

204013391
Deckhand

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7 Posts

Response Posted - 03/29/2004 : 22:16:26

I recently lowered and raised my mast with the A-Frame. I used 10' lengths of 1" conduit. I didn't notice any deflection or bending. I'm glad I had a few extra hands around just in case, but it worked like a charm...except...

I bent one of the outer shroud t-buckles (I left connected as a little side to side protection) pretty bad because I didn't watch them when going up. It got twisted a half turn and...a trip to West Marine was in the cards for me.

There's obviously a lot of force being transmitted if it can bend one of those buckles like that. Caution and Safety can never be overstated.

Troy
Second Wind #3716
www.troyandcheryl.com

Edited by - on

204013391
Deckhand

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7 Posts

Response Posted - 03/29/2004 : 22:17:36

Edited by - on

LeighMarie
Navigator

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USA
188 Posts

Response Posted - 03/30/2004 : 11:10:02

I looked at the rigid 1" conduit, 10 foot section, and it's heavy! But, I now have an electrician all lined up to bend it for me next week and by golly, it's gonna work! It really feels like overkill in getting that heavy of a conduit, but I know when the wind gets blowing while trying to get the mast up it can torque. Besides, I do everything else in "Overkill" mode anyway. It will be nice once I get it set. I don't know how I'm going to actually attach it to the forestay under my furler drum, but I'm sure I'll figure something out.

Edited by - on

svmoxie
Past Commodore

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USA
331 Posts

Response Posted - 03/30/2004 : 13:24:30

Dave,

You don't want the A-frame to connect to the forestay because you will need it free in order to attach it to the stem fitting once the mast is raised. Fasten the jib halyard to the A-frame and run a second line from the A-frame down to the deck, through a snatch block and back to the winch. Once you have the mast up, cleat off the raising line and take the forestay/furler forward and attach it to the stem fitting with the clevis pin.

Edited by - on

LeighMarie
Navigator

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USA
188 Posts

Response Posted - 03/31/2004 : 14:48:45

Clif, We have always been raising the mast using our boom vang (it's an all-rope type) attached to the jib halyard, so it would be feasible to do it that way. A fear I have had is of having the forestay and furler so far up in the air knowing that the furler flops all over the place. But, I think I could use the forestay as once the mast is almost upright there is little pressure anymore needed on the hauling line. I hold the mast upright and we just unattach the jib halyard from the stem fitting (I learned a new term today - stem fitting!) and attach the furler. But, I do like the idea of using the jib halyard and I don't know why I ruled it out earlier. I'll have to revisit that! Thanks!

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deastburn
Captain

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USA
334 Posts

Response Posted - 03/31/2004 : 15:58:37

My A-frame is made of two, ten-foot, 1" galvanized steel electrical conduit, heavy duty. I don't pretend to understand the formulas, but it seems to me that if the a-frame is set up correctly, and everything is inline (as opposed to slightly offset), then there should be little real weight on the actual a-frame itself, since it is being used mostly for leverage and balance.

My jib halyard is attached to a carabiner at the apex of the A-frame, and I run the genny sheet from there through a snatch block attached at the bow back to the main genoa winch in the cockpit. I have an assistant steady the mast, and make sure the bases of the a-frame and the mast are free to pivot as they should.

My goodness, I can stand on two pieces of 1" conduit stretched between two saw horses and not bend them. Are you sure you weren't using one of those new-fangled inventions called plastic?

Edited by - on

frich
Captain

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USA
418 Posts

Response Posted - 03/31/2004 : 17:01:48

On my 1" sections, I bent them in a vice grip. I then used a 2 lb hammer to finish the job.

Frank R
C25 Sk

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Buzz Maring
Master Marine Consultant

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USA
1772 Posts

Response Posted - 03/31/2004 : 17:24:30

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by frich
 On my 1" sections, I bent them in a vice grip. I then used a 2 lb hammer to finish the job.

Frank R
C25 Sk
<hr height="1" noshade id="quote"></blockquote id="quote">

Me, too ...

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