[Public-List] chain plate spacers
dickdurk at atlanticbb.net
dickdurk at atlanticbb.net
Mon Mar 23 17:19:13 PDT 2009
Makes sense to me, Roger, except there is a history of the
original (1/4"?) bolts distorting-something known to
surveyors familiar with Albergs. Shock loading perhaps? I
don't know how you engineer for that.
MichaelGrosh
#220
On Mon, 23 Mar 2009 17:31:44 -0500
"Roger L. Kingsland" <r.kingsland at ksba.com> wrote:
> Gordon et. al. Albergers,
>
> Since properly connecting the rig to the boat is A), so
>important, and B),
> something I need to do soon, I am going to jump on my
>friction vs. shear
> soapbox again hoping others might confirm the logic. I
>have attached a copy
> of an epistle I wrote back in 2004 (please note
>disclaimer) below.
>
> The basic argument is, by putting enough tension
>(tightening) on the bolts,
> the chain plate is forced against the knee/bulkhead with
>enough force that
> it will not slide in the vertical plane (direction of
>load). This
> "friction" connection is quite strong and is the design
>used on most bolted
> connections for steel building structures.
>
> The problem with shear a connection is it relies only on
>the strength of the
> weakest connection material over less than half of the
>surface area of the
> bolts. This makes the shear strength of the bolts
>irrelevant because the
> shear strength of the wood thru which the bolts go is
>less; the bolts would
> elongate the holes in the wood long before the bolt
>would break from shear.
> Also, with a shear connection, only bolts in exact
>alignment with the holes
> in the chain plate would achieve their full shear
>strength.
>
> With a friction connection every bolt applies friction
>so their strength is
> cumulative. I checked the tensile strength of a 1/4"
>304 SS bolt and it is
> about 3,700 pounds. Assuming a 1.3 safety factor (see
>below) and a 6,900
> pound breaking strength of a 1/4" SS wire mainstay,
>sufficient friction
> strength could be achieved with 3 - 1/4" bolts if each
>were tightened to 80%
> of maximum load. So, it appears 5/16" bolts are only
>necessary if one wants
> to move up to 9/32" mainstay wire. In fact, larger
>holes would reduce the
> area of the chain plate and possibly degrade the ability
>of the plate to
> distribute the needed compression load across sufficient
>area (the plate
> could bend at the narrowest part beside each hole).
>
> Another issue is the compressive strength of the
>bulkhead (marine plywood)
> and knees (hardwood). This could be field tested by
>torquing down a bolt
> (does anyone know how to convert tensile strength to
>bolt torque) with a
> couple of layers of wide washers on each side and
>observe if the wood
> crushes. To me, this is perhaps the most important test
>since several
> owners have mentioned water leaks and water damaged
>bulkheads/knees would be
> the weak link in any connection detail.
>
> Does this stuff make any sense or is my version of
>reality drifting yet
> further away from the norm?
>
> Best - Roger 148
>
>
>
> Aug, 2004
> Albergers,
>
> RE the thread on reinforcing the knees, I checked with
>the NA who surveyed
> my boat and learned FG tape has a strength in shear of
>8,000 pounds per
> square inch. The breaking strength of the 1/4" lowers
>is 4,700 pounds. To
> insure the stays break first (comforting thought), the
>chain plates should
> hold 1.3 times the stay breaking load and the knees 1.5
>times, or 7,000
> pounds. The load on the knees is transferred to the
>inside surface of the
> hull (which, like the main bulkhead, is a good, strong
>diaphragm) via the
> vertical FG tape on each side of the knee. Assuming the
>knee is 8" high (I
> haven't measured) or a total length of 16", the
>thickness of the tape should
> be a minimum of 1/16" (7,000 pounds / 16 inches = 440
>pounds/inch; 8,000
> pounds / 440 = 1/18" tape thickness).
>
> I plan to drill a little hole in the tape to check
>thickness but suspect it
> is greater than 1/16." So, the knees are probably
>adequate but, for the
> belt-and-suspender folks, easily reinforced by simply
>adding new layers of
> tape to the existing.
>
> RE the chain plate size, I started looking into the
>shear strength of
> stainless steel and learned what a megapasquale is, and
>also learned its
> strength goes down drastically over 400 degrees so,
>let's hope global
> warming doesn't catch up with us too quickly. Then I
>realized an easy way to
> insure adequate chain plate strength is confirm that the
>minimum cross
> sectional area above or beside the turnbuckle fastener
>pin is at least 1.3+
> times the cross sectional area of the SS stay the chain
>plate supports.
> Since the area of the main stays is 0.20 sq. in. and the
>lowers is 0.11 sq.
> in., chances are the chain plates are more than
>adequate.
>
> Our structural engineers tell me the majority of
>structural failures occur
> at the mechanical connections so, I suspect the weak
>link is the chain plate
> connection at the knees or bulkhead. Unless a friction
>connection is
> employed, the entire load will be placed on the upper
>part of the bolt holes
> in the knees and main bulkhead (shear connection). A
>friction connection
> involves compressing the material between the chain
>plate and backer plate
> (or washers) sufficiently so the friction between the
>different surfaces
> prevents them from "slipping" (like Chinese handcuffs).
>
> The Gougeon Brothers (West System) suggest friction
>connections under load
> (just about everything on a sailboat) be "bonded" by
>adding a layer of high
> compression, adhesive epoxy (West makes a slick powder
>additive) between the
> surfaces to be connected. This insures friction across
>the entire surface
> area, not just the "high points." They also point out
>that the bond must be
> rigid. If the bond is soft and flexible (5200?), the
>load will cause
> movement which will degrade the friction into a shear
>load on the bolts.
> Once this occurs, the bolt shafts work against the holes
>resulting in
> substantial reduction of load capacity and leaks (like
>the toe rail at the
> genoa track?).
>
> The wood knees on #148 appear to have the grain oriented
>vertically so the
> upward load from the stays runs parallel to the grain.
> No big deal with a
> friction connection but real a problem with a shear
>connection because wood
> is much weaker parallel to the grain than perpendicular
>to it. The need for
> a good friction connection argues for backer plates as
>large as the chain
> plate and, perhaps even increasing the size of the chain
>plate.
>
> Assuming the above actually makes sense (time for
>disclaimer; the author is
> not a structural engineer and all said above could be
>total BS; so, rely on
> it under pearl of tumbling mast), my plan to insure
>stays are properly
> fastened is to check thickness and quality of the
>fiberglass connection
> between the knees / bulkhead and the hull. Then, make
>backer plates about
> the same size as the chain plates and fasten them with
>epoxy and a bunch of
> tension on the bolts. My main bulkhead is in good shape
>but I have heard
> some are rotted, particularly if water seal between
>chain plate and deck has
> not been maintained.
>
> Hope this helps sort out the issues, it did for me; but,
>then, I find it so
> easy to agree with myself.
>
> As always (jealous of those with boats in the water),
>
> Roger
>
>
> KSBA
> ________________________________
>
> Architects/Planners/InteriorDesigners/ProjectManagers
>
> 3441 Butler Street
> Pittsburgh, PA 15201
> N 40° 27.8344' W79° 57.9831'
>
> 412-252-1500 ext.101
> 412-779-5101 cell
> 412-252-1510 fax
> r.kingsland at ksba.com
> www.ksba.com
>
> -----Original Message-----
>From: public-list-bounces at lists.alberg30.org
> [mailto:public-list-bounces at lists.alberg30.org] On
>Behalf Of gordon white
> Sent: Monday, March 23, 2009 3:04 PM
> To: public-list at lists.alberg30.org
> Subject: Re: [Public-List] chain plate spacers
>
> Washer-type spacers are a bad thing, if I understand the
>description as to
> how they are used. They apply a bending moment to the
>bolt rather than
> straight shear. Reminds me of a vintage racer whose rear
>axle was a couple
> of inches wider than his suspension setup. He used
>spacers much as you
> describe to bolt the axle into the car. He was running
>on a race track just
> ahead of me when one of the bolts broke, letting the
>car's chassis dig into
> the track. The car flipped over and killed him.
> Not a good thing.
>
> But if you put a shim the full width of the chainplate
>you can mostly
> correct the situation, even though I would not recommend
>it if it could be
> avoided. Certainly up-size the bolts.
>
> - Gordon White
> Brigadoon II
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