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Snap
Joint Technology
for Composite Structures
The Need
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For the past few
decades, the Aerospace industry was the major users for
polymer composites. In the majority of the aircraft composite
structural components, both bolted and/or adhesive bonded
joint was used. Most of the details are similar to those for
metal joints. It was shown from extensive testing on bolted
composite joints that failure always occurs in a catastrophic
manner due to high stress concentration developed at the bolt
locations. Due to the inherent low bearing and interlaminar
shear strengths of composites, these stress concentrations
threaten the downfall of ever piece of the composite
structure. |
Snap
Joint Design |
The Technology
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The optimum composite joint design is the one
capable of distributing stresses over a wide area rather than
to concentrate them at a point. Adhesively bonded joints can
satisfy these requirements, however, most of the adhesives are
brittle, and brittle failure is unavoidable. This was the
motivation of developing what is called the SNAP joint.
The snap joint technology
developed by W. Brandt Goldworthy & Associates, Inc. The
concept is based on similar joining technology used for
connecting wooden parts (wood is considered as natural
orthotropic composites). Also, this technique is very similar
to techniques which were used a decade or so again for
plastic.
The following figure shows a
pultruded structural composite member (A) with one end shaped
as a fir-tree, and therefore has a large load bearing area. In
this figure, part (A) has been snapped into another structural
shape (B). From these figures, on can see that the later shape
has been designed to combine its structural shape with
functionality that allows for the engagement of the
load-bearing surface of member (A). It is possible to
"snap" joint both parts together since part (A) has
been cut for a short distance along it length to provide
enough lateral flexibility to move out of the way when
entering part (B). In order to make this joining concept
successful, the fiber architecture of part (A) must be
designed in such a way that the load bearing surfaces have
higher interlaminar shear strength capacity. Also, it can be
noticed from the figure, that a circular hole was introduced
at the end of the horizontal slot of part (A) to inhibit the
crack propagation along the length of the pultruded member.
Snap
Joint Concept
Hardware for fasternerless snap
joint
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The Benefits
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The applications of
this technology in composite structures will have benefits as
follow:
- The structures are easy to
assembly.
- Installation of structure
members become faster.
- Installation needs smaller
number of labor and equipment.
- Since it use composite
materials, its weight is less than traditional structures.
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Status
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The first prototype
or "Demonstration project" using this joining method
was in designing and construction of three Transmission Tower
Structures near Los Angeles by W. Brandt Goldworthy &
Associates, Inc. and Ebert Composites Corporation. The 1999
CERF Charles Pankow Award for Innovative Applications was
granted to the developers for this innovative composite
transmission tower.
The developers have also proposed an
18-story all-composite structure as a
stack & checkout tower for rockets at Commercial
Spaceport, USKA, Vandenberg Airforce Base, California.
According to the developers, the estimated cost of this
structure is about 20 million dollars including machinery
cost.
This year, through California
Department of Transportation (Caltrans), they have proposed
the design and the construction of a truss structure to carry
highway singes. The project was submitted as a part of the
Federal Highway T-21 program.
More details discussion on this
emerging joining technique as well other similar techniques
will be presented in the separate Chapter in the ASCE
Connection Design Manual which I am writing, and which is
expected to be published by the Middle of year 2000.
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Composite Transmission Tower
Truss Joint
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Barriers
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The snap joining
technique is considered to be one of the optimum techniques to
join composite structural members. However, it has a major
limitation, and can only be used in specific applications.
That is, this method can only be used to transmit axial loads,
which make it ideal for truss-type structures. However, in my
opinion, this method should NOT be recommended when
out-of-plane loads or any shear loads are introduced since the
connection is not design to carry any major bending moments.
Under flexural loads, it is expected that the joint will be
very flexible, and the artificial cracks introduced to members
will propagate and a
complete failure will occur even under moderate service
flexural and/or shear loading.
More details discussion on this
emerging joining technique as well other similar techniques
will be presented in the separate Chapter in the ASCE
Connection Design Manual, and is expected to be published by
the Middle of year 2000.
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Eighteen Story All-Composite
High Rise
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Points of Contact
- Ayman Mosallam, Ph.D., P.E.,
California State University, Fullerton, CA, E-mail: amosallam@fullerton.edu
- David W. Johnson, Strongwell Ebert
LLC, P. O. Box 121071, San Diego, CA 92112 USA, Phone: (619)
234-9401, Fax: (619) 234-4326, E-mail: info@strongwell-ebert.com
- Dr. Clem Hiel, W.
Brandt Goldsworthy & Associates, Inc., 23930-40 Madison
Street, Torrance, California 90505, Phone: (310) 375-4565, Fax:
(310) 375-1146, E-mail: clemhiel@aol.com
References
- Strongwell Ebert LLC Home Page, http://www.strongwell-ebert.com/
- W. Brandt
Goldsworthy and Dr. Clem Hiel, "Composite Structures",
SAMPE Journal, Vol. 34. No.1, Jan/Feb 1998, p. 24-30.
Disclaimer Statement
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Neither the Construction
Industry Institute nor Purdue University in any way endorses this
technology or represents
that the information presented can be relied upon without further investigation. |
MA28 |
