Double I-beam structural joint for connecting fiber-reinforced plastic beams or girders

Beams or girders composed of fiber-reinforced polymers or plastics can be adhesively bonded into structural members by means of an interconnecting joint of such material having an arch structure which is positioned and arranged so that it provides a flexibility that tends to reduce peel failures of the adhesive bonds between the joint and beams or girders.

BACKGROUND OF THE INVENTION 
This invention relates to structural joints for connecting fiber-reinforced 
plastic beams or girders without strength reducing bolts, grooves, or the 
like. More particularly, the invention relates to a double I-beam (DIB) 
joint for connecting fiber-reinforced plastic beams or girders into civil 
engineering structures. 
Prior uses of fiber-reinforced polymers or plastics and the like plastic 
structures are believed to be typified by patents such as the following: 
U.S. Pat. No. 3,100,555 describes a tower structure composed of tubular 
plastic structural members joined by segmented plastic joints which are 
glued together to interconnect the intersecting braces, legs, etc. U.S. 
Pat. No. 3,685,862 describes connectors for joining plastic rods within 
hoops in which the converging rod portions are squeezed together and held 
by driven-in wedges. U.S. Pat. No. 3,975,874 describes a prefabricated 
plastic swimming pool design which proposes side and shoulder portions 
supported by X-shaped molded plastic braces having I-beam cross-sections 
along some portions and flat cross-sections in other locations. U.S. Pat. 
No. 4,177,306 describes laminated plastic girders in which some of the 
fiber-reinforcing materials extend into both of the girders. 
SUMMARY OF THE INVENTION 
The present invention relates to a structural joint or connector for 
joining intersecting fiber-reinforced plastic structural beams or girders. 
The present connector contains a double I-beam in the form of an arch-like 
structure in which a web portion is bounded by, respectively, a pair of 
substantially concentric arcuate flanges and a pair of converging 
generally flat flanges. The generally flat flange portions extend at least 
somewhat beyond each side of the web portion in the form of T-beams in 
which relatively short and outwardly decreasing tapered portions are 
parallel to the web portion of the DIB joint. The present connector is 
sized and shaped so that, in a completed structural joint, the generally 
flat flanges contact and substantially parallel adjacent faces or sides of 
the beams being interconnected with the arch-like portion located away 
from the point of intersection of those beams. And, in the completed 
structural joint the generally flat flanges of the DIB joint are 
adhesively bonded to the adjacent faces of the interconnected beams. 
Where desirable, the strength of a structural joint in which beams or 
girders are interconnected by a DIB joint can be increased by adhesively 
bonding a generally flat plate between the contiguous portions of such 
beams. And, additionally, or alternatively, where such beams are I-beams, 
the strength of the connection can be increased by adhesively bonding 
C-shaped plastic clamps over the contiguous portions of the I-beam flanges 
and DIB joint flanges.

DESCRIPTION OF THE INVENTION 
A main obstacle to the use of fiber-reinforced polymers as structural 
members has been the difficulty of interconnecting such members to form a 
primary load-carrying structure. In fact, the key to the performance of 
nearly all composite structures composed of fiber-reinforced plastics lies 
in joint design and technology. The lack of an efficient structural joint 
design has delayed the potentially large scale application of such 
plastics in the building industry. The conventional methods of joining 
such members, particularly in civil engineering structures, have relied on 
fasteners such as bolts and rivets of the type commonly used with steel 
structures. Such discrete fastening devices limit the strength of a joint 
to much less than the strength capability of the fiber-reinforced polymer 
members which are interconnected. 
It has now been discovered that a joint which uses a fiber-reinforced 
plastic arch structure located away from the point of intersection of 
structural members and attached to the structural members by adhesive 
bonding avoids many of the disadvantages of the previously used methods 
for reinforced polymer members. An important feature of the present joint 
structure is that its flexibility at the adhesive bond location is 
graduated along the bond length in a manner which reduces the tendency for 
peel failures in the adhesive bond. 
FIGS. 1-3 show the present double I-beam or DIB joint, or connector 
element. Such a connector contains an arch-shaped web 1 bounded by a pair 
of arcuate flanges 2 and a pair of generally flat flanges 3. The generally 
flat flanges preferably have end portions which extend beyond the edge of 
the web in the form of tapered T-beams. In a DIB joint in which the flat 
flanges are terminated by T-beam end portions, such as portions 3a, the 
maximum thickness of the webs of the terminal T-beam portions are 
preferably less than two-times the widths of the flat flanges, and the 
flat flange-widths are preferably substantially constant throughout the 
joint. The minimum thickness of such terminal T-beam webs can be zero and 
their lengths and the slopes of their tapering (which can be constant or 
variable) are preferably correlated with the design requirements. The 
shapes and dimensions of all such end portions are preferably the same on 
each joint. 
Where desirable, the outer surfaces of the generally flat flanges 3 can be 
shaped to conform to the shape of the beam to which it is to be attached, 
e.g., by adding an arcuate outer surface to conform to at least some of 
the adjacent portion of a tubular beam, or the like. 
The DIB joint relies on adhesive bonding for load transfer from one member 
to the other. Since this increases the flexibility of the joint, 
particularly at the T-beam end sections of the flat flanges, it minimizes 
the tendency of the joint to fail due to adhesive peeling. 
The disadvantage of using mechanical fasteners such as bolts or rivets in 
connections for fiber-reinforced structures has been described in 
publications such as "Mechanics of Composite Materials" by R. M. Jones, 
McGraw-Hill Rock Company, 1975; "Analysis of Discontinuities, Edge Effects 
and Joints" by G. C. Grimes and L. F. Greenman, Composite Materials, 
edited by L. J. Brautman and R. H. Krock, Academic Press, 1975. 
FIG. 4 schematically illustrates a simple truss structure in which the 
present DIB joints can be used. Such a truss structure can be, for example 
one of two load-carrying trauss panels of a simple bridge configuration. 
The DIB joints can be used to connect all members of the exemplified truss 
structure. All parts of such a structure, including the joint and truss 
members, can be made of fiber-reinforced plastic. 
In general, the dimensions of the DIB joints are chosen according to the 
loading requirements at the various joint connections--and, such loading 
requirements can be determined by known types of structural analysis. 
FIG. 5 shows a connection of the members ea and ed at the joint e of the 
truss structure of FIG. 4. Where those members comprise I-beams 6 the 
flanges of the I-beams are connected to the flat flanges of the DIB joint 
4 by adhesive bonding. This causes the load transfer from one member to 
another via adhesive bonding through the DIB joint. A generally flat plate 
7 is preferably adhesively bonded between the intersecting portions of the 
beams being joined, such as beams 6. Such plates should, of course, be 
shaped to conform to any non-flat portions of the intersecting beam 
surfaces. 
FIG. 6 shows the connection arrangement at joint a of the structure of FIG. 
4. As shown, the arcuate flange portions of the two adjacent DIB joints 
are arranged to lie approximately in the arc of a circle. This tends to 
eliminate any eccentricity in the normal loads acting on the two flanges 
of a member such as the member ad. 
FIG. 7 shows the joint connection at joint d of the structure shown in FIG. 
4. In addition to the DIB joints, lap joints (preferably double lap 
joints) formed by attaching plates 7 can be used to connect the webs of 
members de and dc; and if desired, to connect the flanges of those 
members. A dashed line was used to depict the lower plate on the flanges 
since the effect of that plate was not used in the stress calculations 
described herein. 
FIG. 8 shows the use of C-clamps 8 connected over the contiguous flanges of 
an I-beam 6 and a DIB joint 4. Such fiber-reinforced C-clamps provide an 
additional load transfer. Such clamps are also useful in holding the 
members together during the construction process. In addition, such 
C-clamps provide resistance against peeling of the adhesive bond between 
the flanges of the I-beam and DIB joint. 
The joint connection at joints b and c are similar to those at joints a and 
e. It should be noted again that only adhesive bonding is used to connect 
the various members of the truss structure. Although the fiber-reinforced 
C-clamps behave like mechanical fasteners, they are actually adhesively 
bonded to the I-beam and the DIB joint. Unlike bolts and rivets, these 
C-clamps do not weaken the load carrying capability of the truss members. 
The dimensions of the I-beam and the DIB joint will be chosen once the 
forces carried by the truss members are determined. Assuming that a load 
of 10,000 pounds is applied to the truss of FIG. 2 at joint d, the forces 
in the members of the truss can be easily shown to be: 
F.sub.ae =F.sub.bc =-5774 lb (compression) 
F.sub.ed =F.sub.dc =2887 lb (tension) 
F.sub.ad =F.sub.bd =5774 lb (tension) 
F.sub.ab =-5774 lb (compression) 
These are the forces carried by the truss members at locations away from 
the joints. In calculating the force distributions at the joint locations, 
the effect of the fiber-reinforced C-clamps has been ignored. Thus, the 
adhesive bonding between the I-beams and the DIB joints has been treated 
as the primary load transfer device. Consequently, the use of the C-clamps 
provides a redundant load transfer mechanism which can assure the safety 
and reliability of the joint design. 
The dimensions of the components of the truss structure are determined by 
first selecting the material used for their construction. For this 
particular example, we assume that random short fiber-reinforced composite 
with 60 percent by volume of short glass fibers is used for all the 
components of the truss structure. The mechanical properties of this 
composite are assumed to be 
EQU E=3.times.10.sup.6 psi 
EQU .nu.=0.4 
EQU .sigma..sub.ult =30.times.10.sup.3 psi 
where E refers to the Young's modulus, .nu. refers to Poisson's ratio and 
.sigma..sub.ult refers to the maximum strength of the material. 
A safety factor of 2.5 was used for the design of all parts of the 
exemplified truss structure. For simplicity, the cross-sectional 
dimensions are assumed to be identical for all the members of the truss 
structure. The dimensions of the cross-section of the I-beam were: flange 
and web thicknesses of 0.4 in., a flange width of 2 in. and a beam height 
of 4 in. It can be shown very easily that such a cross-section is 
sufficient to carry the stresses at any location along the length of the 
members. 
The forces and moments acting on member ad at the joint locations have been 
calculated. Comparison of the forces acting on this member with the other 
members of the truss shows that it is the most critical member of the 
truss structure. The normal forces which are acting on the two flanges of 
the I-beam are opposing forces of 5000 pounds each. To prevent buckling of 
the web of the I-beam, these two normal forces must be distributed over a 
sufficiently long portion of the beam. Assuming a simply supported 
condition for the web of the I-beam, the critical buckling stress is 
simply 
##EQU1## 
Using a safety factor of 2.5, the minimum value of l (where l represents 
the length of the flange needed to carry the normal compressive loading) 
##EQU2## 
To account for stress concentrations that might arise at the two end points 
of l, an additional factor of safety of approximately 1.5 was used to 
increase the value of to 4 inches. Since these normal stresses are 
transmitted through the DIB joint to the other members of the truss, the 
distance between the curved bars or concentric arcuate flanges in the DIB 
joint is also chosen to be 4 inches. 
The design dimensions of the DIB joint for the exemplified truss structure 
can be based on simple calculations. The most important parameter is the 
area of the flanges of the DIB joint. A sufficiently large area must be 
chosen for load transfer through adhesive bonding. If these stresses are 
too high for the adhesive used, then a larger area for the flange of the 
DIB joint can be chosen to lower the stress intensity at the adhesive. For 
the present problem, a successful adhesive bonding seems to be attainable 
by means of commercially available adhesives. 
The dimensions for the C-clamps and the lap joints can also be determined 
very easily. They are, therefore, considered known quantities for this 
simple example. 
In general, the structural beams or girders which are interconnected by the 
present DIB joints can have substantially any configuration. Preferably 
the beams have a configuration providing at least one pair of 
substantially flat faces which occupy adjacent sides of the beams at the 
beam intersections. Suitable plastic beam compositions and shapes are 
described in U.S. Pat. No. 4,177,306. I-beams are particularly preferred. 
The beams used can be formed by substantially any procedure for embedding 
reinforcing fibers within a solidified plastic material. The pultrusion 
process is particularly suitable. Examples of suitable plastics include 
the commercially available epoxy and polyester resins such as epoxy 
impregnated graphite fibers, Rigidte fibers from Narmco Development of 
Celanese Corporation, glass fiber and aromatic polyamide fibers such as 
the aramid fiber Kelvar from DuPont, the reinforcing fiber glasses and/or 
glass cloths from Owens Corning Glass Company. Such plastics can contain 
conventional hardeners, cure rate modifiers, etc. The adhesives used in 
the present process can be the commercially available epoxy or polyester 
or the like adhesives for forming plastic structures.