TECHNICAL FIELD

The present invention is in the field of field-assembled box beams, particularly a beam comprised of two identical sections wherein a first section matingly engages with a second section to form a box or tubular structural member.

BACKGROUND ART

Existing two-piece box or tube beam sections have a substantial U-shape wherein the flange sections overlap and a fastener is then inserted or driven through the two overlapping flanges. Because the free end of each of the flanges is not laterally supported, the allowable design strength is significantly lower than the ultimate strength for the section and material of the section (even assuming a factor of safety) when compared to a conventional tubular section or two welded conventional channel sections. For example, in a two-piece box beam constructed of two overlapping C-shaped beam sections, the ultimate strength of aluminum is around thirty- eight (38) ksi and the allowable design strength of the resulting box beam from the overlap of two U-shaped sections is around eight (8) ksi. Thus, the allowable design stress for current field- constructed box beam made from overlapping C-sections is about 21% of the materials ultimate stress. While this configuration of a box beam section provides ease of construction in the field, it results in a very inefficient use of raw materials and restricts the working span lengths.

There are no known currently existing box beam sections that allow a designer to use an increased design stress associated with unitary box beams such as conventional tubular sections or welded channel sections. U.S. Patent Publication No. 2007/0074480 to Kleila, et al (the "'480 application") discusses issues that pertain to the design of currently existing box beams which are constructed from two conventional C-shape sections (extrusions 212 and 214). In Figs. 6 and 7 of the present invention replicate figures 1A and IB of the '480 application. Fig. 6 illustrates a web-buckling failure mode of a box-beam section 210 wherein the thin- walled web 216 displaces outwardly under loading and the displacement and elastic yielding of the material results in failure of the box beam section 210. Fig. 7 show a failure mechanism of box-beam 210 in which flange 218 fails due to local buckling from displacement of the free-end in the upward direction and the other flange 220 fails due to local buckling from displacement of the free-end in the downward direction. Physical testing corroborates the statements made in the '480 application concerning the proper application of the Aluminum Design Manual, Part Al and its application to local flange buckling, but their illustrated deflections in Figs. 6 and 7 may not be correct. As predicted by the Aluminum Design Manual, Part Al, when loaded, depending on size, these beams fail at stresses as low as 20% of the ultimate tensile strength, and the Aluminum Design Manual, Part Al is highly accurate in predicting the stress at which a particular beam will fail in this mode. However, physical testing of the beams in the '480 application in Figs. 6 and 7 do not fail as illustrated in the figures and as generally hypothesized in the art. Testing demonstrates that the failure may occur as a ripple in which each flange yields in both the upward and downward directions which results from the free-edge of the flange not being adequately supported.

The '480 application also discloses self-mating beam shapes that insufficiently encapsulates the unsupported edge to prevent a ripple type failure due to local buckling similar experienced in the embodiment shown in Figs. 6 and 7. In practice, the beam sections shown in the '480 application fail under physical testing at close to the same stress level as the embodiment shown in Figs. 6 and 7 due to the fact that the free edge of the flanges are not laterally supported in both directions and local bucking of the free edge of the flanges is experienced at similarly low stresses like the pure C-shaped sections. This result was unexpected, but the free edge of the flange is not sufficiently laterally supported in both directions and the beam fails due to local buckling of the flange member. There is nothing in the current art or in the '480 application that teaches or suggests that a critical tolerance must be maintained to benefit from allowable design stress of a unitary box beam rather than the C- shaped half section, and the figures of the '480 application clearly illustrate an embodiment that allows for movement of the free end of the flange outside of the critical tolerances. There is no existing teaching or suggestion as to what the critical tolerance is or how it is determined. Thus, the section of the '480 application as described has a shortcoming because it fails to be the structural equivalent of a unitary box beam, such as a similarly sized tubular section, because of the local buckling of the flanges. An increase in allowable design stress allows more efficient use of the material as it allows the same box beam section to have an increased span or, alternatively, reduces the amount of material that must be used for the same span. Thus, there is a need in the art for a box beam section that maintains the ease of use and installation currently provided by the overlapping box beam sections, but also allows the designer of to use an increased design stress of a unitary box beam to improve the material efficiency experienced in the industry providing lower material costs for the contractor and the consumer.

BRIEF DESCRIPTION OF DRAWING

The accompanying drawings form a part of the specification and are to be read in conjunction therewith, in which like reference numerals are employed to indicate like or similar parts in the various views.

FIG. 1 is a sectional view of one embodiment of a self-mating beam section in accordance with the teachings of the present invention;

FIG. 2 is a sectional view of another embodiment of a self-mating beam section in accordance with the teachings of the present invention;

FIG. 3 is a sectional view of two self-mating beam sections of Fig. 1 in a mating engagement;

FIG. 4 is a sectional view of two self-mating beam sections of Fig. 2 in a mating engagement;

FIG. 5 is an enlarged sectional view of the mating engagement of a free end of one beam section with another Beam section of the two self-mating beam sections of Fig. 2;

FIG. 6 is a sectional view of a two-piece box beam in accordance with the prior art and illustrating a web buckling failure mode; and

FIG. 7 is a sectional view of a two-piece box beam in accordance with the prior art and illustrating a flanged buckling failure mode. BEST MODE FOR CARRYING OUT THE IN VENTIO

The following detailed description of the present invention references the accompanying drawing figures that illustrate specific embodiments in which the invention can be practiced. The embodiments are intended to describe aspects of the present invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the spirit and scope of the present invention. The present invention is defined by the appended claims and, therefore, the description is not to be taken in a limiting sense and shall not limit the scope of equivalents to which such claims are entitled.

As illustrated in Figs. 1 and 2, the present invention is directed towards a self-mating aluminum beam section 10 and 10' that can be used to assemble box beam or tubular-shaped structural members in the field. In particular, as shown in Figs. 3 and 4, a box beam can be formed by coupling one length of self-mating beam section 10 or 10' with an opposing second length self-mating beam section 10 or 10' wherein the two sections matingly engage to form the box beam.

Figs. 1 and 2 illustrate embodiments wherein self-mating aluminum beam section 10 or 10' comprises a web 12, a first flange 14 and a second flange 16. Web 12 includes a first end 18 and a second end 20 that define a web length, and an outer surface 22 and an inner surface 24 that define a web thickness.

First flange 14 includes a first end 26 and a second end 28 that define a first flange length, and an outer surface 30 and an inner surface 32 that define a first flange thickness. First end 26 is coupled to web 12 proximate first end 18 of web 12. First flange 14 extends away from inner surface 24 of web 12 wherein second end 28 is generally, and may be referred to herein as, a free end. First flange 14 includes an overlapping portion 34 having one or more longitudinal ribs 36 disposed on its outer surface 30. Overlapping portion 34 is generally orientated substantially perpendicular to web 12, although, angular orientations are within the scope of the present invention. Self-mating beam section 10 and 10' may also include a longitudinal groove 38 that is continuous along the length of self-mating beam section 10 and 10'. Groove 38 is positioned proximate the outer surface 30 of first flange 14 and proximate the intersection of the web 12 and flange 14. Longitudinal groove 38 may extend substantially perpendicular to overlapping portion 34 and self-mating beam section 10 may have a stiffening portion 40 disposed laterally adjacent to groove 38. As shown, stiffening portion 40 may comprise an outer portion 40a and an inner portion 40b. Stiffening portion 40 may extend substantially laterally and perpendicular to groove 38 and overlapping portion 34 as shown in Figs. 1 and 2.

Fig. 2 illustrates an embodiment of self-mating beam section 10' having a similar configuration to self-mating beam section 10, wherein first flange 14 further includes a transition portion 42 which provides an angled transition from web 12 to first flange 14. This configuration may be desirable as it provides a self-mating beam section 10' which may include a stiffening leg 44 which may define a cavity 46 which may receive a molding for windows, a spline for a screen, or fasteners for windows, screen sections, or ceiling or wall sections. For example, as shown in Fig. 2, cavity 46 includes a rounded closed end 70 and transition portion 42 includes an angled face 72 wherein the angled face angles away from overlapping portion 34. In this configuration, securing a spline of a screen in cavity 46 would also act to stretch the screen slightly. Generally, stiffening leg 44 and cavity 46 may be configured to matingly engage or be otherwise complimentary with any panel, window, or screen type, installation method, molding, or fastening system as desired by manufacturer or installer. As shown, groove 38 or 60 may be proximate the end of transition section 42 or 64.

Turning back to Figs. 1 and 2, second flange 16 includes a first end 48 and a second end

50 which define a second flange length and an outer surface 52 and an inner surface 54 that define a second flange thickness. First end 48 is coupled to web 12 proximate second end 20 of web 12. Second flange 16 extends away from inner surface 24 of web 12 wherein second end 50 is generally, and may be referred to herein, as a free end. Second flange 16 includes an overlapping portion 56 having one or more longitudinal ribs 58 disposed on its inner surface 52. Overlapping portion 56 is generally orientated substantially perpendicular to web 12 although, angular orientations are within the scope of the present invention. Self-mating beam section 10 and 10' also includes a longitudinal groove 60 that may be continuous along the length of section 10 and 10' or may be intermittently located along the length of the section 10 and 10'. Groove 60 is positioned proximate the inner surface 54 of second flange 16 an proximate the intersection between the flange 16 and web 12. Since these sections are typically extruded, a continuous groove is preferred. Longitudinal groove 60 may extend substantially perpendicular to overlapping portion 56 and second flange 16 may have a stiffening portion 62 disposed laterally adjacent to groove 60. Alternatively, grooves 38 and 60 may be similarly disposed on inner surface 24 of web 12. As shown, stiffening portion may comprise an outer portion 62a and an inner portion 62b. Stiffening portion 62 may extend substantially perpendicular to groove 60 and overlapping portion 34 as shown in Figs. 1 and 2. The present invention also includes embodiments wherein the stiffening portion (not shown) is adjacent to groove 60, but may be extend at an angle with respect to groove 60.

Fig. 2 illustrates an embodiment of section 10' wherein second flange 16 includes a transition portion 64 which provides an angled transition from web 12 to second flange 16. This configuration may be desirable as it provides an area which may include a stiffening leg 66 which may define a cavity 68 which may receive a molding for windows, a spline for a screen, or fasteners for windows, screen sections, or ceiling or wall sections. For example, as shown in Fig. 2, cavity 68 includes a rounded closed end 70 and transition portion 64 includes an angled face 72 wherein the angled face angles away from overlapping portion 34. In this configuration, securing a spline of a screen in cavity 68 would also act to stretch the screen slightly. Generally, stiffening leg 66 and cavity 68 may be configured to matingly engage or be otherwise complimentary with any panel, window, or screen type, installation method, molding, or fastening system as desired by manufacturer or installer.

Fig. 3 illustrates two of the sections of the self-mating beam section 10 of Fig. 1 in a mating engagement. Fig. 4 illustrates two of the sections of the self-mating beam section 10' of Fig. 2 in a mating engagement. As shown in Figs. 3 and 4, when inside surfaces 24a and 24b of beam section 10a and 10b and lO'a and 10'b are opposing, the flanges and grooves line up so that the flanges of one section are received into the grooves of the other section. Overlapping portion 34a of first beam section 10a and lO'a overlaps with overlapping portion 56b of second beam section 10b and 10'b and overlapping portion 56a of first beam section 10a and lO'a overlaps first overlapping portion 34b of 10b and 10'b. In particular, second end 50b of second flange 16b is received into groove 38a of first flange 14a, second end 28b of first flange 14b is received into groove 60a, second end 28a of first flange 14a is received into groove 60b, and second end 50a of second flange 16a is received into groove 38b. This configuration is especially beneficial for constructing structural box beams at the construction site.

When the ends of the flanges 14 and 16 of both sections are received into the grooves 38 and 60 as shown in Figs. 3 and 4, a fastener 74 is inserted or driven through the overlapped flanges. In one embodiment, fastener 74 is inserted or driven proximate the location of longitudinal ribs 36 and 58. This arrangement engages the longitudinal ribs 36 and 58 of the two sections 10a and 10b or 10'a and 10'b to help resist separation of the two sections in a direction parallel to the flanges 14 and 16.

Once the second ends of each flange 14 and 16 are received into a groove 38 or 60 in the mating engagement, the free second ends 28 or 50 are effectively restrained from lateral displacement in both directions. Tests have shown that this construction increases the allowable design strength of a conventional site-constructed box beam without the mating engagement of second ends 28 and 50 and grooves 38 and 60. In one embodiment of the present invention, the allowable design strength of an extruded aluminum section doubled from eight (8) ksi for a C- shape section to sixteen (16) ksi for a unitary box beam. Thus, substantial material efficiency may be realized using the present self-mating beam section 10 and 10'.

Fig. 5 illustrates the engagement of second end 50b of second flange 16b with groove 38a, but the following principles are applicable to all flange/groove mating circumstances. As shown, flange 16b has a flange thickness Tf and groove 38a has a groove width Wg. Groove width Wg may be slightly greater than the flange thickness Tf to facilitate the flange 16a being received into groove 38a. The allowable clearance, or tolerance of movement, is the difference between the groove width Wg and flange thickness Tf. In order for a pair of mated sections to be designed equivalent to a unitary box beam, the free ends of the flanges of the mated sections can only deflect a certain and limited amount in either lateral direction. Otherwise, the flanges will experience a localized buckling at a loading stress which is less than the design stress for a unitary box beam. In order for a beam to meet the requirements of the Aluminum Design Manual, Part Al, for the section to be considered equivalent to a unitary box beam, the built-up beam section must act in a unitary manner. Thus, it is imperative that the tolerance of lateral movement, and hence the amount of deflection that can be allowed, to prevent failure due to this local buckling mechanism is limited. Turning to Fig. 3 and Fig. 4, the tolerance for movement of second end 50b of second flange 16b within groove 38a, of second end 28a of first flange 14a within groove 60b, of second end 28b of first flange 14b within 60a, and of second end 50a of second flange 16a within 38b is a maximum of around 0.010 inches in order to prevent premature failure of the flange and beam section due to local buckling.

A novel approach to estimating the allowable clearance or tolerance of movement for the free end of a flange of a self-mating beam section is to analyze the buckling of a similarly dimensioned unitary tube shape. For example, a unitary rectangular tube shape that at is nominally two inches by four inches (2"x4") having a wall thickness of fifteen-one-hundredths of an inch (0.15") was used to model one embodiment of the present self-mating beam section. This analysis is based upon a calculation of allowable stress set for aluminum shapes the Aluminum Design Manual ("ADM") produced by the Aluminum Association, 1525 Wilson Blvd., Arlington VA 22209. The allowable stress per the Aluminum Design Manual ("ADM") is approximately sixteen and eight tenths (16.8) ksi for a unitary box beam of these dimensions.

The engineering formulas that apply for the calculation of this deflection for various rectangular geometries, such as conventional tube sections, are as follows:

Stress _max =M _max / Sectional Modulus.

fma ⁼ W _max x 5 x (l) ³ /384 x (the Modulus of Elasticity) x (the Moment of Inertia).

In the above formulas, "M _max " is the maximum moment applied to the beam, "w" is total uniformly distributed load in pounds/foot, "1" is the length of the beam between supports and is used in several of the formulas below, Stress _max equals the maximum stress experienced by the beam when the M _max is applied; f _max is the maximum deflection; W _max = the total load, "Sectional Modulus" and "Moment of Inertia" are properties specific to the geometry of the shape under consideration, "Modulus of Elasticity" is a property of the specific material being used.

In addition, formula 3.4-3-14 of the ADM for normal slenderness limits for a box-beam is: L _b S _c / 0.5C _b (Iy) ^'5 , with "Lb" being the Unbraced length of the beam, "Sc" being the Sectional Modulus of the beam - compression side, "Cb" being a coefficient depending on moment gradient that comes from the ADM, and "Iy" being the moment of inertia of the beam section.

In the example of a unitary rectangular tube shape that at is nominally two inches by four inches (2"x4") having a wall thickness of fifteen-one-hundredths of an inch (0.15") and an allowable design stress of 16.8 ksi, the maximum deflection cannot exceed plus or minus twelve- one-thousandths of an inch (+/- 0.012") in order for the member not to experience a stress level above 16.8 ksi. Thus, for a mated shape of similar net dimensions to function substantially equivalent to the box, it was posited that the maximum deflection of the free ends of a flange of a section 10 or 10' must not exceed plus or minus twelve-one-thousandths of an inch (+/- 0.012"). Thus, in the embodiment of Fig. 5, it was posited that the difference between the groove width Wg and the flange thickness Tf shown in Fig. 5 must not exceed twelve-one-thousandths of an inch (+/- 0.012") for the section to preform substantially equivalent to a unitary box beam and utilize full maximum design strength. In practice, the difference between the groove width Wg and the flange thickness Tf may be limited to around one-one-hundredth of an inch (0.01"). Subsequent testing has confirmed the inventor's hypothesis and the higher allowable stress values were observed as opposed to the lower failure stress experienced when testing the designs of existing sections.

Thus, it was unexpected that the maximum clearance between the flange and the groove can be estimated by the above formulas. These formulas result in one embodiment having the maximum clearance being one-one-hundredth of an inch (0.01"). In other words, if the difference between the groove width Wg and the flange thickness Tf as shown in Fig. 5 is less than one-one-hundredth of an inch (0.01"), then this construction allows the mated section to behave as a box beam and be designed with an increased allowable stress, even though, there may be some clearance 76 between outer surface 80 of flange 14a and inner surface 82 of flange 16b. As further shown in Fig. 5, to provide the minimal clearance between the groove width Wg and the flange thickness Tf, an embodiment of self-mating beam section 10 and 10' may include an outwardly projecting shoulder 78 defining the transition between outer surface 80 of flange 14 or 16 and groove 38 or 60.

The present self-mating beam section 10 and 10' may be made from extruded metal or polymer, with common metals being aluminum, stainless steel, copper, steel, or any other known extruded metal. Any substantially rigid polymer, PVC, or plastic may also be within the scope of the present invention. The present self-mating beam section 10 and 10' may also be cast or injection molded in defined lengths. In one embodiment, the present self-mating beam section 10 and 10' may be made from 6000 series aluminum alloys including 6005, 6066 and 6070, or 5000 series aluminum alloys, including 5050. Any of the materials may have a "brushed' finish that would eliminate or mask cosmetic issues due to fabrication.

As is evident from the foregoing description, certain aspects of the present invention are not limited to the particular details of the examples illustrated herein. It is therefore contemplated that other modifications and applications using other similar or related features or techniques will occur to those skilled in the art. It is accordingly intended that all such modifications, variations, and other uses and applications which do not depart from the spirit and scope of the present invention are deemed to be covered by the present invention.

Other aspects, objects, and advantages of the present invention can be obtained from a study of the drawings, the disclosures, and the appended claims.