US2806561A

US2806561A - Portable structural assembly and interlocking units for constructing the same

Info

Publication number: US2806561A
Application number: US346815A
Authority: US
Inventors: William B Spangler
Original assignee: Individual
Current assignee: Individual
Priority date: 1953-04-03
Filing date: 1953-04-03
Publication date: 1957-09-17
Anticipated expiration: 1974-09-17

Description

Sept. 17, 1957 W.'B. SPANGLER 2,306,561 PORTABLE STRUCTURAL ASSEMBLY AND INTERLOCKING UNITS FOR CONSTRUCTING THE SAME 4 Sheets-Sheet 1 Filed April 5, 1953 rromvtrs 1 57 w. B. SPANGLER PORTABLE STRUCTURAL ASSEMBLY AND INTERLOCKING Sept. 17,

UNITS FOR CONSTRUCTING THE-SAME Filed April 3, 1955 4 Sheets-She et 2 llll ||l Illxlll llll .lllllllllllll-l l lnNlllllllll dl/lZ/AM 5. 5/91/7619? Sept. 17, 1957 w. B. SPANGLER PORTABLE STRUCTURAL ASSEMBLY AND INT 2,806,56 l ERLOCKING UNITS FOR CONSTRUCTING THE SAME 4 SheetsSheet 3 Filed April 3. 1953 INVENTOR.

i MILL/AM B. se /1mm ww I A 0 v P 17, 1957 w. B; SPANGLER 2,806,561

, PORTABLE STRUCTURAL ASSEMBLY AND INTERLOCKING 4 sheets-sheet 4 UNITS FOR CONSTRUCTINQ THE SAME Filed April 5, 1953 III/III syn-46 201mm 3. se /mm Unite PORTABLE STRUCTURAL ASSEMBLY AND IN- TfillslhOCKlNG UNITS FOR C(PNSTRUCTING THE S E i William B. Spangler, Groveton Heights, Va., assignor to the United States of America as represented by the Secretary of the Army The invention described herein may be manufactured and used by or for the Government for governmental purposes, without payment to me of any royalty thereon.

The present invention relates to structural units which may be assembled and mutually interlocked without requiring the use of bolts or other extraneous fastening devices into structural shapes of highly diversified application and character which are intended primarily to be of a portable or transitory character, the invention providing structural units which may be assembled into structural shapes such as portable runways or road surfacings that may be used in single layers or in multiple layers as desired for supporting various loads on subgrades with various load-bearing values. Also, the structural units may be interlocked so as to form bridge beams and decks, Coffer-dams, and other four-sided structures such a storage bins. However, regardless of the particular shape and intended utility thereof which is to be assembled from the units of the present invention, the assembling and interlocking of the individual units will be effected without the use of bolts or other extraneous fastening devices and without requiring the use of any unusual type of tools for effecting the assembling and also the disassembling of the units, both of which operations being effected easily and conveniently; and structural shapes of widely diversi fied character and of any desired strength can be assembled through the provision of the improved units of the present invention which may be assembled interlockingly into either single or multiple layer constructions of highly diversified character while eliminating the use of any kind of fastening instrumentalities throughout the assembled structure.

The principal object of the invention is to provide structural elements which may be assembled or interlocked into a wide variety of structural shapes, the units of the present invention being capable of forming shapes of either a single layer or multiple layers of such units and of wide applicability and utility, the structural shapes resulting from the units of this invention being characterized by the absence of bolts or any other kind of extraneous fastening devices, all as have been indicated above.

Other objects and advantages of the present invention will become apparent as the description proceeds, and the features of novelty will be defined in particularity by the appended claims. I

The invention will be understood more readily by reference to the accompanying drawings, in which Fig. 1 is a plan view of a plurality of panel units of the present invention interconnected to form a runway or a road surface, the view indicating several of the panel units interconnected both laterally and in end-to-en'd relation, parts of the assembled structure being broken out to accommodate the assembly to the size of the sheet of drawings;

Fig. 2 is a longitudinal sectional elevation taken on the line 11-11 of Fig. 1, looking in the direction of the arrows; Fig. 3 is a transverse sectional elevation taken on the States Patent 2,806,551 Patented Sept. 17, 1 957 ice 2 line IlI-III of Fig. 1, looking in the direction of the arrows;

Fig. 4 is a plan view of an assembly similar to Fig. 1, but scaled to show a complete panel unit and fragments of. contiguous units in end-to-end and side-by-side interlocking relation;

Fig. 5 is a plan view similar to Fig. 4, but indicating in greater detail the length and relationship of the side locking bars relative to the end joints between adjacent panels;

'- Fig. 6 is a sectional perspective of panel units assembled as in Fig. 1;

Fig. 7 is a view similar to Fig. 6 but showing the panels interlocked to form a runway or roadway surface composed of multiple layers for increased strength;

Fig. 8 is a view similar to Fig. 7 but showing the lower panel inverted to present a smooth surface to the ground for increasing stability of the assembled shape where the ground is soft and weight distribution over large areas is desirable;

Fig. 9 is a fragmentary sectional perspective showing units of the present invention interlocked into a foursided structural shape; 1

Fig. 10 is a perspective view of one of the end connectors as shown in Fig. l;

Fig. 11 is a side elevation, parts broken out for shortening, of one of the side connector and reinforcing bars;

Fig. 12 is a view of the units of the invention knockeddown and assembled into a package for shipping; and

Fig. 13 is a longitudinal sectional elevation taken on line XIIIXIII of Fig. 12.

Before proceeding with a detailed consideration of the drawings, it may be noted that generally the present invention comprises three different elements or units, the

manner of assembling these units producing the highly diversified character of the structural shapes for which the units are adapted. The present invention comprises three principal types of structural elements, namely, a panel unit, a connector bar, and an interlocking beam side connector. As has been indicated above, theseelements or units are so designed that they may be interlocked Without the use of bolts or any other extraneous fastening means to form various structural shapes of highly diversified. utility, such as, by way of example, a portable runway or road surfacing that may be assembled as a single layer or as a multiple layer structure for supporting various loads on subgrades with various load-bearing values. Other structural shapes which may be assembled from the present units include bridges of any required strength across small obstructions in emergencies; sheet piling to form cotter-dams for use in constructing foundations;

emergency revetments for the protection of bridge approaches, earth dams, or similar earth constructions; bridge beams and decks; storage bins; or virtually any four-sided structure which may be desired.

During fabrication in the production plant, the panel units areequipped with a plurality of connector bars at one end of each panel unit, the opposite end of the unit being open for receiving the connector bars of the next adjacent unit. Specifically, the panel units are equipped with five such connector bars at one end of the unit, although this specific number is not critical. However, it is employed as being suitable for the usual installations of the units, and it is desirable from fabrication considerations to make the parts standardized both as to dimen sions and characteristics. When the panel units are used vention. The connection is made by inserting a flange of the said connector beam into a recess on the edge of the panel and sliding the beam until equal parts of its length rest on each side of the end connection between panels. The recess on the edge of another panel then is slipped over the opposite flange of the connector beam. A second layer of the panels, or as many layers as may be desired'for structural strength, may be secured to the previously placed layer by using interlocking beam side connectors to form a vertical connection.

The accompanying drawings show the structural units of the invention as described above and various structural shapes that can be assembled therefrom. Since the units and the structural shapes derivable therefrom are intended to be readily portable, the individual units of the invention preferably and suitably are composed of some so-called light metal such as aluminum or magnesium, or light metal alloys, such as ,the well-known magnalium which is an aluminum-magnesium alloy containing typically from about 2%10 magnesium, about 1% copper, balance aluminum, or duralumin which is an alloy composed essentially of about 0.5% magnesium, about 0.25 to 1.00% manganese, about 3.5 to 4.5% copper, and 9395% aluminum, with traces of iron and silicon.

The individual units of the invention are .producible conveniently by extrusion, although duralumin can be machined like wrought iron and is notable for its strength, hardness and corrosion resistance :both with respect to chemicals, sea-water, and atmospheres high in humidity and industrial gases. Also, beryllium and suitable alloys thereof, is includible in the light metals suitable for the fabrication of the individual units of the present invention. It will be understood that there is no great criticality in the selection of the materials of construction .entering the composition of the individual units of the present invention, the above giving a fair indication of the desirable physical properties which these units should possess in addition to their lightness for readly portability and quick handling.

The interlocking of the individual shapes of the present invention is accomplished by fabricating the units in conjunction with interlocking elements of the type and characteristics described and claimed in United States Patent No. 2,589,304, issued March 18,, 1952, to William B. Spangler, to which patent further reference will be made hereinafter.

Detailed reference now :may be made to the accompanying drawings, it being recalled that the present invention comprises three fundamental or basic elements, namely, the panel units A; the end connector bars B; and the interlocking side connector beams C.

Considering first the panel units A, these are fabricated with a substantially flat surface 14 interspersed, however, with longitudinally extending spaced traction ribs 16. Extending from the reverse surface 18 of the panel units are a plurality of spaced parallel Webs 20 which extend longitudinally of the panels, there being specifically in practice six of these webs 20 for each panel of standard size, it being the practice to extrude these panels of equal standard size for interchangeability in the assembled shapes.

The web portions 20 terminate in transverse flanges 22 which are provided on opposite, longitudinal edges of .the web portions 20, these flanges 22 extending at right angles ,to the web portions 20 an equal distance from the sides 24, 24' of these web portions. These transverse flanges 22 are formed adjacent to their longitudinaledges with curved lateral hook flanges 26, which'depend inwardly from the outer extremities or edges thereof substantially at right angles to the transverse flanges 22 and parallel with the Webs 20, whereby recesses 28 are formed between the opposite sides 24, 24' of the webs 20 and along the longitudinal edges thereof. The longitudinal -edges 30 of the panel units A are formed similarly with lateral transversely extending flanges 32 which terminate in curved lateral hook flanges 34, each of which is disposed opposite to a similar hook flange 26. Each of the terminal hook flanges 34 depends inwardly from the lateral longitudinal edges of the panels A substantially at right angles to the surface 14 thereof and parallel to the outermost webs 20 adjacent to the lateral longitudinal edges of the panels, whereby a longitudinal lateral recess 36 is formed equal and opposite to an outside lateral recess 28 projecting transversely from the end web 20' along the longitudinal sides of the panels A, thus completing each of the panel units A.

There next will be described the end connector bars B, of which there are provided in practice five for each panel unit, mounted between the six Webs 20 for connecting adjacent panel units A in end-to-end relation. These connector bars B are also extruded units and comprise substantially planar backing portions which have an outer flat surface 38 and an inner flat Surface 40 from which extend longitudinal ribs 42 along each side of the connector bars B, these ribs or flanges 42 being complemental to the recesses 28 between the webs 20 (or 20) and the aforesaid lateral hook flanges 26, so that the end connector bars will fit between such recesses 28 and the reverse planar surface (inner surface) 18 of the panel units A. Figs. 1, 2, and 3 show these end connector bars B in place, and Fig. 10

shows a perspective view of one of these end connector bars B. When these bars B are mounted in an end of a panel unit A, they are forge-welded or otherwise suitably permanently united with such end of the panel units, the opposite end of each panel unit being left open for the insertion of these end connector bars of an adjacent panel so that any desired number of the panels may be interconnected in end-to-end relation, contiguous panels forming an intervening joint such as is designated at 44, as shown in Figs. 1, 4, and 5. Each of the joints such as those shown at 44 is held together in interlocking connection and reinforced by these end connector bars B. Thus, the end connector bars B unite adjoining panel units A to form one integral structure permitting loadsto be transferred from one panel unit to another and causing all the panel units to contribute to the integrity and strength of the entire structural assembly.

The interlocking side connector beams C next will be described, these beams being employed to interlock laterally adjacent panel units in side-by-side relation.

These side connector beams C are substantially in the form of I-beams including webs having transverse flanges on the opposite longitudinal edges thereof. The transverse flanges extend at right angles to, and equal distances from, opposite sides of the webs and are provided with curved lateral hook flanges which depend inwardly relative to the web from the outer extremities of the flanges and parallel to the webs, whereby recesses are formed on the sections adjacent to opposite sides of and along the longitudinal edges of the webs. These side connector beams are similar in every respect to the units described and claimed in the aforesaid Spangler Patent No. 2,589,304 except that the ends of adjacent beams may be beveled complementarily for a rigid interfit.

Figs. 6 and 7 show the shape of these locking beams, a side elevation of which is shown in Fig. 11, this last view showing mid-portions of such a beam broken out for shortening the view, the assembled position of these beams being shown in dotted lines in Figs. 4 and 5, which illustrate a dual purpose for such beams, which dual purpose is to interlock adjacent panel units laterally together while reinforcing and stiffening the joints 44 between adjacent panel units that are connected in end-to-end relation, thereby strengthening each of such joints above the strength and rigidity imparted thereto by the end connector bars B.

These interlocking side connector beams C are produced conveniently by extrusion, as are also the panel units A and the end connector bars B as aforesaid, and

'these beams'C constitute the third principal type unit of the present invention, as has been pointed out above herein. These beams C, since they interlock laterally adjacent panel units A, are disposed horizontally in structural shapes such as runways and road surfaces. As has been noted above, these beams C are essentially I-beams which are duplicates in dimensions for interchangeability without special special selection; and these beams C comprise a web 46 having transverse flanges 48 and 48' provided on opposite longitudinal edges of the web and extending at right angles to the web an equal distance from its opposite sides 50 and 50'. The

transverse flanges

48 and 48 are provided with curved lateral hook flanges 52, 52, respectively, which depend inwardly from the outer extremities or edges of the transverse flanges 48 and 48' substantially at right angles thereto and parallel with the web 46, whereby recesses 54 and 54' are formed adjacent to the opposite sides 50 and 50' of the web 46 and along the longitudinal edges thereof. The

transverse flanges

48 and 48 and the terminal hook flanges 52, 52 fit into the corresponding oppositely disposed terminal lateral recesses 36 and 28 of adjacent laterally disposed panel units A and interlock such adjacent panel units in side-by-side relation, as will be clear from the drawings. The beams C are made with the ends oppositely beveled, as indicated at 58 for reinforcingly interfitting with the ends of an adjacent beam, as will be apparent from the drawings.

The three types of units of the present invention enable the assembling of a wide variety of structural shapes. Thus, instead of a single layer runway or road surface with the panel units A interconnected end-to-end by the end connector bars B and side-by-side by the interlocking side connectors C, this single layer runway or road surface being the simplest structural shape producible from the three fundamental types of units, such runways or road surfaces may be assembled in multiple layers, as shown in Fig. 7 wherein superposed pairs of panel units constructed as has been described above, are interconnected laterally by the aforesaid interlocking side connector beams C placed horizontally as has been described above. These horizontally positioned interlocking side connector beams C are locked together by a further locking beam identical in every respect to the beams C, but positioned vertically as shown by Fig. 7, for example. Since such vertically interlocked beams are identical with the horizontal beams C, the fundamental types of units employed in the invention are not increased in numbers. The superposed panel units A when disposed as shown in Fig. 7 for enhancing mechanical strength in the resulting assembled shape are interlocked horizontally by beams C as has been described above, and interlocked vertically by such beams C turned at right angles, that is, positioned in vertically interlocking position, with web portion 46a, corresponding to web portions 46, positioned vertically, with transverse flanges 48a, corresponding to transverse flanges 48 and transverse flanges 48b corresponding to transverse flanges 48, extending horizontally at right angles to the web portion 46a an equal distance from its opposite sides 50a and 50b. The transverse flanges 48a and 48b are formed adjacent to their longitudinal edges with curved lateral hook flanges 52a and 52b which depend inwardly from the outer extremities or edges thereof substantially at right angles to the transverse flanges 48a and 48b and parallel to the web portion 46a, whereby recesses 54a and 54b are formed on the vertical units adjacent to the opposite sides 50a and 50b of the web portion 46a and along the longitudinal edges thereof.

Therefore, it will be seen that the vertically disposed locking unit shown in Fig. 7 is in fact exactly duplicative of the units C, and is, in fact, one of these interlocking side connector beams C turned through 90 degrees, the transverse flanges 42a and 42b interlocking in recesses 54' of the upper horizontal connector beam C and the recesses 54 the lower horizontal connector beam C. The upper and lower structural shapes are-mutually rigidly interlocked throughout the assembled extent of the shapes.

Fig. 8 shows a still further variety of the shapes obtainable by the assembly of the basic units of the invention. The assembled shape of Fig. 8 is similar to that of Fig. 7 except that the bottom panel units A are inverted with respect to Fig. 7 so as to present the flat surfaces 14 of the units in engagement with the surface of the ground on which the assembled shape is erected. This arrangement is of advantage where the ground or other surface beneath the assembly is soft or yielding and a wide contact area is desired to minimize tendencies of the structural shape, when assembled, from sinking into the soft underlying surface to an excessive extent.

In Fig. 9 there is still a further modified assembly wherein a pair of panel units A is disposed in vertical off-set relation such as in the construction of a four-sided structural shape. In this view, a horizontal panel unit is interlocked with a vertical panel unit through the use of two of the side connector beams C, one of which is horizontally disposed and interlocked at one end with the horizontal panel unit, the other of the beams C being vertically interlocked with the horizontal beam at one end and with the vertical panel unit A at the other end of the said vertical connector beam C.

The reference numbers and their applicability are the same in Figs. 8 and 9 as in Figs. 6 and 7. It is thought that none of these views requires further description, the relationship between the units being obvious from the drawings.

From the foregoing description, and from the showings of Figs. 1 through 11 of the drawings, it will be seen that the present invention comprises a plurality of interchangeable, identical, elongated units adapted to be interlocked together to form various forms of portable structural assemblies or shapes, the units being adapted to be inter locked together with their major transverse axes at right angles to each other, certain of the units being panel units, other of the units being connector bars insertible in an end of a panel unit and receivable in recesses provided therefor in the opposite end of the panel units for connecting adjacent panel units in end-to-end relation, the panel units having flat, substantially rectangular panel portions having end connector bar units permanently mounted in one end and correspondingly recessed in the opposite end for reception of the end connector bars of an adjacent panel unit to be connected in end-to-end relation, and having spaced parallel web portions depending from one side of the rectangular panel portions, certain of the web portions being terminal web portions adjacent to the longitudinal sides of the panel portions, the said longitudinal sides defining a depending inwardly turned edge flange defining a curved recess with the outer surface of the terminal web portions, the remaining units of the invention being side connector beams substantially in the form of I-beams including web portions, the web portions of the panel units and of the connector beams having elongated transverse flanges formed on opposite longitudinal edges of the web portions terminating in curved lateral hook flanges extending along outer longitudinal extremities of the transverse flanges and providing oppositely curved oppositely disposed recesses on opposite sides of the web portions at the longitudinal edges thereof, the interior surfaces of each pair of the recesses with the surface of the web connecting the two recesses being identical in configuration to the exterior surfaces of each of the transverse flanges including the curved lateral hook flanges thereon, the web portions and recesses of the units being identical in size and shape so that when in interlocked relation with each other, the curved hook flanges engaging in and interlocking with the curved recesses in contiguous units of the assembly. Also, as will be seen from the drawings, the beam units are adapted to be interlocked in similar manner with alternate units relatively rotated degrees about their longitudinal axes so that structural assemblies of a plurality of interlocked layers can be constructed, such layers being in diiferent horizontal planes, or in vertical planes, a rigid structural assembly always resulting, regardless of the specific shape of the resulting assembly. The ends of the lateral connecting beam units preferably are oppositely beveled so that a rigid and reinforcing interfit between contiguous ends of adjacent end-to-end beams can be effected.

With further reference to the panel units A of the present invention and the end connector bars B, it will be seen from the drawings that the panel surface from which the spaced parallel web portions depend is a substantially flat or planar surface intermediate the web portions, the panel surface at the junctures thereof with the depending webs having equal and opposite curvatures equal to that of the recesses formed between the said web portions and the longitudinal hook flanges on the transverse flanges 22. Therefore, the end connector bars B having planar backing portions provided with opposite planar surfaces 38 and 40 (Fig. 10) join the longitudinal flanges 42 in a curvature complemental to that between the junctures of the surface of the panel units and the web portions, the flanges 42 snugly seating in opposite recesses 28 of the transverse flanges 22 and interlocking with the inwardly turned edge flanges 26 thereon, so that if the panel units A are fabricated with n number of webs, there will be n1 rend connector bars extending beyond an end of each panel unit, the panel unit and end connector bars being in permanent interlocked relation with each other with the curved hook flanges of each end connector bar being interlocked with the complemental hook flanges on opposite recesses between the web portions and the transverse flange thereof, so that the curved hook flanges on at least one-half of the units B minus one are interlocked in opposing recesses of contiguous web portions. The spaces intermediate the contiguous web portions of the panel units are left open at their opposite ends for receiving similar end connector bars of a successive panel unit in end-to-end relation. It will be seen that through the provision of the end connector bar units B and the side connector beam units C, any number of the panel units may be employed in rigidly interlocked end-to-end and sideby-side relations for constructing a runway or roadway surface of any desired area.

The dimensional equality between the units of each type and the component parts thereof, enables the units to be packed together compactly for shipment as is illustrated by Figs. 12 and 13 of the drawings, one of the panel units being packed so that the web portions of one engage the underside of the adjacent panel surface, the end connector bars B intermediate the web portions being in place and the units C in horizontal position as shown.

Having thus described myinvention, what I claim as new and wish to secure by Letters Patent is:

1. A structural assembly comprising, in combination, a plurality .of interlocking structural units, the assembly including three types of individual structural units, one of which is a side connector beam, another of which is a panel unit, and the third of which is an end connector bar, there being a plurality of panel units locked together, certain of the panel units being interlocked laterally by side connector beams and certain of the panel units being curved hook flanges, the curved hook flanges extending inwardly from the transverse flanges and providing curved recesses on the inner side of each transverse flange, the interior surface formed by each pair of curved recesses and the surface ,of the web portion connecting them being identical in configuration to the exterior surface of each t an vers fl ge, chp nelun t includi sa pan pertion and spaced parallel panel webs extending from one side of the panelportion, each panel web embodying a transverse flange and curved hook flanges, the longitudinal sides .of each panel portion terminating in curved hook flanges, all of the transverse flanges and curved hook flanges of the side connector beams and panel units having identical size and configuration, the exterior surface formed by the transverse flange and curved hook flanges of each side connector beam meshing and interlocking with the surface formed by the outermost panel Web and curved hook flanges of each panel to integrally join adjacent panel units in lateral relation to each other, each end connector bar comprising a channel-shaped member having a web and flanges complemental in size and configuration to a channel shaped recess formed between adjacent panel webs and transverse flanges of the panel units, the web having greater thickness than the flanges, each end connector bar meshing with the channel-shaped recesses of two longitudinally aligned adjaf cent panel units to joint them in end-to-end relation forming an integral structural assembly. 7

2. A structural assembly comprising, in combination, a plurality of interlocking structural units, the assembly including three types of individual units, one of which is a side connector beam, another of which is a panel unit, and the third of which is anend connector bar, a plurality of the panel units being locked together laterally by a plurality ,of side connector beams, a plurality of the panel units being joined in end-to-end relation by end connector bars, each side connector beam having the general form of an I-beam including ,a web and a pair of elongated transverse flanges formed on opposite longitudinal edges of the web, the transverse flanges having extremities terminating in curved hook flanges, the curved hook flanges on the same side of the web extending toward each other and providing curved recesses .on the innerside of .each transverse flange, the interior surface formed by one side of the web and the two curved recesses adjacent to it having a configuration identical to that of the exterior surface formed by the outer side of each transverse flange, each panel unit including a panel portion and spaced panel webs integrally extending from one side of the panel portion, each panel w-eb having a transverse flange and curved hook flanges, the longitudinal extremities of each panel portion terminating in curved hook flanges, all of the transverse flanges and curved hook flanges of the side connector beams and panel units having the same size and shape, the exterior surface formed by the outer side of each transverse flange of the side connector beams meshing and interlocking with the surface of the same size and configuration formed by the outermost panel Web and curved hook flanges adjacent to it and integrally connecting adjacent panel units in lateral relation to each other, each end connector bar comprising a channelshaped member having a thick web portion and rounded flanges, the web having a greater thickness than the flanges, each end connector bar being the complement in size and shape to a channel-shaped recess formed between adjacent panel webs by the sides of the webs and inner sides of the transverse hook flanges and curved hook flanges, each end connector bar interconnecting with the channel-shaped recesses of a pair of longitudinally aligned adjacent panel units to lock them in end-to-end relation forming one integral assembly of mutually supporting and sustaining structural units.

3. A structural assembly as recited in claim 2, in which end joints between adjacent panel units joined together in end-to-end relation by the end connector bars are braced by side connector beams extending continuously across the joints from distances remote on each side thereof, each of the side connector beams having its ends oppositely beveled for reinforcing engagement with an oppositely beveled end .of an adjacent beam in end-to-end engagement.

A st ctu a a s mbly s r c ted in c a m 2, n which the side of the panel portion of each panel unit which is opposite the side from which the panel webs extend is provided with spaced traction ribs disposed longitudinally along the panel unit.

References Cited in the file of this patent UNITED STATES PATENTS 1,274,353 Palmer July 30, 1918 10 Kahn July 24, 1934 Davis Nov. 14, 1939 Wing Aug. 16, 1949 Coleman Dec. 19, 1950 Spangler Mar. 18, 1952 FOREIGN PATENTS Great Britain Sept. 27, 1950