US3000466A

US3000466A - Folding scaffold

Info

Publication number: US3000466A
Application number: US853041A
Authority: US
Inventors: Wallace J S Johnson; Robert E Fisher
Original assignee: Up Right Inc
Current assignee: Up Right Inc
Priority date: 1959-12-01
Filing date: 1959-12-01
Publication date: 1961-09-19
Anticipated expiration: 1978-09-19

Description

'Sept. 19, 1961 w. J. s. JOHNSON- ET AL 3,000,466

FOLDING SCAFFOLD Filed Dec. 1, 1959 4 Sheets-Sheet 1 INVENTORS:

Wallace J..S.Jo/7nson lZoberf E,F/sher WWW ' ATTORNEYS p 1961 w. J. s. JOHNSON ETAL 3,000,456

FOLDING SCAFFOLD 4 Sheets-Sheet 2 Filed Dec. 1. 1959 INVENTORSI Wallace J. .S.dohn$on Roberf' E.Ffsher ATTORNEYS Sept. 19, 1961 w. J. s. JOHNSON ETAL 3,000,466

FOLDING SCAFFOLD Filed Dec. 1, 1959 4 Sheets-Sheet 3 INVENTORS: Wallace J. $.d0hnsan BY Qoberf Ef/Zsher WWW ATTORNEYS p 1961 w. J. 5. JOHNSON ETAL 3,000,466

FOLDING SCAFFOLD Filed Dec. 1, 1959 4 Sheets-Sheet 4 INVENTORS. Wallace J. 5. Johnson BY Qoberf E. Fisher ATTORNEYS United States Patent 3,000,466 FOLDING SCAFEOLD Wallace J. S. Johnson and Robert E. Fisher, Berkeley, Calif., assignors to Up-Right, Inc, Berkeley, Calif., a corporation of California Y Filed Dec. 1', 1959, Ser. No. 853,041

'10 Claims. (Cl. 182-118) This invention relates to support devices, and more specifically involves a cross bracing structure for interconnecting spaced vertical support frames and members. It is believed that this invention provides a novel folding-bracing principle applicable to perhpas several types of devices but having particular significance in the construction of scaifolds.

One object of this invention is to provide an improved support structure including a cross bracing formed with two pairs of pivotally jointed bracing members and having a disconnectable means for selectively joining their pivoted junctions. i

A second object is to provide a support structure including collapsible cross bracing of the kind described and having offset connections permitting the structure to be compactly folded, yet having no loose bracing members.

A third object of this invention is to provide a support structure including disconnectable cross bracing members and having a novel means for joining said bracing members into a rigid bracing structure.

Another object is to provide a support structure including a collapsible cross bracing of the kind describedand having means provided for preventing uncontrolled spreading of the support and bracing members. i

It is a further object of this invention to provide a one-piece collapsible scaffold structure including spaced vertical support frames, said scaffold having no separte braces or loose parts after its erection and requiring no tools for its assembly. Such scaffolds as herein described can be erected by one man in a matter of seconds and the simplicity of the scaffold construction prevents an improper or unsafe assembly.

Still another object is to provide a collapsible scaffold having a pair of spaced vertical supporting frames and including a bracing structure comprising: a pair of first bracing members pivotally joined together at one end and having opposite ends pivotally joined to respective vertical supporting frames; a pair of second bracing members pivotally joined together at one end and having opposite ends pivotally joined to respective vertical supporting frames at points above respective first member junctions; disconnectable means for selectively joining said first and second bracing members proximate their own pivot junctions; a pair of third bracing members pivotally joined together at one end and having opposite ends pivotally joined to respective vertical supporting frames; and means for locking said pivotally connected junction of said third bracing members into a rigid structure.

Anadditional object of this invention is to provide a collapsible scaffold having a pair of spaced vertical supporting frames and including a bracing structure comprising: a first ladder structure having a first pair of spaced bracing members interconnected by steps, a second pair of spaced bracing members pivotally joined to respective first bracing members, said first and second bracing mebers also being pivotally joined in pairs to respective vertical supporting frames; a second ladder structure having a third pair of spaced bracing members interconnected by steps, a fourth pair of spaced bracing members pivotally joined to respective third bracing members, said "ice third and fourth bracing members also being pivotally joined in pairs to respective vertical supporting frames and at points above respective second and first member junctions; whereby said first and second ladder structures complement each other, forming a ladder structure spanning said supporting frames.

Other objects and advantages of this invention will become apparent in view of the drawings and the following description.

In the drawings forming a part of this specification, and in which like parts are designated by like reference numerals throughout the same,

FIG. 1 illustrates a series of elevational views of a collapsible span scaffold as one embodiment of this invention. FIG. 1a is an end elevation while FIG. 1b and 1c are side elevations, respectively, showing the scaffold in a fully collapsed position and a partially collapsed positiOn;

FIG. 2 is a side elevation of the scaifold of FIG. 1 in its initial position for erection;

FIG. 3 is a side elevation of the scaffold of FIG. 1 in its fully erected position;

FIG. 4 is a perspective view of the erected scafiold otherwise illustarted in FIGS. 1 3.

FIG. 5 is the rear elevational view of the disconnectable means emploeyd for joining the cross bracing of the scaffold shown in FIGS. 1-4, the fittings that form the disconnectable means being shown in disconnected juxtaposition and prepared for making a locking engagement;

FIG. 6 is a section taken on lines 6-6 of FIG. 5;

FIG. 7 is a perspective view of the disconnectable means shown in FIGS. 5 and 6 after the fittings have been mated;

FIG. 8 illustrates two elevational views of a second embodiment of this'invention as applied to a ladder scaffolding, :FIG. 8a being an end elevation and FIG. 8b a side elevation, both figures showing the scaffolding in a collapsed or folded position;

FIG. 9 is a perspective view of the ladder scafiolding of FIG. 8 in a fully erected position;

FIG. 10 is an elevational view of the disconnectable means employed for joining the cross bracing of the ladder scafiold, the fittings that form the disconnectable meaiis being shown in disconnected juxtaposition and prepared for making a locking engagement;

11 is a section taken on lines 1111 of FIG. 10'

I IG. 12 is a perspective view of the disconnectable means shown in FIGS. 10 and 11 after the fittings have been mated;

FIG. 13 is a partial transverse section of the locking pin structure employed for locking the fittings together;

FIG. 14 is an elevational view of a pivot junction that is utilized for connecting certain bracing members toits vertical support member;

FIG. 15 is a top plan view of the pivot junction of FIG. 14, taken on lines 15-15 and showing the offset provided;

FIG. 16 is a perspective view of the pivoted link juncture between bracing members 28; and

FIG, 17 is an elevational view of a pivoted hook memher on frame 1 14b.

Referring to FIGS. 1-4 of the drawings, there is shown one embodiment of this invention as conceived for a collapsible span scaffolding. The scaffolding generally comprises a pair of spaced

ladder support frames

10 and 11, each having a pair of vertical support members 12 with horizontal rung members 13, a frame-connecting bracing structure generally designated by reference numeral 14, and a detachable platform 15. The structural members are preferably formed from tubular metal such as aluminum which will provide lightness as well as strength; the platform can be made from plywood material. If desired, the support members 12 may be provided with casters, as shown, for enabling the scaffolding to be rolled into a designated position. These casters are locked by manipulation of an operating arm, such a type of caster being well known and commonly used in the scalfold art. A clasp arm 101 is also provided for retaining the scaffold in a folded position. Arm 101 is pivotally mounted to a horizontal rung member '13 of one frame and detachably connects to a member 13 of the opposite frame.

This invention is more particularly characterized by its bracing structure 14 which includes a first pair of bracing members 16 and a second pair of bracing members 17, both pairs of bracing members being pivotally joined in a manner to be more particularly described. Each pair of bracing members interconnects frames and 11, the ends of said bracing members being pivotally joined to each frame and said second bracing members being connected at points above respective first member connections. As best shown in FIG. 2, the first and second pairs of bracing members are interlocked with the other pair, thereby forming an interlocking V-shaped bracing arrangement. It will be apparent in view of the drawings that as the side frames 10 and 11 are moved away from each other the pairs of bracing

members

16 and 17 tend to unfold and their respective pivot junctions move toward each other until they occupy the erected position, illustrated in FIGS. 3 and 4.

FIGS. 5 and 7 illustrate the manner in which bracing

members

16 and 17 are pivotally jointed and the means for joining said members together. A pair of mated

fittings

18 and 19 is provided at the respective ends of a bracing member 16 and a bracing member 17. Each fitting is complementary to the other, having a receiving pocket 20 and a boss 21. Thus, when

fittings

18 and 19 are brought toward each other in the manner suggested by the arrows in FIG. 5, the boss and pocket of one fitting will be moved toward the pocket and boss, respectively of the other fitting.

It will be noted that

fittings

18 and 19 are rigidly affixed to but one bracing member, the other member of each pair being pivotally and laterally connected from each boss portion. As best shown in FIG. 5, the degree of pivotal movement between boss and pivoted brace member is limited by the engaging contact of a rear fitting surface 22 and the transverse surface 23 formed at the end of the pivoted brace member. It will be understood in view of the drawings that the surfaces 22 and 23 only limit the unfolding type movement of each pair of bracing members. Therefore, bracing members 16 can pivot upwardly (and members 17 can pivot downwardly) without limitation or obstruction until they occupy a completely folded position, as shown by FIG. 1b. The limited pivotal action, as provided by surfaces 22 and 23, prevents uncontrolled spreading of the bracing members while also establishing a solid connection between otherwise pivotally joined bracing members when they are placed into an erected position.

A locking means is also provided

adjacent fittings

18 and 19 for holding them together once they have been mated. For this purpose a spring actuated pinning mechanism 24 is secured to those brace members to which a fitting is attached. The locking mechanism comprises a pin member 25 that transversely pierces its retaining brace member and is spring biased into the path of the fitting on the other brace member. Pin 25 is located at a point such that when the fittings become completely opposed brace member and into a locking position, as best known in the perspective view of FIG. 7.

Details of the pinning mechanism 24, divorced from the related fitting structure, is more particularly shown in FIG. 13. The mechanism, in addition to pin 25, includes a spring 26 and a clip actuating lever 27, and may be installed as a unit in a tubular bracing member. It will be apparent from the drawings that the clip actuating lever is pivotally mounted to pin 25 and is adapted for folding against the outer surface of the brace member as shown in solid line, or may be pivoted to the opposite side of the pin where it operates as a cam for lifting the pin into the broken line position illustrated. Pin 25 is tapered at its end 25a to insure a proper locking of the fittings in their mated position while providing an even earlier locking of the parts. Because of the tapered contact between the pin and the rear contact surface on the opposite fitting, the pin will move behind said fittings rear surface before complete mating of the fitting parts. However, as the fittings are moved closer together a greater pin width emerges from its retaining tubular member thereby maintaining contact with the rear contact surface of the fitting. It will be apparent that the taper provided on the pin could as well have been made upon a fittings rear contact surface since it is only the relative tapering between the parts that is necessary to produce the desired effect.

The bracing structure 14 of the scaifold shown in FIGS. 1-4 also includes a pair of cross brace members 28 that are pivotally jointed and interconnect frames 10 and 11 at pivotally joined ends. The pivot juncture of members 28, as more particularly shown in FIG. 16, comprises a link member 29a which is pivotally connected at either end thereof to the ends of bracing members 28; This pivotal junction enables the axially aligned bracing members 28 to be pivotally folded together. A locking sleeve 2% is provided on one bracing member, and this sleeve is adapted to be moved over the pivotal junction mated, the end of said pin will clear the end of the structurally opposed bracing member. The spring bias will then cause the pin to be moved behind the end of the when the brace members are horizontally aligned, thereby rigidifying said members as a cross brace.

Each of the above described frame connecting pivot junctions is made upon frame offsets to enable the cross members to be folded into parallel adjacency. An example of such an offset is more particularly illustrated in FIG. 14 where member 17 is pivoted upon an axis 30 that is offset from the frame member 12. The fittings for such an offset mounting is provided with a cam lobe 31 that is adapted for engaging a transverse surface portion 32 on the brace member. Accordingly, the outward pivotal movement of brace member 17 is effectively limited to the position shown, although said member may be freely pivoted into parallel adjacency with vertical support member 12. Since each of the frame pivot junctions is provided with a similar type pivot connection, it will be understood that the scaffold is not completely free to assume any position, but that uncontrolled spreading of any one bracing member is prevented. Also, cam lobes 31 and surface portions 32 are designed to contact when each brace member is placed into its normally erected position. Thus, when the scaffold structure is fully assembled an additional point of contact is made, rendering the connections non-pivotal and making the structure more rigid.

FIG. 15 illustrates the particular ofiset arrangement provided at diagonally opposite frame pivot junctions of a bracing member 16 and a bracing member 17. This ofiset structure is inserted to place diagonally opposite members in a common bracing plane, thereby accommodating the cooperation between

fittings

18 and 19. The other member 16 and its opposing member 17 are also disposed for pivoting in a common plane by reason that different from each other pair member but in a plane common to the diagonallyopposed bracing member of the other pair. This relationship of members insures a proper actuation of the bracing structure for effecting a proper mating of fittings 18 and 191 With scaffold structures as above described and shown in the drawings, a folded scaffold unit may be assembled by separating frames and 11 in the manner illustrated in FIGS. 1 and 2. Where one-man operation is employed the vertical members 12 are initially allowed toincline inwardly, thereby maintaining a controlled balance of the structure while the cross brace 28 is straightened and made rigidf With the locking of sleeve 29b across the pivotedlink junction of members 28, the intermediate pivot junctions of

members

16 and 17 will be proximate one another. The operator .will, then, by grasping two diagonally opposite members 16' and 17 move their

respective pivot fittings

18 and 19 toward the other. The pins 25 of each locking mechanism 24 will engage the inclined surface of the diagonally opposite bracing member and be carnmed into a retracted position. Then, by moving the fittings toward oneanother and putting them into a mated position, pins 25 will emerge from behind the diagonally opposite member, locking the fittings together. In this condition and position 'of bracing members, each of the fittings pivot junctions is rigidified by contact of surfaces 22 and 23, and the'frame pivot junctions' are each rigidified by contact of

surfaces

31 and 32.

The spacing between frames 12 is of course established by the lengths of the bracing members. But by proportioning the lengths of cross brace members 28 to the lengths of

cross brace members

16 and 17, the bracing axis of each member 16 will be substantially parallel with the axis of its diagonally opposite member 17. This provides an almost continuous support axis between the frames and the torsional forces applied to the

fittings

18 and 19.

It will be further noted that pivotally joined bracing members are of equal length from their common pivot axis to the support pivot axes upon the frames, and that the common pivot axes of both pairs of bracing members are vertically aligned. This will be an essential condition for a collapsible scaffold which can be compactly folded as shown in FIG. 16. Since horizontal members 28 also form a part of this collapsible structure, the distance between a link pivot connection and the frame pivot axis for the joined member is the same for both members.

FIGS. 8-12 illustrate a second embodiment of this invention as applied to a stairway scaffold. In broad terms of inclusion, the stairway scaffold comprises support frames 110 and 111 having vertical support members 112 and horizontal ties 113 and cross bracing means 114, including stairway cross bracing 114a and a horizontal platform support frame 11412. As with the previously described scaffold structure, casters and clasp arms 101 may be provided.

The stairway cross bracing 114a comprises four pairs of parallel bracing

members

116, 117, 118, and 119, each pair connecting to a horizontal tie 113 upon a pivotal junction that is oifset between the frames, thereby per-' mitting compact folding. Bracing members 116 are pivotally joined to members 119, and members 117 are pivotally connected to members 118, each pair of pivotally joined

members

116 and 119 being pivotally interlocked with a pair of pivotally joined

members

117 and 118. A plurality of steps 120 is provided across

members

116 and 118 thereby forming ladder structures, which, when the scaffold is assembled as illustrated in FIG. 9, provides a continuous stairway spanning frames 110 and 111.

The pivotal connections between bracing members are made with fittings similar to that described for the previous embodiment. However, it will be noted that the pivotally joined bracing members 116 through 119 are shorter in length than the vertical distance between the two horizontal tie members 113 of each frame from which they are supported. Thus, when th scattold is foldedthe fittings take a position between the supporting pivot junctions provided upon the tie members 113. If fittings illustrated in FIG. 5- were'used, portions of the

fittings

18 and 19,- Which define pockets 20, w lie in the pivot plane of one bracing member and p even-t complete folding of the scafiold. Accordingly, clearance has been provided upon each fitting such that there will be no interference between fittings and bracing members. This. clearance k sha e f h cross. brace filfil somewhat different from those previously describ d.

Referring more particularly to FIGS. 10-12, there is shown a pair of mated fittings .121 and 122 which may be used for the ladder scaffold of FIGS. 8 and 9,. Two pairs of these fittings will of course be necessary, one for each four bracing members on both sides of the ladder struc: ture.

Fittings

121 and 122 are rigidly afiixed to one bracing member and have another member pivotally and laterally connected thereto. Each fitting is complemen: tary to the other, having a stop member 123 and a boss .124. When the fittings are moved toward one another, the boss of one engages a lip edge 125 provided by the back edge of the others stop member. It will be noted that the lip edges limit relative movement of the fittings, but being less pronounced than the pocket surface of

fittings

18 and 19, they provide clearance for

members

116 and 118, when upon folding the scaffold, said members are respectively pivoted into alignment with

members

117 and 119. Such a clearance was unnecessary in the previously described structure since the

fittings

18 and 19 extended above or below the frame pivot junctions of the opposite members.

The degree of pivotal movement between a fitting and a pivoted brace member is limited by the engagement of a rear fitting surface 126 and a brace member surface 127 in the same manner and for reasons previously given in describing the

fittings

18 and 19. Also, each pair of fittings is provided with a locking mechanism 24- which operates identically to those devices provided with

fittings

18 and 19. While two locking mechanisms were provided for each pair of fittings in the other scaffold structure, only one such mechanism is used per pair of

fittings

121 and 122 since this allows a single person to operate both pairs of fittings simultaneously.

Horizontal platform support frame 114b is pivotally connected at offset junctions 128 in the same manner as for the stairway cross bracing 114a. The opposite end of support frame 114]) has a pair of hook members 129, that are pivoted to the frame 11 4b (FIG. 17) and are therefore adapted to be pivoted over tie 113 after frames and 111 are spread and frame 114b is raised to the proximate level of tie 113.

To operate the ladder scaffolding into an assembled position as illustrated in FIG. 9, it is only necessary to separate the frames 110 and 111 in the manner described for the span scaffolding of FIGS. 1-4. The horizontal support structure 114b is, however, positioned subsequently to the erection of cross bracing 114a.

Fittings

121 and 122 connect in precisely the same fashion as

fittings

18 and 19, each pinning mechanism dropping'behind the end surface of the opposite fitting as the fittings are mated.

It will be noted from the (folded side view, FIG. 811, that the scaifold frames 110 and 111 are of unequal height. The reason for this is that the scaffolding is designed to receive standard scaffolding unit of other construction on top. Of course, an upper tier scaffolding having the novel ladder bracing structure embodied in the illustrated ground support section is contemplated, and in such case the leg members which supplement vertical supports 112 on frame 111 will be longer than those leg members mating with supports 112 on frame 110. Since additional tiers of scaffolding will be initially 7 braced into a spread position by the vertical supports 112 of the lower tier, it is possible to omit pinning mechanisms from the fittings of upper tiers.

It will be apparent in view of the above disclosure and the drawings that there has been provided a novel improvement in support structures as well as scaffolding constructions. While several embodiments of this invention are shown and described, it is to be understood that various changes in the shape, size and arrangement of certain parts may be resorted to without departing from the'spir'it of the invention or the scope of the attached claims, and each of those changes is contemplated.

Having thus described our invention, what We claim and desire to secure by Letters Patent is:

1. In a support structure having a pair of spaced vertical support members, first and second pairs of first and second bracing members pivotally interconnecting said spaced support members, and the improvement comprising: a pair of mated fittings, said fittings being rigidly attached to the end of a first bracing member and pivotally joined to the end of a second bracing mem ber, each fitting having first and second surfaces thereon extending substantially transversely to the axis of said first bracing member and being axially located for being engaged by corresponding second and first surfaces, respectively, of said other fitting when said fittings are mated.

2.. The support structure and improvement of claim 1 including a means for locking said fittings in an engaged position.

3. The support structure and improvement of claim 1 wherein each fitting provides means for limiting pivotal movement of said second bracing member to establish a predetermined angle between axes of first and second bracing members when said fittings are mated, causing said bracing members to be rigidified.

4. A support structure having a pair of spaced vertical support members and including a bracing means therebetween comprising: first and second pairs of first and second bracing members, the bracing members of each pair being pivotally connected at one of their ends to said support members, a pair of mated fittings, said fittings being rigidly attached to the end of a first bracing member and pivotally joined to the end of a corresponding second bracing member, each fitting having first and second surfaces thereon extending substantially transversely to the axis of said first bracing member and being axially located for being engaged by the second and first surfaces, respectively, of said other fitting when said fittings are mated, a third surface provided on one of said fittings and extending substantially transversely to the axis of the first bracing member to which said one fitting is attached, and a locking pin reciprocally mounted in the first bracing member to which said other fitting is attached, said locking pin being engagab le with said third surface when said fittings are mated.

5. The support structure of claim 4 wherein each fitting provides means for limiting pivotal movement of said second bracing member to establish a predetermined angle between axes of first and second bracing members when said fittings are mated, causing said pairs of bracing members to be rigidified.

6. A collapsible scaffold having a pair of spaced vertical supporting frames and including a bracing structure comprising: a first ladder structure having a first pair of spaced bracing members interconnected by steps, a second pair of spaced bracing members pivotally joined to respective first bracing members, said first and second bracing members also being pivotally joined in pairs to respective vertical supporting frames; a second ladder structure having a third pair of spaced bracing members interconnected by steps, a fourth pair of spaced bracing members pivotally joined to respective third bracing members, said third and fourth bracing members also being pivotally joined in pairs to respective vertical supporting frames and at points above respective second and first member junctions; said first and second pairs of bracing members being interlocked with said third and fourth pairs of bracing members; whereby said first and second ladder structures complement each other, forming a ladder structure spanning said supporting frames.

7. A collapsible scaffold having a pair of spaced verti cal supporting frames and including a bracing structure comprising: a first ladder structure having a first pair of spaced bracing members interconnected by steps, a second pair of spaced bracing members pivotally joined to respective first bracing members, said first and second bracing members also being pivotally joined in pairs to respective vertical supporting frames; a second ladder structure having a third pair of spaced bracing members interconnected by steps, a fourth pair of spaced bracing members pivotally joined to respective third bracing members, said third and fourth bracing members also being pivotally joined in pairs to respective vertical supporting frames and at points above respective second and first member junctions; said first and second pairs of bracing members being interlocked with said third and fourth pairs of bracing members; and means provided adjacent each of said pivot junctions between bracing members for limiting pivotal movement of each bracing member with respect to a pivotally joined bracing member to a position of erection, thereby preventing uncontrolled spreading of said bracing members while providing a rigid connection when said scaffold is erected; whereby said first and second ladder structures comple ment each other, forming a ladder structure spanning said supporting frames.

8. The scaffold structure of claim 7 wherein each bracing member is pivotally mounted to said support frames upon offsets between said frames, allowing said bracing members to be pivoted toward said supporting frames into parallel adjacency therewith.

9. A collapsible scaffold having a pair of spaced vertical supporting frames and including a bracing structure comprising: a first ladder structure having a first pair of spaced bracing members interconnected by steps, a second pair of spaced bracing members pivotally joined to respective first bracing members, said first and second bracing members also being pivotally joined in pairs to respective vertical supporting frames; a second ladder structure having a third pair of spaced bracing members interconnected by steps, a fourth pair of spaced bracing members pivotally joined to respective third bracing members, said third and fourth bracing members also being pivotally joined in pairs to respective vertical supporting frames and at points above respective second and first member junctions; said first and second pairs of bracing members being interlocked with said third and fourth pairs of bracing members; first and second pairs of mated fittings, one fitting of each pair interconnecting a first and a second bracing member, the other fitting of each pair interconnecting the corresponding third and fourth bracing members; each fitting of said pairs having first and second surfaces thereon extending substantially transversely to the axis of one bracing member and being axially located for being engaged by corresponding second first surfaces, respectively, of the other fitting of said pair when said fittings are mated.

10. A collapsible scaffold having a pair of spaced vertical supporting frames and including a bracing structure comprising: a first ladder structure having a first pair of spaced bracing members interconnected by steps, a second pair of spaced bracing members pivotally joined to respective first bracing members, said first and second bracing members also being pivotally joined in pairs to respective vertical supporting frames; a second ladder structure having a third pair of spaced bracing members interconnected by steps, a fourth pair of spaced bracing members pivotally joined to respective third bracing 9 members, said third and fourth bracing members also being pivotally joined in pairs to respective vertical supporting frames and at points above respective second and first member junctions; said first and second pairs of bracing members being interlocked with said third and fourth pairs of bracing members; a rigid horizontal support member interconnecting said frames and having one end pivotally connected to one frame and detachably connected to the other frame; whereby said first and second ladder structures complement each other, forming a ladder structure spanning said supporting frames.

References Cited in the file of this patent UNITED STATES PATENTS Ritter Nov. 29, Miller Dec. 25, Johnson et a1 Mar. 23, Bobst Jan. 12, Hofheimer Aug. 16, Sheard May 8, Mulder Mar. 4, De Palma Dec. 15, Kowalski Dec. 27,