US2263511A

US2263511A - Wall structure and method of producing same

Info

Publication number: US2263511A
Application number: US327712A
Authority: US
Inventors: Harvey B Lindsay
Original assignee: Individual
Current assignee: Individual
Priority date: 1940-04-03
Filing date: 1940-04-03
Publication date: 1941-11-18
Anticipated expiration: 1958-11-18

Description

NOV. 18, 1941. H, LINDSAY 2,263,511

WALL STRUCTURE AND METHOD OF PRODUCING SAME Filed April 5, 1940 3 Sheets-Sheet l Nov. 18, 1941. H. B. LINDSAY WALL STRUCTURE AND METHOD OF PRODUCING SAME Filed April 3, 1940 3 Sheets-Sheet 2 I In.v

VII/II/II/I/A NOV. 18, B. UND Y WALL STRUCTURE AND METHOD OF PRODUCING SAME Filed Apfii 3, 1940 3 Shee'ts-Sheet 3 F57. ,Fzya 7 Patented Nov. 18, 1941 UNITED STATE WALL STRUCTURE AND METHOD OF PRODUCING SAME Harvey B. Lindsay, EvanstonJll. Application April 3, 1940, Serial No. 327,112

12 Claims.

for'many other uses, whether insulated or not,

particularly where a high strength-weight ratio may be desired.

The invention also relates to the tensioning of the sheets on'the framings, more particularly of the order of at least 100 pounds per lineal foot of sheet edge, my present application being a continuation in part of my application for United States Letters Patent Serial No. 214,785, filed June 20, 1938,which latter is a continuation in part of my application for United States Letters Patent Serial No. 187,056, filed January 26, 1938.

As a preface to the statement of my objects in devising the invention, it may be stated that sheet metal, as for example, sheet steel of the lighter gauges, such as 22 gauge and thinner, is ordinarily rolled by the so-called pack-rolling method, and then annealed, such rolling resulting generally in the thickness, and to some degree also the temper, of such a sheet varying considerably over its area, the sheet, as for example of 26 gauge, often being .003" thicker in some portions than others.

Such variations in thickness (as much as and sometimes more) not to mention variations in temper, cause the sheet to present what are called hard" and soft spots, rendering the sheet very far from uniform in the relation between pulling effort and elastic stretch of the sheet, viz., the actual temporary elongation of the metal below its yield point (elastic limit) in the sheet, and thus such a sheet, in use, is subject to yielding at such points of concentrated stress long before the sheet as a whole will yield were the total pull or stress spread evenly over the entire edge of the sheet and consequently over the entire sheet. Such irregularities in sheet metal have therefore presented a condition which must be met in the production of flat wall structures in which the sheets are to be assembled in tensioned condition, with framings, with the sheets in the desired fiat uniformly tensioned condition, thus avoiding stress concentrations and/or waves in the sheet.

Furthermore, the fact that sheets, as for example hot-rolled annealed steel sheets, will stretch only slightly before exceeding their elastic limit, a 36" wide such sheet stretching only about across its width before exceeding its elastic limit, presents an additional condition to be met in the producing of a structure such as above referred to.

Certain of my objects are to provide lightweight metal structures, such as housing or partition structures, and comprising framings and panel sections of sheet metal in which an exceptional and unique strength-weight ratio is sework, rigid to the degree desired without the when subjected to a given pulling force insuilicient to reach the yield point. In other words, assuming the rigid gripping of the sheet edges by means operating to tension the sheet, stretching it within its elastic limit, the tension stresses are then concentrated at certain points or portions of the edges of the sheet though the grip and pulling movement is perfectly uniform along all edges, this usually causing waves or ripples necessity of braces, gussets, diagonals, and similar structure added to the framings or framework, as has heretofore been necessary in lightweight structures such as for example in the case of the specific structures above enumerated.

To provide adequate resistance against the severest wracking and other stresses in the plane of the wall by means of the sheet metal itself, instead of by the usual braces, gussets, diagonals, etc., in the framings.

To provide a unique strength in a panel or assembly of panels against transverse pressure or stress, without permanent distortion thereof, by

the use of light weight sheet metal instead of erecting of the sheets and the framings, under a perfectly uniform tension in all directions and holding them in this condition, such tension being within the elastic stretching limits of the as above pointed out, and in doing so meet the condition, for example, in the case of steel sheets, of the very slight elongation of the sheets before exceeding the elastic limit.

To provide for governing the degree of such uniform tension in the sheets in order to meet varying needs of various uses.

To provide method or means whereby the above-mentioned objects may be attained: and other objects as will be manifest from the following description.

Referring to the accompanying drawings:

Figure 1 is a view in side elevation of a portion of a car body embodying my novel construction with the sheets tensioned in accordance with my novel method.

Figure 2 is a fragmentary enlarged sectional view taken at the line 2 on Fig. 1 and viewed in the direction of the arrow.

Figure 3 is an enlarged sectional view taken at the line 3 on Fig. 2 and viewed in the direction of the arrow showing the sheet-securing bar, forming a part of the wall portion, in spaced relation to the structure with which it cooperates.

Figure 4 is a fragmentary sectional view like Fig. 3 and of the structure therein shown, showing the sheet-securing bar in position preparatory to initiating the operation of tensioning the sheet.

Figure 5 is a fragmentary sectional view like Fig. 3 showing a position occupied by the sheetsecuring bar in the operation of tensioning the sheet.

Figure 6 is a fragmentary sectional view like Fig. 3 showing a modification of the structure, the sheet-securing bar being shown in spaced relation to the structure with which it cooperates.

Figures 7, 8 and 9 are fragmentary sectional I views like Fig. 6 of the structm'e therein shown,

Fig. 7 showing the sheet-securing bar positioned preparatory to initiating the operation of tensioning the sheet, Fig. 8 showing this bar in one position occupied by it in the tensioning of the sheet; and Figure 9 showing the bar in its final seated sheet-tensioned position; and

Figures 10, 11 and 12, fragmentary sectional views like Fig. 3 illustrating several other modifications of the invention.

In the particular construction shown the car body of which the wall structure illustrated is a part, is supported on an underframe comprising cross bearers I3 carried by car trucks (not shown) and a surrounding frame formed of channel side sills, one of which is represented at I4, and end sills.

Extending over and along the end and side sills are angle irons one of which is shown at I5, with a metal sheet, hereinafter referred to, interposed therebetween, the angle irons and sills being secured together in any suitable way, as for example by bolts or rivets.

Superposing, and secured to, the underframe is a skeleton car-body-forming framelike structure comprising portions of the side walls, end walls and roof of the-car.

This frame-like structure comprising both vertical and horizontal members, a'portion of which for one side only of the car body is shown (which is suflicient for illustrating my present invention) is formed of metal bars I6 of channel shape with their channels I! facing outwardly, these bars preferably having oppositely extending flanges I8 on their free edges; openings thus being presented between the bars I6 for receiving metal sheets spanning them to be held in position on the framework by means as hereinafter described.

The various openings provided by the framework are closed with metal sheets I9 of an suitable kind, such as for example sheet steel, the marginal edges of which, flanged as hereinafter described before applying them to the framework, extend into the channels of the several frame bars I6 in which they are engaged by the bars 20, preferably of channel form as shown, which in being moved inwardly in the channels I'I, exert force against the marginal edges of the sheets in a direction to tension the sheets and preferably exert force against these marginal edges forclamping them against the bottoms IIa of the channels H to positively maintain the sheets in tensioned condition, the bottom surfaces I'Ia as shown being substantially normal to the inner surfaces of the adjacent side walls of the channels ll.

The bars 20, forming tensioning members, are forced into the channels I1 and held in seated position therein by any suitable means, as for example screws, rivets or bolts 2|, rendering these bars and consequently the respective sheets individually removable for replacement or substitution in the event they become damaged, or it be otherwise desired to remove them.

To counteract the tension of the sheets on the longer frame members, spreader-bars, one of which is shown at IIia, are inserted as compression members and firmly fastened to the channel bars I6.- These spreader bars also serve to prevent any tendency of the channels to bend open.

In the particular construction shown the metal sheet above referred to as interposed between the angle irons I5 and the sills and which is represented at 22, forms a floor for the car body with its edges upwardly bent as represented at 23 and the terminal portions of its edges flanged as in the case of the sheets I9, these flanged portions extending into the lower horizontal ones of the channels I1 and being tensioned and held in place by the lower ones of the bars 20.

Referring to the construction shown in Figs. 1 to 5, inclusive, which may be typical of all of the marginal edges of the several sheets and represents one form in which the flanges on the sheets may be provided, this construction involves the bending or drawing of the edge of the sheet to form a flange 24 the terminal edge of which is bent back to form a relatively wide hook 25 of substantially arcuate shape in cross section, shown as substantially semi-circular, to receive the adjacent leg of the bar 20, the hook thus being engaged by the bar 20 and becoming deformed thereby upon the moving of this bar inwardly in the channel H, the edge of the sheet becoming clamped firmly in deformed condition against the bottom Ila of the channel in the final movement of the bar 20 to seated position.

As shown in Figs. 3 and 4, the hook 25 is of such depth that when the sheet is initially applied to position on the framework, the bottom surface 26 of the hook is spaced from the bottom "a of the channel l1.

In the movement of the bar 20 inwardly in the channel I! from the position shown in Fig. 4 toward the position in which it clamps the deformed hook portion against the bottom of the channel as shown of the bar 20 in Fig. 2, it causes the hook to become unrolled toward straight condition into engagement at its outer margin with the bottom of the channel as shown in Fig. 5, in which movement of the hook its marginal portion must be free of obstruction, the sheet in this operation becoming somewhat irregularly tensioned, due to the resistance of the hook to deformation, and the irregular resistance of the sheet to elastic stretch as described.

As the bar 20 continues its inward movement from the position shown in Fig. 5 toward the position shown in Fig. 2, it exerts continuing deforming action on the hook 25 in a direction toward the bottom I la of the channel to force or deform the hook into the interior angle between the bottom of the channel and the adjacent side wall of the latter and in this movement increases the tension in the sheet. The force required to do this is less if the sheet will stretch sufllciently so that the margin of the sheet will travel with the toe of the bar and the latter can merely bend the sheet continuously sharper on one line of contact between the toe and the sheet, but greater force is required inthis action, if the sheet will not so stretch and the toe of the bar 20 (on its inward travel) must therefore successively so bend or draw new lines of contact into sharpening angles and necessarily at the same time substantially straighten out the previous lines of bend. This movement of the tensioner bar toe from one line of contact with the hook portion to another lower down thereon (all under severe drawing stress on the sheet hook material) I call slippage of the tensioner bar toe. Hence, as resistance to elastic stretching (tensioning) of the sheet is less opposite the soft portions thereof than opposite the hard portions, the inwardly moving bar toe will continue to draw down the sheet margin (increase the sheet tension) opposite the soft portions while having to bend new lines of sharp angularity opposite the hard portions, or in other words, causing "slippage of the tensioner bar toe opposite the hard portions of the sheet while not opposite the soft portions. This action continues until the tension of the soft portions of the sheet have been thus brought up to that opposite the hard portions, when further inward travel of the bar to its final position in which the sheet is clamped by the bar 20 against the bottom Ila of the channel I! (Fig. 2), simply increases the thus equalized tension of the whole sheet, because of the progressive deformation of the hook along its entire length previously described, until or unless the thus equalized tension in the sheet becomes thetent, in order to make it clear. Actually it is curled s I probably more nearly continuous, but in any event it results in a perfect and uniform tension throughout the sheet.

It is probable that with relatively stiff sheets (e. g. 22 gauge hot rolled annealed steel sheets) some relatively small part of this equalization takes plac before the round hook edge is unciently to contact the bottom of the channel, but this does not seem to be the case with lighter gauges of such sheets.

From the foregoing it will be understood that in operating the tensioning bars at each edge of the sheet, the sheet is placed under substantially uniform tension in both directions throughout its area, the tension produced causing measurable stretching of the metal within its elastic limit.

As it is undesirable to have too wide a channel in the framing channel members (of the preferred light shape shown) which would seriously increase the bending movement and consequent tendency of the channel to spread, it is desirable to have the hook of the sheet flange,

which obviously must be unobstructed in uncurling or opening out, as narrow or of as short a radius as is consistent in providing the action above described. This being the case, and realizing that the greater part of the tensioning effect is obtained after the margin of the sheet contacts the bottom of the channel, particularly in the case of the lighter sheets (e. g. 26.or 30 gauge hot rolled annealed steel sheets), and as the ordinary such sheets require some pulling down by the tensioner bar before they become flat though not in any appreciable tension, and as in such lighter gauges, even to do this may partly uncurl the round hook, it is desirable in such cases to form a hook which will not so uncurl without compensating tensioning effect on the sheet. For this purpose I prefer a hook shaped as shown in Fig. 6, having a sharp bend with a very small radius, say inside radius of the order of .020", and a bar toe of approximate conformation, so that its outer slightly rounded edge is the only line or lines of contact with the sheet hook, thus preventing .its inner edge from spreading the hook by striking the wider curved portion of the hook adjacent its edge.

Referring to the construction shown in Figs. 6 to 9, inclusive, particularly useful in developing relatively high tension values in the sheet within the elastic limit of the metal of the sheet, such as are desirable in railroad cars, trailer bodies, marine structures, etc., the hooks represented at 21 are made of general acute-angular form with erably slightly rounded to conform to the shape of said interior corners as shown at 34.

It will be immediately seen that with the arrangement shown in Fig. 6 the bending leverage. of the tensioner bar toe is so short that very considerable pressure can be exerted by the bar in its inward movement, toward the position in which it clamps the deformed hook against the bottom I'Ia of the channel, Fig. 10, before the hook opens out, during which movement in which the hook remains undeformed several hundred pounds tension per foot of edge may be developed in the sheet, and the remaining small distance the bar travels (after the marginal part of the hook has contacted the bottom of the channel) should be suflicient to equalize and develop the high tension in the sheet obtainable by using this form of hook.

While I have been able to find no formulae whatever governing the forces required for the progressive angular deforming of a given metal sheet as described, I have consistently found it true in all my tests that that force is always less than the force required to reach the yield point (elastic limit) of that sheet; Hence whatever form of hook is used (short of so sharp a bend as to injure the metal) there is apparently no danger of exceeding the yield point of the metal sheets in my invention. This being the case, it is apparent that when the tension in the sheet has been equalized and the force required to further increase the sheet tension is greater than the force required to further progressively deform the flange hook into the corner of the channel, then the sheet edge will not travel with the tensioner bar, but the latter will progressively deform the hook until the final seating and clamping position of the bar is reached, the highest tension reached being maintained.

In order substantially to control this tension, three factors toward such control are available.

(1) The shape of the hook; the greater the resistance to unbending or unrolling, the greater the contribution to tension of the sheet.

(2) The relative stiffness of the metal sheet; the stifier the material of the hook, the greater the resistance (of any given form of hook) to unbending and particularly to successive deforming of the hook material into the corner of the channel, and consequently the greater the contribution to increasing the tension of the sheet.

(3) The distance (when the sheet is in place on the channel) from the bottom of the hook to the inside bottom of the channel; provided this distance is not too great to permit the hook margin in unbending to contact the bottom of the channel as described, then generally speaking,

the less this distance the less the contribution 'sharp bend with its co-ordinating tensioner bartoe, as shown in Fig. 6, as with this arrangement it is possible to produce higher sheet tensions, as stated. Then, where such high tensions are not needed, but it is more important to avoid fatigu of the metal by lower maintained stresses, I reduce the distance from the bottom of the hook to the inside bottom of the channel.

As examples of preferred practice, producing uniform tension in the sheet, and involving the provision of the hook flanges on opposite edges of the sheet each flanged edge having one of the tensioning bars cooperating therewith, I quote the following (all using hot rolled annealed steel sheets and the distances specified between the hooks and the bottoms of the channels being between these parts in the initial positioning of the hooks in the channels with the bodies of the sheets in proper contact with the channel bars [6) Example No. 1 v

26 gauge. Tension desired about lbs. per foot of edge. Round hook (Fig. 3) with inside radius approximately Tensioner bar leg preferably not more than about A, thick. Distance between bottom of hook and inside bottom Ila of channel approximately Exdmple No. 2

26 gauge. Desired sheet tension about 300 lbs. per foot of edge. Round hook (Fig. 3) with inside radius approximately Tensioner bar leg preferably not more than about thick. Distance from bottom of hook to inside bottom l'la. of channel approximately 1?.

Erample No. 3

26 gauge. Desired tension about 500 lbs. per foot of edge. Round hook (Fig. 3) with inside radius approximately Tensioner bar leg preferably not more than about thick. Distance from bottom of hook to inside bottom Ila of channel approximately 3 or hook as in Fig. 6 with its inner radii approximately .020, and the bottom surface 35 of its bend approximately 132-" from the bottom Ila of the channel.

Ezrample No. 4

24 gauge. Desired tension about 1000 lbs. per foot of edge. Hook as in Fig. 6 with inside radius of approximately .020" but the distance from bottom surface 35 of hook to inside bottom Ila of channel approximately It is obvious that, if the sheet is fixedly anchored at one edge and tensioned from the opposite edge only, it is desirable to double the distance between bottom of hook and channel bottom that is required where the tensioning action is coincidentally operated at both edges.

In this general connection it is well to point out that in tensioning a sheet at both of opposite edges or at all edges, the operation should be of the continuous type, viz., partially and successively tightening the various tensioner bar involved, until all have been firmly driven home, similar e. g. to the usual practice in tightening the holding bolts of an automobile cylinder head block.

It is to be noted that if a sheet particularly "hard or particularly soft throughout or having excessive buckles be encountered in duplicating the above examples, a corresponding variation in results must be expected. The above examples represent practice with sheets selected for reasonable flatness from average runs, and with the flanges die-drawn.

The modifications shown in Figs. 10 to 12 represent additional forms of structure in accordance with my invention, by which the results produced by the structures of Figs. 1 to 9, inclusive, may, at least to a material and useful degree, be attained, in the use of any of the structures of Figs. l0'to 12, inclusive, where relatively low tension in the sheet is desired, and as to Figs. 11 and 12 where the metal of the sheet is relatively brittle under bending first one way and then the other. In these cases, while some degree of control and tension possibly may be lost, it will be seen that the actions and their results, caused by the forcing home of the tensioner bar, will be substantially the same, in Fig. 10 after the sheet fiange edge has contacted the channel bottom Ila, and in Figs. 11 and 12 after contact of the tensioner bar toe with the sheet flanges, as in the cases above fully described (Figs. 3 and 6) after the hook has contacted the bottom of the channel.

The construction shown in Fig. 10 involves the bending of th edge portion of the sheet to form a flange 36 terminating in an outwardly bent flange or ledge portion 31 preferably of such width that when engaged and bent by the adjacent leg of the tensioning bar herein represented at 38 and corresponding to the bar 20, it will engage at its terminal edge with the bottom Ila of the channel I! and will become clamped at this portion against the bottom l'la of the channel'in the final seating movement of the bar.

In this construction it will be noted that the bar 39 once its downward pressure has overcome the resistance of the bent edges (of the sheet) to unbending, exerts force against successive portions of th ledge portion 31 in a direction toward the bottom of the channel as the bar moves inwardly in the channel, thus exerting a pull on the sheet placing it in some tension, which pull is much increased after the terminal edge of the ledge portion contacts the bottom of the channel, the bar, in its final movement, clamping the tensioned sheet against the bottom of the channel. 1

The construction shown in Fig. 11 difiers from that shown in Fig. only in the respect that the flange of the sheet and represented at 39 and corresponding with the flange 36, is spaced .from the adjacent side wall of the channel II in the initial assembling of the sheet with the bar l6, and its terminal flange portion represented at 49 and corresponding with the portion 31 of Fig. 10 extends at an obtuse angle to the flange 39 and initially engages at its edge portion with the bottom l'la of the channel. The bar 38 in moving inwardly in the channel thus acts against successive portions of the flanged portion of the sheet to exert a pull thereon because of successive deforming of the flange of. the sheet, placing the sheet in tension and finally clamping the edge of the sheet in tensioned condition, against the bottom "a of the channel In the construction shown in Fig. 12, the edge of the sheet is provided with a. flange 4|, without any reverse bend, the flange 4| inclining toward the medial line of the channel l1 and thus spaced from the adjacent side wall of the channel and being of such width that it engages the bottom ||a of the channel in the initial assembly of the sheet with the bar l6. Thus as the bar 38 moves inwardly it engages successive portions of the flange 4| progressively deforming it and placing the sheet in tension and finally' clamping the edge of the sheet in tensioned condition against the bottom "a of the channel If desired, the flange 36 of Fig. 10 instead of engaging the adjacent side wall of the channel in the initial assembling of the sheet with the channel bar I6, may be bent to extend in spaced relation thereto, as for example as shown of the flange 39 of Fig. 11. Furthermore, the

flanges

24 and 39 of Figs. 3 and 11, may be bent to extend in contact with the adjacent side walls of the channels instead of being spaced therefrom as show It should be apparent from this description and the drawings of various types of hook-portions that can be used, that, generally speaking, the more resistance to deforming the hook portion offers, the greater the'tenslon developed in the sheet. It should be equally evident that the hook must open out and deform, as described, in order to provide the possibility for the equalizing tension as described.

In all of the modifications shown in Figs. 10 to 12, inclusive, as in the case of the structures shown in Figs. 1-9, inclusive, non-uniformity of resistance of the sheet to pulling stress, exerted by the tensioning bar, due to hard and soft spots in the sheet, is'compensated for and final uniform tension throughout the sheet is effected, by slippage, of the tensioning bar as described, during the inward movement of this ,bar to its final position, past those portions of the sheetfiange which are opposite the hard spots of the sheet as well as slippage of the tensioning bar along the entire length of the flange when the equalized tension in the sheet equals the force required for further inward movement of the tensioning bar, as described.

As above set forth one of the factors controlling the development of the final tension in the sheet in the case of Figs. 3 and 6, is the distance of the bend of the hook from the bottom Ila of the channel, the dotted lines in these figures representing different such initial spacings of the hooks. This also applies to Fig. 10- the closer the juncture between the

flanges

36 and 31 is to the bottom of the channel the less the final tension in the sheet. I

In Fig. 11 the closer the juncture between

flanges

39 and 40 is to the bottom ||a of the channel and inFig. 12 the less the angle of inclination of the flange 4| relative to the side wall of the channel II, the less the final tension in the sheet.

If desired, in addition to the spreader bars I60, the several bars l6 may be reinforced at suitable intervals to prevent spreading in the securing thereto of the metal sheets, as by providing clips 42 shown as of U-shape, applied to straddling position on the bars as represented of the clip in Figs. 2 and 3 where one such clip is shown as applied to one of the vertical channel bars IS, the clips being secured to the bars in any desirable Way, as for example by welding them thereto.

The word stretching is used herein and in the claims in its popular sense and not in the sense of producing a permanent elongation of any dimension of the sheet or any portion thereof. In order to develop the full elastic strength of the sheets the tensile stress to which the sheets are subjected must not stretch the sheets beyond their elastic limit.

It is obvious that as to all of the structures shown the tensioning bars may be gradually forced home uniformly about the sheet or one side may be fully seated before initial stress is applied to the opposite tensioning bar, in which case the first applied bar will act only as an anchor.

Considering the action of the tensioning bar on any of the flanged edges of the sheet, to tension the sheet, such action is exerted on such edge in the anchored condition of the opposite edge of the sheet whether the tensioning bars at opposite edges of the sheet are operated simultaneously, or in repeated succession to gradually force home the tensioning bars about the sheet, or the edge opposite the edge being operated on by the tensioning bar is rigidly held, as for example by fully seating the tensioning bar at this edge before initial stress is applied to the opposite tensioning bar or by providing any other desirable bar clamped in any way against, or rigidly connected with, the sheet as for instance by welding it thereto against yielding under the stress exerted by the tensioning means.

It will also be apparent that tension may be applied in one dimension only of the sheet, while securing many of the advantages of the invention.

The term metallic sheet as used herein and in the claims is to be understood to mean a commercial sheet such as is supplied to the trade, which sheets are always unequally resistant to a stretching force, being either thicker or harder or both in some portions than in other portions.

While I have illustrated and described certain particular embodiments of my invention and have disclosed certain procedures for the practicing of my improved method I do not wish to be understood as intending to limit the invention thereto as the embodiments shown may be variously modified and altered and the invention embodied in other forms of structure and the method practiced by other procedures, without departing from the spirit of the invention.

What I claim as new and desire to secure by Letters Patent, is:

1. The method of constructing a sheet metal panel which comprises: applying tensioning stress throughout the length of at least one margin of the sheet against anchoring resistance applied at the opposite margin; said stress sufllcient to cause stretching of the metal of the sheet but within the elastic limit of the material; allowing slippage of the stress applying means relative to the sheet metal at any locations therealong opposite those portions of the sheet offering greater resistance to tensioning than other portions of the sheet; whereby to secure substantially uniform tension on all portions of the sheet; and anchoring the stress applying means to hold the sheet in tension.

2. A structure having an area to be spanned, with a channel member at one edge of said area, a sheet metal member spanning said area having a preformed flange at one of its margins terminating in a reversely bent hook portion at which said sheet extends into said channel in initially spaced relation to the bottom thereof, means at the opposite margin of the sheet metal member anchoring it in place, a tensioning member to enter said channel and be moved inwardly therein in engagement with said hook portion, said tensioning member in said movement bending said hook portion outwardly, saidchannel member being free of obstruction to outward bending of said hook portion by the tensioning member into engagement with the bottom of said channel, whereby said hook portion becomes deformed and tension is exerted on said sheet sufllcient measurably to stretch the same, but within its elastic limit, and means for holding said tensioning member in sheet-tensioning position.

3. A structure having an area to be spanned.

with a channel member at one edge of said area, a sheet metal member spanning said area having a preformed flange at one of its margins terminating in a reversely bent hook portion at which said sheet extends into said channel in initially spaced relation to the bottom thereof, means at the opposite margin of the sheet metal member anchoring it in place, a tensioning member to enter said channel and be moved inwardly therein in engagement with said hook portion,

said channel member being free of obstruction to outward bending of said hook portion by the tensioning member into engagement with the bottom of said channel, said tensioning member, in its initial sheet-tensioning movement bending said hook portion int engagement, at its outer edge portion, with the bottom of said channel and thereafter deformingly forcing the hook portion toward the interior angle of said channel, whereby said hook portion becomes deformed and tension is exerted on said sheet sufllcient measurably to stretch the same, but within its elastic limit, and means for holding said tensioning member in sheet-tensioning position.

4. A structure having an area to be spanned, with a channel member at one edge oi. said area, a sheet metal member spanning said area having a preformed flange-at the one of its margins adjacent said channel member projecting into said channel, the free edge of said flange reversely bent into hook form, the overall depth of said flange being less than the depth of said channel, means at the opposite margin of the sheet metal member anchoring it in place, a tensioning bar to enter said channel and enter and. engage said hooked portion substantially throughout the length of said hooked portion, and means to exert pressure on said bar, said tensioning member adapted to slip relative to said sheet at any 10- cations therealong opposite to those portions of the sheet which offer greater resistance to tensioning than other portions of the sheet in the operation of drawing said sheet, by said tensioning bar, t the desired tension.

5. In a structure of the class described, an assembly of channel members bounding a polygonal area, a sheet metal covering for said area having preformed flanges at its margins projecting into the respective channels, the free edges of said flanges reversely bent into hook form, the overall depth of said flanges being less than the depth of said channels, tensioning bars to enter said respective channels and enter and engage said hooked portions substantially throughout the length of said hooked portions, and means to exert pressure on said bars, said tensioning member adapted to slip relative to said sheet at any locations therealong opposite to those portions of the sheet which offer greater resistance to tensioning than other portions of the sheet, in the operation of drawing said sheet, by said tensioning bar, to the desired tension.

6. The structure of claim 4 in which the surface ofsaid area joins the side of said channel in a curved surface.

7. The structure of claim 4, with the depth of the marginal flange predetermined to be clamped,

by the tensioning bar, against the bottom of the channel when a predetermined tensile stress has been exerted on said sheet metal member, sufficient measurably to cause stretching of the metal within its elastic limit.

8. The structure of claim 5, with the depth of the marginal flanges predetermined to be clamped, by the tensioning bars, against the bottoms of the channels when a predetermined stress has been exerted on said sheet metal member, sufficient measurably to cause stretching of the metal within its elastic limit.

9. A structure having an area to be spanned, with a channel member at one edge of said area, a sheet metal member spanning said area hav-- ing a preformed flange at the one of its margins adjacent said channel member projecting into said channel, the free edge of said flange reversely bent into hook form, the overall depth of said flange being less than the depth of said channel, said hook portion being curved with its concave surface facing in the direction of the outer side of said channel and said flange being substantially fiat, said hook extending at substantially an acute angle to said flange and connected with said flange at a sharp bend the interior angle of which is rounded, means at the opposite margin of the sheet metal member anchoring it in place, a tensioning bar to enter said channel and enter and engage said hooked portion substantially throughout the length of said hooked portion, and means to exert pressure on said bar, said tensioning member adapted to slip relative to said sheet at any locations therealong opposite to those portions of the sheet which offer greater resistance to tensioning than other portions of the sheet in the operation of drawing said sheet, by said tensioning bar, to the desired tension.

10. The method of constructing a structure embodying a sheet metal panel including development of the tensile strength of the sheet metal to resist deformation of the panel in the plane of sioning stress is applied in more than one direction.

12. A structure of the class described comprising a rigid metallic frame; a continuous metallic sheet secured upon said frame by continuous clamping means, under uniform tension in all directlons in the plane of the sheet; the metal of said sheet measurably stretched within its elastic limit thereby developing the tensile strength oi the sheet throughout its area in all directions.

HARVEY B. LINDSAY.