US2892239A - Improved method of erecting shellform concrete structures - Google Patents

Description

June 30, w N

IMPROVED METHOD OF ERECTING SHELL-FORM CONCRETE STRUCTURES 2 Sheets-Sheet 1 Filed June 19, 1952 FIG.

INVENTOR N E F F F I G. 7

wALLA'c wT lMl-l &

ATTORNEY June 30, 1959 w, NEFF 2,892,239

IMPROVED METHOD OF ERECTING SHEILL-FORM I CONCRETE STRUCTURES Filed June 19, 1952 2 Sheets-Sheet 2 l N VENTOR l3 WALLACE NEFF,

. J I BY W ATTORNEY United States Patent IMPROVED arnonon ERECTING SHELL- FORM CONCRETE STRUCTURES Wallace Neif, Los Angeles, Calif Application June 19, 1952,.Serial No. 294,316

3 Claims; (Cl: 25-454) This invention relates" to improvements in building construction, and more particularly to an improved method of erectingshell-form concrete structuresof the continuous-wall, self-supporting type of size suitable for dwellings; buildings, and the like, on an internal pneumatic-form.

Continuing'experience in the erection of concrete shell form" structures as aforesaid on an internal pneumatic f0rm--according to'the methods disclosed in my prior Patents" No. 2270, 229, dated January 20} 1942, No. 2,335,300, dated November 30, 1943, and No. 2,388,701, dated November 13; 1945; establis'hedthe fact that such shells; which are conventionally shaped as'-a dome, seldomcrackor fail in their top portion, even when-unreinforc'ed, because the material of the top-portion of a concrete dome is substantially all in compression. However, faults and crackssometimesdevelop in'the sidewall portions of said'structures, since in every dome there is a horizontal plane called the plane of rupture; below which the material isin tension, due to theradial outwardthrust of the'load of the top'portion. Fromthis plane on down the side wall; horizontal reinforcing'rings were usually applied so as to counteract this outward thrust and. thereby prevent development of unsightly cracks and/or side wallfailures. Such reinforcing rings of necessity acted in the manner of conventional embeddedreinforcement and hence lacked any function of prestres'sing the concrete of the'side wall portions of the structure so as to place same under compression since it-was found that any tensioning of the rings'c'ould result in serious deformation of the corresponding side wall portions of the pneumatic form against which the concrete wasplaced. Moreover, due to the curvature ofthe form side-wall, it was not considered feasible or even possible to apply, wire mesh reinforcement to the pneumatic form in'such manner that it could be pretensioned so as to place thehardened concrete materi'al'of the structu-re-side wall under compression following removal of the. form. This followed from the fact that wire mesh reinforcementcomes in strip form and must necessarily be applied as aseries of stepped diameter, substantially cylindrical wrappings about/the curved'side wall portion of 'the pneumatic form, with the result that, when=the wrappings..were tensionedsagainst the form, the tension varied from too great at the-lower portion-of. each wrapping-to. too little: at the upper: portion thereof. In. actual practice, itwaswin. some cases foundunecessary to relieve the=too great tension occurring, at the-lower portionr of each wrapping as by: severing the wirestrands making reinforcement, thus to prevent suchstrands from cutting, through andthereby impairing if not destroying the pneumatie form According; to-the present; invention the aforesaid. difficnlties-in pretensioning; the side wall reinforcement of concrete-erectedone-pneumatic form, thereby to place thew-material oftthesidewall, like'that of the top portion ofzthe dome, under compression, maycbe simply a yet clfectively overcome by the use of a pneumatic form having a circular floor plan and straight (vertical) orsubstantially straight side walls. Since such a form, upon in flation, is cylindrical in its straight side wall portion,.it can be wrapped with one or more layers of reinforcing mesh which may be pretensioned in various Ways to be described, with assurance that the tension is equally distributed throughout the vertical height of each of the reinforcement wrappings or layers and hence without any danger of the form being damaged consequent to such tensloning.

Preferably also, the pneumatic form employed is one in which the shape of its top portion, and particularly the zone thereof connecting the straight side wall portion with the curved top portion of theform, is effectively controlled. In explanation, whereas in the lower straight wall portion of the form inflation and loading pressures are exerted equally in radially outward direction, inflation pressure tends to deform the curved top portion thereof to the shape of a sphere, while load pressure tends to bulge the top portion outwardly at its lower zone which connects with and merges into the straight side Wall portion of the form. While the opposing pressures aforesaid counteract one another to a degree, and the loading pressure may be utilized to effect a controlled deformation of the pneumatic form when the concrete is placed from the top down, nevertheless it is highly important that no movement or deformation of the pneumatic form occurs after the concrete has started to set. To this end, the form incorporates special reinforcement in the zone or region thereof most likely to deformor bulge outwardly under the weight of thc load placed thereon; and which is effective to control the shape of the form, irrespective of whether the concrete is placed from the bottom up or from the top down.

The above and other features and objects of the invention are set forth in the following detailed description and illustrated in whole or in part in the accompanyingdrawings, in .which Fig. 1 is a side elevation of one type of pneumatic form. for use in erecting shell-form structures according tothe invention, the view beingintended to illustrate the pneumatic form set up on a suitable support or foundation and inflated to its normal load-sustaining pressure.

Fig. 2 is a view similar to Fig. 1, illustrating the-side wall reinforcingmesh applied to the pneumatic form and pretensioned by. the full inflation of the form;

Fig. 3 is a side elevation, partly broken away, illus" trating the completed structure erected on the pneumatic form shown in Figs. 1 and 2, and prior to deflation and removal .of the form;

Fig. 4 is an enlarged view generally similar to Fig. 3, which. illustrates some construction details of the pneumatic form and its. anchoring means;

Fig.. 5 is a view similar to Fig. 2, which is intended to illustrate a preferred method of tensioning the reinforcing mesh applied to the straight side portion of the pneumatic form;

Fig. 6 is a typicalsectiontaken through a portion of the straight side wall portion of the final shell structure;

Figs. 7 and 8 are enlarged views illustrating alternate types of special reinforrnent applied to the pneumatic form for the purpose of controllingits shape in the zone thereof connecting the curved top portion and straight" wall bottom portion; and

Fig. 9'isa vertical section taken through a penumatic form having another type of special reinforcement for controlling'the shape of the form.

Referring to the drawings, reference character 10 r (Figs. 1-4) generally designates an internal pneumatic form for use according to the invention in erecting shellform structures of concrete, which are characterized by a circular floor plan, a dome-shaped top or roof portion curved for strength and for shedding water, and straight (vertical) or substantially straight side walls. Hence',-the pneumatic form is fashioned to a size and shape that when inflated it Will assume the size and shape of the interior of the desired structure which, it will be understood from my prior patents aforesaid, is to be erected thereon by gunniting or otherwise placing concrete on the exterior surface of the form and without the use of any external form. Accordingly, the form 10 has a curved or dome-shaped top portion 11, a straight side wall portion 12, and a circular floor plan. Preferably, the pneumatic form 10 is closed at its bottom as at 13, although, for large-size structures designed for commercial use, the form may be of the open-bottom type disclosed in my prior Patent No. 2,388,701. The pneumatic form 10 may be fashioned from various materials such as canvas, nylon, balloon cloth, etc., and is preferably internally and externally surfaced with rubber or with a rubber-like material such as neoprene.

Although the pneumatic form 10 may be set up on the ground or on scaffolding, it is illustratively shown in Fig. 4 to be set up on a floor slab 16 and is firmly held against uplift forces developed during its inflation by means of a catenary flap 17 which extends continuously along its bottom perimeter, the catenary being in turn fastened to the footing 18 of the floor slab by a cable 19 threaded through eyes 20 embedded in the footing. A nipple 21 (Fig. 1) provides a convenient means by which any suitable gas under pressure, such as air, may be supplied to the interior of the form to effect its inflation.

Due to the curvature of the top portion 11 of the pneumatic form, the top portion of a structure erected thereon is substantially immune to cracks or structural failures, since the concrete material making up said top portion is substantially all in compression, as explained above. According to the invention, the concrete making up the side wall portion of the structure is reinforced in such manner as not only to enable the side wall portion of the concrete structure to withstand the outward thrust of the load of its top or roof portion, but also to place the concrete of the side wall portion under compression, i. e. to effect prestressing thereof, thus to obtain the known advantages of prestressed concrete, such as greater strength and hence more stable structure, reduction in dead load, and economy in amount of reinforcement required.

The desirable prestressing of the concrete of the side wall of the structure is made possible by the use of a pneumatic form characterized by circular (or nearly circular, as will hereinafter appear) floor plan and straight side walls as above, since inexpensive Wire mesh reinforcement may be applied to the straight side wall type of form as a series of cylindrical wrappings, in single or plural layers, and thereupon tensioned uniformly throughout the depth of each wrapping without unwanted deformation of or damage to the form. The aforesaid tensioning of the reinforcement is preferably achieved as follows. Referring to Fig. 5, the form 10 is erected on the floor slab 16 by inflating it to is normal load-sustaining pressure of 41- oz. per sq. in., and thereupon reinforcement comprising wire mesh of a desired width for convenient handling is wrapped tight around the straight side wall portion 12 only of said form. Upon firmly securing the ends of the wrappings, the reinforcement is disposed as bands encircling the form, which are designated 24a, 24b, 24c and 24d. Thereupon concrete is placed over the external surface of the form in a socalled Working down operation as disclosed in my aforesaid prior Patent No. 2,335,300. As a result thereof, tensioning of the reinforcement is effectively brought about consequent to an increase in pressure developing within the straight side wall portion 12 of the form under 4 the load of the concrete first applied to the domed top portion thereof, as causes said side wall portion to tend to expand radially or diameter-wise, a tendency which is resisted by the reinforcement which is accordingly now placed in substantial tension. In addition to the tensioning of the reinforcement as aforesaid, it is also to be noted that there results a distribution of the tension equally throughout the depths of the several reinforcing bands 24a, 24b, 24c, 24d, due to the pressure within the top-loaded form being applied equally in radially outward direction.

Instead of applying the reinforcement as individual bands or wrappings as aforesaid, it may be applied in the form of one continuous length of reinforcing mesh wrapped around the straight side wall portion of the pneumatic form. If desired, multiple layers of the reinforcement may be applied to the form rather than a single layer, as illustrated. Closely spaced, small-diameter reinforcing bars running horizontally about the straight side wall portion of the form may also be used in place of reinforcing wire mesh as above. Thus, regardless of the type or manner of application of the reinforcement, the increase in pressure and resulting tendency of the straight side wall portion of the form to distend radially outwardly under the load of the concrete first applied to the domed top portion 11 of the form will effect the desired substantial tensioning of the reinforcement as described.

Upon the concrete being now forcibly projected on to the external surface of the straight side wall portion of the pneumatic form and its tensioned reinforcement, as by the well known gunnite method in accordance with the teachings of my prior patents aforesaid, the reinforcement in its tensioned state becomes embedded in the continuous layer of concrete built up on the form, as illustrated in Fig. 6. To insure proper depth of embedding, the reinforcing mesh may be pulled away slightly from the outer surface of the form by a hook or other suitable tool as the gunniting proceeds, but usually the force of projection of the concrete is such that it flows behind'the mesh and securely embeds the same.

Full inflation pressure is of course maintained during setting of the concrete, such in turn resulting in the tension on the reinforcement being maintained during the setting period. However, upon the concrete hardening, and upon the pneumatic form 10 being deflated and removed from the resulting shell structure, as through a window or door opening to the interior thereof, the hardened concrete maintains this tension against the urge of the reinforcement to contract and return to its unstressed state. Due to this tendency, the reinforcement places the concrete making up the side wall portion of the structure under substantial compression, even under the load of the concrete top or roof portion of the structure. Thus, failure or cracking of the side wall portion of the structure can occur only if the tensile strength of the reinforcement or the compression strength of the concrete is exceeded, which are conditions hardly likely to occur in structures as herein contemplated, whose side walls, gengrally speaking, are required only to support roof loa As pointed out above, the reinforcement extends only to the upper line of the straight side wall portion of the pneumatic form 10, since it is not feasible to carry the reinforcing wrappings over the curved portions of the form, and it is too expensive to reinforce such portions with reinforcing rings. From this line upwardly, the form curves inwardly-upwardly to merge into the dome shaped top portion proper of the form. In actual practice, it has been found that this merging portion 14 of the pneumatic form tends to deform inwardly as the form is inflated and to bulge and/or stretch in outward direction under the load of the concrete placed on the top portion of the form. Any such deformation occurring during setting of the concrete is highly objectionable and it can-also result in misshapingsofthe concretestructime; .meraih ry, it is desirable to controllthe shape or theinwardly curving orrnerging portion 14 of thetform, which can be achievedlin various ways. One wayiiskillustrated in Figs. 1 5, Tand 81 and comprises a plurality of spaced, horizontally disposed reinforcing strips whichextendabout the above referred to merging portion of the form as generally. illustrated. The. reinforcing strips may take the form of tapes or cords 30 fashioned from woven glass fabric which is substantially non-stretchable and which are preferably applied and secured to the form during manufacture thereof, as by sewing, cementing, etc. The tapes 30 may be secured against the outer surface of the pneumatic form as indicated in Fig. 7, or they may be secured to its inner surface, as in Fig. 8.

Another means of controlling the shape of the pneumatic form throughout its so-called merging portion 14 is illustrated in Fig. 9 and comprises disc-form horizontal membranes 34 distributed at various levels of said merging portion and which are secured about their peripheries to the internal surface of the form so as to be contained within the inner space thereof. The membranes 34 may be fashioned from an open-mesh material having high resistance to stretch, or they may be made from a solid non-stretchable fabric, such as glass fabric. When made of solid fabric, the membranes 34 are formed with one or more openings to maintain equal pressure within the pneumatic form. From the above, it will be apparent that the localized reinforcement such as the reinforcing strips 30 or the membranes 34 is of particular advantage in straight side-wall structures wherein no deformation of the form under the load of the concrete first applied to its top portion is required or desired, as is desirable in the true dome structures characterized by curved side walls throughout.

Without further analysis, it will be seen that the practice of the above disclosed method produces a shell-form concrete structure wherein the material of both the curved top wall and of the side walls is under such a degree of compression that the structure is very strong and stable, and is not subject to failure or cracking. The method disclosed for achieving this desirable result is simple in practice and requires no special skills. The present invention also provides an improved pneumatic form having provision by which its shape when inflated and loaded may be eflectively controlled, thus assuring a fixed shape during setting of the concrete placed thereon, as well as a uniform shaping of all structures built on any one pneumatic form.

While the invention has been described as relating to a method of erecting straight side-wall shells or structures having circular floor plan on correspondingly shaped pneumatic forms, it is to be understood that circular, as applying to the structure floor-plan and to the horizontal section of the form, is to be interpreted as nearly circular or substantially circular, since, in the case of the form, the nearly or substantially circular form tends to become circular under inflation pressure and thereby tensions the reinforcement uniformly as described above, and, in the case of the final structure, limited deviation from true circular floor plan is permissible architecturally. Also, in the .event of the placing of the concrete on the form by the gunniting or trowelling procedures proving too expensive for extreme low-cost construction desired in certain areas and/ or countries, the structure or the outer shell thereof may be erected of inexpensive local brick or concrete block placed against the form or the straight side-wall portion thereof, following pretensioning of the reinforcement as aforesaid, the bricks or blocks being anchored to the reinforcement by grouting, etc.

As many changes could be made in carrying out the above constructions and method without departing from the scope of the invention, it is intended that all matter 6 contained, abo e: descripticu on, in the accompanying drawings shall be interpretcd'as illustrative and not in a limitingsense; 3

Lclaim:

1. The method of constructing shetlifonn shtucmres of the continuous-wall, self-supporting typcr'of size. .suitable for dwellings, buildings and the liketof concrete iircompression by the use of an internal fonnionly, whichlcomprises the steps of providing a normally deflated pneumatic form having circular plan, a substantially straight side-wall portion and a generally dome-shaped top portion and being adapted upon inflation to provide an internal form of the size and shape corresponding to the interior of the intended structure, anchoring the form along its bottom perimeter to a suitable support, inflating said form, wrapping the substantially straight side-wall portion of the form with reinforcing mesh, applying concrete first to the top portion of the form so as to effect an increase in the pressure within the substantially straight wall portion of the form consequent to the load of the so applied concrete and thereby a tensioning of the reinforcing mesh, then applying concrete to the straight side-wall portion of the form in such manner as to build up with the concrete applied to said top portion a continuous layer of concrete over the full external surface of the form and in which the tensioned reinforcing mesh is embedded, maintaining the increased inflation pressure within the form during setting of the concrete, and finally deflating and removing the form from the shell-form structure resulting upon set ting of the concrete.

2. The method of constructing shell-form structures of the continuous-wall, self-supporting type of size suitable for dwellings, buildings and the like of concrete in compression by the use of an internal form only, which comprises the steps of providing a normally deflated pneumatic form having circular plan, a substantially straight sidewall portion and a generally dome-shaped top portion and being adapted upon inflation to provide an internal form of the size and shape corresponding to the interior of the intended structure, anchoring the form along its bottom perimeter to a suitable support, inflating said form, map ping the substantially straight side-wall portion of the form with reinforcing mesh, applying concrete over the external surface of the form and reinforcing mesh from the top portion of the form down, thereby to effect an increase of pressure within the substantially straight sidewall portion of the form consequent to the load of the concrete applied first to its top portion and thereby a tensioning and embedding of the reinforcing mesh, maintaining said increased pressure until the concrete has set, and finally deflating and removing the form from the shell-form structure resulting upon setting of the concrete.

3. In a method of constructing shell-form structures of the continuous wall, self-supporting type and of size suitable for dwellings, buildings and the like of concrete in compression by placing a continuous layer of concrete over the exterior of an inflated pneumatic form constituting the sole form employed and which is deflated upon setting of the concrete, said form being characterized by circular fioor plan, a substantially straight side-wall portion and a generally dome-shaped top portion and having size and shape such that when inflated its exterior surface corresponds to the interior of the intended structure, the steps of wrapping the substantially straight side-wall portion only of the inflated form 'with reinforcing mesh, applying concrete to the external surface of the form in a working-down operation, thereby tensioning said mesh prior to the concrete being applied to said portion, and maintaining the tension on the reinforcing mesh during setting of the concrete, whereby to effect embedding of the tensioned reinforcing mesh in the set concrete, and whereby following deflation and removal of the form the embedded tensioned reinforcing mesh results in the concrete being in compression.

(References on following page) 23923535 :1 1 1 f 1 References Cited in the file of this patent 2,460,662

UNITED STATES PATENTS ggggggi 2,270,229 Nefi Jan. 20, 1942 H 2,585,446 2,308,806 Davis Jan. 19, 1943 6 2,612,675 2,355,300 Neff Nov. 30, 1943 2,616,148 2,382,171 Pomykala Aug. 14, 1945 2,616,149 2,388,701 Nefi Nov. 13, 1945 2,705,360

Van Voorhis Feb. 1, 1949 LeTourneau May 10, 1949 Crom et 211. Dec. 25, 1951 Edwin et a1. Feb.12, 1952 Wread Oct. 7, 1952 Hawes Nov. 4, 1952 Waller Nov. 4, 1952 Leonhardt Apr. 5, 1955

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