US2335300A

US2335300A - Building construction

Info

Publication number: US2335300A
Application number: US420437A
Authority: US
Inventors: Neff Wallace
Original assignee: Individual
Current assignee: Individual
Priority date: 1941-11-25
Filing date: 1941-11-25
Publication date: 1943-11-30
Anticipated expiration: 1960-11-30

Description

' Nov. 30, 1943. w. NEFF 2,335,300

BUILDING CONSTRUCTION Filed Nov. 25, 1941 V 2 Sheets-Sheet l v 151g. 1

W. NEFF BUILDING CONSTRUCTION Nov. 30, 1943.

2 Sheets-Sheet 2 Filed Nov. 25, 1941 i atented Nov. 3i), w th untrue EUING CONSllRtlCliliillN Wallace Neill, Los Angeles, flalii. Appiication November 25, 1941, Serial No. dZiiAWi 24 Claims. ('Cl. 25--l3l.5)

This invention relates to improvements in building construction, and is particularly directed to refinements in the method of constructing shell-form concrete building structures as disclosed in my copending application Serial No. 386,727, entitled "Building construction, filed April 3, 19m,

In the practice of the method of the above referred to application which, speaking generally, consists in forcibly projecting or shooting wet concrete over a large-size pneumatic form to build up thereon a continuous layer of concrete which on. setting provides a self-sustaining concrete shell of size suitable for use as dwellings, hangars, barracks, and the like, I have found that construction is slowed down to some extent if the layer is built up to final thickness in a continued operation, this by reason of the fact that the thicker the layer the longer the setting period, and hence the longer use period of the form.

According to the present invention, I shoot a relatively thin layer of concrete over the pneumatic form which by reason of its thinness sets at a fast rate and hence permits removal of the form at the termination of a relatively shortened setting period, whereupon it is available for immediate re-use at another site. Upon setting of the relatively thin-layer shell, such. may be used as a secondary form for the placing or building up thereon of an outer shell of concrete of desired thickness. Preferably, the exterior of the inner shell, prior to the shooting of the outer shell thereon, is sprayed or otherwise coated with a vapor sealing substance and preferably also with a layer of insulating material which, upon the formation of the outer shell, combine to prevent passage of warm or moisture laden air outwardly, as well as to maintain the inner shell at a more or less constant temperature, thus to prevent the condensation forming on the interior wall surface thereof.

According to a further feature of the present invention, I have discovered not only that an improved shell can be formed, but also that the final shape of the concrete shell can. be controlled within limits, by working from the top down, that is to say, by shooting the top or crown portion of the form first, and thereafter shooting the side and bottom portions of the form. By working from the top down, the load of the corn crete on the top portion of the shell causes a slight distortion of the form which is utilized, in the case of a semi-spherical form and, equivalently when forms of other shapes are used, to flatten the form at its pole and to belly the form outwardly along its sides, thus to lower the ceiling and increase the head room along the sides of the final shell, Moreover, by shooting the shell from the top down, the concrete load carried on the top portion of the form causes the form to adjust itself to its final shape, and the concrete may then be shot against the deformed side portions of the form, with the assurance that no further deformation or change of shape can take place. According to the present invention, therefore, the lilseiihoool of cracks occurring in the shell, particularly in its side portions, due to deformation under the load of the concrete applied to the top portion of the form, is positively precluded.

Other objects will be in part obvious and in part hereinafter pointed out in connection with the following analysis or this invention wherein is illustrated an embodiment of the invention in detail.

In the drawings-=- Fig. 1 is a part sectional and part eievational view of an inflated pneumatic form in accordance with the present invention shown as set up and anchored to a suitable footing;

Fig. 2 is a like view of the form over which has been shot a continuous layer of concrete;

Fig. 3 is a similar view illustrating the completed structure in its preferred form;

Fig. 4 is an enlarged section taken through a portion of the side wall ofthe final shell; and

Figs. 5 and 6 are details of the form anchoring means,

Referring to the drawings wherein is shown one variant of the many shapes of inflatable forms which may be employed within the purview of the present invention, as well as the correspendingly shaped structure formed there-on, reference character it designates an inflated pneumatic form of semi-spherical shape having a curved Wall extending along an arc of constant radius and a fiat-bottom i i adapted to be placed fiat against the ground or equivalent supporting surface.

As indicative of the size both of the form and of the structure capable of being built up thereon, but not limiting the invention thereto, the form shown is illustrative of an actual form which was built to a radius of ll. 4", thus to provide a generally semi-spherical shell having an interior diameter at its floor line of 22' and a maximum ceiling height of approximately 11', and which wasfabricated from a multiply rubberized fabric, with a coating of rubber both inside and out.

The form along its equator is anchored to a circular concrete footing I! which is sunkinto the ground to a depth below the frost level. Set into the footing at spaced intervals are a plurality of anchoring hooks l3, the hooked ends of which extend above the footing and the angled heads I4 of which are embedded therein as more clearly shown in Figs. 5 and 6. The form along its bottom edge or equator is provided with a flaplike lacing strip l6 which, according to the preferred method of. anchoring, is bent back on itself to provide a loop or hem I! which is notched as at H3 at spaced intervals along its length, the notches being spaced tocorrespond to the spacing between the anchoring hooks l3. Curved tie rods 20 of length to extend the distance of about four of the notches, and which are easily insertible into and removable from the loop I], are arranged to span the 'open ends of the notches, thus to provide anchorage for the hooked ends of the anchoring'hooks l3.

When set up and anchored as aforesaid, the form is inflated to the desired pressure which depending on operating conditions, may vary from 1 to 5 lbs. per square inch, and takes its semi-hemispherical shape illustrated in Fig. l. Preferably, the inflated form is covered with a wire reinforcing mesh 22 of approved type. The

conventional coarse-mesh chicken wire has been found suitable for thi purpose.

With the inflated form providing a backing therefor, a thin, continuous layer 23 of wet concrete is next built up on the exterior of the form, preferably by the so-called gunite" method, -according to which a mixture of cement, fine aggregate and water is forcibly projected against the form under pneumatic pressure by a cement gun or equivalent placing apparatus. This layer is built up to about 1 thickness which is sufficient to embed the mesh 22 and to provide upon setting a self-sustaining shell. stood that the pressure maintained within the form enables it to withstand the force of protection, as well as the weight of the applied concrete which is relatively low by reason of the thinness of the layer thereof. It will further beappreciated that according to the invention the continuous shell-forming layer of concrete is built up directly on the exterior of the pneumatic form without the use of an outside form. During the shooting operation, the flap i6 and the anchoring means are shielded from the concrete by a suitable shielding strip disposed about the base of the form.

According to the preferred method of placing It will be underthe concrete, the top or crown portion of the form is shot first and the sides built up last. By working from the top down, the load of the concrete first applied to the top slightly changes the shape of the form, causing its tip or pole portion to flatten out and its sides to belly outwardly or bulge to a limited extent. This deformation is advantageous in that instead of the structure being of true semi-spherical shape, its height or ceiling is lowered and its sides are bellied outwardly to provide increased head room along the sides of the final structure.

It is also within the purview of the invention to modify the shape of the structure relative to that of the form by increasing or reducing the pressure within the form during operation. For

' example, by suitably reducing the pressureafter about the upper third of the form has been shot, the section of the form changes from a semicircular to an approximate ellipse. It will be appreciated, therefore, that the shape of the form is controllable within limits and represents a bale ance between the load of the first applied concrete and the air pressure maintained within the form.

After the so formed thin shellrhas set, the form is deflated and can thereuponlbe moved to the next site and again inflated for shooting a second structure, without awaiting final work on the thin shell whose construction has just been described.

' If desired, the thin shell may be suitably waterproofed and used in that form for economical fireproof buildings to replace tents, barracks, aeroplane hangars, and .the like. However, for dwellings and buildings of the more permanent type, I prefer to use the thin shell as a permanent secondary form for an outer shell of concrete of required thickness, which is built up on the inner ably applied by one of the known spraying methods. The function of this insulating layer is to prevent the transmission of outside temperaturas to the inner shell and thus to maintain the inner shell, insofar as possible, at a uniform temperature.

With the thin inner shell acting as a secondary form, wet concrete is thereupon shot over the in sulation to a thickness of about 2 inches, and bonds thereto, being intimately secured to the insulation by the force of projection of the con- I crete. Upon setting, the outer layer of concrete forms an exterior shell 26 which extends over the entire surface of the inner shell in the manner illustrated in Fig- 3. As will be further observed from Figs. 2 and 3, the concrete forming the outer shell is placed relatively thick below the outward bulge of the side walls, thus to straighten out" the walls adjacent their base line so that the outer face of the final structure extends substantially vertically from the footing i2.

If desired; tie wires may be inserted in the inner shell during formation thereof to hold the mesh properly spaced from the form, and to anchor the insulating material to the shell.

The exterior of the outer shell is thereupon painted or finished in desired manner. The interior surface of the inner shell may be painted or given a thin plaster finish, or' decorated, depending upon the use for which the shell is intended. Under conditions requiring the same, the inner surface of the inner shell can be sprayed with an insulating material, but this appears to be necessary only for localities where changes of temperature are severe, due to the fact that the insulating layer 25 disposed between the shells prevents condensation forming on the interior wall of the inner shell.

By the above described method of construction, structures suitable for dwellings, hangars, barracks, and the like, can be erected in a fraction of the time required for the erection of comparable structures by conventional methods. As the shell is of the continuous arch or span type, it is self-sustaining and eliminates the requirement for joists, beams, studding, girders, and like about the opening frames, end by shooting the concrete about the end over the reletively outer feces thereof, the frames ore permanently into structure in the operation or erectlog the inner shell. Tit will be uriderstoocl that tire form upon deflation is withdrawn through one of the shell openings provided as aforesaid.

While the pneumatic form hos been illustrated rind described es e. semi sphere, as required for the construction of hollow semi-spherical shells, it will be understood that other sl'iopeol forms may be employed, notably that form which in transverse section corresponds to a. section telten through c. serul spherlceil form but which is closegcted cs required to :orovide the so-celled barrel shell or barrel urcli of structure. It is also possible to shape the form to provide e structure havin o. rectangular or square body portion and.

a root provided by lotto-subdivided trierigulur oreos extending clung as gentle curvature and which merge into each other. It reouirecl for particular shape of structure, the form may be provided with cross wires or stiffening sewed in or otherwise secured generally to the top portion of the form corresponding to the roof of the intended structure.

The present invention provides an inexpensive and extremely test method oi erecting shell-like structures suitable for use as dwellings, barracks, hangers end the like. as well as a construction method which is of extremely simple nature and which in practice may be carried out at the exoemiiture of minimum labor and material costs. a As many changes could be made in carrying out the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in e limiting sense.

I claim: a 1. The method of constructing shell-form building structures of the continuous wall; self supporting type suitable for use as dwellings, barracks, hangers and the like by the use of an internal form only, which consists in. arranging on a suitable foundation a normally deflated pneumatic form having a bottom adapted to lie flat on said foundation and side and top wall portcns which upon inflation of the form are adapted to assume a size and shape corresponding substantially to the interior of the intended structure, anchoring the form to the foundation,

inflating the form, applying concrete on to the exterior of said form in such manner as to-build up thereon without the aid of an external form a continuous layer of concrete] which is thin in comparison to the final wall thickness or the structure but which is of the required thickness as to provide upon setting a sot-sustaining slvell, deflating and t ereupon withdrawing the form upon setting of-the concrete, and building up on the exterior of the so-formed shell 8, covering layer of concrete of desired thickness which on eeouooo the form.

internal form only,

setting provides an outer shell superimposed on the first formed shell.

2. The method of constructing shell-form building structures oi the continuous well, self suoporting type suitable for use as dwellings, berrucles, lieugers end the like by the use of on ii1- terzoel. form only, which consists in anchoring to u suite'ble foundation e riormolly deflated poem v cud shape as upon inflation subste-otiolly to the interior oi the cod stru inflating the form, and coolyirig wet concrete against the exterior of the form such mormer es to build up thereon without the old of on external form a continuous layer oi concrete which is thin iucomperison with the final Wall thickness of the structure but which is of the required thickness as to provide upon setting a self-su$tuinir1g preliminary shell, meloteiriing the form inflated until the concrete has set, and building up on the exterior oithe soiormed preliminary shell a covering layer of concrete which upon setting provides an outer shell superposed on the first formed shell.-

3. The method of constructing shell iorru building structures of the continuous well, sell supporting type suitable for use as dwellings, borroclre, hangers and the like by the use of en internol form only, which consists in anchoring to a suitable foundation a normally deflated procurrietic form of size and shape as upon inflation to correspond substsnticlly to the interior of the intended structure, inflatin the form, and forcibly projecting wet concrete against the exterior of the form in such manner as to build up thereon without the aloof an external form a continuous layer of concrete which is thin in comparison with the final wall thickness of the startleture but which is of required thickness as to provide upon setting ing and thereupon withdrawing the form upon setting of the concrete, and forcibly projecting wet concrete against the exterior of the so-formed shell to build up thereon an outer layer of concrete of desired thickness and which upon setting is adapted to provide an outer shell superimposed on the first formed shell.

l. The method of constructing shell-form building structures of the continuous wall, self supporting type suitable for use as dwellings, barracks, hangers and the like by the use of an which consists in anchoring to a suitable foundation a normally deflated pneumatic form of size and shape as upon inflation to correspond substantially to the interior of the intended structure, inflating shooting concrete over the exterior of the form in such manner as to build up thereon without the aid of an external form a relatively thin and continuous layer of the concrete, maintaining the form inflated until the concrete has set to a self-sustaining shell 'formation, thereupon deflating and withdrawing the form from the shell, and finally-placing a covering layer of concrete over the exterior of the soformed shell which upon setting provides an exterior shell superimposed on the first formed 'shell.

5. The method of constructing shell-form building structures of the continuous wall, self supporting type suitable for use as dwellings, barracks, hangers and the like by the use of an internal form only, which consists in anchoring to a suitable foundation a normally deflated pneumatic form of size and shape cs upon inflaa self-sustaining shell, deflc.t

tlon'to correspond substantially to the interior of the intended structure, inflating the form, shooting concrete over the exterior of the form in such manner as to build up thereonwithout the aid of an external form a relatively thin and continuous layer of the concrete, maintaining the form inflated until the concrete has set toa self-sustaining shell formation, thereupon deflating and withdrawing the form from the shell, and finally shooting a layer of concrete over the exterior of the so-formed shell which on setting provides an exterior shell superimposed on the first formed shell.

6. The method of constructing shell-form building structures of the continuous wall, self supporting type suitable for use as dwellings, barracks, hangars and the like by the use of an internal form only, which consists in anchoring to a suitable ,foundation a normally deflated pneumatic form of size and shape as upon inflation to correspond substantially to the interior of the intended structure, inflating the form,- shooting concrete over the exterior of the form in such manner as to build up thereon without the aid of an external form a relatively thin and continuous layer of the concrete, maintaining the form inflated until the concrete has set to a self-sustaining shellformation, thereupon deflating and withdrawing the form from the shell, coating the exterior of the shell with a vapor sealing substance, and finally placing an outer layer of concrete over the coated shell which upon setting provides an exterior shell superimposed on the first formed shell.

'7. The method of constructing shell-form building structures of the continuous wall, self supporting type suitable for-use as dwellings, barracks, hangars and the like by the use of an internal form only, which consists in anchoring to a suitable foundation a normally deflated pneumatic form of size and shape as upon inflation to correspond substantially to the interior of the intended structure, inflating ,the form, shooting concrete over the exterior of the form in such manner as to build up thereon without the aid of an external form a relatively thin and continuous layer of the concrete, maintaining the form inflated until the concrete has set to a self-sustaining shell formation, thereupon deflatingand withdrawing the form from the shell, coating the exterior of the shell with a vapor sealing substance, applying a layer of insulation to the, exterior of the coated shell, and finally placing an outer layer of concrete over the coated shell which upon setting provides an exterior shell formed shell.

8. The method of constructing shell-form building structures of the continuous wall, selfsupporting type suitable for use as dwellings, barracks, hangars and the like by the use of an internal form only, which consists in anchoring to a suitable foundation a normally deflated pneuing substance, spraying the exterior of the coated shell with insulating material thereby to build up thereon a layer of insulation, and finally shooting a, layer of concrete over the insulation which on: setting provides an outer shell superimposed on and insulated from the first formed shell.

9. The method of constructing shell-form building structures of the continuous wall, selfsupporting type suitable for use as dwellings, barracks, hangars and the like by the use of an internal form only, which consists in anchoring to a suitable foundation a normally deflated pneumatic form of size and shape as upon inflation to correspond substantially to the interior of the intended structure, inflating the form, laying a reinforcing mesh over the exterior of the inflated-form, shooting concrete through said mesh and on to and over the form in such manner as to build up thereon a relatively thin and continuous layer of concrete embedding the mesh, maintaining the form inflated until the concrete has set to a self-sustaining shell formation, thereupon deflating and withdrawing the form from the shell, applying a layer of insulation over the exterior of the shell, and finally shooting an outer layer of concrete over the insulation which superimposed on and insulated from the 'flrst' tended structure, inflating the form, laying a reinforcing mesh over the exterior of the inflated form, shooting concrete through said mesh and on to and over the form in such manner as to build up thereon a relatively thin and continuous layer of concrete embedding the mesh, maintaining the form inflated until the concrete has set to a self-sustaining shell formation, thereupon deflating and withdrawing the form from the shell, coating the exterior of the shell with a vapor sealing substance, spraying the exterior of the coated shell with an insulating material until a layer of insulation of required thickness is built up thereon, and finally shooting an outer layer of concrete over the insulation which upon hardening provides an exterior shell superimposed on and insulated from the first formed shell.

11. The method of constructing shell-form building structures of the continuous wall, selfsupporting type suitable for use as dwellings, barracks, hangars and the like, which consists in anchoring to a suitable foundation a normally deflated pneumatic form of size and shape as upon I inflation to correspond substantially to the interior of the intended structure, inflating the form, first placing concrete on the top of the form and then applying concrete to the s ide of the form whereby the load of the concrete placed on the top of the form effects adjustment of the side thereof to final shape prior to the placing of concrete against the latter, and maintaining the form inflated until the concrete has set to a selfsustaining shell formation.

12. The method of constructing shell-form building structures of the continuous wall, selfsupporting type suitable for use as dwellings, barracks, hangars ,and the like, which consists in ae'sasoo anchoring to a suitable foundation a normally deflated pneumatic form of size and shape as upon inflation to correspond substantially to the interior of the intended structure, inflating the form, first placinga layer of concrete over the top portion of theform thereby to obtain a controlied deformation of the form under the load of the concrete and thereupon extending said layer to the sides of the form, and maintaining the form inflated until the concrete has to a self-sustaining shell formation.

13. The method of constructin shell-form building structures of the continuous wall, selfsupporting type suitable for use as dwellings, bar racks, hangars and the like, which consists in anchoring to a suitable foundation a normally deflated pneumatic form of size and shape as upon inflation to correspond substantially to the interior of the intended structure, inflating the form, shooting a layer of concrete over the form from top to bottom thereof whereby the load of the concrete first applied to the top portion of the form effects adjustment of its side portions to final shape prior to the shooting of the concrete thereagainst, and maintaining the form inflated until the concrete has set to a self-sustainhig shell formation, deflating and withdrawing the form from the shell.

14. The method of constructing shell-form building structures of the continuous wall, selfsupporting type suitable for use as dwellings, barracks, hangars and the like, which consists in anchoring to a suitable foundation a normally deflated pneumatic form of size and shape as upon inflation to correspond substantially to the interior of the intended structure, inflatingthe form, first shooting a layer of concrete over the top portion of l the form to obtain a controlled deformation of the side portions thereof under the load of the concrete, thereupon shooting a continuation layer of concrete against the deformed side portions of the form, and maintaining the form inflated until the concrete has set to a selfsustaining shell formation.

15. In the method of constructing shell-form concrete buildings by placing, a continuous layer of concrete over the exterior of an inflated pneumatic form which is deflated upon setting of the concrete, the steps of first placing the concrete on the top portion of the form, and thereupon and prior to applying the concrete to the side of the form adjusting the pressure at which the form is maintained in relation to the load of the concrete placed on the top thereof in such manner as whereby to obtain a controlled deformation of the form.

16. In the method of constructing shell-form concrete buildings by placing a continuous layer of concrete over the exterior of an inflated pneumatic form which is deflated upon setting of the concrete, the steps of placing the concrete over the top portion of the form first, whereby the form adjusts itself to final shape under the load of the so placed concrete, and thereafter, in a working down operation, placing the concrete over the side portions of the form.

17. In the method of constructing shell-form concrete buildings by placing a, continuous layer of concrete over the exteriorof an inflated pneu-' matic form which is deflated upon setting of the" concrete, the steps which consists in first shooting concrete over the form in such manner as to build up thereon without the aid of an external form a thin and continuous layer of concrete suitable for elation surface and side and which upon setting forms self-sustalning shell, deflating and witlidrawlng the form from the shell following setting thereof, and thereupon utilising the resulting shell as a secondary inter= nal form by placing concrete to any desired thick pass over the same.

18. A pneumatic form for use in the construc-- tlcn oi shelLform structures of size and shape suitable for use as dwellings, hangars, barraclrs and the like, consisting of a hollow body having a bottom shaped to lie flat on a horizontal foun elation surface and side and top wall portions which are so shaped that upon inflation the body corresponds to the interior of the intended struc ture, the body being fashioned from rubberized and rubber-surfaced multiply fabric, the body being provided along its lower edge corner with an anchoring strip carrying sectional tie means adapted to cooperate with anchoring hooks provided on the foundation.

19. A pneumatic form for use in the construction of shell-form structures of size and shape suitable for use as dwellings, hangars, barracks and the like, consisting of a hollow body having a bottom shaped to lie flat on a horizontal lountop wall portions which are so shaped that upon inflation the body corresponds to the interior of the intended struc-= ture, the body being fashioned from rubberized and rubber-surfaced multiply fabric, the body being provided along its lower edge corner with an anchoring strip which is notched at intervals along its length and which carries rigid ties ex= tending across the open ends of the notches for cooperation with anchoring hooks provided on the foundation.

20. A pneumatic form for use in the constructlon of shell-form structures of size and shape use as dwellings, hangars, barracks consisting ofa, hollow semi-spherical a bottom shaped to lie flat on a and the like, body having horizontal foundation surface and upwardly extending and continuous side and top walls, the body being fashioned from rubberized multiply fabric and being surfaced with rubber both inside and out, an exteriorly disposed anchoring flap extending along the lower edge corner of the body, and said flap carrying sectional tie rods for cooperation with anchoring means on the foundation.

21. A shell-form concrete structure constituted by a, continuous wall of concrete consisting of a set layer of initially wet concrete placed without the aid of an external form over the exterior surface of an inflated pneumatic form of substantially semi-circular section which is capable of limited deformation under load, said wall extending along a curvature which is the resultant of the semi-circular sectional shape of the form and the deformation of the form under the load of the wet concrete Which is first placed on the top portion thereof and which causes the form to flatten and its side wall portions to bulge outwardly prior to the placing of the concrete thereagainst.

22. A shell-form concrete structure constituted by a continuous wall of concrete consisting of a set layer of initially wet concrete placed without the aid of an external form over the exterior surface of an inflated pneumatic form of substantially semi-circular section which is capable of limited deformation under load, said wall extending along a curvature which, considered transversely, approaches that of the semi-circular section of the form but which is flattened at its top portion and bulged outwardly at its side portions due to the deformation of the form under the load of the wet concrete which is placed on.

the top portion of the form prior to being placed against the side portions thereof. 4

23; A shell-form concrete structure comprising inner and outer shells, the inner shell consisting of a set layer of initially wet concrete'placed without the aid of an external form over the exterior surface of an inflated pneumatic form of substantially semi-circular section which is capable of limited deformation under load, said wall extending along a curvature which is the resultant of the semi-circular sectional shape of the form and the deformation of the form under the load of the wet concrete which is first placed on the top portionthereof and which causes the form to flatten and its side wall portions to bulge outwardly prior to the placing of the wet concrete thereagainst, the outer shell consisting of a set layerof concrete placed over the inner shell and conforming generally to the shape of the latter, the outer shell adjacent its base line and below the bulge of the inner shell having thickness such that the outer face of-said shell from the base lineto the region of said bulge extends substantially vertical. I

24. A shell-form concrete structure comprising inner and outer shells, the inner shell consisting of a set layer of initially wet concrete placed without th aid of an external form over the exterior surface of an inflated pneumatic form side portions due to the deformation of the form under the load of the wet concrete which is placed on the top portion of theform prior to being placed against the side portions thereof, the outer shell consisting of a set layer of concrete placed over the inner shell and conforming generally to the shape of the latter, the outer shell adjacent its base line and below the bulge of the inner shell having thickness such that the outer face of said shell from the base line to the region of said bulge extends substantially vertical.

WALLACE NEFF.