Patent Publication Number: US-3879914-A

Title: Method of making a platform structure

Description:
United States Patent 1 Haller et a1.  
 [ METHOD OF MAKING A PLATFORM STRUCTURE [76] Inventors: Hans Haller; Erwin Wendl, both of Nibelungengasse 40, Graz, Austria [22] Filed: May 24, 1973 [21] Appl. No; 363,529  
  Related US. Application Data [62] Division of Ser, No, 75,851, Sept. 28, 1970. Pat. No.  
 [30] Foreign Application Priority Data Sept, 23. 1969 Austria 9016/69 [52] US. Cl. 52/745; 52/227; 52/262; 52/577 1511 Int. Cl. E04g 21/14 [58] Field of Search 52/262, 747, 227, 743, 52/745, 577; 264/333, 334, 3132;249/18, 26, 144, 50  
 156) References Cited UNITED STATES PATENTS 1.1,1,045 7/1912 Conzelman 52/602 1,592,070 7/1926 Blake 1 52/602 X 1.744.309 1/1930 Henderson 264/336 X 2,101,538 12/1937 Faber r 52/227 2,306,548 12/1942 Lecmhuis 52/602 X 2.344.206 3/1944 Forni 264/336 X 2,357,968 9/1944 Pennington .1 264/336 X 1 Apr. 29, 1975 Prinmr ExaminerErnest R. Purser Assistant E \&#39;arm&#39;nerLeslie A. Braun Auorney, Agent, or FirmKarl F. Ross; Herbert Dubno [57] ABSTRACT A roof platform or deck structure consists of a plurality of elongated prefabricated reinforced-concrete ele ments of inverted-U or cross-section interconnected contiguously and independently self-supporting so that the structure is self-supporting when formed without the need to cast concrete in place The individual elements are formed in a mold having a removable inner member which remains in place after withdrawal of the partially hardened concrete body until the latter becomes at least se1f-supporting.  
 1 Claim, 10 Drawing Figures PATENTEDAPRZSIQYS sum 3 or &#39;3 FIG. 8  
  1 METHOD OF MAKING A PLATFORM STRUCTURE This is a division of application Ser. No. 75.851 filed Sept. 28. I970. now US. Pat. No. 3.767.l53.  
  Our present invention relates to concrete roof platform or deck structures. to an improved method of making such structures. to molds for producing the elements constituting said structures and to a method of making such elements.  
  In the manufacture of structures. especially concrete roofs. platforms or decks. it has been the practice heretofore to provide a plurality of spaced-apart beams onto and about which is cast concrete with the end of a formwork or the like. Upon removal of the formwork. the roof. platform or deck is self-supporting and constitutes a substantially monolithic structure Concrete roofs. platforms or decks have also been made by casting a slab of concrete in place between the supports to create a monolithic structure. Neither of these two systems has proved to be entirely satisfactory. When beams have been used. for example. it has been the practice to dimension them to be as small as possible. thereby limiting the span of the beams and the size of the roof. platform or deck structure to be produced therefrom. Both systems have. moreover, been found to be relatively expensive. in no small part due to the requirement that they be cast in situ. The casting of the roof. platform or deck structure elsewhere and its transportation to the erection site also is no solution because of the high cost.  
  It is the principal object of the present invention. therefore. to provide a roof. platform or deck structure of concrete wherein. however. high transportation costs are avoided and the need for hardening wet concrete in situ is eliminated.  
  It is also an object of this invention to provide an improved concrete roof. platform or deck structure which is capable of spanning large distances between supports.  
  Another object of our invention is to provide an improved method of making a concrete roof. platform or deck structure and a method of making the elements thereof.  
  It is also an object of the invention to provide an improved mold for the production of the structural elements of a concrete roof. platform or deck.  
  These objects and other which will become apparent hereinafter are attained. in accordance with the present invention. in a roof. platform or deck structure spanning a pair of spacedapart supports and composed of laterally contiguously elongated concrete structural elements of substantially identical configuration which are entirely sefl-supporting and unitarily span the supports. the elements being laterally connected to produce a roof. platform or deck structure or deck having substantially the characteristics and strength of a monolithic concrete slab or shell. or stress and strength characteristics exceeding those of a precast or reinforced concrete shell.  
  Our invention is based upon the discovery that the individual elements which are entirely self-supporting between the horizontally spaced support members. can be coupled together in a laterally contiguous relationship. so as to define a substantially continuous roof. platform or deck surface without the application of wet concrete or the pouring casting or other application of concrete in situ.  
  According to a most important feature of this invention. each of the roof. platform or deck elements com prises a horizontal or substantially horizontal web having a vertically extending flange preferably running in the longitudinal direction and unitarily with the web to increase the load-carrying capacity thereof. Advantageously. two or more such webs are provided so that the element has an inverted-U cross-section with the web lying along the bight of the U and a pair of flanges corresponding to the legs thereof. It has also been found to be desirable. in an extension of this principle. to provide a basically channel-shaped element. the flanges or legs of which lie somewhat inwardly of the longitudinal edges of the web; in other words. the web may project beyond the flanges laterally and may be contiguous with the corresponding edge or similar adjoining elements. Then the element will have a ll-crosssection or the configuration TT. The space between the flanges. in the latter case. may be equal to twice the width of the laterally projecting marginal portion of the web. The wall thickness of the flanges are 4 5 cm.  
  Best results are obtained when the flanges converge or diminish in thickness downwardly. i.e.. when the channel defined by the element is approximately of trapezoidal cross-section with the narrow base of the trapezoid at the web and the broad base of the trapezoid between the&#39;free ends of the flanges. The convergence of the flanks of the flanges provides no structural disadvantage and indeed reduces the weight of the elements without materially limiting its load-carrying capacity and furthermore provides&#39;a draft facilitating the withdrawal of the case concrete elements from the mold.  
  The cast concrete elements. according to the present invention. are composed of reinforced concrete and may. if desired. be prestressed in accordance with conventional principles. lf prestressing is used. the cables or tensioning rods may run longitudinally of the structural elements. i.e.. in the direction of the major dimensions thereof. while reinforced structural elements without prestrcss may contain reinforcing rods. mats or grids. When mats or grids are used. we prefer to embed them both in the horizontal web and in the vertical flanges and. preferably. to use a channel-shaped mat or web bent essentially to the configuration of the concrete element and inserted into the mold for casting same. However. longitudinally extending rods. tied together by stirrups and the like. may also be used. Fur thermore. openings may be provided in the webs or flanges to afford access to the means for laterally connecting the concrete elements and we prefer to provide such openings by inserting into the mold for the casting of the structural elements one or more removable members. e.g. of synthetic resin. wood or other materials which do not readily bond to concrete to form knockout removable during assembly. The lateral connection may be accomplished at the flanges or through the webs. the former being preferred and the connection means may include tie rods spanning a number of the structural elements. transversely to their major dimensions. bolt arrangement interconnecting adjoining flanges or any combination of the two. Additionally. transverse flanges. ribs or webs may be provided for further reinforcement. the latter ribs spanning the flanges which run longitudinally close to the edges of the main web.  
  The structural elements may be cast with the desired camber. slope or superelevation required for the roof. platform or deck and may be curved in the longitudinal or transverse directions. When such curvature is presem. all of the structural elements are preferably designed with the same radius of curvature so that the structural elements can be joined longitudinally or laterally contiguously to form a smooth and unbroken roof surface. The elements will also. therefore. fit one another in all positions.  
  According to another aspect of the invention. we provide a mold for the formation of the structural elements which comprises an upwardly open outer shell and an inner shell conforming substantially to the recess or cavity ofthe structural element which is removably received in the outer shell and defines therewith a channel-like mold cavity of inverted-U orfl configuration. The concrete is cast in this cavity. Preferably after the introduction of the reinforcing members into the latter and is compacted and vibrated [jarred or jolted) by vibrating members affixed to the outer shell. According to an important feature ofthe invention. the non-self-supporting and only partially hardened concrete body is lifted from the outer shell of the mold upon the removable inner shell and permitted to set upon the latter while a fresh inner shell is introduced into the mold for the casting of a further structural element thereon. Once the soft concrete has hardened, e.g. to a point at which the concrete body is selfsupporting. the inner shell can be removed.  
  Hence the inner form of the double-form mold has several functions. Firstly. it constitutes the inner wall defining the web and flanges of the structural element. Secondly. it constitutes a support and transport frame or bed for the cast but unhardened concrete. Thirdly. it may constitute a means for facilitating the hardening ofthe concrete. Furthermore. since only the outer shell need be provided with vibrators. according to the present invention. the production of the concrete bodies can proceed with limited capital expenditure and no obstruction of the casting and jolting operation during the hardening ofthe concrete bodies. The system ofthe present invention has the further advantage that the moist concrete bodies, prior to complete setting. may be subjected to further treatment outside the mold at locations which would be inaccessible for the body within the mold. Such operations may include the for mation of openings within the body to permit the introduction of transverse supports or fastening means.  
  We have found it to be particularly desirable and to expedite the removal of the cast concrete bodies prior to complete hardening when the ends of the mold cavity are defined by a pair of end plates fixed to the inner shell and provided with means. e.g. wire loops. cutouts or the like engageable by the hooks ofa crane. Furthermore. since the load-carrying capacity of the platform and the ability of the individual structural elements constituting same to open large distances. depends upon the height of the flanges. we provide means in the mold for adjusting the flange height to the construction conditions. Such means may include insert strips raising the floor of the mold and thereby foreshortening the depths of the mold cavity. such strips preferably being adjustably mounted upon the inner shell. Similarly. we may desire to use a single mold for the production ofa number of elongated structural elements in accordance with the above principles. in which case we subdivide the mold longitudinally with templates having the configuration of the desired cross-section of the structural element and forming the ends of individual mold compartments. Such templates fit within the mold cavity and advantageously serve as patterns for the adjustable walls of the cavity to enable the latter to be readily positioned for a particular cross-section of the structural elements. The templates may be provided with fastening means for mounting them at any desired location along the mold walls and advantageously upon the outer form. Furthermore. the templates can impart the desired terminal angle to the structural elements and may be provided with a mold surface of corresponding configuration.  
  According to another feature of the invention. the hardening ofthe concrete upon the inner form is accelerated by conducting a heating medium through the latter. Advantageously. this heating is accomplished after the removal of the inner form from the mold and a number of such inner forms can be positioned at the setting station to conduct the heating medium. e.g. steam of hot air therethrough.  
  The inner form can be made from sheet metal. this material having been found to be particularly satisfactory when heating of the concrete body from within is desired. Since a number of such inner forms is required. however. and it may be desired to make inner forms associated with each flange height. we may cast the inner forms from concrete and especially lightweight concrete having aggregates of expanded min eral. It has been found to be advantageous in all cases in which a concrete inner form is used. to provide it with a highly smooth surface of. for example, a syn thetic resin. In the preferred case. however. an adjustable inner form is employed and. when a cast inner form is used. it is provided in its interior with conduits for the heating medium. When a flshaped body is to be produced. moreover, it has been found to be advantageous to employ a number of removable inner forms. The inner forms can include a pair of outer members having a cross-section corresponding approximately to half that of the central member. The several inner forms may be connected together by plates, e.g. a bottom plate spanning the floor of the outer form and/or endplates closing the extremities of the mold cavity. The connecting plates may be provided with means enabling the crane to grip the forms and the concrete body carried thereby. In this case also, the spacer templates must have legs shaped corresponding to the fl-shape and extending between the inner forms.  
  The above and other objects. features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:  
  FIG. I is a perspective view of a roof. platform or deck structure embodying the present invention and seen from above;  
  FIG. 2 is a perspective view of the structure as seen from below;  
  FIG. 3 is a side-elevations] view of a mold or form for the production of the structural elements used in the roof. platform or deck structures of FIGS. 1 and 2;  
 FIG. 4 is a plan view of the mold or form of FIG. 3;  
  FIG. 5 is a detail end view. partly in diagrammatic form. of a mold for producing structural elements having an inverted-U cross section according to the invention.  
  FIG. 6 is an elevational view of a template for use in the mold of FIG. 5;  
  FIG. 7 is an elevational view of a strike board for use with the mold of FIG. 5 or the mold of FIGS. 8 or 9;  
  FIG. 8 is a vertical section through another mold according to the invention;  
  FIG. 9 is an end view ofa mold for producing still another structural element according to the invention. and  
  FIG. 10 is a view of the template of FIG. 6 inside elevation.  
  In FIGS. I and 2 of the drawing. we have shown the basic roof. platform or deck construction according to the present invention. the roof. platform or structure spanning a pair of supports shown generally at 27 and 26 horizontally spaced apart.  
  The support 26 is shown to be a grate or the like forming a wall into which the far ends of the structural elements constituting the roof. platform or deck extend to be embedded or cemented with mortar or concrete. The support 27 merely forms a masonry pillar upon which the roof structure rests. It should be understood that the invention is applicable to any type of support structure so that either support may be of the kind which is bonded by cement. bolts. flanges or the like for attachment of the structural elements. or may be the kind upon which the roof. platform or deck rests via a ledge. shoulder of other horizontally disposed surface.  
  The roof. platform or deck structure comprises a plurality of laterally contiguous substantially identical structural elements of channel configuration. the elements being represented generally at 20. The elements are. therefore. of inverted-U cross-section and. to iIlus&#39; trate the invention in some of its many ramifications. structurally different elements have been shown in these Figures although it will be understood that the roof. platform or deck structure will generally consist of elements of only one kind. All of the elements. however. comprises a horizontal web 21. forming the bight of an inverted-U and. monolithic. integral and in one piece therewith. a pair of longitudinally extending flanges 22 lying in vertical planes.  
  Each of the structural elements has a length L sufficient to span the distance D between the supports 26 and 27 and a width W which is a minor fraction of the length L. In general. the length may be ten times the width or more. The width it corresponds to the bight ofthe channel and has a thickness r. of 3 to 10cm (web thickness] while the flanges 20 have a thickness 1 of approximately 4 or 5cm. The structural element 201: (FIG. 2) is shown to have a reinforcement 20a in the form of a mat. grid or mesh bent into channel configuration and extending both along the web 21 of the structural element and the flanges 22 thereof.  
  The element 201). however. is seen to be formed with a reinforcement as constituted by arrays of reinforcement bars which may be transversely interconnected by stirrups (not shown) in accordance with conventional techniques. Tie rods 30 traverse the flanges 22 of the structural elements in the transverse direction and are tightened at the lateral ends of the roof structure via. for example. nuts 31 (FIG. I) to hold the longitudinal edges 200 of the contiguous structural elements 20, 20a. 20b etc. together to form an uninterrupted roof surface.  
  Alternatively. each pair of adjoining members can be coupled together by bolts 32 traversing the respective flanges 22. It has also been found to be advantageous to laterally interconnect the elements by members extending through the webs 21 as represented diagrammatically at 33. access to the bolts being afforded by openings 23 provided in these webs. Additional openings 24 can be formed in the flanges 22 to accommodate the tie rods 30, and transversely extending ribs 25 of a profile resembling that of the flanges 22. may bridge the flanges at longitudinally spaced locations (FIG. 2).  
  At one or more locations along the lower surfaces 22a of the flanges. we may provide pins. studs 35 or the like to which ceiling structures in the form of insulating plates 34 may be affixed. The attachment ofthe insulating members 34 may be accomplished as well by affixing synthetic resin or wood strips 36 to the lower edges of the flanges by positioning these strips in the mold and providing them with projections 37 which are embedded in the concrete.  
  The openings 23 and 24 may. in part. be formed by inserts 38 of wood. foamed synthetic resin (e.g. cellular polystyrene) which can be extracted from the molded member prior to erection of the roof. In FIG. 2. one such knockout is shown in place at 38. The openings 23 may also be conveniently employed to insert transverse supports 39 having footings 40 which bear against the oppositely facing inner surfaces of the flanges 22 to strengthen the structure against the inward force applied by the tie rods 30. The strips 36. of course. permit the members 35 to be mounted on the concrete roof by nailing. stapling or the like. The structure illustrated in FIGS. l and 2. whether produced by lateral tensioning of a support carrying the loosely disposed by contiguous members 20 or by tying the members together in pairs or in groups as previously described. yields a stiff plate which is self-supporting. No application of web concrete in situ is required and the plate has the structural characteristics of a monolithic slab. It is possible to walk upon it immediately after its erection. The tie rods 30 may be encased in concrete concurrently with the formation of webs similar to that shown at 25 and openings 23 and 24 may be closed by surrounding the indicated portions with a false work and pouring concrete through the openings 23. the concrete lying flush with the upper surface of the deck.  
  In FIG. 5. we show an outline (broken lines) of the actual cross-section of the inverted-U or channelshaped members produced. in accordance with the present invention. and diagrammatically illustrated at 20 in FIGS. I and 2. From this FIGURE it will be apparent that the inner flanks of the flanges 22. preferably diverge downwardly with a slight draft so that the space within the channel has the configuration of a trapezoid. the broad base of which is defined by a line connecting the free ends of the flanges 22 while the .narrow base lies along the web 2]. As shown in FIG. 9  
 in broken lines. the outer flanks of the flanges may converge toward the inner flanks so that the flanges taper downwardly in cross-section and are of reduced thickness remote from the web 21.  
  In FIGS. 3- 5 and 9. we have shown forms or molds for producing structural elements in accordance with the present invention. These molds comprise a pair of outer walls 2a and 2b which define the flanks of the concrete element and form a chamber 40 between them into which the concrete may be cast. The walls 20 and 2b extend continuously over the entire length of the form. which latter corresponds to the maximum length ofa concrete element to be produced by the system. The walls 2a and 2b are affixed by bolts 41 to spaced-apart brackets 42 consisting of upright arms 42:! carrying the walls 2a and 2b. and horizontal arms 42h provided with slots 42(&#39; running transversely to the major dimension of the form. Reinforcing ribs 42d interconnect the arms to stiffen the brackets.  
  As shown in FIG. 5. the brackets 42 are mounted upon spaced-apart l-beams l anchored at in to a foundation. e.g. a concrete footing. the l-beams being pro vided with centering screws 7 for proper alignment of the l-beams and. therefore. of the formwork. Bolts 14 pass through the slots 42c and into the upper flange of the l-beam l to adjustably anchor the brackets 42 thereto. By loosening the nuts of these bolts. it is possible to remove the brackets 42 inwardly or outwardly and thereby change the width w of the concrete element to be formed in the mold. The upper edge 2c of each wall 20. 2h. extends continuously above the brackets 42 and thereby serves as a guide for a strike bar. eg as shown at 15 in FIG. 7. Generally. however. the strike bar will merely be a board. the underside of which rests upon the guide edges 21&#39; to strike the concrete flush with these edges.  
  Apart from the outer formwork generally designated 2. we provide an inner form or core generally indicated at 3 and preferably composed of synthetic resin. The form 3 is wholly removable from the outer form 2 by a crane or the like in a manner not otherwise illustrated. The form 3 may be composed ofconcrete. wood or sheet metal and may be designed for a particular configuration or may be adjustable to suit any configuration of the inner cavity and the dimensions of the mold cavity More particularly. the inner mold may have a pair of end plates 5 (one ofwhich is designated in broken lines in FIG. 5) spanned by a pair of bars 9 in the longitudinal direction the bars 9 extending beyond the ends of the mold cavity at 9a and 9b as shown in FIG. 4 to enable the hooks of the crane to engage the inner form and lift it. together with the concrete body cast therearound. from the outer form 2. A sheet metal horizontal member 3:: forms the roof of the inner mold cavity 3 and has. preferably a symmetrically downward camher from a central crest as best seen in FIG. 5. The inner flanks of the flanges 22 are formed by a pair of sheet metal plates 3b and whose lower edges are turned outwardly at 3:! and 30 to rest approximately against the inner surfaces of the walls 2a and 2b of the mold. Hence a full-height flange may be cast within the space defined between the inner and outer shells 2 and 3. respectively.  
  The vertical height of the flanges 22 can. in accordance with the structural requirements, be adjusted by the use of vertically shiftable strips 4 are shown to be of angle profile and comprise a horizontal leg 40 and a vertical leg 4b. the latter being adjustably fixed to the walls 3b and 3(- by screws 40. Hence it is possible to lift the leg 4a from its solid line position there illustrated and shorten the height of the flange 22 to be formed.  
  According to an important feature of this invention. the strips 28, with anchoring pins. studs or projections 28a extending upwardly. can be placed upon the members 4 to be locked into the concrete body in the manner of the strips 36 illustrated in FIG. 2. Strips 28 can be composed of a material to which underlying plates may be connected with screws or the like. It is also possible to mount studs. nuts. screws or like fastening means upon the members 4 for embedding in the con crete or to use wire loops for this purpose.  
  At places corresponding to the openings 23, 24 etc.. the mold is provided with plates of cellular polystyrene. wood or other material readily removable from the concrete body after hardening thereof. As noted earlier. an important aspect of the present invention resides in the removal of the partially hardened and im completely set concrete from the outer shell 1 of the mold together with the inner shell. It has been found to be convenient. if this procedure is followed. to cut the openings 23 and 24 from the soft concrete. To facilitate removal of the inner form from the solidified concrete body and withdrawal of the inner form together with the partially hardened concrete body from the outer form. the end plates 10 may be provided with wire loops engageable by the crane.  
  It has been found to be convenient. moreover. to pro duce a number of concrete elements 20 in a single mold. In this case. templates 5 (FIG. 6) are provided at spaced locations along the mold. these templates having the configuration of the desired concrete element and. therefore. the configuration of the mold cavity. Between each pair of elements 5. a respective structural element 20 is produced. Of course. when each of the structural elements is to be half or less of the length of the mold. two or more concrete bodies may be produced within the mold. Similarly. when the length L is less than 5/1: where S is the total length of the mold. n concrete members 20 can be formed therein.  
  As best seen in FIGS. 6 and 10. the spacer templates 5. which define the extremities of the individual structural elements manufactured in a particular mold and also may serve as a gauge for proper setting of the inner form 3. can comprise a bar Su having a pair of legs 5b and 5c and thereby corresponding to an inverted-U. The bars and legs may be provided with openings 13 traversable ny reinforcing rods extending through th emold cavity so that the templates simultaneously serve as supports and spacers for these reinforcing rods. Moreover. the template 5 has a C-clamp 11 defined by an arm llu overhanging the leg and defining a throat 1112 into which the wall 2b of the outer mold can extend. The template 5 can be tightened in place by a pair of thumb screws 12 traversing the arm llu. To prevent canting of the template within the mold cavity, we provide the template with a bracket 6 having legs 6a and 6b which rest upon the edge 20 previously described. Since the template 5 also determines the angle of the end face of each structural element and it may be desired to have this end face include an angle with the platform surface which is other than we provide the template 5 with a surface 5e inclined at the desired angle B to the horizontal (see FIG. 10). To adjust the leg of the individual structural elements. therefore. it is merely necessary to loosening the screws 12 and shift the template 5 to the desired position along the trough formed by the outer shell 2 of the mold. The various movable parts of the inner form may then be adjusted to rest against the inner periphery 5d of the templates. thereby accurately establishing the dimensions and configuration of the cast body. Reinforcing members can then be inserted through the openings 13 and concrete cast between each pair of templates. The strike board is drawn across the deges 2c of the lateral walls of the mold to smooth the upper surface of the cast body which is permitted to harden until its slump is negligible. Prior to attaining a self-supporting state of the concrete and after decline of its slump to zero. the inner form is lifted out of the outer form together with the nonsupporting cast concrete bodies and carried to a hardening station at which a hot fluid. e.g. steam or hot air. is conducted through the interiors of the inner molds and. if desired. through the pipes 9. Since a number of such inner molds and hardening bodies will generally be provided at this station because of repeated use of the outer form for the casting of further bodies. the inner form may be connected in series or parallel to conduct the heated fluid through them. When each of the bodies has hardened. set or cured sufficiently to be self-supporting. the inner form may be turned over to permit the concrete body to rest on its platformforming surface and the inner form to be lifted therefrom by the crane. Prior to setting or hardening of the body, openings may be cut into the soft concrete as noted earlier and after hardening, and opening-forming inserts may be knocked out as desired. Upon erection of the laterally contiguous bodies into a platform. these openings can be closed. Unused openings 13 in the template may be closed by plugs according to the present invention.  
  In FIG. 9., we have shown somewhat diagrammatically, a mold for the formation of concrete bodies wherein a pair of lateral walls 102a and I02!) on sup ports define the outer form. The inner forms 1030. I03!) and I030 are here shown to be located behind the end plate 10 and to be constituted of sheet metal as shown in FIG. 5 or of concrete as previously described. Conduits 9 may traverse the inner forms to secure them to the end plates I0. the end plate having a wire loop [04! enabling the inner forms and the concrete body carried thereby to be lifted from the mold. The conduits 9 may serve merely to attach the inner forms to the end plates I0 or also as conduits for the introduction of steam or other heating medium into the inner form. The assembly illustrated in FIG. 9 is used to produce a fl-shaped body as represented in broken lines, the flanks of the flanges 122 of which converge downwardly while the spaces defined between these flanks and the respective web I21 form trapezoids with the broad base at the bottom.  
  When the mold illustrated in FIG. 9 is used. the templates must have a corresponding (l-shaped configuration. It is also possible to insert the templates into the reinforcement basket when the latter is constructed outside the mold for insertion together with the reinforcements into the latter.  
  In FIG. 7, we have shown a strike board IS having shoulders lSu adapted to ride upon the edges 2c of the mold and a central tongue I5 1) extending within the mold cavity for establishing the thickness of the web of the concrete body. In FIGS. 3 and 4, we have also shown that the exterior of the mold is provided with vibrators for compacting and jolting the concrete. To facilitate removal of the cast concrete bodies from the inner and outer form, the latter may be oiled as is conventional with concrete formwork or provided with a synthetic-resin coating having low adhesion to the concrete. In FIG. 8. we show a negative form 16 for the casting of inner forms 17 which may be used in place of the sheet metal inner form illustrated in FIGS. 5 and 9. Conduit means may be provided within these concrete inner forms as shown in 16a to conduct a heating medium through the body to promote curing of the concrete. When it is desired to provide inner forms of different height. e.g. as when the flanges of the structural elements are to have different heights. one need only employ a negative form of a depth corresponding to the height of the largest inner form to be made and cast the inner forms to the required depth of concrete. The negative form. as will be apparent. is of trapezoidal configuration.  
  The improvement described and illustrated is believed to admit of many modifications within the ability of persons skilled in the art. all such modifications being considered within the spirit and scope of the invention except as limited by the appended claims.  
 We claim:  
  I. A method of making a concrete platform comprising the steps of:  
 casting a plurality of downwardly open U-section concrete structural elements each having a horizontal web and a pair of downwardly extending flanges running longitudinally along said web in a mold having an outer form and an inner form removable from said outer form:  
 embedding in said flanges inserts lying between the surfaces of the inner and outer form.  
 removing each of the cast-concrete elements on a respective inner form from the outer form prior to the hardening of the concrete element into a selfsupporting condition but after an initial setting of the concrete:  
 hardening the concrete element upon the inner form out of the outer form to a self-supporting condition;  
 casting further concrete elements similarly on respective inner forms in said outer form during the hardening of the previously removed cast-concrete elements;  
 separating each cast-concrete element from the respective inner form only when the cast element has attained a self-supporting condition;  
 assembling the self-supporting elements in laterally contiguous relationship to form a platform and knocking out said inserts to form through openings in said flanges whereby the openings of adjacent cast-concrete elements are aligned;  
 passing reinforcing members through said openings with clearance;  
 fitting between flanges of at least some of said elements. transverse supports sustaining the flanges against inward forces produced by said members; and  
 tensioning the reinforcing members between the outermost flanges on opposite sides of said platform. thereby placing said supports under compression.