Patent Publication Number: US-6988337-B1

Title: Means and method for constructing a fully precast top arch overfilled system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part application of Ser. No. 10/102,921 filed by the same inventor on Mar. 22, 2002 now U.S. Pat. No. 6,719,492, and assigned to the same assignee. The disclosure of the 10/102,921 document is fully incorporated herein by reference. This application also claims priority based on Ser. No. 10/131,526 filed on Apr. 25, 2002, and the disclosure of the application Ser. No. 10/131,526 is also fully incorporated herein by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to the general art of structural, bridge and geotechnical engineering, and to the particular field of overfilled arch and/or cut-and-cover structures. 
     BACKGROUND OF THE INVENTION 
     As discussed in the incorporated document, overfilled arch structures are frequently formed of precast or cast-in-place reinforced concrete and are used in the case of bridges to support one pathway over a second pathway, which can be a waterway, a traffic route, or in the case of other structures, a storage space or the like. The terms “overfilled arch” or “overfilled bridge” will be understood from the teaching of the present disclosure, and in general as used herein, an overfilled bridge or an overfilled arch is a bridge formed of arch elements that rest on the ground or on a foundation and has soil or the like resting thereon and thereabout to support and stabilize the structure and in the case of a bridge provide the surface of the first pathway. The arch form is generally arcuate such as cylindrical in circumferential shape, and in particular a prolate shape; however, other shapes can be used. Examples of overfilled bridges are disclosed in U.S. Pat. Nos. 3,482,406 and 4,458,457, the disclosures of which are incorporated herein by reference. 
     Prior to the structure disclosed in the incorporated 102,921 document, reinforced concrete overfilled arches were usually constructed by either casting the arch in place or placing precast elements, or a combination of these. As used herein, the term “prior art” will refer to structures prior to the structure disclosed in the incorporated 102,921 document. These arched structures rest on prepared foundations at the bottom of both sides of the arch. The fill material, at the sides of the arch (backfill material) serves to diminish the outward displacements of the structure when the structure is loaded from above. As used herein, the term “soil” is intended to refer to the normal soil, which can be backfill or in situ, located at a site used for a bridge structure, and which would not necessarily otherwise adequately support an arch. The terms “backfill,” and “in situ” will be used to mean such “soil” as well. 
     Soil is usually not mechanically stiff enough to adequately support bridge structures of interest to this invention. Thus, prior art bridge structures have been constructed to transfer forces associated with the structure to walls of the structure and/or large concrete foundations at the base of the wall. Such walls have to be constructed in a manner that will support such forces and thus have special construction requirements. As will be discussed below, such requirements present drawbacks and disadvantages to such prior art structures. 
     As discussed in the incorporated 102,921 document, for the prior art structures, the overfilled arches are normally formed such that the foundation level of the arch is at the approximate level of a lower pathway or floor surface of an underground structure over which the arch spans. As discussed in the incorporated document, prior art systems include sides or sidewalls which transfer loads from the top of the arch to foundation. The sides of such prior art arch systems must be sufficiently thick and contain sufficient reinforcement in order to be able to carry these loads and the thereby induced bending moments. 
     Furthermore, as it is necessary to limit the arch loading and bending actions in the top and sides of prior art overfilled arch systems to an acceptable level, the radius of the arch is in practice restricted. This restriction in arch radius leads to a higher “rise” (vertical dimension between the top of the clearance profile of lower pathway surface and the crown of the arch) in the arch profile than is often desirable for the economical and practical arrangement of the two pathways and formation of the works surrounding and covering the arch. This results in a lost height which can be substantial in some cases. 
     Beams or slabs, while needing a larger thickness than arches, do not require this “rise” and, therefore, can be used for bridges accommodating a smaller height between the top of the clearance profile of the lower pathway and the top of the upper pathway. Arches, despite their economical advantage, often cannot compete with structures using beams or slabs for this reason especially for larger spans. However, the larger thickness may result in an expensive structure whose precast elements may be difficult, unwieldy and heavy to transport to a building site. Thus, many of the advantages of beam or slab structures may be offset or vitiated. 
     Furthermore, as discussed in the incorporated document, the foundations for the prior art overfilled arch systems must be substantial in order to carry the arch loading and will require additional excavation at the base of the arch (generally beneath the lower pathway) to enable their construction. 
     For overfilled arches made of precast construction, the incorporation of the required height of the sides or sidewalls of the arch result either in a tall-standing precast element which is difficult and unwieldy to transport and to place or in the requirement of pedestals. 
     The system disclosed in the incorporated document solves these problems by having foundation blocks located behind or near the top of the side walls and against which the arch of the structure bears. The arch delivers all or at least most of its support forces into the foundation blocks. 
     This is an extremely effective system and accomplishes all of the objects set forth for this system in the incorporated document. 
     However, the effectiveness of this structure can be further enhanced by improving the methods used to erect the structure. Therefore, there is a need for a means and a method for building the structure disclosed in the incorporated document Ser. No. 10/102,921. 
     While the cast-in-place (cip) mode of constructing an arch system is suitable for many situations due to its economy and speed, there are certain commercial and technical (site) conditions for which a totally precast structure is preferred. Some of these conditions are:
         time restrictions for on-site installation;   weather conditions, especially low temperatures;   the absence of shuttering and crew suited/trained for the cip construction procedure;   a need to limit the specialist contractors&#39; duties to supplying (and, perhaps mounting) precast elements, in contrast to providing total contractor&#39;s services (and responsibility);   limited clear space, not allowing allowint the use of a shuttering (such as with live train lines at the lower pathway);   special requirements (aesthetic, etc.).       

     Therefore, there is a need for a means and a method for building a fully precast overfilled shallow arch structure such as disclosed in the incorporated document. 
     The precast arch elements in many prior systems are cast on their sides. This requires forms which have walls and also may require special handling of the forms to ensure proper formation of the arch elements. Still further, these elements are generally shipped in the side-on orientation. The elements are then lifted off the transporting vehicle, turned in the air to be oriented in the use orientation (as used herein, the use orientation is an orientation shown in  FIG. 1  herein as well as in  FIGS. 2A–2C  of the incorporated document, and a side-on orientation will have the elements rotated 90° with respect to the orientation shown in these same figures). Side-on formation and shipping has several drawbacks: complicated formwork; special transportation problems; and lifting problems associated with lifting and turning such elements. 
     Therefore, there is a need for a means and a method for forming and shipping a precast arch element such as disclosed in the incorporated document in a use orientation. 
     In the case of relatively large overfills, no connection may be required between adjacent arch elements because the overfilled soil spreads the loads on the overfill surface so that no differential displacements between adjacent elements occur. Differential displacements are caused by loads, such as traffic loads, placed only on one arch element, then on the adjacent arch element, and so on. Such deformations may lead to so called deflection cracking (cracks that propagate from the top of the arch element to the pavement surface). Such deformations should be avoided. 
     For shallow arch applications, shallow overfills are more frequent than high overfills since the shallow arch is preferably used where lost height needs to be minimized. In such a case, with only one or two feet or even only inches of overfill or almost-zero overfill in some situations, the live loads may act on individual elements before being transferred to the next one causing the relative vertical displacements that can be such that the pavement of the system will be cracked due to these relative displacements. 
     Therefore, there is a need for a means and a method for forming an arch system such as disclosed in the incorporated document in a manner that avoids differential displacements between adjacent arch elements of the system. 
     Still further, there is a need for a means and a method for forming an arch system such as disclosed in the incorporated document in a manner that avoids differential displacements between adjacent arch elements of the system even in the situation of a shallow, or even a zero, overfill. 
     OBJECTS OF THE INVENTION 
     It is a main object of the present invention to provide a means and a method for building the structure disclosed in the incorporated document application Ser. No. 10/102,921. 
     It is another object of the present invention to provide a means and a method for building a fully precast overfilled shallow arch structure such as disclosed in the incorporated document. 
     It is another object of the present invention to provide a means and a method for forming, stacking and shipping a precast arch element such as disclosed in the incorporated document in a use orientation. 
     It is another object of the present invention to provide a means and a method for forming an arch system such as disclosed in the incorporated document in a manner that avoids differential displacements between adjacent arch elements of the system. 
     It is another object of the present invention to provide a means and method for forming an arch system such as disclosed in the incorporated document in a manner that avoids differential displacements between adjacent arch elements of the system even in the situation of a shallow, or even a zero, overfill. 
     SUMMARY OF THE INVENTION 
     These, and other, objects are achieved by a means and method for forming an arch system such as disclosed in the incorporated document in which the arch elements are fully precast in a use orientation, then stacked and shipped in a use orientation. It is noted that the term “fully precast” is used herein to mean that the arch element is fully precast and with the exception of some cast-in-place concrete in the footings and in some cases cast-in-place concrete in the crown joints. The arch elements are placed on the foundation blocks in a manner which distributes forces associated with the arch elements to the foundation blocks, as taught in the disclosure of the incorporated document. 
     The formwork is very simple and no counter forms are usually required. Furthermore, there is no need to turn the elements in the air while hanging from a crane. 
     The arch elements can be prestressed by pre-deformation either during movement from the shipping vehicle to the in place location, or in another manner. The prestressing will partly or wholly compensate for the influence of possible outward yield (deformation) of the abutments (foundation blocks). The elements are placed in their pre-deformed shape and come back to their intended and optimal shape when overfilled. 
     The width of arch elements may be limited by the geometric transportation limitations and the weight. The lying down or use orientation has several advantages over the standing way or the side on orientation including the advantages associated with longer elements. For the shallow arches of the present invention, longer elements can be transported (even with footings attached) than with other arch geometries. 
     It is noted that the means and method disclosed herein can be applied to skew arch structures as well as to spans which do not allow one element solutions but which require a crown joint to connect two halves together. Therefore, spans can range from about twelve feet to eighty-four feet or more. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1  is an elevational view of a completed arch structure as disclosed in the incorporated document and which is formed in accordance with the teaching of the present disclosure. 
         FIG. 2   a  is a plan view of a system with skew alignment that can be formed in accordance with the teaching of the present disclosure. 
         FIG. 2   b  is a plan view of a system with curved alignment which can be formed in accordance with the teaching of the present disclosure. 
         FIG. 2   c  is a plan view of a system with an irregular alignment which can be formed in accordance with the teaching of the present disclosure. 
         FIG. 3   a  is a plan view of a curved system which can be formed in accordance with the teaching of the present disclosure showing adjacent arch elements. 
         FIG. 3   b  is a plan view of a skewed system which can be formed in accordance with the teaching of the present disclosure showing adjacent arch elements. 
         FIG. 3   c  is a plan view of a conventional span system which can be formed in accordance with the teaching of the present disclosure showing adjacent arch elements. 
         FIG. 4  is a plan view of a form used to form arch elements in a use orientation in accordance with the teaching of the present disclosure. 
         FIG. 5  is an end elevational view of the form shown in  FIG. 4 . 
         FIG. 6   a  shows an arch element that has been formed in the use orientation being moved in the use orientation. 
         FIG. 6   b  shows a top plan view of the arch element being moved in the use orientation. 
         FIG. 7  shows an arch element having a prestressing element associated therewith. 
         FIG. 8  shows a portion of an arch element in which bores are defined to accommodate tie elements, such as dowel rods or the like. 
         FIGS. 9 and 9   a  show a tie element located in a bore of the arch element. 
         FIG. 10  is a longitudinal section of a plurality of adjacent arch elements. 
         FIG. 11  shows a detail of a connection between adjacent arch elements. 
         FIG. 12  is an elevational view in section of a completed arch system in which adjacent arch elements are connected together in accordance with the teaching of the present disclosure. 
         FIG. 13  is a detail view showing a connection between two adjacent arch elements of a completed arch system in accordance with the teaching of the present disclosure. 
         FIG. 14  is a detail view showing an alternative form of a connection between two adjacent arch elements in accordance with the teaching of the present disclosure. 
         FIG. 15  is an elevational view in section of an arch system showing the arch system during one step in the process of erecting the system in accordance with the teaching of the present disclosure. 
         FIG. 16  is a detail view of an end of an arch element and a portion of a foundation block during one step in the process of erecting the arch system in accordance with the teaching of the present disclosure. 
         FIG. 17  is a detail view of an end of an arch element and a portion of a foundation block during one step in the process of erecting the arch system in accordance with the teaching of the present disclosure. 
         FIG. 18  shows a detail view of one form of an arch element and its footing that is included in the disclosure of the present invention. 
         FIG. 19  shows another detail view of a form of an arch element and its footing that is included in the disclosure of the present invention. 
         FIG. 20  shows another detail view of a form of the arch element and its footing that is included in the disclosure of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
     Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description and the accompanying drawings. 
     Shown in  FIG. 1  is an arch support system such as disclosed in the incorporated document. Reference is made to the incorporated document of patent application Ser. No. 10/102,921 for a full disclosure of the system shown in  FIG. 1 . However, by way of reference, shown in  FIG. 1  is a system  10  which includes an arch span  12 , which also will be referred to as an arch element, or simply an arch, which forms the roof of a void or open space  14  within an earth filled space. Beneath arch span  12 , walls  16  and  18 , which will also be referred to as side walls or retaining walls, retain backfilled earth  20  or excavation edges  22  and  24  of previously existing (in situ) ground material on either side of void or open space  14  above arch space  12 , overfill (earth) material OV is placed to create the plane  36 . The arch and retaining walls may or may not be structurally connected. The art and practice of the present invention enables the arch and the walls to be constructed independently, in different construction phases. The purpose and form of the arch, the retaining walls and the means of founding these two key components of the backfilled and/or overfilled structure will be understood from the teaching of the incorporated disclosure. 
     Structure  10  can be located between first selected area  30  which can be the floor of a void or a lower pathway, and which includes a plane  32 , and a second selected area  34  which can be a roof of a void or an upper pathway which includes a plane  36 . Arch span  12  and overfill (earth) material OV is placed to create the plane  36 . 
     The arch span is founded via arch footings  48  and  50  and foundation blocks  40  and  42  on general earth backfill  20  and/or on in situ soil (the surface of the previously existing (in situ) subsoil having been excavated to that extent). Foundation blocks  40  and  42  are each placed behind corresponding sidewalls  18  and  16  respectively of the overfilled and/or backfilled arch structure during its construction. Arch footings  48  and  50 , formed of concrete and/or reinforced concrete are interposed between springs  44  and  46  which will also be referred to as ends of arch span  12  and the foundation blocks to distribute forces over a wide area thus also reducing the strength and stiffness requirements of the solidified fill material of the foundation blocks. 
     As discussed in the incorporated disclosure, the foundation blocks distribute the concentrated arch support forces at the springs of the arch via arch footings onto a sufficiently large earth backfill area such that the bearing pressure on the volume of (in situ or backfill) earth to which the arch loads are applied does not cause unacceptable displacements, especially in the horizontal direction. 
     As is also shown in  FIG. 1 , a roadway R can be located above the system and can include pavement P with pavement P′ located beneath the system. 
     Shown in  FIGS. 2   a – 2   c  are examples of the type of systems that can be formed using the teaching of the present disclosure. As shown in  FIG. 2   a , the system can include skew elements SB. As shown in plan view  FIG. 2   b , the system can include a round bridge RB having a plurality of trapezoidal arch elements  12 T or an angled system AB with one trapezoidal element  12 T′. Plan views of different arch structures are shown in  FIGS. 3   a ,  3   b  and  3   c  as curved elements CB, skew elements SE and straight elements STE. 
     As discussed above, the method embodying the present invention forms the arch elements in a use orientation. The use orientation for arch element  12  is shown in  FIG. 1 ; whereas, a side on orientation would have arch element  12  oriented at a 9.0° angle with respect to the orientation shown in  FIG. 1 . As also discussed above, forming the arch elements in the use orientation produces several advantages over forming the arch element in a side-on orientation. A formwork  60  is shown in  FIG. 4  in plan view and can be used to form the straight elements STE, and/or the skew elements SE and/or the trapezoidal elements TE. The skew elements can include an angle α. Formwork  60  can include walls, such as  62 , to define the desired shapes as well as outer perimeter walls  64 . Materials and procedures suitable for forming the arch elements are carried out using the formwork and suitable procedures. The formwork is very simple and no counter forms are usually required. The formwork can be lifted up or down on one side of the form as indicated by double-headed arrow  66  in  FIG. 5  to help in placing and vibrating the concrete in the formwork, and to prevent the flow of vibrated concrete by changing the gradient/slope. The lifting can be performed using a suitable jack. The formwork, itself, can be vibrated, and when using the lifting system with suitable jacks, the vibration of the formwork can be done in halves or thirds of the arch element. 
     Once the concrete is poured and has hardened, the elements are moved, in the use orientation, from the formwork to a yard for stacking and from there to a transportation vehicle using a crane or the like. As shown in  FIGS. 6   a  and  6   b , an element  12   x  is attached to a crane (not shown) by a harness  68  which includes two cables  70  and  72  attached to a first surface  74  of element  12   x . As element  12   x  is lifted from the formwork, it will flex under its own weight from an unflexed configuration  12   x1  as shown in solid line in  FIG. 6   a  to a flexed configuration  12   x2  shown in dotted lines in  FIG. 6   a . This flexing can be used to obtain the desired pre-deformation to prestress the arch to partly or wholly compensate the influence of a possible outward yield (deformation) of the foundation blocks when the arch is subjected in its final position to loading. The arch elements are placed in their pre-deformed shape (indicated in dotted line in  FIG. 6   a ) and return to their original shape (indicated in solid line in  FIG. 6   b ) when overfilled. When the elements with the dotted line shape are placed onto the foundation blocks, the foundation blocks will hardly move under the dead weight of the arches only. When all elements have been placed, the overfill is placed which then has a total weight greater than that of the elements alone. This loading condition, the overfill plus the arch dead weight, produces a considerable horizontal thrust are on the foundation blocks. If the foundation block, or blocks, is/are not as stiff as desirable, this loading may push the foundation blocks out by a small amount. Even small movements result in the activation of the earth resistance to a considerable degree preventing further movement of the foundation block. Ideally, the foundation block will move out about as much as the ends of the arch elements have been drawn together by the pre-deformation before installation. If this is the case, the moments introduced by the drawing together of the ends and the opposite moments caused by the outward deformations of the foundation blocks will largely cancel each other out so that the elements—before traffic loads are applied—are in a state of very little moments. This helps to overcome disadvantages created by a certain amount of yielding of the foundation blocks. Should the foundation blocks not yield, the prestressing or pre-deformation is not harmful because it is done only to a degree which is within the allowable limits of the arch design. Furthermore, the moments generated by prestressing are opposite in direction to the majority of moments generated by traffic and are therefore not detrimental to the load carrying capacity of the arch. 
     Prestressing of the arch element can also be effected by structural elements, such as tie rod  80  shown for arch element  12   x2 . Tie rod  80  can include a turnbuckle  82  or the like to set the desired amount of camber, or pre-deformation on the arch element. 
     As discussed above, in some instances differential displacement can occur between adjacent arch elements in a system having a plurality of arch elements. This differential deformation can be prevented, or at least minimized, by connecting adjacent arch elements together once they have been put in place. The connection can transfer shear forces between elements and thereby reduce the relative displacements to zero or almost zero. Additionally, the load carrying capacity is increased since two or more adjacent elements carry the imposed loads in unison. 
     The method embodying the present invention includes connecting adjacent elements in one of several different ways. 
     The first connection is via post-tensioning one or several of the tie elements. This can be effected by introducing tension braces to the tie elements. The post-tensioning force creates friction between the adjacent elements which in turn provides shear resistance. The shear resistance prevents and counteracts differential deformation between adjacent arch elements. 
     A second form of connection is by bolting. Bolting is indicated in  FIGS. 8 through 13 . Holes, such as hole  90  are provided through each arch element. The holes can be defined by placing pipes in the formwork during formation of the arch element. The holes can have a counterbore  92  on each end thereof. The holes in each arch element are located so that the holes in one arch element will be aligned with the holes in an adjacent arch element as shown in  FIG. 10  for adjacent arch elements  12   xa1  and  12   xa2 . A relatively thick steel rod or dowel bar  94  (reinforcement bar) is positioned in the aligned holes such that it extends through the holes in at least two adjacent arch elements. To ensure centricity of the rod, support elements  96  can be located in the arch elements inside the holes. To guarantee a tight fit and proper load transfer, the rod has a sheath  98  surrounding it which can be a thin but tough plastic sheathing. After placement of the rod the sheath is filled with grout (cement plus the sand (or filler) plus water) under pressure. The grout fills the interspace between the rod and the arch element adjacent to the holes. The grout prevents play between the rod and the arch element. The rod or dowel bar becomes, after hardening of the grout, an integral part of the arch element. A space  99  exists between the sheath and the arch element adjacent to the hole and is filled when the sheath expands after insertion of grout under pressure. At ring joints, such as ring joint RJ (see  FIGS. 3   a  to  3   c ), the bar or rod continues between elements. Here also it is surrounded by grout which protects it against corrosion. Since the sheath extends for the entire length of the rod or dowel bar, the grout will not leak out of the sheath before setting. The sheath will expand to snugly fit the hole (or holes). At the joints between the elements, such as joint  102 , the sheath prevents the grout from leaking out. Additionally, as shown in  FIG. 13 , caulking  104  can be applied at the joints to make the structure watertight. 
     It is also noted that in order to produce a bridge from precast elements, it has to be done in several pieces which are each smaller than the entire bridge. These pieces (elements) can be tied together on site using the dowel and grouting system discussed above. 
     It is also noted that due to the rods or dowels the precast arch bridge performs almost as well, deformation and resistancewise, as if the joint (the ring joint) didn&#39;t exist as would be the case with a cast-in-place structure. The whole bridge acts as a homogeneous vault and not a number of individual arch elements, one next to the other. Thus, the rods or dowel bars are an effective means to overcome the drawbacks of precast structures which are separated by joints instead of being homogeneous structures like cast-in-place structures. 
     Still further means can be used to connect adjacent arch elements. Such a further means is indicated in  FIG. 14  and includes a cam  110  in one arch element and a corresponding depression  112  in an adjacent arch element. Each arch element contains both cams and depressions. A cam on one elements is accommodated in an associated depression on an adjacent element to connect the two adjacent elements together. Adhesive can also be applied to the cam and/or to the depression to provide a permanent connection free of play. 
     The foundation of the precast arch element is, in principal, the same as the foundation disclosed in the incorporated document. The foundation will include the foundation block. The arch elements can include an arch footing such as indicated in  FIG. 1  as arch footings  48  and  50 . In the means and method embodying the present invention, the arch footings can be precast together with the arch element as indicated for arch footing  50   p  in  FIG. 18  which rests directly on the foundation block. Another form of the arch footing is shown in  FIG. 19  as arch footing  50   p1  which is cast in place and connected to the arch element which does not contain precast footings. Yet another form of the arch footing is shown in  FIG. 20  as arch footing  50   p2 . Arch footing  50   p2  includes a small footing  50   p2′  that is precast with the arch element and a layer of cast-in-place concrete  50   p2″  between the precast footing and the foundation block. This procedure allows the precast footing to be designed quite small (thus adding only little weight to the precast element) while the concrete (preferably unreinforced) which is cast-in-place between the precast element and the foundation block spreads the footing forces sufficiently to be borne by the solidified earth material of the foundation block. This cast-in-place concrete would be poured after the precast elements are installed in their final position, the latter being provisionally supported on locally protruding parts of the arch element LPP in  FIGS. 3   a  to  3   c  or element  124  of  FIG. 16 . This ensures that the final support will be between the larger part of the arch element and the foundation block via the cast-in-place concrete. 
     This process of placing cast-in-place concrete between the arch element and the foundation block is indicated in  FIGS. 15 to 17  in which arch element  120  has an end area  122 . An element  124  extends out of the end area of the arch element and engages the foundation block when the arch element is initially installed. Reinforced or unreinforced concrete  126  is then cast in place around the arch element end and the foundation block and overfill  128  is subsequently placed on the cast-in-place concrete once this has hardened. Concrete can also be located between the end of the arch element and the foundation block as indicated in  FIG. 15  by cast-in-place concrete  130 . 
     As used herein, the term “prestressing” refers to the condition of an arch element such as shown in  FIGS. 6   a  and  7  prior to placement of the arch element in the system; and the term “post-tensioning” refers to a condition of an arch element after it has been placed. Thus, the elements shown in  FIGS. 6   a  and  7  are prestressed; whereas, adjacent arch elements  12  can be post-tensioned by the action of the dowel rods or by the action of friction of one arch element on an adjacent arch element or by the interlocking action of the elements shown in  FIG. 14 . 
     It is understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangements of parts described and shown.