Abstract:
An improved breakaway steel guardrail post for use in dissipation of impact energy upon impact of the post having an upper post member and a lower post member, a connecting joint, and a mechanism connected to the upper and lower post members for controlling the energy dissipation of the guardrail post about the connecting joint at a predetermined rate. A first embodiment utilizes a cable restraint lopped through openings in the guardrail posts. Other embodiments included straps and fasteners designed to distort or fail at predetermined rates or strengths and combinations thereof.

Description:
This is a continuation-in-part application of co-pending U.S. patent application Ser. No. 09/187,461, filed Nov. 4, 1998, which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to breakaway steel posts for use with a highway guardrail end terminal system or a crash attenuation system. More particularly, the invention relates to a supplemental energy-absorbing mechanism affixed to such steel posts. 
     Existing highway guardrail end treatment systems include: the breakaway cable terminal (BCT), the eccentric loader terminal (ELT), the modified eccentric loader terminal (MELT), the vehicle attenuating terminal (VAT), the extruder terminal (ET), the slotted rail terminal (SRT), the sequential kinking terminal (SKT), and the flared energy absorbing terminal (FLEAT). 
     In all of these systems, breakaway wooden posts, either inserted in foundation tubes (known as BCT breakaway post) or directly installed in the ground (known as controlled release terminal CRT post), are used to facilitate proper breaking of the posts to minimize the potential for snagging on the posts and excessive decelerations on the vehicles in end-on impacts with the terminals. Holes are drilled into the wooden post at and/or below ground level to reduce the cross-sectional area of the post, thus reducing the force required to break the post. Steel breakaway posts based on the slip-base concept have also been developed, but did not receive widespread acceptance due to maintenance problems and higher initial costs. 
     However, there are situations in which a transportation agency may choose not to use wooden posts for environmental concerns or as a matter of policy. In such situations, breakaway steel post would be an alternative. A prior patent of the inventors relates to various conceptual designs for breakaway posts suitable for use with highway guardrail and end terminal systems. These designs have a predictable breakaway force threshold when impacted along the weak axis (in the direction of end-on impacts with the terminal system) while maintaining a sufficiently high bending force in the strong axis (perpendicular to the weak axis in the direction of side impacts) to provide the required lateral stiffness to the terminal system for side impacts. 
     The present invention relates to an improved steel post for use with a highway post-and-beam type guardrail at locations where deflection of the post in the soil is limited or not allowed, e.g., posts embedded in concrete or asphalt in such applications as mow strips. 
     Existing standard guardrail line posts, either wood or steel, require some deflection in the soil to function properly. When a guardrail is impacted, the posts in the immediate vicinity of the impact would typically deflect and absorb some of the impact energy. Deflection of the posts would allow the rail to go into tension and act like a ribbon to contain and redirect the vehicle. If deflection of the posts in the soil is not allowed or limited in such applications as mow strips where the posts are embedded in concrete or asphalt, wooden posts would fracture and steel posts would bend or twist at the base. 
     The present invention relates to various conceptual designs for energy-absorbing breakaway steel guardrail posts suitable for use at locations where deflection of the post in the soil is limited or not allowed. These designs provide a predictable failure or yielding force threshold when impacted to maintain the required lateral stiffness; a mechanism for adequate energy dissipation by the post; and a limit beyond which the lateral resistance of the post is eliminated. 
     SUMMARY OF THE INVENTION 
     The improved breakaway guardrail post of the present invention includes upper and lower post sections which are connected by a specially designed breakaway joint. The guardrail may be provided with a various number of alternative embodiments of controllers attached to the upper post member and lower post member to control the energy dissipation of the guardrail about the breakaway joint at a predetermined rate. The failure or yield is at a predictable force threshold when impacted along either the strong or weak axis while maintaining a sufficiently high bending force in the strong axis to provide the required lateral stiffness to the terminal system for side or strong axis impacts. 
     The present invention discloses several energy-absorbing controller alternatives for breakaway posts used with a highway guardrail or crash attenuation system. All breakaway alternatives involve joining of two sections (upper and lower) of structural steel shape (e.g., I-beam) posts in such a manner that the joint will fail or yield at a predictable force threshold when impacted along the weak axis while maintaining a sufficiently high bending force in the strong axis to provide the required lateral stiffness to the terminal system for side impacts. The lower section of the post is installed in the ground by either means of driving or drill and backfill. The rail element for the guardrail or crash attenuation system is attached to the upper portion of the post. The energy absorbing dissipation controlling mechanisms include: a cable restraint member; a strap attached along the post web; a strap having a U-shaped portion; and straps with slotted openings for receiving fasteners. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a perspective view of a steel breakaway guardrail post without a supplemental energy-absorbing mechanism of the present invention. 
     FIG. 1A shows a side elevation view of a cable restraint embodiment of the present invention. 
     FIG. 1B a top view of the present invention illustrating the cable restraint mechanism. 
     FIG. 1C a perspective view of the present invention with two cable restraint members. 
     FIG. 1D shows a looped cable restraint of the present invention. 
     FIG. 2 illustrates a partial cutaway, side elevation view of an alternative embodiment of the control member the present invention having a strap. 
     FIG. 2A is a top view of the strap embodiment of the present invention. 
     FIG. 3 shows a front elevation view of yet another embodiment of the control member of the present invention having a U-shaped strap. 
     FIG. 3A shows a side elevation view of the embodiment of FIG.  3 . 
     FIG. 3B is a top view of the embodiment of FIG.  3 . 
     FIG. 4 illustrates a front view from the weak axis direction of a third embodiment of the present invention having a strap with two slotted openings. 
     FIG. 4A shows a side elevation view of the strap embodiment of the present invention having a single slotted opening. 
     FIG. 4B shows a top view of the embodiment of FIG.  4 A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning to the drawings, it may be seen in FIG. 1 that a breakaway post  10  has an upper post member  12  and a lower post member  14 . They are joined by connecting joint  16 . (It should be understood that any of the embodiments of co-pending application Ser. No. 09/187,461 may be utilized to form the connecting joint  16 .) The upper end  18  of joint  16  is attached to post member  12  by a first set of fasteners  20  while the lower end  22  of the joint  16  is attached to the lower post  14  by a second set of fasteners  24 . As may be seen the ends  19  and  21  of upper post  12  and lower post  14  are slightly spaced apart when the posts are joined by joint  16  yielding a slight gap  99  between the ends  19  and  21 . 
     In FIGS. 1 and 1A, joint  16  is formed by steel plates  25  on opposite sides of the post  10 . The lower post member  14  is rigidly attached to the plates  25  by four or more bolts  24  (or welding) on each side (flange) of the post  14 . While FIG. 1B illustrates the post  10  as constructed from steel I-beam, other material compositions and composites may be used. 
     The upper post member  12  is attached to the plates  25  by two through bolts  20 . Other connectors such as pins, rods, welds and the like may be used. 
     As shown in the embodiment of FIGS. 1-1C, when impact forces are applied in the weak axis direction I w , one of the two through bolts  20  breaks and the upper post  12  rotates downwardly. The fracture force in the weak direction is:          F   w     =       2        aV     2   /   3         h               where,                            F   w     =     static force required to fail one through bolt about the weak axis.                              a   =     distance between the through bolts.                            h   =       height of                     F   w                     above the through bolts.                                V   b     =     shear strength of through bolts.                                         
     The force required to fail the connection in the strong direction is then          F     3   /   8       =       2        dV     2   /   3         h                            
     when 
     F s =static force required to fail through bolts when loaded about strong axis. 
     d=post depth (shown in FIG.  1 ). 
     The ratio between these two failure forces R f  is shown below. Thus, the post strengths in each direction can be controlled by selecting the bolt size to control V b  and the appropriate ratio d/a.          R     7   /   8       =     d   a                            
     Thus, the connecting joint member or plate  25  is connected to upper post  12  at the upper end  18  by first fasteners  20  to lower post  14  with the lower end  22  attached by a second fastener  24 . The first fasteners have a failure strength less than the failure strength of the second fasteners. When the impact face strikes the upper post member  12  along the direction of the weak axis I w  one of the first fasteners  20  fails and the upper post  12  rotates downwardly about the other of the first fasteners. 
     The improvement of the present invention is shown in FIGS. 1A-1C as a cable restraint  104  or plurality of such restraints  104  and  105 . FIG. 1C shows the cables  104  and  105 , made of materials having well known strength characteristics, passing upper opening  106  in web W u  and lower opening  108  in web W L . The cables each are of a predetermined length L and have first ends  110  and second ends  112 . The ends overlay or overlap each other by predetermined lengths OL 1  and OL 2 , and joined or held together by a cable clamp  114  having a predetermined slip strength (see FIG. 1 D). The cable ends pull through the clamp at a predictable force level, providing rotational resistance or deflection for the post. 
     The clamp  114  is crimped after the cable is slipped through the openings  106  and  108  and overlapped by lengths OL 1  and OL 2 . The length of the free ends  110  and  112  of the cable on either side of the clamp controls the distance (or rotation of the post) through which resistance acts. As the post is loaded, the breakaway mechanism on the flange fails and the cable resists rotation (acting like the post rotating in soil and absorbing energy). 
     Energy dissipation during the cable slipping process is the magnitude of the slip force times the available slip distance. The force required to cause a cable to slip through the clamp is related to the size and type of the cable involved and the number, size, and type of clamping devices used in the joint. The optimum slip force is determined using static testing of various cable types and sizes with different clamping mechanisms. Common methods for clamping two cable ends together include threaded cable clamps or ferrules made from different metals such as copper or aluminum. Threaded clamps have a U shape with two nuts that force a cross piece down onto the cables in the clamp. Ferrules are generally formed in “figure 8” shape with one cable through each of two openings. Ferrules are attached to the cables using a crimping device that deforms the metal around the two cables. Slip forces in these two connections are controlled by the size and number of threaded clamps or the amount of crimping on the ferrule. 
     The available slip distance in the cable splice is the total amount of cable extending beyond the clamp. When forces are applied to such a joint, one cable will usually begin to slip out of the joint before the other. When the end of the first cable approaches the clamping device the force required to pull it through the joint begins to increase due to normal damage done to the cable when it is cut. This increase in force will generally cause the other cable to begin to slip through the joint until maximum extension is achieved. Thereafter, one end of the cable will slip out of the joint and the energy dissipation process is terminated. This behavior is generally independent of the mechanism used to cut the cable. Shearing or sawing cuts cause the end of the cable to expand and torch cuts generate small clumps of metal. In either situation, the slip force increases as the end of the cable approaches the clamp, thereby causing the other cable to begin to slip and allowing the splice to reach maximum extension. 
     The total energy dissipated in each joint is found by experimentally measuring the slip force and multiplying by the total cable beyond the clamping mechanism. 
     In a second embodiment  120  of the deflection control mechanism shown in FIGS. 2 and 2A, a strap  121  is fastened to the web W u  of the upper post member  12  by a first fastener  122  and to the web W L  of the lower post  14  by a second fastener  124 . The fasteners shown in FIGS. 2 and 2A are bolt, nut, and washer combinations but any number of fasteners such as a pin, rivet, or rod will serve the equivalent purpose. The fasteners  122  and  124  pass through openings  126  and  128  in webs W u  and W L  and openings  130  and  132  in the straps, respectively. 
     When the post  10  is loaded, and the primary mechanism  25  on the flanges fails, additional energy in this embodiment is absorbed by the failure of either the web W u  or W L  or the strap  121  that is connected to both top and bottom of the post members. The fasteners  122  and  124  are put into shear from the travel of the adjacent ends of the post. Failure can occur in the strap, web or bolt. In each case this is a shear failure which is a function of material properties and geometry. For the fasteners  122  and  124 , the shear would occur based on strength and cross-sectional area. For the web W a  and W L , and strap  121 , shear is determined by strength, thickness and distance d 1 , d 2  and d 3  of the openings  126  and  128  from the edge of each respective post member or strap. It is predetermined which component will fail based on these parameters. The failure of the flat stock will simply appear as the bolt pulling past its original hole, with a failure of the material parallel to the direction of travel of the bolt. 
     The tear-through process will provide lateral resistance at a relatively constant force as the bolt is pulled through the metal. When the bolt breaks free of the metal, the lateral resistance of the post will be eliminated. 
     It should be understood that a plurality of such straps and fasteners may be attached to the guardrail post. 
     Yet another embodiment of the present invention is shown in FIGS. 3,  3 A and  3 B. Again, a strap  150  having a U-shaped portion  152  is attached at a first end  154  to the upper post member  12  and attached at a second end  156  to the lower post member  14 . As shown in FIGS. 3-3B, the attachment may be made to the flange F of the post member. However, it should be understood that the strap  150  could be attached to the webs W u  and W L . Attachment may be by any conventional means such as welding or bolting. 
     Upon impact and after failure of the assembly  27  on the face of the flange F, the U-shape portion  152  will have to deform and eventually become generally straight. The force required to accomplish this is again related to the thickness and width of the strap  150 , as well as its yield properties. When the strap  150  becomes generally straight, either it will fail in tension or the weld attachment will fail. Energy is absorbed over some fixed rotation and then the post fails. 
     FIGS. 4,  4 A, and  4 B illustrate still yet another embodiment of the present invention. The strap/shear concept is taken one step further by having additional travel for the fasteners  122  and  124  in slots  162  and  164 . The initial deformation of the post after failure of the breakaway mechanism  27  is controlled by the friction between the strap  160  and washers  166  and  168  and the webs W u  and W L  of the beam. This frictional force is dependent on material properties of the respective surfaces and the torque on the fasteners  122  and  124 . When the fasteners reach the end of the slots either the bolt  122  or  124 , web W u , or W L , or strap  160 , will fail in the same manner as previously discussed. 
     FIG. 4A illustrates an embodiment wherein a single upper slot  163  is used while the lower opening  165  for the fastener  124  is a standard sized hole for the fastener. It should be understood that a plurality of such straps with single or double slotted openings may be used on any given guardrail post. 
     Finally, while the description and drawings show each embodiment separately attached to a guardrail post it should be further understood that any combination of the embodiments may be arranged on a particular guardrail to yield the desired energy absorption upon guardrail impact. 
     Although the invention has been described with reference to a specific embodiment, this description is not meant to be construed in a limiting sense. On the contrary, various modifications of the disclosed embodiments will become apparent to those skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended claims will cover such modifications, alternatives, and equivalents that fall within the true spirit and scope of the invention.