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CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional from U.S. patent application Ser. No. 09/074,496, filed on May 7, 1998 by James R. Albritton and entitled “Breakaway Support Post for Highway Guardrail End Treatments” that claims benefit of U.S. Provisional Application Serial No. 60/046,015 filed May 9, 1997. 
     The application is also related to U.S. Divisional patent application Ser. No. 09/835,282 filed Apr. 12, 2001 and entitled “Breakaway Support Post for Highway Guardrail End Treatments”, now U.S. Pat. No. 6/488,268. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to highway guardrail systems having a guardrail mounted on posts, and more particularly, to guardrail end treatments designed to meet applicable federal and state safety standards including but not limited to crash worthiness requirements. 
     BACKGROUND OF THE INVENTION 
     Along most highways there are hazards which present substantial danger to drivers and passengers of vehicles if the vehicles leave the highway. To prevent accidents from a vehicle leaving a highway, guardrail systems are often provided along the side of the highway. Experience has shown that guardrails should be installed such that the end of a guardrail facing oncoming traffic does not present another hazard more dangerous than the original hazard requiring installation of the associated guardrail systems. Early guardrail systems often had no protection at the end facing oncoming traffic. Sometimes impacting vehicles became impaled on the end of the guardrail causing extensive damage to the vehicle and severe injury to the driver and/or passengers. In some reported cases, the guardrail penetrated directly into the passenger&#39;s compartment of the vehicle fatally injuring the driver and passengers. 
     Various highway guardrail systems and guardrail end treatments have been developed to minimize the consequences resulting from a head-on impact between a vehicle and the extreme end of the associated guardrail. One example of such end treatments includes tapering the ends of the associated guardrail into the ground to eliminate potential impact with the extreme end of the guardrail. Other types of end treatments include breakaway cable terminals (BCT), vehicle attenuating terminals (VAT), the SENTRE end treatment, and breakaway end terminals (BET). 
     It is desirable for an end terminal assembly installed at one end of a guardrail facing oncoming traffic to attenuate any head-on impact with the end of the guardrail and to provide an effective anchor to redirect a vehicle back onto the associated roadway after a rail face impact with the guardrail downstream from the end terminal assembly. Examples of such end treatments are shown in U.S. Pat. No. 4,928,928 entitled Guardrail Extruder Terminal, and U.S. Pat. No. 5,078,366 entitled Guardrail Extruder Terminal. 
     A SENTRE end treatment often includes a series of breakaway steel guardrail support posts and frangible plastic containers filled with sandbags. An impacting vehicle is decelerated as the guardrail support posts release or shear and the plastic containers and sandbags are compacted. A cable is often included to guide an impacting vehicle away from the associated guardrail. 
     A head-on collision with a guardrail support post located at the end of a guardrail system may result in vaulting the impacting vehicle. Therefore, guardrail end treatments often include one or more breakaway support posts which will yield or shear upon impact by a vehicle. Examples of previously available breakaway posts are shown in U.S. Pat. No. 4,784,515 entitled Collapsible Highway Barrier and U.S. Pat. No. 4,607,824 entitled Guardrail End Terminal. Posts such as shown in the &#39;515 and the &#39;824 patents include a slip base with a top plate and a bottom plate which are designed to not yield upon lateral impact. When sufficient axial impact force is applied to the upper portion of the associated post, the top plate and the bottom plate will slide relative to each other. If a vehicle contacts the upper part of the post, the associated impact forces tend to produce a bending moment which may reduce or eliminate any slipping of the top plate relative to the bottom plate. Also, improper installation of the top plate relative to the bottom plate, such as over tightening of the associated mechanical fasteners, may prevent proper functioning of the slip base. A breakaway support post is also shown in U.S. Pat. No. 5,503,495 entitled Thrie-Beam Terminal with Breakaway Post Cable Release. 
     Wooden breakaway support posts are frequently used to releasably anchor guardrail end treatments and portions of the associated guardrail. Such wooden breakaway support posts, when properly installed, generally perform satisfactorily to minimize damage to an impacting vehicle during either a rail face impact or a head-on impact. However, impact of a vehicle with a wooden breakaway support post may often result in substantial damage to the adjacent soil. Removing portions of a broken wooden post from the soil is often both time consuming and further damages the soil. Therefore, wooden breakaway support posts are often installed in hollow metal tubes, sometimes referred to as foundation sleeves, and/or concrete foundations. For some applications, one or more soil plates may be attached to each metal sleeve to further improve the breakaway characteristics of the associated wooden post. Such metal sleeves and/or concrete foundations are relatively expensive and time consuming to install. 
     Light poles, sign posts or similar items are often installed next to a roadway with a breakable or releasable connection. For some applications, a cement foundation may be provided adjacent to the roadway with three or more bolts projecting from the foundation around the circumference of the pole. Various types of frangible or breakable connections may be formed between the bolts and portions of the light pole or sign post. 
     SUMMARY OF THE INVENTION 
     In accordance with teachings of the present invention, various shortcomings of previous guardrail support posts associated with highway guardrail end treatments have been addressed. The present invention provides a breakaway support post which will buckle or yield during head-on impact by a vehicle at or near the extreme end of an associated guardrail to minimize damage to the vehicle and provide sufficient strength to direct a vehicle back onto an associated roadway during a rail face impact with the guardrail downstream from the guardrail end treatment. The use of breakaway support posts incorporating teachings of the present invention substantially reduces the time and cost associated with initial installation of a guardrail end treatment and repair of the guardrail end treatment following impact by a motor vehicle. 
     One aspect of the present invention includes providing a breakaway support post having one or more slots formed in the support post to allow the support post to buckle or yield in response to forces applied to the support post in a first direction by an impacting vehicle without causing excessive damage to the vehicle. The orientation and location of the slots are selected to allow the support post to effectively anchor the guardrail to direct an impacting vehicle back onto an adjacent roadway in response to forces applied to the support post in a second direction during a downstream rail face impact. For some applications, one or more plates may be attached to the breakaway support post and inserted into the soil to provide additional support during a rail face impact with the associated guardrail and to provide more reliable buckling or yielding of the breakaway support post during a head-on impact with one end of the associated guardrail. Alternatively, the length of the portion of the breakaway support post inserted into the soil may be increased to enhance these same characteristics. For some applications, the breakaway support post may have a typical I-beam cross section with slots formed in one or more flange portions of the I-beam. Alternatively, the breakaway support post may have a hollow, rectangular or square cross section with slots formed in one or more sides of the post in accordance with teachings of the present invention. 
     Another aspect of the present invention includes providing a breakaway support post having a first portion or an upper section and a second portion or a lower section with the first portion rotatably coupled with the second portion. A pivot pin or other suitable type of rotatable coupling preferably connects adjacent ends of the first portion and the second portion to allow rotation of the first portion relative to the second portion. The pivot pin is preferably oriented during installation of the associate breakaway support post to allow rotation of the first portion when force is applied thereto in one direction and to block rotation of the first portion when force is applied thereto in a second direction. A shear pin or other suitable releasing mechanism may be provided to releasably couple the first portion and the second portion aligned longitudinally with each other. The shear pin and pivot pin are preferably oriented such that during a head-on impact with the end of the associated guardrail facing oncoming traffic, the shear pin will fail and allow the upper section to rotate relative to the lower section and thus minimize damage to the impacting vehicle. For some applications, a release bar or push bar may be attached to the lower section to assist with disengagement of the upper section from the lower section during such rotation of the upper section. During a rail face impact with the associated guardrail, the same orientation of the shear pin and the pivot pin prevents the upper section from rotating relative to the lower section. Thus, the breakaway support post will buckle or yield during a head-on impact to minimize damage to an impacting vehicle and will remain intact to redirect an impacting vehicle back onto the associated roadway after a rail face impact. 
     Technical advantages of the present invention include providing breakaway support posts which are easier to initially install and to repair as compared to wooden breakaway support posts. Major portions of each breakaway support post may be fabricated from standard, commercially available steel I-beams using conventional metal bending and stamping techniques in accordance with teachings of the present invention. One or more metal soil plates may be attached to each breakaway support post to further enhance desired characteristics of yielding or buckling during head-on impact with one end of an associated guardrail to minimize damage to an impacting vehicle and to securely anchor the associated guardrail to redirect an impacting vehicle back onto the adjacent roadway after a rail face impact. Breakaway support posts incorporating teachings of the present invention may be used with a wide variety of guardrail end treatments having various types of energy absorbing assemblies located at or near the end of the associated guardrail facing oncoming traffic. For many applications, breakaway support posts may be satisfactorily installed adjacent to the edge of a roadway without the use of steel foundation tubes and/or concrete foundations typically associated with installing wooden breakaway support posts and other types of breakaway support posts. 
     A further aspect of the present invention includes providing guardrail support posts having a first portion or upper section attached or coupled, at least in part, by a frangible connection, to a second portion or lower section. The support post and frangible connection may be oriented in accordance with teachings of the present invention to resist impact by a motor vehicle from one direction (strong direction), and to yield to impact by a motor vehicle from another direction (weak direction). Preferably, the fragile connection allows the upper portion of the post to deflect slightly and then break off of the lower portion, thus minimizing lifting of the impacting vehicle into the air. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following written description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic drawing showing an isometric view with portions broken away of a highway guardrail system having a breakaway support post with a guardrail mounted thereon in accordance with one embodiment of the present invention; 
     FIG. 2 is a schematic drawing in elevation with portions broken away showing a side view of the highway guardrail system of FIG. 1; 
     FIG. 3 is a schematic drawing in section of the breakaway support post taken along lines  3 — 3  of FIG. 2; 
     FIG. 4 is a schematic drawing showing an isometric view with portions broken away of a highway guardrail system having a breakaway support post with a guardrail mounted thereon in accordance with another embodiment of the present invention; 
     FIG. 5 is a schematic drawing in elevation with portions broken away showing a side view of the breakaway support post of FIG. 4 in its first position; 
     FIG. 6 is a schematic drawing in elevation with portions broken away showing a side view of the breakaway support post of FIG. 5 rotating from its first position to a second position in response to a force applied to the breakaway support post in one direction corresponding with an impact by a vehicle with one end of the associated guardrail; 
     FIG. 7 is a schematic drawing showing an isometric view with portions broken away of a highway guardrail system having a breakaway support post with a guardrail mounted thereon in accordance with a further embodiment of the present invention; 
     FIG. 8 is a schematic drawing in elevation with portions broken away showing a side view of the highway guardrail system of FIG. 7; 
     FIG. 9 is a schematic drawing in section of the breakaway support post taken along lines  9 — 9  of FIG. 8; 
     FIG. 10 is a schematic drawing showing an isometric view with portions broken away of a highway guardrail system having a breakaway support post with a guardrail mounted thereon in accordance with another embodiment of the present invention; 
     FIG. 11 is a schematic drawing in elevation with portions broken away showing a side view of a breakaway support post analogous to the breakaway support post of FIG. 10 rotating from its first position to a second position and separating in response to a force applied to the breakaway support post in one direction corresponding with an impact by a vehicle with one end of the associated guardrail; 
     FIG. 12 is a schematic drawing showing an exploded, isometric view with portions broken away of an alternative embodiment of breaker bars suitable for use with the guardrail system illustrated in FIGS. 10 and 11; 
     FIG. 13 is a schematic drawing in elevation with portions broken away showing a side view of the breakaway support post of FIG. 10 utilizing the breaker bars of FIG.  12  and rotating from its first position to a second position and separating in response to a force applied to the breakaway support post in one direction corresponding with an impact by a vehicle with one end of the associated guardrail; 
     FIG. 14A is a schematic drawing in elevation with portions broken away showing a detail side view of a breakaway support post incorporating a further embodiment of the present invention; 
     FIG. 14B is a schematic drawing in elevation with portions broken away showing another side view of the breakaway post of FIG. 14A; 
     FIG. 15A is a schematic drawing in elevation with portions broken away showing a detail side view of a breakaway post in accordance with still another embodiment of the present invention; 
     FIG. 15B is a schematic drawing in elevation with portions broken away showing the upper portion and the lower portion of the breakaway support post of FIG. 15A disconnected from each other; 
     FIG. 15C is a schematic drawing in elevation with portions broken away showing another side view of the breakaway support post of FIG. 15B; and 
     FIG. 16 is a schematic drawing in elevation with portions broken away showing a side view of the breakaway support post of FIG. 15A rotating from its first position to a second position in response to a force supplied to the breakaway support post in one direction corresponding with an impact by a vehicle with one end of an associated guardrail. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the present invention and its advantages are best understood by referring to the FIGS. 1 through 16 of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     Portions of highway guardrail system  20  incorporating one embodiment of the present invention are shown in FIGS. 1,  2  and  3 . Portions of highway guardrail systems  120 ,  220 , and  320  incorporating alternative embodiments of the present invention are shown in FIGS. 4 through 13. Breakaway support posts incorporating further embodiments of the present invention are shown in FIGS. 14A through 16. Highway guardrail systems  20 ,  120 ,  220 , and  320  are typically installed along the edge of a highway or roadway (not expressly shown) adjacent to a hazard (not expressly shown) to prevent a vehicle (not shown) from leaving the associated highway or roadway. 
     Guardrail systems  20 ,  120 ,  220 , and  320  are primarily designed and installed along a highway to withstand a rail face impact from a vehicle downstream from an associated end treatment. Various types of guardrail end treatments (not expressly shown) are preferably provided at the end of guardrail  22  facing oncoming traffic. Examples of guardrail end treatments satisfactory for use with the present invention are shown in U.S. Pat. No. 4,655,434 entitled Energy Absorbing Guardrail Terminal; U.S. Pat. No. 4,928,928 entitled Guardrail Extruder Terminal; and U.S. Pat. No. 5,078,366 entitled Guardrail Extruder Terminal. Such guardrail end treatments extend substantially parallel with the associated roadway. U.S. Pat. No. 4,678,166 entitled Eccentric Loader Guardrail Terminal shows a guardrail end treatment which flares away from the associated roadway. U.S. Pat. Nos. 4,655,434; 4,928,928; 5,078,366; and 4,678,166 are incorporated herein by reference. When this type of guardrail end treatment is hit by a vehicle, the guardrail will normally release from the associated support post and allow the impacting vehicle to pass behind downstream portions of the associated guardrail. However, breakaway support posts incorporating teachings of the present invention may be used with any guardrail end treatment or guardrail system having satisfactory energy-absorbing characteristics for the associated roadway and anticipated vehicle traffic. 
     Support posts  30 ,  130 ,  230 ,  330  and  530  have a strong direction and a weak direction. When a post is subjected to an impact from the strong direction, the post exhibits high mechanical strength. The strong direction is typically oriented perpendicular to the guardrail. Thus, when the post is impacted by a vehicle in the strong direction (such as when the vehicle impacts the face of the guardrail), the post will remain intact and standing, and the vehicle will be redirected back onto the road. When the post is subjected to an impact from the weak direction, the post exhibits low mechanical strength. The weak direction is typically oriented parallel to the guardrail. Thus, when the post is impacted by a vehicle in the weak direction (such as when the vehicle impacts the end of the guardrail), the portion of the post that is substantially above the ground will either break off or bend over, so as to avoid presenting a substantial barrier to the vehicle. Preferably, the upper portion of the post will deflect slightly and then break off, in order to minimize lifting of the impacting vehicle into the air. 
     One or more support posts  30 ,  130 ,  230 ,  330 , and  530  are preferably incorporated into the respective guardrail end treatment to substantially minimize damage to a vehicle during a head-on impact with the end of guardrail  22  facing oncoming traffic. The number of support posts  30 ,  130 ,  230 ,  330  and  530  and the length of guardrail  22  may be varied depending upon the associated roadway, the hazard adjacent to the roadway requiring installation of highway guardrail system  20 ,  120 ,  220  or  320 , anticipated vehicle traffic on the associated roadway, and the selected guardrail end treatment. As discussed later in more detail, breakaway support posts  30 ,  130 ,  230 ,  330  and  530  will securely anchor guardrail  22  during a rail face impact or side impact with guardrail  22  to redirect an impacting vehicle back onto the associated roadway. Support posts  30 ,  130 ,  230 ,  330  and  530  will yield or buckle during a head-on impact with the end of guardrail  22  without causing excessive damage to an impacting vehicle. 
     Support posts  30 ,  130 ,  230 ,  330  and  530  may be fabricated from various types of steel alloys or other materials with the desired strength and/or breakaway characteristics appropriate for the respective highway guardrail system  20 ,  120 ,  220 , and  320 . For some applications, a breakaway support post incorporating teachings of the present invention may be fabricated from ceramic materials or a mixture of ceramic and metal alloys which are sometimes referred to as cermets. 
     Portions of breakaway support posts  30 ,  130 ,  230 ,  330  and  530 , as shown in FIGS. 1-16, have the general configuration associated with a steel I-beam. Alternatively, the teachings of the present invention may be incorporated into a breakaway support post having a generally hollow or solid, rectangular, square or circular cross section. 
     Breakaway support posts  30 ,  130 ,  230 ,  330  and  530  as shown in FIGS. 1-16, have respective upper portions and lower portions with approximately the same general cross-section. However, for some applications, the upper portion of a breakaway support post incorporating teachings of the present invention may have a cross-section which is substantially different from the cross-section of the associated lower portion. For example, the upper portion may have the general configuration associated with an I-beam, while the associated lower portion may have a general configuration associated with either a hollow or solid cylindrical post or a hollow or solid square post. 
     In FIGS. 1,  2 ,  4 ,  7  and  10 , highway guardrail systems  20 ,  120 ,  220  and  320  are shown having a typical deep W-beam twelve (12) gauge type guardrail  22 . For some applications, a thrie beam guardrail may be satisfactorily used. Other types of guardrails, both folded and non-folded, may be satisfactorily used with breakaway support posts  30 ,  130 ,  230 ,  330  and  530  incorporating the teachings of the present invention. Breakaway support posts  30 ,  130 ,  230 ,  330  and  530  may sometimes be described as direct drive support posts. 
     Various techniques which are well known in the art may be satisfactorily used to install breakaway support posts  30 ,  130 ,  230 ,  330  and  530  depending upon the type of soil conditions and other factors associated with the roadway and the hazard requiring installation of respective highway guardrail systems  20 ,  120 ,  220 , and  320 . For many applications, breakaway support posts  30 ,  130 ,  230 ,  330  and  530  may be simply driven into the soil using an appropriately sized hydraulic and/or pneumatic driver. As a result, breakaway support posts  30 ,  130 ,  230 ,  330  and  530  may be easily removed from the soil using an appropriately sized crane or other type of pulling tool. For many applications, breakaway posts  30 ,  130 ,  230 ,  330  and  530  may be satisfactorily used to install guardrail  22  adjacent to an associated roadway without the use of metal foundation tubes or other types of post-to-ground installation systems such as concrete with a steel slip base support. U.S. Pat. No. 5,503,495, entitled Thrie-Beam Terminal With Breakaway Post Cable Release, shows one example of a breakaway support post with this type of foundation. 
     As shown in FIGS. 1,  2  and  3 , breakaway support post  30  includes elongated body  32  defined in part by web  34  with flanges  36  and  38  attached thereto. Elongated body  32  may be formed by cutting a steel I-beam (not expressly shown) into sections having the desired length for elongated body  32 . A pair of elongated slots  40  and  42  are preferably formed in flange  36  on opposite sides of web  34 . Similarly, a pair of slots  44  and  46  are preferably formed in flange  38  on opposite sides of web  34 . Slots  40 ,  42 ,  44  and  46  are formed intermediate first end  31  and second end  33  of breakaway support post  30 . Slots  40 ,  42 ,  44  and  46  define in part a frangible or yieldable connection between an upper portion and a lower portion of support post  30 . 
     The length of breakaway support post  30  and the location of slots  40 ,  42 ,  44  and  46  will depend upon various factors including soil conditions and the anticipated amount of force that will be applied to breakaway support post  30  during a rail face impact with guardrail  22  and during a head-on impact with one end of guardrail  22 . For the embodiment shown in FIGS. 1,  2  and  3 , slots  40 ,  42 ,  44  and  46  are formed in breakaway post  30  at a location corresponding approximately with the anticipated ground line when breakaway support post  30  is properly installed adjacent to the associated roadway. 
     For one application, elongated body  32  may be formed from a standard steel I-beam with flanges  36  and  38  having a nominal width of four (4″) inches and web  34  having a nominal width of six (6″) inches. Slots  40 ,  42 ,  44  and  46  have a generally elongated oval configuration approximately six (6″) inches in length and one fourth (¼″) inch in width. Slots  40 ,  42 ,  44 , and  46  are positioned intermediate ends  31  and  33  to cause local buckling of the associated breakaway post  30  when properly installed. 
     For the embodiments shown in FIGS. 1 and 2, block  48  is disposed between breakaway support post  30  and guardrail  22 . Block  48  may sometimes be referred to as a “blockout.” For other applications, guardrail  22  may be directly mounted adjacent to end  31  of breakaway support post  30 . During a rail face impact between a vehicle and guardrail  22  downstream from the associated end treatment, block  48  provides a lateral offset between breakaway support post  30  and guardrail  22 . The distance and direction of the lateral offset is selected to prevent the wheels (not shown) of an impacting vehicle from striking breakaway support post  30  during the rail face impact. 
     For the embodiment shown in FIGS. 1,  2  and  3 , breakaway support post  30  includes soil plates  52  and  54  which are attached to the exterior of respective flanges  36  and  38  adjacent to the portion of breakaway support post  30  which will be inserted into the soil adjacent to the associated roadway. For this embodiment, soil plates  52  and  54  have approximately the same thickness as web  34  and are generally aligned with web  34  on opposite sides of respective flanges  36  and  38 . 
     Breakaway support post  30  is preferably installed with web  34  extended approximately perpendicular from guardrail  22  and flanges  36  and  38  extending generally parallel with guardrail  22 . By aligning web  34  approximately perpendicular to guardrail  22 , breakaway support post  30  will provide sufficient support to resist large forces associated with a rail face impact or rail face impact between a vehicle and guardrail  22 . As a result of forming slots  40 ,  42 ,  44  and  46  in respective flanges  36  and  38  and orienting flanges  36  and  38  generally parallel with guardrail  22 , a head-on impact from a vehicle with one end of guardrail  22  will result in buckling or yielding of breakaway support post  30 . 
     The amount of force required to buckle and/or fracture breakaway support post  30  may be decreased by increasing the size and/or the number of slots  40 ,  42 ,  44  and  46  formed in respective flanges  36  and  38 . Alternatively, reducing the number and/or size of slots  40 ,  42 ,  44  and  46  will result in a larger amount of force required to buckle or yield breakaway support post  30 . 
     The orientation of soil plates  52  and  54 , relative to a head-on impact with one end of guardrail  22  will prevent twisting or tilting of breakaway support post  30  during the head-on impact. The additional support provided by soil plates  52  and  54  will increase the reliability of breakaway support post  30  yielding or buckling at the general location of slots  40 ,  42 ,  44  and  46  in response to a selected amount of force applied adjacent to end  31  of post  30  in a first direction corresponding to the direction of a head-on impact with one end of guardrail  22 . Soil plate  52  includes a generally triangular portion  56  which extends above elongated slots  40 ,  42 ,  44  and  46  to provide additional support for breakaway support post  30  and guardrail  22  during a rail face impact. 
     For some applications, the length of elongated body  32  may be increased such that soil plates  52  and  54  are no longer required to provide additional support for the resulting breakaway support post  30 . Eliminating soil plates  52  and  54  will allow a hydraulic or pneumatic hammer to more quickly install the associated breakaway support post  30  and a crane or hydraulic/pneumatic pulling tool to more easily remove a damaged breakaway support post  30 . Alternatively, breakaway support post  30  could be inserted into an appropriately sized concrete foundation and/or metal sleeve. Soil plates, concrete foundation, sleeves and other anchoring devices can be used in any of the posts of the present invention. 
     For some applications, it may be preferable to form a breakaway support post in accordance with teachings of the present invention from an elongated body having a generally hollow, rectangular or square configuration (not shown). Slots  40 ,  42 ,  44  and  46  may then be formed in opposite sides of the resulting breakaway support post which are aligned generally parallel with the associated guardrail similar to flanges  36  and  38 . The other pair of opposite sides preferably extend approximately normal from the associated guardrail similar to web  34 . 
     When force is applied adjacent to end  31  of breakaway support post  30  in a second direction corresponding with a rail face impact between a vehicle and guardrail  22 , web  34  will resist buckling of breakaway support post  30  and provide sufficient support to redirect the impacting vehicle back onto the roadway. 
     Breakaway support post  130 , as shown in FIGS. 4,  5  and  6 , includes elongated body  132  having an upper portion  142  and a lower portion  144  which are rotatably coupled with each other. For the embodiment of the present invention shown in FIGS. 4,  5  and  6 , rotatable coupling assembly  140  is preferably installed intermediate ends  131  and  133  of elongated body  132 . Upper portion  142  and lower portion  144  each have a general configuration of an I-beam defined in part by respective webs  134  and flanges  136  and  138 . Upper portion  142  and lower portion  144  may be formed from a conventional steel I-beam in the same manner as previously described. 
     For the embodiment of the present invention as shown in FIGS. 4,  5  and  6 , rotatable coupling assembly  140  includes a first generally U-shaped bracket  150  attached to one end of upper portion  142 , opposite end  131  and a second U-shaped bracket  152  attached to the end of lower portion  144  opposite from end  133 . Brackets  150  and  152  each have a generally open, U-shaped configuration with extensions substantially parallel to the flanges and protruding beyond the respective webs. A portion of bracket  150  is preferably sized to fit within a corresponding portion of bracket  152 . Pivot pin  154  extends laterally through adjacent portions of bracket  150  and  152  in a direction which is generally parallel with webs  134 . The resulting breakaway support post  130  is preferably installed with webs  134  and pivot pin  154  extending generally normal from the associated guardrail  22 . As a result of this orientation, webs  134  and rotatable coupling assembly  140  including pivot pin  154  allow breakaway support post  130  to sufficiently support guardrail  22  during a rail face impact to redirect an impacting vehicle back onto the associated roadway. 
     In FIGS. 4,  5  and  6 , respective webs  134  of upper portion  142  and lower portion  144  are shown generally aligned parallel with each other. For some applications, the orientation of lower portion  144  may be varied with respect to upper portion  142  such that web  134  of lower portion  144  extends approximately parallel with guardrail  22 . The attachment of brackets  150  and  152  with their respective upper portion  142  and lower portion  144  may be modified to accommodate various orientations of lower portion  144  relative to upper portion  142 . 
     Depending upon the length of lower portion  144  and the type of soil conditions, soil plates  162  and  164  may be attached to lower portion  144  extending from respective flanges  136  and  138 . For some applications, lower portion  144  may be substantially longer than upper portion  142 . As a result of increasing the length of lower portion  144 , the use of soil plates  162  and  164  may not be required. 
     Shear pin  156  is laterally inserted through adjacent portions of brackets  150  and  152  offset from pivot pin  154 . Shear pin  156  preferably has a relatively small cross-section as compared to pivot pin  154 . As a result, when a vehicle impacts with one end of guardrail  22 , shear pin  156  will break and allow upper portion  142  to rotate relative to lower portion  144  as shown in FIG.  6 . Shear pin  156  maintains upper portion  142  and lower portion  144  generally aligned with each other during installation of the associated breakaway support post  30 . 
     The amount of force required to fracture or break shear pin  156  may be determined by a variety of parameters such as the diameter of shear pin  156 , the type of material used to fabricate shear pin  156 , the number of locations (either along a single pin or with plural pins) that must be sheared, and the distance between shear pin  156  and pivot pin  154 . As discussed later in more detail with respect to breakaway support post  530 , as shown in FIGS. 15A through 16, rotatable coupling  540  may be modified to allow upper portion  542  to disconnect and separate from lower portion  544 . 
     Various types of releasing mechanisms other than shear pin  156  may be satisfactorily used to maintain upper portion  142  and lower portion  144  generally aligned with each other during normal installation and use of the associated breakaway support post  130 . A wide variety of shear bolts, shear screws and/or breakaway clamps may be used to releasably attach first bracket  150  with second bracket  152 . 
     When a vehicle impacts with one end of guardrail  22 , force is applied in a first direction to upper portion  142  and will break shear pin  156 . As a result, upper portion  142  will then rotate relative to lower portion  144  as shown in FIG.  6 . 
     FIGS. 7,  8  and  9  show portions of highway guardrail system  220  which includes breakaway support post  230  and guardrail  22 . Breakaway support post  230  includes elongated body  32  and is similar in both design and function with breakaway support post  30 . One difference between breakaway support posts  30  and  230  is the replacement of soil plates  52  and  54  by soil plates  254  and  256 . As best shown in FIGS. 8 and 9, fastener assembly  160  may be used to attach soil plate  254  with elongated body  32 . Fastener assembly  160  includes threaded bolt  163 , hollow sleeve or spacer  168  and nut  165 . The use of soil plate  254  and fastener assembly  160  eliminates some of the welding steps associated with attaching soil plates  52  and  54  to breakaway support post  30 . 
     Soil plate  254  has a generally rectangular configuration. The length, width and thickness of soil plates  254  may be varied depending upon the intended application for the associated breakaway post  230  and the anticipated soil conditions adjacent to the associated roadway. An appropriately sized hole is preferably formed in the mid-point of soil plate  254  and bolt  162  inserted therethrough. The head  166  of bolt  163  is disposed on the exterior of soil plate  254 . Spacer or hollow sleeve  168  is then fitted over the threaded portion of bolt  162  extending from soil plate  254  opposite from head  166 . A corresponding hole is preferably formed in web  34  at the desired location for soil plate  254 . Bolt  163  is inserted through the hole in web  34  and nut  165  attached thereto. 
     For some applications, a smaller soil plate  256  may be attached to the exterior of flange  36  adjacent to web  34 . The dimensions and location of soil plate  256  may be varied depending upon the anticipated application including soil conditions, associated with highway guardrail system  220 . 
     FIGS. 10 and 11 illustrate portions of highway guardrail system  320 , which includes breakaway support post  330  and guardrail  22 . FIG. 11 illustrates an embodiment of support post  330  having narrower breaker bars  350  and  352  than those illustrated in FIG.  10 . Support post  330  includes an elongated body  332  having an upper portion  342  and a lower portion  344 . Upper portion  342  and lower portion  344  each have the general configuration of a steel I-beam similar to elongated body  32  of breakaway support post  30 . 
     Upper portion  342  and lower portion  344  are defined in part by respective webs  334  and flanges  336  and  338 . Upper portion  342  and lower portion  344  may be formed from a conventional steel I-beam in the same manner as previously described. Lower portion  344  may be positioned substantially within the ground. Alternatively, lower portion  344  could be inserted into a concrete foundation and/or a metal sleeve which have been previously installed at the desired roadside location. 
     Upper portion  342  and lower portion  344  are provided with breaker bars  350  and  352 . In the embodiment shown in FIG. 10, flanges  336  and  338  in upper portion  342  are connected to breaker bar  350 , by for example, welds. Flanges  336  and  338  in lower portion  344  may be connected to breaker bar  352  in an analogous fashion. Other suitable connection techniques may be used to couple flanges  336  and  338  of upper and lower portions  342  and  344  to breaker bars  350  and  352 , respectively. For example, as illustrated in FIG. 11, tie straps  362  and  364  may be used, particularly in an embodiment where breaker bars  350  and  352  are narrower than flanges  336  and  338 , as is the case in FIG.  11 . For some applications, breaker bar  352  may be directly attached to a concrete foundation to eliminate the use of lower portion  344 . 
     Breaker bars  350  and  352  are connected to each other by fasteners  358 , which is illustrated by a simple nut and bolt; however, other suitable fasteners may be used with this aspect of the invention. Breaker bars  350  and  352  are preferably formed with chamfered or tapered surfaces  354 . Chamfered surfaces  354  cooperate with each other to define in part a notch or gap between adjacent portions of breaker bars  350  and  352 . Chamfered surfaces  354  extend generally parallel with each other in a direction generally normal to guardrail  22 . An imaginary line  359  can also be drawn through fasteners  358  in the same general direction parallel with chamfered surfaces  354  and normal to guardrail  22 . Imaginary line  359  corresponds with a strong direction for breakaway support posts  330  in which breakaway support post  330  exhibits high mechanical strength. There is a notch or gap on each side of the imaginary line  359 . 
     Chamfered surfaces  354  cooperate with each other to allow upper portion  342  to pivot relative to lower portion  344  during a head-on impact, as illustrated in FIG.  11 . Such pivoting may cause fasteners  358  to break, separating upper portion  342  from lower portion  344  and may therefore substantially minimize damage to a vehicle during a head-on impact with the end of guardrail  22  facing oncoming traffic. The orientation of chamfered surfaces  354  and fasteners  358  relative to each other further define a weak direction for breakaway support post  330  in which support post  330  exhibits low mechanical strength. However, chamfered surfaces  354  do not reduce the ability of guardrail  320  to redirect an impacting vehicle back onto the associated roadway during a rail face impact with guardrail  22 . 
     FIG. 12 is a schematic drawing showing an exploded isometric view with portions broken away of an alternative embodiment of breaker bars suitable for use in guardrail system  320 . Breaker bars  450  and  452  perform similar functions as breaker bars  350  and  352 . Breaker bar  450  includes a flat plate  453  having a protruding member or projection  454 . Breaker bar  452  includes a flat plate  455  having a protruding member or projection  456 . Flat plates  453  and  455  are each formed with two or more apertures  458  for receiving a connecting member, such as mechanical fastener  358 , for attaching breaker bars  450  and  452  with each other. The use of protruding members or projections  454  and  456  allows upper portion  342  to pivot relative to lower portion  344  during a head-on impact, as illustrated in FIG.  13 . Impact from the weak direction for support post  330  will result in bending and preferably failure of connecting members  358 . Failure of connecting members  358  separates upper portion  342  from lower portion  344  and may, therefore, substantially minimize damage to a vehicle during a head-on impact with the end of guardrail  22  facing oncoming traffic. However, protruding members or projections  454  and  456  do not reduce the ability of guardrail  22  to redirect an impacting vehicle back onto the associated roadway during a rail face impact. 
     FIGS. 14A and 14B are schematic drawings with portions broken away showing an alternative embodiment of a frangible or yieldable connection satisfactory for releasably coupling upper portion  342  with lower portion  344  of support post  330 . For this embodiment, breaker bars  450  and  452  are substantially the same as previously described with respect to the embodiment shown in FIG. 13, except for the elimination of protruding members or projections  454  and  456 . A pair of elongated connecting members  458  and a plurality of nuts  460  are preferably provided to maintain a desired gap or spacing between breaker bars  450  and  452 . For the embodiment shown in FIGS. 14A and 14B, elongated connecting members  458  and nuts  460  have matching threads. However, various types of mechanical fasteners and connecting members may be satisfactorily used to position upper portion  332  of support post  330  relative to lower portion  344 . 
     As a result of incorporated teachings of the present invention, support post  330  has relatively low mechanical strength with respect to impact from a direction generally normal to an imaginary line  359  (see FIG. 10) extending through connecting members  358  or  458  as appropriate. This direction may be referred to as the “weak direction.” Connecting members  358  and  458  are preferably formed from materials which will yield and preferably fracture or break to allow upper portion  342  to separate from lower portion  344 . Since there is a gap between the breaker bars  350  and  352  or breaker bars  450  and  452  to either side of line  359  in the weak direction, connecting members  358  or  458  as appropriate will carry substantially all of the force or load from an impact in the weak direction. 
     When support post  330  is impacted from another direction, the resulting force, or at least a component of the resulting force, will tend to place one of the associated connecting members  358  or  458  as appropriate in tension, and will tend to place the other connecting member  358  or  458  as appropriate in compression. Therefore, the mechanical strength of the frangible connection between upper portion  342  and lower portion  344  is substantially greater in the strong direction as compared with an impact from the weak direction. The strongest direction for an impact with support post  330  is from a direction substantially perpendicular to the surface of flanges  338  and  336  and parallel with web  334  (the strong direction). The weakest direction for an impact with support post  330  is in a direction which is substantially perpendicular to web  334  and parallel with flanges  336  and  338 . 
     Spacers with various forms and configurations may be used to separate breaker bars  350  and  352  or  450  and  452  from each other as desired. For the embodiment shown in FIGS. 10 and 11, tapered surfaces or chamfered surfaces  354  form the necessary spacers as integral components of breaker bars  350  and  352 . For the embodiment shown in FIGS. 12 and 13, protruding members or projections  454  and  456  function as spacers to form the desired gap. For the embodiment shown in FIGS. 14A and 14B, nuts  460  cooperate with connecting members  458  to function as spacers to form the desired gap. Nuts  460  that are between breaker bars  450  and  452  may also be referred to as “stops.” 
     For some applications, upper portion  342  and lower portion  344  of support post  330  may be coupled with each other by only one connecting member  358  or  458 . Alternatively, more than two connecting members  358  or  458  may be used depending upon the anticipated application for the associated support post  330 . For some applications, one connecting member  358  or  458  may be provided on the side of support post  330  which is immediately adjacent to guardrail  22 . The associated breaker bars  350  and  352  or  450  and  452  will contact each other on the opposite side of the post, whereby the single connecting member  358  or  458  as appropriate will provide sufficient strength for support post  330  to withstand rail face or side impact with the associated guard rail  22 . 
     Support post  530 , as shown in FIGS. 15A through 16, is substantially similar to previously described support post  130 , except rotatable coupling assembly  140  has been replaced by rotatable coupling assembly or releasable hinge  540 . The embodiment shown in FIGS. 15A,  15 B,  15 C and  16  provides for the separation of upper portion  142  from lower portion  144 . Thus, upper portion  142  will not lift an impacting vehicle. Support post  530  may be formed in part by upper portion  142  and lower portion  144  as previously described with respect to support post  130 . Coupling assembly or releasable hinge  540  preferably includes a first generally U-shaped bracket  550  attached to one end of upper portion  142 , and a second U-shaped bracket  552  attached to an adjacent end of lower portion  144 . Brackets  550  and  552  each have a generally open, U-shaped configuration. A portion of bracket  550  is preferably sized to fit over a corresponding portion of bracket  552 . 
     Pivot pin  554  preferably extends through adjacent portions of brackets  552  in a direction which is generally parallel with webs  134 . Alternatively, pivot pin  554  may be replaced by generally round projections extending from opposite sides of bracket  552 . Bracket  550  preferably includes a pair of slots  572  formed in opposite sides thereof. Slots  572  are preferably sized to releasably engage respective portions of pin  554  which extend from bracket  552 . Slots  572  cooperate with pivot pin  554  to allow rotation of upper portion  142  relative to lower portion  144 , and to allow disengagement of upper portion  142  from lower portion  144 . 
     The resulting breakaway support post  530  is preferably installed with webs  134  and pivot pin  554  extending generally normal from the associated guardrail  22 . As a result of this orientation, webs  134  and releasable hinge  540 , including pivot pin  554 , allow support post  530  to adequately support guardrail  22  during a rail face impact to redirect an impacting vehicle back onto the associated roadway. 
     Shear pin  556  is preferably inserted through adjacent portions of brackets  550  and  552  offset from pivot pin  554 . Shear pin  556  maintains upper portion  142  and lower portion  144  generally aligned with each other during installation of the associated breakaway support post  530 . Shear pin  556  preferably has a relatively small cross-section as compared to pivot pin  554 . As a result, when a vehicle impacts with one end of guardrail  22 , shear pin  556  will break and allow upper portion  142  to rotate relative to lower portion  144  as shown in FIG.  16 . For some applications, push bar  580  is preferably attached to and extends between opposite sides of bracket  552 . The location of push bar  580  on bracket  552  is selected to assist disengagement of slot  572  from pivot pin  554  as upper portion  142  rotates relative to lower portion  144 . See FIG.  16 . 
     The amount of force required to fracture or break shear pin  556  may be determined by a variety of parameters such as the diameter of shear pin  556 , the type of material used to fabricate shear pin  556 , the number of locations (either along a single pin or with plural pins) that must be sheared, and the distance between shear pin  556  and pivot pin  554 . 
     Various types of releasing mechanisms other than shear pin  556  may be satisfactorily used to maintain upper portion  142  and lower portion  144  generally aligned with each other during normal installation and use of the associated breakaway support  530 . A wide variety of shear bolts, shear screws, frangible disks, and/or breakaway clamps may be used to releasably attach first bracket  550  with second bracket  552 . 
     When a vehicle impacts with one end of guardrail  22 , force is applied in a first direction (weak direction) to upper portion  142  and will break shear pin  556 . As a result, upper portion  142  will then rotate relative to lower portion  144  as shown in FIG.  16 . When portions of bracket  550  contact push bar  580 , slots  572  will disengage from pivot pin  554  and release upper portion  142  from lower portion  144 . 
     Although the present invention and its advantages have been described in detail it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the following claims.

Summary:
A support post for a guardrail which resists impact by a motor vehicle from one direction (strong direction) and yields to impact by a motor vehicle from another direction (weak direction). The support post is adapted to receive the guardrail such that the rail face of the guardrail runs generally perpendicular to the strong direction such that the support post resists an impact on the rail face of the guardrail and yields to an impact force on the end of the guardrail.