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  Energy absorbing post for roadside safety devicesUSPTO Application #: 20060027797Title: Energy absorbing post for roadside safety devices Abstract: An energy-absorbing post for absorbing the impact energy of an errant vehicle wherein the impact energy is absorbed by out-of-plane deformation in the material of the post. Out-of-plane deformation is provided by utilizing a through-bolt extending through a splice connection between upper and lower posts sections. Alternatively, out-of-plane deformation is provided by leaving an axial gap on a splice bolts. For terminal applications, a single through-bolt is utilized to allow the upper post section to pivot during end on impacts. Bolt tear out facilitators, including stress concentrators and pre-buckles, or an angled through-bolt decrease any initially high tear-out forces. Direct out-of-plane deformation is provided by extending a tab from a splice plate and connecting the tab to the post, by forming one or more slots in an upstream lateral face of the post and directly welding a splice plate near the slots, or by connecting a bent over splice plate on opposing planar sides thereof to facilitate out-of-plane deformation in a weldment area between the plate and the post. (end of abstract)
Agent: Charles J. Rogers - Houston, TX, US Inventors: Dean L. Sicking, John R. Rohde, John D. Reid, King K. Mak USPTO Applicaton #: 20060027797 - Class: 256013100 (USPTO) Related Patent Categories: Fences, Highway Guard The Patent Description & Claims data below is from USPTO Patent Application 20060027797. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention relates in general to roadside safety devices and more specifically to mounting posts for roadside safety devices. In particular, the present invention relates to improved energy-absorbing breakaway posts for roadside safety devices, such as guardrail, guardrail terminals, and crash cushions, mounted in a foundation of rigid or semi-rigid earthen or artificial materials. BACKGROUND [0002] Many highway agencies across the nation have begun to use barrier layers, such as Portland cement or asphaltic mow strips, to prevent the growth of vegetation under roadside safety devices such as guardrail. Mow strips consist of a narrow strip of pavement placed under the length of a guardrail to limit the growth of vegetation. When Portland cement concrete is used, the guardrail is normally erected first and the concrete is poured around the mounting posts and under the barrier. Alternatively, the guardrail posts may be driven though an asphalt pavement barrier layer laid and compacted in the area of the guardrail. Although mow strips effectively eliminate the growth of vegetation, they also have a profoundly negative impact on the safety performance of roadside safety devices such as W-beam guardrails. [0003] Guardrail posts are normally embedded vertically in soil at a depth that allows the post to rotate laterally upon the impact of an errant vehicle on the face of the guardrail. The guardrail is attached to the post by a bolt placed in a slot in the W-beam element which allows the guardrail to detach from the post when it begins to rotate laterally. Typically, the posts will absorb lateral forces in the neighborhood of 10 kips before rotating in the soil for 1.25 to 1.5 feet in order to absorb approximately 12.5 to 15 kip-ft. The lateral rotation of mounting posts in soil is one of the primary and intended mechanisms by which guardrails dissipate the energy of an impacting vehicle. [0004] When a guardrail post is installed in a rigid foundation, such as a mow strip, the base of the post is prevented from rotating in the soil. Thus, wooden guardrail posts placed in a rigid foundation fracture quickly upon impact without absorbing significant amounts of energy. When wide-flange steel beam posts are placed in a rigid foundation, the post often fails in an unstable manner due to lateral torsional buckling. Initially, high lateral forces of 12 kips or more are generated before a steel post begins to yield. After only a short lateral deflection, a steel post begins to rotate due to lateral torsional buckling, which causes the post to twist until it is loaded about the weak axis. When the post twists until it is loaded about the weak axis, the resistance force drops dramatically and the energy dissipated by the post is greatly reduced. The twisting motion also causes the bolt between the post and the guardrail to slide along the W-beam until it contacts the end of the slot in the guardrail. When the bolt reaches the end of the slot, pullout is inhibited which can cause the guardrail to be pulled below the impacting vehicle with the lateral rotation of the post and thus degrade safety performance of the guardrail. [0005] Full-scale crash testing and accident records indicate that W-beam guardrails installed in rigid foundations are not capable of meeting current safety performance evaluation criteria. (See, e.g., U.S. Dept. of Transp., Federal Highway Admin., Memo No. HSA-10/B64-B (Mar. 10, 2004).) Testing has also shown that this problem is not alleviated by using conventional breakaway guardrail posts that do not absorb energy during fracture. When guardrail posts fail quickly without absorbing sufficient energy, the W-beam guardrail often ruptures and the impacting vehicle is thereby allowed to penetrate through the barrier. Currently, most highway agencies resolve this problem by leaving open areas or cutouts in the mow strips in the area around the posts. Cutouts can defeat the purpose of the mow strip by allowing vegetation to grow up in the area around the posts. Some states attempt to resolve this problem by specifying that the cutout area around posts should be filled with a very low strength grout. However, low strength grouts are difficult to obtain in the field because most construction materials are specified by a minimum strength rather than a maximum allowable strength. Accordingly, the grouts actually used in cutouts are often found to be much stronger than the specified maximum strength and the effectiveness of the guardrail can therefore be seriously compromised. In addition, the installation of mow strip cutouts, whether open or grout-filled, increases the labor associated with the construction of a mow strip and thereby also increases overall costs. [0006] Cold winter weather in northern climates may also present difficulties for roadside safety devices. In these climates, the soil may freeze during the winter to a depth of one foot or more. This type of frozen ground condition can result in the creation of a rigid foundation similar to a concrete mow strip. Unfortunately, there is no known post foundation treatment that mitigates the safety degradation associated with a rigid foundation caused by frozen soil. [0007] There currently exist designs for energy absorbing breakaway posts, such as those described in U.S. Pat. No. 6,254,063 (hereby incorporated by reference). These designs generally utilize two post sections joined together by an energy-absorbing splice and are designed such that the upper post section is intended to break away from the lower section at a predetermined impact force. The energy absorbing post splice is typically created by utilizing cable restraint systems, bending of metal tabs, and/or bolts placed in slotted splice plates. These designs have been shown to absorb significant amounts of energy. However, the cost and/or reliability of these designs is believed to be a concern. Cable restraint designs rely on energy dissipation associated with the friction of a cable slipping through a cable clamp. Similarly, bolts placed in slotted splice plates rely on energy dissipation through friction between the bolt head and the splice plate. Energy dissipation systems that rely on friction can be sensitive to even a minor variance in installation details, such as the application of improper torque when tightening the splice or cable clamp bolts. In addition, the reliability of friction-based systems can also be adversely affected by corrosion of the friction components. Systems utilizing metal tabs that dissipate impact energy by bending are generally more reliable and less susceptible to corrosion, but the energy absorption capacity of these systems is lower and their fabrication cost is higher. SUMMARY OF THE INVENTION [0008] In view of the foregoing and other considerations relevant in the field, the present invention represents an improvement over conventional breakaway guardrail posts to increase energy absorption and thereby allow guardrails and other roadside safety devices to provide adequate safety performance even when installed in a rigid foundation. Further, the present invention provides an effective solution to the problems associated with rigid post foundations created by both concrete mow strips and frozen soils. These and other characteristics of the present invention are achieved by enhancing energy absorption in breakaway posts by facilitating bolt tear-out and the creation of out-of-plane stresses in the connection area of the upper and lower post sections. [0009] In general, a lower post section is mounted in a foundation. An upper post section is vertically aligned and spliced or welded to the lower post section. The upper post section has a generally flat lateral side facing the anticipated direction of a lateral impact. When the upper post section is struck by an errant vehicle, impact energy is absorbed either by bolt tear-out in the connection, or by direct Mode 3 out-of-plane tearing in the splice plate or lateral face of the upper or lower post section. [0010] Several preferred embodiments are described in more detail below, including the following: [0011] Several embodiments utilize a through bolt extending through a splice connection between the upper post section and the lower post section. The through bolt preferably includes a head facing the anticipated direction of a lateral impact and a fastener opposing the bolt head. At least one splice section created by the through bolt does not include additional compressive fasteners that restrict out-of-plane deformation between the underlying splice plate and the post flange. During an impact, energy is absorbed by bolt tear-out of the flange material. By utilizing the through bolt, as opposed to two or more standard compressive fasteners, the through bolt will produce energy absorbing tear-out even when located at greater distances from the edge of the post section. [0012] Several additional embodiments utilize tear-out facilitators to reduce initially high forces required to initiate bolt tear-out. Examples of facilitators described in more detail below include a saw cut located at the edge of the bolt hole in the material undergoing tear-out, as well as an out-of-plane pre-buckle formed in the edge of the bolt hole in the material undergoing tear-out. In other embodiments, initially high bolt tear-out forces may be reduced by orienting the through bolt at a non-perpendicular angle with respect to the material undergoing tear-out. Two or more of the tear-out facilitators may be combined to even further reduce initially high tear-out forces. [0013] The embodiments with a through bolt may also be utilized in terminal applications by using a single through bolt to connect the splice between the upper and lower post sections. By using a single through bolt, the upper post section is allowed to pivot freely during an end on impact while still absorbing impact energy during a lateral impact. To provide added stability in these applications, additional splice fasteners may be utilized by mounting the additional fasteners closer to the edge of the upper or lower post section, by removing post material near the other fasteners to similarly decrease the edge distance, or by removing a vertical slot of post material extending from the edge of the bolt hole to (or near to) the edge of the post. [0014] In an alternative embodiment, bolt tear-out may be facilitated even without utilizing a through bolt by locating a soft or compressible gasket under the head or nut of a splice bolt. Here, upon impact, the compressible gasket material permits angular deflection of the bolt in the hole and thereby reduces the energy required to initiate and sustain bolt tear-out as a means for energy absorption. [0015] Still other embodiments absorb impact energy by direct Mode 3 out-of-plane tearing in the splice plate or lateral face of the upper or lower post section. Here, energy absorption by direct Mode 3 out-of-plane tearing in the splice plate may be accomplished by extending a tab cut out or formed from a portion of the splice plate near the abutting ends of the post sections. One end of the splice plate is rigidly attached to the upper or lower post section by conventional means. The end of the tab is rigidly attached to the other post section such that deflection of the upper post section during an impact absorbs energy by out-of-plane tearing in the splice plate near the tab extension. [0016] Alternatively, direct Mode 3 out-of-plane tearing in the lateral face of the upper or lower post section may be accomplished by forming slots in the lateral face of the upper or lower post section. One end of the splice plate is rigidly attached to the upper or lower post section by conventional means. The other end of the splice plate is welded or attached to the lateral face of the other post section adjacent to the slots. On a lateral impact, angular deflection of the upper post section causes direct out-of-plane tearing in the lateral face of the post at or near the slots. [0017] Still other embodiments provide energy absorption by direct out-of-plane tearing in a weld area between the splice plate and the upper or lower post section. One end portion of the splice plate is rigidly attached to the upper or lower post section by conventional means. The other end portion of the splice plate is bent over on itself and its back side is welded by one or more vertically oriented welds to the other post section. In this manner, the upper and lower post sections are joined by opposing planar sides of the splice plate. Upon impact, angular deflection of the upper post section causes direct out-of-plane loading of the weld material between the back side of the splice plate and the underlying lateral post face. [0018] In any of the embodiments absorbing energy by direct Mode 3 tearing, the generation of out-of-plane forces may be facilitated by locating a small spacer between the splice plate and lateral post face. In this manner, a small out-of-plane angle is formed between the splice plate and the lateral post face such that even initial forces are directed out-of-plane. BRIEF DESCRIPTION OF THE DRAWINGS [0019] The foregoing and other features and aspects of the present invention are best understood with reference to the following detailed description of particular embodiments of the invention, as read in conjunction with and in light of the accompanying drawings, wherein: [0020] FIG. 1 is a partial side view of a flange with splice bolts for joining post sections. Continue reading... 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