ROHDE JOHN R (US)
REID JOHN D (US)
MAK KING K (US)
SICKING DEAN L (US)
ROHDE JOHN R (US)
REID JOHN D (US)
MAK KING K (US)
US6254063B1 | 2001-07-03 | |||
US6264162B1 | 2001-07-24 | |||
US6394241B1 | 2002-05-28 |
1. | An energy absorbing post for roadside safety devices comprising: a lower post section for engaging a foundation upon which the roadside safety device is mounted; an upper post section for receiving the impact of an errant vehicle, the upper post section and the lower post section being generally vertically aligned; a splice connection formed between the lower and upper post sections; and a rod member extending laterally through holes in the splice connection, wherein the rod member includes at least one axial portion thereof that closes the splice connection without opposing compressive fasteners; wherein the material of the splice connection which surrounds said axial portion of the rod member has a thickness t and a plate yield strength σy , and the rod member has a diameter db and an ultimate strength σu , each selected to satisfy the general relationship: 4S T° S) such that the energy of an errant vehicle impacting the upper post section may be absorbed by tearout from the material of the splice connection. |
2. | The post of claim 1 wherein the edge distance extending from the outside edge of the material undergoing tearout to the edge of the hole in said material is more than 1.5 times the diameter of the rod member. 3. The post of claim 1 or 2 wherein the rod member includes a distal end for receiving a fastener, the distal end being oriented away from the anticipated direction of a lateral impact. 4. An energy absorbing post for roadside safety devices comprising: a lower post section for engaging a foundation upon which the roadside safety device is mounted; an upper post section for receiving the impact of an errant vehicle, the upper post section and the lower post section being generally vertically aligned; and a means for attaching the lower post section to the upper post section and facilitating outofplane deformation in the post upon the impact of an errant vehicle on the upper post section; such that at least a portion of the energy of the errant vehicle is absorbed by the outofplane deformation. 5. The post of claim 4 wherein the means for attaching comprises a splice connection between the lower and upper post sections to facilitate outofplane deformation in the material of the splice connection. 6. The post of claim 5 wherein: the means for attaching comprises a rod member extending laterally through a hole in the splice connection, wherein the rod member closes at least one portion of the splice connection without opposing compressive fasteners; and the edge distance extending from an outside edge of the material undergoing tearout to the hole in said material is more than 1.5 times the diameter of the rod member. 7. The post of claim 6 further including a stress concentrator located at the edge of the hole. 8. The post of claim 6 further including a prebuckle formed in the edge of the hole. 9. The post of claim 6 further including a stress concentrator and a prebuckle located at the edge of the hole. 10. The post of claim 6 wherein the rod member includes a distal end for receiving a fastener oriented away from the anticipated direction of a lateral impact. 11. The post of claim 6 wherein the rod member is oriented at an acute angle with respect to the post in a vertical plane extending in the anticipated direction of a lateral impact. 1. |
3. | The post of claim 5 wherein the means for attaching comprises overlapping plates connected by a bolt, wherein the axial distance between the bolt head and nut substantially exceeds the thickness of the overlapping plates, such that angular deflection of the bolt during an impact facilitates outofplane deformation in one or more of the plates. 13. The post of claim 12 further including a compressible gasket positioned between the plates and the bolt head or nut. 14. The post of claim 6 wherein the rod is a single rod member such that the upper post section pivots on the rod member during an end on impact. 15. The post of claim 14 including a splice fastener positioned upstream from the rod member, wherein the edge distance for the upstream fastener is less than 1.5 times the diameter of the upstream fastener. 16. The post of claim 4 wherein the means for attaching comprises: a splice plate oriented in the direction of a lateral impact; and a tab extending from the splice plate and connected to the post such that impact energy may be absorbed by outofplane tearing in the area where tab extends from the splice plate. 17. The post of claim 4 wherein the means for attaching comprises: a generally horizontal slot formed in a lateral portion of the post, the lateral portion facing the anticipated direction of impact; and a splice plate rigidly connected near said slot such that impact energy may be absorbed by outofplane tearing in the lateral portion of the post. 18. The post of claim 17 further including one or more generally vertical slots adjacent to the generally horizontal slot, the generally vertical slots further facilitating outofplane tearing in the lateral portion of the post. 19. The post of claim 17 or 18 further including a spacer located between the splice plate and the post to further facilitate outofplane tearing. 20. The post of claim 4 wherein the means for attaching comprises a splice plate attached on a first planar side thereof to the upper or lower post section, the splice plate including a bent over portion welded on an opposing planar side to the other of the upper or lower post section, such that impact energy may be absorbed by outofplane loading of the weld material. 21. The post of claim 20 further including a spacer located between the splice plate and the post for further facilitating outofplane loading in the weld area. 2. |
4. | An energy absorbing post for roadside safety devices comprising: a lower post section for engaging a foundation upon which the roadside safety device is mounted; an upper post section for receiving the impact of an errant vehicle, the upper post section and the lower post section being generally vertically aligned; a splice connection between the upper and lower posts sections, the splice connection including overlapping splice plates; and a bolt connecting the overlapping splice plates, wherein the bolt forms an axial gap between the bolt head and nut, the gap exceeding the thickness of the overlapping splice plates; wherein the axial gap permits angular deflection of the bolt during an impact to facilitate tearout in one or more of the overlapping splice plates and thereby absorb at least a portion of the energy of an errant vehicle impacting the upper post section. 23. The post of claim 22 further including a compressible gasket positioned in the axial gap. 24. An energy absorbing post for roadside safety devices comprising: a lower post section for engaging a foundation upon which the roadside safety device is mounted; an upper post section for receiving the impact of an errant vehicle, the upper post section and the lower post section being generally vertically aligned; a splice connection between the lower and upper post sections, the splice connection including a splice plate rigidly attached to one of the upper or lower post sections; and a tab extending from a portion of the splice plate, the tab being attached to the other of the upper or lower post sections; such that the tab closes the splice connection, and the energy of an errant vehicle impacting the upper post section is absorbed by outofplane tearing in the material of the splice plate near where the tab extends therefrom. 25. The post of claim 24 wherein the splice plate is oriented facing the anticipated direction of a lateral impact. 26. The post of claim 25 wherein the tab is cut out from the material of the splice plate. 27. The post of claim 24 or 25 wherein the tab is welded to the upper or lower post section. 28. An energy absorbing post for roadside safety devices comprising: a lower post section for engaging a foundation upon which the roadside safety device is mounted; an upper post section for receiving the impact of an errant vehicle, the upper post section and the lower post section being generally vertically aligned; a splice connection between the lower and upper post sections, the splice connection including a splice plate facing a lateral impact and attached to the upper or lower post section; and a generally horizontal slot formed in a portion of the other of the upper or lower post sections, the splice plate being rigidly attached to the post near said slot; such the energy of an errant vehicle impacting the upper post section is absorbed by outofplane tearing in the material of the upper or lower post section near said slot. 29. The post of claim 28 wherein the splice plate is welded to the upper or lower post section near the generally horizontal slot. 30. The post of claim 28 or 29 further including one or more generally vertical slots adjacent to the generally horizontal slot, the generally vertical slots further facilitating outof plane tearing in the post. 31. The post of claim 28 or 29 further including a spacer located between the splice plate and the post to further facilitate outofplane tearing in the post. 3. |
5. | The post of claim 29 further including: one or more generally vertical slots adjacent to the generally horizontal slot; and a spacer positioned between the splice plate and the post; wherein the generally vertical slots and spacer further facilitate outofplane tearing in ■ the post. 3. |
6. | An energy absorbing post for roadside safety devices comprising: a lower post section for engaging a foundation upon which the roadside safety device is mounted; an upper post section for receiving the impact of an errant vehicle, the upper post section and the lower post section being generally vertically aligned; a splice connection between the lower and upper post sections, the splice connection including a splice plate facing a lateral impact; a first portion of the splice plate is rigidly attached on a first planar side to the upper or lower post section; and a second portion of the splice plate is side bent over and an opposing second planar side thereof is welded to the other of the upper or lower post sections; such that impact energy of an errant vehicle is absorbed by outofplane tearing in the weld area. 3. |
7. | The post of claim 33 wherein the welding includes one or more generally vertical welds. 3. |
8. | The post of claim 33 or 34 further including a spacer located between the splice plate and the post to further facilitate outofplane tearing in the weld area. |
where: Fb = Minimum force required to initiate plate tear-out σy = Plate yield strength t = Plate thickness db - Bolt diameter The shear strength of the through bolt must be sufficient to initiate the tear-out process. The strength of the through bolt can be approximated by,
F - σ»^
where: Fv = Bolt shear strength σu = Ultimate strength of bolt material Ab = Area of bolt in the shear plane Accordingly, in order to initiate tear-out, the relationship between the size and strength of the bolt and the thickness and strength of the plate material much be such that,
In the equations above, it should be noted that as the bolt diameter is reduced, both the tear- out initiation force and bolt strength diminish. However, the reduction in bolt strength is directly related to the square of its diameter. Accordingly, bolt strength tends to decrease much more rapidly. Larger diameter through-bolts 40 produce higher tear-out forces and higher post energy absorption, but the deflection of the post at failure is not significantly affected by the bolt size, but rather is controlled primarily by the post edge distance d. As bolt size increases, the resistance to bolt tear-out can become so large that the post fails in other modes, such as fracture through the flange 30. Also, smaller diameter through-bolts 40 may not have sufficient shear capacity to produce long tear-out distances because the bolt itself tends to fracture in shear before sufficient force is generated in the material of flange 30. While though bolts 40 have been illustrated throughout the figures, it should be understood that other elongate rod members could be used instead. It has been found that Grade 5 through-bolts 40 with diameters between 9/16 inch and 1 inch appear to provide the optimal behavior and produce consistent energy dissipation through tear-out. Higher grade through-bolts 40 with smaller diameters may also be able to provide adequate shear capacity to facilitate energy dissipation through tear-out. Optimally, the threaded portion of the bolt is kept out of the shear plane (i.e., oriented on the back of the post) to improve the reliability of energy absorbing posts with long tear-out distances. Here, in some instances the bolts themselves can fracture in two pieces due to stress concentrations in the threaded portion of the bolts located on the back side of the post and thus greatly restrict energy dissipation. Ideally, an energy absorbing post in accordance with the present invention will exhibit little or no plastic deformation until the lateral load reaches a desired level, typically 10 to 12 kips. The ideal post would then sustain the initial force until it reaches the desired deflection limit, when the post would finally fracture completely. As shown in Figures 4 and 5, long through-bolts 40 (e.g., Figure 3) do not achieve this ideal behavior because the forces required to initiate tear-out are much higher than the force required to sustain the process. As discussed below, this undesirably high initial tear-out force can be reduced by one or a combination of several methods, including stress concentrators to facilitate tear-out, dimples located in the post flange to serve as pre-buckles, and by using through-bolts 40 that form a non-perpendicular angle with respect to the flange 30. As shown in Figure 6 A, a saw cut 100 or other stress concentrator can be placed on the edge of the hole 20 in the post flange 30. Alternative stress concentrators might include a v-cut or a square-shaped hole 20 (not shown). As indicated in Figure 6B5 the stress concentrator allows the material forming the post flange 30 to deform at lower initial bolt shear loads in order to produce out-of-plane deformations. These out-of-plane-deformations allow the material of the flange 30 to be loaded in a Mode-3 fracture condition to initiate tear- out. The initial deformation of the post, facilitated by the stress concentrator, also allows the though bolt 40 to become angled relative to the post flange 30, which further facilitates tear- out. Alternatively, as shown in Figure 7, a dimple 110 or other out-of-plane deformation may be placed at the bottom of the hole 20 in the post flange 30 to serve as a pre-buckle for reducing the otherwise high forces associated with initiating local buckling of the post. The dimple 110 provides an out-of-plane deformation to facilitate generation of Mode 3 shear stresses in the material of flange 30 as soon as the post becomes loaded. These out-of-plane shear stresses reduce the lateral forces required to initiate tear-out in the flange 30 and thereby facilitate more effective energy absorption. As shown in Figure 8, a combination of a saw cut 100 or other stress concentrator and a pre-buckle such as dimple 110 can be used in conjunction to further reduce the forces required to initiate tear-out. A saw cut 100 can be placed below the through-bolt 40 (not shown) and a small out-of-plane deformation, such as dimple 110, can be formed at the top of the saw cut 100. This double initiator embodiment further assures that out-of-plane shear stresses will be applied to the post flange 30 immediately and that a crack will facilitate better energy dissipation and lower stress levels. In similar fashion, one or more of the facilitators can be combined in any of the alternative through-bolt embodiments described below. In yet another alternative embodiment of the present invention, as shown in Figure 9, the through-bolt 40 (or other rod member) can be placed at an acute angle α relative to the post flange 30 to also produce out-of-plane shear stresses in the post flange 30. As shown in the embodiment of Figure 9, the through-bolts 40 can be placed at an angle α of approximately 80 degrees relative to the post flange 30 by cutting holes 20 located 1 inch higher in the rear flange 30 than the holes 20 in the front flange 30. Thus, installing the through-bolt 40 at an acute angle α can also facilitate tear-out and a more effective energy absorption. As would be recognized by persons skilled in the art with reference to this specification, the facilitators described in connection with Figures 6A, 6B, and 7 could also be combined with the angled-through bolts embodiment of Figure 9 to even further facilitate tear-out. In Figures 3 and 6-9, splice plate 60 is shown as attached outside the flange 30 and, accordingly, bolt tear-out occurs in the material of the flange 30. Here, tear-out is facilitated, even with large edge distances d (see Figure 1) because the through-bolt 40 does not restrain the material of flange 30 against out-of-plane deformations. Accordingly, those of ordinary skill in the field with reference to this specification would recognize that alternative configurations may facilitate tear-out in the material forming other components of the present invention. Figures 1OA through 1OC depict examples of other such configurations. As shown in Figure 1OA, tear-out may be facilitated in splice plates 60 by locating splice plates 60 on the back side 50 of flange 30 and using through-bolt 40 to permit out-of-plane deformations in splice plates 60. Further, as shown in Figure 1OB, splice plates 60 may be omitted entirely to facilitate tear-out in the material of flange 30 in the upper post section by employing an upper post section with a flange separation distance slightly less than the flange separation distance of the lower post section, such that the flange 30 of the lower post section overlaps the flange 30 of the upper post section. Similarly, as shown in Figure 1OC, splice plates 60 may be omitted to facilitate tear-out in the material of flange 30 in the lower post section by employing a lower post section with a flange separation distance slightly less than the flange separation distance of the upper post section, such that the flange 30 of the upper post section overlaps the flange 30 of the upper post section. So long as through-bolt 40 does not also include structure such as nuts 16 restraining out-of plane deformation in both upper and lower post sections (e.g., Figure 2), tear-out will be facilitated to provide enhanced energy absorption of the breakaway post. Alternatively, as shown in Figure 1 IA, a soft compressible washer or gasket 70 may be placed between the nut 16 on bolt 10 and the material of flange 30, or as shown in Figure 1 IB, the compressible gasket 70 may be placed between the head 14 of bolt 10 and the material forming flange 30. In these embodiments, bolt 10 need not extend through the entire connection because the compressible gasket 70 permits local buckling of the flange material and angular displacement of the bolt 10 to facilitate the generation of out-of-plane stresses and bolt tear-out in the material of flange 30. This design relies primarily on the use of a bolt 10 that is too long to allow the clamping of the plates between the head 14 and the nut 16. When lateral loads are applied to the post, such a longer bolt 10 rotates until the bottom of the bolt head 14 and the top of the nut 16 contact the splice plate and post flange respectively. Ideally, the bolt 10 and compressible material of gasket 70 will be sized to allow the bolt 10 to rotate sufficiently to provide out-of-plane shear stresses to be applied to the material of flange 30. Further note that the compressible material of gasket 70 is primarily recommended to eliminate post vibration which could occur with an overly long splice bolt 10. Hence, the gasket 70 could be omitted entirely without adversely affecting the safety performance of the energy absorbing post. As would be appreciated by persons of ordinary skill in the field, guardrail posts used for mounting end terminals or crash cushions should break easily during end-on impacts. The present invention includes embodiments adapted for use in these end-on impact applications. For example, while side-impact applications might ordinarily utilize a plurality of through-bolts 40, alternatively, as shown in Figure 12, the present invention is readily adapted for terminal applications by utilizing a single through-bolt 40 to form the splice between the two breakaway post sections. In this embodiment, the attachment between the guardrail and the guardrail post maintains the post in an upright position until the post is struck by the terminal (generally by the impact head) during a head-on collision. Thereafter, the single though-bolt 40 provides a pivot to facilitate angular deflection of the upper post section. Full-scale crash testing has shown that a design utilizing a single through-bolt 40 in accordance with this alternative embodiment provides both adequate energy dissipation during lateral redirection impacts and also performs well during end-on impacts. The embodiment utilizing a single through-bolt 40 should also improve the performance of guardrail line posts by reducing the effects of a wheel snagging on a post. Wheel snagging has been shown to produce heavy damage to the front suspensions of light trucks, which can lead to vehicle rollovers during guardrail impacts. The single through-bolt 40 embodiment of the present invention eliminates this problem by allowing the post to rotate when it is struck by a vehicle's wheel, thereby greatly reducing both vehicle loading and suspension damage. Alternatively, as shown in Figures 13A-C, two through-bolts 40 can be used in a breakaway guardrail post for terminal applications if the flange material below the upstream bolt 4OU has a low tear-out distance d, and therefore a lower resistance to tear-out during end on impacts. To create such a low tear-out distance d, the material of flange 30 below the upstream bolt 4OU can be removed (Figure 13A), or a slot 120 may be placed below the upstream bolt 4OU (Figure 13B). In this embodiment, the tear-out distance d for the upstream bolt is reduced to allow the post to easily breakaway during end-on impacts with the terminal. If a slot 120 is placed in the flange 30, the slot could even be extended all of the way from the bolt to the edge of flange 30, as indicated in order to even further reduce the fracture energy associated with an end-on impact. Those of ordinary skill in the field would recognize, with reference to this specification, that other alternative locations for bolt 4OU may be selected to reduce its associated edge distance d. For example, as shown in Figure 13 C, the upstream bolt 4OU may be placed much lower on the post flange 30 in order to similarly reduce the tear-out distance d. Also facilitators such as those described in connection with Figures 6A, 6B, and 7 can be employed in the terminal embodiment of Figures 13A-C. Alternatively, additional fasteners could be employed to close the splice connection between the upper and lower post sections, and thereby add nominal stability to the post, (e.g., small shear pins, not shown) so long as the force required to shear such fasteners does not adversely affect the ability of the post to easily rotate during an end on impact, and the force required to shear or tear-out such fasteners does not adversely affect the lateral energy-absorbing characteristics of the invention during a lateral impact. As shown in Figures 14A-F, another alternative embodiment of the present invention involves loading the splice plate 60 or the flange 30 of the post to allow direct Mode 3 out-of- plane tearing of the splice plate. Figures 14A-F demonstrate mechanisms for loading the post splice plate and the post flange in Mode 3 out-of-plane shearing when a lateral load is applied to the top of the post. Although these figures show specific examples for loading the splice plate 60 or post flange 30 to facilitate tearing, a person of ordinary skill in the field with reference to this specification would be able to substitute alternate structures for loading the splice plate 60 or the post flange 30 in order to allow direct Mode 3 out-of-plane tearing of the splice plate. Referring to Figures 14A-D, there is shown an embodiment of the present invention for loading the splice plate 60 to facilitate out-of-plane tearing. In this embodiment, a tab 130 is cut in the splice plate 60 and the tab 130 is thereafter bent outward by 90 degrees or more. The front flange 30 of the upper portion of the post is then welded to the tab 130. Those of ordinary skill should recognize that the weld 132 between the tab 130 and the post flange 30 must be of sufficient strength to propagate the out-of-plane crack in the splice plate 60. Here, a wider tab (indicated as width w in Figure 14B) will provide a greater weld length without greatly increasing the forces required to propagate the cracks. A conventional splice plate 60 may be welded (as shown in Figure 14C) or bolted (not shown) to the back of the upper and lower post sections. As would be understood by persons skilled in the art with reference to this specification, in any of the embodiments shown in Figures 14A-D, the conventional splice plate 60 located on the back of the upper and lower post sections may be omitted simply by welding the lower post section to the upper post section along a line 136 between the upper and lower post sections (as shown in Figure 14D). When a lateral load is applied to the top of the post in the embodiment of Figures 14A-D, the front flange 30 is placed in tension. The tension load is transmitted into the tab 130 in the splice plate 60. As the tab 130 is pulled upward, an out-of-plane tearing stress is applied to the base of the tab 130 and it begins to tear away as shown in Figures 14D. Here, the saw cuts or stamping process used to form the tab 130 generate points of high stress concentration that will quickly lead to the formation of a crack in the material of splice plate 60. The force-deflection behavior of this embodiment is controlled by the fracture resistance of the splice plate 60. Fracture resistance is related to the strain energy release rate and the thickness of the material forming splice plate 60. Classical fracture mechanics can be used to aid in the selection of the material and thickness of the splice plate 60. It should be noted that, due to the existence of the tear formed by the saw cut or stamping process used to form the tab 130, crack initiation does not produce potentially undesirable large initial post loads. Further, crack propagation occurs at a relatively constant force. Thus, in this embodiment, energy absorption by out-of-plane tearing produces relatively flat force-deflection behavior until the crack propagates through the top of the splice plate 60 and thereafter the upper post will easily deflect laterally about the rear splice plate 60. Figures 15A-C illustrate a similar embodiment that produces out-of-plane tearing in the post flange 30. In this embodiment, a small generally horizontal slot 140 is created in the post (e.g., punched out of the flange 30) and the splice plate 60 is welded to the flange 30, just below slot 140. When tensile loads are applied to the post flange 30, the misalignment between the post flange 30 and the splice plate 60 thereby causes a moment to be applied to the flange 30 just below the slot 140. This moment produces out-of-plane deformation that creates out-of-plane tearing stresses at the ends of the slot 140 and eventually leads to Mode 3 tearing of the post flange 30. In this embodiment, vertical slots 142 may be added to facilitate initial out-of-plane deformation of the flange 30 and initiate the Mode 3 tearing. As would be recognized by persons of ordinary skill with reference to this specification, this embodiment is merely another example of many involving out-of-plane tearing of the post flange 30 or splice plate 60. Figures 16A-C illustrate another embodiment of the present invention in which energy is absorbed by direct out-of-plane tearing. In this embodiment, the top of the splice plate 60 is bent over on itself and its back side is welded directly to the upper or lower post section, or to an intermediately plate (not shown) attached to the upper or lower post section. The welding process used can be either fillet welds on the edge of the splice plate or resistance seam welding to produce lines in the middle of the splice plate. In Figures 16A and 16B, splice plate 60 is shown removed from the post in order to indicate the general area where welding 150 fastens the bent over portion of the splice plate 60 to the post section. The other end of the splice plate 60 may be rigidly attached to the other post section by conventional means. Upon impact, a moment is applied to the upper post section as shown in Figure 16C. The displacement of the upper post section causes direct out-of-plane tearing in the area of welding 150, thereby absorbing impact energy. Note that when fillet welds are used, the weld material is loaded in a conventional Mode III, out-of-plane tearing condition. However, when resistance welding is used, the out-of-plane loading actually produces Mode I crack-opening loading condition on the weldment. hi either case, out-of-plane loading of the weld material allows the post to efficiently dissipate impact energy. Figure 16C further illustrates the use of a small spacer 160 placed between the post section and splice plate 60 in the embodiment of Figures 16A and 16B. The use of spacer 160 facilitates the generation of immediate out-of-plane stresses by creating an angle between the plane of the splice plate 60 and the area where it is attached to the upper or lower post section. The use of such a spacer 160 thereby tends to decrease initially high loads in the force-deflection curve, such as shown in Figures 4 and 5. As would be recognized, spacer 160 may also be utilized in like manner between the splice plate 60 and the post section in the embodiments shown in Figures 15A-B. From the foregoing detailed description of several specific embodiments of the present invention, it should be apparent that novel and non-obvious, energy-absorbing breakaway posts for use with various roadside safety devices, including those mounted in a rigid foundation, have herein been disclosed. Although specific embodiments of the invention have been disclosed in some detail, this has been done solely for the purposes of describing various features and aspects of the invention. Moreover, it is contemplated that various substitutions, alterations, and/or modifications may be made within the spirit and scope of the invention. Such may include but are not limited to the substitution of rods or other rigid elongate members for through-bolts, the substitution of splice plates integral with the upper or lower post section, or the substitution of splice plates located on the flange back side, as well as the implementation details known to those of skill in the art to which the present invention pertains. Accordingly, the scope of the invention is defined by the following appended claims.