U.S. patent application number 10/913738 was filed with the patent office on 2006-02-23 for energy absorbing post for roadside safety devices.
Invention is credited to King K. Mak, John D. Reid, John R. Rohde, Dean L. Sicking.
Application Number | 20060038164 10/913738 |
Document ID | / |
Family ID | 35756553 |
Filed Date | 2006-02-23 |
United States Patent
Application |
20060038164 |
Kind Code |
A1 |
Sicking; Dean L. ; et
al. |
February 23, 2006 |
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.
Inventors: |
Sicking; Dean L.; (Houston,
TX) ; Rohde; John R.; (Lincoln, NE) ; Reid;
John D.; (Lincoln, NE) ; Mak; King K.; (San
Antonio, TX) |
Correspondence
Address: |
CHARLES J. ROGERS
600 TRAVIS STREET, SUITE 7100
HOUSTON
TX
77002-2912
US
|
Family ID: |
35756553 |
Appl. No.: |
10/913738 |
Filed: |
August 7, 2004 |
Current U.S.
Class: |
256/13.1 |
Current CPC
Class: |
E01F 9/635 20160201;
E01F 15/0461 20130101 |
Class at
Publication: |
256/013.1 |
International
Class: |
E01F 15/00 20060101
E01F015/00 |
Claims
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 .sigma..sub.y, and the rod
member has a diameter d.sub.b and an ultimate strength
.sigma..sub.u, each selected to satisfy the general relationship:
.pi..sigma. u .times. d b 2 4 .times. 3 .gtoreq. .sigma. y
.function. ( t .times. .times. d b + 4 .times. t 2 3 ) ##EQU4##
such that the energy of an errant vehicle impacting the upper post
section may be absorbed by tear-out 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 tear-out 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 out-of-plane 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 out-of-plane 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 out-of-plane 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 tear-out 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 pre-buckle formed in the
edge of the hole.
9. The post of claim 6 further including a stress concentrator and
a pre-buckle 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.
12. 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 out-of-plane 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 out-of-plane 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 out-of-plane 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 out-of-plane 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
out-of-plane 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
out-of-plane 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 out-of-plane
loading in the weld area.
22. 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 tear-out 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
out-of-plane 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
out-of-plane 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 out-of-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
out-of-plane tearing in the post.
32. 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
out-of-plane tearing in the post.
33. 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 out-of-plane tearing in the weld area.
34. The post of claim 33 wherein the welding includes one or more
generally vertical welds.
35. The post of claim 33 or 34 further including a spacer located
between the splice plate and the post to further facilitate
out-of-plane tearing in the weld area.
Description
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.
[0021] FIG. 2 is a top plan view of a spliced connection common
among the prior art.
[0022] FIG. 3 is a top plan view of a spliced connection utilizing
through bolts in accordance with several embodiments of present
invention.
[0023] FIGS. 4 and 5 plot force versus deflection during an impact
on the energy absorbing posts in accordance with the several
embodiments of present invention.
[0024] FIG. 6A is a partial side view of a flange section with saw
cut tear-out facilitators.
[0025] FIG. 6B is a partial perspective view of a post section with
a partially deformed saw cut tear-out facilitator.
[0026] FIG. 7 is a partial perspective view of a post section with
a pre-buckle tear-out facilitator.
[0027] FIG. 8 is a partial perspective view of a post section with
a combined pre-buckle and saw cut tear-out facilitator.
[0028] FIG. 9 is a partial side view of a splice connection
utilizing an angled through bolt in accordance with an embodiment
of the present invention.
[0029] FIGS. 10A-C are partial cut-away side views of alternative
through bolt embodiments of the present invention.
[0030] FIGS. 11A and 11B are partial side views of an alternative
embodiment of the present invention utilizing a compressible gasket
to facilitate angular displacement of splice bolts.
[0031] FIGS. 12A and 12B are alternative side views of an
embodiment of the present invention adapted for terminal
applications.
[0032] FIGS. 13A-C are side views of an additional embodiment of
the present invention adapted for terminal applications.
[0033] FIGS. 14A-D are alternative side views of an embodiment of
the present invention adapted for absorbing impact energy by direct
tear-out in the splice plate.
[0034] FIGS. 15A and 15B are alternative side views of an
additional embodiment of the present invention adapted for
absorbing impact energy by direct tear-out in the post flange.
[0035] FIG. 15C is a perspective view of the embodiment shown in
FIGS. 15A and 15B.
[0036] FIGS. 16A-C are alternative side views of a further
embodiment of the present invention adapted for absorbing impact
energy by direct tear-out in a welded area between the splice plate
and post flange.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] Refer now to the drawings in which the depicted elements are
not necessarily shown to scale and like or similar elements are
designated by the same reference numeral throughout the several
views.
[0038] In several embodiments, the present invention utilizes bolt
tear-out as a mechanism for energy absorption. Bolt tear-out
normally occurs when there is insufficient material between the
edge of a bolt hole and the edge of a metal plate. In this
situation, in-plane shear stresses produced by the bolt bearing on
the plate material exceed the capacity of the metal and the plate
fails in a double Mode-II fracture. Traditional structural design
guidelines recommend that bolt tear-out can be prevented by
increasing the bolt edge distance (the distance from the bolt hole
to the margin or edge of the bolted material) to at least 1.5 times
the bolt diameter. (See Shigley, J. E. & Mischke, C. R.,
Mechanical Engineering Design 360 (5th ed.) (1989) (noting that
failure due to tear-out "may usually be neglected" with large edge
distances).) As shown in FIG. 1, when a bolt 10 with a lateral side
12 is forced laterally into the edge of a hole 20 in a steel flange
plate 30, the flange material is loaded in a Mode 2, in-plane shear
condition. As is generally understood, Mode 1 fracture is
associated with tension stresses, Mode 2 fracture is associated
with in-plane shear failure and Mode 3 fracture arises from
out-of-plane shear stresses in the plate material.
[0039] With a large edge distance d, the material resistance is
sufficient to prevent the bolt 10 from tearing out from the
material of flange plate 30 due to Mode 2 fracture. In this case,
the bolt 10 tears out from the material of flange plate 30 when two
cracks develop in the margin material of flange 30 (as indicated by
arrows 13 on each side of the bolt 10) and eventually a small piece
of the material forming flange 30 is torn out. On the other hand,
buckling in the material of flange 30 allows the material of flange
30 to deform out-of-plane and become loaded in Mode 3, out-of-plane
shear.
[0040] As shown in FIG. 2, when short bolts 10 are used in a post
splice, as common among prior devices, the material forming flange
30 is clamped in plane between the bolt head 14 and the nut 16 so
as to generate a high resistance to local out-of-plane buckling. In
these prior systems, the potential for energy absorption by bolt
tear-out is thereby greatly restricted.
[0041] FIG. 3 illustrates a cross-section view of an embodiment of
the present invention wherein the breakaway post is depicted as
having an I-beam cross-section. As should be understood by those
skilled in the field with reference to this specification, other
beam sections could be employed, e.g., round or box-beam, so long
as the beam section forms generally opposing vertical walls of
metal material (typically steel) with a definite wall thickness
between the exterior surface of the post and its interior surface,
here flange back side 50.
[0042] As shown in FIG. 3, when a through-bolt 40 is used in a post
splice in accordance with the present invention, the flange back
side 50 is not restrained in plane by the through-bolt 40. In this
embodiment, through-bolt 40 is a generally cylindrical rod member
extending through the post which does not include a fastener (e.g.,
a threaded nut) compressing the flange back side 50. Accordingly,
local out-of-plane buckling of the material forming flange 30 is
uninhibited. Further, as the material of the post flange 30 begins
to buckle, the post rotates laterally and the through-bolt 40 thus
begins to displace at a non-perpendicular angle to the flange 30,
thereby accentuating the out-of-plane shear loading. As a result,
bolt tear-out can occur, even with large edge distances d, provided
that through-bolts 40 are employed for the splice such that the
flange back side 50 is unrestrained.
[0043] FIGS. 4 and 5 provide the results of dynamic testing of
breakaway posts joined by a splice in accordance with an embodiment
of the present invention using through-bolts 40 positioned at two
edge distances. Deflection of the upper post section is indicated
on the x-axis (in inches). Post load is indicated on the y-axis (in
1000 pound units). From this it can be seen that that the lateral
force required to initiate tear-out is much greater than the force
required to sustain tear-out. This initially high force is required
to initiate buckling of the material forming flange 30 prior to the
onset of tear-out. It can also be seen that the amount of post
deflection at the point of final fracture increases as the edge
distance (distance d in FIG. 3) increases. The area under the
force-deflection curves, i.e., the energy absorbed by the post,
also increases as the bolt edge distance increases. However, when
the edge distance (distance d in FIG. 3) exceeds 5 inches,
resistance to tear-out can become too great and the post can fail
in other modes. It should also be understood that, with reference
again to FIG. 3, a splice utilizing through-bolts 40 provides the
most reliable energy dissipation characteristics when the bolt head
14 is placed on the front or tension side of the post such that the
threaded portion of the through-bolt 40 does not interfere with the
bolt tear-out process.
[0044] Local buckling of the material surrounding the through bolt
helps to initiate Mode-III tearing of the plate material used in
the splice. Although classical solutions can be used to predict
local buckling of metal plates, these analysis techniques are not
readily applicable to the dynamic loading conditions associated
with the present invention. However, dynamic testing of a large
number of bolted splice configurations has shown that the tear-out
initiation forces are at least equal to the force required to yield
all of the material below the through bolt and create slip planes
on each side of the bolt as approximated below. F b = .sigma. y
.function. ( t .times. .times. d b + 4 .times. t 2 3 ) ##EQU1##
where:
[0045] F.sub.b=Minimum force required to initiate plate
tear-out
[0046] .sigma..sub.y=Plate yield strength
[0047] t=Plate thickness
[0048] d.sub.b=Bolt diameter
[0049] 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 v = .sigma. u .times. A b 3 ##EQU2##
where:
[0050] F.sub.v=Bolt shear strength
[0051] .sigma..sub.u=Ultimate strength of bolt material
[0052] A.sub.b=Area of bolt in the shear plane
[0053] 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, .pi..sigma. u
.times. d b 2 4 .times. 3 .gtoreq. .sigma. y .function. ( t .times.
.times. d b + 4 .times. t 2 3 ) ##EQU3##
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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 FIGS. 4 and 5, long through-bolts
40 (e.g., FIG. 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.
[0058] As shown in FIG. 6A, 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 FIG. 6B,
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.
[0059] Alternatively, as shown in FIG. 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.
[0060] As shown in FIG. 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.
[0061] In yet another alternative embodiment of the present
invention, as shown in FIG. 9, the through-bolt 40 (or other rod
member) can be placed at an acute angle a 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 FIG. 9, the through-bolts
40 can be placed at an angle .alpha. 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 .alpha.
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 FIGS. 6A, 6B, and 7 could also be combined with the
angled-through bolts embodiment of FIG. 9 to even further
facilitate tear-out.
[0062] In FIGS. 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 FIG. 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. FIGS. 10A
through 10C depict examples of other such configurations. As shown
in FIG. 10A, 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 FIG. 10B, 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 FIG. 10C, 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., FIG. 2), tear-out will be
facilitated to provide enhanced energy absorption of the breakaway
post.
[0063] Alternatively, as shown in FIG. 11A, 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 FIG. 11B, 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.
10561 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 FIG. 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.
[0064] Alternatively, as shown in FIGS. 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 40U 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 40U can be
removed (FIG. 13A), or a slot 120 may be placed below the upstream
bolt 40U (FIG. 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 40U may be selected to reduce its
associated edge distance d. For example, as shown in FIG. 13C, the
upstream bolt 40U 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 FIGS. 6A,
6B, and 7 can be employed in the terminal embodiment of FIGS.
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.
[0065] As shown in FIGS. 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. FIGS. 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.
[0066] Referring to FIGS. 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 FIG. 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 FIG. 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
FIGS. 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
FIG. 14D).
[0067] When a lateral load is applied to the top of the post in the
embodiment of FIGS. 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 FIGS. 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.
[0068] FIGS. 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.
[0069] FIGS. 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 FIGS. 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 FIG. 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. In either case, out-of-plane loading of
the weld material allows the post to efficiently dissipate impact
energy.
[0070] FIG. 16C further illustrates the use of a small spacer 160
placed between the post section and splice plate 60 in the
embodiment of FIGS. 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 FIGS. 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 FIGS. 1 5A-B.
[0071] 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.
* * * * *