U.S. patent number 3,782,126 [Application Number 05/255,202] was granted by the patent office on 1974-01-01 for stressed skin safety trenching box.
Invention is credited to John R. Pavese.
United States Patent |
3,782,126 |
Pavese |
January 1, 1974 |
STRESSED SKIN SAFETY TRENCHING BOX
Abstract
A safety trenching box is provided having a "stressed skin" wall
construction which permits the box to be strong, yet
light-in-weight. Each of the wall members of the box has a pair of
end columns which are adapted to be bolted to spreader members of
varying widths, so that the box may be used in trenches of
differenet size. The wall members are constructed of one or more
flexible plates which are curved or "pre-stressed" about a
substantially horizontal axis, so that the lateral forces exerted
by the sides of the trench are borne by tension and compression
stresses in the flexible plates or "skin" of the wall members.
Inventors: |
Pavese; John R. (Paramus,
NJ) |
Family
ID: |
22967286 |
Appl.
No.: |
05/255,202 |
Filed: |
May 22, 1972 |
Current U.S.
Class: |
405/282;
52/455 |
Current CPC
Class: |
E02D
17/08 (20130101) |
Current International
Class: |
E02D
17/06 (20060101); E02D 17/08 (20060101); E21d
005/12 () |
Field of
Search: |
;61/41A,41,455,456,457,458 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Shapiro; Jacob
Attorney, Agent or Firm: Maikis; Robert A.
Claims
What is claimed is:
1. A safety trenching box adapted to be disposed in an excavated
trench or the like to prevent the sides of the trench from
collapsing comprising
a pair of spreader members; and
a pair of earth-retaining wall members, each of said wall members
having
a pair of end columns connected to said spreader members,
face plate means disposed between said end columns adjacent the
sides of the trench, and
back plate means disposed between said end columns adjacent the
center of the trench, said back plate means being connected between
the top and bottom edges of said face plate means,
at least one of said plate means being continuously curved with a
concavo-convex curvature about a substantially horizontal axis, to
prevent the wall member from buckling under laterally applied
loads.
2. A safety trenching box as claimed in claim 1 wherein
said face plate means comprises a flexible face plate curved to
form a convex surface facing the sides of the trench and a concave
surface facing the center of the trench.
3. A safety trenching box as claimed in claim 2 wherein
said back plate means comprises a flexible back plate curved to
form a convex surface facing the center of the trench and a concave
surface facing the concave surface of said face plate, and
compression plate members are disposed between the concave surfaces
of said face plate and said back plate and connected between the
end columns of the wall member.
4. A safety trenching box as claimed in claim 3 wherein
tension cables are connected between the end columns of each wall
member adjacent said back plate.
5. A safety trenching box as claimed in claim 3 wherein
stiffener plates are disposed between the concave surfaces of said
face plate and said back plate and are connected to said
compression plate members to form a double convex frame for
supporting the face plate and the back plate.
6. A safety trenching box as claimed in claim 5 wherein
a top plate is disposed between the top edges of said face plate
and said back plate and is connected between the end columns of
each wall member, and
a bottom plate is disposed between the bottom edges of said face
plate and said back plate and is connected between the end columns
of each wall member.
7. A safety trenching box as claimed in claim 6 wherein
the top edges of said face plate, top plate and back plate are
connected together and disposed within a first horizontally
extending cap member, and
the bottom edges of said face plate, bottom plate and back plate
are connected together and disposed within a second horizontally
extending cap member.
8. A safety trenching box as claimed in claim 2 wherein
said back plate means comprises a substantially coplanar plate
connected between the end columns of each wall member.
9. A safety trenching box as claimed in claim 8 wherein
a second back plate is connected between the end columns of each
wall member, said second back plate being connected between the top
and bottom edges of said coplanar plate adjacent the center of the
trench and being curved to form a convex surface facing the center
of the trench and a concave surface facing said coplanar plate.
10. A safety trenching box as claimed in claim 8 wherein
tension cables are connected between the end columns of each wall
member adjacent said coplanar plate.
11. A safety trenching box as claimed in claim 2 wherein
said back plate means comprises a plurality of plano-convex plates
connected between the top and bottom edges of said face plate at
spaced points along the length thereof.
12. A safety trenching box as claimed in claim 11 wherein
said face plate and said plano-convex plates are disposed within a
channel-shaped member with the convex surface of the face plate
abutting the web of the channel-shaped member and the top and
bottom edges of the face plate connected to the flanges of the
channel-shaped member.
13. A safety trenching box as claimed in claim 11 wherein
tension cables are connected between the end columns of each wall
member adjacent the straight edges of said plano-convex plates.
14. A safety trenching box as claimed in claim 1 wherein
said back plate means comprises a flexible back plate connected
between the end columns of each wall member and curved to form a
concave surface facing the side of the trench and a convex surface
facing the center of the trench, and
said face plate means comprises a plurality of plano-convex plates
connected between the top and bottom edges of said back plate at
spaced points along the length thereof.
15. A safety trenching box as claimed in claim 14 wherein
a cover plate is connected between the end columns of each wall
member abutting the straight edges of said plano-convex plates.
16. A safety trenching box as claimed in claim 15 wherein
an angle-shaped member is connected between the end columns of each
wall member, said angle-shaped member having the edges thereof
connected to the top edge and the convex surface of said back
plate.
17. A safety trenching box as claimed in claim 15 wherein
tension cables are connected between the end columns of each wall
member adjacent the convex edges of said plano-convex plates.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to construction industry safety
devices and more particularly to an improved safety trenching box
which has a high degree of strength and a low weight.
2. Description of the Prior Art
During the digging of trenches and the performance of other earth
excavation work, safety trenching boxes are often employed to
prevent the walls of the excavation from collapsing and injuring
workmen in the excavated area. For example, when sewer lines are
installed, a narrow but fairly deep trench is dug and workmen enter
the trench to install the sewer pipe. A safety trenching box or
"safety drag box" is often placed in the trench to protect the
workmen from falling earth and stone caused by the collapse of the
walls of the trench. Since the earth and stone encountered in
construction work may exert very large lateral forces on the sides
or "walls" of the trenching box, the box is customarily fabricated
of steel to provide the high structural strength required. In many
of the prior art types of safety trenching boxes, the vertically
disposed wall members of the box are formed by a frame consisting
of a number of structural steel or bent plate steel columns and
beams which are covered by sheet steel to provide a smooth outer
surface in contact with the walls of the trench. The two wall
members are separated by a pair of spreader members which are
disposed at the front and rear of the box to separate the wall
members a distance corresponding to the desired width of the
trench. The spreader members are made in sets of different lengths
and are usually bolted to the wall members, so that a single safety
trenching box may be employed in trenches of different widths.
Because of the high structural strength required, prior art safety
trenching boxes have been fabricated of structural steel or bent
plate steel members and consequently are very heavy. Since the
boxes are often dragged through the trench as the excavation work
progresses and are frequently hoisted in and out of the trench by a
crane or other piece of construction equipment, it is extremly
desirable to make the box have the lowest possible weight. When the
safety trenching boxes are manufactured, they must be shipped to
the customer and are thereafter frequently transported long
distances between jobs. Consequently, it is desirable that the
weight of the boxes be reduced as much as possible to minimize
shipping and handling charges. Additionally, a suitable safety
trenching box should utilize a minimum number of parts to
facilitate manufacture and repair. In order to prevent injuries to
workmen using the box, the box should be so constructed that earth
and stone are prevented from accumulating on the walls of the box,
so that such accumulations will not fall on workmen in the box when
the box is moved or otherwise disturbed in position.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a safety trenching box
which exhibits the high structural strength required for use in
earth excavation work and yet is of compact size and relatively low
weight.
It is a further object of this invention to provide a safety
trenching box which may be manufactured and repaired with
substantial savings in labor and materials.
It is a still further object of this invention to provide a safety
trenching box having a strong, light-weight construction which
minimizes shipping and handling costs and allows large boxes to be
handled with standard construction equipment.
It is another object of this invention to provide a safety
trenching box which is so constructed as to minimize accumulations
of earth and stone on the surfaces of the box, to thereby reduce
injuries to workmen using the box.
Briefly, the safety trenching box of the invention comprises a pair
of earth-retaining wall members and a pair of spreader members
connected between the wall members for separating and laterally
supporting the wall members against the lateral forces exerted by
the sides of the trench. Each wall member has a pair of end columns
connected to the spreader members. Face plate means and back plate
means are disposed between the end columns of each wall member and
the back plate means are connected between the top and bottom edges
of the face plate means. At least one of the plate means is given a
concavo-convex curvature about a substantially horizontal axis, so
that the wall members are prevented from buckling under the lateral
forces exerted by the sides of th trench. If desired, tension
cables may be connected between the end columns of each wall member
to increase the strength of the box for wall members of substantial
length.
In one embodiment of the invention, both the face plate means and
the back plate means comprise flexible plates having concavo-convex
curvatures and a double-convex frame is disposed between the curved
plates. Another embodiment of the invention utilizes a curved face
plate and a substantially coplanar back plate. A third embodiment
of the invntion contemplates the use of a curved face plate and a
back plate means comprising a plurality of plano-convex plates
connected between the top and bottom edges of the face plate at
spaced points along the length thereof. In a fourth embodiment of
the invention, the back plate means comprises a flexible back plate
having a convex surface facing the center of the trench and a
concave surface facing the side of the trench. The face plate means
in this embodiment of the invention comprises a plurality of
plano-convex plates connected between the top and bottom edges of
the back plate at spaced points along the length thereof. A cover
plate is then connected between the end columns and arranged to
abut the straight edges of the plano-convex plates.
The nature of the invention and other objects and additional
advantages thereof will be more readily understood by those skilled
in the art after consideration of the following description taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a perspective view of a safety trenching box constructed
in accordance with the teachings of the present invention;
FIG. 2 is a perspective view of the rear portion of the safety
trenching box of FIG. 1 of the drawings;
FIG. 3 is a full sectional view taken along the length of one of
the wall members of the safety trenching box shown in FIGS. 1 and 2
of the drawings;
FIG. 4 is a full sectional view of the wall member of FIG. 3 of the
drawings taken along the line 4--4 in FIG. 3;
FIG. 5 is an enlarged fragmentary view of a portion of FIG. 4 of
the drawings showing the details of the cable sleeves employed in
the wall member of the safety trenching box of the invention;
FIG. 6 is an enlarged fragmentary view of a portion of FIG. 3 of
the drawings showing the details of the anchoring means employed
for the cables used in the wall members of the invention;
FIG. 7 is a perspective view of a wall member constituting an
alternative embodiment of the invention with a portion of the face
plate or "skin" broken away to reveal details of construction;
FIG. 8 is a perspective view of a wall member constituting another
alternative embodiment of the invention; and
FIG. 9 is a perspective view of a wall member constituting still
another alternative embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
Referring now to FIG. 1 of the drawings there is shown a safety
trenching box constructed in accordance with the teachings of the
present invention. The box comprises a first wall member 10, a
second wall member 11, a front spreader member 12 and a rear
spreader member 13. When the box shown in FIG. 1 of the drawings is
lowered into a trench or other excavation, the wall members 10 and
11 prevent the sides of the trench from collapsing and filling the
trench with earth and stone. The spreader members 12 and 13 are
usually bolted to the wall members so that spreader members of
different sizes may be employed to permit the trenching box to be
used in trenches of different widths. The box is also provided with
a pair of front cutting edges 14 and 15 so that minor outcroppings
of earth and stone may be sheared off as the box is dragged through
the excavated trench by a piece of construction equipment, such as
a crane, for example. The rear spreader member 13 is shown in FIG.
2 of the drawings as being mounted on the wall members 10 and 11
near the top of the trenching box to provide for a suitable
clearance between the bottom of the spreader member 13 and the
floor of the trench. The size of the clearance depends upon the
diameter of the pipe or other equipment to be installed in the
trench and the space needed by workmen during the course of
construction.
The construction of the front and rear spreader members is not
critical provided the members are capable of handling the large
lateral forces produced by the pressure of earth and stone on the
wall members 10 and 11. In FIG. 2 of the drawings, the rear
spreader member 13 is shown as comprising a top plate 16 which is
welded to side plates 17 and 18. A truss capable of handling the
lateral loads is formed by braces 19, 20, 21, 22, 23, 24 and 25
which are welded together. A steel plate or sheet 26 is welded to
the top plate 16, the end plates 17 and 18, and the truss members
19 through 25 to complete the assembly. It will be noted that the
truss members 19, 21 and 25 form an arch which permits a
substantial clearance to be obtained between the bottom of the
spreader member and the floor of the trench. Since the front
spreader member 12 differs in construction from the rear spreader
member, it will be explained in conjunction with the the following
description of the wall members 10 and 11 of the trenching box.
The detailed construction of wall member 11 is shown in FIGS. 3 and
4 of the drawings wherein it is seen that the wall member comprises
a front column member 30 and a rear column member 31 to which the
front and rear spreader members are respectively bolted. The column
members 30 and 31 are of hollow construction and may be formed of
welded steel plate or by combinations of steel plate and channel
members, for example. The open top and bottom of each column member
may be closed by a cover plate 32 to prevent the accumulation of
water or dirt in the column. The column members 30 and 31 are
connected together by compression plates 33, 34 and 35 and by a top
plate member 36 and a bottom plate member 37. In practice, the
compression plates 33, 34 and 35 and the top and bottom plates 36
and 37 are usually formed of continuous lengths of plate steel and
are welded to the column members 30 and 31 to form a light-weight,
but strong frame. It may be noted from an inspection of FIG. 4 of
the drawings, that, for reasons which will be explained
hereinafter, the center compression plate 34 is of greater width
than the compression plates 33 and 35. A face plate 38 of flexible
material, such as steel plate or steel sheet, for example, is bent
over the surface of the frame and is welded to the top plate 36,
the compression members 33, 34 and 35, and the bottom plate 37, so
that a convex surface is formed facing the side of the trench and a
concave surface is formed facing the center of the trench.
In a similar fashion, a back plate 39 of flexible steel plate or
steel sheet is welded to the top plate 36, the compression members
33, 34 and 35, and the bottom plate 37. The back plate 39 forms a
convex surface which faces the center of the trench and a concave
surface which faces the concave surface of the face plate 38. From
an inspection of FIG. 4 of the drawings, it is believed apparent
that the cross-sectional area of the wall member 11 has a
double-convex shape. A first row of stiffener plates 40 are placed
at spaced points along the length of compression member 33 between
the facing concave surfaces of the face plate 38 and back plate 39.
The stiffeners 40 are welded to the member 33 and the top plate 36
and are suitably contoured to provide a double-convex,
cross-sectional area for the frame and wall member. A second row of
stiffeners 41 are provided between compression plate 35 and bottom
plate 37, while a third row of stiffeners 42 are provided between
compression plates 33 and 34. Finally, a fourth row of stiffeners
43 are welded in place between members 34 and 35 to complete the
frame. The four rows of stiffeners 40, 41, 42 and 43 are suitably
contoured to provide the overall double-convex configuration for
the frame and the wall member 11. A horizontally disposed steel cap
member 44 is welded in place along the top of the wall member to
provide additional strength for the welded connection between the
top plate 36 and the face and back plates. The cap member 44 also
provides a smooth surface for the top of the wall member 11. In a
similar fashion, a continuous steel cap member 45 is welded to the
bottom of the wall member 11.
The foregoing described construction provides a wall member which
is extremely strong but light in weight. From an inspection of FIG.
4 of the drawings, it may be seen that when a lateral force is
exerted on the face plate 38 of the wall member in the direction of
the arrow 46, the face plate or "skin" 38 tends to flex and to
resume a coplanar shape. Since the back plate 39 is a flexible
plate and is connected to the top and bottom edges of the face
plate, it tends to "straighten out" and to assume a coplanar shape.
The compression members 33, 34 and 35 prevent the face plate and
back plate from becoming coplanar and are conseqeuntly placed in
compression. Accordingly, the back plate 39 is placed in vertical
tension so that it functions as a tension plate. Since the face
plate and back plate are given concavo-convex curvatures about a
substantially horizontal axis, horizontal tension and compression
stresses are also set up in these plates which resist the lateral
force exerted by the sides of the trench. Since plate or sheet
steel has a very high tensile strength and a good compression
strength, it is readily apparent that the arrangement disclosed is
capable of handling very large loads with a light-weight
construction. In prior art types of safety trenching boxes, such as
the box shown in U. S. Pat. No. 3,593,528, granted July 20, 1971 to
John R. Pavese, for example, the wall members are formed by a
substantially coplanar steel face plate which is welded to a very
heavy frame formed by structural steel beams and columns, so that
the high tensile and compression strength of steel sheet
construction is not utilized to advantage in handling the lateral
load imposed on the box. The safety trenching box of the present
invention, however, permits the frame of the wall member to be
fabricated of relatively thin steel plate and utilizes the high
stress point of steel or other suitable materials in the back plate
and face plate of the wall member to resist the lateral loading.
Accordingly, a light-weight safety trenching box is provided having
the required structural strength. It is believed apparent from the
foregoing description that the frame member may be easily assembled
of light-weight components, thereby minimizing labor and material
costs in the construction of the box.
The construction thus far described for the wall members 10 and 11
of the safety trenching box offers a strong, but light-weight,
construction for boxes of average length. For trenching boxes of
substantial length, however, it may be desirable to provide
additional support to prevent buckling of the wall member. To this
end, provision is made in the wall member shown in FIGS. 3 and 4 of
the drawings for the use of steel reinforcing cables. As
illustrated, openings are formed in the stiffener rows 42 and 43
adjacent the compression plate members 33, 34 and 35 and pipe
sleeves 47 are disposed therein and welded to the adjacent plate
member. When the lateral force 46 exerted by the sides of the
trench is applied to the face plate 38, a large horizontal tension
stress is created in the back plate 39 in the vicinity of the
compression members 33, 34 and 35. Consequently, the tension cables
are located adjacent the back plate 39 so that they will bear this
tension stress. FIG. 5 of the drawings shows one of the pipe
sleeves 47 welded in place adjacent compression member 35. A steel
cable 48 is threaded through each of the pipe sleeves 47 on a
particular plate member and extends the full length of the wall
member between the end columns 30 and 31. The cables 48 may be
anchored at each end to the columns 30 and 31 by any convenient
means. In the illustrated arrangement, however, wedge anchors 49
are utilized to make s secure connection which will permit the
cables to be placed in tension during assembly of the wall
member.
One of the wedge anchors for end column 31 is shown in enlarged
view in FIG. 6 of the drawings. As seen in FIG. 6, a pipe sleeve 50
is disposed within the hollow end column 31 to provide additional
strength for the column at the point where the cable passes through
the column. The cable 48 is threaded through the pipe sleeve 50 to
the outside of the end column 31 and a cylindrical collar 51 is
placed over the free end of the cable. A conical wedge 52 is then
placed over the free end of the cable and inserted between the
cable and the collar 51, so that as the wedge 52 is hammered or
driven into place, the pressure of the collar 51 against the wedge
causes the cable to be firmly clamped in place. Accordingly, as the
tension on the cable 48 is increased, the wedge 52 tends to be
pulled further into the collar 51, thereby increasing the clamping
force on the cable. While the disclosed arrangement may be used for
both ends of each of the cables in a wall member, in some
situations it may be convenient to weld or clamp button heads or
mushroom heads to one end of each of the cables and to employ the
wedge anchor arrangement on the other end. Regardless of the
anchoring method employed, however, when the wall members of long
boxes are assembled, the cables are placed in tension, so that they
serve to bear a portion of the horizontal stress created in the
back plate by the lateral force exerted by the sides of the
trench.
Referring again to FIG. 1 of the drawings, it will be noted that
the front vertical cutting edge 14 of the box is mounted on the
hollow end column 30. In practice, the cutting edge 14 may be
formed by steel plates which are welded together to provide an
extension of triangular cross-section, so that a sharp, vertical
edge is formed on the front of the wall member. In order to provide
additional support for the front cutting edges 14 and 15, the
plates forming each cutting edge are provided with a horizontally
extending flange 53 to which is bolted support members or struts 54
and 55. The struts 54 are welded at one end thereof to the flange
56 of an I beam forming the front spreader member 12, while the
strut 55 is bolted between the flanges 53 on the front cutting
edges 14 and 15. By virtue of this arrangement, the front spreader
member 12 not only serves to separate and support the wall member
10 and 11, but also serves to support the front cutting edges 14
and 15. This permits the wall members to remain substantially
parallel while the trenching box is dragged through a trench by a
crane or other piece of construction equipment, so that the cutting
edges 14 and 15 shear off projecting earth and stone on the walls
of the trench. Again, the front spreader member 12 may be made in
different sizes to permit the trenching box to be used in trenches
of different widths. Both the front spreader member 12 and the rear
spreader member 13 are bolted to the hollow end columns 30 and 31
on the walls of the box to permit easy installation and
removal.
Referring now to FIG. 7 of the drawings, there is shown an
alternative embodiment of the invention wherein the face plate is
curved and the back plate means takes the form of a coplanar plate.
In this arrangement, a substantially flat or coplanar steel plate
60 is welded between the top and bottom edges of a face plate 61. A
second back plate 62 is also welded between the top and bottom
edges of the plate 60. The face plate 61 is given a convex
curvature facing the side of the trench and the back plate 62 is
given a convex curvature facing the center of the trench, so that a
lateral force on either the face plate 61 or back plate 62 is
converted into a vertical tension stress in the plate 60. Again,
this arrangement results in the double-convex, cross-sectional area
utilized in the wall member of FIGS. 1 through 6 of the drawings. A
top cap member 63 and a bottom cap member 64 are respectively
welded in place on the top and bottom edges of the wall member thus
formed to provide added strength for the welded joints and a smooth
top and bottom surface. When a lateral force is exerted on the face
plate or "skin" 61 of the wall member, the face plate tends to
straighten out and thereby places the flat plate 60 in horizontal
and vertical tension, so that this plate bears the lateral thrust
exerted by the walls of the trench. In a similar fashion, a lateral
thrust exerted against the second back plate 62 of the wall member
also causes the plate 60 to be placed in tension. This construction
again utilizes the high tensile strength of steel and similar
materials to permit the wall to take very large loads with a
ligh-weight construction. Hollow end columns (not shown), similar
to columns 30 and 31 in the embodiment of FIGS. 1 through 6 of the
drawings, are employed to terminate the wall member shown in FIG. 7
of the drawings and provide anchoring points for the spreader
members. If desired, prestressed tension cables may be connected
between the end columns within the envelope formed by the face
plate and back plate to provide additional strength for boxes of
long length. In order to bear the maximum horizontal tension
stress, the cables should be disposed adjacent the second back
plate 62 or the flat plate 60 if the second back plate is
eliminated.
Another alternative embodiment of the invention is shown in FIG. 8
of the drawings. In this arrangement, the back plate means
comprises a plurality of plano-convex plates 70 which are disposed
in a channel-chaped member 71. A face plate 72 of flexible
material, such as steel plate or sheet steel, for example, is
welded along the convex edges of the plates 70 and disposed within
the channel-shaped member 71. Accordingly, the plurality of
plano-convex plates 70 are connected between the top and bottom
edges of the face plate 72 at predetermined spaced points along the
length thereof. A horizontally disposed cutting edge 73 is formed
on the bottom of the wall member by means of welded steel plates 74
and 75. Finally, tension cables 76 are passed through the plates 70
adjacent the straighr or "plano" edge of the plates to provide for
extra support if required. The tension cables 76 are anchored in
end columns (not shown) similar to the columns 30 and 31 in FIGS. 1
through 6 of the drawings. The same end columns may, of course,
serve as the mounting members for the spreader members of the
trenching box.
This embodiment of the invention operates in a manner similar to
the embodiment shown in FIGS. 1 through 6 of the drawings and the
embodiment shown in FIG. 7 of the drawings in that the straight
edge 77 of each of the plates 70 is placed in tension when a
lateral force is applied to the face plate 72 of the wall member.
The lateral force exerted by the walls of the trench on the outer
surface 78 of the channel member 71 causes the curved face plate 72
to flex in a direction tending to reduce its curvature, so that the
straight edge 77 of the plates 70 are placed in tension to bear the
load. Since the face plate 72 is given a concavo-convex curvature,
it will bear the horizontal tension stresses in the curved portions
of the plate adjacent the top and bottom edges thereof. The tension
cables 76 may be employed in boxes of long length to bear this
horizontal tension stress. Although the channel-shaped member 71
could be eliminated from the disclosed construction, it does
provide mounting support and stability for the entire assembly.
Referring now to FIG. 9 of the drawings, there is shown a wall
member for a safety trenching box constituting another alternative
embodiment of the invention. In this embodiment of the invention,
the face plate means comprises a plurality of plano-convex plates
80 which are connected by means such as welding to spaced points
along the top and bottom edges of a curved back plate 81. A cover
plate 82 is welded to the straight or "plano" edges of the plates
80 and is adapted to receive the lateral forces exerted by the
sides of the trench. A bent plate or angle-shaped member 83 has its
edges welded to the top edge of the back plate 81 and to the convex
surface of the back plate to provide added strength for the wall
member. Tension cables 84 are disposed in the wall member adjacent
the convex edge of the plates 80 and are anchored to end columns
(not shown). The end columns, as in the embodiments of FIGS. 1-6,
FIG. 7 and FIG. 8 of the drawings may be conveniently formed of
bent plate steel. The plates 80, the back plate 81, the cover plate
82 and the angle-shaped member 83 may be fabricated of steel plate.
The plates 81 and 82 and the member 83 are connected to the end
columns (not shown) by means such as welding, so that the wall
member can be connected to the spreader members of the trenching
box.
In this embodiment of the invention, when a lateral force is
exerted on cover plate 82 by the sides of the trench, the back
plate 81 is placed in horizontal tension in the vicinity of the
tension cables 84. The areas of the back plate near its top and
bottom edges are placed in horizontal compression, so that a stable
structure is produced. For wall members of relatively short length,
the angle-shaped member 83 and the tension cables 84 may be omitted
since these elements serve to assist the back plate 81 in bearing
the horizontal stresses produced by the lateral force exerted by
the walls of the trench. It will be noted that the bottom edge 85
of the wall member provides a horizontally disposed cutting edge
which may be used to shear-off projections of earth and stone on
the trench walls as the box is lowered or driven into place in the
trench. If desired, however, the cover plate 82 need not extend the
full height of the wall member, so that an open space is left
between the bottom edge of the cover plate and the bottom edge 85
of the wall member to prevent earth, stone, water and other matter
from accumulating in the hollow wall member. This embodiment of the
invention also provides a strong, light-weight wall member which
may be easily fabricated from steel plate or other materials having
satisfactory stress characteristics.
It is believed apparent that many changes could be made in the
construction and described uses of the foregoing safety trenching
box and many seemingly different embodiments of the invention could
be constructed without departing from the scope thereof. For
example, it is believed obvious that other frame configurations
could be employed to produce the stressed skin feature of the
invention. Accordingly, it is intended that all matter contained in
the above description or shown in the accompanying drawings shall
be interpreted as illustrative and not in a limiting sense.
* * * * *