US3703219A - Manhole ladder step unit - Google Patents

Abstract

A non-metallic manhole ladder step unit of high-strength plastic molding composition, having spaced cantilevered side beams to be embedded in a wall and a cross beam to form a ladder step. High strength and good molding characteristics are obtained by forming the beams with longitudinal walls which are vertically deep but transversely thin, joined in an inverted-U or other mutually reinforcing relation. For best characteristics, a thermosetting molding composition is used, containing at least 20 percent and preferably 30 to 50 percent reinforcing glass fiber, 25 to 60 percent mineral filler, preferably hydrated alumina, and 20 to 50 percent thermosetting resin, such as polyester or epoxy resin. Thermoplastic compositions, especially high temperature compositions containing glass fiber reinforcement, may also be used when lower strength and flame characteristics are acceptable.

Description

I United States Patent 1151 3,703,219

Jones [4 1 Nov. 21, 1972 [54] MANHOLE LADDER STEP UNIT Primary Examiner-Reinaldo P. Machado [72] Inventor: David P. Jones, Indianapolis, Ind. Atwmey TraSk' Jenkms Hanley [73] Assignee: Construction Products Corporation, 57 ABSTRACT Indianapolis, Ind.

A non-metalhc manhole ladder step umt of highl Filed: March 1971 strength plastic molding composition, having spaced [21] APPL 124,679 cantilevered side beams to be embedded in a wall and a cross beam to form a ladder step. H1gh strength and good molding characteristics are obtained by forming [52] US. Cl ..l82/46, 182/90 the beams with longitudinal walls which are vertically 51 11 1.01. ..E06c 9/04 deep but transversely thin joined in an inverted or [58] Fleld of Search ..l82/46, 90 other mutually reinforcing relation For best charac teristics, a thermosetting molding composition is used, [56] References cued containing at least percent and preferably to UNITED STATES PATENTS percent reinforcing glass fiber, 25 to 60 percent mmeral filler, preferably hydrated alumma, and 20 t0 H0llaender percent thermosetting resin uch as polyester or 3,404,751 10/1968 Nosworthy ..l82/46 epoxy resin Thermoplastic compositions, especially 1,639,210 8/1927 Brown ..1s2/90 high temperature compositions containing glass fiber 1,944,384 1/1934 West ..182/ reinforcement, may also be used when lower Strength 2,064,803 12/1936 Grove ..1s2/90 and flame Characteristics are acceptable. 2,215,129 9/1940 Morris ..l82/9O 8 Claims, 5 Drawing Figures PATENTEBunve: I972 I I, I ll Fig. 5

'INVENTOR DAVID P. JONES s= W 11g g- BY '6 WV I I AT TORNE S MANHOLE LADDER STEP UNIT BACKGROUND OF THE INVENTION It is a common and desirable practice to provide permanent ladder steps in the walls of manholes of sewer and other underground passage systems, as by embedding the ends of step units in such walls as they are made. As a very minimum requirement, each step unit must be capable of supporting a single workman and hencemust have a load strength of at least say 300 pounds. Preferably, however, they should have a load strength several times greater than that amount, and should also have characteristics to meet high safety requirements and severe environmental conditions. They should have high physical load strength to support workmen and their equipment with a large margin of safety, and should have high impact strength both for that reason and to withstand impacts of equipment and other objects being dropped into or removed from the manhole. Since they are exposed continuously to damp conditions and may be exposed to a variety of chemical liquids and vapors, they must resist rusting and attack by such chemicals. They should also withstand heat and be non-burning. They must also satisfy severe economic requirements and must be inexpensive to purchase and install, and should provide a long life in the face of adverse conditions.

In the past, the most commonly used manhole ladder steps have been made as iron castings, in different grades or weights. A typical heavy-weight iron step unit is a cast U-shaped bar of solid l-inch square cross section, while other iron step units are of somewhat lighter weight and cross section. In use, these are often coated with a heavy coating of asphalt paint. Recently, there has come on the market a plastic-encased steel step unit consisting of a length of deformed %th-inch reinforcing rod bent to a U-shape and covered with a molded casing of polypropylene plastic having an inner porous body and an outer solid shell. I refer to this below as a plastic-encased steel step.

The commonly used cast iron steps have acceptable load strength and heavy cast iron steps have acceptable impact strength, but both are subject to severe rust and corrosion under the environmental conditions of a sewer manhole. The plastic-encased steel steps have good corrosion resistance in their original state, but the plastic shell tends to crack under service and impact loads and the cracks then permit rust and corrosion to occur about the inner steel rod. The plastic softens under heat, and will support combustion, although at a slow rate.

The present invention provides a manhole ladder step which is entirely composed of a homogeneous high-strength molding composition, made in a configuration which provides good load-bearing and impact strength. In its preferred form and composition, the molded step provides load and impact strength comparable to that of the cast iron steps, it is substantially impervious to rust and corrosion, and it has high flame and heat resistance. It is thus adapted to provide good physical characteristics and to have an indefinitely long service life.

In accordance with the invention, molded step units are formed in a generally U-shaped configuration, comprising two spaced side beams adapted to be mounted with one end thereof embedded in the face of a concrete or masonry wall and to project therefrom as cantilever beams, and a cross beam forming aladder step at the projecting ends of the side beams to form a ladder step spaced from the face of the wall. The step unit is desirably molded as an integral body with the beams of a configuration which gives both good molding characteristics and high strength. To this end, each of the side and cross beams is 'formed of integrally joined longitudinal walls which extend the full length of the beam, and each beam in cross section includes at least one wall which is deep in the load bearing plane and preferably extends the'full vertical depth of the beam, and at least one wall disposed at an angle to the vertically-extending wall to reinforce it. Each such wall desirably has a molding-section thickness less than onehalf the overall dimension of the beam in the direction of such thickness. A preferred step unit has beams formed of a pair of spaced longitudinal side-walls of full vertical depth and of small thickness, interconnected by a horizontal wall at the top in a mutually reinforcing relation, so as to form an inverted U-shaped cross section. Other mutually reinforcing wall relations may be used, for example an inverted V-section, an l-l-section, a T-section, etc.

The step unit is molded from a high-strength molding composition. For highest physical strength together with high resistance to rust and chemical corrosion, as well as temperature stability and non burning characteristics, l preferably use a thermosetting molding composition containing a substantial proportion of reinforcing fibers such as glass fibers.

A preferred composition may contain from 20% to 50% of a thermosetting resin. A number of suitable thermosetting resins are known, of which polyester resin and epoxy resin are especially desirable examples but which also include melamine-formaldehyde, urea, phenolic and other known thermosettingresins.

For strength reinforcement, the thermosetting composition desirably includes from 20 to 50 percent and most desirably at least 30 percent of glass fiber. This may be distributed uniformly throughout the composition or may be concentrated in those parts subject to tension or bending.

For high strength and especially for flame and heat resistance, the thermosetting composition desirably also includes from 25 to 60 percent of mineral filler. The fillers which may be used include aluminum and magnesium oxides, fly ash, mica, barytes, zircon, silica, whiting, lime stone, and asbestos. The preferred filler is hydrated alumina which is known for use in molding compositions for electrical insulating elements and which is especially advantageous to provide flame resistance and non-burning characteristics, and this is preferably included to the extent of at least 20% of the total composition.

In cases in which lower physical strength and less temperature and flame resistance is acceptable, thermosetting molding compositions may also be used. In such case, the composition may contain a lower proportion of glass fibers or even none, and will ordinarily not contain any substantial proportion of mineral filler. A glass-reinforced thermosetting composition may contain from 50 to percent thermosetting resin, from 20 to 50 percent glass fiber, and from O to 10 percent pigment. Organic thermoplastic binders which may be used include such binders as polyethylene, polypropylene, polyamide, acetal, acrylic, and others. The molding compound used is desirably one having a high thermo-softening temperature of say 325 F. and higher.

The accompanying drawing illustrates the invention, and shows a preferred embodiment. In such drawing,

FIG. 1 is an isometric view of a manhole ladder step unit embodying the invention;

FIG. 2 is a section taken on the line 22 of FIG. 1',

FIG. 3 is a plan view of the unit shown in FIG. 1;

FIG. 4 is a front elevation of the unit shown in FIG. 1, with parts broken away to show one side leg in section on the line 44 of FIG. 1; and

FIG. 5 is a vertical section of a manhole equipped steps as shown in FIGS. 1 to 4.

The manhole ladder step unit 8 shown in FIG. 1 is a generally U-shaped molding having two side legs 10 forming mounting beams and a front leg 12 forming a step. The side legs are adapted to have their free ends embedded in a concrete or other masonry wall, as indicated in FIG. 2. The side beams 10 may have a length of 10 inches and be embedded to a depth of about four inches to support the cross beam 12 about 6 inches from the face of the wall. Each beam leg 10 is of inverted U-shaped section, with two longitudinal side walls 16 extending substantially the full length of the beam and having a section thickness of the order of 3/16 inch and a section depth of the order of 1 k to 2 inches at the deepest point along the beam. The walls are spaced apart to give an overall beam thickness of approximately 1 inch and the walls are joined at the top by a transverse wall 18 of similar section thickness. As shown in FIG. 2, the beam is of greatest depth at its mid-portion, and tapers to a smaller depth toward the front where it joins the cross beam 12. At the rear, the upper surface of the beam has a downward sloping portion 22 terminating at an upstanding boss portion 24 which serves to interlock with the wall in which it is mounted.

The cross'beam 12 is likewise of inverted channel section, as shown in FIG. 2. Its two outer side walls 26 may have the same wall thickness and depth as the wall 16 of the side beams, and are similarly spaced apart to form a downwardly open channel 27. They are joined at the top by an interconnecting wall 28 which over at least a portion of the length of the cross beam 12 may have an angular cross-sectional shape to present a sharp ridge at the top of the step. The ends of the cross beam 12 are desirably provided with upstanding bosses 30 to prevent the user's foot from slipping off the end of the step. The downwardly open channels 17 and 27 of the side and end beams may be in open communication with each other, and the inner side walls 16 and 26 of the side and front beams may be joined by a curved section 36 of the same section thickness as the other side walls. Desirably the cross beam 12 is formed with a straight top surface andwith a progressively decreasing depth from its middle toward its ends where it joins the side beams 10.

The manhole shown in FIG. 5 comprises a bottom section 40, two intermediate sections 42 and 44, and a top section 46 formed to provide an eccentric opening 48. The manhole is covered by a cover plate 50 mounted in 'a ring 52 at ground level and supported on the upper manhole section 46 by a collar 54. The manhole sections 42, 44 and 46 are of preformed concrete, and have ladder steps 8permanently mounted in them in the course of their manufacture. It is conventional to mount ladder steps in such manhole sections, as by forcing the steps into the concrete before it has set, or by molding cavities in the concrete and setting the steps in grout in such cavities. I

EXAMPLE r Manhole ladder step units shown in FIGS. 1 to 4 were molded of a molding composition of the following formula:

polyester resin, pigmented and catalyzed, 30 parts fiberglass, 30 parts hydrated alumina, 40 parts The ingredients were mixed to form a molding dough, weighted portions of the dough were charged into a heated mold shaped to form step units as shown in the drawing, and such units were molded under heat and pressure in a conventional manner.

Comparative tests were made of the resulting nonmetallic manhole ladder step units, in comparison with commercially-available steps of light cast iron, heavy cast iron, and plastic-encased steel as described above. For bearing load tests, the steps were mounted in supporting blocks and were subjected to bearing loads in a hydraulic tester, applied at a 1 inch area at the center of the front beam or step of the unit. Tests were carried out at room temperature and at 0 F. In each case, the load was increased until failure occurred, the results were as follows:

990 lbs. 30 bend Severe plastic crack.

650 lbs. 30 bend Severe plastic crack.

These tests show that the molded plastic step of Example I had bearing strength nearly as high as that of light cast iron steps both at ambient temperature and at 0 F., and six times greater than the aforesaid bare minimum of 300 lbs.

Impact tests were carried out on other samples of the same step units. Again the units were mounted in supporting blocks and were subjected to progressively increasing impacts applied at a 1 inch area at the middle of the front beam or step portion of the unit. The impact loading was increased to the point of failure. The results were as follows:

Impact loading at which 79.5 Ft. Lbs., 79.5 Ft. Lbs., Plastic Fracture Plastic Fracture 106 Ft. Lbs., (Not tested at Severe deformation. 106 Ft. Lbs.)

These tests show that non-metallic manhole steps in accordance with Example I have an impact strength equal to or better than that of the commercially-available heavy cast iron steps tested, and better than the other units tested.

Corrosive liquid exposure tests were made on the molded plastic unit of Example I and on the commercially-available plastic encased steel step unit. Corresponding tests were not carried out on the cast iron units because known experience indicated that such cast iron units would not withstand such tests. The step units were submerged for 72 hour periods in different corrosive liquids, as follows:

sodium hydroxide, concentrated sodium hydroxide, percent solution ammonium hydroxide, concentrated ammonium hydroxide, 10 percent solution sulfuric acid, concentrated sulfuric acid, 10 percent solution hydrocholoric acid, concentrated hydrocholoric acid, 10 percent solution phosphate solution In every case, the molded plastic unit of this Example l and the commercial plastic-encased steel unit gave satisfactory corrosion resistance in each of the liquids named, except in concentrated sulfuric acid, which severely attacked both units. The plastic and steel step units used were new units in which the plastic shell contained no cracks.

This series of corrosive liquid tests is considered to be very severe and to show that the molded plastic steps of the present invention have a high corrosion resistance of a character which is not obtained with the commonly used prior cast iron step units.

The molding composition used is Underwriters Flame Classified SEQ (Self-Extinguishing Without Dripping or Glowing). Test of water absorption of the composition show percentage water absorption after 24 hours immersion to be 0.1 percent.

EXAMPLE II Molded plastic ladder step units are made as in Example I of a composition of the following formula. Epoxy resin, polymer and monomer 30 parts, fiber glass 30 parts, filler 40 parts. The ingredients are mixed in known manner and molded under heat and pressure conditions sufficient to form and cure the composition. The resulting step units have properties similar to those of Example I.

EXAMPLE III Examples I and II may be repeated, using other thermosetting molding resins, such as melamine formaldehyde resin, urea resin, phenolic resin, and other known thermosetting resins.

With all the resins of Examples I, II, and III, the proportions of components may be varied. Fiber glass should be present in sufficient amount to give the high physical strength. The filler, especially hydrated alumina, also contributes to physical strength and provides desirable flame resistant properties. The binder must be present in sufficient amount to serve its function as a binder in the molded article, but some increase over that minimum amount is usually desirable to improve flowability of the composition in the molding operation and thus to facilitate molding. In general, the molding composition and the molding operation follows known practices, and selection of proportions and conditions are made in accordance with conventional practice to provide the desired high physical strength and physical properties.

EXAMPLE IV Where highest performance characteristics are not required, it is permissible to use thermoplastic resins instead of using thermosetting resins. Thermoplastic resins which may be used include polyethylene, polypropylene, polyamide, acetyl, acrylic, and others. The choice of resin used and the formulation of the composition should be such as to give a molded product having a high softening temperature, preferably one above about 325 F. With especially high strength thermoplastic composite resins, such as some recent ABS composition, high strength may be obtained without fiber reinforcement, but I prefer to use glass reinforced formulations.

A glass-reinforced thermoplastic molding composition maybe used which contains:

20 to 50 percent glass fiber 50 to percent thermoplastic resin 0 to 10 percent pigment.

Step units as shown in the drawing are molded from such a thermoplastic composition by conventional procedures. The thermoplastic molded plastic step units will not have as high strength or as good flame and temperature resistant properties as those of Examples I and II, they may be made to have usable strength characteristics and lend themselves to convenient molding practices.

Iclaim:

l. A manhole ladder step unit, comprising a pair of spaced side beams adapted to be mounted with one end thereof embedded in the face of a concrete or masonry wall and to project therefrom as cantilever beams, and a cross beam at the projecting ends of the side beams to form a step spaced from the wall,

each said beam comprising integrally-joined molded walls which extend longitudinally of the beam substantially the full length thereof, and including in cross section at least one wall which extends substantially the full vertical depth of the beam and at least one wall disposed at an angle thereto to reinforce the same, each such wall having a molding-section thickness less than one-half the dimension of the beam in the direction of such thickness,

said beams being entirely composed of a highstrength-resinous molding composition and from 0% to 50% by weight of reinforcing glass fiber.

2. A manhole ladder step as defined in claim 1, wherein said beams are molded of a thermosetting molding composition containing, by weight,

20 to 50 percent fiber glass 20 to 50 percent thermosetting molding resin 25 to 60 percent mineral filler.

3. A manhole ladder step unit as defined in claim 1 wherein said beams are molded of a thermosetting molding composition containing, by weight,

LII

7. A manhole ladder step as set forth in claim 1 in which said side and cross beams are of inverted channel shape, having spaced vertically-disposed longitudinal side walls integrally joined adjacent the top by a connecting wall.

8. A manhole ladder step as set forth in claim 3 in which said side and cross beams are of inverted channel shape, having spaced vertically-disposed longitudinal side walls integrally joined adjacent the top by a connecting wall.

. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 703.219 v Dated flgygmbgr 31, 1,222

lnvent fl David P. Jones I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 18, change "weighted" to weighed Column 4, line 43, In the table: the second Word "deflection" appearing in the second column of the table should be read as part of the third column.

Column 4, last line, In the table, the words "Tread Bend" should be read as part of the second column of the table.

Signed and sealed this 1st day of May 1973.

(SEAL) Attest: I I

EDWARD H. FLETCHER, JR. ROBERT GOTTSCHAIK Attesting Officer Commissioner of Patents DRM PO-1050 (10-69) USCOMM-DC 60376-P69 U.5, GOVERNMENY PRINTING OFFICE: l9l9 0-366-334

Claims (7)

1. A manhole ladder step unit, comprising a pair of spaced side beams adapted to be mounted with one end thereof embedded in the face of a concrete or masonry wall and to project therefrom as cantilever beams, and a cross beam at the projecting ends of the side beams to form a step spaced from the wall, each said beam comprising integrally-joined molded walls which extend longitudinally of the beam substantially the full length thereof, and including in cross section at least one wall which extends substantially the full vertical depth of the beam and at least one wall disposed at an angle thereto to reinforce the same, each such wall having a molding-section thickness less than one-half the dimension of the beam in the direction of such thickness, said beams being entirely composed of a high-strength-resinous molding composition and from 0% to 50% by weight of reinforcing glass fiber.

2. A manhole ladder step as defined in claim 1, wherein said beams are molded of a thermosetting molding composition containing, by weight, 20 to 50 percent fiber glass 20 to 50 percent thermosetting molding resin 25 to 60 percent mineral filler.

3. A manhole ladder step unit as defined in claim 1 wherein said beams are molded of a thermosetting molding composition containing, by weight, 30 to 50 percent fiberglass, 20 to 50 percent thermosetting resin, 25 to 60 percent hydrated alumina.

4. A manhole ladder step unit as defined in claim 3, in which the thermosetting resin is a polyester resin.

5. A manhole ladder step unit as defined in claim 3, in which the thermosetting resin is an epoxy resin.

6. A manhole ladder step unit as defined in claim 1 in which said beams are molded of a thermoplastic molding composition containing from 20 to 50 percent glass fiber and from 0 to 10 percent pigment.

7. A manhole ladder step as set forth in claim 1 in which said side and cross beams are of inverted channel shape, having spaced vertically-disposed longitudinal side walls integrally joined adjacent the top by a connecting wall.

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