ANALYSIS AND DESIGN OF PRE-STRESSED CONCRETE SLEEPERS

BY

YUSUF M. HASHIM, HASSAN ABBA MUSA

M.TECH. STRUCTURAL ENGINEERING, SHARDA UNIVERSITY

AT

the ballast and the subgrade. Prestressed concrete sleepers

are supplied ready for laying straight from the factory,

meaning that all the reinforcing has been built into the

sleeper and prestressed in accordance with the standards.

They were first of all used in France round about in 1914

but are common since 1950 and later developed and used

by the British and German federal railways after the Second

World War.

GENERAL FUNCTIONS AND

REQUIREMENTS OF SLEEPERS

• To provide support and fixing possibilities for the rail

foot and fastenings.

• To sustain rail forces and transfer them as uniformly as

possible to the ballast bed.

• To preserve track gauge and rail inclination.

• To provide adequate electrical insulation between both

rails.

• To be resistant to mechanical influences and weathering

over a long time period.

TYPES OF PRESTRESSED SLEEPERS

The most commonly used Pre-stressed sleepers which have

been adopted by the railways of various countries are:

1. Twin-block sleepers: These are connected by a coupling

rod or pipe filled with concrete and containing high-tensile

bars for compressing the concrete in the blocks as

illustrated in fig. 1

2. Longitudinal sleepers located continuously under the rails

and connected by flexible tie bars for gauge retention as

illustrated in fig 2

3. Mono-block sleepers: These are Beam-type single-piece

prestressed concrete sleepers, and have roughly the same

dimensions as timber sleepers.

The advantages of the twin-block sleeper over the mono-block

sleeper are: well-defined bearing surfaces and also high lateral

resistance in the ballast bed. The advantages of beam-type

sleepers are their lower cost and high flexural stiffness. They also

provide greater measure of rigidity to the track if the rails are

tightly fastened to the sleepers, preventing rotations at the seating

and buckling of the rails.

DESIGN CONSIDERATIONS OF SLEEPERS

The principal function of rail road tie is to distribute the wheel

loads carried by the rails to the ballast. Although the sleeper

is a simple determinate structural element, it is not possible

to precisely evaluate the loads to which a concrete sleeper is

subjected to during its service life, due to the various

uncertainties and complexities inherent in the variables

involved, together with the inadequate knowledge of the

dynamics of the track structure.

The various factors influencing the design of sleepers are;

1. The static and dynamic loads imposed on rail seats which

depend upon;

• The type of track (straight or curved)

• It’s construction and standard of maintenance

• The axle load and their spacing

• The running characteristics, speed and standard of

maintenance of vehicles.

2. The ballast reaction on the sleeper, which is influenced

by;

• The shape of the sleeper.

• It’s flexibility and spacing.

• The unit weight of the rail.

• The standard on maintenance of the track.

• The characteristics of the ballast.

END OF FIRST PRESENTER

The “permissible stress” method is currently most commonly used to design sleepers. However, the permissible stress principle does not consider the ultimate strength of materials, probabilities of actual loads, and the risks associated with failure, all of which could lead to the conclusion of cost-ineffectiveness and over design of current pre-stressed concrete sleepers. Recently, the limit states design method, which appeared in the last century and has been already proposed as a better method for the design of pre-stressed concrete sleepers. The limit states design has significant advantages compared to the permissible stress design, such as the utilisation of the full strength of the member, and a rational analysis of the probabilities related to sleeper strength and applied loads.

First Concept

DESIGN OF PRE-STRESSED CONCRETE SLEEPERS

introduction

Codes of practice for design of pc sleepers

Indian standard (IS: 12269 – 1987) with amendment No.6 of June 2000, Specification for 53-S grade cement for manufacture of concrete sleepers.

Indian Railway Standard Specification for Pre-tensioned Pre-stressed Concrete Sleepers, For Broad Gauge And Metre Gauge , Serial No. T-39-85 (Fourth Revision – Aug’ 2011).

Australian Standard (AS 1085.14-2003), Railway track material Pre-

stressed concrete sleepers

design of pc sleepers

(A) - GENERAL CONDITION

1. TYPE AND SPACING

The sleepers shall be pre-stressed concrete sleepers intended for track designs using centre-to-centre spacings of sleepers of 500 mm to 750 mm. The sleepers are designed as fully prestressed sections where the limiting stresses are based on the fatigue resistance of the concrete.

2. SHAPE AND DIMENSIONS

The depth and width of the sleeper may vary throughout its length. The minimum length of the sleeper shall be determined by the bond development requirements of the pre-stressing tendons, and the base width shall then be determined by the allowable bearing pressure.

3. CLEAR TENDON COVER Minimum clear concrete cover to tendons at the soffit of the sleeper shall be 35 mm. Elsewhere, the minimum clear concrete cover to tendons generally shall be 25 mm with the exception that the tendon may be exposed at the end faces.

(B) - DESIGN FORCES

1. VERTICAL WHEEL LOAD

The design static wheel load shall be specified by the purchaser. NOTE: For information about dynamic effects on wheel loads and sleepers, See Appendix

2. QUASISTATIC LOAD

The Quasistatic load is the sum of the static load and the effect of the static load at speed. It includes the effects of the geometrical roughness of the track on vehicle response and the effect of unbalanced superelevation.

The dynamic load is the load due to high frequency effects of the wheel/rail load interaction and track component response. A minimum allowance of 150 percent of static wheel load shall be used.

3. DYNAMIC LOAD

The combined quasistatic and dynamic design load is the sum of the static load, the allowance for the effects of the static load at speed and the allowance for dynamic effects and shall be not less than 2.5 times the static wheel load. Therefore, the combined quasistatic and dynamic design load factor (j) shall be not less than 2.5.

4. COMBINED VERTICAL DESIGN LOAD FACTOR (J)

FIGURE 4.1 AXLE LOAD DISTRIBUTION FACTOR (DF)

The proportion of vertical load taken by a single sleeper resulting from a single wheel may be determined from the following equation provided the values used in the equation can be determined:

where Qx = the load carried by any sleeper, per rail, in kilonewtons, for a single wheel at distance x x = distance from the sleeper to the wheel load, in metres zx = rail deflection at distance x from a point load s = sleeper spacing, in metres Q = static wheel load, in kilonewtons λ = (k/4EI)0.25 k = track modulus, in megapascals E = Young's modulus for the rail steel, in megapascals I = second moment of area for the rail section, in metres

RAIL SEAT LOAD The value of the rail seat load (R) shall be based on the impact and load distribution factors determined in accordance with Clauses 4.2.1 and 4.2.2 and shall be calculated as follows:

R = jQ DF/100 .

BALLAST AND BALLAST PRESSURE The maximum ballast pressure shall be determined from loading conditions similar to those for the maximum positive bending moment at the rail seat (see Clause 4.3.2.1 and Table 4.1). This maximum ballast pressure )( ab p is based on a uniform pressure distribution beneath each rail seat and is calculated using the appropriate equation from Table 4.1.

Table 4.1 maximum ballast pressure

LATERAL LOADS In order to prevent gauge-widening under traffic, fastenings shall restrain the rail from lateral movement when a lateral load is applied at the rail head in addition to vertical wheel load as specified in Clause 4.2.1.1. AS 1085.19 provides requirements for resilient fastenings (see Section 5).

LONGITUDINAL LOADS The rail shall be restrained to avoid excessive longitudinal movement. A minimum longitudinal restraint force of 10 kN per rail seat shall be allowed. Maximum movement of the rail relative to the rail seat under such a load shall not exceed the values given in AS 1085.19 (see Section 5).

RAIL SEAT NEGATIVE DESIGN BENDING MOMENT The rail seat negative design bending moment (MR−) shall be not less than 67 percent of

the rail seat positive design bending moment or 14 kNm, whichever is greater.

MOMENTS AT CENTRE CENTRE POSITIVE DESIGN BENDING MOMENT The maximum positive bending moment of the sleeper shall be based on a pressure distribution beneath each rail seat, similar to that shown in Figure 4.2(a). The length of the ballast pressure distribution beneath each rail seat and the centre positive design bending moment (MC+) shall be calculated from the appropriate equation given in Table 4.3.

CENTRE NEGATIVE DESIGN BENDING MOMENT The maximum negative bending moment shall be taken to occur at the centre of the sleeper, under partially or totally centrebound conditions producing tensile stress at the top and compressive stress at the underside of the sleeper. The value of the centre negative design bending moment (MC−) for track gauge of 1600 mm and greater is based on a ballast pressure distribution as shown in Figure 4.2(b) and is calculated as follows:

The value of the maximum centre negative bending moment (MC−) for track gauge of 1435 mm is based on a uniform distribution of ballast pressure on the sleeper soffit and is calculated as follows:

LOSS OF PRESTRESS The loss of pre-stress shall be determined by the methods specified in AS 3600. For preliminary design, a value of 25 percent may be assumed. NOTE: A lower value of total loss may be considered if it can be proven by testing (see Paragraph E7, Appendix E).

DEVELOPMENT LENGTH AND END ZONE The development lengths of tendons shall comply with AS 3600. Bursting and spalling forces shall be assessed and reinforcement provided if needed.

Thank you !

ANY QUESTIONS TO BE CLARIFIED

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