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Design Calculator

This calculator simplifies concrete pipeline design calculations based on the recommendations in BS EN 1295-1 "Structural design of buried pipelines". It offers all the basic values; from external design loads (W_e), to bedding factors (F_m) taking into account the pipe crushing strength (F_n). It then offers advice on what type of bedding to use. The calculated load (W_e), which is the total load a concrete pipe in a trench is required to sustain, is used in the design formula as follows:

design load formulae

Where:

F_m is the required bedding factor.
W_e is the total external load on the pipe
F_se is the factor of safety.
F_n is the standard BS test strength for concrete pipes (this is termed ‘W_t’ in BS EN 1295-1).

Test strength of pipe (F_n): The test strength of a concrete pipe may be referred to as F_c or F_n Values are specified in BS 5911:2002. For a reinforced concrete pipe F_c is the load which the pipe will sustain without developing a crack exceeding 0.30mm in width over a length of 300mm and F_n is the load which the pipe will sustain without collapse, irrespective of crack width. However, to further simplify the procedure it is more straightforward to use the maximum test load F_n and apply a factor of safety (F_se) of 1.5 for reinforced pipes.

Safety (F_se): These are taken to be F_se = 1.5 for reinforced pipes and F_se = 1.25 for unreinforced pipes. Pipe with diameter ranges between DN300 to DN600 are always unreinforced. In the DN675 to DN900 pipe size range some manufacturers make unreinforced pipes and pipes above DN900 are always reinforced.

Design Load (W_e): There are a number of simplified tables (TRRL tables) that can be used to obtain design load values. These are based on pipe size, cover depth, trench width^* and nature of installation (under main road traffic, light road traffic, or fields) and can be found at pages 20, 21, and 22 of the CPSA Technical Guide No. 1.

Bedding Factor (F_m): The bedding class to be used will depend on the bedding factor (F_m) calculated. A bedding factor under 1.9 will require type B bedding, bedding factors between 1.9 and 2.2 require type S bedding, and type A bedding is used if the bedding factor exceeds 2.2.


Figure 1. Class A Bedding	Figure 2. Class B Bedding	Figure 3. Class S Bedding

Using the Design Calculator:

The Design Calculator (below) incorporates different values for W_e and F_n depending on the type of pipe, cover depth, and activity above ground. It also carries out the basic calculations associated with the formulae above.
To use the calculator you will need the following information:

Size/ internal diameter (DN) of the pipe**
Cover depth of pipe installed (in metres)***
Factor of Safety F_seshould be 1.25 or 1.5 only
Activity above ground/ nature of road.

The Calculator will then advise on what bedding class to use.

Notes:
* The calculations are based on recommended trench widths ranging from 0.7 metres for a DN225 pipe, up to 4.15 metres for a DN3000 pipe. Designers will have to refer to the TRRL tables to learn more about the trench widths assumed.
** Please note that the calculator accounts for pipes sizes between DN225 and DN3000 only.
*** Pipes at depths under 0.9 metres or over 8 metres might need special consideration and, therefore, not accounted for in this calculator. Please contact CPSA for advice.

Disclaimer:
Whilst every care is taken to ensure the accuracy of the "Design Calculator", CPSA cannot accept any liability arising from its use. It is offered as a guide, and not a substitute for the services of a suitably qualified engineer

Calculated Results

Using the Design Calculator: