Structural Design Calculator
The CPSA Calculator App is now available on your App Store.
This calculator App has been developed to optimise the structural design and material cost of your underground sewer pipeline system.
Test strength of pipe (Fn): The Structural design calculator test strength of a concrete pipe may be referred to as Fc or Fn Values are specified in BS 5911:2010. For a reinforced concrete pipe Fc is the load which the pipe will sustain without developing a crack exceeding 0.30mm in width over a length of 300mm and Fn 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 Fn and apply a factor of safety (Fse) of 1.5 for reinforced pipes.
Safety (Fse): These are taken to be Fse = 1.5 for reinforced pipes and Fse = 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 (We): There are a number of simplified tables known as TRRL tables (also found in BS 9295: 2010) that can be used to obtain design load values. These are based on pipe size, cover depth, trench width and loading applied to the installation (i.e. main road traffic; light road traffic; fields) and can be found at pages 27, 28, and 29 of the CPSA Technical Guide.
Bedding Factor (Fm): The bedding class to be used will depend on the bedding factor (Fm) calculated. Bedding Classes N, F, or B can be used for a bedding factor not exceeding 1.1. For bedding factors between 1.1 and 1.5 a bedding class F or B can be used. Bedding Class B can also be used for bedding factors up to 1.9. For bedding factors between 1.9 and 2.2 only a bedding Class S can be used.
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Using the Structural Design Calculator:The Structural Design Calculator (below) incorporates different values for We and Fn depending on the type of pipe, cover depth, and activity above ground. The Structural Design Calculator also carries out the basic calculations associated with the formulae above.
To use the calculator you will need the following information:
Nominal diameter (DN) of the pipe
Cover depth of pipe installed (in metres)
Activity above ground/ nature of traffic.
Factor of Safety Fse = 1.25 for unreinforced pipes DN225-600mm; 1.5 for reinforced pipes DN675-2100mm
The Structural Design Calculator will then advise on the acceptable bedding class(es) available.
Design DataPipe Diameter (mm):
Cover Depth (m):
ResultsTotal External Load We = kN/m
Designed Strength of Pipe Fn (also known as Wt) = kN/m
The required bedding factor Fm =
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Note: Caution should be exercised when installing a pipe using Classes F or N. The contractor will need to insure that the ground is adequately stable to support a Class F or N pipeline bedding
(1) Cover depths are rounded up to the nearest 0.20m, e.g. cover depth 2.65m is rounded to 2.80m.
Trench width is based on “medium” trench widths as per TRRL Simplified Tables.
Please refer to the TRRL tables to learn more about the trench widths assumed.
The calculator accounts for nominal pipe sizes between DN225 and DN2100.
Pipes at cover depths under 0.9 metres or over 8 metres may need special consideration and are not accounted for in this calculator. Please contact CPSA for advice.
Whilst every care is taken to ensure the accuracy of the "Structural 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.