Structural calculation of Krah pipes for buried pipes
The static calculation tool in the Krah Mickey software provides structural static calculations following either the design manual M55 of the American Water Works Association (AWWA) or the Australian/New Zealand standard AS/NZ2566.1. Both documents are worldwide spread and accepted.
The Mickey static tool is now getting completely revised and provides in future also a reviewable print out with input data, intermediate and final results. To simplify the handling the input can be done in imperial or in metric units.
All results are calculated accordingly. Reasonable assumptions are already preadjusted, so for example the recommended bedding angle of 120° and in general good bedding situation is already premised. The values for strength and flexural modulus are suggested for the typical pipe materials and all considered load-times. For native and embedment soil the modulus of soil reaction is considered accordingly relevant tables of the standard. But the user has always the possibility to make changes in the values. After all inputs are made, the software follows the equations and tables of the manual to calculate all external dead and live loads. For the dead load a practical and conservative approach for the designing is considered, arching is ignored and it is assumed that the dead load on the pipe equals the weight of the column of soil directly above the pipe. Arching tends to transfer some of the weight of the backfill to the soil beside the pipe and thus reduces the load on the pipe. The main live loads are traffic load, surcharge load, groundwater table and vacuum load.
Following final results are calculated:
- Wall buckling proof (AWWA M55, equation 5-10)
- Ring deflection proof (AWWA M55, equation 5-8)
- Vacuum stability proof (AWWA M55, equation 5-14)
- Compressive stress proof (AWWA M55, equation 5-15)
Furthermore additional proofs and interesting key-factors of the pipe are calculated, like stiffness and internal pressure load capacity. All proofs are made for different loading-times: short term, mid term (3 month) and long term (50 years). Due to calculation for different load times any peak-loads for example by internal vacuum load or by changing ground water table can be better considered. Of course more proofs and analysis can always be made, but the results provide already a good basis. We are open to integrate more proofs or to make changes if needed – just send us your ideas and comments.
This structural calculation is done according to AWWA M55, PE PIPE - DESIGN AND INSTALLATION, chapter 5 “External Load Design”. All mentioned numbers for tables (Tab) and equations (Eq) refer to this standard. The results provide a good basis for further statical considerations and examinations. Some additional aspects like e.g. internal pressure resistance, pipe stiffness etc. are scanned and mentioned. The user is requested to verify the input data and results according to local requirements and if applicable for kind of project.
The structural calculation presumes adequate handling of pipes according to manufacturer’s recommendation and installation of pipes according EN 1620 or equivalent. For pipe bedding, Krah Pipes recommend a standard bedding angle of 120° and backfilling with compactable granular soil with little or no fines (GW,SW).
Author: Krah Pipes GmbH & Co. KG