Can you ‘tune’ a hydraulic strut by pre-loading to achieve a defined stiffness value?
24 July 2018
MP500 Hydraulic Release Nuts
Groundforce uses high-load capacity hydraulic props for supporting deep basements. A feature of these props is that the on-board hydraulic ram facilitates pre-loading of the support system by pushing back against the supporting surface – typically a diaphragm wall, concrete or sheet piles.
The ability to take out ‘slack in the system’ as well as inducing pre-load at a nominal level, has the effect of reducing wall deflections. Traditionally, steel frame propping solutions tend to rely on the inherent stiffness of the supporting sections both axial and flexural, as the main tool to resist deflections. Hence there is a tendency to design for stiffness as opposed to structural capacity; resulting in an over-designed solution.
A more economical solution
Groundforce frequently gets requests for a comparative and hopefully more economical solution based on our proprietary hydraulic equipment: stiffness value is often quoted as a design parameter.
Pure hydraulic systems are unlikely to be able to provide an equivalent section stiffness compared to a steel member due to compression of the column of oil inside the hydraulic ram. Mechanical hydraulic isolation devices are a possible alternative, however, they add a degree of complication compared to the simplicity of installing hydraulic systems – the latter being one of the main selling features of proprietary systems.
Research project partnership
To demonstrate that pre-loading props is an efficient way to compensate for lack of inherent structural system stiffness, Groundforce partnered with geotechnical consultant specialist Atkins Global (Dubai) – an expert in the FE analysis of geotechnical problems. To move this research project forward, we visited Dubai on a couple of occasions to scope out the terms of reference for the project as follows:
Using soil profiles typical of excavations in London and Dubai, calculations were made to compare the performance of temporary steel and proprietary hydraulic propping.
Numerical modelling using Plaxis software indicates that pre-loading allows the lower-stiffness hydraulic prop to match the performance (in terms of prop load and wall displacement) of a higher-stiffness steel prop, without pre-load.
In addition, the low prop stiffness of the hydraulic prop also helps to mitigate loading induced by thermal effects. Furthermore, where limiting the horizontal deflection of the wall is critical, the application of a calibrated pre-load can improve the economy of the temporary works by reducing the prop size.
Pre-loaded props are sometimes associated with increased bending moment and shear force values in the retaining wall. The finite element models produced for this research prove that, for the cases considered, that the use of pre-loaded low stiffness hydraulic props achieved the same performance as stiff steel props, with no impact on the wall reinforcement.
The project results demonstrated that applying a carefully chosen pre-load can allow low-stiffness hydraulic props to match the performance of high-stiffness steel props; both in terms of wall deflection and induced structural shear forces and bending moments.
The results for both prop types were remarkably close to each other, which is encouraging in promoting the use of controlled pre-load with lower stiffness props to provide an economic temporary works solution. Based on the results of the research, pre-loads of 50 to 75 per cent of the prop working load achieved the required stiffness performance.
To read the research project in its entirety, follow this link.
Visit the Groundforce Ireland website www.vpgroundforce.ie.