It would be most remiss of me not to firstly wish all readers a happy, prosperous and healthy 2018.
It’s certainly been a while since I posted a technical blog, as my colleagues in marketing keep reminding me. I can only apologise to followers for lack of recent content. However this is an obviously good time to make a meaningful New Year’s resolution to get back to regular postings; so here goes.
This is a technical blog after all, so this article will attempt to answer the question:
Can you “Tune” a Hydraulic Strut by Pre-loading to Achieve a Defined Stiffness Value?
As most readers will know, Groundforce have pioneered the use of high load capacity hydraulic props for supporting deep basements for quite some time. One of the advantages of these props is that the on-board hydraulic ram provides an added benefit of being able to pre-load the support system by pushing back against the supporting wall i.e. d-wall, concrete / sheet piles. The ability to take out the “slack in the system” as well as inducing pre-load, all be it at a fairly nominal level, is clearly of benefit in reducing wall deflections. In our experience, most traditional steel frame propping solution 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.
During the value engineering stage of the project, we often get called in to provide 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 (to a steel member) due to the “squashiness” of the column of oil inside the hydraulic ram. Mechanical hydraulic isolation devices can help but add a degree of complication to the simplicity of installing hydraulic systems. The latter being one of the main selling points of proprietary systems.
Cutting to the chase, we have been working for some time on a research project, the purpose of which is to demonstrate that pre-loading props is an efficient way to compensate for lack of inherent structural system stiffness. Our project partners are geotechnical consultant specialists Atkins Global (Dubai) who are experts in the FE analysis of geotechnical problems. Unfortunately (Ha Ha) we had to visit Dubai a couple of times to scope out the terms of reference of the project as follows.
Using soil profiles typical of excavations in London and Dubai, calculations have been carried out 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 paper prove that, for the cases considered, the use of pre-loaded low stiffness hydraulic props achieved the same performance as stiff steel props, with no impact on the wall’s reinforcement.
We were very pleased that the results of the project 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% of the prop working load achieved the required stiffness performance.
There is little point in me regurgitating the paper in detail here as it is available in the public domain. For those of you that would like to know more, I would encourage you to click this link. The paper is relatively short and has been written in plain English, so should be an easy bedtime read!
Until next time….
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