We’ve all learned a lot from mentors who were instrumental in shaping us during our early career and continue to do so.
That has been one of my core values – to also reciprocate the same to young engineers that get the opportunity to work with us.
So, a few months ago, one of our clients offered us a job to design a jib crane footing that was to be placed inside a warehouse that had an existing slab.
This presented a great opportunity for one of our graduate engineers who had joined us in January, fresh from uni, to get a first-hand experience on how some of the teachings that they’ve acquired from uni differs from the real engineering world!
So we assigned one of our graduate engineers Brandon Westdorp to work on the crane footing design following our client’s requirements – the client had space limitations and wanted a deeper footing rather than a wider one. The maximum plan dimension of the footing could not exceed 3.5 m
Brandon was ready and willing to take up this task obviously with the help of our senior engineers.
Below is the procedure that Brandon followed and what he had to say on some of the things he learnt while working on the project once complete.
5T Crane Footing Design Criteria Followed
Pad footing spreadsheet
- Used for preliminary design
- Doesn’t take into account pressure on the sidewall of footing
- Strand 7 was used to check the crane stability
- 48MPa/m face support as good soil
- Force applied in orthogonal and 45-degree directions
- The preliminary design showed a 3.5m x 3.5m x 1m footing was required for overturning
- The model found a 3.0m x 3.0m x 1m was okay for overturning but the preliminary size was required for deflection control.
- The displacement is important as this could cause a fatigue issue
- Surface slab prevents easy displacement
What I (Brandon) Learned While Working On The Project
Some things that the jib crane footing design taught me that differ from university teachings include;
When using Finite Element Analysis (FEA) modelling to determine an efficient size for the footing, compression-only face support is used to represent the soil reaction pressure on the side and base of the footing.
Finding an appropriate value for this reaction force was something I didn’t learn at university as we didn’t do any design using FEA. With help from senior engineers, I came to understand the typical values used for this and how to make the assumptions.
Limitations of the Spreadsheet
The spreadsheets we use to aid in our design process are based on a general design type. Using these tools requires understanding where the limitations are. For example, the spreadsheet applies soil reaction as a force rather than pressure, giving a less accurate result.
Using an FEA model, I can better represent the soil reaction pressure and design a less conservative footing.
University syllabus teaches considerations for serviceability including this in the design process. They look at immediate serviceability but not any complications that may occur in the design life long-term.
Cranes aren’t something I covered in my time at university, any differences in the requirements such as serviceable deflection were new to me.
The jib crane footing design highlighted this. As there is some soil deformation when the crane is above the corner of the footing (within serviceable limits), it needs to be considered if this will compound with use and cause serviceability issues over time.
The geotechnical syllabus includes the design of footings and calculation of soil capacity but doesn’t review how to make assumptions. Assuming support reactions and bearing pressure capacity can be tricky without information and limited engineering experience.
This is a topic not covered when learning design at university. Things like construction joints, isolation joints and needing to provide details for conduit for power supply were only mentioned. This is something I’ve slowly been picking up from advice of senior co-workers and requirements in designs. For this jib crane design, the power supply conduit is placed in the new slab and also required cutting into the existing slab for placement.
I reflected on my own beginnings after reading this. I was thrown in the deep end and I learned a lot, real quick. FEA was different back then (card input!!), but I did learn quickly that the principles learned at uni were not generally the methods used in the real world. What came clearer later, was that the fundamental principles learned at uni formed the basis of understanding of how things work and how structures behave – and knowing these made it a lot easier to select the practical design methods used in the design office.
Thanks for your contribution, Brandon. This case study was originally presented at one of our Friday arvo team sessions – an initiative the guys created themselves.
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