Over the years I've contemplated posting something like this, but have never gotten around to it. Some recent discussions prompted me to bite the bullet and post it.
This is my take on a pier design spreadsheet that should be fairly simple for the lay person to use. You enter your desired pier height, what it's supporting (mount, scope, camera weights totalled), what kind of pier you want to use (eg circular hollow section, square hollow section, square concrete, circular concrete), set some movement limits, and it will output the minimum sized pier to meet those criteria.
The input cells are the orange ones. All other cells and worksheets are protected so the formulae and section properties don't get changed/corrupted by accident. Hopefully it's all pretty self-explanatory.
Quick summary of assumptions:
- It does not model vibrations. Having read around, asked a few questions, and chatted to a few experienced pier/observatory builders, we agreed that for appropriately sized piers based on the other criteria in the spreadsheet, vibrations should not be a critical factor. There are too may sources of vibrations; the frequency, magnitude of them is impossible to model simply, and it's not worth pursuing that when they're not going to increase the size of the pier anyway.
- The sizing of the pier is based on a horizontal/lateral load being applied on the top of the pier, causing a deflection. This is the same model as proposed on the often referred to Cloudy Nights post on
Pier Design Fundamentals.
- Similarly, the default minimum load applied (2kg) is in the same order as the 5lb load suggested in the above CN post, but can be overridden.
- I've included a function for the weight the pier is supporting, and use a fraction of that load to be applied horizontally. This is to deal with mounts carrying quite large loads where the above-mentioned 2kg arbitrary load may start becoming a bit light on.
- The deflection limits of 0.5arcsecs and 0.1-0.5pixels (whichever is worst) were also borrowed from the CN post and discussions with other IIS members.
The steel section sizes are based on Australian Tube Mills availability. There's probably other section sizes available, and they can easily be added to the spreadsheet later.
Edit: I've now added a "rat cage" section to check the impact of it on stiffness/deflection. To use it, though, you must use the "custom section" bottom part of spreadsheet.
I'm happy to open discussion about what are acceptable deflection limits, and any other improvements within reason.
Future additions/improvements I am thinking of are:
- suggest reinforcement for the concrete options