Treadmill Desk Platform
Note: I've created a forum thread here for discussion of this project and some of the questions/concerns outlined below.
Project goal
Create a platform that can safely hold a person seated in an office chair, elevating the surface ~18" above floor level, above the belt/deck of a treadmill, to make the seated position the correct height for the working surface. A secondary goal - as with all good projects - is learning!
Project members
Background/rationale
I've set up my most powerful desktop PC (specs are at the bottom for the fellow computer nerds) at my new treadmill desk, but I'm only good for 2-3 hours on the treadmill most days, and would like to still use my workstation there in a seated position when my energy/physical state doesn't allow continued standing, or simply when movement is making typing/productivity difficult. Ideally this platform will be easily moved on/off the treadmill, allowing for easily alternating positions throughout the day without hassle… i.e., walking while reading/doing e-mail, sitting while typing heavily/coding/concentrating, alternating regularly for breaks - perhaps as part of the Pomodoro Technique.
While I'm down to ~185lbs now (and hopefully still dropping as I continue using the treadmill instead of my previous 99.9% sedentary lifestyle), I'd like the platform to support at least 250lbs, to accommodate the weight of the chair and the occasional cat in the lap. The chair will be your standard cheap office chair with ~5 caster wheels (although perhaps in the future, something more ergonomic like one of those expensive Aeron or Leap chairs that were the rage during the dot com boom - so accommodating up to 300lbs would be ideal for some wiggle room).
Note that while I regularly watched The New Yankee Workshop on PBS and similar shows as a kid, and have done a little bit of shop work for a handful of projects years ago, I haven't had the tools or resources to do anything for 10+ years now, and possess limited real-world knowledge and experience with woodworking, metal work, structural engineering, etc. However, I am certainly eager to keep learning and making my entire life - and am thrilled to have access to the resources - and great minds - at DMS!
Initial planned solution
I've mocked up a wooden platform in Google SketchUp (model attached, screenshots below for easy reference). A 3/4" thick plywood surface, roughly 42" x 48", will be supported underneath by a frame of 2" x 4" support members, which are elevated by three separate 4" x 6" beam 'legs' on each side of the platform / straddling the treadmill.
A 1" x 1" lip will be added above the plywood on the perimeter of the surface, to help prevent the office chair from rolling off the platform.
The size and spacing of the legs ensures the 2x4 support members are all supported solidly by the legs, as seen in the side view - note that all beams at each joint overlap the legs.
Concerns regarding the cross support members under the plywood deck
I'm not sure how best to join them, or if they will be solid enough, to provide the necessary support. In the mockup, they're simply shown as butt joints, but I doubt that would be feasible. While they should certainly be well-supported in the areas where these support joints meet above a leg, the interior joints will bear a lot of the load from the chair above and must be strong enough to distribute that load to the legs.
I'd think creating a cross lap / halving joint (milling halfway through each support beam where they cross and gluing together) would be ideal, although I haven't been trained on the cabinet saw (where a stacked dado blade would make that easy). Does DMS have a router with a dado bit, or any other alternative ideas that might make the cross lap joints easy? With some good wood glue, would 2x4's actually be strong enough for this scenario with that sort of cross lap joint? (since those are really closer to 1.5" thick, the lap joint would be milling down to just 3/4" of material on each support, which seems a bit thin, although I believe once the joint is glued back together it might even be stronger than the original 2x4, if done well)
Another option I considered would be to drill holes in the supports and use hardwood dowels and glue to join them; this should be easy on the tooling side, but I'm assuming the lap joint would be stronger overall. Of course, the easiest route might be some metal angle brackets and screws holding the butt joints together, but I'm worried about the strength of that option - and I'd rather try something less familiar, so I can learn a bit more.
Overall I think the 3/4" plywood will certainly provide a decent amount of support and load distribution on its own, so I'm hoping the 2x4 supports underneath will be sufficient as long as those interior joints are solid enough.
Alternative ideas
I think the 4" x 6" beam legs are likely overkill, and perhaps changing the outer joints on the support 2x4s to lap joints as well - or using a metal plate or something similar to provide the support for the butt joints / load distribution to the legs - would allow for somewhat smaller legs? Would that be worth pursuing to reduce overall weight and perhaps costs?
Should I consider metal framing & legs for this instead of this all-wood construction for the support substructure? I've got no experience with welding but would love to learn. I'm assuming metal would be more costly despite likely requiring a lot less overall material to provide sufficient strength; I'm not sure what the net effect on overall weight of the platform would be.
Future goal
I plan to eventually shorten the leg height, to allow for incorporating spring-loaded caster mechanisms that will allow the platform to roll smoothly when lightly loaded (i.e., just the platform with an empty chair on top, no person). When adding the weight of a person, the spring mechanism would lower and lock the casters in place to prevent undesired movement of the platform. In the interim, I plan to simply use felt sliders on the bottom of the wood legs to slightly ease movement of the platform, while I get a better feel for the final weight of the platform with chair to determine the specs required of the spring mechanism. Preliminary research in this direction turned up some self-braking compression caster options, such as these - 6 of these should support up to 450lbs, but I'm not sure what the braking specs are / how to adjust the braking weight threshold. More research will be done here once I've determined the final platform design/specs.
Computer/treadmill specs
The rig has 64GB of DDR3 1866 RAM, a 6-core 3.2Ghz / 3.8Ghz Turbo i7-3930K processor, dual 256GB Samsung 840 SATAIII SSDs in RAID0 for blazing performance, which are regularly backed up to dual 2TB spindles in RAID1 for safety (RAID0 SSDs would be a very bad idea otherwise). I've got a SAPPHIRE Radeon 6870 1GB graphics card - certainly not high end, but I'm not a gamer at all, this is a productivity rig - primarily for virtualization, as I develop software for distributed architectures and need to test a variety of infrastructure scenarios. Dual 24" monitors provide ample viewing area for monitoring several VMs at once. A water cooling setup was necessary due to filling all 8 memory slots in close proximity to the CPU (on the ASUS P9X79 Deluxe motherboard).. no room for a sufficient passive cooling setup, given the large heatsinks on my RAM.
It's a pretty darn fast machine :)
The treadmill desk is a LifeSpan TR1200-DT which I am loving so far. Only goes up to 4mph, but that's a great brisk pace for speed walking / lightly jogging while watching videos, and ~3mph is my max for reading comfortably; ~2.5mph is the max for typing. At my company's office we have a couple of the much more expensive Walkstation treadmill desks, and those only go up to 2mph - I sometimes find that frustratingly slow now that I've gotten used to mine :) Of course, those have a motorized work surface that can be adjusted for the varying heights of multiple users - definitely needed for the shared office environment they are in.
