We are exploring the adoption of a new foundation option for Hello Hubs. We like the 4 post structure, but the current specification calls for a below-grade portion for each "post" (picture below), to provide stability and to resist overturning of the structure during high winds.
(standard Hello Hub steel frame structure, with sub-grade post foundation)
We are working on the option of using (readily availible) 4-post server racks as the primary structural componant (picture below this paragraph). Many of the racks have high strength specifications, offer modular mounting options, and can be shipped flat-packed (will be great for multiple-hub builds). They are generally a slightly smaller footprint than our current spec (3' x 2' instead of 3' x 3'), but this should be ok as we have been working hard to reduce the power consumption of the system and the result is that we have a reduced number of PV panels to hold up.
Additional specs on this 4-post rack: http://goo.gl/bXlzmR
One of the big effects of using this type of structure will be is that the foundation could be a concrete slab, which is simpler and quicker to install. This new structure and possible foundation requires us to look closely at several engineering challanges, we're working to calculate the following:
- Overturning. Dimensions (and volume/weight) of concrete necessary to resist overturning.
- Diagonal bracing. Dimensions and locations of solid rectangular steel bracing sheets.
- Optional plate and anchor system. Can we set the 4-post structure on plates and anchor via soil anchors ?
- Optional how much weight required to avoid the need for anchors/slab all-together, adding batteries may be cost effective (and would certainly be time-saving).
Here's the main specs we're working off of:
- 4-post section is 7'H x 3'W x 2'L
- Structure weight, including solar panels is 300lbs
- Battery weight (attached to base of structure) 350lbs
- 42 SqFt (9' x 6') of roof area / PV panels, slightly inclined to a long side (depends on solar conditions @ the site).
- Size and load specifications on rack can be found here: http://goo.gl/bXlzmR
I'll update here as we have solved, and add to the documentation section as well. Interested in any thoughts or discussion the structure. In the meantime, here's a render of the Hello Hub using the dimensions from the 4-post rack.
