Hi all (new here!)
I'm looking to build a greenhouse into a ravine on my land. I've built a little kit greenhouse before, but this is obviously a much bigger task. The general idea is to "roof off" a section of the ravine and build walls at either end. Here's what I was thinking as far as the design:
1) 60x200mm** rectangular profile aluminum rafters angling up from the ground (~25°) up to the roof peak (aka, over the middle of the ravine), with 600mm between beams. The same sort of beams would also be used on the vertical walls that descend angled-down into the ravine and close it off at each end.
2) top beam: an additional 60x200mm**, attached to the side beams with angle brackets
3) crossbeams (also profile aluminum), 60x120mm**, at regular intervals as specified by the panels; attached to the rafters by angle brackets
4) Rafters anchored into the ground, each with a large screw anchor rather than concrete (I'd prefer to have the structure not be legally classified a "permanent"
)
5) Standard attachment of the panels (twinwall polycarbonate, UV outer coating, antifog inner coating), including an aluminum cap on the ridgeline. I'm thinking about installing tall vertical bolts every 100mm or so in the cap (or screwing on something else that has periodic protrusions) to act as vortex generators and reduce lift in heavy winds.
6) Concerning the walls that descend into the ravine, I was thinking about bolting sheet aluminum on inside of the support beams and either sheet aluminum or galvanized sheet steel on the outside, with insulation (maybe rock wool, that's common and cheap here) in-between. I may need to paint the outside wall so that it's not annoyingly reflective.
7) The in-ravine walls wouldn't extend all the way to the ground; instead, I'd have a large heap of rock/gravel at the base - dense enough to stop wind from just blowing through, but sparse enough to let water flow through in the event of a flood.
8) I'd be running electricity, cold water, and hot water to it (trenched, of course). I have some spare, standard household radiators for the hot water for heating - I was just thinking of nothing more complicated than staking them into the ground. I have LED grow lights, which I'd use for supplemental winter lighting; I'd hang them periodically from the roofline. I wasn't thinking about installing spray watering at this point, but might in the future; I'd just use a hose. Electricity would be setup by a certified electrician. Not yet decided on whether to have a plumber do the water - honestly, leaks wouldn't exactly be a tragic thing, as my cold water is free and leaked hot water is a benefit.
9) Misc: one door in each end of the ravine (thinking about just hand-making something, with rubber weather sealing, hinges, and a simple latch on each); and one or two vent openers in the ceiling, with the vents handled like the doors (honestly, I'm not sure vent openers are needed here.... the hottest it's ever gotten here is ~25°C/77°F)
Still working on finding out whether it will be legally necessary to pay an engineer to do the structural design.... hope not. :Þ I'd rather spend the money on extra reinforcement than on an engineer to tell me that the extra reinforcement isn't necessary.
The distance across the ravine depends on how far down into it I want to build; at the very top it's about 23 meters across. I was thinking something like 21m (across the ravine) x 10m (along the ravine), with the ability to keep expanding it down the ravine as needed in the future. Which is another reason why I want to keep the in-ravine walls light and simple rather than concreting them - to make it easier to expand the greenhouse as needed.
I put asterisks next to those beam dimensions above because I ran some preliminary engineering calculations for a wind blowing perpendicular to the ravine and it looks like they're totally overkill. If I assume a safety factor of 2 then 6061 aluminum leaves me 138 MPa of yield strength to work with. With a maximum wind speed of 70 m/s (category 5 hurricane), a drag coefficient of 2 (a triangular shape generally is 0,6) and a 25° slope then I come up with a roof pressure of 2,5kPa. In a simplistic case (a perfectly rigid ridgeline position and force evenly distributed), that would be a load of only 25mN/mm^3 on the beams. For the stated 200x60mm beams, assuming 6mm wall thickness, I get a moment of inertia of 68 million mm^4, and a stress of 0,91MPa, versus the available 138 MPa, and a deflection of only 2,75mm. So unless I'm doing something wrong, that's way overkill. If I reduce the beams to 100x50x5mm, I get a stress of 4,7MPa and max deflection of 28,33mm - still way, way under limits.
Now, that's for wind blowing perpendicular to the ravine, which is where the strongest winds on my land are. I haven't done the calcs for "down the ravine" because that's more complicated, I'd really need a FEA model for that (the more I angle the in-ravine walls, the better it will tolerate the winds... plus I really don't want winds putting torque on the ground anchors) Another caveat is that aluminum fatigues, so I really don't want it going anywhere close to its limits at regular intervals. There also will be more stress at the attachment points (any tips for reducing that?)
I considered 304 stainless instead of aluminum for the frame. Looks like its price per kg is double aluminum, and while its ultimate tensile strength is unsurprisingly 63% more than 6061 aluminum, I was shocked to see that its yield strength is actually lower. Aluminum bends more under stress, but I don't really care about that. So yeah, aluminum looks like the right choice. I checked and galvanic corrosion (bolts, brackets, anchors) doesn't look to be a problem between aluminum and various steels (incl. galvanized)
I'd like to work with just bolts, no welding (I have a mig welder, but it's not set up for aluminum) if possible. Going with aluminum instead of steel also makes it easier to drill/cut - although I'm considering ordering the aluminum with all of the holes pre-drilled to save myself time. But then I'd run the risk of having made a mistake, or them making a mistake, and having none of the pieces fit, so it's a hard choice.
So what do you all think.... is this a crazy plan on my part, or is this realistic? And if so, do you have any tips / suggestions? They'd be quite welcome!
I'm looking to build a greenhouse into a ravine on my land. I've built a little kit greenhouse before, but this is obviously a much bigger task. The general idea is to "roof off" a section of the ravine and build walls at either end. Here's what I was thinking as far as the design:
1) 60x200mm** rectangular profile aluminum rafters angling up from the ground (~25°) up to the roof peak (aka, over the middle of the ravine), with 600mm between beams. The same sort of beams would also be used on the vertical walls that descend angled-down into the ravine and close it off at each end.
2) top beam: an additional 60x200mm**, attached to the side beams with angle brackets
3) crossbeams (also profile aluminum), 60x120mm**, at regular intervals as specified by the panels; attached to the rafters by angle brackets
4) Rafters anchored into the ground, each with a large screw anchor rather than concrete (I'd prefer to have the structure not be legally classified a "permanent"
)5) Standard attachment of the panels (twinwall polycarbonate, UV outer coating, antifog inner coating), including an aluminum cap on the ridgeline. I'm thinking about installing tall vertical bolts every 100mm or so in the cap (or screwing on something else that has periodic protrusions) to act as vortex generators and reduce lift in heavy winds.
6) Concerning the walls that descend into the ravine, I was thinking about bolting sheet aluminum on inside of the support beams and either sheet aluminum or galvanized sheet steel on the outside, with insulation (maybe rock wool, that's common and cheap here) in-between. I may need to paint the outside wall so that it's not annoyingly reflective.
7) The in-ravine walls wouldn't extend all the way to the ground; instead, I'd have a large heap of rock/gravel at the base - dense enough to stop wind from just blowing through, but sparse enough to let water flow through in the event of a flood.
8) I'd be running electricity, cold water, and hot water to it (trenched, of course). I have some spare, standard household radiators for the hot water for heating - I was just thinking of nothing more complicated than staking them into the ground. I have LED grow lights, which I'd use for supplemental winter lighting; I'd hang them periodically from the roofline. I wasn't thinking about installing spray watering at this point, but might in the future; I'd just use a hose. Electricity would be setup by a certified electrician. Not yet decided on whether to have a plumber do the water - honestly, leaks wouldn't exactly be a tragic thing, as my cold water is free and leaked hot water is a benefit.
9) Misc: one door in each end of the ravine (thinking about just hand-making something, with rubber weather sealing, hinges, and a simple latch on each); and one or two vent openers in the ceiling, with the vents handled like the doors (honestly, I'm not sure vent openers are needed here.... the hottest it's ever gotten here is ~25°C/77°F)
Still working on finding out whether it will be legally necessary to pay an engineer to do the structural design.... hope not. :Þ I'd rather spend the money on extra reinforcement than on an engineer to tell me that the extra reinforcement isn't necessary.
The distance across the ravine depends on how far down into it I want to build; at the very top it's about 23 meters across. I was thinking something like 21m (across the ravine) x 10m (along the ravine), with the ability to keep expanding it down the ravine as needed in the future. Which is another reason why I want to keep the in-ravine walls light and simple rather than concreting them - to make it easier to expand the greenhouse as needed.
I put asterisks next to those beam dimensions above because I ran some preliminary engineering calculations for a wind blowing perpendicular to the ravine and it looks like they're totally overkill. If I assume a safety factor of 2 then 6061 aluminum leaves me 138 MPa of yield strength to work with. With a maximum wind speed of 70 m/s (category 5 hurricane), a drag coefficient of 2 (a triangular shape generally is 0,6) and a 25° slope then I come up with a roof pressure of 2,5kPa. In a simplistic case (a perfectly rigid ridgeline position and force evenly distributed), that would be a load of only 25mN/mm^3 on the beams. For the stated 200x60mm beams, assuming 6mm wall thickness, I get a moment of inertia of 68 million mm^4, and a stress of 0,91MPa, versus the available 138 MPa, and a deflection of only 2,75mm. So unless I'm doing something wrong, that's way overkill. If I reduce the beams to 100x50x5mm, I get a stress of 4,7MPa and max deflection of 28,33mm - still way, way under limits.
Now, that's for wind blowing perpendicular to the ravine, which is where the strongest winds on my land are. I haven't done the calcs for "down the ravine" because that's more complicated, I'd really need a FEA model for that (the more I angle the in-ravine walls, the better it will tolerate the winds... plus I really don't want winds putting torque on the ground anchors) Another caveat is that aluminum fatigues, so I really don't want it going anywhere close to its limits at regular intervals. There also will be more stress at the attachment points (any tips for reducing that?)
I considered 304 stainless instead of aluminum for the frame. Looks like its price per kg is double aluminum, and while its ultimate tensile strength is unsurprisingly 63% more than 6061 aluminum, I was shocked to see that its yield strength is actually lower. Aluminum bends more under stress, but I don't really care about that. So yeah, aluminum looks like the right choice. I checked and galvanic corrosion (bolts, brackets, anchors) doesn't look to be a problem between aluminum and various steels (incl. galvanized)
I'd like to work with just bolts, no welding (I have a mig welder, but it's not set up for aluminum) if possible. Going with aluminum instead of steel also makes it easier to drill/cut - although I'm considering ordering the aluminum with all of the holes pre-drilled to save myself time. But then I'd run the risk of having made a mistake, or them making a mistake, and having none of the pieces fit, so it's a hard choice.
So what do you all think.... is this a crazy plan on my part, or is this realistic? And if so, do you have any tips / suggestions? They'd be quite welcome!
