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Discussion Starter · #1 · (Edited)
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!
 

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Need a picture of the area you plan on building this.
Still trying to figure out why you would want to build a green house below grade like that and have to deal with moisture, and lack of sun.
 

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Discussion Starter · #5 · (Edited)
https://farm3.staticflickr.com/2940/14719248606_4f9f32ca21_c.jpg
https://farm8.staticflickr.com/7595/26506159200_99efa07969_h.jpg
https://farm8.staticflickr.com/7338/26754057296_f6fbd8d54b_c.jpg
https://farm8.staticflickr.com/7060/26754066066_8a6406196a_c.jpg
https://farm6.staticflickr.com/5596/14739891454_359796b2b9_c.jpg
https://farm9.staticflickr.com/8737/16598490490_d31da3f4d4_h.jpg

The ravine appears at the top, near the road in the third picture. It slowly grows and deepens as it descends into my canyon (I'm not ambitious enough to try to turn a canyon into a greenhouse, lol! ;) Greenhouse redwoods, anyone? ) The top part (including where the greenhouse will be) is dry for most of the year; water only flows through it after long periods of rain and during the spring thaw. The bottom part of the ravine (as one nears the canyon bottom) is usually wet (from groundwater seeps), except during dry periods.

As for why: in-ground greenhouses (often called "pit greenhouses", or sometimes "walipini") are a time-honored tradition. They're more energy efficient, thermally stable, and wind resistant than fully above-grade greenhouses. They're usually built without floors - very simple construction - leaving plants with access to the water table. They also take less glazing material. This does mean less light, but the drop in the amount of light is proportionally less than the reduction in the amount of glazing, since top glazing lets in more light over the course of a day than side glazing.

Wind is the biggest factor on my land. Iceland is one of the windiest places on Earth:

http://cleantechverdict.com/wp-content/uploads/2014/02/3tier_5km_global_wind_speed.jpg

... hence why I used 70m/s in my calculations, and why I've been considering such large, frequently-spaced beams. In a lot of countries people just use metal hoops with plastic sheeting over them to make greenhouses. Around here, that would last about three days. ;)

Re: flooding: indeed, as an integral part of the design, my approach is not to try to stop water - just let it flow right on through - hence, having the base of each in-ravine wall be a big pile of loose rock. I considered just having a "drain" and trying to divert water around the greenhouse, but I came to the conclusion that I don't want any hindrances to the water flow that could "back up". My thoughts were that if too much air flows through the rock (I don't think it would) I'd hang some loose, slightly weighted plastic sheeting on the inside of the greenhouse over the rocks, which would block wind but which water could push away at will.

We had one of our biggest spring floods in recorded history recently, so I took video and measurements of how bad it got there, and plan to allow for floods 2-3 times higher than that.
 

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Sorry I didn't read your entire post. I am focusing on water through the structure. Im in NJ. The houses go by 10-50-100 yr flood cycles. Insurance requirements can become costly. In your case, keeping your hard work from being swept away may become costly. Even if in regular cycles, silt build up could be a pain. Sorry for the lecture, but if you want to live with nature, you shouldn't change the nature.
 

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Discussion Starter · #7 · (Edited)
I have no concerns over the water. It's the wind that concerns me.

This isn't some sort of river. Its "catchment basin" consists of nothing more than a small portion of the hill upslope from my land. It is literally impossible for it to accumulate some sort of huge torrent - there's just not that much ground "upstream" of it. There's a ravine like this every several hundred meters in my valley. Ravines like this form more from erosional slumping of wet, unanchored soil than from scouring by aggressive waters. The rocks in it aren't even smoothed. Likewise, the main floods are snow melt. There's no "debris" sitting on top of snow to silt up/clog drainage. When the snow melts, the ground beneath is still frozen solid; melt procedes downward. There's no branches or other stuff to blow in - this is Iceland. The worst type of debris we get is plastic blown away from farmers' hay bales. I've seen what flowing water does to it - it doesn't stop anything. Lastly, even if the rocks did somehow block and the water backed up, the water would blow out a polycarbonate panel to get in; I'm going to order extras ;)

I have a good understanding of the water here and it doesn't bother me. I also have a good understanding of the wind, and it really does bother me. We don't get hurricanes, but we get storms equivalent to them - often multiple per year (mainly winter/spring), plus many lesser gales. You wouldn't believe the sort of things they toss around / rip down (loaded shipping crates, concrete sheds, etc). Hence part of the reason for wanting the greenhouse in a low point (aka, a ravine) rather than with big vertical walls sitting out in the open.

I want to make sure it's strong. Very, strong. As mentioned, I've built a small kit greenhouse before, and then did a good number of modifications to it. I've done other construction projects such as reinforcing a steel shipping crate with steel I-beams to bury it underground as an underground shed. But this project would be my biggest yet, and I want to make sure that I approach the construction approach itself reasonable manner (which is why I started this thread :) )

I've started modeling the design in Blender so help with planning - see the screenshots below. It's still a work in progress - the front wall doesn't have its perlins yet, and the back wall hasn't been designed at all, there's no doors, and most of the bolts and angle brackets aren't in yet. Some changes to what I originally wrote:

1) I've switched to a three-axis reinforcement instead of two-axis reinforcement for the sides (I'll probably just use two for the front and back). Since the front and back present steeper angles and the greenhouse is wider than it is long, I want to help direct force from down-ravine winds into the edges and resist torsion. This presents some complication on the angle brackets to bolt everything together, however, since they don't meet at 90 degree angles. They don't even meet at 60 degree angles (I tried to use equilateral triangles... I failed); the triangles are 51,77°x64,11°x64,11°. However, there are so many brackets that would be used that I could probably just have them custom made. Alternatively I could probably find 50° and 65° brackets and call that good enough.

2) I shrunk the width down a bit and increased the length a bit. That means that the greenhouse would start a little bit down into the ravine, but not too much. The main reason was that I wanted to keep the rafters down to 12m, since that's the maximum size that fits in an ISO container, so I figure that would save money on shipping. Now, I could certainly use smaller rafters (if 6m or less I could transport them to my land in a trailer rather than a flatbed), but I'd have to bolt them together.

3) The more I look at this as I work on putting the brackets and bolts in, the more I feel that I'd want to have everything precut and predrilled. Now, I don't have experience cutting and drilling aluminum (I understand it's easier), but I *hate* drilling steel; perhaps my drill is bad or something, but it usually takes me like 5 minutes per hole, and I'm always breaking bits. And there's going to be a *lot* of holes. Also, I'll admit that I'm not that neat with my drilling or cutting.

4) Assembly: I've been including some small, 30° brackets to attach pairs of rafters in Vs; the thought was that I could assemble Vs, stand them up (the aluminum is only about 0,6-1,8kg/m, depending on wall thickness), stake the first one in place, stand up an adjacent ones, then start bolting them together with the ground-level purlins... then more Vs, then more purlins, and so on. The purlins would also form a climbing surface.

5) I removed the single monolithic ridgeline beam, replacing it with segments (just like the purlins)... the reason being that a single beam would really complicate assembly (since I don't have a crane, and crane rental costs are absurd)

6) I switched from 60x200mm aluminum to 40x80mm. The former would have not just been expensive, but my calculations showed it to be way overkill. And not just calculations - I went outside and verified them by taking a piece of profile aluminum, setting bricks on it, and measuring the deflection, and punching those figures into my spreadsheet.

7) I'm no longer concerned with fatigue. I looked up aluminum fatigue curves, and unless the structure is frequently getting up to strain levels a good fraction of the ultimate yield strength, then it's not a concern. Even with wind flexure "cycles" every few seconds every day of every year, the time to failure by fatigue would be measured in millenia due to how small of a fraction of the yield strength the "normal" cycles go through.

8) I've spaced all rafters out by 70cm. I checked into local greenhouse construction and with 8mm twinwall polycarbonate they normally use 60cm. I'm looking to use 12mm twinwall polycarbonate, which is significantly stronger; 12mm twinwall at 70cm spacing should be stronger than 8mm at 60cm, according to the charts I've seen.

9) In addition to wind, I do have concern of sheep trying to walk onto the roof and falling through. But there's obviously solutions to that.

Thoughts? My main strength concerns are with the connections - do you think that a bracket with several bolts at each intersection would yield a secure structure? I really don't want to have to weld it... And do you think that this sort of gridded-shell structure would be strong, or do you think I'd have to use trusses / internal reinforcements? I was avoiding that just simply due to how many connections I'm already going to have to make (because of how many rafters one needs for attaching the polycarbonate securely).

The polycarb (plus U/H joints, etc) in bulk will be ~5k $USD. The aluminum in bulk should be something like $10k (rough ballpark). I haven't yet tried to add up the bolts, angle brackets, sheet aluminum or insulation yet. Excavation costs for trenching, wiring, plumbing, etc, each of those is probably another $1-2k USD. Rock, another $1,5k... soil/compost, free from the neighbors' horses.... another $2k for structure anchoring, whether excavation/fill or individual screw-in anchors.... also, a door... vent openers... ~$400 for domestic shipping... Hmm, what am I forgetting...
 

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Discussion Starter · #9 · (Edited)
Hmm, I just had a thought. Instead of using angle brackets on each angle of each of those joins, I should be using gusset plates, shouldn't I? So that there'd be only one per intersection, rather than having to have an angle bracket at each corner of each intersection. The downside would be that it would take a plate on each side, which would make the top surface raised at the joints and a potential complication for panel installation.

Hmm, now that I think about it, if it works out to be economical to have things custom stamped/cut, I could have a sort of hybrid gusset/bracket that just slides onto each joint from the underside and is then bolted on. Could save a ton of work, as the bolts would just be to hold the bracket in place.

It's the joining aspects that I'm the least confident about in this project.
 

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It's the joining aspects that I'm the least confident about in this project.
Indeed, the brackets shown above don't look too good. If you run through-bolts, you will simply collapse the tubing. If you put nuts on the inside, you will not be able to access the nuts (unless you get fancy on the bracket design). Possibly look for a structural "system" rather than inventing it yourself. A consultation with the correct structural engineer (might be hard to find one that does something like this commonly) could be worth the investment.
 

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Discussion Starter · #11 · (Edited)
Indeed, the brackets shown above don't look too good. If you run through-bolts, you will simply collapse the tubing. If you put nuts on the inside, you will not be able to access the nuts (unless you get fancy on the bracket design).
Indeed, they were designed as run-through, and that was indeed a concern - either fastening too tightly and crushing the tubing, or too loosely and not securely gripping the aluminum with the brackets. I considered the possibility of aluminum profile with an internal grid pattern, which is often used for structural applications, but I never really was comfortable with the general attachment concept.

I feel a lot better about the possibility of custom stamped stainless steel brackets that fit around the entirity of each joint (just sliding on from underneath), where bolts are only used to keep the bracket in place. That should be very strong and easy to install. The only question is how much it would cost and what the minimum order would be, due to tooling costs. As designed above, there are two main types of intersections between the purlins and the rafters (six-way and three-way), but there's also special cases on the edges. Hmm... I wonder if I could narrow it down to just two or three types of brackets....

Possibly look for a structural "system" rather than inventing it yourself. A consultation with the correct structural engineer (might be hard to find one that does something like this commonly) could be worth the investment.
Hmm, could you clarify what you mean by "structural system"?

I know having an engineer do the design would run me something like $10K USD. So as mentioned earlier, I'd rather put in $10K more reinforcement than necessary than have to pay that. But then again, it doesn't matter how much reinforcement there is if the joints aren't reliable...
 

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T-Slot extrusion would likely end up costing a fortune when you consider all the brackets and T-nuts.

By system, I mean something like this: https://deckadvisor.com/2012/06/20/goodbye-wood-deck-framing-hello-steel-deck-framing/
This is galvanized steel framing, not aluminum. Still rather rare over here, but possibly more common in Europe. But the methods and components for connection have (hopefully) all been engineered, tested, and proven over time. You have a lot of angles in your design. Going to be cheaper if you can simplify it somewhat.
 

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Discussion Starter · #14 · (Edited)
I'll definitely see what I can do about simplification. But there's a balance when it comes to strength. A simple two-axis reinforcement is subject to sheer stress - it can collapse by rotating at the joint. To bend a three axis structure at a joint means to put the third axis into tensile and compressive stress. In that deck example, I don't see any purlins at all, it seems to be (except at the ends) just single-axis reinforcement.

One obvious simplification that runs to mind would be to increase the number of purlins (but make them correspondingly smaller so the net reinforcement would be the same), so that the three-element intersections on the sides all go away and there are only six-element intersections. So that would mean one type of bracket for each side intersection, one type for each of the front and back edges (I think**), and one type for the top ridgeline. I think a friend who was looking into having some custom parts made at one point quoted a tooling cost of something like $200 USD per part, but don't quote me on that...

Geodesics domes would be nice, but domes don't work with standard polycarbonate panel systems. Panels are rectangular, and are arranged to have the channels making long vertical runs, for drainage / humidity reasons; also, they're designed to bend somewhat on one axis, but not two. Now, if there were "geodesic tunnel" kits, that would be great. :)

** Ed: after thinking about it, I think it would take two types for the edges, since a part isn't inherently equivalent to its mirror image. So four total types of brackets.
 

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Discussion Starter · #15 ·
Concerning custom bracket types, I see two possibilities... #1) like this:

http://www.gazebospareparts.co.uk/photos/1.615475IMG_4159.jpg

(except out of stainless, and for rectangular profile aluminum)... or #2) something like #1 but missing its top so that instead of sliding over the rafters, it slides on from underneath. I'd expect #2 to be cheaper to manufacture, and it would give me an unbroken top surface for attaching the polycarbonate H-channels, but would rely on its bolts somewhat to resist out-of-plane bending (in-plane bending wouldn't be very affected).

Hmm, I need to think about how assembly would go if the brackets were of type #1...
 

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Discussion Starter · #17 · (Edited)
Thanks, but that's timber - it doesn't help concerning aluminum. I have enough experience with small-scale greenhouses to know that I absolutely don't want a wood-framed one. Plus, wood is in general not exactly an ideal construction material for Iceland regardless - we don't exactly have abundant local sources, and it's heavy/bulky for its strength, so it's not cheap to ship in.

I appreciate the effort, though! :)
 

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Discussion Starter · #19 · (Edited)
Okay, so I've been working more on this, mainly on the brackets to join together aluminum segments. Failing to find any strong hex brackets online, I've been working to design one. And from my understanding that CNC folding is the cheapest approach for moderate volumes, I've been trying to design one that can be folded. Which is surprisingly harder than I would have expected! ;) But here's what I have so far. It's three pieces:



The bottom ones fit over the rafters, alternating forwards and backwards, sliding freely. So once the purlins are positioned, one slides up from underneath and its 180°-rotated counterpart slides down from the top. Then two of the middle bracket fit on - one from each side, between the two purlins on each side of the joint. Lastly, a cap (top piece) fits onto the bottom. All of the holes line up so that two short M10 bolts on each side (or one very long one on each side) attach everything securely together.

Here's the pieces unfolded, and what they assemble into (bottom):



Here they are (top, then bottom) around an intersection:




A few details. Here's a closeup of those crosstabs on the top:



Folding puts a lot of constraints on the shape, but I noticed I had a bit of free metal to work with that I could make a tab that would run off from one side of the rafter-bracket to the other, and I figured that'd be nice for added strength. I'm not sure how well a CNC folder can deal with these sort of "gradual bends" or how I'd mark that on the plans.. but anyway. The other downside of the tab is that it doesn't make for a complete bolt hole, only a half-hole:



The bolt hole diameter is 12mm... I figured it'd be good to have a little extra if using a M10, but maybe I should shrink it closer to the bolt diameter?

(more in just a sec!)
 

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Discussion Starter · #20 ·
On the bottom, the reason for the added cap becomes clear when we take it off:



Neither the rafter brackets nor side brackets can cover the middle. Which would be the weakest place, especially if I have to use one of these brackets to join rafter segments! Note that the ridges on the rafter bracket aren't just for added strength, they're necessary to make the folding work (more to the point, I've discovered during my folding testing that they actually need to be bigger). So, here's how the cap fits on:



Where possible, rather than having the metal wrap around the beams 360 degrees, I added an extra 90-degree bend for a "lip" for extra strength (since a bent bracket isn't welded closed..well, it could be, but that'd be extra money per bracket):



The side brackets, as drawn, have to have cuts to have a tab that can be bent up so that it can be joined at the bolt holes:



I've used tolerances of 0,25 to 0,5mm between pieces - do you think that's a reasonable amount? Also, the brackets, as drawn, are 1mm stainless. Again, do you think that's reasonable? Obviously, in most places it's layered, so in practice it's thicker.

So, I designed it, then I unbent it... and that's where I am now. I discovered, as mentioned, a few changes that I had to make during unbending it, so the next step will be to re-bend it... and possibly repeat the cycle a few times until I have something that works both unbent and bent. And to do my best to ensure that nothing hits anything while sliding the pieces on.

A note about the current design: as it stands, nothing is ever actually bolted into the aluminum beams. technically, everything is free to slide. However, in that sort of geodesic grid, there's not actually any room for anything to slide to, everything is locked into everything else. I figured I'd avoid the expense of having tons of holes drilled into the aluminum (and extra holes in the brackets). Do you think this is reasonable or not?

My game plan would be - after getting feedback here, and getting to a version that I think works in both folded and unfolded mode - to 3d print miniatures in plastic and make sure that they all fit in the real world, and then have them manufactured for real. My hope is that the estimate for the brackets would come in at maybe 50 cents a piece (because I'd need a couple thousand). I don't know if that's realistic. I figured I'd start talking to suppliers while waiting for the 3d prints. Does this sound like a realistic approach?

As for the framing, I've been talking with suppliers. It's hard to find any to guarantee the material properties of their profile aluminum, but I think I've got a couple that will. As designed, it's calling for 100x25x1mm (thin wall but large diameter) 6061 T6 aluminum. This is based on calculations that a 15m beam (the length of the rafters on one side) would be at 1/4 of their load limit under a 70m/s (156mph) wind when faced at a roof angle of 25°, 70cm beam spacing, and a drag coefficient of 2,3, and experience a deflection of 21cm. Now, that's assuming that the backside side "doesn't count", because any force on it should be largely in linear compression, which is much stronger, and often it's even reversed (backsides of roofs often get more suction than compression, which is why they lift off when not properly nailed down). But even if we treat it as a fully unsupported beam, that would be 1/2 of the load limit and a deflection of a couple meters. Furthermore, it's not just the rafters, there's also the purlins transferring load around. So, without having done a FEA simulation, that sounds reasonable, right? Still, I can't help but wonder if I should be giving some huge safety margin rather than just 4-fold... the aluminum estimates seem to be coming in at about $2,5k per tonne and the amount needed in the above scenario would be about $1 tonne. Maybe I should double the wall thickness to 2mm and thus double the safety margin? 1mm just seems so darned thin for such a large beam... but then again, the fact that it's so large is what makes it so strong - if the thickness was halved the stress would triple. Its sort of like using a truss - you're not using much metal but you're not spreading it out. Still... I think I'd sleep better at 2mm. ;) (local suppliers don't even sell such large beams at 2mm, only 3mm).

So anyway, that's the status update. Thoughts?
 
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