greenhouse Greenhouse project

Hi all,
As we're building a new freezer at work, I was able to get 27 pine beams (1800x70x70mm). These are untreated, unshaved beams that were used as spacers between heavy PIR insulation panels. I did some brain storming on what I could build with these, as I love to DIY. I made a quick sum up on what I was going to need, forgive the crappy 3D and icons instead of words (saves me translation on different forums).
The plan is to build a frame using as many beams as possible (for weight, stability and wind resistance). I came to a carcass which I would like to cover with heavy HDPE foil (the only thing I could find that is UV resistant, transparent en heavy duty.. It's normally used as a moisture barrier under laminate floors). I'm planning to dig out the center to get a bit more vertical grow space (about 40cm I figure). In the middle I want to dig a walking trench with slightly deeper sides (to make sure it's not a water reservoir instead). To increase venting when it's really hot, I plan to add 4 venting windows in the roof (manually operated). I figure these are easiest to put them over the full length of the four sections.
Pictures only show the carcass, but I want to put thinner 1"x1"  beams on the gutters to pull the foil over. I also want to raise the center so that you get an angled (18.5 degrees) roof to help with channeling rain water off the roof.
Here are some quick drawing;

early drawing to see what I can make with 27 beams

Top view of the carcass

Cutaway with the trench

Final drawing (I might add some diagonal supports if I don't deem it sturdy enough when building - but I want to keep as much light available as possible). 
I'm still open for suggestions on how to apply the foil (tack it?) and to combat wind (my main concern). I might add four "spoons" on the vertical lower beams on the wind side (the other side will be shielded by a closed wooden fence) where I can place about 3 or 4 concrete 1' x 1' x 1.5" tiles on). Would this be enough?
Please share any ideas, pictures, concerns etc and do not hijack the thread with hotdogs, wine or bunnies... :shh: 
I'd need 3 more beams, which I don't have. I only get 27 of them, so I made a drawing of what I could eventually make (as large I can possibly get the floor). I opted for the low horizontal beams for anchoring (for wind protection, it does get quite windy here). I also have limited space so I want to put it in as one side of my garden.
7.55 x 1.94 is about 14.65 m².. That's about 24' 9" x 6' 4", which is about 158 square feet for you Americans. If I would use 2x 13 beams I could only get 2x 5.3m² with a sloped roof on the back. So about 30-40% less volume and floor space.
I can purchase some more beams, but than the project would cost a lot more in the end. It's also hard to find unimpregnated wood to use. I'm planning to keep it untreated and hang the foil over the lower beams, hoping the horizontal lower beams are dry enough to keep them from rotting. As this one lasts me 3-5 years I'm happy (offcourse with replacing the foil once every season). I'd also love to make it of all the same sized beams to make it look a bit profesional instead of shelter built by a homeless person.
Hmm I was actually hoping for a bit more feedback. As it is next to my garage, I was wondering if it would be better to place a small fan in the greenhouse to aid with cooling in the scorching hot summer days? (We only get about 5 Hot days, but I don't want them to kill my plants).

No body here is an aid DIYer?
I'm no constructionologist, but it looks a little fallsy-downsy, especially as you mention high winds. A continuous bottom plate is preferable, eaves to direct run-off out and away. Not entirely sure where you live in Europe, but snow weight would be a concern if left covered over the winter. Google Image "post and beam greenhouse", should give you a few ideas.
It's just the frame, they will be mounted with reinforced galvanized steel corners (ordered 150x150 reinforced corners instead of 50x50mm regular corners. I will also add 45 degree beams on the first, middle and last bows. I will add additional diagonal supports if it is not sturdy enough. I'm also getting a bit more beams than anticipated, so I will go head and add a horizontal beam om the end as well. 

(Worthless without a pretty picture offcourse).
Please also don't forget that this is just the carcass (heavy beams) and that it will be furnitured with smaller planks to tension the foil on, install a door, etc that should also add a lot of sturdiness to the actual structure. The roof will get a ridge to slope the roofing. and to mount a water hose to install a tap, electricity, lighting etc.
The project also received a new name, it's now "Unternehmen Asgard" (i.e. operation Asgard). That's what you get if you ask german military personnel to help out.. : -)
Costs made so far, dollars are rounded off;
Paint (garden epoxy) -  36 euro (43 dollar)
500 torx screws - 7 euro (8 dollar)
new (clean) hammer - 6 euro (7 dollar)
2 paint brushes  - 4 euro (4 dollar)
5x spare torx bits - 5 euro (6 dollar)
2mm metal drills - 3 euro (3 dollar)
woodfile - 3 euro (3 dollar)
Metal work (corners etc) 67 euro (80 dollar)
Wood -  0 euro (0 dollar)
total = 131 euro (154 dollar)
So far I'm only planning to purchas 3 uv resistant PET (mylar) tarps of 8 x 2.2 meter as I still have some left from my laminate floor. They are about 17 euro (20 dollar each). So for about $250 I hope to have a functional greenhouse. This isn't as bad as anticipated and it should keep me busy during the cold winter months that are ahead of us.  I'll try to upcycle as much materials as possible.
there a reason you're doing timber with corners instead of just bending some steel pipe?
the flat top will not be very good at shedding precipitation / wind?
edit: oh i see now you mention an angled roof.
Free timber, no pipe bending tools, more expensive connections and I love working with timber. Regarding the roof, as mentioned it will not be a flat top.

Btw thanks Miguelovic, those eaves are indeed crucial (else the foil will go green quite fast). Didn't think about that.
Glad part of it was helpful, I'm guilty of skim-reading far too often. I haven't much experience with greenhouses, just helping others to make them less fallsy-oversy and to dial in the environment. Interested to see how this develops, pit greenhouse is my prime choice when I get a chance to build one from the ground up/down.
Hmm I'm getting some help now from someone with a bit more building know how, but also not an expert.
New carcass (as I will be getting some more beams)

The idea for the supports;

(Trying to get sturdiness from the top beams)

(Trying to get the sturdiness from the middle row)

Received the iron work today (it looks like some small metal, but the reinforced corners
are 150mm x 150mm x 50mm and are 3mm thick and can take 200kg each... or in 'murrican:  6" x 6" x 2" and 1/8" thick... takes ~440lb)
The lower beams are now overlapping the beams, which should provide more sturdiness, better/stronger connections and will only cost me a little of the floor space. I also remeasured the beams and they are 76mm x 76mm instead of 70mm x 70mm (so a bit stronger/heavier than anticipated, should improve sturdiness and provides a lot of "meat" to drill and screw in).
Not yet, I have a few raised beds that I need to dig out and flatten to put the green house on. I have a lot of help to do that, but we keep ending up drinking instead of digging. It's a cultural thing I guess. I also didn't pre assemble anything in my garage yet, as my garage is about mid 20s (fahrenheit).

I do have all hardware now, paint, tools etc. Just need to buy some foil, but I figure I'll wait until spring for that.

I'm now in doubt if I should build a bunker or a greenhouse. Though I will need to clean out the pollution first (strange silver coloured bugs with teeth)

Damn that was a pile of dirt that need to come out. Approx 5.5x2.2x0.3 m = 18 ft x 8 ft x 1 ft (my raised beds). They where still full of roots, so it was hard digging. It got me about 3.6m³ of dirt (~128 cubic feet).
Never thought I'd be digging foxholes once more..
From an engineering point of view you need a few "tweaks".   LOL   First your top, cross beams need to be perched on top of your columns.   You should also tie each side in with diagonals to minimize torsion.  In this simple form consider using wire and turnbuckles for adjusting, cheap and functional.  Do it on all axis, the door end will need it also but more complicated.  The wire is only to hold all sides square.
I'm actually told by an architect (though just a student) that I should put the middle, crossing beams between the other beams instead of on top of it. The main reason was because it won't really bare weight, to instead it will be hit by wind side ways. Though I had to make sure I had extra strong connections. The red on the top picture are diagonal beams to prevent torsion and hold everything square. I used thin red lines as it was fast amd didn't "clog" up the picture. I also have a lot of new extra heavy connection material (1/4" thick steel 90 degree corners) that are very wide.

So I'm pretty sure it will hold up, my main concern is anchoring it against strong wind. The design stays the same as the last pic, though only with three instead of for segments.
Pfeffer said:
Hmm I was actually hoping for a bit more feedback. As it is next to my garage, I was wondering if it would be better to place a small fan in the greenhouse to aid with cooling in the scorching hot summer days? (We only get about 5 Hot days, but I don't want them to kill my plants).
No body here is an aid DIYer?
I asked about making it two bays for a reason. Have you not seen the stuff I've built? If you are worried about this being flimsy use some earth anchors.
If you're worried about it blowing over just pour a couple of piers under the corners and probably one in the middle and have an anchor bolt come out of that will go through your base support. Then bolt it down.
I also agree with Nuclieye that some angle bracing would be recommended. I know you said you're using this steel corner braces, but over time, moisture and movement, along with the wood expanding and contracting from weather and temperatures will cause your fasteners of those braces to loosen. It can become more wobbly at that point and set you up for failure.
Around the door you could put that clear corrugated plastic material and screw it on to keep that all square. Otherwise your door can get out of square and won't open/close right. I build houses for a living and I've seen a lot of bad stuff happen when people don't brace off correctly. You would also be amazed at how strong sheeting and drywall are when it comes to holding something together. Even if you put that plastic material or wood only 2' up it would greatly improve the strength of the whole building.