Jump to content

NLCbanner2024.jpg.2478be509670e60c2d6efd04834b8b47.jpg

My observatory has no roof

sharkmelley

By sharkmelley
in DIY Observatories

Recommended Posts

dmahon

dmahon

Posted
Posted

I do hope there are no Obby's out there just sitting on the ground, the roof could be fine if held down, but the whole structure could end up over the fence in a neighbours garden...loads of sand bags in all 4 corners might help.....

There are hundreds of thousands of sheds just sitting on the ground. That is the default installation method.

Link to comment
Share on other sites
  • Replies 120
  • Created
  • Last Reply
astronymonkey

astronymonkey

Posted
Posted

On reading some stuff on the internet the biggest forces are on the edges of a flat roof and also along the ridge if there is a gable. Somthe mechanism seems to be thatbthe roof is peeled away which explaines when you see images on the news of flat roofs bening lifted in ths way.

in addition to this the forces are increased where there is an overhang or eaves arrangement and since a lot of observatory roofs have an overhang then this will increase the risk.

with this in mind I think it would be wise to make sure the walls are bolted of fixed to a solid footing, make sure the roof can be locked onto the structure when not in use, and the finally installing some sort of ratchet tie down which tethers the roof corners down to the floor slab to act as a storm proof measure when heavy wind is forecast. Once installed maintainace should be an annual thing to check nothing has warped, rotted or become loose and after that its probably in the lap of the gods.

cheers

Link to comment
Share on other sites
Zakalwe

Zakalwe

Posted
Posted

Right, this is my plan.

My roof is currently secured with 4 x 8mm turnbuckles, one in each corner.

I have a steel pier thats bolted down to a large concrete plinth (just under a cubic metre of concrete). The pier is bolted down with 12mm studs (200mm long) and chemical fixers. I am going to get 4 tie-down rings and slip them over the protruding studding and tighten them down with another M12 nut. These will provide a lashing point for some nylon ratchet straps. The ratchet straps will loop through the roof braces. Using the same locations as the pier fixings will mean that there are no additionaltrip hazards.

If this lot shifts, then there would be nothing that I could have done otherwise.

http://www.ebay.com/itm/1-4-Steel-D-Ring-Rope-Chain-Tie-Down-one-hole-2-400-Truck-Trailer-ATV-Strap-/370638124647

Link to comment
Share on other sites
Gina

Gina

Posted
Posted

Strapping the roof to the pier seems a good idea :)  Think I'll look into that myself.  My obsy itself is pretty heavy with 3 beams 10ft x 7" x 3" and 7 or 8 joists and floorboards on top of that plus the weight of the walls.  The beams are tied down to the 6 concrete support blocks (which are about 18" cube) with embedded wall ties and wood screws into the beams - two to each block.  I didn't want it to move in high winds :D  However, we are on a SE facing hillside and we do get very strong winds.  Worst are east or south-east - SW winds are reduced a bit by high ground, trees and a house.

Link to comment
Share on other sites
Zakalwe

Zakalwe

Posted
Posted

It blew a gale here last night, with stronger winds expected. I lay in bed listening to the wind howling over the roof. At 12:30 I got up and strapped the roof down with another couple of straps, and pulled the C11 and EQ8 into the house. If the obsy took off, then at least the expensive bits of kit would be OK.

Link to comment
Share on other sites
sharkmelley

sharkmelley

Posted
  • Author
Posted

Typical wing loadings for gliders and small aircraft are around 30 to 50 kg/m2 and they take off at around 40 to 50 knots.  Your roof, with an area of 15 m2, could generate lift of 450 to 750 kg.  The lift increases as the square of the speed so doubling the speed will quadruple the lift.

There's easily enough force here to toss a 500 kg roof around, and you need to be serious about tie downs.  Attaching the roof to the building may result in the entire thing taking off.

Chris

Wow!  That easily explains why 650kg of roof took off!  I only had it secured with a couple of hasp and staples to prevent it sliding back on the runners.  Never imagined that something of that weight could take off - I was relying on its ownweight to hold it down.   A painful lesson learnt.

Mark

Link to comment
Share on other sites
sharkmelley

sharkmelley

Posted
  • Author
Posted

Scopes are OK.  Laptops are still on the radiator drying out. Mounts are drying out.  Assorted leads and cables have dried out.  I haven't tried powering anything up yet but I'm pretty sure all is OK.

Boxing Day morning I put a temporary static roof in place covered with a tarpaulin. 

post-19658-0-78476800-1388136936_thumb.j

It survived last night's storm but the tarpaulin leaked a bit so I've just come back from the builders mercahnts with a heavy duty one.  So I can finally begin to dry out the inside of the observatory.  Looks like the wet has caused no permanent damage.

Mark

Link to comment
Share on other sites
Zakalwe

Zakalwe

Posted
Posted

It looks like a very exposed position- great for LP but there doesn't look to be much to slow the wind down before it hits the obsy.

Fingers crossed that everything is OK and you can get it sorted without too much hassle. Be careful handling sheet materials in this wind...they can easily catch the breeze and take you off a ladder.

Link to comment
Share on other sites
Macavity

Macavity

Posted
Posted

Yes, there does seem to be a theme of open countryside. Was monitoring windspeed at observatory height, till about 4 a.m. :p

Despite being "coastal" and at 200m, the *gusts* (predicted to be 60mph) never exceeded 30 mph? Though perhaps like waves at sea (that sink ocean liners) it the one in one hundred... thousand... more(!), you have to worry about. But maybe I should be thankful for the close proximity of neighbours and stuff? Pretty sure it would have been bloomin' windy down on the "prom" - Almost always is.  :o

Link to comment
Share on other sites
itmo

itmo

Posted
Posted

umm aircraft wings generate most lift from the front topside of the wing, where the best low pressure area is generated. The way for make sure a 747 doesnt bounce back into the air after landing is to (automatically) trigger what is called spoilers to "spoil" the airflow over the main lift generating area. I wonder if one could use spoilers or vortex generators to "spoil" the lift over a roof. 

Link to comment
Share on other sites
Macavity

Macavity

Posted
Posted

umm aircraft wings generate most lift from the front topside of the wing, where the best low pressure area is generated. The way for make sure a 747 doesnt bounce back into the air after landing is to (automatically) trigger what is called spoilers to "spoil" the airflow over the main lift generating area. I wonder if one could use spoilers or vortex generators to "spoil" the lift over a roof. 

Might be the way to go. Wooden slats with offset blocks to allow the rain to run under (naturally!)?

More stuff on this: http://www.science-projects.com/HurricaneHouse/HurriHouse.htm  ;)

Link to comment
Share on other sites
SnakeyJ

SnakeyJ

Posted
Posted

umm aircraft wings generate most lift from the front topside of the wing, where the best low pressure area is generated. The way for make sure a 747 doesnt bounce back into the air after landing is to (automatically) trigger what is called spoilers to "spoil" the airflow over the main lift generating area. I wonder if one could use spoilers or vortex generators to "spoil" the lift over a roof. 

This sounds promising, along with Astromonkey and Alien13's comments - time for some wind tunnel tests?    I was having a browse around the web this morning looking for suggestions - while the effect is quite well known, the standard building approach seems to be re-enforcement and bonding with gussets - not terribly helpful where the roof is designed to be removable.

Bizibuilder designed a very simple and effective locking pin arrangement for my observatory roof - with a horizontal pin in each corner of the sliding roof locating in to a receiver (steel plate with hole) fixed to the wall frames.    Once locked in the closed position a drop bolt secures the roof in it's locked position.  

This works very well and has stood up thus far, but it does make me question the strength of the bonding between the walls and the concrete base the observatory is mounted on - I might revisit this with some additional bolts when time/conditions allow.

Do we have an aerodynamics expert in the house who could advise on retrofit strips to spoil airflow over the roof - if this could be done without adding excessive weight, but producing a useful reduction in lift, it might be a useful feature for future builds and as a retrofit for existing obs roofs.

Link to comment
Share on other sites
rwg

rwg

Posted
Posted

I was just chatting with my neighbour, who is an aeronautical engineer, about this problem. His conclusion was that if you can break the smooth airflow over the roof away from the surface, you will wipe out most of the lift - essentially what happens when an aircraft wing stalls. A single upright bar along the apex of the roof, just a few inches high should be enough to force the flow away from the surface on the downwind side, leaving turbulence near the surface. This will help a lot.  A few more of these parallel to the apex - one half way up each slope, one at the bottom of each slope - should really kill the wing effect.

Obviously as a belt and braces approach, I'd say hold the roof down to the walls too - I use these : http://www.ebay.co.uk/itm/Over-centre-catch-receiver-300kg-/320986553831?ssPageName=ADME:L:OC:GB:3160

Robin

Link to comment
Share on other sites
itmo

itmo

Posted
Posted

yes the idea of a spoiler is to spoil the airflow over the front part of the wing. Vortex generators on the other hand generate vortexes which make the flow attach on the wing to generate lift. If I remember correctly one of the problems with roofs is asymmetric lift , basically the lift is lifting  the roof off and pushing it back with drag. The drag is something roofs cant handle that well, especially if combined with snow or water load. I would try to look at patents for airplane spoilers. 

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.