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| Title: | wanting to make a dome out of fence posts? | |
| Posted on: | 10/15/2008 3:56:42 PM | |
| Category | Questions and answers | |
| Author | martinuk | |
| Status | Open | |
| Views | 12752 | |
| Replies | (18) | |
| Wanting to make a dome out of fence posts. does anyone have any info on how i can connect the posts? I have seen some connectors that i can buy but cant find them for using posts. Im going to be making the dome 21m diameter and it will be covered with material and will have to withstand strong sea winds. would be grateful for any help. Many thanks. | |
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| [ Comments 18 ] |
| posted by Colin | 16/10/2008 14:18:59 |
| Hi What dimensions are the posts? The strength comes from the geometry so you don`t need anything too major. | |
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| posted by martin1986 | 19/10/2008 15:00:35 |
| they will be two and a half inch wide posts would this be sufficient? The dome will be a 6v dome with the largest pole being 8 feet. Many thanks. | |
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| posted by admin | 19/10/2008 20:10:34 |
| Wow that's a huge dome, I would think that the 2.5 inch posts would be big enough, you're going to need very strong hubs... What did you have in mind for the hubs? | |
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| posted by Colin | 19/10/2008 20:36:43 |
| Hi Martin A 2.5" strut 7.5ft long may flex a bit under load. I use a general rule of thumb for timber struts where the ratio of length to slenderness shouldnt exceed 24:1, 3.75" timber would be better (ie 2x4) but you may get away with the 2.5" if you plan to use a lightweight covering material and no-one is going climbing.. 10m off the ground :) You`ll need at least 6" diameter tubular hubs to provide sufficient room for the struts unless you chamfer the ends. With the hubs the longest struts will be around 7ft. | |
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| posted by martin1986 | 20/10/2008 15:15:15 |
| Hey, For the cover im getting a 0.55mm pvc fitted cover, it will weigh approx 450kg. Im working on quite a large project. Im going to building several domes in fuerteventura. So all the domes need to also be able to withstand the strong sea winds. domes in sizes from 7m up to 21m and possibly a 30m dome. Also does anyone know of any pages on the internet with diagrams of the making of the hubs? Do you do a design service? Many thanks. | |
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| posted by Colin | 20/10/2008 15:37:25 |
| Hi Martin Is there any reason for choosing the fence posts, ie, you have literally hundreds of them laying around? For the larger domes i`d opt for at least 2x4 timber and then you may get away with 4" steel hubs. The 21m dome presents almost 700m2 of surface area to the wind so it will need to be very strong and well anchored to the ground. You have set yourself quite a task as there are 196 hubs on a 6v. Do you have access to a decent drill press and a metal cutoff saw? With two fixings at each end the struts you`ll have to drill over 2200 holes accurately in the steel hubs..a good challenge :) | |
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| posted by colin | 20/10/2008 16:23:01 |
| A conservative estimate of the total weight assuming 2x4 struts with 90mm dia steel hubs, fixings and 450kg cover weight comes to around 4 tonnes for the 21m dome. If 90mm dia hubs prove too small you may need to go to 114mm dia which adds 130kg approx. Doesn`t seem too bad, the windloading is the biggest factor and conservatively it may place upwards of 75 tonnes of loading on a dome that size. | |
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| posted by martin1986 | 20/10/2008 17:26:29 |
| Hey, Thanks colin, 2200 is alot of holes! especially when Im going to build 2 x 21m domes! Originally I was going to make the dome out of conduit but was advised that It was not going to me strong enough for a large dome. I read on some site that fencing post were a good cheep alternative. I was only making a 6v as the fence posts came in 8 foot lengths and to achieve the 21m diameter it needed to be a 6v to keep the poles under 8 foot. How would I work out the angle that I would need to cut the 2x4 at? how would I fix the 2x4 to the steel hubs? Many thanks for your help. Martin | |
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| posted by Colin | 20/10/2008 19:53:38 |
| Hi Martin The strut angles vary from 4-6 degrees. The hubs will need to be drilled pretty accurately so its best to make a jig especially if you plan to build several domes. 8ft strut lengths tend to amplify errors, a 5 degree error at the hub would equate to a 9" swing at the other end of the strut. Steel would be ok for struts but it wouldn`t be cheap or as friendly for the environment as timber. You`d need to use 32mm diameter galv steel tube for a 21m dome..you`d need a good press to squish it :) | |
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| posted by martin1986 | 21/10/2008 00:37:07 |
| Cheers colin. Think im going to do some more research into the cost of making a steal structure. Do you think I could get away without having hubs if i used galvanized steel and just connected the struts with just one bolt. Also when I first looked into getting steel tube they was quite a few grades of steel and wasn't sure which one to use, will any 32mm galv steel tube do the job? thanks. :) | |
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| posted by Colin | 21/10/2008 13:41:16 |
| Single bolt fixings would be ok, 10-12mm stainless steel. The steel needs to be a reasonable structural grade similar to that used for polytunnel hoops,in fact, it may be worth enquiring if a local tunnel firm could supply the tubing..they`d probably get it at a good price. Steel struts present three potential problems, secure cover attachment is the first one.Secondly the strut stacking height at the hubs will be 24mm if the tube has a 2mm wall thickness. Last but not least, in your very nice climate the steel will get very hot which may cause damage to the covering, so don`t forget to factor in the cost of anti hotspot tape :) | |
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| posted by Colin | 21/10/2008 18:43:27 |
| Judging by the weight of common structural steel tube, the dome could weigh less than 2.5 tonnes including the cover. | |
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| posted by martin1986 | 23/10/2008 11:31:42 |
| I found my seel tube, getting galvanized seamless steel pipe 32mm diameter with a 3.25mm wall thickness. Its quality is Q235 what ever that means, the other option was the same diameter, etc, but was Q345 grade, was 3 times the price. have searched the internet but cant find the difference, is anyone a steel expert? | |
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| posted by colin | 23/10/2008 13:56:00 |
| Hi Martin 235 is plenty strong enough. The Q prefix denotes the steel is tempered and quenched.The 3.25mm wall thickness is too much..you`ll never squash it :) Try to find something with a 2mm wall, think along the lines of trampoline frame or polytunnel hoop. | |
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| posted by martin1986 | 29/10/2008 19:30:34 |
| Hey, I am importing the steel as im going to he getting a minimum of 4000feet, 3.25mm is a standard size (is going to be alot cheeper) Was going to get a 12 tonne press break, Do you think this would be strong enough? | |
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| posted by colin | 29/10/2008 19:57:57 |
| If the steel is tempered its going to be fairly tough to squish, 12T may not be enough. I`d ask the supplier how much tonnage is required to do it..they should be able to find out or maybe even test it for you if they have the machinery :) If you are importing the steel it would be better to try to obtain the 2mm wall thickness if at all possible,your transport costs should be much lower. The dome frame will weigh substantially more with 3.2mm versus 2mm. | |
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| posted by colin | 29/10/2008 20:07:02 |
| Don`t forget the stacking effect of the steel at the hubs. You`ll have a minimum of 6.5mm per squished strut end, a 6 way hub equates to a 40mm stack which could prove interesting when fitting the covering. I`m sure the tube supplier would be happy to send you some short sections of the tube so you can test it for suitability. | |
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| posted by martin1986 | 09/01/2009 18:25:39 |
| Hey, Do you know How i can find out what thickness/type nuts and bolts I will need for the structure? Many thanks | |
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