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Ballast for Truss Arch


psyfrog

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Ok guys I need your help with this one.

I work fo an event agency specialized on sports located in Germany. For several bike race we use a truss archway at the finishing line. On our last event a government official approched us telling me that the ballastation of the construction is to light. Our archway is 4,71 m high and 8,42 m broad and german law requires a ballast of 2,6 t per side(1,3 t on every foot), ridiculous. Usually we used stones for ballastation but the expenses of transporting 5,2 tons of ballast to the venue are just to high. We thought about using water tanks, but then again there is no space for four dice like water tanks that can each hold 1300 l of water an our finishing line. The next problem is that the ballst should not exeed a hight of max. 1m because nothing but the advertisement is ment to be seen or else the sponsors would be bitching around.

I know its complicated but really hope some of you can provide some inspiration for me beacause I´m really stuck with this one

 

http://imageshack.us/a/img827/5964/ueberbau.pnghttp://imageshack.us/a/img109/2105/1718001page001.jpg

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A difficult one and the weights seem larger than I would expect though it depends on where that kentledge was applied. The forces involved are substantial. For normal marquee wind limits of 55 mph, you are looking at 18,000 foot lbs of turning moments from what is effectively a 130 square foot sail on top of an 18 foot mast.

 

My personal solution would be to extend the bottom struts beyond the guy wires and attach kentledge out there. If that were possible there are plenty of plastic water tanks available that hold 650 litres each. One meter by one metre by 600mm high roughly.

 

However, this is dependent on which codes the official is working from and their interpretation so I would strongly recommend you talking to a structural engineer familiar with your regulatory framework. This is obviously a safety issue and there can be no room for guesswork. BR is a brilliant source of knowledge and information but on this one, get a man who can!

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Check out Spirafix ground anchors. A company I work for in the UK uses them to ballast all of their outdoor stages. They are solidly built, and are pretty easy to knock in. They will allegedly go through asphalt too (though I wouldn't like to be the one trying!). The biggest ones will hold up to 1600kg each, and you can combine up to three anchors on a base-plate to create a single anchor point.
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Baring in mind that you have created essentially a 12m "sail" I think 2 ton of ballast per direction is low.... a light 30mph wind on that surface area is going to be roughly the same as hitting the frame with a 1ton object. You either need some proper ballast on those legs or you need to investigate staking it to the ground (rule of thumb, a 1m stake properly driven in to the ground will match 1ton of good ballast) which will obviously take up much less transportation room but presents some complications of its own.
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My first thoughts as well Tom, Cheeseweasel, however looking at that photo and imagining trying to bash pins in along Berlin's equivalent of the Mall seemed somehow a forlorn hope.

 

Check the Victory Column in the background, that finish line is right in front of the Brandenburg Gate and right in the middle of the road. If that is a typical site then gaining permissions through German bureaucracy is going to cost more than any amount of trucking fees.

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One solution I have seen used before is to use suitably branded vehicles as all or part of your ballast. Admittedly it doesn't meet your criteria for maximum one metre height but suitably logoed up, it might be made to work.
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having designed many outdoor structures in my career I dont think that 1.3t x 4 ballast is entirely unreasonable. as has been said before you have created an extremely large sail with all that branding.

 

water ballast is a great cost effective solution but you need a lot of it to achieve the desired end weight. so if you have a physical limitation then that is not good.

 

have you looked into lead weights. as lead is heavier than steel you can reduce the footprint of the ballast considerably. the trade off is price as lead is more expensive.

 

also (as Kerry said) if you increase the length of the base of each tower then you should be able to reduce the amount of ballast down.

 

there is no real solution to all this. if the specified ballast is 1.3t x 4 then that is what you need to use. if the cost of transportation of it is high then pass the cost on to your client. you need to weigh up the different costs involved and present the best one to your client.

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Kentledge - new word to me...

 

I always assumed it was the brand name or manufacturer of the big concrete ballast blocks you see around, known (in the UK at least) as kentledge.

But no, apparently it is a historical word for scrap iron used as ship ballast, "perhaps from Old French quintelage" according to the dictionary. Another useless fact learned...

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From the photo, it looks like the signage is solid. Would the use of a screen-printed mesh banner reduce the efficiency of the structure as a sail, and therefore allow the authorities to require reduced ballast?

Something like this, perhaps (although more permeable alternatives may well exist):

http://www.sunbabaeventbranding.co.uk/2012/09/ultramesh/

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Mesh is better but no-where near as good as you'd imagine - the actual surface area/sail is only reduced by 25% BUT the turbulence caused by the air passing through the holes reduces the benefit significantly; I'd only expect to see a 10-15% reduction in bracing/ballast on something mesh based and that would have to be weighed up against the reduced image density/clarity that mesh printing offers. The giant stage wraps commonly seen at festivals are usually an exceptionally open mesh (so that they have a significant wind benefit) that looks terrible up close but terrific at a distance because they are such low-res.
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most structural engineers that I know dont care if the surface is mesh or not. they just see a big sail (mesh or solid) when working our their calculations. so I would not count on this reducing the ballast in anyway.

 

the stage wraps at festivals etc are mesh for another reason. they normally hide ugly speaker hangs and still allow reasonable acoustics.

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The next problem is that the ballst should not exeed a hight of max. 1m because nothing but the advertisement is ment to be seen or else the sponsors would be bitching around.

 

 

If its the safest way for doing it keeping the sponsor happy is not a valid excuse.

Im sure theyd rather have the needed ballast than their logos all over the structure as it falls over onto people

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I don't know if a similar thing exists but we've used Heras fencing feet for this sort of thing before. The slots you get in most designs mean you can get a ratchet strap through them to secure them to the structure and their small individual size means you can make them quite discrete. Although we've never done that much weight before so the number of blocks you need might get silly.
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