Ground support collapse Jerusalem

[Junior8]

In the notes with the calculations the weight of the actual structure (frame and canopy) are not considered at all because that weight is considered insignificant compared to the upwards force as a result of the wind getting under the canopy and from the aerofoil effect.

 

The canopy is considered as a enormous aerofoil and, if googled, the lifting force on an aircraft wing comes mainly from above the wing itself. To this end we have stanchions inside the structure which have no function whatsoever in supporting the weight of the canopy, but, are there to prevent the canopy and frame being lifted vertically! And ending up in England.

 

Well yes - but to anybody who's worked with demountable structures in the real world that wouldn't need saying. The plain fact is that you can do all the calculations you like but in the real world structures can behave in quite strange ways when subjected to wind and it needn't be that strong either. Twelve years ago I was interviewing a market trader at Ocean Village in Southampton on a pleasant summer day. He was a fashion worker with a box iron frame stall, fitted with integral hanging rails, measuring probably 20x40 feet fully loaded with stock. The all up weight was probably near three tons all told. Suddenly out of nowhere a gust of wind blew under the canopy and lifted the whole lot in the air moving it six inches along and dropping it again. OK there were no weights on the legs but nobody would have bothered on that day either. Like many such stall and covers, of all sizes, the main mechanical and structural integrity is provided by gravity. Had the roof principals lifted high enough for the bar end pins to come out of their sockets (I never thought they were too long again) it could have been very nasty. Outdoor workers know there is no point relying on weights alone, I have seen 56lbs easily dragged sideways by a guy rope, and that there are times when you simply have to give up the struggle and take the sheets off.

[ramdram]

Well J8, that bit might have needed saying to the engineers who designed the entire "system". Did we not read too that on earlier occasions (so not an unknown occurrence) the guys were tied to the military vehicles/jeeps AS WELL AS when the wind got up.

 

Having said that a thought occurs in that quite a few decades ago I used to be taken to the circus. The big top was anchored by many, many metal stakes driven into the ground...I know because I tripped over one. Year or two later I went to the Bath and West Show in the same place, again the marquees were anchored with huge metal stakes.

 

If you have ever used one of those metpost doodads then you will know that getting even the smaller versions out of the ground is very hard work (well for me anyway, ** laughs out loud **).

 

Perhaps these days it is frowned on to make holes in the ground and you are obliged to use a weight? Did we not read that one of our members was obliged to use water ballast? Strikes me all this "new" technology is not quite as effective as a stake in the ground...at least when it is ground and not a concrete base say. I would suggest then that rather more weight in the anchors dept' is needed.

[Simon Lewis]

.... and that there are times when you simply have to give up the struggle and take the sheets off.

 

 

Something that seems to be common in the past few collapses: there was no quick method of making the structure safe. The Jerusalem ground support had lighting suspened on truss under the main structure, but cabling seemed tied off to the main towers, and there were bars on the towers too. It would have been difficult to lower either system quickly.

 

The Indiana roof structure had been taken up, lowered onto steels and then secondary loads (lighting, video etc.) added and flown out. There did not appear to be a method of removing panels to reduce wind loading, and the main motors (apparently) would not handle the fully loaded structure.

 

Of course, this isn't wrong as such, but given the control measure of at least one system is "if it gets too windy, lower the structure", it seems strange that the systems are erected without the ability to be dropped in the time period needed if this is to be a workable control measure.

 

Simon

[paulears]

I'm quite happy to admit I know little about these structures so can I ask a few questions based on what I do know, and common sense?

 

Looking at the pictures, the structure seems to be large format truss, with a very long left right span, supported by smaller cross section truss. Now - in my head, this suggests the top rectangular section weighs considerably more than the supports, even before rigging the kit. Thinking back to my meccano days this suggests a structure that has fundamental stability problems, and the forces, if the upright truss starts to move outside the safe area would be very high - and to my untrained eye, make me wonder about the forces necessary to keep it stable, once it starts to go. tethering each corner with a cable and ground weight, to me as an amateur in this area, would appear to be insufficient. Since these topics started, I've been looking at concrete blocks I see laying around, and seeing a fairly small loader lift one up and move it made me think how easily a structure would do the same thing once it started to move.

 

Being a structure beginner, why is cross bracing never used? To my meccano brain, surely another piece of truss starting at the front bottom going up to the top of another vertical support and then fixed rigidly would produce something much, much more stable. Cost rarely seems to be a driving factor, so surely a couple of rigid truss triangles at the sides and again at the back would provide 90 degree horizontal to vertical angles that could not change, even with high winds. Looking at the typical stage style structures, they seem to be lacking all the features the Victorians built into their structures that had little maths and excess metalwork - and are still standing. On the Seafront in Great Yarmouth is a 100 year old 'greenhouse' .

http://www.english-heritage.org.uk/content/images/landing-pages/square-images/winter-gdns-gt-yarmouth.jpg

All that keeps this up are triangles - even with a huge capture area for wind from all four directions. The glass sometimes fails (hence why it's not open), but despite the lack of maintenance, it never falls over. I just cannot understand why truss structures get built without these very basic physics features. Did anyone else make things at school where you designed stable structures in physics? A heavy roof held up by cocktail sticks mounted vertically needed very little side force to collapse - yet with a few glued on cross-beams, side force simply lifted the bottom from the table on one side - the structure remained it's integrity.

 

Why are inherently unstable structures produced, then kept upright with a bit of string and blu-tac (concrete and cable)?

[timsabre]

Why are inherently unstable structures produced, then kept upright with a bit of string and blu-tac (concrete and cable)?

 

For the self-climbing type structures, as most of them are, the aim is to be able to build it with minimum working at height, and fit it in the minimum number of trucks. Guy lines take up no space compared to truss and in theory you still get your triangles. Obviously not as strong as truss... but strong enough until the calculated limits are exceeded...

[Junior8]

Obviously not as strong as truss... but strong enough until the calculated limits are exceeded...

 

Or until the rigid side of the triangle fails!

[ramdram]

Why is cross bracing never used? A very good question. Well Paul have a look at this:

 

Concepts for designing stiffer structures

 

from here:

 

personalpages.manchester.ac.uk/staff/tianjian.ji/.../d2-concepts.pdf

 

This paper is full of diagrams of nothing but cross bracing for scaffolding design.

 

In fact the paper cites a scaff structure failure in '93 which failed because of no cross bracing and even states, (my italics):

 

...the cause of the incident may be explained. It can be seen that in

this scaffolding structure no diagonal (bracing) members were

provided, i.e. no direct force paths were provided, which produced

a structure that did not have enough lateral stiffness and it

collapsed under wind loads alone.

 

So even if this was the first instance of wind caused failure, and that is extremely unlikely, it has been known for the best part of 20 years that lack of rigid cross bracing is a contributory factor in the failure of scaff structures.

 

Ref the self climbing structures..it begs the question if you cannot brace them adequately or only to certain lateral forces then why are they being used in known windy locales..at all?

 

From the recent threads it is difficult to ignore the seeming fitness of such arrangements...and question the faith of the designers of said design arrangements.

[kerry davies]

Splitting this up into constituent parts.

Simon, I was trying to work out a method statement for one of the earliest small climbing roof structures with Penn Truss when I did exactly what you point out as a No-No. The difficulty comes when add-ons are incorporated without redesigning and re-engineering the totality.

 

Ground anchors and Kentledge is a subject all its own. The most crucial element of any TDS is the anchorage and Kentledge is only used where fixed anchorage cannot be used. Mutamarq, the marquee mans bible, states that every anchorage point not pinned should have "several tonnes" of Kentledge. The normal cubic water carriers hold 7,000 litres or 7,000 Kg of water and neither Jerusalem nor Indianapolis got anywhere close to what would be acceptable in the UK. 1 cubic metre of concrete is about 2000 Kg and the Jersey barriers go up to around 3,000 Kg each only.

 

Neither would we use guy lines at the angles seen at Indianapolis or place Kentledge on a garden wall as is rumoured at Jerusalem. When we are expecting more than 25 MPH we double pin each marquee leg and add a load strap on a multi-pinned plate at each marquee leg.

 

From my PoV the design of aluminium frameworks similar to those discussed has a major flaw in that the tower bases are often not fixed. Any slight sway or bend from the precise vertical creates forces tending to move the base which then exacerbates the instability of the entire structure. That is why Kentledge for stages or big marquees is shown attached to or guyed very close to the vertical supports if not built into the bases.

 

Paul says that there is little maths in Victorian structures but since the time of the Pharoahs and Archimedes there has been a knowledge of "Sacred Geometry" which is purely a layer of spirituality (or mumbo-jumbo depending on your PoV) on top of genuine scientific and engineering principles such as the 3-4-5 triangle, Fibonacci sequence, golden ratio etc. What a lot of people don't understand is that cast iron actually gets stronger with age and at Ford we left our castings out in the open to strengthen them before machining. Odd but true.

 

On a lighter note for Ramdram, a little bit of interweb looky has revealed that there are neither trees nor marquee companies on Shetland, what do they know that those in Tornado Alley do not? If you want to see some tidy scaff bracing go back to early Knebworth, Live Aid and U2 tours in the 70's, 80's and 90's. Methods don't change much.

 

And for Junior, Hi Roger, half a hundredweight is peanuts, I once saw a 14 stone cazzy fly 30 feet in the air on a circus tent rig. Some people don't listen when you tell them never to wrap a rope around the hand.

[Junior8]

And for Junior, Hi Roger, half a hundredweight is peanuts, I once saw a 14 stone cazzy fly 30 feet in the air on a circus tent rig. Some people don't listen when you tell them never to wrap a rope around the hand.

 

Perhaps I should have said this was on a 2x2 stall but I agree with the general point. As for not pinning I really do think many are too precious about this. With modern corkscrew pins, which have much greater restraining qualities for the cross section of the bar, and a bit of care it is surely possible to pin into most surfaces with minimal and easily repairable damage. And for years now I have been covering town centre regeneration projects - often mentioning 'entertainment space' - where numerous anchorage points for all kinds of purposes could be easily and cheaply included as part of surfacing and utility work but seldom are. Decorative novelty fountain? Yes! Cast iron street furniture? Big Screen? Bring them on! Tie-downs? I'm sorry what do you mean....

[paulears]

Ramdram - any chance you could fix the link - it's broken!

 

Kerry - just behind the structure I mentioned was 98 year old theatre - which was in a very poor condition, so it was demolished, and rebuilt in modern materials to look exactly the same, but now contains a bowling alley. The only things re-used was the Victorian metalwork - which was determined to be in excellent condition,so was cleaned and painted and is in use again holding up the structure in it's second century. The structural engineers were in so I asked them about suspending a long truss from the beam - and enquired about load? They asked what I had in mind and smiled. It seems the new materials in the building are so much lighter that the capacity was huge, now it wasn't holding up the roof. You do have to be impressed with the Victorians.

[ramdram]

Paul, see what you mean about duff link. If you copy and paste this:

 

Concepts for designing stiffer structures

 

into google you will see the usual listings and the link is the first one.

 

Ref Victorian design, my dear old dad (ex RN, sailed with Nelson, he swore...and latterly craft teacher) explained that cast iron, once covered in rust and marine crustaceans, as in the archetypal "seaside pier", would last "forever" as once the cast iron was covered in a layer of rust then said rust layer prevented any further rusting, or more likely the rusting process was slowed down almost to a standstill.

 

If you have ever been to Kew Gardens and seen the glass houses then you might appreciate cast iron...and then the later stuff...all brilliant.

[paulears]

I managed to pursued Google to give me the page, because your link was completely missing this time - how very odd?

 

Anyway - here it is click here

[the kid]

My question in relation to this topic, and like Paul I have little knowledge on the subject. How should stages be in relation to the roof? Clearly above :P but should legs be outside of the stage area or inside (ie placed on top), both being strapped down etc. Is there anything to say or is it down to MI ?

 

I ask because I have been looking around at various stages some seem to have a roof round the stage, but a very few on the stage.

[kerry davies]

Any way that you want it, Kid, as long as it is designed so to be.

 

Stage covers can be staging mounted, spanning staging units or fully integral. You say you haven't seen any stage mounted yet the trailer based kit is nearly all "stage" mounted or integral. The early Orbits were all stage mounted and Saddlespans often are. The really big stages are nearly all integral systems and the free-standing roof systems are normally seen in stadia.

 

Whichever design is used the anchorage is the bottom line and in these two cases that is what failed initially. It is important to note that the Jerusalem unit was not a roof but a trussing framework and whether it was even designed as a free standing, structurally engineered construction is questionable.

 

If you google the IStructE guide to temporary demountable structures (which is free somewhere on t'interweb) that will answer your questions and the Star Events Staging website is informative on much of what puzzles.

 

As for the Victorians over-engineering their structures I sometimes think our labour-saving, mass-produced, just-in-time-resource-flow, cheap-as-chips culture doesn't engineer at all. I see no sign of it on the Jerusalem job in particular but Indianapolis was also an accident waiting to happen.

[ramdram]

Kerry, your last para. Did the Victorians over engineer? Or do "we" under engineer these days (in some circumstances) in the perhaps over confident belief that "our" new fangled truss is "better"designed or fabricated?

 

The photos of the truss seemed to indicate the welds were not all that brilliant.

 

Perhaps the Victorians did not have all the maths behind their designs but we are surrounded, still, by Victorian structures everywhere.

 

(If for example you look at the Tamar railway bridge (1859) then it is looking tatty because "we" have neglected to keep up the paintwork...yet it still conveys fortune favoured foreign folk from that country attached to Cornwall to the delights of our Riviera, beaches included, and even our picturesque moorlands, haunt of the invisible Cornish black pumas and similar...revealed only to those who have self medicated with the likes of substantial volumes of Tribute or Doom...ahem.)