Temporary Structures

[DSA]

I have been meaning to post these for a few days, and whilst I realise that Kerry Davies has already posted a link to the SCOSS report in another thread (See here), I think this is an extremely important topic which deserves a thread in its own right (it would be deviating from the main topic if discussed on the linked thread).

 

Therefore, if relevant, please read the following (concise) reports relating to temporary structures. The SCOSS (Standing Committee on Structural Safety) alert document is a short report on the matter, whilst the CROSS (Confidential Reporting on Structural Safety) report is the publication of a concern received in confidence by the panel (number 276 is the relevant one from the newsletter). The two reports have obvious links in that they deal with similar issues, however are different in content.

 

SCOSS Alert: Link

 

CROSS Report (within Newsletter No. 25): Link

 

These documents first came to my attention via the CROSS newsletter, which I receive on a regular basis. I am posting them here as I believe that they would be of significant interest to the Blue Room community, and may spark some discussion.

 

David

 

P.S. Moderators, you may wish to split off the SCOSS discussion from the other thread.

[cfmonk]

Very interesting. In particular the bit about using IBC tanks as ballast as this is extremely common. It would be nice to know what they considered correct practice in that area.

 

Chris

[johnlinford]

The problem that strikes me here is two-fold - firstly, we've not had an accident in the UK since 2006, and I suspect things have improved with the high-profile incidents abroad in the last year.

 

Secondly, and more importantly, for all the good intentions in the world by those designing the structures, the people putting it up on site are unlikely to have any great understanding of structural engineering, and probably won't appreciate that not filling up fully the IBC tanks, or moving the guy line slightly to make the promoter's path work will have much further reaching effects.

 

This is also not helped by those "in charge" of events often being PR agents or promoters with not a jot of technical knowledge between them, and not appreciating what's been on the plan for months and why we can't "just move it a little bit that way because I didn't realise...".

[ImagineerTom]

Correct / best practice is stakes (or multiple stakes through a ground anchor). A 1.2m steel stake, properly angled (into a wide variety of ground types) will take a metric ton of pull quite happily with no movement.

 

Water tanks & concrete ballast might "weigh" a ton but can be quite easily pushed/slid along the ground by a single person if there's a bit of gravel/debris underneath (acting as rollers) as they rely entirely on friction for their movement stability. I've refused to use ballast for precisely this reason and have gone to great lengths to come up with a solution using stakes precisely because they are infinitely more reliable and stable than ballast.

 

Switch to a "screw" stake and you can multiply the strength dramatically - there's a 40cm screw stake we use occasionally that'll take 3/4 ton; Some of the mega-tent providers have 1.4m screw stakes that are tested to 4ton!

 

T

 

(Family business is circus tents so wearing my tentmaster's hat)

 

 

[Simon Lewis]

The problem that strikes me here is two-fold - firstly, we've not had an accident in the UK since 2006,

 

Bearded Theory stage collapse 2009...

Birmingham crowd barrier 2009...

[kerry davies]

Thanks David, really good to have someone from the engineering safety point of view here.

Chris, good that a relative newcomer to the game is honest enough to ask relevant questions. The problem with Kentledge is not so much that it isn't effective as that promoters moan about the sight and the cost so it is sometimes most relevant in absence.

Tom, great to have a born tentman on board to share hands-on experience, I am not specifically tent based but as an outdoor event guy know much more than most BR members. Frightens me sometimes! :o

John hits the nail on the head, it isn't about good practice, it is how do we get promoters to shell out for that with no monetary return obvious to their accountant.

 

Just to make an opinionated statement, the UK is well ahead of the game in structural safety but our industry is international and there are too many accidents occurring overseas which affect the UK industry. The US has just issued a couple of pages of guidance whereas we had the IStructE guide 5 years ago with 100 pages. We have the rule book, if you like, now we need to get everyone to play by those rules. Hopefully the ILMC Production Meeting (IPM) which is discussing the subject in March will come up with something (see My link for attendees and agenda.)

 

We need more people willing to say no to promoters, as the engineer in Cheltenham did to the racing Festival, and be able to stand by that decision under enormous pressure. Two supposedly good aspects of the events industry are the "can-do" attitude and "the show must go on" culture. Both of these have their downsides and we need to address them. Sometimes the show MUSTN'T go on and often you CAN'T DO and an expert has to be brought in.

 

The instance where an external CROSS "reporter" examines our practices is a good move and the IPM meeting where we can address the problems with the promoters present is one way forward, IMO. We do need however to put our own house in order and make those good practices the norm.

[cfmonk]

Hi Tom,

 

Thanks for that. I know stakes are always a preferred option, they're a damn site quicker to knock in the ground rather than wait for Mrs Miggin's garden hose to spit out 1000 litres per tank! However, clients tent to get a bit shirty when you ask to knock 40 x 2.5" holes into their shiny new tennis court / driveway or you are building on top of reclaimed land where you are absolutely NOT allowed to penetrate past 15cm for fear of releasing black death (or some such) such as most of the land outside the O2. Therefore using ballast has to be done at times. Furthermore the specifications from both Losberger and Roder (who make my tents) refer to WEIGHT per leg and do not specify that base plates need further protection from slipping laterally (if it did then they would have to specify the strength of the material being used in stakes etc).

 

So using ballast is not the worst thing in the world and can be done safely (or else I wouldn't be doing it), we secure the baseplate to the tank to prevent slips and then guy down from the top of the leg to the base of the tank (N.B. not the cage of the tank!) as well as having the tank on the base plate. Now, I would consider this adequate as it exceeds the manufacturers requirements for weight on base plates and further applies forces in all the correct places (based on my (yes I know short) four years experience) plus the combined experience of all the other companies who I know use the same method (and are who I learnt it from) who I have a good amount of professional respect for.

 

This is getting rambling so I shall finish!

 

My point is that it is not particularly useful to include a line which says that often the securing methods used with IBC Tanks are insufficient without specifying neither WHY (is it because they are often absent / underfilled / moved a bit or because they are fundamentally unsafe or because people do not secure base plates to them) nor specifying what the correct practice for ballasting should be in excess of the specifications of the designers of the original equipment.

 

Chris

[ianl]

doesnt it come down to what your trying to do with your balast?

 

in a simplified way...

 

balast on a tent is generally at the bottom of a leg, in order to stop the leg lifting when the wind gets under the tent, therefore little risk of sliding

 

balast on the big ground suport stages is often used a long way away from the bottom of the leg with a wire attached to the top of the structure at 45 deg (or so) to try to stop the top of the structure blowing sideways compared to the feet. this is obviously where sliding would be a big problem and pegs should be used.

[ImagineerTom]

Yeah, ballast on shed marquee legs is a very different situation to guy rope ballast (either for stages or tensile tents) because it usually is combating a direct upward force (so it's working in its optimum plane) and there's usually a half dozen short stakes in the plates stopping sideways motion. What's being discussed here is positions for guys and there's usually a fair range of possible positions for the guys (so you can choose positions where staking is possible by using a slightly longer or shorter guy that gets you to the patch of ground that will take a stake) whereas shed marquees have only a few inches variation possible on their base-plates.

 

I'm well aware that there's complications with using stakes but it's always been worth the hassle because we've ended up with good strong points of a known strength that we know can't move. You're talking to someone whose family have put lots of stakes in at the O2, at various stately homes, world heritage sites and all the other "problem" locations you could possibly imagine. A combination of good planning from the outset and a range of different stake styles mean we've yet to find a situation where we've needed to use ballast. Digging a couple of test holes to find out what's under the ground, conducting a few pull tests to see what the ground will take. lifting a few paving slabs or piercing thru a bit of tarmac (which is so easy to repair afterwards) and spending a couple of hours playing around with CAD to work out if by moving the structure 2ft to the left enables you to use better stake positions are all the sort of things any good tent-master will do and there's really no reason why these sorts of practices aren't used for outdoor stages. I honestly can't see any legitimate reason why you should NEED to use ballast instead of properly sited stakes/ground anchors for guy ropes / outriggers.

 

 

 

 

//gets off soapbox//

 

 

[Junior8]

Switch to a "screw" stake and you can multiply the strength dramatically - there's a 40cm screw stake we use occasionally that'll take 3/4 ton; Some of the mega-tent providers have 1.4m screw stakes that are tested to 4ton!

 

T

 

(Family business is circus tents so wearing my tentmaster's hat)

 

That's good advice. The first time I saw these being demonstrated for portable structures I was very impressed and the smaller ones go through tarmac with very little damage easily repaired by a quick patch up. I have seldom read so much good sense as in the post immediately above this one.

 

After reading the reports I would once again draw event organisers' attention to Mr Justice Fosket's judgement in the Dreamspace incident.

[kerry davies]

We are getting a little specific with this. Chris asked why correct practice was not explained in the document which is fair enough. The answer is that the reporter is a local authority officer giving a subjective view and not a technical audit. Those erecting structures using kentledge may be the problem, but as John writes, it could equally be the promoter/client responsibility. The writer does not go any further than reporting the situation.

 

As far as never having found places where staking was impossible I have to disagree with Tom, besides the tennis courts at 5 star luxury hotels belonging to famous people I can take him to a regular site in Bristol where the Cat scan shows up a cobweb of 3 phase supplies just below the surface and two feet to the "left" is on a boat! You could also get murdered if you even thought of putting a two metre spike through the surface at the Millennium Stadium in Cardiff.

 

The subject of structural safety is a broad one with a multitude of aspects like kentledge and staking and, generally speaking, we are pretty conscientious as an industry in the UK. Simon's reminder however means that we cannot afford complacency. The current topics seem to me to be about the culture of safety in temporary structures at events and how we improve that culture. Not necessarily about individual techniques.

My link

Take a look at section 6 and note 6.3 where there is reference to a HSL report on ground anchors. I cannot find a free copy but it can be bought from the British Library, allegedly!

[cfmonk]

Your solution to staking at the O2 would be greatly appreciated. We have been told in absolutely no uncertain terms that we have the ballast and not stake. If we don't agree to that we don't get the job!

 

Would much rather stake, quicker, cheaper (on transport) and I don't need to bother with a stand pipe permit!

 

Chris

[ImagineerTom]

@ Junior - Thankyou

 

@ kerry - 3phase power cables under the ground. Either get a proper survey done and place your stakes accurately. If it's a regularly used site then consider arranging for some permanent anchor points to be installed or use of of the non-conductive, non-cable-piercing stakes that are out there designed for similar situations. Putting marquees on to tennis courts is really something only ever done using sheds or other small structures so is irrelevant to this discussion. My uncle once put a big-top up on the roof of a famous London cinema 30 years ago to host the after-party of a film launch. Again careful planning enabled them to use existing points and clever brailing to create all the anchor points they needed without using a single bit of ballast.

 

@cfmonk - PM me and I'll take you through some options/suggestions (it's a big site with plenty of variations)

 

 

[tom w]

A few years ago the construction industry changed with regards to water-based ballast solutions. The principle being that any breach would lose some or all of the water/ballast. Water ballast now has to be subdivided into separate chambers so that a single breach will only lose a fraction of the whole.

Our industry is generally only a few beats behind adopting most of the legislation which is applied to the construction industry. We had previously used water ballast bags which locked directly to the layher bays. We now use concrete blocks beneath or beside the structure. On some occasions the ballast is placed onto a lower deck. Either way, the load is applied directly to the legs of the platform. The obvious downside - haulage.

As for stakes. They are absolutely a good solution, but not for every occasion. Some sites charge per hole. One job I did many years ago now had 100 tonne of water ballast built into the structure. The alternative was around 60 holes charged at £7000 per hole. Now that's a well paid ground-keeper!