Подробнее на сайте: https://themes-wp.org
http://live-code.ruhttp://gromder.net Go to top of pagehttp://collect-pc.ru
http://codingway.ru http://run-pc.ru
http://hightechcomp.ruhttps://python-scripts.com http://python-3.ru http://collect-computer.ru
http://computermonster.ruhttp://hardcomputer.ruhttp://build-biz.ruhttp://pro-java.ruhttp://pythonlearn.ru

Newsletter 1

White Paper – Examining temporary structures

Issue 1 – Post seminar 2012. Circulated 17/04/2012

How strong does a temporary structure need to be to meet current guidance? Ask a hundred engineers and you will get 110 answers. The current guidance on wind loading of Temporary Demountable Structures (TDS) is confusing at best.

The question we ask at this point is, “How strong does a TDS need to be, to be safe?”. In this document, we will attempt to cut through the fog, examine the issues and provide a simple conclusion to clarify the situation. Where we cannot find a conclusive answer, we intend to query the relevant bodies for such a clarification.

Let us consider this fact. The temporary structures industry in the UK has an admirable safety record. But prudence and legislation require the designers to strive to improve year on year. The unfortunate incidents of last summer is making many of us look at our own working practices. TDS such as concert stages, tents, display structures and vehicles, temporary screens, temporary roofs, canopies, lighting towers, speaker gantries and other structures as simple as fencing need to be engineered to resist the environmental and imposed loads which are typically applied in their day to day use. At our recent seminar, Amanda Ramsden of Momentum asked, “Why don’t we design stages that can’t fall down? A simple answer is that we can, but at what cost? The million dollar question is “How do we design, build, install and safely use temporary structures which balance safety performance against the cost of doing so?”

Designers of TDS have no legislation which covers the design and manufacture of such structures and have to rely on their own experience and judgement based upon published guidance, and if common sense prevails, current guidance. This guidance I believe should be compulsory to provide a level playing field. This LAW, would then cover most day to day engineering tasks. Parameters which are clearly defined are easy to meet. Costs will be more controllable with the use of predefined parameters, whereas safety standards could be compromised by building structures to the most favourable guidance instead of to the locational parameters which may be encountered.

Following our recent seminar in Sheffield on Examining Temporary Structures, a few issues were highlighted;

  1. - Five different guides for wind loadings, two of which are superseded, are quoted in;
  1. EN BS 13782 (Temporary structures –Tents) 2005
  2. The IStructE, Temporary demountable structures. Guidance on procurement, design and use, Third Edition 2007
  3. BS EN 1991-1-4: 2005 - Eurocode 1. Actions on structures. General actions. Wind actions.
  4. BS 6399 Part 2 1997 - Loading for Buildings. Part 2 Code of practice for wind loads.
  5. CP3 Chapter V Part 2 : 1972 – Wind loads
  1. - Building regulations have the force of law but generally are not applied to TDS. Yet the HSE have issued a clarification of the situation at this time which states that the question of whether CDM will apply on larger marquees and tents will be judged on a case by case basis.  This is not an acceptable situation for long term planning as CDM tends to be site specific and therefore site specific documentation would need to be prepared for individual or perhaps every job.
  1. - Many stages such as ground support rigs (a type of stage roof with vertical columns that relies on bracing cables for most of the lateral stability) and open frame canopies require site to site variations in the method of placement of bracing cables. More documentation work may need to be done if the contractor changes specification from site to site.

The FUD Factor

The variance in advice leads to Fear, Uncertainty and Doubt. TDS 111 (The IStructE, Temporary demountable structures, Guidance on procurement, design and use. (Third Edition) (2007)), which gives excellent guidance on many areas of temporary structure use, states that BS 6399 should be used for calculating surface wind pressures, when the new EN BS 13782 supersedes (or appears to supersede) the older standard. Highlighting this is not intended to criticise a valuable and useful document like TDS 3 but to show the confusion that is present in ALL guidance at this time. My opinion is that the most current standards and guidance should be considered when designing new structures and that older structures or developments of older structures should use the standard in use at that time .  This is  the viewpoint of TDS 3 but it does not show the current situation.

EN BS 13782 has a definitive guide to loadings which should be a minimum.

Table 1 — Wind loads

 

height: h

pressure: q

m

N/m2

h < 5

500

5 < h < 10

600

10< h <15

660

15 < h < 20

710

As simplification, the values given in Table 1 may be applied with the distribution shown, Contrary to the pressures specified in Table 1, a reduced pressure of 300 N/m2 may be applied in the case of tents

with a width of 10 m or less and height of 5 m or less.

Alan White, of Alan White Design and Newsletter 1 | Special Structures Lab has done a study of the effects of using the different codes and guidance. By simple calculation it can be seen that there isn’t much difference between the codes and guides in any case. So why not use one standard? See his results in the table below.

Code

CP3 Part V

BS6399

ENBS 1991-1-4

EN BS 13782

IStructE

Wind Speed m/s

33.88

31.77

29.42

31.29

30.82

Wind Pressures kN/m2

0,70

0.62

0.53

0.60

0.58


Conclusions from the Seminar

I was asked many questions at the seminar which I promised to answer as soon as it was possible. The following response is the result of studying those questions from the attendees of the seminar. Please bear in mind that as a designer and engineer I must state that these are my personal views, not those of any body or organisation. If not sure on any issue, take professional advice.

Definition of Temporary Demountable Structure

Any structure that can be readily and completely dismantled and removed from the site between periods of actual use. The structure may or may not be authorised for use at the same site from season to season or from year to year. Not to be confused with relocatable buildings that may only be relocated once or twice in their lifetime.

Wind Loading

Wind loadings on temporary structures are basically what determine every part of the structure, be it a fabric covered stage roof, a hospitality marquee or a big top type structure or pole marquee. The wind loads are calculated using coefficients of pressure (Cp) that depend on wind speed and direction, the slope of the roof surface, and the area of the surface. These forces are distributed by the roof of the structure into the main members such as trusses or the poles on a tent. From there the forces have to be accepted by either bearing pressures on the ground, by the stage or platform on which the structure is stood, or by the tie backs such as side guys and bracing cables. The efficiency of these ground plates, the ground itself, the supporting stage or platform and the ground anchors are what eventually accept the wind forces and other loads, yet these are the very parts which tend to fail first as they are often unknown prior to arriving on site with a temporary structure.

The difference in surface loading between (say) 28 metres per second wind speed pressures and 36 metres per second which is nearly double. This is because surface pressures increase with the square of the wind speed.

Stakes and anchors

The question of anchors came up at the seminar. As everyone knows, when the stake or anchor is underground we can’t see what is there. The stake could be 600, 800 or 1200 millimetres long. More than twenty years ago I came up with a scheme, jokingly called Rudi’s Rings! The name was kept as it was thought that it would help installers remember it. This scheme is so simple I still don’t know why no one uses it. The different lengths of stakes are colour coded with a different colour for each length. The examiner knows that if the stakes have a red ring and the frame is 12 metres span with four stakes on each base plate it is probably OK.  It needs all contractors agree to use the same scheme or the examiner will not know which is which with without a chart. See the chart below, more on http://www.specialstructures.com/rudi-rings-tent-stake-standard.html.

Guidance

Examining temporary structures can and will be more efficient and more transparent when variables and inconsistencies in guidance are removed. The examiner needs to take an holistic approach to the inspection. He or she may have never seen the structure being examined before and as with all professionals, will have to fall back on experience, training and basic engineering knowledge. The examiner needs to identify the main constructional elements, assess the risks involved with each part of the erection and use phases of the installation and consider which procedures are being used to minimise the risk. A history of previous installations recorded in the TDS Technical Handbook would allow a different perspective.

Suspended Loads

With stages, the structure is not a stand-alone issue. The roofs of stages are used to suspend large and heavy loads often with minimal lateral restraint. In touring shows such as Walt Disney On Ice, Holiday On Ice, MTV Music Awards or on many, many festival stages, tents are required to suspend huge lighting, sound and effects rigs with Disney using a 44,000 kilo rig and at MTV Roma 90,000 kilos of Mark Fisher’s finest effects for world TV use.

So in addition to the basic structure, the roof must also be looked upon as a load- bearing structure over and above the environment loads such as wind. For this reason when a basic structure changes from simple cover against the elements to a temporary venue to house a performance, additional examining procedures come into play.

Both of these examples show the need for the examiner to not only concentrate on the stand-alone structure but also to consider the impact of what equipment is being used in or near or suspended from the structure. The additional loads are often not considered as the structure is examined immediately after erection and before ‘load in’ starts. Ground bearing pressures on major elements of the structure will be affected by the imposition of the additional loads. Often, prior to the erection of the structure, minimum soil bearing pressures can be determined without the time constraints of the build. I heartily recommend that more engineering protocols and controls are put into place for those installations that are large enough to have a major social and economic impact on the locale.

Self imposed standards for the industry

Historically, temporary structures were dismantled and tents lowered to the ground if bad weather was imminent. In today’s financial environment, where festivals turn over tens of millions of pounds and the local area sometimes double that, it is unthinkable for the organisers to consider shutting a stage, so the solution is to have higher standards, even if self-imposed, to ensure that events are safe and that technical, financial and artistic risks are minimised.

The best possible way that the event organiser can ensure that a stage is not shut due to a preventable procedure is to establish higher standards for main stages with huge audiences in front of them and tented stages where the crowds may take shelter in bad weather. Although not all of the facts are known about the happenings at Pukklepop, the youtube videos clearly show people running into the big top for shelter while it is in the process of collapsing. On the six pole tent, five poles had collapsed leaving one pole standing with an incredibly small steel wire rope cable holding it. Although it finally gave way apparently, that last pole staying up probably saved many lives. Less than one hundred pounds (in money), the cost of a length of wire, saved many lives.

Based upon this apparent occurrence (If anyone has conflicting evidence from Pukklepop, please advise me.), perhaps we should take extra precautions when an audience or festival crowd may try to enter a stage that is already at capacity. Perhaps the wind loading criteria should be higher if the structure could be used as shelter in emergency.

Use of anemometers attached to structures

At the seminar, Professor Pavic pointed out that the use of anemometers on tents and stages cannot be trusted as the wind flow over the structure can influence the reading by a large factor. More work is needed on where to position the anemometers.

Future design of temporary structures

Engineers are required by law, professional ethics and building codes to exercise due diligence in their work. Only after a thorough appraisal of the structure including the use of sophisticated analysis tools and of course previous experience with similar structures whose performance is known, can the engineer sign a document which passes the design as shown in the documentation as safe. Under the proposed future implementation of CDM (The Construction (Design and Management) Regulations 2007) which in any case is current UK law, although not implemented at events at this time, the contractor must involve the designer and the client in the design certification and signing off process.

This can be helped by keeping historical data of the ground conditions at sites with regular year on year usage to provide data for the design of ground anchors. Information such as this and the possible future use of live performance measuring of structural members as described by Professor Pavic at the seminar can help to make future structures inherently more efficient and safer than their predecessors while providing advance warning of potential failure.

Competent persons

Current government legislation such as CDM and guidance’s such as TDS 111 require the structure to be transported, erected and signed off by a ‘competent person’. The usual way for government to define competency is by qualifications. In the event industry many of the jobs carried out week in and week out are undertaken by people who have no qualifications at all. This does not mean that those people who erect temporary structure don’t know what they are doing, far from it. The owners and bosses of these very expensive items of equipment (a stage with roof starts at £100k for festival use and a large festival stage tent can cost more than £200k) will not let just anyone go out and erect them.

What is needed is a method of certification of those industry professionals who have the necessary knowledge and experience. There have been many other industries that have gone through this process with future generations being brought in with qualifications in place.

Link wind speed standards to costs

Why not link the extra costs of additional bracing, kentledge (ballast), trussing, additional rigging, larger ground anchors etc to the higher wind speeds that additional work will allow? The contractor could offer the event organiser a ‘shopping list’ for certification for (say) 60, 80 and 100 mile per hour winds (27, 36 and 45 metres per second). The lower the wind speed decided upon, the more chance there is of the likely hood of those winds becoming real. The higher the wind speed specified, the LESS likelihood there is that those adverse weather condition will occur, allowing the event organiser a safety cushion against shutting the stage or the site. The customer could then use their event safety teams to determine which standard would be applied.

Weather

Contractors often joke that the weather is the responsibility of the client. Their contract for the hire of the equipment will clearly state the performance levels of the structure. Much time is spent working on emergency procedures in the event of high winds or torrential downpour or local flooding of the site. Why not simply raise the level at which emergency procedures are started? The risk of hazards such as bad weather becoming real, are immediately much lower. There will be a short tem impact on installation times and an attempt by contractors to raise prices but before too long all that would settle down and the event would be inherently safer.

Finally

Safety engineering is a modern part of the events industry, and one of the reasons that our industry has a relatively good safety record. When I first went to the Isle Of Wight festival in the early 70’s we would never have dreamed where the industry would go and where it has ended up at today. The events of the summer of 2011, industry forums and events like our seminar will all help the industry professional to provide a higher level of competence and higher safety standards generally. But this is not a one way ticket. We designers need feedback, the industry needs feedback on  these issues. We need a forum for all parts of the industry to discuss new issues arriving. If anyone is interested in helping with such a forum, let me know.

It is only good practise for engineers to share experiences and discoveries for the good of all. It has to make sense for event organisers to share common safety experiences. So to all who read this document, please share your experiences.

Rudi Enos

Sheffield - April 2012