White Paper – Examining temporary structures
ISSUE 2 – POST SEMINAR 2012
In this issue we provide the contact details for all seminar attendees as agreed at the seminar. We also provide an overview of stakes and staking.
1 Carl Ryder kindly gave us a link to the official report on the Indiana State Fairground collapse -
http://www.in.gov/sfc/files/041212_TT_Final_ISFC_Report.pdf. Sally Ann Dodd sent this link - http://www.thorntontomasetti.com/news/spotlight/199-indiana_state_fair_commission_investigation_report.
2 I must apologise, in the last newsletter I credited a comment by Abigail Mathews to Mandy Ramsden, advanced senility creeping in... Sorry Abigail, but both Mandy and myself agree with your question “Why don’t we design stages that don’t fall down?”
3 STAKES AND STAKING
The average mobile structure is invariably held down with a higher level of force than it is held up. Proven many times by analysis, each side guy is capable of having a greater load placed upon it than the stake on a temporary structure can carry in soft ground.
This basic fact proves what all Tentmasters have known for many years;-
which is: in bad weather put extra stakes in.
Staking is the single most important variable to the erection of a marquee/stage/mobile structure. Soft ground conditions, changeable weather patterns, the type of sub surface soil, or weather it has rained recently or not, all make a tremendous difference to the load bearing capabilities of the ground anchor, and the tent master must use his/her discretion on this point more than any other.
There are varying thoughts on what angle and distance from the tent the stakes should be inserted at. The answer to both of these used to be as follows :-
1 The angle of the stake, in other words it's vertical position.
The traditional thoughts on stake angle was that the stake should be nearly perpendicular to the line of pull. This thinking also concluded that the angle of pull from the top of the side-pole to the anchor should be not less than 45 degrees; if there is a 2.2 metre (7 foot) side pole, then the stake should be 2.2 metres (7 foot), from the bottom of the pole.
2 If the stake is placed too close to the side wall of the tent, the rope will be pulling up (and out of the ground) more than across the head of the stake and this will reduce its effectiveness.
3 If the stake is too far from the side wall of the tent then it will be pulling out but not down, allowing the roof to lift.
4 The angle of the stake to the surface. Obviously stakes should never be inclined towards the tent, as the results of this would be obvious.
Modern thinking and site tests have resulted in different thinking. EN BS 13782 clearly shows stakes being tested vertically. I haven't been able to discover why the writers of the guidance arrived at this. My thoughts are always with the school that angles the stake. If the stake is not angled and the ground compresses, then the stake will be angled towards the tie back, increasing the chance of the stake pulling. When a stake is driven into the ground the soil around it is compressed due to the volume of soil being replaced by the stake. The compressed soil will withstand more force. This is just one of the reasons why a larger diameter stake will always perform better than a small one.
Lets go back to the concept of vertical stakes. I am not going to go into the issue of soil conditions at this point. (Partly because this is where I am undertaking basic research, basic research is what I’m doing when I don’t know what I’m doing).
Soil becomes fluid under pressure such as a stake applying a force. Technical terms such as thixotropy, viscosity and hysteresis come into play when describing these situations. Basically it comes down to the questions; how sticky is the soil? And how much will it compress?
(Thixotropy is the property of certain gels or fluids that are thick (viscous) under normal conditions, but flow (become thin, less viscous) over time when shaken, agitated, or otherwise stressed. They then take a fixed time to return to a more viscous state . In more technical language: some non-Newtonian pseudoplastic fluids show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. - Wikipedia)
My thoughts on the issue of angle are clear but I can’t prove what I believe; that the stake is more efficient under varying conditions than if vertical. Perhaps someone smarter than I am will point out why I am wrong. I would welcome an explanation.
Let’s not worry about theory and discuss what is known. Depending on the friction, stickiness or adhesion of the soil to the stake (choose any one), the stake, if subjected to a force greater than the soil around the stake can accept, will pull out of the ground with a significant amount of the ground soil still attached. This holding cone (my definition) or ground wedge as it has been called, is a cone of soil or compressed material roughly cone shaped which is about the shape of an ice cream cornet. The result is simple; if we imagine a holding cone attached to a one metre long stake (39 inches) and we compare with a similar example with a stake that is a metre and a half long, the difference in holding capability is not one and a half, it is approximately 3.6 times, ; the difference in volume between the two holding cones. So the benefit of longer stakes becomes apparent.
Double staking.: The concept of the holding cone becomes important when using multiple stakes. There is no point putting multiple stakes in the same piece of ground. That area of soil will only accept a limited amount of upward force. Not that multiple stakes are a bad idea, just that the layout must be optimised to make best use of them. The average Big Top company at a festival carries several tonnes of steel bar around the country every weekend through the summer. More efficient use of anchors saves purchase costs, transport and the energy required to drive and remove them, which would make a significant gesture towards environmental issues. And be more efficient, which is where my thoughts lie.
Double staking allows the stakes to not only improve the holding force but provide a factor of safety and even if they are positioned wrongly (as above) two are always better than one. Multiple are always better than singles. To improve the efficiency fully, move the anchors into their own holding cones., The overlapping area of soil will reduce the total holding capability. If tying stakes together, better to use a bar or strip with holes in it, that allows the multiple stakes to act together.
Other issues, however basic.: If the diameter of the stake is too small, it will bend and allow the top of the stake to deform towards the load. If the soil is too loose, the stake will slice through the ground and also fail. Recently disturbed ground will provide a million unknowns. Filled ground will only provide a decent anchoring restraint if well settled and compressed. Tarmac or asphalt covering fill is very tricky. The thickness makes a difference and of course the consistency. Don't assume that the asphalt will hold like concrete, it definitely will not.
Although some contractors still use wooden stakes, these are generally frowned upon in the industry. As the contractor can never be totally confident that the wood has no rot in it or any such other problems, so steel is almost universally used. With tThe average marquee stake being is 700 - 900mm long with a diameter of 20mm (2-3 feet with a diameter of 3/4 inch).
I will finish with some simple guidelines on holding down temporary structures. An average marquee or tent stake of one inch or 25 millimetres mm (1 in) diameter which is one metre long will probably on average accept a force of 600 to 800 kilogram’s (or 1,300 to 1,750 pounds) in soil and possibly as much as double that if the ground consists of well compressed hard core. If the stake is larger, a stake which is twice the length and twice the diameter will possibly hold as much as three times that.
If we accept that a typical uplift on a tent or stage roof is .5 Kn/2 (kilo Nnewton/ per metres squared2), then allowing for a factor of safety of roughly 1.5 then a typical frame marquee or stage roof of approximately 20 metres by 20 metres (22 ysds x 22 yds) will have a total uplift of approximately twenty tonnes. Assuming four metre bays for the framework (an assumed rather than real average) and eight stakes per bay plus gable legs, then taking the previously mentioned figure of 600 kilogram’s we see that with a total of one hundred plus stakes will have be required to restrain the twenty tonne upward lift;. Oonly 200 kilogram’s each. Except that the four stakes on each base-plate are acting on the same ground area. So we divide by four. Eight hundred kilos each is not a large factor of safety, which shows why stake placement can make such a difference.
A typical pole tent or rental marquee has a better chance of providing the required holding down force. If we assume a typical 12m x 24m (40ft x 80ft) twelve metre by twenty four metre pole tent or eighty foot by forty foot tent then the uplift can be as high as fifteen tonnes. A pole tent or traditional marquee has the side poles and therefore the anchors spaced on roughly one metre or three feet centres. Which means that each anchor is only called upon to accept two hundred kilos, none of which are using the same area of soil.
To finish this section on stakes, let’s be clear; with the technical advances in engineering software and an increased knowledge of the performance of these temporary structures from historical data, we can more or less simulate real world situations while the proposed structure is still on the computer screen. What we don’t know, even on sites which are used year on year, is how the ground will perform, particularly if it changes from dry to wet.
So the only answer to “How much load will that stake accept?” is, test it and I will tell you.
Alan, more to come here, I will send when you have approved the above text.
Sheffield - April 2012