Alan presented his notes with explanatory narrative. We dont't have the transript yet but will put the audio up when we get it.
Alan showed how the code guidance can be used to calculate the amount of load that soil can accept.
Rudi Added the following;
The installation of temporary structures is a complex and continuously variable series of compromises. There is no 'best way' to erect a structure. Some methods vary from contractor to contractor even with similar structures.
"Every day, man is making bigger and better fool-proof things, and every day, nature is making bigger and better fools. So far, I think nature is winning."
The difference between 'HAZARD' and 'RISK'. Electricity is dangerous, it is hazardous. But we all handle electricity every day. When we first get up in the morning, sleepy eyed. When we pour ourselves through the front door late on a Friday night a little worse for wear. When we are excited or not taking care. Yet the risk of being electrocuted is not high in normal use.
People use the terms HAZARD and RISK indiscriminately, but they mean quite different things. A HAZARD is an object or circumstance which has the POTENTIAL to cause harm, while a RISK is the PROBABILITY of that HAZARD causing harm.
A classic example is passenger flight. In passenger flight we are sitting in a pressurised can travelling at over 500 MPH and 20,000 feet above the ground. But statistics show that flight is the safest form of travel. The RISK of the hazard becoming real is slight due to the many, many thousands of man hours spent on minimising risk by the flight industry.
Erecting temporary structures is HAZARDOUS. There are many areas of the erection sequence of most structures where RISK must be managed. So, let us look at the main types of stages first. Stages with fabric roofs can be considered in the same way as frame tents for erection purposes. Where the covered stage differs vastly from the frame tent is in use. The stage will have a continuously varying configuration, sometimes at the same event. As with all things there are exceptions, but in general, stage roofs suspend large packages of light, sound and effects, tents do not.
Tents and stages both share a simple feature which examiners can see for themselves easily. Depending on the design, some structures are inherently safer than others. If you have a ground support type stage of a big top type tent, there is a period during the erection sequence when the structure is potentially not fully stable. If a wind above 25 metres per second or 55 MPH is blowing, it would probably be better not to erect the structure unless additional measures are taken such as additional guying. Until a ground support stage or big top type structure is fully erected, stabilised and tensioned it is Not Fully Stable.
If you have a structure where the frame is fully hoisted into position and made completely fast and secure before the fabric roofing elements are inserted, or the suspended load, then the erection sequence is inherently safer. If the roof fabric is inserted for example, using Keder grooves, a beaded edge on the fabric, then the RISK of is also reduced.
The operator of the structure must put in place measures which reduce the RISK. On ground support stages it is possible to use 2 stage hoisting to lift the main roof which accepts environmental loads. First, fully stabilise the roof using braces, guys or ballast and using pre-rig techniques hoist the light rig second.
The examiner should be able to see braces and guys without too much trouble, if not, ASK the crew chief of the erection team. If they don't know, your doomed.
Everybody understands anchors. Stakes, pegs, duck bills. Or they think they do. Quite a bit of work has been done on analysing ground and soil conditions under load. Here we show a test of 2 king poles with a giant 30 metre truss. The total test load on both king poles was 45,000 kilograms or 45 tonnes. The indentation in the grass when we removed the metalwork was only 75 millimetres. This example shows that in good dry conditions, normal soils can accept large loads. When we add water to the equation everything changes. The composition of the soil becomes more fluid and allows the anchors to move. A certain amount of movement of ground anchors is normal as the soil accepts the load placed upon it. On a big top type structure it is normal to see as much as 50 mm of compression in the soil in the direction towards the structure.
Let us look at how ground anchors work. If there is water present, the anchor has no resistance to vertical lift except suction in the hole. Sideways forces can 'SLICE' the ground if the soil is soft. To prevent this, it is normal for stakes to have 'SPADES' or 'PADDLES' welded to the side of the steel stake. The stake must have a large enough diameter that it does not bend. Wood should never be used for ground anchors although it does grip the ground better but rotting cannot be accurately measured. The best anchors are screw or helical anchors. They hold the ground in a positive way and usually can hold many times the force of a straight anchor. The most holding force we have measured in soil with a helical anchor is 28 tonnes.
If an anchor pulls out of the ground in normal conditions it will bring a large amount of soil with it. The amount of soil varies with constituency of the soil. This clump or cone of soil is crucial in understanding why stages and more so, screw anchors work. When a typical screw anchor pulls out of the ground they bring a cone of soil attached to the screw with them. This I call the holding cone. The holding cone provides a very good example of why longer stakes work. The cone is roughly 60
degrees to each side of the stake or anchor. In simple terms longer stakes will support a higher load the deeper you go into the ground, purely on bearing area.
But if you consider the holding cone, as in the slide showing now (number ??), the difference in holding force is not the difference between the length of the stake, but the difference between the volume of the holding cones. If we assume that example number 1 is 1 metre into the ground and example number 2 is 1 and a half metres into the ground, the difference in holding force is the difference between the volume of examples 1 and 2 is 3 .37 times.
In the example shown, much of the holding cone is above ground and of no use, so the first third of the anchor doesn’t do much.
The ground conditions the structure is sited upon, soft soil conditions, nearby drains or underground power supplies.
Make sure that all stakes or anchorage's are secure and not "pulling".