Factory Wind Ratings Recommendations

Overview

When assembling any marquee or temporary structure it is essential that it is adequately held down. If it is not held down properly in high winds the consequences can be catastrophic with damage to buildings, vehicles and most importantly the risk of serious injury to anyone in the vicinity. There are 3 main ways of holding the tent down:
1. Stakes or other ground anchors
2. Weights
3. Bolts

Ground pull test

The first step is to check the ground is capable of holding a marquee at up to 50mph winds. To do this a ground pull test must be performed by a qualified engineer. This involves driving stakes into the ground where the final stakes will be. A vertical pulling force should then be applied to the stake, if the stake moves at all below 110kg of pulling force, the ground is not capable to hold the feet of the marquee at 50mph winds. Proof of passing a pull test must be retained.

Pre assemble site checks

It is essential that for every site a full risk assessment is carried out to ensure the safety of structures in high winds. This should include inspection of soil/floor conditions and the exposure of the site to prevailing winds. Forecasts should be monitored and plans should be in place to deal with extreme conditions. The customer must be made fully aware of their responsibilities to keep the tent sealed and not leave doors open in high winds, and stand by crews to add more guy ropes or stakes should be available if the forecast dictates. Simply put, making sure the tent is correctly assembled and held down on every job means it is extremely unlikely you will lose one!

Weather forecasts and preparations

If forecasts are predicting that wind speeds will be above those that the tent is designed for, and there is sufficient time, then the structure should be taken down and stowed safely as it normally would. If the conditions occur whilst the structure is in use, the priority is to evacuate everybody to a safe distance upwind of the tent. If the conditions allow, the PVC roofs should be removed first to stop the aerofoil effect of the PVC skin. Walls can be removed afterwards to stop the shear force applied. If conditions are so extreme that it is not possible to remove the PVC covers as normal, then they can be cut out with a sharp bladed knife around the keder. This may release sheets of PVC to be blown downwind, but is preferable to allowing the framework to fail as this can do considerably more damage.

Securing a marquee on soft ground

When using stakes, to meet the 50mph wind loading ratings the included stake must be installed as per the instructions and meet the ground pull test minimum requirements. Soil with high clay content will give a very good holding down force, whereas sandy or rocky soil will not have as much cohesive friction with the side of the stake. We would refer you to the IFAI study on soil types and equivalent holding down forces required. Every job will have to have an assessment of the soil carried out to ensure the correct number of stakes is used. Additional stakes and guy roping of the structure, and inserting stakes at an angle are all good methods of increasing the holding down force on a tent. Screw down anchors can be used as an alternative to stakes in sandy soil, and it is essential the manufacturers recommendations are followed when using these.

Securing a marquee on hard ground

Wind rating tests have not been performed on a hard surface, and therefore cannot be guaranteed. Where the structure is assembled on a hard surface i.e. on concrete, and it is allowable, the tent can be bolted to the ground either mechanically or by use of a chemical resin fixing. The stake pull test can then be performed.

Sufficient sealing

Marquees should always be constructed according to the instructions. The advertised wind ratings are based on a correctly assembled and braced tent with no openings. In high winds it is essential that all opening into the tent are closed, and where access is required it is preferable not to open any panels on the windward side of the structure. If high winds are forecast it is possible to increase the holding down force of the tent with additional guy ropes. These can be attached to stakes, vehicles or any other heavy load present so long as there is no risk of pulling up. If extreme weather is forecast the preference is to stake down the entire structure if there is time to do so safely. Alternatively simply removing the PVC covers will mean the loads on the structure will be dramatically reduced, and in emergencies covers can be cut out with a knife instead of being removed as normal. Once this has happened the entire site would have to be roped off and kept clear as there will still be the danger of framework blowing out.

How a marquee can fail

Wind damage can occur in a number of ways. Tears in the fabric could appear which will allow the wind to get inside the structure and create uplift, which will increase the risk of the anchorages failing. The structure could be blown sideways and then trip over itself, or simply be lifted from one side and peeled open. There is also the possibility of the frame buckling, but this is mostly when there are other forces acting on the tent than wind alone such as build up of ice or snow. Further guidance is available from MUTA, IFAI, and Temporary Demountable structures