Archive for December, 2013

Christmas Opening Hours

Monday, December 23rd, 2013

Merry Christmas to all of our DIY Marquee customers and anyone else who pops in to read our mutterings and mumblings.

Today (Monday 23rd) is our last day in the office, the factory will open again on Monday 6th January though some of us will be in on Thursday 2nd and Friday 3rd in case some of those marquees bought as xmas presents need changing for a different size.

Thanks for reading throughout the year and see you in 2014!

Spencer

Should I remove the sides of my marquee in strong winds?

Monday, December 16th, 2013

No, a marquee is far more wind resistant with the sides on rather than off. For some reason there are many people who believe that leaving the sides off a marquee ‘lets the wind pass through’. I will try to explain below why this is not true and why leaving the sides off of a marquee actually makes it far more susceptible to the wind than leaving the sides on.

Marqueeinwinddiagram

The diagram above shows how the wind passes over a marquee and how any wind going in to the marquee gathers and lifts the roof up.  The best way to avoid this is simply to have the sides on the marquee facing the wind and have the marquee well anchored down.

Please note that this article is dealing with marquees in strong winds but not extreme weather. Marquees should not be left erected in extreme weather conditions.

The people who advocate taking the sides off in strong winds believe that the above scenario is best solved by removing the remaining side panels, thus allowing the wind to pass harmlessly through the other side of the marquee. It doesn’t. There are two factors in play if the sides are off.

i. Wind building up under the marquee roof – this still happens even if that other side is taken off. Don’t believe me? Try walking across a field in windy conditions with an umbrella – by taking the sides off your marquee you’re turning it in to one very large umbrella.

ii. Lift being created by the wind that DOES pass through the marquee:

Marqueeinwinddiagram2This shows the wind passing over or through the roof only marquee. The wind passing over the top of the marquee has much further to travel so has to go faster to keep up. This may ring some bells for those who stayed awake during Physics lessons:

Marqueeinwinddiagram3The reason a wing attains lift is by the air passing over the top at a greater speed than underneath and so the differing pressures caused creates lift.

A marquee roof isn’t as aerodynamic as a wing but a lifting force will still be created, not only that but the steeper the roof the more force there will be – another reason for having a 20degree roof/3m apex on 6m marquees as discussed last week.

This post isn’t intended as scare-mongering, it is purely aimed at dispelling a myth. At DIY Marquees we try to encourage all of the marquee hire companies we help to follow best practices:

  • Ensure the marquees are well anchored down, that means tie down kits as a minimum.
  • Leave the walls on any side likely to face strong winds (have the entrance on a different side)
  • Never take all of the sides off a marquee in strong winds
  • Know your marquees limitations -if Michael Fish states there definitely won’t be a hurricane then resign yourself that the marquee has to come down (this is rare, it happened once in the 10+ years I ran a hire business).

Thanks for reading, as always feedback is gratefully received.

Spencer

Why buying 6m marquees with 3m apex height/20 degree roofs are the best option

Monday, December 2nd, 2013

Most commercially used marquees in Britain use a 20degree angled roof (3m high apex for 6m marquees with 2m leg height) as that gives the greatest wind-resistance by design, obviously it still needs well anchoring down.

When designing a marquee we consider the likely weather the marquee has to withstand in Britain: rain, snow and wind.

Rain is reasonably straight forward, any waterproof cover with enough angle on the roof for run off would do the job.

Snow and wind are perhaps surprisingly treated together, this is because it is very difficult to design for both in the same marquee. Consider alpine chalets that have very steep roofs to prevent snow settling – if you used this design in a marquee it would make it far too susceptible to wind as described below. So in general marquees in Britain are designed to be strongest against wind and are not rated for snow-loading (hire companies get round this by leaving heaters on to prevent snow settling).

The taller a marquee is the greater the wind force it has to resist. The strength of this force is increased by the square of the height increase so if the height of a marquee is doubled the force it has to resist actually goes up by 4 times as much. The lower the apex height the more wind resistant a structure will be.

As the roof angle increases the apex gets higher and the surface area of the roof increases. By the law used in hydraulics the pressure per square metre is constant, if the area increases then there is more upward thrust when the wind gets inside the marquee. To reduce the likelihood of damage again the apex should be as low as possible.

It’s no coincidence the industry standard for a pitched roof is 20degrees (making it a 3m apex for 6m marquees on a 2m leg), it’s been found to be the best angle for helping the wind to roll over the roof of the marquee.

By way of example here’s a photograph of the 2012 Showmans Show (2013 not available as yet). The marquees highlighted in red use a 20 degree pitched roof, the ones highlighted in blue don’t use a pitched roof (they are curved or peaked) and the ones highlighted green are marquees that we can’t recognise as 20 degree pitch roofs or not. Note that 20 degree pitch roofs are by far the most popular and the marquees that may use other angled roofs are the smaller ones – In 3m and 4m widths the overall roof areas are smaller where you can use a higher apex/steeper roof with fewer problems.

showmans20degreeroofs

When buying a larger marquee look for those that feature a 20 degree roof pitch, on a 6m wide marquee on 2m legs that means a 3m high apex. They may cost a little more to manufacture but it is worth it. There’s a reason it is the industry standard.

Thanks for reading

The above was written by Colin King, former principal lecturer in Engineering.

Translated in to ‘normal’ English by Spencer!