Large sign – Wind issues?

[Nick Minall]

This maybe of some use to you… click here

[Neil Churchman]

I would imagine that over a 5m span, if you want to use only three posts that the diameter would need to be larger than 76mm, and/or use a rectangular or square section steel post which is structurally sounder than a circular tube.

Also consider how deep you are going to dig the holes for the posts which is just as important as dimensions of sign posts. If the ground is not compact which might be the case where building has taken place in recent years, you may need to dig out a significant square hole (perhaps 750mm x 750mm x 750mm) for the post and fill in with concrete.

There’s probably a British Standard on wind loading for road signs if you want to do it by the book!

happy digging 🙂

[Lee Attewell]

Hiya Mike,

Over engineer the bugger mate…Charge the client for the extra and sleep well at night.

Have a look at freeway signs and see what sort of structure they use for a similar sign.

[Shane Drew]

Hi Mike,

My rule of thumb is a post every 1200mm for a long span in a harsh environment (especially using a small diametre post like 75mm). As Lee says, easier to sleep sound at night knowing you have gone that bit extra.

In a high wind area, I make sure 30% of the pole height is in the hole, in concrete, and I’d use square posts with a substantial thickness.

Our councils here have a minimum requirement, but I always exceed that to be safe……

Cheers

[Steve Morgan]

A bit late I know but it may still be of interest.
A friend of mine who is an architectural engineer told me that wind loading is a complex subject, however he recommends that a rule of thumb figure to work with is 15lbs/sq.ft for each ‘mile per hour of wind speed’. That will give you some pretty frightening numbers to consider!
And for what it’s worth – I think that round tube is much stronger than square, especially in bending.

Steve

[Simon Clayton]

quote Steve Morgan:

A bit late I know but it may still be of interest.
A friend of mine who is an architectural engineer told me that wind loading is a complex subject, however he recommends that a rule of thumb figure to work with is 15lbs/sq.ft for each ‘mile per hour of wind speed’. That will give you some pretty frightening numbers to consider!
And for what it’s worth – I think that round tube is much stronger than square, especially in bending.

Steve

I would agree, that’s why road signs have round poles.. Maybe take a look at a motorway sign of around the same size (if possible) and take some dimension’s from that.. When i worked at oldham signs the petrol station gantry signs had a root cage and concrete base of around 3 cubic meters can’t remember the exact sizes but something like 2m X 1.5m X 1m

[David Rogers]

quote Steve Morgan:

……………..A friend of mine who is an architectural engineer told me that wind loading is a complex subject, however he recommends that a rule of thumb figure to work with is 15lbs/sq.ft for each ‘mile per hour of wind speed’. ……….Steve

Whew! That figure is kinda over rated!!

So in a 1mph ‘wind’ there is a 15lb load on a 1′ square panel…or to get it into real world figures – my ‘front elevation’ is under 10 square feet – I weigh (mumbles) 180lb so in a 2mph wind (10sq /ft x 15lbs x 2mph = 300lbs of force) ….I would blow away. Not bleedin’ likely!!

From practical experience – I can hold an 8×4 of aluminium square-on into a 20mph wind – maybe 50 or 60lbs of force exerted. Nick’s calculations (sailing site) appear more realistic IMHO….and in line with those figures.

Dave

ps. Personally, I’d do 4x 100mm poles for that sign. 75mm may take the load – but are just so spindly looking and will sway & flex like hell!!

Oh, and get STEEL poles…not aluminium – they can crack if it does flex!!

[Colin Crow]

Another tip, if using fully hollow poles, is to insert a cheap steel scaffold pole to give added strength. Obviously this doesn’t work with the slotted type poles.

[Shane Drew]

quote Simon C:

quote Steve Morgan:

A bit late I know but it may still be of interest.
A friend of mine who is an architectural engineer told me that wind loading is a complex subject, however he recommends that a rule of thumb figure to work with is 15lbs/sq.ft for each ‘mile per hour of wind speed’. That will give you some pretty frightening numbers to consider!
And for what it’s worth – I think that round tube is much stronger than square, especially in bending.

Steve

I would agree, that’s why road signs have round poles.. Maybe take a look at a motorway sign of around the same size (if possible) and take some dimension’s from that.. When i worked at oldham signs the petrol station gantry signs had a root cage and concrete base of around 3 cubic meters can’t remember the exact sizes but something like 2m X 1.5m X 1m

With respect guys, a square pole is stronger than a round pole. The maths is way too hard for me to explain here, but it has something to do with the corners adding to the strength in a backbone sense.

My engineer that fabricates my large signs, always uses square if maximum strength is a requirement.

I think councils use round poles because it is a) a cheaper option and b) all the fixing systems they have are for round poles.

All the oversized overhead signs on our freeways here are on square poles, or if they are on round poles to conform to a design requirement, the poles are very thick.

I agree with David Rogers too. 100mm would be my preferred minimum size for a sign this size.

Cheers

[Neil Churchman]

Sorry guys – I agree with Shane.

A square or rectangular post is structurally stronger than a round post.

The reason a lot of sign-makers, including myself probably prefer to use round poles is because they are a bit cheaper, but mainly because if the pole is slightly out of position, it’s normally no big deal.

Imagine a square post being out of square with the other posts,
the sign would not fit flush to the poles.

[Peter Normington]

I’m almost certain that if you compare weight, cross section and wall thickness, round is stronger, hence such things as oars, golf club shafts sailboat masts and scaffolding are a round cross section.

Peter

[Nick Minall]

Have a look at this may help with the round/sguare thing!

[Simon Clayton]

quote Peter Normington:

I’m almost certain that if you compare weight, cross section and wall thickness, round is stronger, hence such things as oars, golf club shafts sailboat masts and scaffolding are a round cross section.

Peter

100% true… that is why bike frames are round or oval not square.. Just look at an egg, put a dozen in a carton, then a board on top you could virtully stand on it without them breaking, try the same with boxes made of the same wall thickness and mass… The reason square section ais used over round is because it easier

[Mike Grant]

Thanks Guys for your comments. The customer actually said to go for the 4 posts version before I posted this, but I just needed the confirmation and advice. I might even do the scaffold pole bit as a just in case.
An interesting argument against round v’s square!

Apologies to Rob and thanks for changing the title, with hindsight mine was a bit vague 🙄 😳

[Shane Drew]

quote Nick Minall:

Have a look at this may help with the round/sguare thing!

Interesting Nick. The engineers comment "As for the two shapes (of the same size) calculated, square was proven to be # 1 (more cross sectional material) and round #2." is what I was led to believe.

I must confess I didn’t read it thru as I was asleep before I got to the end of the page 😳

[David Rogers]

quote Simon C:

100% true… that is why bike frames are round or oval not square.. Just look at an egg, put a dozen in a carton, then a board on top you could virtully stand on it without them breaking, try the same with boxes made of the same wall thickness and mass… The reason square section ais used over round is because it easier

Might pick you up on your comparisons, as the round v square is about strength in resisting lateral deflection (bending).

Bike frames: (can be made in box section as a ‘design’ feature), but the frame material is NOT designed to resist any significant lateral forces – only compression & tension. Prime example is my current bike. The tube walls range from well under 1mm thick for my downtube (aluminium, frame – sounds like an empty coke-can if you ping it) to a max of 3mm for those with a small cross section. eg, chain stays. All of the strength & loads is maintained in the joints, not the tubes – being mainly gusseted welds & machined pivots. Torsional deflection (twisting) is kept to a minimum by using massively oversized main tubes. There is virtually no strength in the sidewall, I could actually crush it quite easily!

Eggs: The eggs in your analogy again are in compression – not experiencing any lateral deflection. Besides, they are not round tubes, but a complex structure capable of resisting compression through it’s vertical axis alone.

IMHO (could be wrong) square IS stronger (3" sq v 3" dia) and more rigid BUT the round tube is far superior at distributing the stresses and less prone to catastrophic failure by bending / flexing far more before the structure fails completely.

Dave

[Steve Morgan]

Just read all of yesterdays stuff about the wind, oops! how to make a fool of yourself.
I bet none of my friends buildings will blow down! must ask him about that.
I think the round versus square in most fabrication instance is ease of production, it’s easy to join 2 round tubes with a mitre cut to form a right angle, try cutting 2 round tubes to form a ‘T’
Steve

[Steve Morgan]

Dave,
Does the egg in compresson distribute the load away from the point of load in the same way as a brick arch?
Machined pivot points – sounds like a mountain bike? One of my road bikes has a carbon frame, I’d love to know how thick that is.
Steve

[David Rogers]

Sorry for going O.T. – should maybe move this!

quote Steve Morgan:

Dave,
Does the egg in compresson distribute the load away from the point of load in the same way as a brick arch?

More like a domed roof. The more conical the structure the better the forces are directed along the axis and not into ‘expanding’ the perimeter, that’s why nearly spherical eggs just squash!

quote :

Machined pivot points – sounds like a mountain bike? One of my road bikes has a carbon frame, I’d love to know how thick that is.
Steve

Yes, currently on a Scott Fx-1 cross-country, few years old now. Down tube so thin you can flex the sidewalls with your thumbs! I’d guess your C.F. is pretty thin too…just stiffer!

[Steve Morgan]

May this be the last on the subject – I spoke to the engineer who told me that 15lbs/ft² is as a rule of thumb figure for wind pressure and he confirmed that it was correct. It’s just that you don’t multiply it by anything! That’s the maximum.
Dave your frontal area of less than 10ft² (I hope it’s a lot less than that) is safe until you go out in a storm force 10 and if you can see tiles being blown off the roofs don’t take that 8×4 outside.

Steve

[Craig Brown]

quote Mike Grant:

I don’t know the maths to work this out so is there anyone out there that could advise me properly.

Thanks
Mike

As this thread suggests wind loading on larger signs is a problem we all face from time to time and if an engineer or architect is involved in the project they will often want structural drawings and calculations. If you don’t have the in-house facility to do this and want to take a belts and braces approach on your larger signs I’d recommend a company called Russell & Havard Consultants.
They specialize in no nonsense, cost effective structural advice for the sign industry. They have always come back to me with a quick response to any queries I’ve had, they are also fully insured and their design and calculations are relevant to the current British Standards.

Simon Havard at
Russell & Havard Consultants
Tel 01306 632840

I hope this is a helpful addition to the thread

Regards Craig B

[Hugh Potter]

quote Steve Morgan:

Dave your frontal area of less than 10ft² (I hope it’s a lot less than that) is safe until you go out in a storm force 10 and if you can see tiles being blown off the roofs don’t take that 8×4 outside.

Steve

hehe :lol1: i should test this theory tonight while fishing the end of eastbourne pier, when i have my floatation suit on, my frontal sq footage is considerably increased, it’s forcasting a force 6-7 tonight, so should be fun ! had trouble even standing on the shingle during a match last thurs night, that was force 7-8 !

interesting topic though, thanks for posting it !

[David Rogers]

quote Steve Morgan:

May this be the last on the subject – I spoke to the engineer who told me that 15lbs/ft² is as a rule of thumb figure for wind pressure and he confirmed that it was correct. It’s just that you don’t multiply it by anything! That’s the maximum……Steve

Still well over engineered to a factor of 5 or 6 times over…better safe than sorry!! Nothing like building in a GOOD safety margin, and it keeps down building flex…well they don’t flex too well!!

Cheers for clarifying that – makes much more sense now!

Dave

[Peter Normington]

We just had our Saturday curry, will inform you all later about the high wind issues 😉

Peter