Simple question about levers.

[Sorcerer]

I work in temporary traffic management, we use stands and bases to support our signs which look like these http://www.trafficmanagementltd.co.nz/store/3/145/Sign-Stand-Base

 

A problem we frequently encounter is that the wind blows over the sign, so to counteract this we place a sandbag on the feet of the base.

 

My question is, where is the best place to put the sandbag to minimisee the chance of the sign blowing over?

 

At first thought I chose the very end of a foot, because it is the maximum distance from the fulcrum and therefore provides maximum downward force countering the wind. But when thinking that through, the wind would conversely have the best leverage to lift the sandbag in that position. So does that mean the best spot is near the center of the base, where it would be hardest for the wind to lift it, like it is hard to move a door from a spot near the hinge?

[michel123456]

I work in temporary traffic management, we use stands and bases to support our signs which look like these http://www.trafficmanagementltd.co.nz/store/3/145/Sign-Stand-Base

 

A problem we frequently encounter is that the wind blows over the sign, so to counteract this we place a sandbag on the feet of the base.

 

My question is, where is the best place to put the sandbag to minimisee the chance of the sign blowing over?

 

At first thought I chose the very end of a foot, because it is the maximum distance from the fulcrum and therefore provides maximum downward force countering the wind. But when thinking that through, the wind would conversely have the best leverage to lift the sandbag in that position. So does that mean the best spot is near the center of the base, where it would be hardest for the wind to lift it, like it is hard to move a door from a spot near the hinge?

I think you are thinking too much.

The "like it is hard to move a door from a spot near the hinge" part is wrongly interpreted. The wind does not push near the hinge, the wind always push upon the sign. The reverse is happening: the sandbag pushes against the wind and you should put it as far away from the centre as in correct your first thought.

The other way is to make holes in your sign.

[Sorcerer]

I think you are thinking too much.

The "like it is hard to move a door from a spot near the hinge" part is wrongly interpreted. The wind does not push near the hinge, the wind always push upon the sign. The reverse is happening: the sandbag pushes against the wind and you should put it as far away from the centre as in correct your first thought.

The other way is to make holes in your sign.

Thanks, could you clear something else up? Where is the fulcrum? Is it on the furthest leg in relation to the direction of the wind (ie SE corner leg when the wind is NW) or is it in the center of the base?

 

Suppose the prevailing wind with intensity to knock over a sign is a North West wind, should I then place my sandbag on the leg of the base closest to the north west?

 

Unfortunately the signs must be a single solid reflective piece, adding extra holes would stop them being compliant.

 

Also they must be collapsible in case a car runs them over, so that prevents adding a brace to make a support against the wind.

 

What about aero foils could you add something which creates down force to counter the sideways then tilting force of the wind?

Edited October 24, 2015 by Sorcerer

[Janus]

I work in temporary traffic management, we use stands and bases to support our signs which look like these http://www.trafficmanagementltd.co.nz/store/3/145/Sign-Stand-Base

 

A problem we frequently encounter is that the wind blows over the sign, so to counteract this we place a sandbag on the feet of the base.

 

My question is, where is the best place to put the sandbag to minimisee the chance of the sign blowing over?

 

At first thought I chose the very end of a foot, because it is the maximum distance from the fulcrum and therefore provides maximum downward force countering the wind. But when thinking that through, the wind would conversely have the best leverage to lift the sandbag in that position. So does that mean the best spot is near the center of the base, where it would be hardest for the wind to lift it, like it is hard to move a door from a spot near the hinge?

There's a couple of ways to approach this. Let's say you have four sand bags to use. If you know that the wind is going to be constantly from one direction, then you best bet is to put all four bags on the ends of the upwind legs.

The reason being that, in this situation, the fulcrum will be the at the ends of the downwind legs, and then they will be the furthest from the fulcrum, while the effect of the wind on the sign will be to lift the base at the point where the legs meet, which is closer to the fulcrum.

 

Now consider a situation where the wind direction is erratic and you can't predict the direction. If you put a bag on the end of each leg. If the wind comes from the West, the Eastward sandbags will be sitting at the fulcrum and only the Westward bags will be doing any good.

One the other hand, if you put all four bags near the center of the base, you reduce the effect of the Westward bags but increase the effect of the Eastward ones.

So let's look at the numbers: We'll assume 100 lb bags and a 4x4 ft square base.

With the bags on the ends, this puts 200 lbs 4 ft from the fulcrum and 200 lbs on the fulcrum, since the weight on the fulcrum doe not add to the torque, we get 4x200 = 800 ft-lbs.

With the bags at the center, we have 400 lbs 2 ft from the fulcrum, for 400x2 = 800 ft-lbs.

 

So in the case where you have to allow for wind from any direction, it does not matter whether you put the bags on the ends or the center.

Edited October 24, 2015 by Janus

[michel123456]

There's a couple of ways to approach this. Let's say you have four sand bags to use. If you know that the wind is going to be constantly from one direction, then you best bet is to put all four bags on the ends of the down wind legs.

The reason being that, in this situation, the fulcrum will be the at the ends of the downwind legs, and then they will be the furthest from the fulcrum, while the effect of the wind on the sign will be to lift the base at the point where the legs meet, which is closer to the fulcrum.

 

Now consider a situation where the wind direction is erratic and you can't predict the direction. If you put a bag on the end of each leg. If the wind comes from the West, the Eastward sandbags will be sitting at the fulcrum and only the Westward bags will be doing any good.

One the other hand, if you put all four bags near the center of the base, you reduce the effect of the Westward bags but increase the effect of the Eastward ones.

So let's look at the numbers: We'll assume 100 lb bags and a 4x4 ft square base.

With the bags on the ends, this puts 200 lbs 4 ft from the fulcrum and 200 lbs on the fulcrum, since the weight on the fulcrum doe not add to the torque, we get 4x200 = 800 ft-lbs.

With the bags at the center, we have 400 lbs 2 ft from the fulcrum, for 400x2 = 800 ft-lbs.

 

So in the case where you have to allow for wind from any direction, it does not matter whether you put the bags on the ends or the center.

When you get that kind of coinciding results you must check with other numbers.

[J.C.MacSwell]

There's a couple of ways to approach this. Let's say you have four sand bags to use. If you know that the wind is going to be constantly from one direction, then you best bet is to put all four bags on the ends of the down wind legs.

 

up wind legs?

Edited October 24, 2015 by J.C.MacSwell

[Janus]

When you get that kind of coinciding results you must check with other numbers.

Not in this case, as it is pretty straight forward. Moving the bags from the ends to the center halves the moment arm while doubling the weight at the end of the arm, so it doesn't matter what the mass of the bags or the size of the base.

In fact, it doesn't matter where you place the bags on the legs as long as they are equidistant from the center.

 

Assume A base side of B and a Mass of M for each bag.

If we move all the bags a distance of D (as measured in the wind direction) inward, then the torque of the windward side is

2M(B-D)

And for the leeward side:

2MD

 

The total torque is

2M(B-D)+2MD

2M(B-D+D)

2MB

 

D does not factor into the final answer.

up wind legs?

Oops, yes I meant upwind. And I will edit that. Thanks for pointing it out.

[Sorcerer]

There's a couple of ways to approach this. Let's say you have four sand bags to use. If you know that the wind is going to be constantly from one direction, then you best bet is to put all four bags on the ends of the down wind legs.

The reason being that, in this situation, the fulcrum will be the at the ends of the downwind legs, and then they will be the furthest from the fulcrum, while the effect of the wind on the sign will be to lift the base at the point where the legs meet, which is closer to the fulcrum.

 

Now consider a situation where the wind direction is erratic and you can't predict the direction. If you put a bag on the end of each leg. If the wind comes from the West, the Eastward sandbags will be sitting at the fulcrum and only the Westward bags will be doing any good.

One the other hand, if you put all four bags near the center of the base, you reduce the effect of the Westward bags but increase the effect of the Eastward ones.

So let's look at the numbers: We'll assume 100 lb bags and a 4x4 ft square base.

With the bags on the ends, this puts 200 lbs 4 ft from the fulcrum and 200 lbs on the fulcrum, since the weight on the fulcrum doe not add to the torque, we get 4x200 = 800 ft-lbs.

With the bags at the center, we have 400 lbs 2 ft from the fulcrum, for 400x2 = 800 ft-lbs.

 

So in the case where you have to allow for wind from any direction, it does not matter whether you put the bags on the ends or the center.

Thanks this is helpful for the larger signs where we use multiple sandbags. The small ones however only use 1, sometimes 2 when 1 just isn't enough, that in the case of being on a hill or just in some crazy natural spot of a wind eddy.

 

I can reasonably predict where the strong winds will come from, in summer the foehn wind is a North West that is the most gusty and in winter plus spring/autumn storms it's almost always a direct southerly or a South West wind. I normally do account for this by putting more bags on the up wind leg (I hope you meant upwind,you said down wind,but from the latter statement of distance from fulcrum I can see you meant up).

In fact, it doesn't matter where you place the bags on the legs as long as they are equidistant from the center.

Funny you should say, I've seen donut shaped sand bags designs for the small bases. They're not used often though, the cost is high in comparison to a regular sandbag with very little practical benefit and they are inefficient at using space when stored/transported.

 

Any ideas on how effective an aero foil would be and what kind of design it would have? It'd be easy enough to have it free spinning so it'd self align with the wind.

[michel123456]

Not in this case, as it is pretty straight forward. Moving the bags from the ends to the center halves the moment arm while doubling the weight at the end of the arm, so it doesn't matter what the mass of the bags or the size of the base.

In fact, it doesn't matter where you place the bags on the legs as long as they are equidistant from the center.

 

Assume A base side of B and a Mass of M for each bag.

If we move all the bags a distance of D (as measured in the wind direction) inward, then the torque of the windward side is

2M(B-D)

And for the leeward side:

2MD

 

The total torque is

2M(B-D)+2MD

2M(B-D+D)

2MB

 

D does not factor into the final answer.

Oops, yes I meant upwind. And I will edit that. Thanks for pointing it out.

Maybe you are correct but it needs a graph. The height of the signs plays a role in the moment arm. Also, say the wind is south and assuming the legs are north/south, then the west & east bags act exactly as if they were in the centre. The only difference is taken by the position of the south and north bags. So I don't know.

[studiot]

I am suprised your base uses the unstable configuration of four feet (legs?).

 

UK portable signs generally adopt the stable tripod configuration.

These also usually incorporate cross members that can transfer the moment and are ideal for hanging the counterbalance sandbags on.

 

http://www.barriersdirect.co.uk/car-security-mirrors-signage-c1171/warning-signs-tape-c1083/folding-high-vis-signs-chapter-8-class-2-p1102

 

You should always weight all four legs of your stand since even if we consider the wind steady it will still vary.

The varying wind will induce varying moments which will make the stand shift or walk over time and can even slough off the sandbags.

 

You should also consider a plate or ring (better) base connecting your legs.

This will also stabilise the bags in position by (wind) load redistribution.

 

It is, however, good that you use sandbags, not lumps of concrete eg kerbs, as I have seen.

 

In the UK contractors who do this may be liable if anyone runs into the lump of concrete.

Sandbags are physically and financially safer.

 

Edited October 24, 2015 by studiot

[Sorcerer]

Those tripods wouldn't be compliant with our TTM. Stands must be collapsible and stand no more than 15cm, I think, when hit by a vehicle so they don't anchor it/make it swerve.

 

There's also no room for supplementary signs and the stand looks too short, it must be 1.5m for lower traffic volume work and even bigger for higher level roads.

https://www.nzta.govt.nz/resources/code-temp-traffic-management/copttm.html

 

There would've been a lot of money to be made in design and manufacture of TTM gear about 5 years ago before the earthquake hit. But the markets kind of flooded now. Plus once a company is using 1 type of gear mixing in new stuff becomes untidy.

 

From New Zealand's code of practice for temporary traffic management :

 

Sign stands and/or supports must be designed to ensure they:

 

will not cause significant damage to a vehicle if struck by one

 

are stable under all reasonably expected weather conditions and air turbulence from passing traffic, and

 

will not present a hazard to vehicles, including bicycles, after being knocked or falling over, ie the signs support and stand must lie relatively flat with no part more than 150mm above the ground surface.

 

Sandbagging is an effective method of securing signs. Signs must not be secured by hanging a weight from any part of the sign. Concrete and heavy steel (truck wheel rims, welded water pipe, etc) must not be used as a base for signs.

 

Where ballast is used on a sign stand or base it must:

be designed so that it cannot roll

 

be constructed from hessian, rubber or plastic bags containing a soft granular material, and

 

be no higher than 300mm above ground level.

 

Sign bases must:

be designed so they cannot roll

 

be able to be placed/disassembled to a height equal to or less than 150mm

 

be designed to break away from the rest of the sign support system on impact.

Edited October 24, 2015 by Sorcerer

[studiot]

That is one of the difficulties with different regulations in different parts of the world, though I am suprised that yours are so different from those in the UK.

 

Unfortunately I expect there is one common factor,

 

Having to cater for the crass stupidity and perfidy of some drivers who make things unsafe for the rest of us.

 

[Sorcerer]

That's all part of the job. Our codes so different because our roads our city layouts and our population is very different. I imagine traffic management on one of your main motorways is pretty similar to our traffic management on our motorways though.

 

You probably have 100 times the volume of traffic, but once it reaches our kind of volumes safety is pretty much maxed out, bar just closing the road.