Wind speed versus the force on objects like flower pots and deck boxes

[scott simpson]

i was wondering if anyone had a site that catalogued windspeed effect on heavy objects? I found several sites that talk about trees and shingles, I'm interested in flower pots and deck boxes. If someone could shoot me an equation would allow me to calculate various objects

 

[scott simpson]

on a related note, is it better to leave small gaps between plywood sheeting to allow air around during a hurricane, or attach them without gaps between to try to keep all wind out?

 

[tech99]

i was wondering if anyone had a site that catalogued windspeed effect on heavy objects? I found several sites that talk about trees and shingles, I'm interested in flower pots and deck boxes. If someone could shoot me an equation would allow me to calculate various objects

Find out the once in ten year wind speed (say) for your area. Assume about 40psf at 100mph. Force is proportional to Vel^2.
Then check there is likely to be enough friction with the ground (assume mu=1) and enough weight for stability.
Allow a big safety factor.

 

[scott simpson]

My quantitative physics is nonexistent, can you give me the formula taking into account weight? I'm assuming force = velocity squared?

 

[A.T.]

I'm assuming force = velocity

https://en.wikipedia.org/wiki/Drag_equation

 

[tech99]

My quantitative physics is nonexistent, can you give me the formula taking into account weight? I'm assuming force = velocity squared?

I am using USA units here, which I am a bit wobbly with after years of metric!
Force due to the wind = 40 V^2 / 100 in pounds, where V is wind speed in mph.
To avoid sliding, (weight x mu) must be more than the wind force.
Turning moment due to the wind = force x height of centre of pressure. Let's assume it is force x half height of object.
Righting moment is weight x half the width. Distances in the same units.
Righting moment must be more than overturning moment.

 

[A Lazy Shisno]

Wind effects? Hmm. Well the two things I can think of that would be most involved with that are sliding and tipping.

First, let's determine the force of the wind acting on the object. The drag equation states that

$$F_D=\frac{\rho C_DA}{2}v^2$$

Where $\rho$ is the air density, $C_D$ is the drag coefficient of the object (this depends on shape), $A$ is the cross-sectional area of the side that the wind is hitting (for a rectangular box, this will just be the area of the side the wind is hitting, for a cylindrical shaped plant pot, this will be the area of the flat side if you cut it in half), and $v$ is the wind speed.

Now, to make sure your object doesn't slide you must make sure the drag force is less than the force of friction between the object and the ground, which can be found using

$$F_f\approx \mu F_N$$

Where $\mu$ is the coefficient of friction of the material of the object and the material of the ground, and $F_N$ is the normal force (which will just be the force of gravity $F_g=mg$ on a non-inclined surface).

As long as $F_f>F_D$, the object shouldn't slide.

Next, to find when the objects will tip, we must first find the moments exerted by the forces acting on the objects.

The moment of gravity and the normal force can be found using

$$M_{g/N}=F_gw$$

Where $F_g$ is the force of gravity on the object and $w$ is the width of the object (in the direction of the wind).

Next, the moment of drag and friction can be found using

$$M_{D/f}=F_Dh$$

Where $h$ is the height the force is being applied at, which, because the force of the air is applied everywhere on the surface, will just be half the total height of the object.

And so, as long as $M_{g/N}>M_{D/f}$, the object is safe from tipping.

Are there any other effects you can think of that you would like analysed?