Tipping an object over due to wind forces 3

[dpsmith85]

I have a question about something I have been working on lately. I want to make sure that I have the concepts right. The basic question is I have a trailer with a large mast attached to it and am wondering if it will tip over or not. I've attached a picture for reference and some calculations. The picture attached shows the trailer from a rear view with the 1500lbs reaction forces being the tires. The mast is 40ft tall and made of 6" x 6" square tubing. The surface area of the mast would therefore be 20sqft. My goal was to compare the moment of the wind force to the moment of the trailer itself. If there is anywhere I messed up it is in evaluating the moment of the trailer. I evaluated at 90mph wind and found that the trailer would tip when comparing the moments. Is this approach correct? Does anyone have any good advice on the subject? Also, bear in mind this purely a concept so I can understand the physical principles. I'm not really worried about if something could actually be built the way I've described it.

 

[AELLC]

Error appears to be the calc for stability moment of trailer -

It should be 3000 lbs x 2.5 ft, not 1500 lbs x 2.5 ft.

 

[MiketheEngineer]

While I might not agree with 415 lbs - I get something just over 500 lbs using F = .0033 V^2. But lets go with that.

Sum Moments at Fb:

22' x 415 = 9130 ft-lbs

Divide by 5'

9130 / 5' = 1826 at each wheel from the sign.

Where do the 1500 lb= "UP" reactions come from.

Lets say the trailer weighs 3,000 lbs - I think that is what you are saying and assume it is at the middle.

Again sum force at Fb"
( could use 1500 x 5')

+22' x 415 lbs - 2.5' x 3,000 lbs

or is 22' x 415# greater than 5' x 1500#

Answer is yes. It will flip!!

 

[dpsmith85]

Thanks for the quick responses. When you guys answered something clicked in my head. (I don't know if that's a good or bad thing). Check out the attached file and see if it looks a little better. The idea that I had was to sum forces and calculate the reaction forces at both tires (FA and FB). If the reaction force at either one was in the negative direction that would mean that the trailer was tipping. Let me know if you agree with the logic or not.

MikeTheEngineer...you're probably correct about the wind forces I googled it and found that equation which may be wrong...I think it included a gust factor which may be why the answers were different.

Thanks again.

 

[AELLC]

dpsmth85,

In general, when you look at Overturn moment - Stability moment = Net moment, and the structure (or object) is symmetric in gravity load (such as the trailer), you are looking at sum of moments around either the left or right wheel's contact on ground, using a sign convention such as counterclockwise moments are negative and clockwise moments are positive.

For example, if you take the wind direction as left to right, you would sum moments about the right wheel because that would be the center of the trailer's overturn rotation, and if the result (Net moment) is positive, then you do not have stability.

 

[desertfox]

Hi dpsmith85

I haven't checked whether your wind force formula is correct or not, so I just took the value you gave originally, you need only to take moments about one wheel of your trailer ie:-

2.5*3000= 7500lbsft

compare this with the moment on the mast due to wind:-

415*22= 9130lbsft

The fact that the latter figure is greater than the former means the trailer will tip as stated by others.

desertfox