Survival wind speed meaning?

I don't know for certain, but I suspect that the 42m/s is the service (working stress) load and the 48 (could that be 48.8m/s?) is the ultimate load to work with the factors.

You would be right but if 42m/s was service the customer will write 42m/s as "operational wind speed", but now it is written as "basic wind speed".?!

You should be talking to your customer, not us. We can't educate you long distance. And if we are wrong, who will you blame?

And more to the point: Who will the customer blame?

I suspect that he should be following up on the code document, not the client. It is the Engineer's job to see that his work complies with the code.

Code document? What "code document"?

I will say as the others that I am not sure about the interpretation but I give you my feelings to be used in discussions with your client.

The 42 m/s which correspond to 151 km/h is a value for "normal wind range" although high, the second one is for extreme situations 175 km/h being a hurricane which could occur but at low probability and frequency.

I further think that the whole dimensioning could be done for the 42 m/s and a verification for stresses< elasticity limit and stability of tower on foundation against the ultimate wind. Survival means that neither the lattice will not collapse nor the tower will fall down. Since wind forces are proportional to W^2 the ratio is 1.35. It may be necessary, for tower stability, to consider the safety coefficient with respect to the highest wind speed.

It would be great if after you discussed with your customer you will inform us about what he meant.

First, I'm really worried about that word "I." If your team includes a soils engineer to help with the foundation, and a guru who's put a few towers, then maybe this job is not outside your firm's core competency. If you're The Man, please ask yourself: does this contract pay enough to compensate for the risk exposure? Towers that fall down, or that have foundations that settle, b-a-a-a-a-d for business, worse if litigation is involved.

Yes, correct design constraint is worst-case scenario of 48.8 m/s, or roughly 110 mph. That "basic wind speed" is ambiguous. If it means "average daily speed," then 42 m/s is too high by, oh, an order of magnitude. Maybe the coast of Nova Scotia or the Antarctic winters have sustained wind speeds of that magnitude, but as an additional design constraint, the number doesn't mean much. Then again, if "basic wind speed" really means "max velocity of a gust" then I'm suddenly thinking moment arm, modulus of elasticity, resonant frequency, and inspection frequency, as in bolt tension from the foundation all the way up.

I suspect the client has not given you all the pieces of the puzzle. The biggest omission is mass: how much of what is he going to hoist up there? If he's thinking "wind turbine" I dunno if lattice is the way to go. If he's thinking "microwave," then my gut feeling is that lattice is probably fine.

You might find this helpful: http://www.imea.org/EnergyInformation/AESI_MetTowerPrimer.pdf

Good luck, but I think you're going to need a whole lot more than that.

Both are important !

While the basic wind speed pertains the 'average max wind speed' in a particular season under consideration, the 'survival wind speed' is the peak wind speed pertaining to the transient phenomena...

The reiforce members should obviously therefore be designes by taking into account the 'survival wind speed', as well as the 'tilt angle' of the side facing the average wind direction. However, the 'column tear strength' of the columns and the 'crushing strength' of walls facing the 'average wind direction' should be designed keeping in mind the 'persistent basic wind speed'.

Your idea of a safety factor of 1.5 seems ok if the building is not to be in an area where phenomena like tornados and cyclones do not occur.

In a case otherwise, the safety factor has to be enhanced accordingly, as well as the 'height to base area' ratio needs a consideration.

Ashok Shukla

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