Veeeee

V (pronounced `Veeee’, in wood engineering, in any particular usage, generally stands for one of two things. As pertaining to loads and forces in (inside of) wood structural members themselves, V stands for `Shear’, in particular `horizontal shear’, and specifically to wood, `shear parallel to grain’. We won’t talk about that kind of V here; instead, we’ll talk about V (also pronounced `Veeee’), being wind speed.

I strongly suspect the V comes from the word `velocity’. Velocity; in pure Engineering / Physics, is velocity `speed’ with direction, a so-called `vector’ quantity.  Wind, as it may hit a structure, is typically deemed to come from `any’ direction, but we crunch numbers with it as a `scalar’ quantity (no direction component).  Wind results in pressures, acting normal to the surfaces upon which the wind `hits’, and either positive (pushing on the surface), or negative (suction).  Further, the forces and stresses in the members of a structure, resulting from wind, most certainly have directionality; but these directions are understood member-by-member, as scalar quantities (and handled by scalar equations).

The precise definition of (wind speed) V is somewhat of a moving target. For a while it was defined as `Fastest Mile’, determined from how long a packet of wind (say, in a storm) would take to transit a mile; and then the fastest such packet, in the Imperial System of Units, expressed in miles-per-hour (mph). I doubt meteorologists and engineers actually followed a packet of wind in a storm, but simply took the average wind speed over which that wind would take to go a mile. Some day I’ll look up the detailed history.

Then it was decided to change the definition to … `3-Second Gust’.  So, average speeds over 3-second time periods were examined, and the fastest was picked.  I hope it’s no leap to understand that in any particular wind storm, the fastest `3-second gust’ would be a higher number (speed) than the `fastest mile’. After all, a 60-mph fastest-mile-wind would have been averaged (the fluctuations of highs and lows smeared) over a minute, as compared to (a burst of wind of) 3 seconds. During the transition time from fastest-mile to 3-second gust, there were (and I’m sure still are, somewhere) helpful tables to convert one to the other.

But there’s more! This wind speed, again as pertains to building design, is the speed measured, or presumed, some 33 feet above the ground, and with a certain `exposure’ (wind unobstructed by other buildings, trees, etc.). We’ll discuss exposure later. The point for now is that, if you understand a bit about `boundary layers’ in fluids (liquids and gasses, to include air), you know that as one gets closer and closer to the flow boundary, in this case flow of air (wind), and the boundary being the ground, the wind (speed) decreases. In fact, theoretically, the flow speed decreases to zero right at the boundary. In building design, though, obviously, our building sticks up above the ground; we never assume zero wind speed; in fact, in no case may we consider a speed lower than that at 15 feet (above the ground). Going up the boundary layer higher and higher from the ground, the wind speed increases, and, obviously, continues to increase above that valued deemed at 33 feet.

So, for starters, we need to know which `V’ we’re talking about: fastest-mile or 3-second gust. We also need to know the Exposure Condition of our building, and the heights involved. At the time of this writing it appears that we’ve pretty much left `fastest-mile’ winds in the rearview, and are dealing with the 3-second gust winds. But now we’re battered with having to sort out something more complicated, the `recurrence interval’ of the wind to be considered, i.e., the wind that occurs `once-in-50 years’, or `once in some-other-huge amount of years’, and terms such as VASD, VULT, V(nominal), V(basic), V(Risk Category I), V(Risk Category II), and so on. Yikes. Explanations to follow …