Volume 35, Number 5, May 2000

FenestrationFocus

What’s the Big Deal?

how to determine wind load and design load pressure on buildings, part one

by Craig H. Wagner, P.E.

If you’ve ever picked up a building code or design standard intent on determining the wind design pressure for a building, and found yourself in an extreme state of confusion, relax, you are not alone. You may, however, be wondering why it has to be so complicated.

In its simplest form, the process involves converting wind speed to wind pressure. Sounds easy enough. Wind is air with mass traveling at a given velocity. Convert it to pressure, right? So what’s the big deal? Why do these wind engineers have to make it so complicated? It has been suggested that they make it complicated so that only an engineer can do it.

Look, all they have to do is establish a design wind speed for every possible terrain, in every geographical location, for every possible type and use of building, in every possible setting. Then, they must establish what pressures the wind will impose at any location on every surface of every possible size, shape and configuration of building for all possible wind directions. Next, they need to take the extreme variations in pressures that exist on the building surfaces at any instant in time, and give us a nice uniform pressure to work with. Simple, right?

OK, I’m trying to make a point. Whether you’re amused or not, the fact is that the effect of wind on buildings is an extremely complex phenomenon. Research engineers strive to reduce these complexities to a practical set of equations and coefficients that can be used in practice to design buildings safely for wind. So, what are these complexities and how do the design provisions deal with them? Before we go on, let’s first dispel a few myths.

The wind design criteria is not something imagined or conjured up from some ivory tower theory, but is derived from a huge database of actual wind pressure measurements:

• Wind speed is not constant and surface pressures are not uniform;

• Negative pressure really exists;

• Internal pressure really exists;

• All codes are not created equal.

Codes and Standards

There are three major model codes used in the United States: the BOCA National Building Code, the Standard Building Code and the Uniform Building Code. Each of these codes publish wind load design provisions. In addition, they each recognize Chapter 6 of the standard, ASCE 7 Minimum Design Loads For Buildings And Other Structures for wind design loads. ASCE 7, in one of its many versions, is the foundation of all the wind load provisions used in the United States. The most significant change in wind load provisions used today occurred with ASCE 7-95, when the use of 3-second gust speed replaced fastest-mile speeds for the basic wind velocity used in design. What this means is they don’t record wind speeds as they have in the past. The wind load provisions in the current major model codes, including the new International Building Code, are all still based on fastest-mile wind speeds. Since the building codes also recognize the latest ASCE 7 by reference, this essentially gives you two options for determining the design pressures. In addition, some codes, and ASCE 7-98, now provide a simplified method that, depending on your building, may provide a third method. This can, of course, be controlled by a well-written project specification that dictates which criteria to use. Even more ideal would be a project that specifies the design pressures. This sure would level the playing field for the bidding, wouldn’t it?

Wind Velocity

OK, so either by choice or by directive, you’ve determined which criteria you’re going to use. Now you have to select the basic wind velocity. That’s easy. Find your project location on the wind speed map and hope that it doesn’t fall in one of those special regions where “anomalies in wind-speed values exist.” Also, be careful, your competitor may interpolate between the wind speed contours and use a lower velocity than you. That’s allowed.

There are a variety of other factors that need to be taken into consideration such as exposure classification, variation in surface pressures, negative pressure, internal pressure, and design pressure. These will all be discussed in part two of this column, which will appear in the June issue.

Craig H. Wagner, P.E. is a licensed professional engineer and senior project engineer for Architectural Testing Inc., in York, Pa. Fenestration Focus appears monthly with rotating columnists.

USG