July 18, 2022
Author: Brad Bartilson, Product Manager
How to select doors for safe pedestrian Access in regard to wind force variables
Could the wind force a door to open so harshly, even pushing someone backward as they begin opening it? Absolutely. To protect those entering a structure from such a scenario, how might we measure the impact of wind force, aperture size and pressure-related factors so that we can design the optimal door solution? There are many variables to take into consideration. What may seem like a simple calculation, isn’t. Let’s take a look at some of these complexities.
For example, the wind could be of steady velocity, or it may be accelerating/decelerating (i.e., gusting). The wind may gust at precisely the right moment and direction to impart a great thrusting force. The building’s HVAC and/or clean room air filtration systems could also aid in pulling the door inward when cracked open, as it creates vacuum conditions. If the same door is located on the opposite side of the building, however, the force can be minor and inhibit HVAC effects.
Site effects that resist wind (such as a building sitting among hills/valleys, trees, fences or other obstructions) also affect wind impact. Door size, weight and opening/closing mechanisms play into the analysis as well. Door designers perform these calculations according to code requirements, and architects/specifiers need an understanding to correctly specify a door. However, anyone associated with the door industry can get a valuable appreciation of the effects of wind by understanding these relationships and getting a sense of the potential magnitude of forces involved.
Wind Impact Variables
- Direction
- Velocity
- Acceleration
- HVAC System
- Door Type
- Door Geometry
- Door location
- Building Location
- Building Geometry
- Surrounding Features
- Orientation
Make everything as simple as possible, but not simpler.” Albert Einstein
Designing for the Maximum Condition
While all the aforementioned factors can greatly impact the force on a door and make for complexity, developing a simplified model to measure wind effects can provide a preliminary sense of the magnitude of force to be seen. Toward this, a common engineering practice is designing for the maximum condition. And this simplified preliminary analysis forms the basis for satisfying standards and codes.
In this simplified look, the maximum force and related moment that a door will experience is that condition when the maximum wind condition is presented directly at the door (wind vector pointed directly at the door – “head on”). Examining the ASCE code formulation for the velocity pressure on the door (as seen in figure below), we see the maximum condition is at the leading edge of the building facing the wind (when examining the case of pushing the door inward).
Observing the velocity pressure equation, the building and its surrounding features are factors that would only serve to decrease the pressure from that of a worst-case scenario of the door standing in free space – i.e., factors Kd, Kz, Kzt are <1 (if applied). Thus, we arrive at the worst-case of this formulation by assuming a free-standing door having no wind-resisting surrounding features, with the wind directly facing the door.
As presented in our first blog article on the effects of wind, How to Design and Select Doors for High Wind and Hurricanes, using the basic wind speed as input, the velocity pressure equation is arrived at by exacting the area’s maximum wind conditions from ASCE 7-10 tables and figures of area and risk categories. With the basic wind speed in hand, the corresponding velocity pressure for our simplified model for the maximum condition is directly calculated, and we can proceed to determine the moment and force on the door.
Crunching the Numbers
This simplistic model is summarized by the maximum velocity pressure exerted across the door surface. Physics dictates that the sum of the pressure exerted across the entire area of the door exerts a total Fw(force)=(P) Pressure x Area (Ft2). For the case of a swing door (hinged at the edge), the force imparted by the wind as one opens the door using the handle is arrived at by moment balance: Fw * (1/2 door width)=Fhand * X (distance from hinge to door handle). Note that in the case of a door with an operator, it resists the wind force Fw through its set of linkages, as opposed to our hand operation.
The following equations summarize the calculation of pressure that the wind is exerting on the door.
Pressure (pdf) = 0.00256 x v2 (mph)
Total Force on door (lbs) = Pressure (psf) * Area (ft2)
Total Moment = Total Force x 1/2 door width
Force to Open Door (at handle) = Moment / “X”
Since the pressure is a function of the wind velocity squared, we see that every 5 mph increase in velocity does not have an equal impact, e.g., the force increases about 2.4 lbf from 5–10 mph; whereas, it increases 18.1 lbf from 65–70 mph. Examining the magnitude of force at wind speeds of just 25 mph, an average human will find the force enough to cause difficulty, easily upsetting their balance. As wind speeds above 25 mph are common in most locations, we see that wind is a common and important factor in designing and selecting doors.
Drawing Some Conclusions
What door is best for high wind conditions? From our formulation above, one would choose the smallest area door. However, there is little “wiggle room” on door size because minimum door width and height are fixed by the need to accommodate a large human, movement of furniture/equipment in and out of the building and, of course, building standards and codes.
As to door type, is a swing door or sliding door a better option? Consider your experience of having your car door caught by the wind. As it swings further out, the angle of wind to the door becomes closer to the ideal (90 degrees or “head on”), with the force increasing as the sine of that angle increases. The car door, being a swing door, catches more of the wind as it opens further. Conversely, a sliding door offers only one angle to the wind. And for this reason, it is the door type chosen for high-wind applications.
Horton is a premier, longstanding provider of hurricane-rated doors with patented technologies to address the requirements of storms and high-wind applications. You can view Horton’s offerings here: Hurricane Rated & HD-Storm Sliding Door Systems - ProfilerStorm® Sliding Doors.