28 FEBRUARY 2003
Frost Depth, Basic Wind Speed, Ground Snow Loads
ANSI/ASCE 7-95, Minimum Design Loads for Buildings and Other Structures
American Society of Civil Engineers
1015 15th Street NW , Suite 600
Washington DC 20005
NOTE: Use UFC 3-310-01, Load Assumptions for Buildings, for reference on some
frost depth and ground snow load values.
Suggestions For Use. The dry bulb, wet bulb, and humidity ratio values in
Figure 2-1 are peak load conditions and are used for sizing mechanical equipment.
Design guidance determines the frequency of occurrence design is to be based upon.
Dry Bulb Temperature. The 0.4% dry bulb temperature value is seldom
used for sizing conventional comfort control systems but is sometimes appropriate for
mission-critical systems where equipment failure due to high heat would be
unacceptable. Using the 0.4% value for equipment sizing requires that the engineer
consider its operation at less-than-peak design conditions. In the past, oversized
cooling equipment has been incapable of modulating during the more common range of
operating conditions, yielding comfort control problems. Also, over-sized equipment
cycles on and off more frequently, increasing maintenance costs and failing to remove
enough moisture to maintain humidity control.
2-184.108.40.206 Similar special considerations apply to the extreme low dry bulb
temperature. Heating equipment designed for extreme conditions must be evaluated
carefully to ensure that it will modulate properly to maintain comfort at less extreme
outdoor temperatures that occur in 99.6% of the hours during the year.
2-220.127.116.11 The mean coincident value for humidity at the 0.4% peak dry bulb
temperature is not the highest moisture value and must not be used for design of
humidity control systems. The mean coincident value is the arithmetic average of all the
moisture levels that occur when the dry bulb temperature is high; however, the highest
moisture values typically occur when the dry bulb temperatures are lower.
Wet Bulb Temperature. High wet bulb temperature is used for sizing
cooling towers and other evaporative equipment.
Peak Humidity Ratio. Peak humidity ratio is used for sizing
dehumidification systems. Peak moisture condition usually represents a higher
enthalpy (total heat) than peak dry bulb condition. Consequently, engineers use the
peak moisture condition to cross-check operation of a system that may be primarily
intended to control temperature.
Coincident Wind Speed. Coincident wind speed allows the engineer to
accurately estimate latent loads due to infiltration of humid air in the summer and of dry
air in the winter.