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Important Note – These are descriptions to changes implemented in WoodWorks Shearwalls for version 10.0 and may not reflect current behavior
ASCE Commentary C27.3.2 indicates that the 0.00256 constant used in the calculation of wind pressures in Eqn. 27.3-1 corresponds to average air pressure at sea level and if sufficient weather data are available a value based on actual air pressure can be used. Therefore the program now allows you to adjust this value as follows
In the Site Information dialog, a group box called Mass Density Constant has been added, with the following data inputs:
Altitude of building site in feet defaulting to 0.
The following choices for the density used to reference Table C27.3-2. We chose not to allow minimum density.
Average
Maximum
An edit box called Ambient air density, with unit label lb/cu.ft. You can override what is calculated from the other selections. .
A text field with the resulting constant that is used in Eqn. 27.3-1 appears, but can not be changed.
Calculation of Wind Velocity Constant
The arithmetic in C27.3.2 indicates that the equation 27.3-1 for the velocity pressure is just the dynamic pressure term in the Bernoulli equation:
q = ½ ρv2,
So the constant which is 0.00256 is at sea level is calculated as
169 / 162 ρ/g
g = gravitational acceleration = 32.174 ft/s2
ρ = ambient air density in lbm/ft from Table C27.3.-2 (lbm = pounds mass) , The table is interpolated for altitudes that are not listed.
The constant 169 / 162 in this expression comes from ½ * (5200 / 3600)2, the latter two numbers being conversions from feet to miles and seconds to hours, respectively, because v is in miles per hour. g is a conversion factor between pounds of mass and pounds of force. (Note that this g should not be confused with the g that appears in Bernoulli’s equation in the hydrostatic term ρgz for the gravitational effects on heavy fluids, which does not apply here).