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A ground elevation factor Ke has been added to account for the effect of altitude on velocity pressure qz in equation 26.10-1 (Section 26.10.2). Ke is determined from Table 26.9-1 or the equation in Note 2 to that table. The equation
Ke = e-0.000119zg
with zg in meters, is based on the formula for change in pressure p or density ρ with altitude z,
p/p0 = ρ/ρ0 = e -gz/RT
g is acceleration due to gravity, R the gas constant of air, and T is temperature in Kelvin = 288 degrees.
Previously a procedure in C27.3.2 was used to modify the factor 0.0256 in the equation for qz for the change of air density with altitude. The density was taken from a table, and the factor is just ρg in the expression p = ρgv2 for velocity pressure. Now the sea level factor 0.0256 is always used and then modified by the Ke factor.
Note that the concept of minimum, average and maximum density based on factors such as temperature, weather, season and latitude has been dropped and the new formula is based on density at 15 degrees C. Note too that the densities shown in ASCE 7-16 Table C26.9-1 do not correspond exactly to any of the minimum, maximum, or average densities from 7-10 C27.3-2, so this is a substantive change to the calculation of velocity pressure in all cases other than sea level pressures.
The data group Velocity pressure coefficient (C27.3.2) in the Site Information dialog has been renamed Ground elevation factor Ke.
The input of the air density category (minimum, average, maximum) and ambient air density, and the display of resulting mass density constant, have been removed. The Altitude input has been retained, and the resulting Ke factor is shown.
The calculation of the mass density constant has been removed and it is now always 0.0256. The Ke factor is calculated using the equation in Note 2 of Table 26.9-1 and applied to the velocity pressure qz in equation 26.10-1.
In the Wind Load Generation Details,
Ke has been added to the Definitions, as has ground elevation zg. The definition for what we called d, the mass density constant, has been removed.
Ke has been added to the equation for q, which has also been changed to show the number 0.0256 instead of the symbol d for air density. The equation for Ke has been added and the one for d removed.
The value calculated for Ke has been added to the Data (all loads) section.
A Partially Open category has been added for those buildings that are not Open, Enclosed, or Partially Enclosed. Previously Enclosed buildings were defined as not Open or Partially Enclosed, but now they are defined by having opening area in each wall less than the lesser of 4 sq. ft. or 1% of the wall area.
The internal pressure co-efficients GCpi for Partially Open walls are the same as for Enclosed, +0.18 and -0.18, so this change does not impact pressures generated.
In the Site Dialog, Partially open has been added to the list of Enclosure choices. The wording Partly Enclosed has changed to Partially enclosed.
These changes are also reflected in the Site Information section of the Wind Load Generation Details.
The program applies the new rules to the estimation of enclosure classification that is triggered by a button in the Site dialog. Note that a "wall" is taken by Shearwalls to mean an entire building face.
Internal pressure GCpi coefficients 0.18 and -0.18 are applied to the Partially Open walls when selected.
A limit has been added to the end zone width a for low-rise structures greater than 300 m in each dimension, with roof slopes less than 7 degrees. For these structures a is limited to 0.8 times the mean roof height h.
This limit is applied to those structures designed using the Envelope Procedure. The ordinary end zone width is defined as
a = max (min (.1 D, 0.4h), max ( 0.4D, 3ft) ).
where D is the least horizontal dimension of the building. For buildings whose least horizontal direction is greater than 300 ft, this is now
a = min (max ( min (.1 D, 0.4h), max ( 0.4D, 3ft) ), 0.8h).
End zones defined for the Envelope Procedure for MWFRS loads in Fig 28.3-1 have elevated pressure coefficients relative to the rest of the building face, and for C&C wall loads defined in Fig 30.3-1 have elevated coefficients for negative pressures (suction).
Note that there are few wood buildings with both dimensions greater than 300 ft.