GCp and Roof Zone Changes in ASCE 7-16

While most structural engineers have noticed the obvious changes to the wind speed maps included in ASCE 7-16, many may not be familiar with other changes within the document.  One of the most significant changes in the new ASCE 7-16 wind provisions is changes to the roof pressure coefficients and roof zones.

In November 2018, Don Scott, SE, from PCS Structural Solutions, presented ASCE 7-16 Wind Provisions – Changes Affecting the Design Provisions.  Don covered the changes to the new wind speed maps and explained how to determine the design wind speed.  He also presented the significant changes to the roof pressure coefficients, and explained the effects of the new Elevation Factor.

In the past, the roof pressure coefficients included in ASCE7 were determined from wind tunnel studies dating back to the 1970s.  Since that time, there have been tremendous strides made in the study of aerodynamics of low-rise buildings and validation of wind tunnel studies using full scale models.  Using new studies from NIST and TTU, the previous roof pressure coefficients and roof zones were shown to be unconservative.  Thus, the new coefficients and roof zones shown in ASCE 7-16 more accurately represent the actual roof pressures found in recent studies, and the roof zones are mostly dependent on the height of the building rather than the plan dimensions.  This can be a significant change to future roof designs, depending on your specific location.

Also, the new provisions now include uplift to gable overhangs. Other changes include the addition of hip roofs angles to include divisions from 0 to 20 degrees, 20 to 27 degrees, and 27 to 45 degrees. The Commentary now includes the equations for all roof pressure coefficients, so engineers can program these into spreadsheets.  The Elevation Factor has also been included into the main body of the standard, which had previously been only in the Commentary.  This Elevation Factor allows for a reduction in wind pressures depending on the site specific altitude.

Overall, the changes to the wind provisions in ASCE 7-16 may certainly affect your future projects and is very dependent on your site location.  Although the roof pressure coefficients have increased along with the roof zones, these increases may be offset by the reduction in wind speed and inclusion of the Elevation Factor.  However, certainly the new roof pressure coefficients will increase cladding pressures on the roof within hurricane coastlines.  In all cases, the roof zones are larger than previous editions, and better reflect actual conditions.


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