Our wind load factors map is a resource for engineers designing structures for wind loads in Seattle. Our map shows wind exposure and terrain adjustment factors based on the wind load provisions in the American Society of Civil Engineers’ (ASCE’s) Minimum Design Loads of Buildings and Other Structures (ASCE 7). You can use our map when you are using ASCE 7 in your designs.
The color-coded areas identify areas in Seattle where DPD will allow factors to be used in calculating wind loads that may differ from the values in ASCE 7. We will accept the topographic factor (Kzt) values on this map as default values for Kzt instead of the calculated values required by Section 18.104.22.168 of the 2005 edition ASCE 7-05 (Section 26.8.2 of the 2010 edition). In addition, where ASCE 7 would ordinarily require the use of factors associated with Exposure Category D (shoreline areas with at least 1 mile of water upwind — see Section 22.214.171.124 of ASCE 7-05 or Section 26.7.3 of ASCE 7-10 for a more precise definition), we will accept the use of the factors associated with Exposure Category C in the areas shown on the map. Other shoreline areas are not required to be designated as Exposure Category D.
Use of the mapped factors will be accepted by DPD without question, but engineers may also choose to justify the use of factors they derive, as the mapped values may not be 100 percent accurate in all cases.
The engineer or designer may determine their own values if they feel these values are not correct. Derivations need to take into account topography and winds in all directions.
Maps courtesy of Scott Beard, SEAW Wind Committee.