January22

Proposed WindBorne Debris Definition

by Liz Kovach,

On 1/12/16 the BBRS accepted the HBRAMA proposed WINDBORNE DEBRIS definition below by unanimous vote.

Areas within hurricane-prone regions located in accordance with one of the following:

  1. Within 1 mile (1.61 km) of the coastal mean high water line where the ultimate design wind speed, Vasd, is 130 mph (58 m/s) or greater.
  2. In areas where the ultimate design wind speed, Vasd, is 140 mph (63.6 m/s) or greater; or Hawaii.

Converting from Vasd values of 130 and 140 mph yields Vult values of about 170 and 180 mph, respectively. These Vult wind speeds do not occur anywhere in MA.  So the windborne debris will not be applicable anywhere in MA.

The definition change will eliminate mandatory Opening Protection requirements i.e. impact windows, shutters, plywood panels.

The Wind Zone tables in the 9th Edition code (IRC 2015) will list the Vult wind speeds.

The conversion of Vult to Vasd will be found in the following IRC 2015 section –

R301.2.1.3 Wind speed conversion.

Where referenced documents are based on nominal design wind speeds and do not provide the means for conversion between ultimate design wind speeds and nominal design wind speeds, the ultimate design wind speeds, Vult  of Figure R301.2(4)A shall be converted to nominal design wind speeds, Vasd, using Table R301.2.1.3.

The Islands and most of the Cape are listed in the 9th Edition Tables with an ultimate (Vult) design wind speed of 140 mph.

When converted to nominal (Vasd) design, it is equivalent to a basic wind speed of 108 mph.

wind speed conversion table

The 9th Edition Code process continues and Public Hearing will be announced in the coming months. 

There will be no concurrency period this time around but the BBRS has said they plan to give a 6 month start date notice once the code is promulgated. 

I expect we will not see the 9th Edition until late this year or Jan 2017.

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