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Residential Design Pressure

Frequently Asked Questions

Exposure

Exposure B

For buildings with a mean roof height of less than or equal to 30 feet (9144 mm), Exposure B shall apply where the ground surface roughness, as defined by Surface Roughness B, prevails in the upwind direction for a distance of at least 1,500 feet (457 m). For buildings with a mean roof height greater than 30 feet (9144 mm), Exposure B shall apply where Surface Roughness B prevails in the upwind direction for a distance of at least 2,600 feet (792 m) or 20 times the height of the building, whichever is greater.

Exposure C

Exposure C shall apply for all cases where Exposure B or D does not apply.

Exposure D

Exposure D shall apply where the ground surface roughness, as defined by Surface Roughness D, prevails in the upwind direction for a distance of at least 5,000 feet (1524 m) or 20 times the height of the building, whichever is greater. Exposure D shall also apply where the ground surface roughness immediately upwind of the site is B or C, and the site is within a distance of 600 feet (183 m) or 20 times the building height, whichever is greater, from an Exposure D condition as defined in the previous sentence.

Risk Category

This is the risk associated with the use of the building.

RISK CATEGORY
NATURE OF OCCUPANCY
I
 Buildings and other structures that represent a low hazard to human life in the event of failure, including but not limited to:
  • Agricultural facilities.
  • Certain temporary facilities.
  • Minor storage facilities.
  • Screen enclosures.
II
 Buildings and other structures except those listed in Risk Categories I, III and IV.
III
 Buildings and other structures that represent a substantial hazard to human life in the event of failure, including but not limited to:
  • Buildings and other structures whose primary occupancy is public assembly with an occupant load greater than 300.
  • Buildings and other structures containing Group E occupancies with an occupant load greater than 250.
  • Buildings and other structures containing educational occupancies for students above the 12th grade with an occupant load greater than 500.
  • Group I-2 occupancies with an occupant load of 50 or more resident care recipients but not having surgery or emergency treatment facilities.
  • Group I-3 occupancies.
  • Any other occupancy with an occupant load greater than 5,000.a
  • Power-generating stations, water treatment facilities for potable water, wastewater treatment facilities and other public utility facilities not included in Risk Category IV.
  • Buildings and other structures not included in Risk Category IV containing quantities of toxic or explosive materials that:
    • Exceed maximum allowable quantities per control area as given in Table 307.1(1) or 307.1(2) or per outdoor control area in accordance with the Florida Fire Prevention Code; and
    • Are sufficient to pose a threat to the public if released.b
IV
 Buildings and other structures designated as essential facilities, including but not limited to:
  • Group I-2 occupancies having surgery or emergency treatment facilities.
  • Fire, rescue, ambulance and police stations and emergency vehicle garages.
  • Designated earthquake, hurricane or other emergency shelters.
  • Designated emergency preparedness, communications and operations centers and other facilities required for emergency response.
  • Power-generating stations and other public utility facilities required as emergency backup facilities for Risk Category IV structures.
  • Buildings and other structures containing quantities of highly toxic materials that:
    • Exceed maximum allowable quantities per control area as given in Table 307.1(2) or per outdoor control area in accordance with the Florida Fire Prevention Code; and
    • Are sufficient to pose a threat to the public if released.b
  • Aviation control towers, air traffic control centers and emergency aircraft hangars.
  • Buildings and other structures having critical national defense functions.
  • Water storage facilities and pump structures required to maintain water pressure for fire suppression.
How to Determine the Zone 4 or 5 and Explanation for End Wall

               To determine the design pressures, you will need to enter the opening size and determine if the opening is in zone 4 or zone 5. The diagram below shows each zone, the dimension “a” is the end wall displayed below, so from the corner and form a distance of “a” it falls under zone 5.
If only a portion of the opening is within this area, assume zone 5.

Determine zone 4 or 5
Height above the ground

             Height from ground level to the middle of the roof height.

Height above the ground for a house
Wind load

            Ultimate wind speed, this is the speed design, it is determined by the building code.

wind speed wind load florida
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Exposure represents the adjustments due the characteristics of the terrain surrounding the building.

Exposure B.For buildings with a mean roof height of less than or equal to 30 feet (9144 mm), Exposure B shall apply where the ground surface roughness, as defined by Surface Roughness B, prevails in the upwind direction for a distance of at least 1,500 feet (457 m). For buildings with a mean roof height greater than 30 feet (9144 mm), Exposure B shall apply where Surface Roughness B prevails in the upwind direction for a distance of at least 2,600 feet (792 m) or 20 times the height of the building, whichever is greater.

Exposure C.Exposure C shall apply for all cases where Exposure B or D does not apply.

Exposure D.Exposure D shall apply where the ground surface roughness, as defined by Surface Roughness D, prevails in the upwind direction for a distance of at least 5,000 feet (1524 m) or 20 times the height of the building, whichever is greater. Exposure D shall also apply where the ground surface roughness immediately upwind of the site is B or C, and the site is within a distance of 600 feet (183 m) or 20 times the building height, whichever is greater, from an Exposure D condition as defined in the previous sentence.