High wind and extreme weather systems · Lesson 1 of 4

High wind and extreme weather systems: principles and terminology

Why the roof perimeter is a leading place for wind failures, the six hazards a building enclosure faces, the difference between a gust and a sustained wind, what the hurricane and tornado scales do and do not say, how wind loads a roof edge, and the stop-work boundary.

You will be able to: Name the weather hazards that load or damage a building enclosure, say which scale or standard speaks to which, and define gust, sustained wind, suction and internal pressure well enough to read a wind report without misusing it.

12 min · 12 slides · 10 questions

All lessons in this module

Slide 1 of 12

Why the perimeter goes first

SPRI's edge-metal seminar concludes that edge metal failure is a leading cause of roof failures.

FEMA P-2181 says the roof border is at more risk from wind uplift and wind-driven rain than the rest of the roof.

An SPRI slide citing FM Global reports that in a study of built-up roof losses, 59% occurred because the roof perimeter failed.

Sources

Videos for this lesson

What this video shows: IBHS's 41-second clip of its July 2012 full-scale test of masonry strip-mall-type buildings: its description says a wall separated from the building during a 110 mph gust after a 2x4 shattered the window, and gives the one-minute sustained equivalents.

Video: How Commercial Walls Fail in Extreme Wind Conditions by Insurance Institute for Business & Home Safety (IBHS), on YouTube. Chosen from the video's published description.

Glossary

Basic wind speed: The map wind speed in ASCE 7 that sets design wind loads; SPRI labels the ASCE 7-10 and 7-16 map speeds 3-sec peak gust values, and the speed depends on the edition and the risk category of the building.
Sustained wind: Wind averaged over a period such as one minute; the National Hurricane Center uses maximum sustained wind for hurricane categories, and IBHS quotes one-minute equivalents of gust speeds.
Gust: A short peak in wind speed; SPRI labels the ASCE 7-10 and 7-16 map speeds 3-sec peak gust values, and the National Weather Service describes EF estimates as three-second gusts.
Saffir-Simpson scale: The National Hurricane Center scale that rates hurricanes 1 to 5 from maximum sustained wind speed only, without storm surge, rainfall flooding or tornadoes.
Enhanced Fujita scale: The National Weather Service tornado rating, EF0 to EF5, a set of wind estimates based on damage rather than measurements.
Components and cladding: Parts of the enclosure, such as coping, edge metal and wall cladding, that receive wind loads directly and pass them on to the structure.
Windborne debris: Objects carried by wind that strike an enclosure; blown-off copings, edge flashing and loose gravel are both victims and sources of it.
Internal pressure coefficient (GCpi): The factor for pressure inside the building that adds to outward load on the enclosure: 0.18 for an enclosed building and 0.55 for a partially enclosed one in SPRI's table.