High wind and extreme weather systems · Lesson 3 of 4

High wind and extreme weather systems: methods and worked application

A six-step screening method, a fully worked example for a parapet coping, how velocity pressure grows with the square of speed, two pressure bases, where a manufacturer value fits, a second example from SPRI, and the limits of the screening.

You will be able to: Carry a documented wind screening for one roof edge from site facts to demand numbers, show two safety-factor bases, set a manufacturer's published value beside the result without a verdict, and decide what the engineer must be handed.

12 min · 11 slides · 9 questions

All lessons in this module

Slide 1 of 11

The screening method in six steps

A documented screening goes from site facts to numbers the engineer of record can check:

Site facts: location, risk category, height, exposure
Adopted ASCE 7 edition and basic wind speed
Velocity pressure from speed, height, exposure
Roof zones, coefficients and internal pressure
Pressures on the basis the engineer chooses
Hand-off: tornado check, tested assembly, gaps

1: Site facts: location, risk category, mean roof height and exposure
2: The adopted ASCE 7 edition and basic wind speed from the jurisdiction, never a storm category
3: Velocity pressure from the speed squared, the height and the exposure
4: Roof zones and coefficients, with internal pressure for enclosed or partially enclosed buildings
5: Pressures on the basis the engineer chooses: strength level, allowable stress or a safety factor
6: Hand-off: tornado check, tested assembly, substrate and what AFS cannot confirm

The order the AFS screening method follows, from the ASCE 7-16 low-rise method. The amber box is the screening's own limit: outside it the engineer of record's analysis is needed.

Sources

Design Center wind uplift & edge screening method - Architectural Flashing Supply
ASCE 7 (Wind Design Seminar presentation, 2023): St. Petersburg example, ASCE 7-22 and tornado slides - SPRI

Videos for this lesson

What this video shows: A SOPREMA technical support manager demonstrates, per its description, how to calculate wind design pressures for low-slope roofing using ASCE 7-16. It is a roofing membrane maker's walk through the method, not about edge metal.

Video: Low Slope Roofing Wind Design: ASCE 7-16 Calculations by SOPREMA USA, on YouTube. Chosen from the video's published description.

Glossary

Velocity pressure (qh): The wind pressure at roof height, 0.00256 x Kz x Kzt x Kd x Ke x V squared in psf with V in mph; it grows with the square of the speed.
Kz: The velocity pressure exposure coefficient, which depends on the terrain exposure and the height above ground; the AFS screening gives 0.982 for Exposure C at 30 ft.
Kd: The wind directionality factor, 0.85 for components and cladding in SPRI's examples.
Ke: The ground elevation factor added in ASCE 7-16; SPRI notes that 1.00 is a permitted conservative value.
Strength level: The wind load from the map speed used with strength design; allowable stress design takes 0.6 times it.
Allowable stress design (ASD): A design basis that applies a 0.6 factor to the ASCE 7-16 strength-level wind result, as SPRI describes.
Basis A and basis B: The two pressure presentations the AFS screening shows: strength level times 2.0, and strength level times 0.6 times 2.0. The engineer of record chooses.
Tested assembly: The exact product, metal, gauge, cleat, splice and fastener spacing that an accredited lab tested to ES-1; a capacity statement is only valid for that assembly.