Velocity Pressure qz Step by Step - How to Calculate qz with Kz, Kzt, Ke (ASCE 7-22 Eq. 26.10-1)

2:26Published October 31, 2026Narrated, with captions and a full transcript

How do you calculate wind velocity pressure qz? By ASCE 7-22 Equation 26.10-1, qz = 0.00256 Kz Kzt Ke V² in psf, with V in mph. Kz comes from Table 26.10-1 for the exposure category at each height z, and the directionality factor Kd is NOT in this equation in ASCE 7-22 - it moved into the design pressure equation, Eq. 27.3-1. Reusing a spreadsheet from an earlier edition double counts it.

This is Step 1 of a full Chapter 27 Directional Procedure worked example on a gable-roof house (V = 100 mph, Exposure C, mean roof height h = 22 ft): Kz from Table 26.10-1, Kh interpolated at the mean roof height (0.916), one full hand substitution at z = 15 ft (21.76 psf), the velocity pressure profile at every height the building needs (qh = 23.45 psf), and a sensitivity check on exposure and wind speed.

Watch the full worked example Open the Wind Loads module

Transcript and details

Questions this video answers

What is the velocity pressure equation in ASCE 7-22?: qz = 0.00256 Kz Kzt Ke V² (Eq. 26.10-1), in psf with V in mph.
Is Kd in the velocity pressure equation?: No. In ASCE 7-22 Kd moved into the design pressure equation, Eq. 27.3-1; putting it in qz as well double counts it.
Where does Kz come from?: Table 26.10-1, by exposure category and height z, interpolated linearly between tabulated rows.
How do you get Kh at the mean roof height?: Interpolate Table 26.10-1 at h - here 22 ft between the 20 ft and 25 ft rows, Kh = 0.916.
What is qz at 15 ft for 100 mph, Exposure C?: 0.00256 x 0.85 x 1 x 1 x 100² = 21.76 psf.
Which surfaces use qh instead of qz?: The leeward wall, both side walls, the roof and the internal pressure all use qh; only the windward wall varies with height.
How much does exposure category change qh?: Exposure B drops it to 16.28 psf (31% lower); Exposure D raises it to 28.06 psf (20% higher).
How much does a 10% higher wind speed change pressure?: About 21% - pressure grows with V², so 110 mph gives qh = 28.37 psf.

How to Calculate Wind Loads — Directional Procedure (ASCE 7-22 Chapter 27)

Transcript

The narration word for word, 409 words, from the video's own captions. Select a time to play from there.

Velocity pressure qz - the equation

0:00Step one, velocity pressure. The velocity pressure equation. Zero point zero zero two five six, times K sub Z, times K sub Z T, times K sub E, times V squared, in pounds per square foot with V in miles per hour. Notice what is missing: K sub D, the directionality factor, is not in this equation in ASCE 7-22 — it moved into the pressure equation instead, equation 27.3-1. Reusing an old spreadsheet from an earlier edition here double counts it. Keep that in mind; we will use K sub D properly in Step 3.

Kz from Table 26.10-1

0:35K sub Z comes from Table 26.10-1. Here are the rows bracketing the heights this building needs, values exactly as tabulated, for exposure C.

Interpolating Kh at the mean roof height

0:46Between tabulated rows, interpolate linearly. At the mean roof height, twenty two feet — the number we found from the building drawing a minute ago — K sub H comes out to zero point nine one six. That twenty-two-foot mean roof height is the same number we found from the elevations at the very start — it is what makes this interpolation possible.

Hand substitution at z = 15 ft

1:08One full substitution, so the arithmetic is not a black box. At fifteen feet, Exposure C gives K sub Z equal zero point eight five. Zero point zero zero two five six, times zero point eight five, times one, times one, times one hundred squared — twenty one point seven six pounds per square foot.

The qz profile at every height

1:25And the full velocity pressure profile at every height this building needs. Twenty three point four five pounds per square foot, at h, is the workhorse value — the leeward wall, both sidewalls, the roof, and the internal pressure all use q sub H. Only the windward wall varies with height and uses q sub Z directly.

Sensitivity - exposure category and wind speed

1:43Same building, same height, run through the same engine three ways. Exposure B instead of C drops q sub H to sixteen point two eight — thirty one percent lower. Exposure D raises it to twenty eight point zero six — twenty percent higher. And wind speed matters even more: a ten percent increase, to one hundred ten miles an hour, raises q sub H to twenty eight point three seven — a twenty one percent jump in pressure, because pressure grows with velocity squared. Terrain roughness and wind speed are worth more than most member size decisions — classify them honestly, they deserve the first minutes of any wind calculation, not the last.