When to Use the Envelope Procedure - ASCE 7-22 Chapter 28 vs Chapter 27 for Low-Rise Wind Loads

When can I use Chapter 28 instead of Chapter 27?

0:00Six questions worth closing on. Whenever the building is low rise, per section 26.2: mean roof height under sixty feet, and not exceeding the building's least horizontal dimension, enclosed or partially enclosed, with a flat, gable, or hip roof. This house qualifies easily. Where it applies, the envelope method is usually faster — one velocity pressure, table read zones, four defined load cases, no direction by direction bookkeeping. It also gets the Exposure B K sub Z equals zero point seven zero footnote, which Chapter 27 does not have. A building that fails the low rise test — taller than sixty feet, or taller than it is wide — must use Chapter 27 instead, which has no such geometric limits.

Why doesn't internal pressure change the base shear?

0:44Because the internal pressure acts on every interior surface with the same magnitude, its horizontal resultants on opposite walls are equal and opposite. On this house, the positive internal pressure case lowers the windward zone to six point seven eight psf and deepens the leeward suction to negative eleven point nine nine; the negative case does the reverse. But both cases sum to the identical nineteen thousand eight hundred forty three point one eight pounds of base shear.

1:09It absolutely does matter for roof uplift, though — there is no opposing surface to cancel against — and for components and cladding, and for any single wall line, where you keep whichever sign is worse.

What are the E zones and the dimension a?

1:22Wind tunnel studies found that pressures concentrate near the building corner facing the oncoming wind, so the figure boosts coefficients in strips near a reference corner — zone one E windward wall, two E and three E roof, four E leeward wall, each within twice a of that corner. The dimension a is the smaller of ten percent of the least horizontal dimension and zero point four h, with a floor at four percent and three feet — here, three feet, so the strips are six feet wide. Because the wind can approach any corner, the pattern is technically applied at each corner in turn; for a symmetric building like this one, one orientation per direction captures the envelope.

Why is the base shear smaller parallel to the ridge?

2:01Two reasons. First, the along wind walls are the small ones — six hundred fifteen square feet of gable end, against one thousand eighty square feet of long wall in the transverse direction. Second, nothing else helps: the side walls carry equal suction on both faces, netting to zero across the building, and the roof planes are parallel to the flow, so their large suctions produce uplift but no along wind force at all. What is left, eight thousand nine hundred thirty nine point five five pounds, is under half the transverse shear — low enough that the minimum design wind load takes over instead.

What is the Section 28.3.3 total horizontal load check?

2:32Section 28.3.3 says the total horizontal shear cannot be less than what you get by neglecting the wind forces on the roof entirely. On a pitched roof, the windward slope's suction acts perpendicular to that slope, which gives it a small upwind horizontal component — so including the roof can actually reduce the computed base shear, and the code refuses to let you take that credit. There is an exception for buildings using moment frames as their lateral system, which genuinely do benefit from modeling the roof pressures as applied. This wood frame shear wall house takes no such exception — in StructSuite, the moment frame attestation is an explicit checkbox, unchecked by default.

When does the Section 28.3.6 minimum govern?

3:12And this last question is entirely about that projected-area picture, so here it is again. Whenever the computed envelope shear falls below sixteen psf on the projected wall silhouette plus eight psf on the projected roof silhouette, checked per direction. For this house, the floors are nineteen thousand six hundred eighty pounds perpendicular to the ridge and nine thousand eight hundred forty parallel to it — and three of the four load cases fall short of their computed value and get lifted to the floor. Suburban Exposure B terrain, moderate wind speeds, and torsional cases are the classic triggers, and for light frame construction, the minimum governing is the rule rather than the exception.

Reproduce this in StructSuite

3:53Enter this building in StructSuite's Wind module — Chapter 28, this geometry, one hundred fifteen miles an hour, Exposure B — and every table in this example appears on screen and in the printed calculation package.