Wind Load Calculation Step by Step - Envelope Procedure for Low-Rise (ASCE 7-22 Ch. 28)

Wind Load Calculation - Envelope Procedure

0:00This is a complete, no steps skipped MWFRS wind calculation, this time by ASCE 7-22, Chapter 28 — the Envelope Procedure — on the building this method sees most often in practice: a two story, sixty by thirty foot suburban house with a four in twelve gable roof. We go from the basic wind speed to the velocity pressure, through every GC sub p f zone, pressure, and surface area for all four load cases, resolve each zone into base shear and roof uplift, and finish with the minimum design wind load check — which for this house governs three of the four cases. Every number here was computed by the same engine that runs StructSuite's Wind module.

The building - what we're designing for

0:40Here is the building. Sixty feet by thirty feet in plan, ridge along the long dimension. An eighteen foot eave, a twenty three foot ridge, a four in twelve roof pitch. Mean roof height: eighteen plus twenty three over two, twenty point five feet. Hold onto that number — almost everything in this chapter is measured relative to it. And every input this example needs, on one page.

1:08Chapter 28 is a low rise only method: mean roof height under sixty feet, and not exceeding the building's least horizontal dimension. Here, twenty point five feet clears both — it is well under sixty, and well under the thirty foot width. That is the whole qualifying test. The payoff for meeting it: the GC sub p f coefficients of Figure 28.3 dash 1 are pseudo pressures, calibrated from wind tunnel tests to envelope the worst structural actions — shear, uplift, frame moments — over every wind approach angle at once.

1:40So four defined load cases replace the direction by direction bookkeeping Chapter 27 requires. And never mix the two: Chapter 28's coefficients already include the gust effect, so there is no separate G factor here, and Chapter 27's C sub P values must never be substituted into this chapter's pressure equation. Four inputs before any equation. K sub D, zero point eight five for buildings. Exposure B — urban and suburban terrain with closely spaced obstructions, the correct call for most residential subdivisions, judged by the terrain upwind of the site, not the lot itself. Enclosed — openings small enough that the area ratios in section 26.12 are satisfied — giving an internal pressure coefficient of plus or minus zero point one eight. And worth remembering: a failed garage door reclassifies the building as partially enclosed and triples that internal pressure to

2:30plus or minus zero point five five. That is an opening protection decision, not an architectural one.

Step 1 - Velocity pressure qh (the Exposure B footnote)

2:40Step one, velocity pressure — and Chapter 28 has a rule Chapter 27 does not. The same shape of equation as Chapter 27, and the same trap: K sub D is not in this equation. It applies later, inside the pressure equation. Reusing an old spreadsheet that bakes it in here double counts it. K sub H comes from Table 26.10 dash 1, at the mean roof height — twenty point five feet. And here is the Chapter 28 specific rule.

3:09Table 26.10 dash 1 directs that for buildings designed with the envelope procedure, in Exposure B, at heights under thirty feet, K sub Z is taken as zero point seven zero flat — not the smaller interpolated value the table's own rows would otherwise give you. That interpolated value, something like zero point six two to zero point six six, applies only to the directional procedure. Skipping this footnote is the most common Chapter 28 error, and it runs unconservative — so here, K sub H is zero point seven zero, not zero point six two four.

3:43Zero point zero zero two five six, times zero point seven zero, times one, times one, times one hundred fifteen squared — twenty three point seven zero pounds per square foot. And unlike Chapter 27, there is no height varying profile up the windward wall in this method — every single pressure in this calculation uses this one value. That is part of what makes the envelope procedure fast.

Step 2 - Internal pressure

4:07Step two, internal pressure. Twenty three point seven zero, times zero point eight five, times plus or minus zero point one eight — plus or minus three point six three pounds per square foot. Both signs get checked: positive is the building pressurized, wind pushed in through windward openings; negative is depressurized, suction pulling through leeward openings. Here is the fact worth understanding, not just memorizing: internal pressure cancels out of the base shear. The same internal pressure acts on every interior surface at once, so on opposite walls of equal area, its horizontal push on one side exactly cancels its pull on the other. The two internal pressure signs give different pressures at every individual zone, but the same total base shear. It does not cancel for roof uplift, though — there is no opposing surface to cancel against — nor for components and cladding, or the

4:55design of a single wall line. There, you keep whichever sign makes that particular check worse.

Step 3 - The edge-zone dimension a

5:04Step three — a single dimension that controls where the highest pressures live. Figure 28.3 dash 1's higher, quote, E, unquote coefficients apply near the corner where wind effects concentrate — the wind tunnel data shows pressure spikes right at the building corner facing the oncoming wind. Their extent is set by a: ten percent of the least horizontal dimension, three feet, against zero point four times the mean roof height, eight point two feet — the smaller governs, three feet — checked against a lower bound of three feet as well. So a equals three feet, and every E zone in this building lives within six feet, twice a, of the reference corner.

Step 4 - The zones of Figure 28.3-1

5:41Step four — naming the surfaces. For the transverse direction, wind perpendicular to the ridge — that is load cases one and three — the envelope method numbers the surfaces: zone one is the windward wall, two the windward roof slope, three the leeward roof slope, four the leeward wall. Each of those has an E twin, in the six foot end strip. For the longitudinal direction, wind along the ridge — load cases two and four — zone five is the windward gable end, six the leeward end.

6:07The two long side walls become one and four, the two roof slopes become two and three, and again E strips sit within six feet of the windward end. Drawn out, load case one: the windward wall pushed, both roof slopes and the leeward wall pulled. The six foot strip at the reference corner, shaded, takes the higher E coefficients; the remaining fifty four feet of the building takes the ordinary zone values.

Step 5 - External pressure coefficients GCpf

6:32Step five, the coefficients themselves — and a roof angle that does not land on a tabulated row. The transverse table is written for roof angle rows at zero to five degrees, twenty degrees, and thirty to forty five. Our four in twelve roof works out to eighteen point four degrees — between the tabulated rows, so we interpolate. Zone one, worked in full: zero point four zero, plus eighteen point four minus five, over twenty minus five, times zero point five three minus zero point four zero — zero point four zero plus zero point eight nine six times zero point one three — positive zero point five one six.

7:06Zones two and two E, by contrast, do not move at all between these rows — only zone one shifts with angle here. The longitudinal table, for load case two, is a single row — it does not vary with roof angle at all, because the wind runs along the ridge regardless of pitch. And the torsional cases reuse both of these tables, adding a reduced pressure zone on the half of the building away from the reference corner — at just twenty five percent of the basic value.

7:37And the sign convention: positive GC sub p f is pressure toward the surface, negative is suction away from it. On this house, only zones one and one E, the transverse windward wall, and five and five E, the longitudinal windward gable, are positive. Every other zone is suction.

Step 6 - Design pressures p

7:54Step six turns coefficients into pressures. Q sub H times K sub D times the bracket — the zone coefficient minus the internal coefficient. Q sub H times K sub D comes out to twenty point one four pounds per square foot, and every zone gets two pressures, one per internal pressure sign. Here is the full table — every zone, both signs, exactly as StructSuite prints it.

8:20Two worked lines. Zone one, with positive internal pressure: twenty point one four times zero point five one six minus zero point one eight — six point seven eight pounds per square foot. And zone two E — the corner roof strip: twenty point one four times negative one point zero seven minus zero point one eight — negative twenty five point one eight. The strongest suction anywhere on this building, which is exactly why roof fastening at corners is always the first components and cladding conversation on a real project.

Step 7 - Zone areas, case by case

8:48Step seven — and this is where most hand calculations go wrong, so every area gets its formula. Two facts anchor every area in this step, both read straight off this elevation: the roof's slope length, and the six-foot end-zone strip. The roof plane's slope length: half the width over cosine of the roof angle — fifteen over cosine eighteen point four degrees — fifteen point eight one feet. And the E strips run six feet, twice a, from the reference corner. Everything below has to add up to the same building totals:

9:17two side walls of one thousand eighty square feet each, two gable ends of six hundred fifteen each, two roof planes of nine hundred forty eight point six eight each. Load case one areas.

9:35Load case two: the side walls and slopes keep the same shapes, running along the length, and the gable ends come in. The full gable end: the eighteen foot wall rectangle plus the five foot rise triangle above it — five hundred forty plus seventy five — six hundred fifteen square feet. Load case three, torsional, perpendicular: the case one pattern on the reference half of the building length, twenty five percent zones on the other half.

10:02And load case four, torsional, parallel: each gable end split at mid width, full pressure on the reference half, twenty five percent on the other. Check: fifty five point five plus two hundred fifty two plus three hundred seven point five comes to six hundred fifteen square feet exactly — the corner strip, the reference half, and the torsional half tile the gable end with nothing left over.

Step 8 - Forces, base shear, and uplift

10:24Step eight — turning pressure times area into the forces a lateral system actually resists. Four rules turn these pressures into structural actions. Walls carry their full force horizontally, and the leeward wall's suction adds to the base shear, pulling the building downwind, the same way it worked in Chapter 27. Side walls that run parallel to the wind — that is zones one and four in the longitudinal cases — contribute zero along wind shear, because their equal suctions on opposite faces of the building cancel each other out. Roof slopes contribute their sine-theta component along the wind only when the wind is perpendicular to the ridge, and every roof zone contributes its cosine-theta component to uplift, in both wind directions. And section 28.3.3: the total horizontal shear cannot be less than what you get by simply ignoring the roof — because on a

11:11pitched roof, the windward slope's suction actually pulls slightly upwind, and the code refuses to let you bank that credit. There is an exception for buildings using moment frames as the lateral system; this wood frame shear wall house takes no such exception. Load case one, with positive internal pressure shown — the negative case sums to the identical shear, exactly as we said internal pressure would. And here is section 28.3.3 in action. The roof's own net along wind component comes out to negative one thousand four hundred forty four pounds — the stronger windward slope suction actually leans the building slightly upwind. Include that, and the raw sum drops below the walls-only value. That is precisely the situation the code's floor exists for: the total horizontal shear may not be less than the walls-only twenty one thousand two hundred eighty seven point five

12:02two pounds, so that walls-only number is what governs. Roof uplift is not affected by this floor at all — summing F cosine theta over all four roof zones gives negative twenty eight thousand five hundred ninety point eight six pounds, upward, because on a four in twelve roof the cosine factor keeps almost all of that suction working vertically. Load case two: only the gable ends act along the wind at all.

12:26Roof uplift for this case comes to negative twenty six thousand seven hundred twenty three point four five pounds. Load case three, torsional perpendicular, runs the same rules over the split pattern: the sum including the roof comes to twelve thousand seven hundred forty six point seven five, walls only to thirteen thousand six hundred eighty two point four — section 28.3.3 governs again, and the walls-only number wins. Load case four, torsional parallel, gives five thousand seven hundred eighteen point four seven pounds of shear. But the actual point of these two cases is not the magnitude of the shear — it is the twist. Full pressure on one half of the building and twenty five percent on the other loads the diaphragm and the wall lines unevenly, and that uneven loading is what a torsional case is checking for.

13:14Drawn out, load case two: only the gable ends push and pull along the wind. The long side walls and the roof planes run parallel to the flow, so their suctions produce uplift, but contribute nothing to the along wind shear.

Step 9 - Minimum design wind load

13:27Step nine — the same floor Chapter 27 has, worked for this building. The eighteen foot eave and the five foot roof rise both come from this same elevation. The same projected area logic as Chapter 27. Perpendicular to the ridge, the silhouette splits into a wall band and a roof band. Parallel to the ridge, the gable end triangle is wall area, so the whole silhouette takes the wall rate.

13:53Perpendicular to the ridge: sixty by eighteen, a thousand eighty square feet of wall at sixteen psf, seventeen thousand two hundred eighty pounds. Sixty by five, three hundred square feet of roof at eight psf, two thousand four hundred pounds. Nineteen thousand six hundred eighty pounds total. And parallel to the ridge, the end silhouette — six hundred fifteen square feet, all of it wall — at sixteen psf: nine thousand eight hundred forty pounds.

14:21And the comparison, case by case — this is where the minimum earns its keep. For this suburban house, the minimum governs three of the four load cases. And notice how the two floors stack: without the section 28.3.3 walls-only rule, load case one's raw shear of nineteen thousand eight hundred forty three point one eight pounds would have cleared the minimum by less than one percent — uncomfortably close. The walls-only floor lifts it to twenty one thousand two hundred eighty seven point five two, a comfortable eight point two percent margin.

14:50For light frame buildings in Exposure B terrain, the minimum governing is the rule, not the exception — and the torsional cases still have to be evaluated for the twist they apply, even when the minimum replaces their shear magnitude.

Summary of results

15:07And the summary. Velocity pressure, the governing shear in each direction, roof uplift, and the minimum wind load check — every governing number from this building in one place. Remember, every one of these is a strength level W value. For allowable stress design, wind enters at zero point six W — apply that factor after the governing comparison, never before it.

FAQ - when can I use the envelope procedure?

15:33Six 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.

16:17Because 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.

16:42It 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. Wind 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

17:25turn; for a symmetric building like this one, one orientation per direction captures the envelope. Two 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.

18:05Section 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.

18:44And 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.

19:26Enter 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.

Now in StructSuite - entering the same building

19:40Now let's enter this exact building in StructSuite's Wind module, starting from a blank project, and watch it reproduce every number we just found by hand.

App Step 1 - Select Design Procedure

19:49Step one. Chapter 28 this time — the Envelope Procedure. Same two radio buttons we saw in the Chapter 27 video; this building qualifies for this one because it clears the low rise test we talked about a few minutes ago.

App Step 2 - Geometry & Configuration

20:06Step two, geometry. Sixty feet, parallel to the ridge. Thirty feet, normal to it. And here the module offers something the Chapter 27 video did not need: enter the roof as a rise over run slope directly — four over twelve — instead of converting to degrees yourself. Four, twelve. The module converts it to eighteen point four three degrees on its own. And the eave height, eighteen feet — the one dimension we actually know.

20:40Ridge, twenty three feet. Mean roof height, twenty point five. Eave, eighteen. Exactly the three numbers the hand calculation used. And notice the note right above the schematic: h must be at or under sixty feet, and at or under the least horizontal dimension — thirty feet here. That is the low rise qualifying test we talked about, checked by the app in real time against the geometry you just entered.

App Step 3 - Risk Category

21:03Step three. Risk Category II, same as before.

App Step 4 - Basic Wind Speed

21:11Step four. One hundred fifteen miles an hour — a higher speed than our Chapter 27 example, from a different site.

App Step 5 - Wind Load Parameters

21:22Step five — the same five sub-panels as the Directional Procedure video, because these parameters belong to Chapter 26, shared by both chapters. Only the values differ this time. Kd, the same MWFRS row: zero point eight five. And this time, Exposure B — urban and suburban terrain, closely spaced obstructions — the reference photographs make the difference between this and Exposure C obvious at a glance. Kzt at the shortcut, one point zero.

21:54Ke at the shortcut, one point zero. And Enclosed, selected directly from the table.

App Step 6 - Kz / Kh

22:04Step six — and this is exactly where the Chapter 28 specific footnote we spent real time on lives. The same Table 26.10 dash 1 as before — but look at the Exposure B column at fifteen feet and below: zero point five seven, with zero point seven zero in parentheses and an asterisk. That parenthetical value is the envelope procedure override we worked through by hand — the module is showing you both numbers side by side, so you can see exactly which one applies to which procedure, in the same table.

App Step 7 - Velocity Pressure

22:36Step seven. Q sub H, using the correct zero point seven zero — twenty three point seven zero pounds per square foot. The one value every pressure in this method will use.

App Step 8 - External Pressure Coefficients

22:49Step eight is the big one — the torsional exception check, the edge zone dimension, and the full coefficient tables for all four load cases, together. First, the torsional exception. Three conditions that would let you skip the torsional cases — none of them apply to this two story house, so torsional cases are required: yes. And right below it, the edge zone dimension, worked automatically from the same geometry we entered: length sixty, width thirty, mean roof height twenty point five — the same inputs, the same derivation we did by hand a few minutes ago.

23:23And here is something worth pausing on: an isometric drawing straight out of the code figure itself, not a flat elevation. Load case one — you can see zones one through four running around the building, with the E strips shaded darker at the windward corner, and the two-a dimension marked right on the drawing. Load case two — the same building, wind now along the ridge, with zones five and six appearing on the gable ends.

23:47Load case three, the torsional perpendicular case — the T zones split the building down its length. And load case four, torsional parallel — the T zones this time splitting the gable ends. And below each drawing, the actual coefficient table it illustrates — every roof angle row the code publishes, plus a row calculated at our exact eighteen point four degree pitch, so you never have to interpolate by hand.

App Step 9 - Design Wind Pressures p

24:13Step nine — and this single table does more work than its name suggests. Load case one, every zone, in one row each: the coefficient, both internal pressure columns, the resulting pressure, the zone's area, the total force, and that force already split into its horizontal and vertical components. Read zone one: nine hundred seventy two square feet, six point seven eight pounds per square foot, six thousand five hundred eighty seven pounds of force — all horizontal, none vertical, because a wall's force is already horizontal. And zone two E, the corner roof strip: negative twenty five point one eight psf, the same strongest suction we found by hand.

24:50Load case two adds zones five and six for the gable ends, and here you can watch the horizontal and vertical split do real work: the roof zones, two and three, show almost all of their force landing in the vertical column — uplift — with only a small horizontal component, exactly the geometric rule we described by hand: roof slopes contribute their cosine component to uplift regardless of wind direction, but their sine component to shear only when the wind is perpendicular to the ridge.

App Step 10 - Pressure & Force Diagrams

25:18And the last step — every load case, every quantity, drawn. The sidebar groups every view into load cases, resultant forces, and the minimum design wind load — and it already tells you what governs. Load case one: governs base shear, governs uplift. Load case two: governs below the section 28.3.6 minimum — meaning its own computed shear did not clear the floor, and the minimum took over instead. Load case three: the same tag, governs below the minimum.

25:50And load case four: governs below the minimum as well. Three of the four cases, exactly as the hand calculation found. And here is the minimum wind load itself, one more time, before the app draws its own version of the same picture. The module's own minimum design wind load drawing: sixteen psf on the wall band, eight psf on the roof band, on this building's actual elevation — ridge twenty three, h twenty point five, eave eighteen. Nineteen thousand six hundred eighty pounds, perpendicular to the ridge. And look at the line above the drawing: the maximum strength-level shear from all four load cases is twenty one thousand two hundred eighty seven point five two pounds — the same walls-only floored value from load case one — with its allowable-stress-design equivalent, twelve thousand seven hundred seventy two point five one, printed right alongside it. Every number

26:38from this video, on one screen.

Hand calculation vs. StructSuite - every number matches

26:43One last comparison before we close. Mean roof height, twenty point five feet both ways. Velocity pressure, with the Chapter 28 footnote applied, twenty three point seven zero both ways. Zone one pressure, six point seven eight; zone two E, the strongest suction, negative twenty five point one eight — both ways. The edge zone dimension, three feet. The section 28.3.3 walls-only floor, twenty one thousand two hundred eighty seven point five two pounds. The minimum wind loads, nineteen thousand six hundred eighty and nine thousand eight hundred forty. And three of the four load cases governed by that minimum — both ways, to the case.

27:19Every figure, table, and formula in this video came from the written guide. Every field you watched us enter in the app is a number this video derived first, by hand, straight from the code. And every one of the four load cases ASCE 7-22 requires got checked, drawn, and compared against the minimum — not just the one that looked worst. Read the article, or watch this video — either way, you get the same building, checked the same way, twice.