Wind Load Calculation Step by Step - How to Calculate MWFRS Wind Pressure (ASCE 7-22 Ch. 27)

Wind Load Calculation - Directional Procedure

0:00This is a complete, no steps skipped MWFRS wind calculation for an ordinary building — a two story, gable roof wood frame house — by ASCE 7-22, Chapter 27, the Directional Procedure. We go from the basic wind speed all the way to the four design wind load cases of Figure 27.3 dash 8 — the part of this chapter most engineers meet for the first time on a plan check correction. Every number here was computed by the same engine that runs StructSuite's Wind module, so you can reproduce any line of this in the app and get exactly these values.

The building - what we're designing for

0:35Here is the building. Forty eight feet by thirty two feet in plan. An eighteen foot eave, a twenty six foot ridge, and a roof pitch of twenty six point six degrees. Because that pitch is over ten degrees, the code wants a mean roof height, not the eave height — average the eave and the ridge: eighteen plus twenty six over two, twenty two feet. That single number, h equals twenty two feet, is going to reappear constantly, because almost everything in this chapter is measured relative to it. Two wind directions are carried all the way through: perpendicular to the ridge, and parallel to it.

Choosing the inputs - where every value comes from

1:11Four selections set the stage before a single equation runs. Here is every input this example needs, on one page, before we touch a single equation. The first, and most consequential, choice is exposure category — look at the terrain upwind of the site, not the site itself. Category B is urban, suburban, or wooded terrain with closely spaced obstructions — most residential subdivisions. Category C is open terrain with scattered obstructions under thirty feet — open country, grassland, and the default whenever B or D don't clearly apply. Category D is flat, unobstructed terrain and water — shorelines, mud flats. This house sits at the edge of open fields, so we choose C. Hold onto that choice — it alone moves every pressure in this calculation, as we'll see in a moment.

1:58Next, the directionality factor, K sub D, from Table 26.6 dash 1. It accounts for the low probability that the strongest wind blows from exactly the direction that produces the worst pressure coefficient on your building — two maxima that rarely coincide. A rectangular building gets the biggest credit, zero point eight five. A round chimney looks the same from every direction, so it earns no credit at all — one point zero.

2:24Third, the enclosure classification, which sets GC sub p i, the internal pressure coefficient, from Table 26.13 dash 1. This is about openings — doors, operable windows, vents — and it has real teeth: a garage door failure can turn an enclosed building into a partially enclosed one and triple the internal pressure. We take Enclosed for this house. And two more, quickly. The topographic factor, K sub Z T, stays at one point zero unless the building sits on the upper half of an isolated hill, ridge, or escarpment. The ground elevation factor, K sub E, is one point zero at sea level, and may be reduced with altitude — taking one point zero is always the conservative choice. In StructSuite, Step 3 presents every one of these as the real code table with your row highlighted, and the enclosure classification can be computed from opening areas instead of

3:14simply asserted. One thing worth settling before Step 1: why Chapter 27 at all? This building actually qualifies for both procedures. Chapter 28, the Envelope Procedure, is limited to low rise buildings — height under sixty feet, height under the lesser of length and width, and a flat, gable, or hip roof — all true here, and where it applies it often produces the lowest pressures. Chapter 27 has no such limits: it works for any height and geometry, which is why it is the method you can always fall back on — and it is the procedure that carries the Figure 27.3 dash 8 design wind load cases this example is built around. Whichever you pick, use it consistently: never mix Chapter 27's C sub p pressures with Chapter 28's GC sub p f zones in the same calculation.

Step 1 - Velocity pressure qz

4:05Step 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 dash 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.

4:41K sub Z comes from Table 26.10 dash 1. Here are the rows bracketing the heights this building needs, values exactly as tabulated, for exposure C. Between 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.

5:13One 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. And 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.

5:48Same 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.

Step 2 - External pressure coefficients Cp

6:24Step two, the external pressure coefficients. Back to the building, because every coefficient in this step is read off its shape. For walls, windward is always positive zero point eight, sidewalls are always negative zero point seven. The leeward wall depends on how deep the building is along the wind — its length over breadth ratio. Interpolating that ratio for our building gives the leeward coefficient. Now the roof, wind perpendicular to the ridge. H over L parallel is twenty two over thirty two, zero point six nine. At this pitch, the windward slope actually carries two coefficients — the table gives both a suction and a pressure value between twenty and thirty degrees, and both have to be checked as separate load cases: negative zero point three three, and positive zero point one five, interpolated at our twenty six point six degree pitch.

7:17The leeward slope is simpler: negative zero point six. And wind parallel to the ridge: the slope stops mattering, and the roof behaves like a flat roof, loaded in strips measured from the windward edge. With H over L parallel at zero point four six, under one half, the strips are: negative zero point nine from zero to eleven feet and eleven to twenty two feet, negative zero point five from twenty two to forty four feet, and negative zero point three beyond that — each one paired with an alternative value of negative zero point one eight that must also be checked.

Step 3 - Design pressures p

7:51Step three, turning coefficients into pressures. Here is where K sub D finally enters — in this equation, not the velocity pressure one. Q, times K sub D, times G, the gust effect factor, times C sub P, minus the internal pressure term. Q equals Q sub Z for the windward wall, Q sub H everywhere else, and the internal Q is Q sub H for an enclosed building. And every single surface gets computed twice — once with positive GC sub P I, once with negative — because the interior can push or pull, and different elements are governed by different signs.

8:26One full substitution on the windward wall at the mean roof height: twenty three point four five, times zero point eight five, times zero point eight five, times zero point eight, minus the internal term — thirteen point five six, minus or plus three point five nine. Nine point nine seven pounds per square foot with positive internal pressure, or seventeen point one four with negative. And once more on a suction surface — the leeward wall. Negative twelve point zero six pounds per square foot. Notice the sign logic: on the windward wall, positive internal pressure fights the external push, so the negative internal column gave the bigger load there. On a suction surface like this one, positive internal pressure adds to the pull instead. That is the entire reason every surface in this chapter carries two columns.

9:12Here is the complete table, wind perpendicular to the ridge, exactly as StructSuite's Step 7 prints it — every surface, both internal pressure signs. And wind parallel to the ridge, loading the gable end. Positive acts toward the surface, negative — suction — acts away from it. Drawn out, wind perpendicular to the ridge, positive internal pressure case: the windward wall is pushed, and everything else — both roof slopes and the leeward wall — is pulled. At our twenty six point six degree pitch, both roof slopes end up in suction here; the positive zero point one five windward case is the separate check we mentioned a moment ago.

9:51Worth knowing, even though it changes nothing about how you use the table: the C sub P coefficients are not derived from an equation. They are envelopes of decades of boundary layer wind tunnel measurements on instrumented building models, refined by full scale field studies. The gust effect factor traces back to A G Davenport's gust loading factor research from the nineteen sixties. That lineage is exactly why interpolating between tabulated pitch and depth ratios is legitimate — the tables sample a continuous measured surface, not a formula.

Step 4 - Base shear and the Figure 27.3-8 load cases

10:23Step four — turning pressures into the story shear the lateral system actually resists. The horizontal drag comes from the signed sum of windward and leeward wall pressures — internal pressure acts on every interior surface at once, so it cancels out of the total shear — plus, since our roof pitch is over ten degrees, the horizontal component of the windward minus leeward roof pressures. But a single direction at full pressure is not the whole story: section 27.3.5 requires the MWFRS to be designed for all four load cases of Figure 27.3 dash 8.

10:59Case one: full design pressure, one principal axis at a time. Case two: seventy five percent of case one's pressure, plus a torsional moment from shifting the resultant fifteen percent off center. Case three: seventy five percent of case one, but on both axes at once — a stand-in for a quartering wind. Case four: seventy five percent of case two, which works out to fifty six point three percent of full pressure, plus torsion, on both axes.

11:21Each plan here is drawn with the same forty eight by thirty two proportions as our actual building plan. Why do these exist at all? Real wind is not obliged to blow along a principal axis. An oblique, quartering wind loads two faces at once — that is case three. And gust structure plus plan asymmetry twist a building even when it looks perfectly symmetric — cases two and four. What case one alone misses: corner columns, where seventy five percent on two axes combined can exceed one hundred percent on a single axis, and torsionally sensitive layouts. ASCE 7 does exempt some buildings from the torsional cases through Appendix D criteria referenced in section 27.3.5 — including certain one and two story light frame buildings — but verify those conditions before dropping cases two and four.

12:12And here is StructSuite's own summary — all four cases, both directions. Reading this table, the forty eight foot face — the long side of this plan — is the one that governs shear, because it presents more sail area to the wind. The forty eight foot face direction governs the overall shear, at nineteen thousand nine hundred sixty three pounds — your shear wall lines perpendicular to the ridge have to resist that. The minimum design wind load is a floor under this result, which we work out in full next; here it is exceeded, but not by a wide margin. And remember the force level: wind enters the allowable stress design combinations as zero point six W. Nineteen thousand nine hundred sixty three, times zero point six, eleven thousand nine hundred seventy eight pounds — that is what the NDS shear wall design actually sees.

13:00As with seismic, labeling the force level correctly is the difference between a clean review and a correction letter.

Step 5 - The minimum design wind load, worked in full

13:09Step five — the floor every analytical wind calculation has to clear. Every analytical wind calculation ends at the same checkpoint. Section 27.1.5 — and its Envelope Procedure twin, 28.3.6 — sets a floor: the MWFRS wind load cannot be less than sixteen pounds per square foot times the wall area, plus eight pounds per square foot times the roof area, both projected onto a vertical plane normal to the wind, applied simultaneously. The rule exists because Chapter 27's pressures can legitimately come out small — a low mapped wind speed, sheltered exposure B terrain, a favorable C sub P — and the code wants a hard lower bound tied to nothing but geometry. Two things trip people up: it is a projected area load, not a surface pressure — you flatten the silhouette onto a plane and push horizontally, you don't

13:57chase the wind around every roof slope — and it is checked per direction, so a comfortable margin one way cannot excuse a shortfall the other way. Both panels here look perpendicular to the wind — the same section along the wind convention as the earlier pressure diagram. The dashed line above the eave on the left panel is the projection plane; there is no wall up there, the eight psf acts on the flattened roof silhouette above it.

14:28The eighteen foot eave and the eight foot roof rise both come straight from this elevation. Looking along the ridge, the wind sees the forty eight foot long face. The silhouette splits at the eave: the roof's vertical rise is half the building width times the tangent of the pitch — sixteen feet times tangent twenty six point five seven degrees, eight feet. Wall projection: forty eight times eighteen, eight hundred sixty four square feet. Roof projection: forty eight times eight, three hundred eighty four square feet. Sixteen times eight sixty four, plus eight times three eighty four: thirteen thousand eight hundred twenty four plus three thousand seventy two — sixteen thousand eight hundred ninety six pounds.

15:05Looking across the building, the wind sees the thirty two foot gable end — and here is the subtlety worth a highlight. The triangle between eave and ridge is the gable end wall, a vertical wall surface, not roof. So the whole silhouette takes sixteen psf: thirty two times eighteen, plus one half times thirty two times eight, five hundred seventy six plus one hundred twenty eight, seven hundred four square feet of wall, and zero roof area, because the roof planes project onto that same triangle and are not counted twice. Sixteen times seven oh four: eleven thousand two hundred sixty four pounds.

15:39The check, per direction: computed base shear against the minimum, both directions. Run this identical house in Exposure B instead — a suburban lot — and the case one shears drop to thirteen thousand six hundred thirteen pounds perpendicular to the ridge, and nine thousand three hundred seventy six parallel to it. But the minimums do not move, because they depend only on geometry. Now section 27.1.5 governs in both directions. For light frame buildings in sheltered terrain, that is the rule, not the exception — which is why this check belongs in every submitted package, not just the ones where it looks close.

Summary of results

16:17And the summary. Velocity pressure, both wall pressure tables, all four load cases, and the minimum wind load check — every governing number from this building in one place.

FAQ - when does the minimum design wind load govern?

16:29Two questions worth closing on. The projected-area picture again, because this question is entirely about that geometry. Whenever geometry beats pressure. The minimum scales only with the building's silhouette, while the analytical pressure scales with wind speed squared, exposure, and the pressure coefficients. The classic triggers are a low mapped wind speed, sheltered Exposure B terrain, and geometries with small pressure coefficients. And remember: both sides of that comparison are strength level W values — apply zero point six W afterward, for the allowable stress design combinations.

17:08And this question is about the four load cases we just built, so here they are again. Because oblique winds load two faces at once, and asymmetric pressure distributions can twist a building even when its plan looks perfectly symmetric — effects case one alone cannot capture. In this example, case one governs the overall base shear, but cases three and four turn out to set the governing roof uplift, at twenty one thousand one hundred twenty five pounds. That split — one case governing shear, different cases governing uplift — is a typical pattern for symmetric low rise buildings, and it is exactly why the code makes you check all four rather than trusting the one that looks obviously worst.

17:48Enter this exact building in StructSuite's Wind module — Chapter 27, this geometry, one hundred miles an hour, Exposure C — and every table in this example appears on screen and in the printed calculation package, including the Figure 27.3 dash 8 case summary.

Now in StructSuite - entering the same building

18:06Now 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

18:15Step one, choosing the procedure. Two radios, right at the top: Chapter 27, the Directional Procedure, or Chapter 28, the Envelope Procedure. This building actually qualifies for both — we already talked about why Chapter 27 is the one that carries the Figure 27.3 dash 8 load cases. Chapter 27. The wizard reshapes its own steps around that choice from here on.

App Step 2 - Geometry & Configuration

18:42Step two, geometry — and this is where we get to watch the app do something the hand calculation did on paper. Plan dimension parallel to the ridge: forty eight feet. And normal to the ridge: thirty two feet.

18:59Roof angle, twenty six point five seven degrees — entered directly in degrees. There is also a rise over run option, if that is how your drawings give it to you; the module converts either way. This is the moment worth watching closely — we are about to give the app only ONE of the three heights on this elevation, and see it find the other two on its own.

19:21Three ways to tell the module how tall this building is: ridge height, mean roof height, or eave height — pick whichever one you actually have. We know the eave height, eighteen feet, so that is the one we enter. And there it is: ridge, twenty six feet. h, twenty two feet. Eave, eighteen. The exact three numbers the hand calculation used — the app derived two of them from the one we actually typed, using the same geometry we worked out by hand. And notice the schematic even reports the roof as a six to twelve slope — the same twenty six point five seven degree pitch, in the other units a framer would use.

App Step 3 - Risk Category

19:55Step three, risk category — straight out of Table 1.5 dash 1. Category II — everything except the low-hazard, high-hazard, and essential-facility exceptions. That covers this house.

App Step 4 - Basic Wind Speed

20:08Step four. One hundred miles an hour, read off the ASCE hazard tool for this site and risk category — the module links straight to it if you need to look yours up.

App Step 5 - Wind Load Parameters

20:21Step five bundles the five parameters we spent the first few minutes of this video choosing by hand — and here they are, as five collapsible sections, each citing its own code section. Kd: the real Table 26.6 dash 1, every structure type it covers, not just the one row we need. We pick the main wind force resisting system row for buildings — zero point eight five. Exposure: and here the module does something worth calling out — it shows real reference photographs for B, C, and D, not just the code's written description. Exposure C: open terrain, scattered obstructions under thirty feet. This is the single choice that moved every pressure in this calculation, and the app makes you look at a picture of what you're actually choosing.

21:05Both K sub Z T and K sub E offer the same shape of choice: a one click shortcut for the ordinary case, or the full code method one click further in. K sub Z T defaults to a shortcut at one point zero for a building not sitting on a hill, ridge, or escarpment — or you can switch to the full Figure 26.8 dash 1 hill shape method if it does. K sub E similarly lets you simply take one point zero at any elevation, which the code always permits, or look up the small reduction Table 26.9 dash 1 allows at altitude. This building sits on ordinary, low lying ground, so both shortcuts apply.

21:41Use K sub Z T equals one point zero, for buildings not on hills, ridges, or escarpments. And use K sub E equals one point zero, permitted for all elevations. And the enclosure classification. You can select straight from Table 26.13 dash 1, as we do here — Enclosed — or calculate it from your actual opening areas per section 26.12, which is what you would do if you were not sure a building qualified, or if a plan reviewer wanted the arithmetic shown. We assert Enclosed directly, matching the hand calculation.

App Step 6 - Kz / Kh

22:15Step six is a read-only step: the full velocity pressure exposure coefficient table. Every row of Table 26.10 dash 1, for all three exposure categories, from zero feet up to five hundred. You do not have to interpolate by hand — the module already knows where our twenty two foot mean roof height falls between the twenty and twenty five foot rows.

App Step 7 - Velocity Pressure

22:38Step seven turns that table into an actual pressure. q sub H, at the mean roof height — the workhorse value every surface but the windward wall will use.

App Step 8 - External Pressure Coefficients

22:48Step eight lays out the full Figure 27.3 dash 1 — not just the rows we need, the whole figure, for both wind directions. Walls: windward always zero point eight, sidewall always negative zero point seven, and leeward keyed to the length over breadth ratio — exactly the table we read by hand. And the full roof table for wind perpendicular to the ridge — every height to length ratio, every angle from ten degrees to vertical, not just our row. Our building sits between the zero point five and one point zero H over L rows, at our twenty six point six degree pitch.

23:24This is the same pressure picture from the hand calculation — every one of those coefficients in the table just now is a cell this drawing turns into an arrow. And the flat-roof-strip table for wind parallel to the ridge — the same strips we computed by hand, zero to H over two, H over two to H, H to two H, and beyond.

App Step 9 - Design Wind Pressures p

23:45Step nine turns coefficients into pressures — for every surface, every height, both internal pressure signs. The windward wall, height by height, because it is the one surface that does not simply use q sub H. Read the twenty two foot row: nine point nine seven pounds per square foot with positive internal pressure, seventeen point one four with negative. The exact numbers from our hand substitution, to the decimal.

24:09And every other surface — leeward, sidewalls, roof — each at its own q sub H based pressure, both internal pressure columns, exactly as printed in the report.

App Step 10 - Design Wind Load Cases

24:19Step ten is the heart of this chapter — the four design wind load cases, computed in full, plus the Appendix D exemption check and the minimum design wind load. First, the Appendix D exemption check: five radio options, each one a path that would let you skip the torsional cases. We take no exemption, matching the hand calculation, so all four cases run. The same four cases we already met — and now the app is about to run every one of them, on this actual building, not just describe them.

24:50Case one: full pressure, each principal axis on its own. Reference wall pressures for both the x and y directions, ready to combine into the total shear. Case two: seventy five percent of case one's pressures, plus the torsional moment — and here are the actual eccentricities. E sub X, plus or minus four point eight feet; e sub Y, plus or minus seven point two feet. The same fifteen percent of face width we found by hand, now run in all four sign combinations, with M sub Z computed for each — five thousand seven hundred nine pound feet per foot at its largest.

25:24Case three: seventy five percent of full pressure, on both axes at once — the quartering wind stand-in. And case four: case two's combination on both axes together — the heaviest torsion case. Below it, the module works out the horizontal shear at the base directly from these pressures — this is where the nineteen thousand nine hundred sixty three pound governing shear we quoted earlier actually comes from. And here is the minimum wind load check, the same projected-area picture from the hand calculation — this time drawn by the app itself, to this building's actual proportions.

25:59The module reproduces the Commentary figure, splits the silhouette at the eave, and prints the sixteen and eight psf arithmetic line by line — the same sixteen thousand eight hundred ninety six and eleven thousand two hundred sixty four pounds we found by hand, for both directions.

App Step 11 - Visualization

26:15And the last step — seeing all of this drawn, for any case you want to inspect. Design pressures, wind perpendicular to the ridge, positive internal pressure — drawn on the actual building elevation, arrows scaled to the pressure they represent. This is the sidebar's first group: the design pressures themselves, for every case and both internal pressure signs. A second group of views shows the resultant forces — the pressures collapsed down to the shear and moment they actually produce — and a third group steps through the four Figure 27.3 dash 8 cases directly. Case one, full pressure, one axis at a time.

26:52And the fourth group draws the minimum design wind load itself — the sixteen and eight psf projected areas, on this building's silhouette, the same picture we just saw the module compute in Step 10. Every number in this video, drawn.

Hand calculation vs. StructSuite - every number matches

27:06One last comparison before we close. Mean roof height, twenty two feet both ways. Velocity pressure, twenty three point four five both ways. The windward wall pressure at the mean roof height, nine point nine seven and seventeen point one four, both ways. The eccentricities for the torsional cases, four point eight and seven point two feet, both ways. And the minimum design wind load, sixteen thousand eight hundred ninety six pounds perpendicular to the ridge and eleven thousand two hundred sixty four parallel to it — both ways, to the pound.

27:37Every 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 actually requires got checked — 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.