Seismic Base Shear Step by Step - How to Calculate V (ASCE 7-22 ELF Worked Example)

Seismic Base Shear - ASCE 7-22 ELF Procedure

0:00This is a complete equivalent lateral force calculation — the ELF procedure of ASCE 7-22, section 12.8 — on an ordinary building: a two story, gable roof wood frame house with an unoccupied attic. We do the honest weight takeoff first, because computing W correctly is half of a seismic calculation and most worked examples hand it to you from nowhere. Then the design spectral accelerations, the Seismic Design Category, the system factors, the period, the response coefficient, the base shear, how it distributes up the height, and the diaphragm forces — which are not the same forces, and that catches people. Every number here was computed by the same engine that runs StructSuite's Seismic module, so you can reproduce any line of this in the app.

The building - what we're designing for

0:47Here is the building. Forty feet by thirty in plan — twelve hundred square feet. Two stories, ten feet each. A six in twelve gable roof, twenty six point six degrees, over an unoccupied attic. Plywood sheathed shear walls, light frame diaphragms. And here is the one idea that decides this whole calculation: the amber dashed line. In a house with a flat ceiling and an attic above it, the ceiling diaphragm is the top of the structure as far as earthquake is concerned. The roof and attic mass acts there, at twenty feet. The structural height, h sub n, is twenty feet — not the twenty seven and a half foot ridge. Hold onto that line; almost every number in this video is measured from it.

Choosing the inputs - where every value comes from

1:27Seven choices set the stage before a single equation runs. Plan and heights off the drawings. The system, a bearing wall light frame system with wood structural panels, out of Table 12.2 dash 1 — we come back to that in step three. Risk Category two, which is a house, giving an importance factor of one point zero. The ground motion — S sub M S, one point five g, S sub M one, zero point nine g, S sub one, zero point four five g, and T sub L, eight seconds — from the ASCE Hazard Tool for the site address and site class. And the diaphragms, which wood light frame construction is permitted to idealize as flexible under section 12.3.1.1. That last one is not a throwaway; it earns a discount later.

Step 0 - Effective seismic weight W: the honest takeoff

2:11Step zero — the weight takeoff, which is where this calculation is actually won or lost. Section 12.7.2 says the effective seismic weight is the dead load, plus a few add ons: the actual partition weight, permanent equipment, twenty five percent of storage live loads where storage actually occurs — an attic used for storage would pick that up — and part of a heavy flat roof snow load. Ordinary roof live load does not count; nobody is standing on the roof during the earthquake. And the reason to take this seriously is at the bottom: base shear is C sub S times W, a straight line.

2:44Every pound of roof tile or extra gypsum shows up directly in the force every shear wall in the building has to resist. The assumed unit dead loads for this house. Roof, fifteen p s f. Ceiling, ten. Exterior walls, eleven. Interior partitions, five p s f over the floor area. Second floor, twelve. And note the first line carefully — fifteen p s f of sloped surface. That word matters, and it is the next thing we fix.

3:11Roof dead load is estimated per square foot of roof surface, but the takeoff is done on the horizontal plan area — so you multiply by the slope length factor. For a six in twelve pitch that is the square root of twelve squared plus six squared, over twelve: the square root of one hundred eighty over twelve, one point one one eight. Fifteen p s f becomes sixteen point eight p s f of plan area — a twelve percent penalty that a flat roof simply does not pay.

3:37Everything in the takeoff comes off this drawing: twelve hundred square feet of plan, a thirty foot gable face with a seven and a half foot rise, and a hundred forty foot wall perimeter. The roof level takeoff. Sloped roof: fifteen p s f, times the slope factor, times twelve hundred square feet — twenty thousand one hundred twenty four pounds. The ceiling, joists plus gypsum plus insulation: twelve thousand. The two gable end triangles, eleven p s f on two halves of thirty by seven and a half: two thousand four hundred seventy five. The upper half of the second story walls, eleven p s f on a hundred forty foot perimeter, five feet tall: seven thousand seven hundred. And the upper half of the partitions: three thousand. Total, forty five thousand two hundred ninety nine pounds — call it forty five thousand three hundred.

4:25Follow each of those loads down its own members and you see why they all arrive at the same elevation. Roofing, sheathing and insulation load the rafters, and the rafters bear at their feet on the top plates of the second story walls. The ceiling gypsum and insulation load the ceiling joists, which bear on those same top plates. The gable end triangles are stud walls standing on the end wall top plates. So in an earthquake, the inertia of every pound of roof, attic framing and ceiling enters the shear walls at the plate line — twenty feet — because that is where the ceiling diaphragm is. There is no diaphragm at the ridge and none mid slope. The ceiling is the highest surface in this building that can collect horizontal force and hand it to the walls.

5:05The second floor, at ten feet, is simpler. Floor assembly, twelve p s f over twelve hundred square feet: fourteen thousand four hundred. The walls tributary to this level — the top half of story one plus the bottom half of story two, so a full ten feet of wall on a hundred forty foot perimeter: fifteen thousand four hundred. Partitions, half above and half below: six thousand. Thirty five thousand eight hundred pounds.

5:29Add them: eighty one thousand one hundred pounds. That is the number the whole rest of this calculation multiplies. This is worth a pause, because it is the difference between the two kinds of house. With a flat roof there is one horizontal plane at the top — roofing, joists, insulation, and the ceiling gypsum below all live in the same assembly — and the roof is often the lighter level of the building. An attic house stacks two planes up there: the sloped roof surface with its twelve percent slope penalty, plus a full ceiling assembly, plus the gable end triangles, all tributary to the ceiling diaphragm. The result here is that the roof level, at forty five thousand three hundred pounds, is twenty seven percent heavier than the floor below it. Seismic force follows mass, so this one modeling decision reshapes the entire force distribution. Watch it

6:14happen in step six.

Step 1 - Design spectral accelerations SDS and SD1

6:19Step one, the design spectral accelerations. ASCE 7-22 hands you the site adjusted ground motion as S sub M S and S sub M one — the risk targeted maximum considered earthquake, already adjusted for site class. You read those off the Hazard Tool for your address. The design values are two thirds of them. Two thirds of one point five is one point zero g. Two thirds of zero point nine is zero point six g. And one more number we will use over and over: the corner period of the design spectrum, T sub S, is S sub D one over S sub D S — zero point six seconds.

6:55Why two thirds? Because the code's collapse safety logic assumes a properly detailed structure carries a margin of about one and a half against collapse under the maximum considered earthquake. Divide the rare motion by that margin and you land on the design level motion the load combinations are actually calibrated to — that is the two thirds. And the maps themselves are risk targeted: since ASCE 7-10, they aim at a uniform one percent probability of collapse in fifty years, rather than a uniform ground motion hazard. Commentary section C11.4 has the whole argument.

Step 2 - Seismic Design Category

7:30Step two, the Seismic Design Category — and establish it early, because it drives almost everything downstream. Section 11.6 works in a fixed order, and it is worth following it exactly. First the short circuit: where the mapped S sub one is zero point seven five g or more, the structure is Seismic Design Category E — or F for Risk Category four — immediately, with no tables involved. Ours is zero point four five, so that does not trigger. Otherwise you enter both tables with your risk category: Table 11.6 dash 1 with S sub D S, Table 11.6 dash 2 with S sub D one, and you take the more severe of the two answers, irrespective of period.

8:12Table 11.6 dash 1, keyed on S sub D S. Ours is one point zero g, which is well past the zero point five g threshold in the bottom band. Risk Category two, so the first column: category D. And Table 11.6 dash 2, keyed on S sub D one. Zero point six g, past the zero point two g threshold, Risk Category two: category D again. Both tables give D, so this is Seismic Design Category D. When they disagree, the worse letter wins. There is also a permission in section 11.6 to use Table 11.6 dash 1 alone, where S sub one is under zero point seven five and four conditions all hold — one of which, the approximate period being under zero point eight times T sub S, this building will satisfy in step four. Here both tables agree anyway, so the

8:58permission changes nothing. But for a building sitting right on a table boundary it can move the category down a letter, and that is worth knowing about.

Step 3 - System factors R, Omega-zero, Cd

9:10Step three, the system factors — and the table itself teaches the lesson here, so we read our row and its neighbours. Our building is row A fifteen: light frame wood walls sheathed with wood structural panels. R is six point five, omega zero is three, C sub d is four, and it is permitted in Seismic Design Category D up to sixty five feet. Now look at its neighbours. Row A seventeen — the same walls, but with shear panels of all other materials, gypsum sheathing for instance — has an R of two. And in the B group, building frame systems, the same wood panel walls that are not carrying gravity get an R of seven.

9:47Three questions, in order. What resists the lateral force? Wood structural panel shear walls. Do those same walls also carry gravity? In a house, yes — so this is a bearing wall system, category A, not a building frame system, category B. And is the system permitted in this Seismic Design Category and at this height? The right hand columns of the table say category D, up to sixty five feet; our twenty feet is fine. That bearing wall penalty — six point five against seven — exists because a wall that is busy carrying the roof has less deformation capacity left to spare when it also has to yield laterally.

10:23And this is not a small choice. C sub S divides by R, so with everything else held fixed: R of six point five, the wood structural panel row, gives a base shear of twelve thousand four hundred seventy seven pounds — this example. R of two, the gypsum sheathed row, gives forty thousand five hundred fifty — three and a quarter times the force, and a thirty five foot height limit on top of it. Sheathing choice is not a drafting detail; it is the difference between designing for twelve kip and forty one kip. R rewards systems with tested ductility — the framework came out of ATC three dash oh six in nineteen seventy eight, and its modern quantification is FEMA P six ninety five.

11:02One more thing from this table. The tabulated omega zero of three may be reduced by a half for structures with flexible diaphragms, under a footnote to Table 12.2 dash 1 — so ours is two point five. The rationale is worth understanding rather than memorizing. Omega zero anticipates how much force the system can redistribute into an element that has to stay elastic — collectors and their connections being the classic case, in section 12.10.2.1. A rigid diaphragm redistributes aggressively: as some walls yield it shifts load into the stiffer ones. A flexible diaphragm delivers load by tributary area no matter what yields, so the credible overshoot is smaller. In StructSuite that is a checkbox in step four, and the effective two point five then flows into every overstrength combination automatically.

Step 4 - Approximate period Ta

11:52Step four, the approximate fundamental period. The approximate period is C sub t times h sub n to the x, with C sub t and x from Table 12.8 dash 2. Four of the five rows are frames — steel and concrete moment frames, eccentrically braced frames, buckling restrained braced frames. A wood shear wall building is not any of those, so it lands in the last row: all other structural systems. C sub t of zero point zero two, x of zero point seven five. And because it is not a frame, the zero point one N shortcut in equation 12.8 dash 9 is not available either.

12:27And the height that goes into it is this one — h sub n equals twenty feet, measured to the ceiling diaphragm, not to the ridge. Zero point zero two times twenty to the zero point seven five. Twenty to the three quarters is nine point four five seven, so the period is zero point one eight nine seconds. And look where that sits: well below T sub S, zero point six seconds. This building is on the flat plateau of the design spectrum. Short, stiff buildings ride the top — which is exactly why equation 12.8 dash 2 is going to govern C sub S in a moment.

13:01Two observations. First, refining the period buys nothing for this building — on the plateau, C sub S does not depend on T at all. Second, if you did compute a period from a computer model, section 12.8.2 caps the period you are allowed to use at C sub u times T sub a, with C sub u from Table 12.8 dash 1 — one point four, for an S sub D one of zero point three or more. One point four times zero point one eight nine is zero point two six five seconds. Still on the plateau. That is the sort of thing worth checking before you spend an afternoon building the model, not after.

13:34Now, the question that catches everyone. Why twenty feet and not the roof? Follow the code's own chain. Equation 12.8 dash 8 does not define h sub n at all — section 12.8.2.1 sends you to a definition in section 11.2, and that definition is two sentences, each handling a different kind of house. Sentence one is the general rule: the vertical distance from the base to the highest level of the seismic force resisting system. Not the highest point of construction — the highest level of the system. In this house that is the ceiling diaphragm at the plate line; the rafters and sheathing above it carry gravity only, and brace nothing. So sentence one governs, and h sub n is twenty feet. Sentence two answers the question sentence one leaves open: when the sloped roof itself is the highest level of

14:17the system, where on the slope do you stop? The answer is the average height of the roof. That is the cathedral ceiling house — take the ceiling joists out and the sloped sheathing becomes the top diaphragm, so h sub n is twenty plus twenty seven and a half over two, twenty three point seven five feet. Never the ridge, in either case. An attic above the top diaphragm adds mass — we lumped all of it at the ceiling — but it adds no height.

14:45Drawn side by side. On the left, our attic house: the ceiling diaphragm is the top level of the system, the roof above it is ghosted because it is gravity only, and the whole roof and attic mass acts at twenty feet. On the right, the same geometry with the ceiling joists removed: the sloped sheathing is now the top diaphragm, h sub n rises to the mean roof height of twenty three point seven five feet, and the mass rises with it. And the spectrum at the bottom settles the question of whether the difference matters. Zero point one eight nine seconds and zero point two one five seconds — both of them land on the flat plateau, well short of T sub S at zero point six. So C sub S, and therefore the base shear, is identical either way. What

15:25does change in the cathedral version is the model: the top level in step six moves up to twenty three point seven five feet, and every story force lever arm and the overturning moment rise with it.

Step 5 - Response coefficient Cs and base shear V

15:38Step five — the response coefficient, and the base shear itself. C sub S is a basic value fenced by an upper bound and two lower bounds, and it pays to write all four out rather than trusting the one you remember. Equation 12.8 dash 2 is the basic value: S sub D S over R over I sub e — one point zero over six point five, zero point one five three eight. Equation 12.8 dash 3 is the upper bound, zero point four eight eight, but it is only active for periods beyond T sub S, and ours is not — so it does not bite. Equation 12.8 dash 5 is the lower bound, zero point zero four four, comfortably satisfied. And equation 12.8 dash 6, the near fault floor, is only required when S sub one is zero

16:19point six g or more; ours is zero point four five, so it is not triggered at all. Equation 12.8 dash 2 governs. So the base shear: zero point one five three eight times eighty one thousand one hundred pounds — twelve thousand four hundred seventy seven pounds, about twelve and a half kip. The house has to resist a horizontal force equal to fifteen point four percent of its own weight. And notice the chain of causation you just walked: heavier building, bigger V, linearly. Every pound of roof tile shows up in this number, which is one reason light wood frame construction performs so well, economically, in seismic country.

16:57And a sensitivity check the other way. Move this identical house to a site with an S sub D S of zero point seven five and an S sub D one of zero point four five — still Seismic Design Category D — and C sub S drops to zero point one one five four, giving nine thousand three hundred fifty eight pounds. Twenty five percent less force, same building. Which is the useful framing: the two levers an engineer actually controls are the system and the weight. The site is whatever the address says it is.

17:26One more thing before we distribute it. Twelve thousand four hundred seventy seven pounds is E — the seismic load effect in the strength design load combinations. Wood design under the NDS is allowable stress design, where the seismic term enters as zero point seven E: eight thousand seven hundred thirty four pounds. That is the number your shear wall tables actually see. StructSuite tabulates every seismic result at both levels and labels them, because mixing the two is among the most common plan check corrections there is.

Step 6 - Vertical distribution: story forces Fx

17:57Step six — distributing that base shear up the height of the building. The base shear distributes in proportion to weight times height, raised to the power k. With a period of zero point one eight nine seconds — under half a second — k is one, so it is simply weight times height. The exponent grows toward two for long period buildings, to mimic the higher mode whip effect that throws extra force at the top of a tall, flexible structure. A two story house is nowhere near that.

18:24Two masses, two heights — forty five thousand three hundred pounds at twenty feet, thirty five thousand eight hundred at ten. That is the entire input to this step. Weight times height: nine hundred six thousand at the roof, three hundred fifty eight thousand at the second floor, one million two hundred sixty four thousand in total. Divide through and the roof gets zero point seven one six eight of the base shear — eight thousand nine hundred forty three pounds.

18:48The second floor gets the remaining zero point two eight three two — three thousand five hundred thirty four. And the last column carries both through to the allowable stress level: six thousand two hundred sixty, and two thousand four hundred seventy four. Drawn to scale, and the picture makes the point better than the arithmetic does. The roof level takes seventy two percent of the base shear. Height does part of that — it is twice as high — but the attic's mass does the rest, because this level is also twenty seven percent heavier than the one below it. Compare a flat roof version of this same house, where the roof is the lighter level and takes closer to sixty percent. The attic changed the design of every shear wall in the upper story.

19:30Two things fall straight out of those story forces. The story shears: below the ceiling level, eight thousand nine hundred forty three pounds — just the roof force. Below the second floor, both forces accumulate: twelve thousand four hundred seventy seven, which is of course the base shear again. Those are the shears your two stories of shear wall lines have to resist. And overturning, section 12.8.5: at the second floor, eight thousand nine hundred forty three times ten, eighty nine thousand four hundred thirty one pound feet. At the base, eight thousand nine hundred forty three times twenty plus three thousand five hundred thirty four times ten — two hundred fourteen thousand two hundred pound feet, about two hundred fourteen kip feet. Overturning is what becomes hold down tension and end post compression at the ends of each wall.

Step 7 - Diaphragm design forces Fpx

20:14Step seven — the diaphragm forces, which are not the forces we just computed. Each diaphragm, with its chords and collectors, gets its own force. Equation 12.10 dash 1 is the running average acceleration from that level up to the roof, applied to that level's own weight. And it is fenced: not less than zero point two times S sub D S times I sub e times the diaphragm weight, and not more than zero point four times the same. Those two bounds are the whole story of this step.

20:42At the roof, equation 12.10 dash 1 gives eight thousand nine hundred forty three pounds — the roof force divided by the roof weight is the roof's own acceleration, so the equation just hands the F sub x value straight back. But the minimum is nine thousand sixty, and it is larger. At the second floor the equation gives five thousand five hundred eight, and the minimum is seven thousand one hundred sixty. The minimum governs both diaphragms.

21:06And read the last two columns: nine thousand sixty and seven thousand one hundred sixty at strength level; six thousand three hundred forty two and five thousand twelve at the allowable stress level. Look at how close that is at the roof. Eight thousand nine hundred forty three from the equation, nine thousand sixty from the minimum. The floor wins by about one percent — and that near miss is not a coincidence of these particular numbers.

21:33Here is the structural algebra, and it is worth seeing once. At the roof, equation 12.10 dash 1 reduces to the roof force over the roof weight — the roof's own acceleration under the ELF pattern. For this building that is eight thousand nine hundred forty three over forty five thousand three hundred: zero point one nine seven g. A hair under the zero point two g floor. And with an R of six point five and k equal to one, a two story light frame building's ELF accelerations always sit right around that floor. So for ordinary wood buildings, expect equation 12.10 dash 2 to control your diaphragm design more often than 12.10 dash 1 does.

22:12Skip the minimum check here and the roof diaphragm is one percent under designed — but the floor below it is thirty percent under designed, five thousand five hundred eight against seven thousand one hundred sixty. The reason the floor exists at all: recorded floor accelerations in real earthquakes routinely exceed the smooth ELF pattern at individual levels. A single diaphragm can see a momentary spike even while the building's total base shear matches design. Commentary section C12.10 has the measurements.

Summary of results

22:43And the summary. Weight, response coefficient, base shear, both story forces, the base overturning moment, and both diaphragm forces — at strength level and at the allowable stress level, side by side. Every value here comes from the functions that power StructSuite's Seismic module, which is exactly what we are going to do next: open the app, enter this building, and watch these numbers come back.

FAQ - weight, period, and Fpx

23:08Three questions before we move to the app. The first one is entirely about this picture. Never the ridge. Which of the two sentences in section 11.2 applies depends on whether a ceiling diaphragm exists, not on whether the roof happens to be pitched. Flat ceiling with joists and an attic above: the ceiling diaphragm is the highest level of the seismic force resisting system, the rafters above carry only gravity, so h sub n stops at the plate line — twenty feet here. Cathedral or vaulted ceiling with no ceiling joists: the sloped sheathing is itself the top diaphragm, the pitched roof sentence applies, and h sub n goes to the mean roof height — the average of eave and ridge, twenty three point seven five feet for this geometry.

23:52The second is about these forces, and the ones we drew right after them. F sub x distributes the base shear for designing the vertical system — the walls. Those forces act concurrently, and they accumulate into story shears and overturning moments. F sub p x designs one diaphragm, and it is fenced between zero point two and zero point four times S sub D S times I sub e times that diaphragm's weight. The critical difference in practice is that the F sub p x forces are not concurrent: each diaphragm is checked for its own force, separately, and they are never summed into a story shear. And because a single floor can momentarily exceed the smooth ELF acceleration pattern during shaking, the code floors the diaphragm force — which for ordinary two story wood buildings with an R of six point five

24:36means the minimum usually governs, as it did at both levels of this example. The overstrength factor exists to protect elements that must remain essentially elastic while the walls are yielding — collectors and their connections being the classic case. The tabulated value anticipates force redistribution: a rigid diaphragm can shift load into stiffer elements as others yield, pushing local demands well beyond the design values. A flexible diaphragm delivers force by tributary area regardless of what the walls do, so the credible overshoot is smaller — and the footnote to Table 12.2 dash 1 permits omega zero minus a half. For this building's wood structural panel walls, three minus zero point five: two point five.

Now in StructSuite - entering the same house

25:23Now let's enter this exact house in StructSuite's Seismic module, starting from a blank project, and watch it reproduce every number we just found by hand.

App Step 1 - Building geometry and weights

25:33Step one, and notice the app starts where the hand calculation started — with the weights, not with the ground motion. One level to begin with, and three columns: the seismic weight at that level, and then a choice — story height, h s x, or height from base, h x. Whichever one your drawings give you; the module fills in the other. Add story, and there are the two levels of this house. Levels are numbered from the bottom up, so level one is the second floor and level two is the ceiling diaphragm.

26:02Level one: thirty five thousand eight hundred pounds — the floor assembly, the walls tributary to it, and the partitions, exactly the takeoff we did by hand. And its story height, ten feet. Watch the read-only column to its right: the app is accumulating h x from the base as we go. Level two: forty five thousand three hundred pounds — the sloped roof with its slope factor, the ceiling, the gable triangles, and the upper half of the second story walls, all lumped here because this is where the diaphragm is.

26:32This is the modeling decision the whole video turns on, and the app is simply recording it: two masses, at ten feet and twenty feet. Ten feet again for the second story height — and h x for level two comes out at twenty. And there they are: total weight W equals eighty one thousand one hundred pounds, and h sub n equals twenty feet, the structural height per section 11.2. The two numbers the hand calculation spent its first several minutes deriving — and the app computed the second one from the story heights we typed, exactly as the code defines it.

App Step 2 - Site class and spectral parameters

27:07Step two, the ground motion. Table 20.2-1 in full, every site class with its shear wave velocity range. We take D — medium dense sand or stiff clay. And read the blue note under it: the site class is required even when you enter the spectral values by hand, because section 12.8.1.3 uses it later — one of that provision's criteria excludes site class E and F. S sub one, zero point four five g. This is the value that decides whether the Seismic Design Category short circuit fires, and whether the near fault floor on C sub S applies. Neither, at zero point four five.

27:44T sub L, eight seconds — the long period transition period, off the Figure 22 dash 14 maps. It never matters for a short building like this one, but the module asks, because beyond T sub L the C sub S upper bound changes form entirely. Now the part worth watching. The short period pair is bidirectional: enter either S sub M S or S sub D S, and the module computes the other through equation 11.4 dash 1. We have S sub M S from the Hazard Tool — one point five g — so we type that, and S sub D S appears as one point zero zero. That is the two thirds we worked out by hand, done for us.

28:22Same for the one second pair. S sub M one, zero point nine g, gives S sub D one of zero point six. Which also fixes T sub S at zero point six seconds — the corner period that is about to tell us which C sub S equation governs. Site class and all four spectral parameters, entered once, at the top of the calculation.

App Step 3 - Risk Category, Ie, and SDC

28:43Step three bundles the risk category with the Seismic Design Category, because the second one needs the first. Table 1.5 dash 1, in the code's own words. Category two is the residual one — all buildings except those listed in categories one, three and four — and that is this house. Below it, Table 1.5 dash 2 highlights the matching importance factor: I sub e equals one point zero. And here is what we did by hand, done book style. Table 11.6 dash 1, every band, with our cell highlighted: S sub D S of one point zero g, Risk Category two, category D.

29:20Table 11.6 dash 2, same treatment: S sub D one of zero point six g, Risk Category two, category D again. And the module states the reasoning rather than just the letter: S sub one is under zero point seven five, so assign the more severe of the two tables; Table 11.6 dash 1 gives D, Table 11.6 dash 2 gives D, Seismic Design Category D. That is what a plan reviewer wants to see — why the building is category D, not just that it is.

App Step 4 - Seismic force-resisting system

29:50Step four, the system — and this is the whole of Table 12.2 dash 1, not an excerpt. The table's lettered sections, collapsed: bearing wall systems, building frame systems, moment frames, dual systems, cantilevered column systems. Exactly the structure the printed table has. And notice the Seismic Design Category column headings — the D column is already highlighted, because Step 3 decided that. Open A, bearing wall systems. Every row: masonry, concrete, cold formed steel, cross laminated timber — with R, omega zero, C sub d, and the height limit in each Seismic Design Category.

30:26Row sixteen: light frame wood walls sheathed with wood structural panels rated for shear resistance. R of six point five, omega zero of three, C sub d of four, and sixty five feet in category D. The same row we read out of the excerpt by hand — and in the app you can see the rows above and below it, which is where the R equals two lesson lives.

30:46And the footnote b checkbox, which only appears when the selected system's omega zero is two point five or more: structure has flexible diaphragms, so omega zero may be reduced by a half. Check it, and the effective value reads two point five — the reduction we argued for in step three, now flowing into every overstrength combination downstream.

App Step 5 - Period determination

31:08Step five, the period. Four methods: the approximate period from C sub t and h sub n, the zero point one N shortcut for moment frames, the masonry and concrete shear wall formula, or entering a period directly from a model. We take the approximate period, which is what the hand calculation used. And h sub n is read only here — twenty feet, brought forward from the story heights in Step 1. You cannot accidentally type the ridge height into this field, because the field is not yours to type into; it is the sum of the story heights you already declared.

31:43Table 12.8 dash 2, with the row selected automatically from the system we chose in Step 4. Wood structural panel shear walls map to all other structural systems: C sub t of zero point zero two, x of zero point seven five. The module tells you which Table 12.2 dash 1 row produced that mapping, so the chain is auditable rather than magic. And the calculation, written out the way a calculation package should be: the values and where they came from, then the substitution, then the result. Zero point zero two times twenty to the zero point seven five — zero point one eight nine seconds. To the digit.

32:21Below it, the response spectrum control periods: T sub S equals S sub D one over S sub D S, zero point six seconds, T zero is zero point one two, and the project period of zero point one eight nine seconds is reported as sitting on the plateau — the flat, constant S sub D S part of the spectrum. Which is the conclusion we reached by hand, stated by the app before we ask for C sub S.

App Step 6 - Cs and base shear V

32:44Step six. First, an option we are not using but which is worth seeing: the section 12.8.1.3 maximum S sub D S provision, which lets some regular, short period Risk Category one or two buildings cap S sub D S at one point zero. Turn it on and the module checks all seven criteria for you — story count, period, site class, risk category, R — attesting only the two that are genuinely a judgment call. We leave it off, matching the hand calculation.

33:15The design response spectrum, drawn for this building's own parameters, with the project period marked in red. You can see at a glance that the red line lands on the flat part — and that the dashed lower bound is far below the curve, which is the zero point zero four four floor we checked by hand. And every equation, not just the governing one. Equation 12.8 dash 2, zero point one five three eight. Equation 12.8 dash 3, the upper bound, zero point four eight eight — stated, then set aside, because 12.8 dash 2 is the smaller of the two. Then the lower bounds: zero point zero four four times S sub D S times I sub e, and the near fault equation 12.8 dash 6, which does not appear at all because S sub one is below zero point six g. C sub S equals

33:57zero point one five three eight. Equation 12.8 dash 1. Zero point one five three eight times eighty one thousand one hundred — twelve thousand four hundred seventy seven pounds at strength level, and the module prints zero point seven V right underneath it: eight thousand seven hundred thirty four pounds for the allowable stress combinations. Both levels, both labelled, every time.

App Step 7 - Vertical and horizontal distribution

34:22Step seven distributes it. Level, weight, height, C sub v x — and then two blocks of three columns: story force, story shear and overturning moment at strength level, and the same three at zero point seven E. Eight thousand nine hundred forty three at the roof, three thousand five hundred thirty four at the second floor. The same numbers, in the same order, as the table we built by hand.

34:45And every row opens. Here is the roof level: k equals one because the period is under half a second, C sub v x written out as weight times height over the sum, the substitution, F sub x equals C sub v x times V, then zero point seven F sub x — each one carrying the load combination section it belongs to, section 2.3.6 for strength and 2.3.5 for allowable stress. This is the level of detail a plan checker can follow line by line without asking you a question.

35:15And the overturning moment at the base, section 12.8.5: two hundred fourteen thousand two hundred pound feet at strength level, one hundred forty nine thousand nine hundred forty at zero point seven E. That is the moment that becomes hold down tension at the ends of your shear walls.

App Step 8 - Diaphragm design forces

35:31Step eight, the diaphragms — as a separate step, because they are a separate check. The equation value, the minimum, and the maximum, in their own columns, and then the governing F sub p x. Nine thousand sixty at the roof, seven thousand one hundred sixty at the second floor — the minimum in both rows, exactly as we found. Having all three side by side is the point: you can see at a glance which bound is driving each diaphragm, instead of taking one number on trust.

35:59Open the roof row and the comparison is written out: equation 12.10 dash 1 gives eight thousand nine hundred forty three, the zero point two S sub D S I sub e minimum gives nine thousand sixty, the maximum gives eighteen thousand one hundred twenty, and the governing value is the minimum. Both force levels again at the bottom.

App Step 9 - Force diagrams

36:19And the last step — all of it, drawn. Four panels on one height scale, at the strength level. The elevation with the story masses and the F sub x arrows, the story shear step diagram, the overturning moment, and then the diaphragm forces on their own separate elevation. Compare that first panel with the figure from the hand calculation — the same two arrows, the same base reaction, the same building.

36:45The elevation. Masses and heights on the building, forces on arrows to the left, arrow lengths to scale, and the base shear reaction at the bottom. Eight thousand nine hundred forty three and three thousand five hundred thirty four, resolving to twelve thousand four hundred seventy seven. The story shear, as a step diagram — the accumulation we did in our heads, drawn. Eight thousand nine hundred forty three above, twelve thousand four hundred seventy seven below.

37:13Overturning, zero at the roof growing to two hundred fourteen thousand two hundred pound feet at the base. And the diaphragm forces, deliberately on their own lighter elevation, one arrow per level, each labelled with which bound governed it. They are drawn apart from the other three panels for a reason: each F sub p x is a separate design check on its own diaphragm. They are never concurrent, and they never sum into a story shear — so drawing them on the same building as the F sub x arrows would say something untrue.

37:44And one radio switches every panel to zero point seven E, with the force level stated on the figure itself so a reviewer reading the printed report can never mistake which level they are looking at. Six thousand two hundred sixty, two thousand four hundred seventy four, eight thousand seven hundred thirty four at the base.

Hand calculation vs. StructSuite - every number matches

38:03One last comparison before we close. Weight, eighty one thousand one hundred both ways. Structural height, twenty feet both ways. The design spectral accelerations, one point zero and zero point six. Seismic Design Category D. R of six point five, omega zero reduced to two point five, C sub d of four. The period, zero point one eight nine seconds. C sub S, zero point one five three eight. Base shear, twelve thousand four hundred seventy seven pounds. The story forces, eight thousand nine hundred forty three and three thousand five hundred thirty four. Base overturning, two hundred fourteen thousand two hundred pound feet. And the diaphragm forces, nine thousand sixty and seven thousand one hundred sixty — both minimum governed, both ways, to the pound.

38:47Every 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 the check that quietly decides most wood diaphragm designs — the zero point two S sub D S floor — got made, in both places, rather than assumed away. Read the article, or watch this video. Either way, you get the same house, checked the same way, twice.