Building Model Build Spec

A ten-storey reinforced concrete residential building on a 60′ × 52′ footprint, with a grade beam level, a lift core shear wall, and a stair/lift machine room poking above the roof.

The work splits into three phases, and they have to happen in order. Each one feeds the next, so a mistake in phase 1 shows up as garbage in phase 3.

Three bays each way. The X bays are A–B–A; the Y bays are B–16′–B, where that middle 16′ strip is the fixed 7′ + 9′ corridor from the plan and does not scale with your serial number.

Story data

Enter these in Edit › Stories and Grid Systems › Modify/Show Story Data. Build them bottom-up. Only the ten typical floors are similar to each other — make Story 1 the master and set Stories 2–10 as similar to it, which is what lets you assign loads once.

Two concrete grades, one steel grade. The trap is that the slabs and stair are 4000 psi while everything else is 4500 — define both materials before you define any section.

\( E_c = 57000\sqrt{f'_c} \) psi — 3824 ksi and 3605 ksi respectively. ETABS fills these in once you type f′c, but check them; a wrong E changes every drift and period you report.

Define › Section Properties › Frame Sections for the line elements, › Slab Sections and › Wall Sections for the shells.

Sixteen columns, three section types, and a pattern that is not symmetric. This is the one part of the model you cannot infer — you have to read it off the plan.

The grid gives you sixteen intersections: four X axes × four Y axes. What it does not give you is which of C1, C2 and C3 sits at each one. The plan in your assignment has a C1 at an interior position on the second row, which breaks any rule of thumb like “corners are small, interiors are big.” If you guess, you will get a plausible-looking model that is wrong in a way nobody catches until the column schedule.

Transcribe the layout into this table from your PDF before you draw anything:

Columns are drawn once and then replicated up the building. Draw the GB-to-Story-1 lift at the grid intersections, select all of them, and use Edit › Replicate › Story to carry them to Story 10. Orientation matters for C1 and C2 since they are rectangular — the 18″ dimension normally runs parallel to the beam that frames the weaker direction, so check the plan for which way each one is turned.

Four load patterns carry gravity, and the mistake that ruins models is putting a load on the wrong kind of object — area loads onto slabs, line loads onto beams, and never both for the same physical thing.

Define › Load Patterns. Self-weight multiplier is 1 on DEAD only and 0 on everything else. If you leave it at 1 on the superimposed pattern too, you count the whole concrete frame twice.

Dead load accounting

The 5″ slab weighs 62.5 psf on its own and ETABS adds that itself through DEAD. Your superimposed dead load is only the finish and partition allowance:

Wall load — the 425 lb/ft line

This is a 5″ brick wall carried on a beam. It goes on beams, not slabs, and only on the beams that actually have a wall over them: the full perimeter at every floor, plus the interior partition lines shown on your architectural plan.

Live load

Live load reduction: ETABS will not apply it unless you switch it on, and for a ten-storey column it makes a real difference — see Check 1, where it takes the accumulated live load from 133 kip down to 53 kip.

Define exactly these four at Define › Load Combinations. Not ETABS's auto-generated set — these specific four, with these names.

Two of these are one-directional by design. FDLEQy only covers seismic in Y and FDLWx only covers wind in X, which is what the brief asks for. Be aware that means the combination set is not a complete design envelope — a real design needs EQx and Wy too, plus the negative senses of each. Say so on your sheet rather than quietly implying the four are sufficient.

For the Beam AB envelope in section 11, take the worst of FDL, FDLEQy and FDLWx at each station. Under the lateral combinations the support moments grow and can reverse sign, which is exactly why the bottom steel has to be continued into the supports.

Both are auto-lateral loads in ETABS. Enter the code parameters and let it build the storey force distribution — then check the total against section 10.

Choosing R

You have a dual system: moment frames plus a lift-core shear wall. The R value depends on the detailing level you commit to, and it is a factor-of-1.3 swing in your design forces, so it is not a throwaway choice:

Both rows assume the frames can independently resist at least 25% of the base shear, which is the defining condition for a dual system. Check that in your model by running the frames without the wall — if they cannot, it is a wall system and R drops again.

The circled C3 at grid 2-B, at ground floor, where it is carrying everything above it. This is the check that tells you whether your load assignment is right, because it depends on every pattern at once.

Is 18 × 18 enough?

For a tied column the code caps the pure-axial capacity at \( \phi P_{n,max} = 0.80\phi[0.85f'_c(A_g - A_{st}) + f_y A_{st}] \) with \(\phi = 0.65\). At \(A_g\) = 324 in²:

Beam AB runs along the top grid line, Y = 52′, at Story 1. Three continuous spans of 21′ – 18′ – 21′, section FB 12×20.

The ACI approximate moment coefficients are legitimate here, and worth using because they give you an independent number that owes nothing to your model. Check the conditions first — all four hold:

Shear

\( \phi V_c = 0.75 \times 2\sqrt{f'_c}b_wd = \) 21.1 kip, against a maximum \(V_u\) of 23.9 kip at the face of the first interior support. Stirrups are needed but the demand is small — \(V_s\) required is only 3.6 kip, so the maximum spacing \(d/2\) governs everywhere.

The global check. If your seismic weight is wrong, everything lateral is wrong, and this catches it in five minutes.

This is what the AutoCAD long section has to show: where the bars are, where they stop, and why they stop there.

Pull the moment envelope from ETABS first. Take the worst of FDL, FDLEQy and FDLWx at every station along the beam, not just the FDL diagram — under the lateral combinations the support moments grow and the end-span support moment can reverse.

Curtailment rules that set those dimensions

Four drawings, and each one has a specific job. Draw them at a real scale in model space and plot through a layout — do not draw to fit the paper.

Layer discipline

Annotate every bar as count – size – length, for example 2-#7 × 12′-6″, and give each a bar mark that matches a bar bending schedule on the same sheet. A drawing with bars but no schedule cannot be fabricated from.

A 15% gap between hand check and model is normal. Beyond that, it is almost always one of these six things, roughly in order of how often it happens.

Quick reference