Auto Image Collision Repair · Woodstock, GA · 2-story steel frame, 8″ hollowcore deck, two boundary conditions to solve
The unifying principle: hollowcore's prestress is in the bottom of the plank. It wants to span between two real supports in positive moment. Anywhere it has to cantilever, you're working against the prestress. The cleanest details on every edge of this building come from the same move: add a real bearing point so the plank can stay simply supported. On the soil-nail-wall side that's a cap beam on top of the wall. On the balcony side that's the knee brace you've already drawn, or a cantilevered steel beam under the plank.
Terminology used here. The vocabulary mixes structural-steel and precast — to keep things unambiguous:
Column — primary vertical at the building grid; full multi-story height (or spliced near a floor level). Carries the building's gravity and lateral loads.
Post — shorter vertical that carries only its own tributary load. Examples: rail post, parapet post, partial-height member at a balcony edge. Not on the building grid; doesn't pick up other framing.
Girder — primary horizontal at the column line; spans column-to-column and supports the deck (plank ends bear on it). Frames into columns at each end.
Beam — secondary horizontal that frames into a girder. With hollowcore, planks span girder-to-girder directly so there are usually no intermediate beams in the floor field. Reserve the word for cases like edge beams, header beams at openings, or the perimeter member supporting a balcony tip.
Knee brace — diagonal compression strut from a column up to the underside of a girder; supports a cantilevered or partially-supported edge.
Topping — CIP concrete over the plank with a rebar mat; provides diaphragm action and any negative moment.
Plan view — orienting the section cuts
Plank span direction set first, so every following cross-section is unambiguous. Cores and strands run with the span (along the long axis of each plank).
Plan view (north up). Hollowcore planks run N–S, perpendicular to the soil nail wall on the south face. Each plank's long axis spans from the inboard perimeter beam at the north column line to the cap beam atop the south wall. Section A–A is a vertical (N–S) cut, looking east — so in every following cross-section the plank appears in long elevation with cores and strands running with the span (left = north, right = south).
Convention used below. Per PCI standard practice, the plank in long elevation is drawn as a clean silhouette — cores aren't depicted in long view because they all sit behind the visible face. Prestress strands are shown as a continuous bottom line. A small cross-section inset in the legend shows what the same plank looks like cut transversely (cores as circles). The cap beam / soil nail wall is drawn here on the south face — if your wall is actually on the north (or another) face, the cross-sections still apply geometrically, just flip the compass labels.
Side A — Hollowcore deck meets slab-on-grade over a permanent soil nail wall
Section A–A. The shotcrete face of the soil nail wall is a flexural skin sized for lateral soil pressure between nails — it has incidental vertical capacity but no AASHTO wheel-load rating. A CIP cap beam on top of the wall is the missing piece that lets you put deck loads and SOG edge loads at this elevation.
Section A–A (vertical N–S cut through the plank, looking east). Plank in long elevation per PCI convention: clean silhouette in this view; cores are visible only in the cross-section inset (lower-right). Strands run continuously along the bottom in the span direction (here, north on left → south on right). Both wheel loads have a complete vertical path to subgrade.
Side A — why not just cantilever the plank over the shotcrete?
Cantilever the plank past the wall. Standard hollowcore (long view, clean silhouette per convention) has bottom strand only. At the cantilever support the top fiber goes into tension and cracks. Also asks the shotcrete face to do something it wasn't designed for.
Cap beam approach (clean). Plank simply supported on the cap beam. Vehicle load has a complete vertical path: cap → shotcrete in pure compression → toe footing → subgrade.
Story-on-story — column continuity through the deck
You asked what happens at the F1 deck level where the column from below has to continue up to support the roof. The short answer: the deck never carries the column-on-column load. Both column pieces are continuous through the deck plane. Two ways to do it:
Single piece (no splice). One column from foundation to roof. For your 24 ft total (10 ft basement + 14 ft upper) this is well under the ~39 ft / 12 m single-piece rule of thumb. Erection lifts a 24 ft column once, then frames in.
Spliced ~4 ft above the F1 deck. Lower column tops out 4 ft above the deck, upper column splices on with bolted end-plates. The 4 ft height is AISC convention so floor-edge safety cables can anchor to the column above the new deck. Both pieces are still continuous through the deck plane — the splice is well clear of it.
In both cases the deck-to-column connection at the F1 level is identical: the column passes through, the F1 girder frames into it via shear-tab connections, the plank bears on the girder, and the topping wraps the column with non-shrink grout in a 1–2″ gap. The deck never sees the column load — it only carries its own gravity through the girder.
Continuous column. One column per grid line, foundation to roof. F1 girder frames into the column at the deck elevation; roof girder frames in at the roof elevation. Plank bears on the girder; topping wraps the column. No splice operation.
Spliced column. Splice ~4 ft above the F1 deck. Both column pieces are still continuous through the deck plane — the splice doesn't bear on the deck. F1-girder-to-column connection is identical to A.
Close-up of the F1 deck-to-column intersection (transverse section across the plank, at the column line):
The deck stays in its lane: it carries its own gravity through the girder to the column. The column carries everything (its tributary deck weight + everything above it) straight down to the foundation. The grout-filled gap between plank ends and column is non-structural — it just keeps the topping diaphragm continuous across the column.
Side B — Hollowcore balcony cantilever
Same plank, opposite problem. Per the CROSS-Safety panel and PCI cantilever design notes, retrofitted hollowcore balconies have failed in service:
“A cantilever has no redundancy so the described fixing methodologies are basically unsafe... [tension] might be resisted by the tensile capacity of concrete but that is fundamentally unreliable and the mode of failure is brittle.” — CROSS-Safety expert panel, on retrofitted steel balconies fixed to hollowcore
Four ways to do this safely. Three of them keep the plank in positive moment.
Continuous plank with the cantilever as a back-end of a longer span. Possible but the riskiest path. Per CROSS, do not retrofit.
The cleanest answer. Brace converts the cantilever into a simple span. Plank bears on perimeter beam at one end and the outboard sub-beam (carried by the brace) at the other.
Continuous W-section runs from interior column out past the edge column with a moment connection. Plank lays on top in two simply-supported pieces.
PCI standard detail E17.0. Hollowcore stops at the building edge; reinforced solid slab cantilevers past it, cast integral with the topping.
Balcony detail — knee brace approach (deep dive)
Of the four options above, this is what your model shows. The knee brace converts the cantilever into a simple span — same plank, same bearing, same prestress logic as any interior bay. The trick is in the three connections: where the brace lands top, where it lands bottom, and how the deck terminates at the balcony edge with the rail post. Numbered callouts in the elevation correspond to the three sub-details that follow.
Section A–A through one balcony bay. The plank simply spans from the column-line stub girder to the outboard balcony edge beam — same load case as any interior bay. The knee brace transfers the balcony reaction down at an angle to the column at a lower elevation. Three connections (numbered) drive the detailing — close-ups follow.
Top end of the brace: gusset plate welded to the bottom flange of the edge beam in the shop, brace bolted to gusset in the field. Pure compression connection — no moment, no fire-rated weld required at the joint.
Bottom end of the brace: gusset welded to the column's east flange in the shop. Field-bolted. The brace pushes the column inboard at the bottom — your structural engineer needs to check the column for that horizontal kick (and the F1 girder above for the equal-and-opposite reaction).
Outer edge of the balcony: plank ends on the edge beam with neoprene pad, topping continues 1–2″ past the plank as a small curb, rail post anchors through the topping to anchor studs welded to the top flange of the edge beam. Rail / guardrail loads bypass the plank entirely.
The bearing connection — close-up
Two views of the same connection. Left: section across the plank (perpendicular to span) shows two abutting plank ends meeting on a single W-beam — this is the only diagram in the explainer where you see cores in cross-section. Right: section along the plank (parallel to span, matches Section A–A) shows one plank end landing on the cap beam.
Same connection — different section orientation. Left: section across the plank (the only place cores appear as circles). Right: section along the plank, matching Section A–A.
What the SE will want to look at
Side A: cap beam sized for hollowcore reaction + SOG edge load + AASHTO HS-20 wheel patch + load distribution to top-row soil nails. Wall toe footing sized for combined dead + live coming down through the shotcrete.
Side A: drainage at top and behind the wall — the cap pour can't dam the wall's drainage path.
Side B: if you're going with the knee brace, the brace works in compression under gravity; check the column below the brace tip for the added axial / moment, and check the perimeter beam for the kicked-in horizontal component.
Side B: diaphragm continuity at the balcony — transverse rebar in the topping that crosses both supports keeps the plank tied into the building diaphragm.
Negative-moment design at any cantilever support is brittle. Per CROSS-Safety: never rely on hollow void infill as the only top-tension path — calculate it explicitly with rebar in the topping.
CROSS-Safety caution. The dangerous detail is hollowcore cantilevering with no engineered top reinforcement, where someone has “just filled the cores with concrete” and called it good. Failure is brittle — you don't get warning. If you cantilever the plank, the top steel (in the topping or as top-strand in the plank) does the work, full stop.
Reference details and photos
PCI Hollowcore Details bundle — includes E17.0 (cantilever solid slab balcony at plank ends) and E18.0 (integral side cantilever balcony): pci.org/PCINE/.../Hollowcore_Details
CROSS-Safety report 620 — cautions on retrofitted steel balconies fixed to hollowcore: cross-safety.org/.../620
Diagrams are schematic. Bay dimensions, beam sizes, rebar, joint hardware, and brace geometry shown are illustrative — not for construction. Final sizing by the project structural engineer.