fix(rebar): stack the three C9·C10·C11 폐합 to the right so they do not overlap

The three 폐합 of one position were drawn at the same x, so their U bars overlapped. Loop i now sits at x + i·2d and its
삿갓 one bar (d) beside its U, so no U meets another loop's U or 삿갓. The 철근표 counts and lengths are unchanged.
The rule is geometry only; the spec (§4.3) needs the stacking note.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

src/core/rebar/coping.ts

@@ -252,6 +252,12 @@ export function capLength(x: number, coping: number): number {
   return CAP.flat + 2 * lCap;
 }
 
+/**
+ * Step (mm) between the three 폐합 at one position, to the right (수정 2026-10-23): a 삿갓 sits one bar (d) beside its U,
+ * so the next loop's U starts two bars on. Loop i is then at x + i·step and its 삿갓 at x + i·step + d, with no overlap.
+ */
+export const SHEAR_LOOP_STEP = 2 * NOMINAL_DIAMETER.H25;
+
 /** Pieces per position: 3 closed stirrups × (U + cap) = 6. Total 270 for C9 + C10 + C11 (document). */
 export const PIECES_PER_POSITION = 6;
 

src/view/rebar3d.ts

@@ -2,7 +2,7 @@ import * as THREE from 'three';
 import type { PierModel } from '../core/geometry';
 import { assemblyEndOffset, assemblyJs, b10Levels, b9Bars, FOOTING_DEFAULTS, gridPositions, grid, layers, legLength, legOffsets, sideDepth } from '../core/rebar/footing';
 import { bottomPiece, MAIN, widthLayout } from '../core/rebar/coping-main';
-import { C14_INSET, c14Bars, CAP, COPING_REBAR, shearBars, shearBarsMulti, shearUHeight, copingSideBars, SIDE_HOOK_LEG, stairBars, undersideAt } from '../core/rebar/coping';
+import { C14_INSET, c14Bars, CAP, COPING_REBAR, SHEAR_LOOP_STEP, shearBars, shearBarsMulti, shearUHeight, copingSideBars, SIDE_HOOK_LEG, stairBars, undersideAt } from '../core/rebar/coping';
 import { columnRebar, COLUMN_P1 } from '../core/rebar/column';
 import { barsAcross, BLOCK_REBAR } from '../core/rebar/bearing-block';
 import { DEFAULT_MATERIAL, FckByMember, hookLength, Material, materialFor, NOMINAL_DIAMETER } from '../core/rebar/common';
@@ -139,13 +139,15 @@ export function buildRebar3D(m: PierModel, common: Material = DEFAULT_MATERIAL,
         const h = shearUHeight(x, L * 1000) / 1000;
         const bot = top - h;
         const capBot = top - (h - CAP.cover / 1000);
-        const xc = xm + mm(NOMINAL_DIAMETER.S25);
         for (let i = 0; i < 3; i++) {
           const a = legN(i);
           const b = legN(i + 3);
-          addBar(name, 0, [xm, bot, a], [xm, bot, b], dShear, COLOR.shear);
-          addBar(name, 0, [xm, bot, a], [xm, top, a], dShear, COLOR.shear);
-          addBar(name, 0, [xm, bot, b], [xm, top, b], dShear, COLOR.shear);
+          // The three 폐합 stack to the right, so no U meets another loop's U or 삿갓 (SHEAR_LOOP_STEP).
+          const xu = xm + mm(i * SHEAR_LOOP_STEP);
+          const xc = xu + mm(dShear);
+          addBar(name, 0, [xu, bot, a], [xu, bot, b], dShear, COLOR.shear);
+          addBar(name, 0, [xu, bot, a], [xu, top, a], dShear, COLOR.shear);
+          addBar(name, 0, [xu, bot, b], [xu, top, b], dShear, COLOR.shear);
           addBar(name, 0, [xc, top, a], [xc, top, b], dShear, COLOR.shear);
           addBar(name, 0, [xc, top, a], [xc, capBot, a], dShear, COLOR.shear);
           addBar(name, 0, [xc, top, b], [xc, capBot, b], dShear, COLOR.shear);

tests/rebar-shear-loops.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest';
+import { buildPier } from '../src/core/geometry';
+import { NOMINAL_DIAMETER } from '../src/core/rebar/common';
+import { PIECES_PER_POSITION, SHEAR_LOOP_STEP } from '../src/core/rebar/coping';
+import { buildRebar3D } from '../src/view/rebar3d';
+import { loadP1 } from './helpers';
+
+const d = NOMINAL_DIAMETER.H25;
+
+describe('C9·C10·C11 폐합 (코핑철근.md §4 · 수정 2026-10-23)', () => {
+  it('the loop step leaves room for a 삿갓 (one bar beside its U) before the next loop\'s U', () => {
+    // Loop i: U at x_i, 삿갓 at x_i + d. Loop i+1: U at x_i + step. Clear if step − d ≥ d.
+    expect(SHEAR_LOOP_STEP - d).toBeGreaterThanOrEqual(d - 1e-9);
+  });
+
+  it('three 폐합 per position and 6 pieces each: the count and the 철근표 are unchanged', () => {
+    expect(PIECES_PER_POSITION).toBe(6);
+    const rb = buildRebar3D(buildPier(loadP1()));
+    // C10 has 9 positions, each with 3 loops of 6 bars (2 U legs, U base, 삿갓 top + 2 legs) = 6 bars per loop.
+    expect(rb.byName.get('C10')!.length).toBe(9 * 3 * 6);
+  });
+});