diff --git a/src/core/rebar/column.ts b/src/core/rebar/column.ts
index 23bac4c..b210319 100644
--- a/src/core/rebar/column.ts
+++ b/src/core/rebar/column.ts
@@ -27,6 +27,8 @@ export interface ColumnInput {
   ctcCoupler: number;
   /** P2 커플러를 P1보다 올리는 높이. */
   dzCp2: number;
+  /** 커플러는 P1 위 끝에서 이 거리(mm) 이상 아래에만 둔다 (커플러 여유, 가정 2500). */
+  couplerTopMargin?: number;
   ctcP3: number;
   ctcP5: number;
   ctcP4: number;
@@ -46,7 +48,11 @@ export function columnInputFor(model: PierModel, base: ColumnInput = COLUMN_P1):
   const seg = model.segments.find((g) => col.s >= g.s0 - 1e-9 && col.s <= g.s1 + 1e-9) ?? model.segments[0];
   const Hc = Math.round((model.levelBottom - model.footings[0].top) * 1000 * 10) / 10;
   const hcop = Math.round((seg.top - model.levelBottom) * 1000 * 10) / 10;
-  return { ...base, Hc, hcop };
+  // C3 (상면 3단) = the lowest coping top −127 (C1 cover) −2·57.2 (layer spacing 상면 주철근 단 간격); P1: 43.170.
+  // P1 top = C3 − 100 (계산 2026-10-23), so e_P1 = step top − (C3 − 0.1) = 485 on P1.
+  const c3 = model.copingRefTop - 0.127 - 2 * 0.0572;
+  const eP1 = Math.round((seg.top - (c3 - 0.1)) * 1000);
+  return { ...base, Hc, hcop, eP1 };
 }
 
 export const COLUMN_P1: ColumnInput = {
@@ -54,7 +60,7 @@ export const COLUMN_P1: ColumnInput = {
   Hc: 12703,
   hcop: 2644,
   c: 125,
-  eP1: 200,
+  eP1: 485,
   eTop: 1623,
   Hf: 2000,
   nDiv: 46,
@@ -113,7 +119,10 @@ export function columnRebar(x: ColumnInput = COLUMN_P1, m: Material = DEFAULT_MA
   const zTop2 = x.Hc + lD2;
   // Bottom of P1 and P2: 확대기초 하면 2단 (B7) 위 + (d_B7 + d_P1)/2 (기둥철근.md §3.2, the same for both).
   const zBot = -x.Hf + layers(x.Hf).B7 + (NOMINAL_DIAMETER.S19 + dS32) / 2;
-  const couplerLevels = Array.from({ length: Math.max(0, Math.floor(zTop1 / x.ctcCoupler) - 1) }, (_, i) => (i + 1) * x.ctcCoupler);
+  // Couplers at k·ctc from the footing, kept at least the margin below the P1 top (커플러 여유).
+  const couplerMargin = x.couplerTopMargin ?? 2500;
+  const couplerCount = Math.max(0, Math.floor((zTop1 - couplerMargin) / x.ctcCoupler));
+  const couplerLevels = Array.from({ length: couplerCount }, (_, i) => (i + 1) * x.ctcCoupler);
   // Pieces of each main bar between its couplers (기둥철근.md §3.2): the bottom piece carries the 90° hook (16.5·d = 525).
   const p1Hook = Math.round(hookLength('main90', dS32));
   const p1Points = [zBot, ...couplerLevels, zTop1];
diff --git a/tests/column-model.test.ts b/tests/column-model.test.ts
index 54668ee..38b0248 100644
--- a/tests/column-model.test.ts
+++ b/tests/column-model.test.ts
@@ -10,7 +10,7 @@ describe('기둥 철근 높이는 모델에서 (Hc · h_cop, 승인 2026-10-23)'
     expect(input.hcop).toBeCloseTo(2644, 0);
     const col = columnRebar(input);
     expect(col.zTop2).toBeCloseTo(14539, 0);
-    expect(col.zTop1).toBeCloseTo(15147, 0);
+    expect(col.zTop1).toBeCloseTo(14862, 0);
   });
 
   it('taper h2 = 3000: Level underside 39.411, Hc 11203, P2 top 13039 (= Level underside + l_d)', () => {
@@ -21,7 +21,7 @@ describe('기둥 철근 높이는 모델에서 (Hc · h_cop, 승인 2026-10-23)'
     expect(input.hcop).toBeCloseTo(4144, 0);
     const col = columnRebar(input);
     expect(col.zTop2).toBeCloseTo(13039, 0);
-    // The P1 top is anchored to the step top, so it does not move with h2.
-    expect(col.zTop1).toBeCloseTo(15147, 0);
+    // The P1 top is anchored to the step top (and C3), so it does not move with h2.
+    expect(col.zTop1).toBeCloseTo(14862, 0);
   });
 });
diff --git a/tests/rebar-column.test.ts b/tests/rebar-column.test.ts
index fdff22a..4b797d6 100644
--- a/tests/rebar-column.test.ts
+++ b/tests/rebar-column.test.ts
@@ -10,8 +10,8 @@ describe('기둥 철근 C4 (기둥철근.md, plan C4)', () => {
     expect(c.angleStepDeg).toBeCloseTo(360 / 46, 9);
   });
 
-  it('heights: P1 top 15147, P2 top ≈ 14539 (l_d of S32 at fck 27, 1836.9 ≈ the document 1836)', () => {
-    expect(c.zTop1).toBe(15147);
+  it('heights: P1 top 14862 (= C3 43.170 − 0.1, e_P1 485), P2 top ≈ 14539 (l_d of S32 at fck 27, 1836.9 ≈ the document 1836)', () => {
+    expect(c.zTop1).toBe(14862);
     expect(Math.abs(c.zTop2 - 14539)).toBeLessThan(1);
   });
 
@@ -86,10 +86,10 @@ describe('RB-C07 P4 삭제 규칙 (기둥철근.md §6.2)', () => {
 });
 
 describe('기둥철근 P1·P2 마디와 바닥 (기둥철근.md §3.2)', () => {
-  it('bottom at −1697.5 (−H_f + z_B7 + (d_B7 + d_P1)/2), P1 pieces 4697.5 + 525 / 3000 ×3 / 3147', () => {
+  it('bottom at −1697.5 (−H_f + z_B7 + (d_B7 + d_P1)/2), P1 pieces 4697.5 + 525 / 3000 ×3 / 2862', () => {
     expect(c.zBot).toBeCloseTo(-1697.55, 1);
     expect(c.p1Hook).toBe(525);
-    expect(c.p1Pieces.map((v) => Math.round(v))).toEqual([5223, 3000, 3000, 3000, 3147]);
+    expect(c.p1Pieces.map((v) => Math.round(v))).toEqual([5223, 3000, 3000, 3000, 2862]);
   });
 
   it('P2 pieces 4847.5 / 3000 ×3 / 2389', () => {