diff --git a/src/core/rebar/summary.ts b/src/core/rebar/summary.ts
index 14450b8..487f994 100644
--- a/src/core/rebar/summary.ts
+++ b/src/core/rebar/summary.ts
@@ -182,7 +182,7 @@ export function rebarSummary(model: PierModel, m: Material = DEFAULT_MATERIAL):
   const lineCount = (line: number) => c14.filter((b) => b.line === line).length;
   const omitted = c14.filter((b) => !b.legs).length;
   rows.push({ member: 'coping', name: 'C14', size: 'H19', count: c14.length, length: c14Length(model.copingB * 1000), note: `위 2 줄 ${lineCount(1)} · ${lineCount(2)}본씩, 아래 ${[3, 4, 5, 6].map(lineCount).join(' · ')} (합 ${c14.length}) · 연직 발 생략 ${omitted}개 (하면 밖)` });
-  const stairs = stairBars(model.copingL * 1000, model.copingB * 1000, model.copingRefTop);
+  const stairs = stairBars(model.copingL * 1000, model.copingB * 1000, model.copingRefTop, m);
   rows.push({ member: 'coping', name: 'C15', size: 'H19', count: stairs.c15.length, length: stairs.c15[0].length, note: '계단부, 가장 높은 구간 (거꾸로 U)' });
   rows.push({ member: 'coping', name: 'C16', size: 'H19', count: stairs.c16.length, length: stairs.c16[0].length, note: `계단부, 3·4구간 (${Math.round(stairs.c16[0].length)} / ${Math.round(stairs.c16[stairs.c16.length - 1].length)})` });
 
diff --git a/src/main.ts b/src/main.ts
index 5738139..6b74fbc 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -225,7 +225,7 @@ function setupScene(): void {
   rebar3d = null;
   const pier = activePier();
   if (pier?.model) {
-    rebar3d = buildRebar3D(pier.model);
+    rebar3d = buildRebar3D(pier.model, state.material);
     rebar3d.group.visible = state.stage === 'rebar';
     world.add(rebar3d.group);
     const scene = buildPierScene(pier.model);
@@ -926,6 +926,8 @@ function setMaterial(key: keyof Material, raw: string): void {
     return;
   }
   state.material = { ...state.material, [key]: value };
+  // The 3D bars depend on the material too (their lengths), so the scene is rebuilt with it.
+  setupScene();
   renderAll();
 }
 
diff --git a/src/view/rebar3d.ts b/src/view/rebar3d.ts
index 4205e7a..ee98600 100644
--- a/src/view/rebar3d.ts
+++ b/src/view/rebar3d.ts
@@ -5,7 +5,7 @@ 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 { columnRebar, COLUMN_P1 } from '../core/rebar/column';
 import { barsAcross, BLOCK_REBAR } from '../core/rebar/bearing-block';
-import { hookLength, NOMINAL_DIAMETER } from '../core/rebar/common';
+import { DEFAULT_MATERIAL, hookLength, Material, NOMINAL_DIAMETER } from '../core/rebar/common';
 import { stretchMatrix } from './scene';
 
 /**
@@ -34,7 +34,7 @@ const COLOR = {
 
 const mm = (v: number) => v / 1000;
 
-export function buildRebar3D(m: PierModel): RebarScene {
+export function buildRebar3D(m: PierModel, mat: Material = DEFAULT_MATERIAL): RebarScene {
   const group = new THREE.Group();
   const byName = new Map<string, THREE.Mesh[]>();
   const Ms = m.stretches.map((_, k) => stretchMatrix(m, k));
@@ -87,7 +87,7 @@ export function buildRebar3D(m: PierModel): RebarScene {
     // at each end to the C1 level. Each bar is two pieces spliced at the column centre (overlap l_s). The slope runs x_L0 from each end (T형).
     const xL0 = L / 2 - COPING_REBAR.levelWidth / 2000;
     const under = (x: number) => undersideAt(L * 1000, x * 1000, m.endBottom, m.levelBottom);
-    const lap = bottomPiece().ls / 2000;
+    const lap = bottomPiece(mat).ls / 2000;
     xs.forEach((n, j) => {
       const z = (x: number) => under(x) + 0.127;
       const left: [number, number, number][] = [
@@ -166,7 +166,7 @@ export function buildRebar3D(m: PierModel): RebarScene {
       }
     }
     // C15 (거꾸로 된 U) and C16 (L형) on the stair (코핑철근.md §7): 11 bars each across the width.
-    const stairs = stairBars(L * 1000, B * 1000, topEl);
+    const stairs = stairBars(L * 1000, B * 1000, topEl, mat);
     for (const s of stairs.c15) {
       const n = mm(s.n);
       const ld = mm(s.leg);
@@ -187,7 +187,7 @@ export function buildRebar3D(m: PierModel): RebarScene {
   }
 
   // Columns: P1 / P2 main bars on their radii, ties P3 / P5 / P4 as rings, P6 crossties across the column.
-  const col = columnRebar(COLUMN_P1);
+  const col = columnRebar(COLUMN_P1, mat);
   const zBot = col.zBot; // mm
   const ring = (name: string, k: number, sc: number, y: number, r: number, d: number) => {
     const N = 36;
diff --git a/tests/rebar3d.test.ts b/tests/rebar3d.test.ts
index 9cc810b..0d04b5d 100644
--- a/tests/rebar3d.test.ts
+++ b/tests/rebar3d.test.ts
@@ -2,6 +2,7 @@ import { describe, expect, it } from 'vitest';
 import * as THREE from 'three';
 import { buildPier } from '../src/core/geometry';
 import { buildRebar3D } from '../src/view/rebar3d';
+import { DEFAULT_MATERIAL } from '../src/core/rebar/common';
 import { loadP1 } from './helpers';
 
 const rb = buildRebar3D(buildPier(loadP1()));
@@ -103,3 +104,18 @@ describe('확대기초 B9 · B10 · B11 · B12 in 3D (확대기초철근 (1).md
     expect(rb.byName.get('B12')!.length).toBe(47 * 3);
   });
 });
+
+describe('3D 철근 모델은 ③ 공통 설정 fck를 따른다', () => {
+  const model = buildPier(loadP1());
+  const height = (scene: ReturnType<typeof buildRebar3D>, name: string) => {
+    const b = new THREE.Box3();
+    for (const mesh of scene.byName.get(name)!) b.expandByObject(mesh);
+    return b.max.y - b.min.y;
+  };
+
+  it('C15 stair bars are shorter with a higher fck, as in the 철근표 (stepDevLength)', () => {
+    const base = height(buildRebar3D(model), 'C15');
+    const high = height(buildRebar3D(model, { ...DEFAULT_MATERIAL, fck: 35 }), 'C15');
+    expect(high).toBeLessThan(base - 0.05);
+  });
+});