:root{--ink:#20354d;--muted:#526578;--line:#d5dee6;--soft:#f3f6f8;--positive:#b85c45;--negative:#355b86;--inner:#18888b;--outer:#bd772b}*{box-sizing:border-box}body{margin:0;background:#fff;color:var(--ink);font-family:"Microsoft YaHei","Noto Sans SC",sans-serif;font-size:14px;line-height:1.65}main{max-width:1140px;margin:auto;padding:24px 22px 18px}header{margin-bottom:20px}.eyebrow{font-size:12px;letter-spacing:.12em;color:#637486;margin:0 0 4px}h1{font-family:"Noto Serif SC","Microsoft YaHei",serif;font-size:27px;line-height:1.4;margin:0 0 8px;font-weight:700}header>p:last-child{margin:0;color:var(--muted)}h2{font-size:18px;margin:0 0 14px}button,input,select{font:inherit}button{background:white;color:var(--ink);border:1px solid #b9c7d2;border-radius:4px;padding:6px 12px;cursor:pointer;font-weight:600;line-height:1.55}button:hover{background:#edf3f7}button:focus-visible,select:focus-visible,input:focus-visible{outline:3px solid #66abbc;outline-offset:2px}button[aria-pressed=true],button.primary{background:var(--ink);border-color:var(--ink);color:white}button.primary:hover{background:#385773}select{background:white;border:1px solid #b9c7d2;border-radius:4px;padding:6px 7px;color:var(--ink)}input[type=range]{accent-color:#355b86;cursor:pointer;width:100%;margin:10px 0 0;height:20px}input[type=number]{width:75px;border:1px solid #b9c7d2;padding:5px;color:var(--ink);border-radius:3px}input[type=checkbox]{accent-color:var(--ink)}.controls{border-top:3px solid var(--ink);padding-top:14px;margin-bottom:14px}.tensions{display:grid;grid-template-columns:1fr 1fr;gap:26px}.tensions label{display:block;border-left:4px solid var(--inner);padding:0 14px;font-weight:700}.tensions label.outer{border-color:var(--outer)}.tensions output{font-family:Consolas,monospace;font-size:21px;float:right;margin-left:9px;line-height:1.5}.tensions span{font-size:12px;color:var(--muted);font-weight:400;margin-left:8px}.inner input{accent-color:var(--inner)}.outer input{accent-color:var(--outer)}.optimization{display:flex;gap:10px;align-items:center;flex-wrap:wrap;margin-top:15px}.optimization label{display:flex;gap:8px;align-items:center}.cap{white-space:nowrap}#constraints{font-size:12px;margin:9px 0 2px;color:var(--muted)}#status{font-size:13px;margin:0;min-height:24px;color:#22645f}#status.invalid{color:#a8342d;font-weight:600}.bridge-view{border:1px solid var(--line);background:white}.view-head{display:flex;justify-content:space-between;align-items:center;gap:12px;background:var(--soft);padding:10px 12px}.quantity,.views{display:flex;gap:6px}.views button{font-size:12px;padding:5px 8px}.quantity button{font-size:13px}#stage{height:500px;position:relative;overflow:hidden;background:#fbfcfd}canvas{display:block;width:100%;height:100%;touch-action:none}#labels{position:absolute;inset:0;pointer-events:none}.label{position:absolute;transform:translate(-50%,-50%);font-size:12px;line-height:1.35;white-space:nowrap;background:rgba(255,255,255,.94);padding:2px 4px;color:var(--ink);border-radius:2px}.label.cable{font-weight:700}.label.outer{color:#a15b19}.label.inner{color:var(--inner)}.label.force{color:var(--positive);font-weight:700}.label.section{font-size:13px;background:#fff;border:1px solid #b4c5d1;padding:3px 6px}.label.diagram{color:#597084;font-size:11px}.label.invalid{color:#ac3028;background:#fff1ed}.drawing-note{position:absolute;left:13px;top:10px;color:#617386;font-size:11px;line-height:1.6;pointer-events:none;background:rgba(255,255,255,.85);padding:4px 6px}#loading{position:absolute;inset:44% 0 auto;text-align:center;color:var(--muted)}.legend{display:flex;flex-wrap:wrap;gap:10px 20px;padding:10px 14px 2px;font-size:12px}.legend span:before{content:"";display:inline-block;vertical-align:middle;width:15px;height:3px;margin-right:6px;background:var(--positive)}.legend span:nth-child(2):before{background:var(--negative)}.legend label{margin-left:auto;color:#647382}.scale-note{margin:3px 14px 8px;font-size:12px;color:var(--muted)}.section-position{display:flex;align-items:center;gap:12px;padding:0 14px;font-size:13px;white-space:nowrap}.section-position input{margin:0;max-width:420px}.section-position output{font-family:Consolas,monospace;min-width:65px}.operation{color:#6c7c8c;font-size:12px;margin:7px 14px 12px}.results{margin-top:24px}.summary{display:grid;grid-template-columns:1fr 1fr;gap:24px;margin-bottom:15px}.summary p{margin:0;border-left:3px solid #bccbd7;padding-left:13px}.summary span{display:block;font-size:12px;color:var(--muted)}.summary strong{display:block;font-family:Consolas,"Microsoft YaHei",monospace;font-size:24px;line-height:1.7;letter-spacing:-.03em}.summary small{font-size:12px;color:var(--muted)}table{border-collapse:collapse;width:100%;font-size:13px}th,td{padding:8px 10px;border-bottom:1px solid var(--line);text-align:right;white-space:nowrap}th{font-weight:500;color:#586d81}thead{background:var(--soft)}th:first-child{text-align:left}.reactions td{font-family:Consolas,monospace;font-size:17px}.negative-reaction{color:#ad352b;font-weight:700}.balance{font-size:13px;color:#2a655e;margin:10px 0 3px}.tower-force{font-size:12px;color:#637487;margin:3px 0 13px}.inner-text{color:var(--inner)}.outer-text{color:var(--outer)}.relationships{margin-top:26px}.relationship-lines{display:grid;grid-template-columns:1fr 1fr;gap:14px}.relationship-lines p{margin:0;padding:11px 12px;background:#f4f7f9}.relationship-lines p:last-child{grid-column:1/-1}.relationship-lines b{font-size:13px;display:block}.relationship-lines span{display:block;font-family:Georgia,"Microsoft YaHei",serif;font-size:18px;margin:5px 0}.relationship-lines small{display:block;font-size:12px;color:var(--muted)}.equation{font-family:Georgia,"Microsoft YaHei",serif;font-size:16px;background:#f5f7fa;padding:10px 12px;line-height:1.8}.definition,footer p{font-size:12px;color:#627487;line-height:1.8}footer{border-top:1px solid var(--line);padding-top:18px;margin-top:24px}footer h2{font-size:14px;margin-bottom:7px}footer p{margin:5px 0}@media(max-width:900px){main{padding:19px 15px 16px}h1{font-size:24px}.optimization{gap:9px}#stage{height:465px}.tensions{gap:15px}.view-head{padding:8px;gap:8px}.quantity button{padding:5px 9px}.legend label{margin-left:0}.summary{gap:16px}.summary strong{font-size:22px}.relationship-lines span{font-size:16px}}
:root{--stay-control-size:26px}
/* The controls are projected onto the physical stays, not a detached slider. */
.optimization{margin-top:0}.tensions{display:flex;align-items:center;flex-wrap:wrap;gap:10px 22px;margin-top:14px}.tensions label{display:flex;align-items:center;gap:3px;padding:0 9px;white-space:nowrap}.tensions input[type=number]{width:86px;margin-left:8px;font-family:Consolas,monospace;font-size:16px}.tensions span{margin-left:3px}.precision-title{font-size:12px;color:#65788a;font-weight:400}.direct-hint{margin:0;padding:8px 13px;background:#fff;font-size:12px;color:#536b7e;border-bottom:1px solid #e3e9ed}.drag-key{display:inline-block;background:#18888b;color:#fff;border-radius:50%;width:19px;height:19px;text-align:center;font-weight:bold;margin-right:4px}#drag-status{font-weight:600;color:#244c66;margin-left:10px}#stage{height:425px}#direct-overlay{position:absolute;inset:0;width:100%;height:100%;pointer-events:none;overflow:visible;touch-action:none}.stay-hit{stroke:transparent;pointer-events:stroke;cursor:ns-resize}.stay-control{pointer-events:all;cursor:ns-resize;outline:none;--stay-color:#18888b}.stay-control.outer{--stay-color:#bd772b}.stay-grip{fill:#fff;stroke:var(--stay-color);stroke-width:2.7px;filter:drop-shadow(0 1px 2px #0002)}.stay-grip-glyph{fill:var(--stay-color);font-size:17px;font-weight:bold;pointer-events:none}.stay-value-bg{fill:#fff;stroke:var(--stay-color);stroke-width:1px}.stay-value{fill:var(--stay-color);font-family:Consolas,"Microsoft YaHei",monospace;font-size:11px;font-weight:700;pointer-events:none}.stay-control:focus-visible .stay-grip,.stay-control:hover .stay-grip,.stay-control.active .stay-grip{stroke-width:4px;fill:#fffbe8}.stay-control.slack .stay-grip{stroke-dasharray:3 2}.section-hit{stroke:transparent;pointer-events:stroke;cursor:ew-resize}.section-control{pointer-events:all;cursor:ew-resize;outline:none}.section-control>circle{fill:#fff8d6;stroke:#705315;stroke-width:2px}.section-control>circle.section-core{fill:#705315;stroke:none}.section-control:focus-visible>circle,.section-control:hover>circle{stroke-width:3px}#bridge-readout{position:absolute;left:14px;bottom:10px;padding:6px 9px;background:#fff;border-left:3px solid #b99a43;box-shadow:0 1px 5px #263b4e15;pointer-events:none;font-size:12px}#bridge-readout b{color:#5a6b79;font-weight:400;margin-right:10px}#bridge-readout-value{font-family:Consolas,"Microsoft YaHei",monospace;color:#173c56}.drawing-note{font-size:10px}.operation{color:#536b7e}.is-dragging{cursor:ns-resize}@media(max-width:900px){#stage{height:380px}.tensions{gap:10px}.tensions label{padding:0 7px}.tensions input[type=number]{width:78px}.tensions .precision-title{width:100%;margin:0}.tensions span{font-size:11px}.direct-hint{font-size:11px}#bridge-readout{font-size:11px}}
.girder-zero-outline,.girder-zero-line,.girder-reference-curve,.girder-result-curve{fill:none;pointer-events:none;stroke-linecap:round;stroke-linejoin:round}.girder-zero-outline{stroke:#304b60;stroke-width:3px;opacity:.8}.girder-zero-line{stroke:#fff;stroke-width:1.3px}.girder-reference-curve{stroke:#717d86;stroke-width:1.4px;stroke-dasharray:5 4;opacity:.9}.girder-result-curve{stroke-width:2.5px}.girder-result-curve.positive{stroke:#aa432e}.girder-result-curve.negative{stroke:#244f7c}
/* Input -> force transfer -> visible girder response. */
:root{--deformation:#7952a6;--vertical:#2371a1;--horizontal:#52647b}
.force-chain{display:grid;grid-template-columns:1fr 18px 1fr 18px 1.12fr;align-items:center;gap:8px;margin:0 0 20px;background:#f4f7fa;padding:12px;border-radius:5px}.force-chain p{margin:0}.force-chain small,.force-chain span{display:block;font-size:11px;color:#5e7182}.force-chain b{display:block;font-size:13px;line-height:1.7}.force-chain i{color:#8fa1af;font-style:normal;font-size:20px}.force-chain p:first-child{border-left:3px solid #18888b;padding-left:9px}.force-chain p:last-child{border-left:3px solid #7952a6;padding-left:9px}.preset-note{font-size:12px;color:#436077;margin:10px 0 0;background:#f5f8fa;padding:7px 9px;border-left:2px solid #a9bac7}.optimization button{font-size:12px}.force-selection{display:flex;align-items:center;gap:8px;padding:9px 12px 0;flex-wrap:wrap}.force-selection>span{font-weight:700;font-size:12px}.force-selection button{font-size:12px;padding:4px 9px}.force-selection button[data-force-group=inner][aria-pressed=true]{background:#18888b;border-color:#18888b}.force-selection button[data-force-group=outer][aria-pressed=true]{background:#bd772b;border-color:#bd772b}.force-selection small{font-size:11px;color:#687d8c}.force-summary{margin:0 12px;padding:7px 0;border-bottom:1px solid #dbe4ea;font-size:12px;color:#435f73}.force-summary b{margin-right:9px}.force-summary .force-t{color:#18888b}.force-summary.outer .force-t{color:#aa651e}.force-summary .force-v{color:#2371a1}.force-summary .force-h{color:#52647b}.force-comparison{margin:20px 0 0;padding:16px 12px 8px;background:#f8fafb;border:1px solid #dbe4ea}.force-comparison h2{font-size:16px;margin:0 0 5px}.force-comparison p{font-size:12px;color:#567083;margin:0 0 6px}.force-comparison svg{width:100%;display:block}.force-comparison .comparison-conclusion{font-size:13px;font-weight:600;color:#274c60;margin:5px 4px 2px}.stay-value{font-size:10px}.stay-control.selected .stay-grip{stroke-width:3.7px;fill:#fff8df}.force-arrow-layer{pointer-events:none}.force-arrow-label{font-size:12px;font-weight:700;font-family:Consolas,"Microsoft YaHei",sans-serif;paint-order:stroke;stroke:#fff;stroke-width:5px;stroke-linejoin:round}.force-arrow-label.t{font-size:13px}.force-arrow-label.angle{font-size:11px}.force-arrow-line{fill:none;stroke-linecap:round;stroke-linejoin:round}.deformed-reference{fill:none;stroke:#718292;stroke-width:1.5px;stroke-dasharray:6 4}.deformed-edge{fill:none;stroke:#7952a6;stroke-width:3px;stroke-linejoin:round}body[data-quantity=deflection] .girder-result-curve{stroke:#7952a6}body[data-quantity=deflection] #legend-positive:before,body[data-quantity=deflection] #legend-negative:before{background:#7952a6}body[data-quantity=deflection] .girder-zero-line{stroke:#718292;stroke-dasharray:6 4;stroke-width:1.5px}body[data-quantity=deflection] .girder-zero-outline{display:none}body[data-quantity=deflection] .girder-reference-curve{stroke:#b5a9c3}body[data-lesson=forces] .relationships,body[data-lesson=forces] .results,body[data-lesson=forces] footer{display:none}body[data-lesson=forces] main{padding-bottom:12px}body[data-lesson=forces] .force-comparison{margin-top:14px}#stage{height:430px}#deformation-note{color:#7952a6;font-weight:600}.force-chain+p{margin-top:0}@media(max-width:900px){#stage{height:420px}main{padding-left:12px;padding-right:12px}.quantity button{font-size:12px;padding:5px 7px}.view-head{flex-wrap:wrap}.force-chain{gap:6px;padding:10px}.force-chain b{font-size:12px}.force-arrow-label{font-size:11px}.force-arrow-label.t{font-size:12px}.force-summary{font-size:11px}.legend{gap:8px 12px}.direct-hint{font-size:11px}}
/* Section 2.5 already supplies the heading and model assumptions. */
body[data-lesson=forces] header{display:none}body[data-lesson=forces] main{padding-top:8px}body[data-lesson=forces] .force-chain{margin-bottom:12px}body[data-lesson=forces] .controls{padding-top:10px;margin-bottom:10px}body[data-lesson=forces] .tensions{margin-top:9px}body[data-lesson=forces] #constraints{margin-top:6px}body[data-lesson=forces] .optimization{gap:7px}body[data-lesson=forces] .optimization label{font-size:12px}body[data-lesson=forces] .preset-note{margin-top:8px}
.response-summary{display:flex;align-items:baseline;gap:4px 14px;flex-wrap:wrap;margin:0 12px;padding:8px 0;border-bottom:1px solid #dbe4ea;font-size:12px}.response-summary>span{font-size:11px;color:#65798c}.response-summary>b{font-family:Consolas,"Microsoft YaHei",monospace;font-size:12px}.response-summary small{font-weight:400;font-size:10px}.response-summary em{width:100%;font-style:normal;font-size:10px;color:#65798c}.response-w{color:#7952a6}.response-m{color:#a44731}
.tensions input[type=number]{width:104px}