{
  "schema": "bridge-teaching-section/1",
  "evidence_type": "teaching_assumption",
  "source": "原创解析教学例题；材料力学、平截面与换算截面方法",
  "coordinate_system": "x桥向/y横向/z向上，z=0为毛截面形心",
  "render_mapping": "(x-L/2,z+2,-y)",
  "units": {"length":"m","force":"N","stress":"Pa","display":["mm","m","kN","kN·m","MPa","με"]},
  "geometry": {"span":20,"width":0.6,"depth":1.2,"area":0.72,"I":0.0864,"rebar_count":8,"rebar_diameter":0.032,"rebar_effective_depth":1.1,"selected_section_x_min":2.4,"selected_section_x_max":17.6},
  "materials": {"Ec":30000000000,"Es":200000000000,"teaching_ft":2400000,"teaching_fy":400000000,"gamma_N_per_m3":25000},
  "loads": {"selfweight_N_per_m":18000,"additional_dead_N_per_m":8000,"uniform_live_N_per_m":10000,"effective_prestress_N":3000000,"straight_tendon_eccentricity_below_centroid_m":0.35},
  "display": {"rebar_diameter_factor":2,"human_height_m":1.7,"deformation_factor":50,"deformation_scope":"gross elastic bending displacement and rotation only; disabled for RC transformed section; raw strain/stress unchanged","prestress_bundle_display_diameter_m":0.11,"force_arrow_size":"symbolic","crack_trace":"direction symbol, not solved crack"},
  "models": ["gross-section linear elastic beam plane stress","uncracked transformed RC pure bending","cracked transformed RC pure bending; tensile concrete ignored","given straight eccentric prestress superposition"],
  "pc_ui_cases": ["selfweight + given effective prestress","previous + additional dead load","previous + uniform live action"],
  "analysis_excluded": ["solid FE","real construction path","support conversion","age-dependent stiffness","prestress loss","crack width and propagation","nonlinear shear transfer","capacity and code compliance"]
}