docs: def bld_rafter_deflection(length=-9, force=-9, E_mod_elasticity=-9, I_moment_of_intertia=-9):
      """
      calculate rafter deflections - see test_calc_building_design.py for 
      Sample values for equations below from Structures II course
      """
      
      
      if length == -9:
          length = float(input('enter rafter length : '))
          
      if force == -9:
          force = float(input('enter Force or weight applied to roof : '))
      
      if E_mod_elasticity == -9:
          E_mod_elasticity = float(input('enter modulus of elasticity x10**5 (Steel beam example=2.1) : '))
      
      if I_moment_of_intertia == -9:
          I_moment_of_intertia = float(input('enter members "moment of intertia x10**6" (for Steel beam 410UB53.7=188 ) :'))
          
      res = {}
      
      res['max deflection - centre load'] =  (1 * force * (length ** 3)) / (48  * (E_mod_elasticity * 10**5) * (I_moment_of_intertia * 10**6))
      res['max deflection - distrib load'] = (5 * force * (length ** 4)) / (384 * (E_mod_elasticity * 10**5) * (I_moment_of_intertia * 10**6))
      
      pprint(res)
      return res