def deck_props(DD, DR, B, TT, TB): 
    """Properties of a trapezoidal deck when given deck depth (DD), rib depth (DR)
    rib spacing (B), rib width at top (TT), rib width at bottom (TB)
    - P is the length of the deck on the underside (topside is ignored)
    - A is the cross-sectional area of concrete per rib
    - D_AVE is the average depth of concrete
    
    NB At the moment no flexural properties are calculated"""
    A = DD * B + 0.5 * (TT + TB) * DR
    P = B - TT + TB + ((TT - TB)**2 + 4 * DR**2)**0.5
    return {'P': P, 'A': A, 'D_AVE': A / B}