Bearing these general observations in mind, the ordinary simple method of truss analysis yields the tabulated statement of stresses given below for the three types selected for consideration. The first case to be treated is that of Fig. 19, which represents the Pratt truss type. The moving load is supposed to pass across the bridge from right to left. The plus sign indicates tension and the minus sign compression. [Illustration: FIG. 19.] _Stress in c₁_ = + (¹/₇ + ²/₇) _W_ sec _a_ = ³/₇ _W_ sec _a_. _Stress in T₄_ = + (¹/₇ + ²/₇ + ³/₇) _W_ sec _a_ = ⁶/₇ _W_ sec _a_; ” ” _T₃_ = + [(¹/₇ + ²/₇ + ³/₇ + ⁴/₇) _W_ + _Wʹ_ + _W₁_] sec _a_ = (¹⁰/₇ _W_ + _Wʹ_ + _W₁_) sec _a_; ” ” _T₂_ = + [(¹/₇ + ²/₇ + ³/₇ + ⁴/₇ + ⁵/₇) _W_ + _2wʹ_ + _2w₁_] sec _a_ = (¹⁵/₇ W + 2wʹ + 2w₁) sec _a_; ” ” _T₁_ = + (_W_ + _W₁_). _Stress in P₃_ = -(⁶/₇ _W_ + _Wʹ_); ” ” _P₂_ = -(¹⁰/₇ _W_ + 2_Wʹ_ + _W₁_); ” ” _P₁_ = -3(_W_ + _Wʹ_ + _W₁_) sec _a_. _Stress in L₁_ =_Stress in L₂_ = + 3(_W_ + _Wʹ_ + _W₁_) tan _a_; ” ” _L₃_ = ” ” _L₂_ + 2(_W_ + _Wʹ_ + _W₁_)tan _a_ + + 5(_W_ + _Wʹ_ + _W₁_) tan _a_; ” ” _L₄_ = ” ” _L₃_ + (_W_ + _Wʹ_ + _W₁_) tan _a_ + 6(_W_ + _Wʹ_ + _W₁_) tan _a_. _Stress in U₁_ = _-Stress in L₃_ = -5(_W_ + _Wʹ_ + _W₁_) tan α; ” ” _U₂_ = - ” ” _L₄_ = -6(_W_ + _Wʹ_ + _W₁_) tan α; ” ” _U₃_ = ” ” _U₂_ = -6(_W_ + _Wʹ_ + _W₁_) tan α. It is easy to check any of the chord stresses by the method of moments. As an example, let moments first be taken about the panel-point 5 in the lower chord, and then about the panel-point _c_ in the upper chord. The following expressions for the chord members _U₁_ and _L₄_ will be found, and it will be noticed that they are identical with the stresses for the same members given in the preceding tabulation, the counter-members, shown in broken lines, being omitted from consideration as they are not needed. _R.2p_ - (_W + Wʹ + W₁_)_p_ _Stress in U₁_ = ------------------------------ _d_ _p_ = 5(_W + Wʹ + W₁_) ---- = 5(_W + Wʹ + W₁_) tan α. (29) _d_ _R.3p_ - 2(_W + Wʹ + W₁_) . 1½_p_ _Stress in L₄_ = ---------------------------------- _d_ = 6(_W + Wʹ + W₁_) tan α. (30) [Illustration: FIG. 20.]