of diagonal cracks, the de ection at mid-span increased and the
rigidity of the beams gradually decreased with the increase in
shear span-to-depth ratio. It was observed that the reduction
of the rigidity is closely related to the occurrence of inclined
cracks. The rst and second decreases in rigidity were displayed
at the occurrence of the diagonal cracks to join the loading
points and the corners A and B, and C and D, respectively,
as shown in Fig. 2. The concrete strength affects the rigidity
of beams and its effect on the rigidity gradually increased as
the shear span-to-depth ratio reduces. However, the rigidity of
the beams with openings was scarcely in uenced by concrete
strength due to the deteriorating rigidity with the existence of
the opening. The shape ratio of the web opening is not related to
the mechanical behavior at the initial loading stage, but its effect
apparently appeared after the appearance of diagonal cracks