rib shortening

huangyhg

rib shortening
in the aashto load factor design load combinations iv, v, vi there is a term (r + s + t), the r is defined as rib shortening.  i have a good handle on s & t, but am unsure of what rib shortening is... can someone explain to me in what type of structure one would have to consider this effect? does it need to be considered in a spread box beam - concrete deck superstructure? if so maybe you could point me in the right direction to determine how to calculate that force.
rib shortening is taken into account for the design of an arch or a rigid frame.