AASHTO LRFD 7th Edition Section 3.6.5 - Vehicle Collision Force CT

[PaulProvost]

When analyzing an existing pier column for vehicle impact capacity, a 600 kip static load is placed at 5 feet above ground level. All of the current analysis examples I've seen assume a dual plane shear failure mode. Each plane extends through the pier column at 45 degrees above and below the point of impact. This all seems to make sense. However, looking at pictures of failed pier columns due to these types of impacts, I'm curious why only the concrete and horizontal stirrups shear capacity is calculated for resistance? The main vertical reinforcement passes through the shear failure planes at the same angle as the stirrups (45 degrees), is fully anchored at both ends, is confined by the stirrups, and shows similar damage after these impacts. So why cannot the cross sectional area of the main vertical reinforcement be included with the stirrups when calculating the shear capacity contribution of the reinforcing steel? It's not a beam where the main reinforcing steel is in a constant state of compressive or tensile stress. It's stress level at any given time can be assumed to be fairly minimal, particularly on short columned (15 to 25 feet) piers.

I would appreciate any input that would clarify this restriction, and comment on its real world legitimacy. Thank You.