Concrete Shear Reinforcement of integral precast and cast-in-place assembly

[designrider]

In a construction sequence where precast stringer beams pass orthogonally thru a cast-in-place supporting girder beam to become an integral assembly, can the stirrups of the precast section be considered stirrups participating in the shear resistance capacity of the girder beam in the region of overlap?

 

[CELinOttawa]

This depends on the support conditions, bonding a acheived and relationship between the two beams, but typically this can be achieved. Whether part of a beam is cast in advance and the encased into the remainder is a moot point, good detailing and load paths are what makes for the behaviour.

 

[dcarr82775]

In my opinion, no.

 

[designrider]

Thanks for the comments. My thoughts: the vertical leg of a stirrup will behave as such shear regardless if it was part of the precast or CIP section. However, a 'closed' stirrup with horizontal legs does not really exist here since the horizontal portion of the stringer beam stirrups are parallel to the girder beam. Therefore, are the equations for the concrete capacity (Vc) nullified because it is not a confined core in this region?

 

[dcarr82775]

I assume the CIP beam is supporting loads other than just the PC beam. You can not count on the shear strength of the PC beam to aid in the CIP beam unless it is carefully detailed to transfer the loads between the two. The stirrup orientation isn't the problem. At the face of the PC beam you essentially have a CIP beam with a big hole in it. The beam with the hole needs to carry the shear across the PC beam including the load from the POC beam. You could drill all sorts of dowels into and through the PC beam to do the job but why bother with a precast beam at that point.

 

[CELinOttawa]

Load path, detail, bond and (you're right) confinement. Full stop.

 

[Engineerataltitude]

x

 

[designrider]

I agree with dcarr82775 that the conservative approach is to consider the girder beam as a "CIP beam with a big hole in it". But to assume the beam passing through is equivalent to a void seems excessively conservative. The stringer must have a better justified contribution to shear than 0. Avoiding this detail is not an option...although that would be my preferred choice!