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Beam - Critical Sections for Shear and Moment - Precast Concrete

Beam - Critical Sections for Shear and Moment - Precast Concrete

Beam - Critical Sections for Shear and Moment - Precast Concrete

(OP)
ACI 318-14 specifies the locations of the critical sections for beam shear and moment.
These locations (d from face of support, and at face of support respectively) are conditioned
on the beams being "built integrally" with the supports.

I have some doubt as to whether the above would apply to a precast structure where the beams
connect into the face of the columns by means of mechanical couplers.
At the top level of the structure the beams run continuously above the columns and mechanical couplers
connect the bottom of the beam to the top of the column.

In this case, if not considered "integrally built", the critical sections may be closer to the support
and thus, the required moment and shear will increase.

If anyone has more information on this, it would be appreciated.

RE: Beam - Critical Sections for Shear and Moment - Precast Concrete

ACI 318-11, Section 11.1.3.2 states "For prestressed members, sections located less than a distance h/2 from face of support shall be permitted to be designed for Vu computed at a distance h/2."

In ACI 318-14, the similar section is 9.4.3.2.

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RE: Beam - Critical Sections for Shear and Moment - Precast Concrete

(OP)
JAE, thanks for the reply. I should have mentioned in this case the precast beams and columns will not
be prestressed. I'm more convinced now the beam should be designed for the shear and moment at the face
of the support. I suppose that will also depend on the details of the connection - mechanical couplers in this case.

RE: Beam - Critical Sections for Shear and Moment - Precast Concrete

The critical things to have in place to take shear at "d" away from the support are:

1) Loads applied to the top of the beam.

2) Support reaction applied near the bottom of the beam.

Monolithic / non-Monolithic is immaterial. A simple span beam on bearings is a great example where there is no continuity but the reduction definitely applies.

It's tough to say without seeing your connection detail but, if it's the case that reaction shear will be predominantly transmitted through a column/beam compression block near the bottom of the beams, then I would argue that this is little different from the monolithic case and the reduction is valid.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Beam - Critical Sections for Shear and Moment - Precast Concrete

I guess prestressed or not prestressed- the ACI references above describe the conditions for taking Vu at h/2 or d/2 and as KootK states.

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