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jeffhed (Structural)
3 Sep 08 14:29
I have a project where a 50" square ventilation shaft must be cut into an existing 11'-0" tall concrete sub grade wall that is also acting as grade beam between conrete piers.  The opening is centered in the neutral axis of the wall/beam and in the mid span of the wall/beam.  The opening is located below the compression area and above the steel in the bottom of the wall.  The question now is what kind of forces are going to be around this opening?  The top remaining concrete header will be sufficient for the gravity loads above the ventilation shaft, but our biggest concern is the force concentrations around the new opening.
civilperson (Structural)
3 Sep 08 14:43
By definition, the bending force at the neutral axis is zero.  Is it really the NA or is it the centroid of the concrete area? (Not the same).  Line the edge of the hole with steel plate to achieve shear transfer and radius the corners by coring to reduce stress concentrators.
jeffhed (Structural)
3 Sep 08 15:22
civilperson,
The reinforcement in the wall should be symmetrical about the opening, so at this point I am assuming that the opening is located in the NA.  The calculated depth of the compression block is well above the top of the opening, so we are somewhere close.  If I understand you correctly, line the sides of the opening with double angle, bent plate, or flat plate and bolt to the sides of the opening to acheive what ever the shear force is at that point.  Top and bottom will only be alinged to proved for attachment of the steel grate?
RVSWA (Structural)
11 Sep 08 1:56
This is a question within your question.  Until recently I would have approached the calculations the same way and I even agree with the proposed retrofit.  I have not yet chased the Strut and Tie model out completely so I ask, does this proposed region fall within the compression area of a strut?  Could someone comment?  
civilperson (Structural)
11 Sep 08 15:44
Yes, a compression strut could be modeled in the hole location.  However, there are many, (approaching infinite), strut and tie models which can be used for any disturbed region of reinforced concrete.  Many different models with no compressive strut through the proposed hole location is also possible and viable for analysis and design.  The theory says that each of the models represents a stress condition greater than the actual.

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