Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Cut a hole in an existing grade beam/wall

Cut a hole in an existing grade beam/wall

Cut a hole in an existing grade beam/wall

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.

RE: Cut a hole in an existing grade beam/wall

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.

RE: Cut a hole in an existing grade beam/wall

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?

RE: Cut a hole in an existing grade beam/wall

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?  

RE: Cut a hole in an existing grade beam/wall

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close