Shear failure of precast concrete beam during lifting 1

[pizzalover]

Hi there,

Could you please help me with the calculation of punching shear that occurred during lifting?
The precast beam (0.6m x 0.6m x 1.5m long) has two 50mm cast-in conduits for lifting and Nylon sling will be used to lift the beam. My thinking is the failure mode would be shear wedge above the conduit location.
I am still a junior engineer and a bit struggling with this.
FYI, i use Australian standard. Thanks so much.

 

[KootK]

What is the nature of the "conduit"? Are we talking a stiff steel pipe or a flexy plastic tube? If it's the latter, then your lifting stresses will be concentrated at the sides of the beams which may complicate things a bit. If it's the former, then unreinforced, two crack, one way shear as you've sketched it is probably a good place to start.

I very much dislike trying to sort out connections like this from first principles. Testing almost always seems to indicate that such designs wind up being unconservative. I prefer to use connection hard ware with rated/tested capacities whenever possible.

You might conservatively assume that all of the weight of the beam is supported at one side of the conduit and then design that area as plain concrete punching shear on a three sided breakout frustum.

 

[SWComposites]

Has a beam already failed? If so, post photos. If not, do a very conservative analysis.

 

[bugbus]

If it were me I would ignore the punching shear capacity and just add some supplementary reinforcement to anchor the potential wedge to the rest of the beam. Same principle is used in AS 5216 for post-installed anchors.

Conservatively I would consider that due to friction, the slings might support the beam on diagonally opposite corners, so that you would potentially have one half of the weight of the beam supported at the edge of one conduit. Running some quick numbers, a single N12 bar at each face would be plenty in this case. But I would probably put a third in the middle just for peace of mind.

 

[Tomfh]

Why not use the published capacities of cast in anchors?

 

[KootK]

If it were me I would ignore the punching shear capacity and just add some supplementary reinforcement to anchor the potential wedge to the rest of the beam.

The rebar is a great idea for safety. OP should definitely do that. At the same time, I would be inclined to still do the punching shear check. In my precast work, it is usually the case that our clients expect the members to be installed with no significant cracking present. Cracking at install begets piece rejection. Since the rebar bits mentioned would do nothing until after the anchorage frustums crack themselves into existence, such cracking would represent a serviceability failure for the piece in my world.

 

[bugbus]

KootK, good point, I agree with that 100%