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Tilt Up Wall Stress Checks?

Tilt Up Wall Stress Checks?

Tilt Up Wall Stress Checks?

(OP)
Hey folks,

I rarely get involved in means-and-methods in my job, but I am part of a group that has to review some specifics related to precast concrete panels.  The panels will be cast flat and then raised from the forms by attaching anchors to one edge and lifting from the forms causing bending along the short axis of the panel.  However, the panels are comparatively thin at 6.5 inches and 4 inches.  What is the flexural stress limit for lifting panels?  We need to make sure the minimum concrete strength has been obtained before the contractor is permitted to strip the forms.  According to my ancient code book, the value appears to be 0.21 x 7.5 x sqrt (f 'c).

RE: Tilt Up Wall Stress Checks?

5 sqrt(f'c) and maybe a factor of 1.2-1.5 for stripping, suction issues. Make sure your lifting insert has the shear capacity for the lift, perpendicular to the face.  

RE: Tilt Up Wall Stress Checks?

I think I used to use around 2 or 3*sqrt(f'c) since the concrete is pretty fresh. The idea is to keep the panel from cracking during the lift so there are no cracks for the owner to see upon final erection.   

RE: Tilt Up Wall Stress Checks?

I think that 2-3 is conservative, but if you can keep the stresses in check with this, you will ensure a crack free(from lifting) panel

RE: Tilt Up Wall Stress Checks?

If you have any openings, thin legs, etc. controlling the theoretical stress may not be enough.  The corners act as stress risers.  Some panels which may work in the finished structure can't be lifted in the way you describe, but rather have to have face anchors or be cast on a tilting deck.

RE: Tilt Up Wall Stress Checks?

Precast and tiltup use 5000 psi minimum prior to stripping and handling. Easily achieved in 12 hours with modest heat and 6 sack mix.

RE: Tilt Up Wall Stress Checks?

I think I have always seen 4000 psi used on tilt-up.

RE: Tilt Up Wall Stress Checks?

(OP)
Thanks guys.  We specify 5,000psi minimum strength for precast panels.  The engineer wanted to use 5.0 x sqrt(f 'c).  It seems he has some degree of support for this value.

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