Precast Panels and Horizontal Reinforcement for Crack Control

[TVY]

Hi All,

Long time reader, first time posting.

I've noticed some consultants (in Australia) design precast panels for low reinforcement ratios, mainly complying with the ratios provided in AS3600 CL 11.7.1 (0.0015 for vertical, 0.0025 for horizontal).

Assuming you can satisfy all design actions for the above ratios, what is the justification to not comply with Cl 11.7.2 for the horizontal reinforcement? If you are in a B1 exposure, this brings you up to 0.006 for horizontal reinforcement.

Consider double height perimeter panels of say 8m high x 3m wide in a multistorey building

The panel will be restrained at the top and bottom by the dowels, at any point slabs tie into the panels and at stitch plate locations, if these are used.

The only justifications I can think of that support not complying with Cl 11.7.2 is the panels being a narrow width means there is not a significant shrinkage / thermal stress is built up. Also, that before the dowels are grouted and the slabs tied in, the panels are unrestrained while initial shrinkage stress is occurring.

If you were to adopt the approach of neglecting Cl 11.7.2, at a minimum i'd expect you need to provide additional horizontal reinforcement at these points of restraint.

The only reference i've seen that somewhat discusses this is in the AS3600 commentary C9.5.3.3, "For precast, prestressed flooring systems, where the width of individual precast elements is less than 2.5m, the requirements for reinforcement in the secondary direction may be waived."

Does anyone know a reference that specifically supports or refutes ignoring Cl 11.7.2 for precast wall panels?

 

[Tomfh]

Precast panels are effectively free to shrink horizontally in most cases, as joints spaced every few metres—around 3m in your case—permit movement. The dowels and panel connections provide only minor restraint anyway, so if tensile stresses do build up (e.g. if the panels are all welded together), the connections typically fail locally before the panels themselves develop full shrinkage cracks.