AS3600 Mesh in Suspended Slabs

[handex]

A builder just mentioned to us that he has heard people saying that mesh is not allowed as the primary reinforcement in suspended slabs in the new AS3600 code. I have only briefly looked through the code (mainly the new fire requirements), so can anyone help point out whether or not this is true?

 

[TXStructural]

The link below is a PDF covering elevated slabs from the 2001 code:

http://www.sria.com.au/services/pubs/TN6.pdf

And the link is located on this page, which has a few others on the topic:

http://www.sria.com.au/services/pubs/main.html

I do not know about the latest edition (2009), but I would imagine that SRIA would have addressed such a change.

 

[IDS]

The new code has a requirement that if Class N reinforcement is replaced with Class L then the structure must be redesigned.

It's not just a matter of providing the equivalent area because there are many restrictions on the use of Class L in suspended slabs, and the strength reduction factors are lower, but it is not prohibited.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rscassar]

L-class mesh (or low ductility reinforcement) gets an additional 0.8 factor applied during strength calculations. When you perform calculations on the ultimate strain in the reinforcement at failure for cross-sections reinforced with L-class mesh you will find the strain gets close to the rupture strain for the reinforcement. So the failure mechanism then transfers from a ductile failure back to a brittle failure.

There is no direct substitution of low-ductility reinforcement for normal grade reinforcement.

 

[rapt]

General restrictions on its use are listed below! There is a further restriction on its capacity as mnetioned by others of .8. This is now in the capacity reduction factor which is reduced to .64 from .8 if Class L steel is used (Tables 2.2.2 and 2.2.3).

1.1.2 Application
(c) Reinforcing steel of Ductility Class L in accordance with AS/NZS 4671—
(i) may be used as main or secondary reinforcement in the form of welded wire mesh, or as wire, bar and mesh in fitments; but
(ii) shall not be used in any situation where the reinforcement is required to undergo large plastic deformation under strength limit state conditions.

NOTE: The use of Ductility Class L reinforcement is further limited by other clauses within the Standard.

17.2.1.1
Ductility Class L reinforcement shall not be substituted for Ductility Class N reinforcement unless the structure is redesigned.

 

[rapt]

handex

The other problem from a design/construction point of view is the depth to the design steel. When the mesh is layed on site, it is never done as your would require it to ptovide the most efficient lapping. You tend to end up with 4 sheets of mesh lapping in the worst way, resulting in a complete mess.

And you layers tend not to be what you asked for, so you need to assume the worst in both directions!