Draft AS/NZS4671 proposing rules for up to 750MPa reinforcement

[Agent666]

Anyone else think this is a recipe for disaster waiting to happen, on site practices with respect to grade 500 are pretty poor (at least here in NZ), limited respect for bending/rebending requirements, contractors inadvertently welding it, or welding without proper welding procedures in place, incorrect bending/fabrication causing bars to fracture during handling, differences between QT and MA bars, etc.

It seems like throwing a higher grade out there is feeding fuel on the fire, one of their justifications seems to be it will lead to more economic design, less use of materials which I guess is valid if governed by ULS. Seems like the divide between ULS and SLS design might need a lot more consideration, stuff that used to work at SLS by virtue of having enough reinforcement area might not work with 33% less bar area.

My reading is that in NZ at least we have rules for stirrups providing confinement allowed to be up to 800MPa, but everything else is limited to 500MPa being used (though obviously no product on the market to meet the rules at present). Often confinement reinforcement is also shear reinforcement, so whats the point.

Seems like design standards will need further rules for higher grades, given the difficulties in updating our standards this doesn't seem like its coming anytime soon.

Does AS3600 currently allow use of >500MPa?

Thoughts?

 

[hokie66]

Count me as one who thinks it is ridiculous and stupid. I thought going from 400 to 500 was too much of a stretch (pun intended). As you said, area of reinforcement matters for all things related to serviceability.

 

[rapt]

Agent666

AS3600-2018 now allows up to 800MPa but limits it to 600MPa for ultimate strength calculations. 800Mpa is allowed for confinement.

You are correct that in flexure, crack control and deflections will be adversely affected so the higher strength steel will not be as useful. The assumption is that initially at least it will be used in congested areas such as columns and for confinement.

Unfortunately sometimes things are accepted in design codes against the advice of others! Apparently some of us are very old-fashioned and not sufficiently open to change as we came from the age of the dinosaurs.

 

[Agent666]

Regarding columns, in NZ at least to get the bar bond to work through joints if there is a plastic hinging region adjacent to a column face (bond related to bar diameter and yield stress and curvature of the beam) you need large columns and beam depths for 500 grade bars than for grade 300, along with that comes larger capacities which sort of negates the use of higher grades again. For example for a 25 diameter grade 750 bar (extending the grade 500 rules) your internal columns would need to be 1000mm deep to prevent the bars loosing bond and slipping through the joint.

On confinement, the only area where I can see places where confinement steel doesn't double up as shear reinforcement is perhaps the intermediate ties in a column going across the minor direction, or ties across a walls thickness. The confinement requirement isn't that high in these scenarios, usually governed by minimum spacing to prevent bar buckling which means often a grade 300 10mm or 12mm cross tie bar will work easy.

Maybe I'm missing the logic here for its practical uses or the worlds turned upside down on me and no one informed me

 

[Agent666]

Not to mention lap lengths/bar development getting 50% longer.... adding more congestion?

 

[Settingsun]

Is 800 MPa for confinement usable? I'd have thought the strain to achieve that stress would mean not really confined. Do the strength gain and ductility survive the cracking?

 

[rapt]

Agent666,
Reread the last sentence of my last post. We are both hanging by our toes!

steveh49
Apparently tests show it works.

 

[rapt]

Agent666

RE your shear vs confinement comment, there is no reason why 800MPa reinforcement cannot be used and confinement calculations would use the full 800MPa but shear calculations would be limited to 600MPa.

Whether or not using 800MPa reinforcement is worthwhile in that case would depend on which controls.

RE development and congestion, yes development would be 8/5's longer so the congested length of a column would be that much more, but congestion itself would be less with 5/8 the number of or area of bars over the longer length.

The steel is available as N grade. But presumably not E grade which would be required in NZ.

 

[IDS]

ACI have a recent publication on this topic:
439.6R-19 Guide for the Use of ASTM A1035/A1035M Type CS Grade 100 (690) Steel Bars for Structural Concrete

https://www.concrete.org/publicatio...llectionsearch.aspx?m=abstract&MCP_DocID=1128

I have only scanned the document quickly, but it does refer to tests on the use of high strength steel for confinement:

Numerous studies (Muguruma and Watanabe 1990;
Muguruma et al. 1991; Sugano et al. 1990; Budek et al.
2002; Stephan et al. 2003) conducted on the use of high-strength
reinforcement (fyt up to and greater than 120,000 psi
[827 MPa]) show that there is no detriment to using high-strength
transverse reinforcement as column confinement.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/