AS3600 Compression Laps

[OzEng80]

AS3600 states the 20db is required to develop a bar in compression. It is also states that to achieve a lapped splice in compression (0.125fsy-22)db = 40db (for fsy=500) (32db if you have ligs) is required.

What is the basis of this difference? The commentary gives some history but not reasoning.

Also what are the scenarios where a bar is required to be developed in compression?

Do column starters bars need to be developed in compression and the vertical bars lapped accordingly (32db)? Or is this only required if fixity is assumed in the connections?

Thanks!

 

[asixth]

It seems that this post has turned from the interpretation of compression splice lengths to tension splice lengths.

Regardless, there seems to be a continual back and forth occuring between members of the code committee relating to section 13 of AS3600.

I think alot of the debate is coming from the application of the k2 factor, where slabs should always have a value of 2.4 applied, not 1.7 because the clear spacing between bars should never exceed 150mm if the maximum allowed spacing of reinforcement in slabs is 300mm.

The one item that all parties are able to agree upon however is the revision of the lower limit setout from 25k1db to 29k1db.

 

[OzEng80]

Asixth could you expand on your comment about k2? I interpret the clear distance between parallel bars developing stress as being the bar spacing – bar diameter – splice bar diameter. So for N12-300; 2a = 300-12-12 = 276mm > 150mm.

Thanks.

 

[rapt]

OzEng80,

The alternate interpretation to this is that 2 bars lapping over a length are both developing stress and their spacing is = bar dia. Therefore k2 = 2.4 for all lapping slab bars as spacing < 150. Even if the bars are a non contact lap at
300 centres, the clear spacing of the lapped bars is still < 150.

According to two current code committee members who were involved in the original development of those rules back in 1984, this is the way they meant it to be interpreted. Apparently everyone has done it incorrectly since then based on the C&CA manual by Ferguson and walsh's book.

 

[apsix]

Well, I've certainly learnt something.
The method of presentation doesn't help.
If it always applies it should be written into the equation as such or otherwise clarified.

 

[OzEng80]

I didn’t think the splicing bars counted…. Developing stress vs transferring stress?

Figure 1(b) in the ASEC paper shows and defines Sc as being centre to centre spacing of bars developing stress, less two times the bar diameter. It does (in)conveniently have staggered splices which confuses this.

So for N12-300; 2a = 300/2 – 12 = 138mm?

What would happen if you spliced an N16 (at 300crs) with 2N12? Is 2a = 300/3 -12 = 78mm?

Thanks again!

 

[rapt]

OzEng80,

That is the discussion that is going on at present. According to the people who helped develop the old rules, it was supposed to count. A bar in a lap is developing stress.
The people who were not involved in developing the old rules are adopting your logic, but then, they were not involved so how would they know the intent of the code.

I agree that the current wording is bad. That is being sorted out in the next release of the code.