AS3600 8.2.12.4 & Stirrup Cogs 1

[OzEng80]

AS3600 – 8.2.12.4 requires stirrup spacing to be reduced by 20% when the stirrup hook/cog is located in the tensile zone of a beam. Why is it then that all standard beam details and the basic industry standard is to have cogs at the top of beams? Obviously critical shear coincides with negative moment and lends itself to having cogs at the bottom of beams. I have done this since I first saw this clause but the prevalence of cogs in tops of beams has convinced me that I better check that I am not missing something…?

Any comments?

Thanks!

 

[hokie66]

OzEng80,

You are not missing anything, but I suspect that lots of people have never paid attention to this clause.

 

[rapt]

Ozeng80,

The problem is that on site, cogs are generally placed at the top as an industry standard. To avoid problems most people have left them at the top after this rule was introduced. Saves hassles on site.
As an aside, the person who had this added to the code was of the impression that it would not have much effect on the amount of ties used as designers would simply design and detail -ve moment areas with the cogs at the bottom and +ve moment areas with the cogs at the top!!! Got a few good laughs anyway (but he was serious).
Anything other than cogs at the top (what steel fixers are used to) will cause problems on site when it is done wrongly and you have to tell them to pull the whole thing apart and start again (if it is ever checked properly).

Many people have been doing the calculations correctly as RAPT has been allowing for it since the clause came into the code (user controlled but default at the top)!

 

[hokie66]

Rapt,

You would know: is there a similar provision in the ACI code?

 

[OzEng80]

Thanks for your responses. I currently detail the cogs on the bottom and put notes such as 'cog as shown'. I have even taken to underlining anything that I anticipate them doing wrong! Makes for brilliant ammunition on site!

RAPT – The 20% reduction seems pretty severe, did the learned gentleman who made the change have quantifiable evidence to warrant it? After all there is no penalty for developing other steel in tensile zones (AS3600 ch 13)….

Thanks

 

[asixth]

I have seen a bridge crosshead in our office be detailed with cogs on the bottom, being in the tensile zone at low shear regions. These cages are built-up outside the form before being craned in.

 

[asixth]

Sorry, I don't have access to my code or commentary at the moment but has there been testing to show that a reduction in strength occurs when ties are located in the tension zone?

 

[csd72]

This clause has only been introduced a few revisions ago.

 

[rapt]

Sorry, been away on a weeks holiday and have not been monitoring this site.

Answering several of the above,

No other code that I know of has a similar provision.

Standard prictice on building sites is cogs at the top. You would have to do very detailed site supervision if you were going to define bottom, and expect some arguements with some very large steel fixers when you tell them to pull their beams apart and start again! The +ve moment zone in a beam is about 70% oif the span length and cogs at the top suit this area, so I would still put them at the top for this reason. If you cannot fit your shear reinforceemnt in at the supports because of the 20% reduction in that area then I suppose you could switch them around but I would tend to make my beam a bit bigger so the problem goes away and still put them at the top.
And Yes it does seem severe. I am not sure where the 20% came from. I understand there was testing to justify it. I am checking to see if I can find out more about the testing. I will report back if I can find out anything.

Yes, for special structures like pier cross-heads which normally have very good supervision switching the cogs would be logical.

General development length theory says that if there is a confining force, then the developmennt length reduces. I think this is included as an option in the latest draft (still not approved for release and we still do not know when it will be!!! Maybe KRud will force it to come out this year to help the printing and training indutries and to give consultasnts something to do!!).

I think it came into the code in 2001!