Column laps and ties

[blihpandgeorge]

Does anyone know of a requirement for tie(fitment) spacings in lap zones in columns? AS3600 discusses tie spacings generally, and also when using high strength concrete and if columns are stock (Lu<5D), but i cant find anything on lap zones.

 

[rscassar]

I believe it may be in section 13, and says minimum 3 ties sets at laps. Also additional tie sets were bar cranks occur. It's not current to AS3600-2018 but the CIA handbook Z6 is a good reference.

Does your question come from whether ties at half centres are required over lap regions.

 

[blihpandgeorge]

yes, the question is whether ties at half centres are required over lap regions

i was working up some new details to address the requirements of 10.7.4.3 (ii) (b) for Lu<5D and when i redrew the detail i realised there didnt appear a 'need' for ties at half centres

 

[rscassar]

yes, i noticed that as well. I worked for a few companies where laps at half centres was the adopted procedure. I remember seeing it written somewhere a long time ago, may have been the 2001 or 2009 code or possibly even the ACI code. But now I adopt the Z6 detailing of minimum 3 and 2 additional at bar cranks.

 

[rscassar]

do u agree with the Lu/5*D ties requirements. This was previously a detail for IMRF requirement (mu=3) but has made it's way into the code for all columns, irrespective of whether mu=1 has been adopted

 

[blihpandgeorge]

i can see the logic that short/stocky columns with L<5D will attract load and need additional ductility to deal with the load they will attract with their stiffness. i can only assume this is the code writers response to seismic design in Aus is poor, and detailing is done worse. i would rather this approach of a blanket rule, rather than having to do a more detailed analysis to demonstrate the amount of confinment reo needed to give a ductile connection.

i have raised this L<5D requirement a few times, including with some design software companies, and it is not well known about.

 

[rscassar]

Yeah, I used RCC and it won't use this provision unless IMRF is checked. I don't mind the rule, but would rather the provision be:

if ( Lu < 5*D.x) then max_spacing = 0.5*D.x
if ( Lu < 5*D.y) then max_spacing = 0.5*D.y

And not:

if ( Lu < 5*D.x) then max_spacing = 0.5*D.y

 

[blihpandgeorge]

i see where you are coming from but for a typical blade columns of say 1000x300, this would give ties at 500cts which wont do much for confinment. given the smallest realistic typical 0column width is around the 200-250 dimension this sets ties at 100-125 where confinement would be good

i was under the impressions RCC was updating their software to pick this up without needing to tick the IMRF box - that was my understanding over 12 months ago from some correspondence i saw but i havent actually checked it

 

[rscassar]

Yeah, but the 1000x300 column we are set to the maximum spacing of 300 centres. My understanding is that this is more a column shear requirement than a core confinement requirement (I could be wrong). Will really be anticipating the commentary for this. I know this wasn't part of your original question but I didn't get any responses to my thread so thanks for the discussion.