Ties in columns with L<5D 1

[blihpandgeorge]

Hi

Looking at CL 10.7.4.3 (b) (iii) of AS3600-2018 that is new - it appears to be adding confinement reo for 2D for very stock columns, presumeably to make sure they act in a ductile manner if overloaded?

In practice, a column in resi building would be say 250x800 with a 3m storey height, so using the 800 value for D this would lead to very close ties (as per an IMRF) for a very typical column.

Is this how others interpret this?

Thanks

 

[sdz]

CL 10.7.4.3 (b) (iii) where Lu ≤ 5D the spacing of the fitments shall not exceed the requirements of Section 14.5.
Since it doesn't say Dc then D=800.
From 10.7.4.3 (b) (ii) the spacing should be 250mm


14.5.4 Columns
At each end of the clear height of a column within a storey, the longitudinal reinforcement shall be restrained by closed ties for a distance from the end equal to the greater of the
maximum dimension of the column cross-section, or one sixth of the least clear height between consecutive flexural members framing into it.
The spacing of the closed fitments shall not be greater than required by Clauses 10.7.3 and 10.7.4 and the shear requirements of Clause 14.5.2.2(b), (c) and (d) and the smallest of the
following limits:
(a) 8 times the diameter of the smallest longitudinal bar enclosed.
(b) 24 times the diameter of the fitment bar.
(c) One-half of the smallest cross-sectional dimension of the column.
(d) 300 mm.

So it would seem that fitments are required at 125 c/c over a length of Dmax=800 (or 1/6 the clear height) even if it is not in an intermediate moment resisting frame. You would need an extra 3 fitments at each end.

That's how I interpret it but it doesn't seem particularly onerous.

 

[hokie66]

I'm just guessing, but that sounds like an attempt to control cracking in short columns which occurs due to thermal and drying shrinkage volume change of the supported slabs. All too often, stubby columns suffer because they try to resist these lateral loads without success. One area where this has been a problem is short column beside ramps in parking decks.

 

[blihpandgeorge]

hi sdz

i agree with your interpretation of 125 cts, however i want to explore your length of Dmax or 1/6 of height. You note reference to Cl 14.5.2.2(d), and this is the one that specifies that ties are to be placed over 2D from a support.

Thus if D = 800, 2D from support = 1600mm from floor and 1600mm from ceiling, and thus ties at 125cts for full height over a 3m storey. It this part of 2D that i find onerous when compared to previous version. I'm not saying it is flawed, only want to see if i am interpreting correctly.

Thanks

 

[Agent666]

I believe the 2D is a rather simplified application of capacity design and an attempt at protecting the potential plastic hinge regions at each end of the member.

What it is saying is if it hinges/yields (which will occur at the member ends in a column with no significant lateral concentrated loads over its length), is that you need additional confinement in these hinge regions to ensure it can deform/rotate in these regions in a dependable manner whilst still being able to support the gravity load. Basically, holding the core of concrete together with sufficient confinement that it can still carry the gravity load even if the cover concrete spalls. Better confinement = better ductility.

To compare to the NZ code, for a column hinge you need to detail closer ties. This spacing is even more onerous than the AS3600 spacing, in the most onerous cases this is minimum of 6 x minimum longitudinal bar diameter or 1/4 of the least cross-sectional dimension. This applies over a region of anywhere up to 3 x the depth of the member in the direction of loading. How far this distance is at each end is related to the level of axial load, but as an absolute minimum for lightly loaded columns it is 1 x the member depth at each end. Outside these regions over the middle portion of the column the spacing is relaxed to 1/3 of the least cross-sectional dimension or 10 x minimum longitudinal bar diameter.

Even our normal gravity provisions are more onerous than your seismic provisions.

NZ codes have had all this stuff in one form or another since the early 1970's, yes nearly 50 years ago now.... it's nice that you guys are finally joining the party in a limited capacity

 

[Settingsun]

Still at single digit death toll from EQ concrete failures. It's just intuitive to us so we don't need code rules.

 

[Trenno]

Or rather we've been lucky to not have had a moderately intense earthquake in a major city?

EDIT:
Out of pure procrastination, I decided to fact check Steve. 9 people died from the Reinforced Concrete Newcastle Workers Club collapse in 1989. The other 3 were due to an awning tearing away/down a masonry facade onto the pedestrian walkway below. The last death was due to shock. The only other deaths due to earthquakes in Australia was in Warooka, SA in 1902 when two people suffered heart attacks.

 

[rapt]

Everyone thinks Australia is not seismically active. Based on the last 200 years of recorded history. But seismic activity has a history of low and high activity. Just because it has been low does not mean it will stay that way. Return periods of larger quakes are normally measured in 100's (for 5-7's) to 1000's of years. So 200 years of history does not tell us much about the future possibilities.

There have been 13 earthquakes of magnitude 6 or higher in Australia over the last 150 years, when the country has been very seismically inactive historically. Fortunately none of them were in major population centers.

Newcastle was only a 5.6 and it killed 13, on a day when there was a major transport strike and on Dec 28th during the normal Christmas 2 week shut down so many businesses were closed and there there were far fewer people in the city than normal.