Diagonally reinforced Concrete Coupling Beams in Australia 3

[Gishin1]

Hi there

Has anyone ever specified, seen, or heard of projects utilising diagonally reinforced concrete coupling beams in Australia? I know there is no recommendation or advice for it in AS3600, and my own experience (and from what I've seen others doing) is design & detailing of coupling beams as standard beams; longitudinal bars and closed ties.

I'm wondering if we will see more of a necessity for this design approach due to the shear over-strength now explicitly required in AS3600-2018. From what I've seen, a large portion of RC buildings in Australia have either a)not been designed for seismic at all, or b) no consideration of shear over-strength, therefore shear loads not being too large to deal with.

Given that the latest 2018 Seismic code increased SEQ loads by 60% (0.05 to 0.08 hazard factor), and AS3600 require shear be increased by x2 - x2.6, I wonder if conventionally detailed coupling beams are the right approach.

I have seen papers saying things along the lines of 'deep lintels to be designed using strut & tie', but no details are ever given, and I would think that the strut and tie approach would result in the diagonal reinforcement.

From what i've seen and heard, I doubt the industry would accept diagonal reinforcement, deeming it too difficult, time consuming, and costly to implement.

 

[rscassar]

No, diagonally reinforced concrete coupling beams in Australia is not common. NZ probably and I think they may have even invented it. ACI has special requirements for it given the span to depth ratio and average shear stress across the section.

It's funny that we are considering shear design to be increased by 2.6 assuming most buildings previous where designed with mu=2.0. It caught me out when it was introduced into the Australian concrete code, but the more I start reading thru ACI it seems that the amplification from the overstrength factor should have always been applied to shear forces induced by earthquake.

Diagonally reinforced coupling beams does look difficult. Whether future revisions of AS3600 has similar requirements to DRCB as ACI I don't know.

 

[rowingengineer]

Yes have seen them on a few projects, I think a number of engineers started to adopt this type of detailing in my local area after Gilbert did a strut tie model of the situation early 2010 to 2012?. However haven't been involved with a project in that area since 2016 or even a project that would need them since 2015 so don't have great knowledge of current practice.

The local area was north Queensland encase you were wondering.

 

[Gishin1]

rscassar, yes it completely makes sense to amplify the shear forces. Unfortunately, I (like most others) were taught the static EQ loads calc from AS1170.4 with no thought to what Mu and Sp were actually doing. I know its not common, if done at all, but I was wondering if any engineers have started to consider it as a necessity given the high shear forces.

rowingengineer, do you have a link or any more information on the strut-tie model by Gilbert?

 

[Agent666]

You're probably aiming for a ductility capacity of greater than 3 with a diagonally reinforced coupling beam, in that case doesn't AS3600 direct you to NZ concrete code for the design where there are comprehensive rules for the design and detailing (including overstrength and factoring in the appropriate amount of slab reinforcement for 5he overstrength check). Hell even if you were aiming for or require only mu=2.0, just use the NZ provisions.

https://engineervsheep.com

 

[rscassar]

Yeah, I got to have a good read thru the NZ code. This is it here right, standards NZ just gives it away for free download

https://www.standards.govt.nz/shop/nzs-3101-1-and-22006/

?

 

[Agent666]

Yeah, that's the one. Free to ensure engineers are using the latest versions at all times (a criticism of the Royal Commission into 2011 quakes).

https://engineervsheep.com

 

[IDS]

It's "interesting" that all NZ standards seem to be free, except for the joint AS/NZ ones.


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

Yes, Australian standards are not free. Is NZ the only country that provides free standards?

 

[IDS]

There are many standards available for no cost. See:

https://archive.org/details/publicsafetycode?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[GTD_18]

To be honest - when I read clause 14.6.6 I assumed that the amplification was to be applied to the section of pier/wall only not the coupling beams (wrongly - now I think on it).
looks like most of the “premier” software packages only apply it to pier sections too - interesting...
an updated commentary would be useful - when is expected?

 

[Agent666]

You amplify based on whatever your ductile yielding mechanism is. If its purely cantilevered wall, then amplify the moment capacity of the wall to the overstrength capacity, use this ratio inclusive of any over design.

For example if your elastic analysis shows wall moment is 1000kNm and shear 300kN, but your final design results in a nominal design capacity of 1200kNm, then you're overdesigned by a factor of 1.2. Analysis of the wall overstrength capacity might show your overstrength capacity is 2000kNm. Therefore you'd scale up your analysis design shear by 1.2*(2000/1200) = 2.0.

Not sure how AS code works exactly for capacity design, but in NZ for any overstrength check a phi = 1.0 can be utilised as you're not interested in a design safety factor as you're already assuming every element in the system is at it's 95% characteristic capacity. So the overstrength demand is an upper bound, whereas the design capacity is a lower bound (based on 5% characteristic capacities).

Capacity design is all about ensuring your mechanism can perform in a dependable manner, i.e. flexural hinge forms, and shear capacity is sufficient to support this without a shear failure occurring.

For the case of a diagonally reinforced wall/coupling beam system, the diagonal reinforcement is usually the yielding element, potentially with a flexural hinge at the base of the piers depending on the relative stiffnesses/strengths, so the rest of the system outside of the diagonal reinforcement design needs to be designed to support this mechanism forming (i.e. checked for the overstrength capacity of the diagonal reinforcement occurring at every level). So in effect you're ensuring the piers can support the higher shear and flexural demands in the joint regions and beyond so the seismic energy can be dissipated in a predictable manner via the yielding of the diagonal reinforcement in tension and compression. You're not factoring up the diagonal reinforcement and designing it for a higher demand, thats where the demand is coming from for the rest of the system based on the capacity you've provided. Usually you're trying to match the design capacity as closely as possible to the demand.

https://engineervsheep.com