AS1170.4 Draft 2

[Trenno]

New AS1170.4 draft is out for public comment, closing 18/01/24.

Seems like reo rates may be on the rise again with the introduction of these two proposed admendments.

 

[Retrograde]

Where can you download the draft from?

 

[Trenno]

Register to Standards Australia and then search the public comment drafts for 1170.4.

This link may or may not work.

 

[Euler07]

Does this mean that a 51m tall building can be designed lighter than a 49m tall building?

 

[rscassar]

Thanks Trenno, I'll download and give it a read. It's good to see that the scaling up to 70% is being re-introduced

thread744-501807

 

[human909]

Can anybody elaborate for me why exactly the minimum value of 70% is present? Is it just to stop engineers going crazy with their dynamic analysis and getting things wrong? Or is there another more scientific approach?

I ask because calculating the fundamental period T1 based on height alone seems to me to be an extremely crude method. It completely ignores the structure's width/depth which can be just as important as height. (Though it naturally depends on the type and design of the structure.)

This provision has always bothered me. Because it is common for me to have two structures of identical heights next to each other with vastly different stiffness's and fundamental period. And yet the simplistic T1 height approach means they calculate out to having the same fundamental period.

 

[human909]

Also it seems like they have attempted to clarify the contour maps of Hazard Design Factors (Z) but instead actually made is even LESS accurate.

I have seen it commented upon that because 0.08 never actually appears on the maps that some engineers were thus using 0.09. They seem to have attempted to fix this issue by using a single 0.08 point on each map. However the result of this suggest that interpolation from the contour to that point. Which is both a bit perverse and also changes depending on which map you are using.

Why they don't have the area where the minimum 0.08 is suitable as a shaded or hashed region is beyond me. It would be far more clear than using contour maps incorrectly.

(My understanding has been that 0.08 was introduced as a minimum value, not that the actual assessed seismic values for regions shifted monumentally 2018. Yet they seem to have struggled in the previous revision, and this revision, to produce a contour map that represents this in a clear and accurate manner.)

 

[Trenno]

Can anybody elaborate for me why exactly the minimum value of 70% is present? Is it just to stop engineers going crazy with their dynamic analysis and getting things wrong?

That's exactly my understanding. It's to stop non-representative favourable stiffness assumptions (eg cracking walls beyond oblivion and unrealistic Soil-Structure interaction).

And yet the simplistic T1 height approach means they calculate out to having the same fundamental period

On the other hand, it's often the case when we model squat buildings in ETABS it'll tell us that the structure is much stiffer than the what the code T1 formula predicts.

 

[rscassar]

it's a difficult website to navigate and the make suggestions. Has anyone made comment on the pounding / building separation section?

 

[rscassar]

I'm going to make a comments and suggest building separation of adjacent buildings to be greater than the sum of the maximum displacement of both adjacent buildings. Separation of two adjacent floor diaphragms set apart by a building movement joint where shear transfer across the joint is required to be separated by the SRSS of the maximum displacement of both adjacent diaphragms at the building joint location.

 

[Just Some Nerd]

I'm going to make a comments and suggest building separation of adjacent buildings to be greater than the sum of the maximum displacement of both adjacent buildings.

I imagine this getting through would instantly cause a shitstorm, getting that information for a previously constructed adjacent building is going to be nigh impossible a lot of the time. I imagine it would be more practical to update building/planning codes to enforce a setback of all structures X distance from a boundary or between buildings.

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Why yes, I do in fact have no idea what I'm talking about

 

[rscassar]

The boundary setbacks are already in place for new build. Building renovations get the benifit of remaing on the property boundary. It comes up in Sydney a bit, developers choose to refurbish over rebuild to get that extra sellable floor area. My gripe is the separation at building movement joints.

 

[Agent666]

What rscassar notes is pretty typical requirement the world over. More of a shit storm would your building collapsing and it being attributed to pounding of an adjacent buildings floor against your columns.

I always find it amusing how the Australians resist change because you just haven't had your watershed moment in terms of a significant widespread seismic event. It'll come one day.

https://engineervsheep.com

 

[Trenno]

To me it just seems engineers should be able to determine the mandated buiding base shears from something more than an equation introduced 15 years ago and with the only variables being height and lateral system.

Something, Something 'significant figures'...

 

[SRL V1.0]

What are you thinking Trenno?
An updated imperial formula? Or a 'minimum FEA modelling standard' type approach?
I feel like until the AS better contemplates more complex building behaviour (torsion modes etc.) the 'minimum base shear % of static' is the codes best defence against engineers trying to work around these clauses.

 

[Retrograde]

I always find it amusing how the Australians resist change because you just haven't had your watershed moment in terms of a significant widespread seismic event. It'll come one day.

I do not think that Australian engineers resist change any more (or less) than engineers in other parts of the world.

What we would like is more clarity from those drafting the Codes.

 

[BJI]

I don't believe it is just a matter of structural robustness, rather linked to the maximum permitted stiffness discontinuity between levels.

Unlike a lot of the other commentaries, the AS1170.4 commentary was updated in 2021 and can be downloaded for free at the following link:

https://aees.org.au/product/as-1170-4-commentary-2021/

 

[Just Some Nerd]

I should clarify my previous post, apologies for any confusion :/ I'll preface this by making it clear I have no objections to the separation, but I have a lack of faith in the industry to not oppose anything that might lose a few inches of space, namely in basements. I'm probably overestimating the pushback but I'm quite cynical about the industry in Aus / Sydney in particular. Should also note I mostly work on mid to high rise, typically residential or mixed use so I see the same few things over and over.

There are a number of buildings I've done inspections for in the past (note - was not working in design, and certainly would not have done so at that particular company) where parts of the building generally do not feel like they've been adequately separated from a neighbour. Typically this is in situations where there's a wall just above the natural ground level between the shoring/capping beam and the main ground floor slab above - this doesn't seem to generally be included in setback requirements. In once particular case for a tower in a developing business district, the new build was sandwiched into a narrow plot between existing structures and it felt like the wall on one side would probably crash into the tower next door once you got high enough with enough wind or seismic activity. Shoring systems in general will often find themselves constructed right up to the boundary, even right up against adjacent structures/footings - the focus usually gets put on not undermining the support of the nieghbouring structure and lateral movement tends to get overlooked. The threat is pretty self apparent, and the few mm of compressible filler i've seen used as a "fix" is not what I'd call a good solution to the problem either. I figure the money makers are gonna geta bit pissy if they have to start separating their basements further from boundary lines when it's already a struggle trying to make numbers of cars spots work with the fact you need columns for a building to stand up

I always find it amusing how the Australians resist change because you just haven't had your watershed moment in terms of a significant widespread seismic event. It'll come one day.

I don't think engineers here resist change so much as they just don't understand seismic so well to begin with, plus a bit of lamenting the fact that we often cop a bit of blame for when we prioritise safety. Or worse, we cop the blame when the dollar amount being spent on reinforcement goes up for reasons out of our control.

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Why yes, I do in fact have no idea what I'm talking about

 

[Smoulder]

Is real world seismic design better advanced so 70% minimum isn't worthwhile? I don't think so but do any of the top seismic design codes abandon minimums?

 

[Agent666]

In NZ we don't typically have any minimum like that related to a percentage of base shear from a period empirical equation.

Only time it mentioned is for timber multistorey design guidance as we borrow a bit from North American practice where that seems to be the norm. Even then it's limiting period to 2 x the empirical equation output based on height is what I recall. Depending on where you are on the spectrum that could be a bit less than 70% of the base shear.

https://engineervsheep.com