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AS1170.4 Draft

AS1170.4 Draft

AS1170.4 Draft

(OP)
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.



RE: AS1170.4 Draft

Where can you download the draft from?

RE: AS1170.4 Draft

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

This link may or may not work.

RE: AS1170.4 Draft

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

RE: AS1170.4 Draft

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.

RE: AS1170.4 Draft

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.)

RE: AS1170.4 Draft

(OP)

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

Quote (human909)

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.

RE: AS1170.4 Draft

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

RE: AS1170.4 Draft

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.

RE: AS1170.4 Draft

Quote (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.

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

RE: AS1170.4 Draft

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.

RE: AS1170.4 Draft

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

RE: AS1170.4 Draft

(OP)
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'...

RE: AS1170.4 Draft

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.

RE: AS1170.4 Draft

Quote (Agent666)

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.

RE: AS1170.4 Draft

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-2...

RE: AS1170.4 Draft

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 dazed

Quote:

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

RE: AS1170.4 Draft

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?

RE: AS1170.4 Draft

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

RE: AS1170.4 Draft

Quote (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.

Exactly. Surely we should be considering stiffness and mass to find the natural period?

I have no complaints against an simple formula that limits engineers from making unconservative choices but surely we should have enough faith in engineers to be able to do a basic stiffness and mass calculation? I'll be the first to admit that my seismic knowledge is far from immense surely we can do better.

RE: AS1170.4 Draft

Glad some sort of base shear scaling was reintroduced, however it seems to me that the 50m cut off does not make much sense, should be something gradual, not full 70% scaling at 49.9m and no scaling at all at 50.1m. This will lead to some engineers to propose developers that have structures close to 50 m tall to slightly increase the height in order to massively reduce the seismic loads

RE: AS1170.4 Draft

Quote:

This will lead to some engineers to propose developers that have structures close to 50 m tall to slightly increase the height in order to massively reduce the seismic loads

On one hand, I agree with you, on the other hand, would be really good to get some additional headroom for transfer slab depths...

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

RE: AS1170.4 Draft

Why do so many of the Australian standards insist on these hard cut-off points where one rule suddenly stops applying and another one kicks in, just because something is now 1mm longer than it was before?

RE: AS1170.4 Draft

Bugbus

It is not just Australian codes. All do it. I have been trying to remove as many as possible in the codes I am involved in.

I have passed this comment on to the 1170.4 committee in case no one else has in response to the draft comment requests. Hopefully it is not too late.


RE: AS1170.4 Draft

The public draft is open for comment until 25th January next year.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: AS1170.4 Draft

I just received confirmation, following a comment I have left on the draft, that buildings taller than 50m will now require 50% base shear scaling. Still a big jump from 70% to 50% at 50m but that's a great improvement from the previous 70%-0%.

RE: AS1170.4 Draft

Still irks me that they didn't opt for a smooth transition, to be honest.

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

RE: AS1170.4 Draft

(OP)
Be interesting to read the commentary (in ~5yrs time) to see how these scaling factors were derived.

RE: AS1170.4 Draft

It irks me to no end that a group of people can write a code and ostensibly have a rationale for every decision made in it, but take 5 years to actually communicate that rationale to the people wanting to use the code. Aussies are inexperienced enough with seismic as it is, the less guesswork the better.

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

RE: AS1170.4 Draft

Is anyone able to send a copy of the draft through?
The download location is inaccessible now with the comment period finishing up.

RE: AS1170.4 Draft

Quote (Just Some Nerd)

Still irks me that they didn't opt for a smooth transition, to be honest.

Yeah I did propose a smooth transition through a simple linear equation in my comment however it looks like the committee prefers to adopt a more simplistic approach.

RE: AS1170.4 Draft

I'm glad to see that I'm not crazy and that the seismic hazard maps ARE contour maps. I still see engineers and online resources failing to interpret a contour map properly.

My issue (a little pedantic) is their approach in this update to remove the confusion actually adds to the confusion. By marking 0.08 low points on the contour map in inconsistence places it gives false and misleading information. If 0.08 is the minimum they need the 0.08 contour on the map!




EDIT: I suppose I should spend less time complaining here and more time commenting on the draft standard. banghead

RE: AS1170.4 Draft

I actually think that a shaded area would make it very clear where it is 0.8 so people don't waste time trying to read different contour lines. If the site is in the green area, it's 0.08, if it's elsewhere, then it should be fairly easy to tell because the contours will actually be there. H

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

RE: AS1170.4 Draft

Unfortunately they did not mention the new 80% limit on dynamic analysis in the Concrete Institute webinar.

RE: AS1170.4 Draft

Quote (Just Some Nerd)

I actually think that a shaded area would make it very clear where it is 0.8 so people don't waste time trying to read different contour lines. If the site is in the green area, it's 0.08, if it's elsewhere, then it should be fairly easy to tell because the contours will actually be there. H
Oh I agree! I don't know why they didn't include a shaded area in the actual standard.

RE: AS1170.4 Draft

Quote (Retrograde)

Unfortunately they did not mention the new 80% limit on dynamic analysis in the Concrete Institute webinar.

Assume you meant 70%.

I thought about asking a question, however I think it's quite clear now they will introduce 50% scaling for building higher than 50m and 70% for lower than 50m, not much needing clarification there. Don't necessarily agree with the sudden change happening at 50m however I am happy some scaling will be required no matter what, and happy enough this will be clear and the same for everyone. Currently with no scaling required I saw some engineers applying no scalign at all even when going aggressive on stiffness reduction factors on their structure, which is pretty crazy - while I've been accused of being too conservative because I was applying scaling factors to my spectrum case. Glad this will be no more.

RE: AS1170.4 Draft

Got a notification from SAI Global that AS1170.4 2007 has become superseded however I see no new standard been released yet.
Does anyone know if we will be having the new earthquake load standard today, or in the next coming days?

RE: AS1170.4 Draft

AS1170.4-2024 has just been released

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