AS3600_2018 - clause 6.2.4.2 for seismic modelling 3

[GTD_18]

Hi All,

I am just curious on what other teams/offices are doing with clause 6.2.4.2 (in AS3600_2018) wrt to the effective section properties of walls for seismic events?
• Uncracked = tensile stress < mean char flexural tensile strength
• Cracked = tensile stress > mean char flexural tensile strength.
So, from the above it is assumed that the initial seismic modelling should be completed on Igross, then assess the tensions vs rupture capacity and crack the section based on applied axial load.

Personally, up to this point, I had been initially adopting the “uncracked” wall value of 0.7Ig from ACI-316 and cracking from from here if required.

Just curious to see the design approach from others?

 

[rscassar]

I would think cracked section properties are more appropriate. Considering the force demand and displacements experienced for earthquake loading/event are relatively high, I use the cracked values. Uncracked values can be adopted but the task of running through and verifying that sections remain uncracked is very onerous, I think it's more appropriate to go to the upper bound condition being cracked section properties.

 

[rapt]

If it is uncracked with mu = 1 and Sp = 1 then it is uncracked. Assuming it is cracked anyway is not logical.

 

[Agent666]

Real earthquakes don't agree with following code limits. Real earthquakes are not limited to imparting only mu=1.0 and Sp=1.0 actions into a structure.

Also, you'd need to consider the period would be much stiffer if uncracked, so seismic loads would potentially be quite a bit higher (until it actually cracks), oftem loads would be higher momentarily than those actions sufficient to crack a section.

At some point it will crack basically.

Most seismic codes require considering sections to be cracked to some degree for determining the response and resulting seismic actions. Effectively shortcutting your analysis to a cracked level of response, because you're ending up there sooner or later whether you like it or not.

https://engineervsheep.com

 

[rscassar]

while we are discussing earthquakes. With the introduction of Section 14, are people now designing buildings for mu=1 and sp=0.77 (non-ductile).

 

[GTD_18]

Initially I think that the idea of the restraint reo spooked a few of the contractors which led to using Mu=1 as the baseline design but now we are generally back to using Mu=2 with all the detailing requirements that this brings.

Interesting responses to the above – kind of in line with what I hear in the office too.

 

[rscassar]

I don't know. For low-rise and medium-rise. I don't really see a benefit from taking mu=2.0. I'm more for mu=1.0, double the forces and design for the actions that result.

 

[GTD_18]

I'm on the other side of the fence - design as Mu=2 (if applicable) and detail as such. I think once the detailing requirements of the new code become "standard" on site then they'll be much less pushback.

 

[blihpandgeorge]

i agree with GTD_18, design for mu=2 and detail accordingly

 

[GTD_18]

It looks like the new AS1170.4 commentary has provided difference referenced Icr values - refer to pg 70 - table C7.2

 

[bugbus]

GTD_18, I'm trying to locate this

Would you mind giving a quick summary of the values?

 

[IDS]

Download link for the new AEES commentary on AS 1170.4:

AEES has published an update to the AS1170.4-2007 Commentary - 2nd Edition (2021) which is available as a free download by following

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

to our website shop.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[bugbus]

Thanks

 

[Agent666]

I'd be taking the values table C7.2 with a few grains of salt, the values seem miles off anything realistic if you were to actually assess an average section and determine the effective properties.

Table C7.1 is much more in line with what I'd expect to get based on my experience.

But it does highlight that it is an estimate, and it is hard to categorise into a one value fits all scenarios approach.

NZS3101 also provides some guidance in the form of a similar table if you're after another opinion.

https://engineervsheep.com

 

[blihpandgeorge]

the Australian Engineering Seismic Society has just released a commentary on AS1170.4 including recent amndt and it has some guidance on section properties. Guide is free from their website. I am not affiliated with them but feel they do some good work.

 

[captain_slow]

Just chiming in to say that I am seeing a lot of design to mu = 1 and Sp = 0.77 for mid-rise structures in Melbourne. Designers generally adopt cracked sections of around 0.35-0.4 Ig.

It seems that the non-ductile assumption along with section cracking produces designs that look very similar to AS3600-2009. From personal experience, it is extremely difficult to convince project leads to adopt Section 14 detailing and minimum reinforcement requirements.

I really like the recent Amendment 2 additions to the end of Section 11 - the minimum dowel requirements were badly needed for precast construction. The (very) commonplace typical detail of N20 dowels at 3000 max CTS had to go. Now we have to wait and see how long it takes to trickle through the industry...

 

[rscassar]

Yes,

Displacements: No change, displacement check always made on mu=1.0 (2009 & 2018).
Shear: All vertical elements to be designed for mu=1.0 (2018). Columns and Walls.

Flexural (or axial forces for wall/pier segments in Walls)
--> mu=1.0, design for all forces, tension and compression, that arise from analysis. Base shear approx 8 to 20% of building weight.
--> mu=2.0, Base shear approx 4 to 10%. Tie limitations are required for most elements in the building, walls and columns, to the extent that shear & compression capacity of walls & columns exceeds that mu=1.0. Tension demand and design requirements are alot less than mu=1.0. This provision I don't comprehend how the building is more ductile if it has less "tension capacity" in the regions of higher tensile stress than taking mu=1.0.

 

[rscassar]

If mu is taken of 2 or 3 and it results in a higher reinforcement requirement and a lower structural capacity. ???.

 

[Agent666]

rscassar, sounds like you're struggling with the concept of ductility as whole.

Ductility is just controlled damage, allowing the formation of controlled mechanisms (flexural usually) to dissapate seismic energy and to ultimately allow for more economic designs (which arguably need to be demolished due to the damage we are accepting in adopting ductility to lower the overall base shear). But think of it as achieving the life safety requirements, potentially at a lower cost overall than adopting an elastic design at the expense of some more onerous detailing.

The reality is as well in some countries such as NZ we have to use ductility, as we are dealing with seismic loads of more than 100% of the building weight otherwise, that becomes uneconomic. Often even though the overall amount of reinforcement for a ductil designmight be similar, the forces on the foundations after consideration of overstrneght might be considerably lower than designing for an elastic or nominally ductile design.

One aspect that designers seem to struggle with is that even at in an elastic design, if a big enough event comes along then you'll still form your mechanism and won't neccessarily have the remainder of the structure protected from say a shear failure in the case of a cantilevered wall system.

In a real seismic code (not any AS code really) aspects related to capacity design ensure these mechanisms can form and behave in a dependable manner and that the rest of the structure is protected. You're designing certain critical regions to actually yield, and protecting the remainder so you do not form any undesirable mecahnisms. I'll admit your code has gotten better, but it still has some really weird rules on some things from an outside perspective compared to more modern/developed seismic design codes.

Quite frankly some of the rules adopted in Australia and the way people seem to then manipulate them is a joke. Not a funny one. Designers seem to expend a lot of effort (as evidenced in these forums and my own personel dealing with Australian designers) in miscontruing and talking themselves out of doing what just needs to be done. Sure you can be smart about design, but it's a fine line between being smart and dumb as far as seismic design goes, the earthquake will find you out one way or another.

Most likely in my opinion they do this because they have not been educated with seismic design at the forefront of their design education, they simply lack the knowledge and experience to understand the concepts involved, but also old school thinking in the contruction industry and the inability to flex to take on board the evolving requirements with respect to seismic design. That old school we've never had to do it this way before attitude from the contractor and design side of things.

Hopefully at Australian Universities there is now a new breed of engineers being trained and coming through that do have the theoretical knowledge to know why things need to be done this way due to the increased focus on the seismic design side of things in your concrete code, and they won't be so manupulative and blases with the rules as a result.

You really need a serious earthquake that results in widespread damage and potentially loss of life to change your industries way of thinking about seismic design. NZ has been throuh this and learned (and continues to learn) the lesson. We still have our issues with the quality of designers, no easy fix to this I guess.

If mu is taken of 2 or 3 and it results in a higher reinforcement requirement and a lower structural capacity. ???.

If you're ending up with more longitudinal reinforcement then you're doing something wrong? If you're talking about more transverse reinforcement then yes, you'll have this as aspects like confinement in axial members and antibuckling requires more onerous requirements when dealing with ductility and potential for overstrength forces. I suspect because your seismic loads are so low, that the decision to go with a ductile design isn't being forced on you like it is in other countries as the only means of actually producing a building that makes economic sense.

https://engineervsheep.com

 

[Settingsun]

even at in an elastic design, if a big enough event comes along then you'll still form your mechanism and won't neccessarily have the remainder of the structure protected from say a shear failure in the case of a cantilevered wall system.

In a real seismic code (not any AS code really) aspects related to capacity design ensure these mechanisms can form and behave in a dependable manner and that the rest of the structure is protected. You're designing certain critical regions to actually yield, and protecting the remainder so you do not form any undesirable mecahnisms. 

What happens when a bigger earthquake exhausts/exceeds the ductility limit?