AS3600 - Seismic design loads for foundations 1

[li0ngalahad]

Hi, I have recently had a discussion with a colleague regarding seismic design of foundations. In his opinion foundations, like rafts supporting shear walls, should be designed with the same design loads as for the suprstructure, i.e. if I have a building with limited ductile shear walls, foundations should be designed with the same seismic loads, i.e. resuced by a factor of Sp/mu. I was arguing that unless the foundation can be idealized as a flexural member where plastic hinges can form, you cannot consider the foundation as ductile and therefore must be designed with an overstrength factor of mu/Sp. He was arguing back that if you do so, you also need to check overturning, bearing pressures with the same load and for a small building with not a great deal of gravity load on shear walls, and very high natural frequency, you would always come up with huge foundations otherwise they would alwasy overturn or reach excessive pressures. Speaking with other colleagues Id say most of them don't have a strong opinion on this (I suspect they have not much of an idea about it) but they generally seem to lean on designing foundation with threduced design loads, based on their past experience.

For me it makes sense to design the foundaiton for the un-reduced seismic loads mu=1 Sp=1, however seems to be common practice in Australia to just create a model based on a certain ductility and design everything based on the reduced design loads Sp/mu, however the more I get familiar with seismic design, the less this approach seems correct.

The code seems to be silent on this (correct me if I'm wrong). I really think the commitee members are being a bit too optimistic with the Austalian engineers comptence in regards to Seismic matters - recently speaking with some graduates, seismic is still something that is barely mentioned in the civil Engineering courses and they come out from uni not knowing a single thing on this matter.

AS1170.4 table 6.5(A) lists mu and Sp factors for a series of walls and frames structures, and then specifies that for "Other concrete structures not listed above" mu = 2 and Sp=0.77
Obvioulsy this table was superseded by table 14.3 on AS3600-2018 however there's no "Other concrete structures" in the list. There is a note however saying that "Structures and systems not covered in the above table shall have structural ductility factor (mu) and structural performance factor (Sp) determined by a rational analysis.", I cant say I understand the meaning of this phrase fully.

So my question to you is, can we design foundations for the reduced load Sp/mu? and if so, what is the logic behind this?

Thanks for you input as always.

 

[Gishin1]

Hi lion,

This is obviously going to need to be investigated on a case by case basis, but I'd think even if you have combined lateral systems landing on a shared raft, often the compression side of one element is going to be counteracting the tension side of another, unless all the elements are side by side in-plane. ie, the raft probably doesn't have a single point or toe about which it is rotating.

This should put the raft into double curvature between the elements, and the tension developed in the raft is reduced by 0.77/2.6 as well, due to the same yielding. As above, the amount to which the raft can lift-off is a function of the tension steel in the walls/cores, so I still think in this situation, you only need to consider lift off due to the reduced tension load. The raft is still behaving 'flexurally', therefore the tension loads are reduced.

My comments on the compression/shear overstrength remain the same.

Thats my thoughts, but again, it needs to be assessed on a project by project basis, considering geometry, setout, magnitude of loads etc, importance levels & redundancy...and how confident the RPEQ is signing off the F15/F16!

 

[li0ngalahad]

Thanks Gishin1 for sharing your thoughts. So we can say that as long as the raft is dominated by flexure and not by shear, then it can be assumed to being able to behave in a ductile manner and therefore we can design it, and even check stability, with the "reduced" loads. I'd imagine in that case we would need to follow the detailing required in a slab being part of a moment resisting frame, when detailign the raft reinforcement.

What if it's a fully piled raft for a structure detailed as moderatly ductile for example (mu=3) - what design loads should we communicate to the pile designers? Because a piled raft will likely behave as a strut-tie non flexural element, I would think the design and the global overturning moments for pile design should be multiplied by mu/Sp. Would this be also your approach?

 

[Gishin1]

yeah, i guess so., which will probably be the case for most 'true' raft slabs, and the spans aren't really between each individual column/wall/core, but the entire raft dishing under the load. I think that the flexural tension demands can be reduced by 0.77/2, but shear would need to be designed for over-strength, to ensure the tension steel can strain. Your comment about moment-resisting frames is exactly what up said further up on 14 Mar 22. CL14.5.2.1 gives detailing rules for your moment frame beam longitudinal reinforcement, but for shear it say the maximum shear force, E, is to be taken as twice the prescribed load (which in my mind is 2xE, or just use Mu/Sp = 2.6, or use Sp=1, so Mu/Sp = 2).

For piled design's, I've typically given the contractors a compression load of Fz x 2/0.77, but a tension load of Fzx0.77/2, as that is what the tension load in the walls over is, but is this the right approach...? And for non-flexural/strut&tie design, again, i'm not 100% sure. Section 14 says squat walls should be designed as non-flexural, strut&tie, non-ductile elements. So, I'm guessing we can't reduce the tension force, or let the tension steel yield and strain by x2.6, because the angle of the compression struts will collapse and the system could fail, but at least there is no shear to deal with, just compression struts and tension ties.

This is why I think we will start seeing diagonally reinforced coupling beams in Australia now (I've already seen some on a major project (not my job)). As engineers are now amplifying the shear force, its just far to high to design for as a traditional beam, so a diagonally reinforced, strut&tie coupling beam resolves both the moment and shear in one system.

Anyways, thats my thoughts on all of it.