RC Concrete walls/footings to AS3600 and 1170.4

[MrJB]

Is anyone able to clarify the link between the mew and Sp values for RC walls and which values are relevant for designing footings to AS 1170.4 and AS 3600?

Section 11.2 of AS3600 asks for an analysis to be performed with mew of 1 and Sp of 1 to determine if there is any compression/tension in the structural walls. If the full earthquake load needs to be applied elastically does the standard imply the structural walls may need to take this full load? If so, should the footing loads be calculated with a mew of 1 and Sp of 1?

An EDCII building using the equivalent static analysis procedure with limited ductile structural walls results in a mew of 2 and Sp of 0.77. This results in 38% of the full earthquake load being applied elastically and the remainder being applied inelastically. Is the 62% of the load being applied inelastically just absorbed by the structure without the load path being resolved through the footings? If the base reactions can be adopted for footing design it seems that considerable economy and time savings during construction could be achieved by increasing the detailing to cover moderately ductile structural walls. Unless other load cases govern (ie wind or gravity), the footing loads could be reduced by nearly half which must be more efficient than adding in additional ligs in boundary elements and staggering splices etc as required by section 14 of AS3600. Can anyone clarify?

 

[rscassar]

essentially AS1170 need to be updated to reflect the new revision of AS3600. AS3600 defines any wall which doesn't have any for of seismic detailing as non-ductile mu=1.0. AS1170 defines a limited ductile building as mu=2.0. More so than this, AS3600 drives any building which isn't designed as mu=1.0 to essentially be designed as mu=3.0.

I don't like the wall provisions particularly with the ties requirement in boundary elements for a limited ductile wall, 38% of the full earthquake load is used in analysis however full earthquake load is required for the shear design of walls regardless of whether mu=1.0 or mu=4.0 is adopted. Reading through other codes, it seems that these boundary elements isn't necessary required for limited ductile walls. I could be wrong.

 

[rapt]

The wall situation has been explained several times in other posts.

The wall needs to be designed for the mu and Sp values for the ductility level being designed for.

But the the building and wall will eventually experience the full earthquake load after yielding and plastic deformation of the structure.

So the wall (and the structure) need to be detailed for the full loading. In the case of the wall, if it is in compression under the design loading but in tension under the full earthquake loading, then detailing it as a wall in compression will lead to a brittle failure of the wall as it does not have sufficient reinforcement to ensure ductile action in tension. So the weall needs to be detailed for the condition under full loading and needs minimum tension reinforcement to ensure Brittle Failure does not occur.

If the effect of full loading on the footing were to change the action of the footing, then I would assume similar considerations would be required, eg overturning on a footing (whether pressure under the footing is purely in compression or there is uplift tension) would be dependent on the full load, not the design load as the footing will experience the full load.

Basically in any area of the design of a building, if full earthquake load changes the way a member acts or resists loads, then the decision on the way the member acts must also consider the effects under full earthquake load to ensure that the building is detailed for the full earthquake load and can act as designed as the load increases from the design load to the full load.

 

[Tomfh]

The whole topic is very murky. I've asked various engineers what mu should the footings (and overall stability) be designed for, and they all have different views.

We're pretty bad at this stuff in Australia.

 

[Smoulder]

https://www.eng-tips.com/viewthread.cfm?qid=478032[/URL]]It can be easy at times to get lost in all the prescriptive seismic code requirements, but good seismic design boils down to three basic steps:

1. Select an appropriate seismic system.
2a. Design the fuse in your seismic system as close as possible to a DCR=1.0 [design capacity ratio].
2b. Detail the fuse to withstand the amount of ductility that you expect of it.
3. Design the remaining members in the seismic load path for the capacity of the fuse.

A code-based design won't always follow these steps to the letter, but try to think about what each code provision is trying to accomplish behind the curtain and where it fits within those steps.

EQ force design lets you reduce the design forces because the fuse creates a limit by losing its stiffness along the way. The structure still deflects the full amount as determined by the earthquake motion, which is the real load. If you say the wall is in compression under the EQ load then youre implicitly saying it didn't lose stiffness at full EQ deflection because it didnt crack. The sp/mu=1 analysis mimics that full deflection as a check that the wall is actually in compression over its entire section. If it is going to crack at full deflection then it needs appropriate reinforcement.

For the footings, see Dekers step 3. Take the upper limit capacity of the fuse considering accuracy in predicting structural capacity.

 

[MrJB]

Thanks everyone for responding. It seems the footing system will only take the full earthquake load once the walls have developed plastic hinges or exceeded their capacity. If the walls are designed and detailed correctly this should not occur. It would seem excessive then to design the footings for a mu of 1 and Sp of 1 although the nature of the design actions on the footings still needs to be considered uner this combination.

Determining the loads using the equivalent statical analysis appears to be a more conservative method for a 30m +/- building as well over using the dynamic analysis. This just adds an extra degree of comfort to the design. Is it common for a higher level of earthquake detailing to be adopted for robustness? The IMRF section has some sensible provisions for slabs etc that the limited ductile shear walls section doesn't require.

rscassar - Would be very nice to see 1170.4 and AS3600 come more into line on the earthquake front. Harmonising with AS 3700 would be a bonus as well. Hard to believe that a 190 thick concrete wall with 2 layers of reo really has the same ductility as a 200 series blockwork wall with a single layer of reo.

 

[rscassar]

yeah, I have noticed that as well. As it currently written, a non-ductile concrete building is mu=1.0 and a non-ductile masonry building is mu=1.25. This doesn't seem correct. I think AS1170.4 needs and amendment or new revision to come into line with AS3600.

 

[rapt]

MrJB,

if the walls are designed and detailed for mu = 2 and Sp = .77, then the ultimate design loads are based on about 40% of the expected sway. The remaining 60% of the sway is post ultimate capacity and post development of plastic hinges. That is how you are designing it to act if using mu and Sp not equal to 1.

The full sway still occurs, just 60% of it is resisted plastically by the walls.

There are ongoing discussions with AS3700 committee to "improve" it!