New Zealand EQ Detailing & Design Issues

[Trenno]

Happy new year all,

I'm working on a few buildings which will be built in Papua New Guinea. As such, it has been noted that some of these multi-storey building may come under scrutiny with regards to their detailing to NZS1170.5 for the region's high seismic loads. Lateral systems will generally consist of RC or masonry corefilled blockwork shear walls. Horizontal structure will typically be flat plate.

I have access to the current NZ design codes.

I'm under the impression that there are a number of detailing requirements according to the NZ code, but can't quite locate them. Are there any NZ engineers out there able to point me in the right direction? Are there any special issues I need to be careful of?

 

[Trenno]

Preferred seismic details for RC stairs?

 

[rowingengineer]

Trenno,
First of all we need to establish how just how far you want to go into NZ design requirements and how far you are pushing the design. What structural ductility factor are you looking to adopt in your design. This will set what force/detailing requirements you need to meet (hte more ductile the structure the higher the detailing requirements. Also we need to set just what level you need to design for NZ, having only done limited designs in PNG, I wouldn't know. NZ has a lot of different references for design see below.

http://www.building.govt.nz/practice-advisory-13

http://www.building.govt.nz/practice-advisories-index

http://www.concretesociety.org.nz/index.php/nzcs-publications/nzcs-technical-papers

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Trenno]

Hi RE,

Thank you for the links. The note regarding egress stair safety is great. I couldn't open your last link however; "This account has been suspended."

With the building at hand, I think we are going to be alright (strength wise) aiming for reinforced masonry shear walls with limited ductility. So ductility factor of 2 and performance factor of 0.77. This is with close-spacing reinforcement, say N16-200 vert. and N12-400 horz. "Z" (peak ground acceleration) is 0.16 for Port Moresby.

AS3700 essentially just tells us to tie the shear walls into the slab/footing with starters top and bottom. I would have usually done this anyway.

The NZ masonry code also talks a bit about not lapping bars in potential plastic hinge zones. I can't see any other unique detailing requirements.

 

[Agent666]

Read through the concrete standard as well (NZS3101), it offers a better view of capacity design procedures and seismic detailing, much of which is applicable in principle to masonry design. You will be required to do a capacity design if you are designing to a ductility of 2. You didn't say where you are located so not sure if you currently follow some seismic requirments or not, or if the whole seismic design is new to you.

You will find that a grade 500 16 diameter bar at 200 ctrs will fail the balanced strain conditions for out of plane bending, if you have out of plane actions in the walls you need to satisfy this still (i.e. under face loading).

The masonry code NZS4230 is a bit dumbed down in its approach in my opinion (and quite poorly ordered in the info it presents which can make it a challenge to find what you are after sometimes), there are a lot of relaxations when compared with the concrete code as well in terms of what would be considered good seismic detailing (for example masonry wall is singularly reinforced and can design for ductility, but concrete code doesn't let you do this, an upcoming amendment to the concrete code has proposed for singularly reinforced walls that a strength reduction factor of 0.7 is adopted, essentially meaning walls end up being designed for a ductility equivalent to 1.0. this is due to the poor performance of singularly reinforced walls in the Christchurch earthquakes, especially in multistorey construction)

I think it would be fair to say in NZ that not many people actually design ductile masonry structures, in that concrete is usually the better option or they just design masonry elastically.

Longer laps are required, and offset laps in potential plastic hinge regions in masonry (I think 105 bar diameters for grade 500 bars vs 70 bar diameters normally). Just read the standard cover to cover as there are other requirmenets like confinement and antibuckling in columns that need to be satisfied.

At a ductility of 2 you can use an Sp of 0.7 as per table 3.2 as well.

I would be very hesitant to use a ductility of 2 in PNG as the standard of construction and average skill of the labour force just isn't that high. You will require a large degree of onsite supervision to get a standard that you might be used to in your home country.

 

[rowingengineer]

agent666,

What would you suggest would be a better ductility factor? 1.25 and sp=1.0? I think it is very hard to design for poor workmanship, you don;t really know what the safety factor should be.

Trenno,

http://www.nzcma.org.nz/document/27...of_Reinforced_Concrete_Masonry_Structures.pdf

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Trenno]

Great post Agent666. I'm at 'design engineer' level with a few years of experience under my belt - based in Brisbane.

Masonry block is fairly cheap and common in PNG. I did a calc on a 7m long x 5m high 190 RC block wall and it could pretty much take the full seismic load (in plane shear) of the two storey building by itself. There are other shear walls we can use to take out eccentricities and the like.

We aren't necessarily required to design to NZ codes, but it has been raised by local engineers over there.

 

[Agent666]

Rowingengineer, a difficult question to answer without knowing more about the standard of work produced over there. Main point I was making was the risk of getting something wrong on site is higher, and for a ductile structure to perform like designed, you need things to be built with a reasonable degree of correctness.

I used to work for a company that did a lot of work in Indonesia, we learnt pretty early on that single storey structures in masonry were generally ok, but for two storey we always provided a separate steel gravity structure and just used the masonry as the facade. The workmanship was pretty poor, they didn't seem to get the concept of completely filling the walls with blockfill, compaction of the concrete just wasn't something they fully grasped or were equipped to deal with
As a result we worked around it in the design, being more conservative where warrented. We still had our fair share of construction nightmares though!

Trenno, if you can get something to work at a ductility of 1.25 then I'd just use that with an Sp of 0.9. Especially so if you actually have a large number of walls that won't really yield at the ductility of two as you seem to be suggesting. If this is the case you will have an excess of strength and the actual ductility is possibly less than 2.0. Only if your wall design caapcity is exactly matching the design actions at ductility of 2 will the building actually be responding at a ductility of 2. If by virtue of having a lot of longer stronger walls the strength is in excess of 2 times the ductility of 2 forces then the structure is capable of actually taking the ductility 1.0 forces without yielding so this should actually be your design ductility.

Earthquake will use up whatever capacity you provide, essentially until something yields, then this is your actual real ductility once some yielding mechanism forms.


Trenno, if the wall is 5m high without any intermediate restraints then it is too slender, the max slenderness in NZ masonry code is 20 times the thickness, i.e. 3800mm tall is the max height between restraints for a 190 thick wall. Its even less if the wall is cantilevered from memory. Restraints can be vertical or horizontal.

 

[Trenno]

Hi Agent666,

I have the following situation. You mentioned restraints could be vertical. If we put in a vertical post, as indicated on the sketch, I wonder if we could then span the block horizontally between the post and return?

 

[Agent666]

Yeah I don't see why not. Might want to consider a small return on the block wall or a 400x400 pilaster that way you don't have to connect the post to the masonry wall (and post likely to be less stiff than the masonry anyway).

 

[Trenno]

As a follow up question, do either of you (Agent666 or RE) know of any required to have 100% corefilling for non-load bearing masonry partitions? In particular to the NZ code?

As a first pass, I'll be having a look at what the EQ loads are in accordance with Section 8, AS1170.4 and going from there. Generally we are restraining the top of the blockwork through angles to the roof purlins.

 

[Agent666]

Not that I'm aware of, the code covers partially filled construction as well as fully filled construction.

Only difference in design is you can only use the filled cell width as your design width vs the entire wall for flexure, similar for face loaded shear. This is offset by the lower loads due to lower self weight, but it can make the wall fail the balanced strain conditions due to the deeper compression block depth for a given level of reinforcement compared to a fully filled construction with the same level of reinforcement.

 

[sdz]

If you want to keep your design time and headaches down design as an elastically responding structure. It makes everything a lot easier.