AS3700 - Section 10.3 - Seismic Slip Joints

[Trenno]

Hi all,

We rarely dip our toes into residential structural engineering... so have come across a weird conundrum in regards to bracing for a 2 storey townhouse with a suspended slab supported by unreinforced masonry walls.

From what I have read from different sources is that the slab acts as a diaphragm and is braced by literally the friction between the walls/slab slip joint.

Now I find it weird that the bracing resistance is based essentially on a total bed area (total length of wall x thickness)(refer to extract), thus the direction of the walls have no influence on the lateral stability in a seismic case.

Now say you have a building with the suspended slab and walls below as shown. For the EQ Direction B, im concerned that the slab will move to the right and it will all just collapse like a stack of cards.

Am I right in saying that the orientation of the walls below has no effect on the overall lateral stability / bracing of the slab? "This equation applies to both in plane and out of plane shear"

 

[Trenno]

Just to clarify my total resistance = Vd = Kv . Fd . (L . t)

Vd is equal for both directions.

 

[rowingengineer]

I think your pack of cards analogy is correct.

Personally I would provide reinforced masonry walls with N12's at 600-1200 crts, with a 1 course Bond beam below the slab.

I would also enclose the bays 2 and 3 back walls of the building to get bracing requirements.

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]

Reviewing a very similar project designed by a different company, the engineer seems to have relied on all that I have mentioned above, yet with small return nibs in the perpendicular direction at the ends of the walls only.

I would rather tie in the slab too - but seems unreinforced clay masonry is the go on this project.

Anyway, it would be good if anyone can confirm I am interpreting this clause and design philosophy correctly? I might have to ask my old small buildings lecturer...

 

[Ingenuity]

Back in my days consulting in Sydney, I used to use "HERCUSLIP" Link for cast-in-place slabs and clay masonry.

I think you are going to need walls in the perpendicular direction...

 

[asixth]

Yup. Lacks robustness and I would try not to get associated with it.

Who asks for a peer review for a block of townhouses?. Unless they think the structure is over the top and with a 2oomm thk rc slab I say it would be over the top. Should just have a timber framed floor.

 

[nonplussed]

The equation you refer to in the original post is only for checking the capacity of the concrete to brick interface. It is not a check of the brick wall capacity or a check of the global stability of the structure.

To check the wall capacity refer to AS3700 cl 7.5.4.1 (shear capacity - horizontal planes). Most townhouses will have adequate capacity in the unreinforced masonry to satisfy the low earthquake loading. If you have a special case with limited bracing walls, like in your diagram, then one common solution is to use concrete columns to resist the earthquake loads (or any other suitable system). But only do this if the architect does not allow extra cross walls.

As for the 200 thick slab, this is the minimum thickness to meet noise requirements in the BCA I think.

 

[Trenno]

Yeah we usually stick to the 200thk slab for those reasons, nonplussed.

Thanks for the explanation of the code requirements, I'll look into it this morning.

We do have return nibs at the end of most longitudinal walls, I'm sure these will help a little.

 

[Trenno]

So, "AS3700 cl 7.5.4.1 (shear capacity - horizontal planes)" is the same equation I referred to earlier, except I have read that we should assume the shear bond strength to be 0, thus only the 2nd term applies.

I know see that performing this check is only part of the lateral stability picture, as shown in the below example. For Direction B, with no shear walls to resist the loads, stability cannot be achieved.