AS3700 - Section 10.3 - Seismic Slip Joints
AS3700 - Section 10.3 - Seismic Slip Joints
(OP)
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"
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"





RE: AS3700 - Section 10.3 - Seismic Slip Joints
Vd is equal for both directions.
RE: AS3700 - Section 10.3 - Seismic Slip Joints
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.
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RE: AS3700 - Section 10.3 - Seismic Slip Joints
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...
RE: AS3700 - Section 10.3 - Seismic Slip Joints
I think you are going to need walls in the perpendicular direction...
RE: AS3700 - Section 10.3 - Seismic Slip Joints
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.
RE: AS3700 - Section 10.3 - Seismic Slip Joints
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.
RE: AS3700 - Section 10.3 - Seismic Slip Joints
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.
RE: AS3700 - Section 10.3 - Seismic Slip Joints
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.