Multi Storey masonry design under EQ load

[CRQUT]

Hi,

For multistorey buildings I was taught to design the cores and nominated shear walls (if needed in addition to the core/s) to take the lateral EQ load. This approach is no longer valid. There is plenty of threads on here to explain why (special shout out to Rapt for his input to those and all threads related to AS3600-2018).

My question is:

Is it possible to design a multistorey building to NCC 2019, governed by EQ, constructed with reinforced masonry (ie 190 block) that is taller than 15m and has a mu = more than 1.0.

I am just trying to gauge what the general consensus is among peers and industry so thank you in advance for any long or one word answer replies.

I should mention I appreciate the NCC requires ties and AS3700 assumes vertical bars can be restrained by an annulus of grout.

I also appreciate the many many mulltistorey buildings that have been built using blockwork (which are yet to see their design load..).

thanks,

CRQUT

 

[blihpandgeorge]

For clarity - the NCC requires ties and you can't rely on on the grout annulus as per AS3700 (I'm not sure if that is what you are referring to above).

 

[Agent666]

While I have no Australian Masonry experience...

In NZ the masonry code has no height limitations for masonry structures provided you undertake the appropriate capacity design if undertaking a ductile design. There are design rules for confining plates in the compression regions of centrally reinforced walls that act to achieve the required confinement. These are a plate with voids aligning with the cells that are cast into the joints to effectively do the same job as closed ties

Having said that I've never seen anyone ever attempt a ductile multistorey design though and used these, but if you were after inspiration to actually do the design perhaps adopting the NZ standard instead of AS masonry standard is an appropriate approach. My understanding for ductile design AS3600 allows the use of NZS3101 for example.

There would be a strong preference here in NZ to instead use precast concrete panels if anyone ever mentioned masonry greater than 2 storeys I would think.

https://engineervsheep.com

 

[Enhineyero]

For multistorey buildings I was taught to design the cores and nominated shear walls (if needed in addition to the core/s) to take the lateral EQ load. This approach is no longer valid. There is plenty of threads on here to explain why (special shout out to Rapt for his input to those and all threads related to AS3600-2018

I don't follow. Why is it not valid? Walls are very stiff compared to moment frames and will attract majority of the lateral load.

Is it possible to design a multistorey building to NCC 2019, governed by EQ, constructed with reinforced masonry (ie 190 block) that is taller than 15m and has a mu = more than 1.0.

Yes it is possible if stress are low enough and the wall is restrained at every 3m or so.

 

[rapt]

The columns still need to be designed for the loads they will attract and the inter-storey drift they will experience (actual, not design value if mu/sp > 1), and be detailed to survive this.

Good luck with your block walls.

 

[Enhineyero]

I agree with rapt, though columns will attract negligible lateral loads if the frame is not analysed/designed as a moment-resisting frame.

Actions to columns due to Interstory drift tends to be low if you apply limits to the inter-storey drift say L/300 to L/500.

take for example interstorey drift of 10mm, Height = 3000. say 300 x 300 col. F = k (delta). k = 3EIcr/L^3. E = 27000mPa (concrete), I = 675E6, Icr = 0.5 x I = 337.5E6, k = 1012.5

Force to displace by 10mm = 10mm x 1012.5 = 10kN, M* = 30kN.m

try 300x300 f'c = 25mPa with 1% reo (8N12) Axial capacity is around 1000kN (for M* = 30kN.m). For me, its not an alarming amount of bending moment considering that its based on the max. limit for interstorey drift.

 

[blihpandgeorge]

@enhineyero i agree with the intent your calc, however interstorey drift of L/500 or even L/300 is typically used under SLS wind.

Under ULS Seismic, the limit is 1.5% of storey height which can be a big number (h/66). As RAPT has noted, this drift is not simply the seismic drift from analysis, as the deflection from a ULS analysis need to be scaled by mu/sp (AS1170.4 5.4.4 and 6.7.2).

 

[rapt]

And if the moment on the column is that low, is it cracked? I might not be Icr as defined!

 

[Enhineyero]

@Blihpandgeorge - I would still use a higher limit to avoid high stresses acting on the shear wall. Thanks for pointing out the scaling of drift (factor of 2 for heavily reinforced masonry), forgot that in the column calculation.

 

[Enhineyero]

@ Rapt - Zcol = 4.5E6, f'ctf = 0.6(f'c)^0.5 = 3mpa, Mcrack = 13.5kN.m, hence cracked. if N*/A - M*/Z exceeds f'ctf then it is cracked.