RC Walls vs Reinforced Block Masonry Wall on Earthquake Sensitive Buildings

[RayGon]

I was asked if it is possible to replace the rc walls with rc block masonry walls on a building.

The rc walls are not part of the lift-core and stair shafts hence it can/may be assumed as gravity walls. The stress levels are quite small (less than 3.0MPa).

I designed the rc walls with two layers of reinforcements as there are reversal loading during earthquake and the reinforcement on each face of the wall will provide the required degree of ductility and confinement to keep the walls structurally sound during earthquake if it ever happen.

However, in the AS3700 there are no mentioned about providing reinforcements to both faces of the block walls (even in AS3700 Section 10).

Can anyone provide a guidance on this issue.

 

[rscassar]

My personal preference is that any lateral and load-bearing element is to be insitu concrete. Masonry is very limited in load-bearing capacity and I am not completely convinced on the strength equations given in AS3700. Construction wise blockwork is hard to control on site, reinforcement isn't tied and often is used for prodding in-lieu of vibration, the engineer cannot control adequate vibration of grout filling and poor compaction is not visible as the blocks prevent visibility of compaction. Lapped reinforcement likely geta placed too far apart reducing force transmission, and the segmentation of the blockwork doesn't fill me with any confidence for shear capacity.

Blockwork in my opinion should only be used for cantilevered retaining walls <3.0m and non-loadbearing internal partition walls.

This probably doesn't provide you with guidance you asked and really is my personal preference of the matter.

 

[rapt]

My understanding is that there is work being done to align the AS3700 rules with AS3600. I do not know how far this has progressed but as usual it will take time.

Just because a wall is not part of the liftcore/stair walls does not mean it is gravity only in an earthquake. Any wall connected to the frame will experience earthquake actions and needs to be able to resist whatever effects it attracts.

 

[Retrograde]

I do not have any issue using blockwork walls as load bearing provided they are designed in accordance with AS3700.

If you are worried about earthquakes, model the stiffness of the walls in your lateral analysis and design for the resultant moments and shears.

 

[RayGon]

Thanks for everyone's input.

I do understand that all concrete walls(may they be load-bearing or not and may they be part or not of the horizontal shear/sway structures framing) must be provided with reinforcement that will keep the walls able to support the design loads during and hopefully after an earthquake. In Concrete Code AS3600 we do this for walls by providing 2 layers of reinforcements each face as per AS3600 Clause 14.6.7 stipulated. However, in AS3700 i cant find a clause that mentioned the same requirements for 2 layers of reinforcement on each face of block walls on earthquake detailing requirement.

My question is - In AS3600 we place 2 layers of steel each face for concrete walls, should it then be required to do the same for rc block walls - 2 layers of reinforcement of each face?

Retrograde
I would do that but the core walls and the frame of the structures had already been designed and documented to take the full horizontal loading. I do not really want to run the analysis again. (just want a short cut lol). The walls in questions are the walls that are not part of the earthquake system that had been documented as concrete walls. In trying to save some time and cost it was requested to change these walls to block walls. But if we change the rc walls to block walls should we require the block walls reinforcements to 2 layers each face of block walls or will a single reinforcement will be enough?

rscassar
In As3700 Equ 8.5.1 includes 0.4 steel contribution of the compressive capacity of block walls. While As3600 Simplified method completely disregarded the contribution of steel reinforcement in the compression capacity calculation.

Rapt
Thank you very much for the up date. It would be very interesting to see a draft. I think they should just move the unreinforced and reinforced block walls to AS3600 as a separate section.


I do think that block walls will need to have reinforcements to each face same as what we would require an rc wall to have in this instance even when AS3700 have not specified it explicitly. But is this correct?
If this logic is not correct then maybe I can just use block walls with single reinforcement instead of RC wall with double steel reinforcement each face.

 

[rscassar]

AS3600 Simplified wall method excludes major axis bending and I'm my opinion makes it an irrelevant clause for RC walls/blades in concrete buildings (as we know any element integral with the frame will attract some level of shear and moment). If column tie reinforcement is looking to be excluded from the design than N* has to be less than 0.5*phi.Nu (at least that's my approach to wall design).

Generally speaking, 190 blockwork will have reinforcement placed centrally. Nominating reinforcement each-way each-face in 190 blockwork will essentially be a 4-layer cluster of reinforcement placed relatively central. I don't believe there is a requirement to place reinforcement in each face for blockwork.

 

[Retrograde]

The walls in questions are the walls that are not part of the earthquake system that had been documented as concrete walls.

My understanding is that the philosophy of the new edition of AS3600 is that you cannot just assume some walls are not part of the earthquake system and then ignore any lateral requirements on those walls. Although this has been standard practice in Australia for decades the code is moving away from this approach. The "non-earthquake" walls will still be subject to the same drift displacements as the core. As RAPT said, the philosophies of AS3600 and AS3700 do not really align so it is a judgement call. My personal approach is to not second guess the codes. If AS3700 allows it, then I am happy to design it as such.

I agree with rscassar in that it is impractical to detail 2 layers of reinforcement in a 190 wall. I have done it in a 290 wall and even then they always struggle on site to install it correctly.

 

[RayGon]

rscassar
I agree with you about the placement of reinforcement and how crowded it will be for two layers of reinforcement. In paper using every millimeter available inside the block walls shell N12 bars each face both H and V will fit. But,it will never gonna be built how we show it in our drawings.

Retrograde
Yes that's my understanding too. I had a mentor some 30years ago and I remember him telling me "... Design the cores to take the 90% to 95% of the horizontal actions and distribute the rest to the columns and walls in the building...". During that time, computer modelling is a novelty and not many consulting firm can afford a computer. I believe this approach now will be phased out due to the new codes (dinosaurs like me will be extinct soon). This is why some of the walls (mostly those walls that are perpendicular to the direction of the forces) not used in the earthquake model the design used is the simplified calculations with single reinforcement.

I find that I can carry out manual calculations (spreadsheets) quicker than modelling the structure (old school) with a good degree of accuracy hence I still do this for my design. The only difference now, for walls not included in the earthquake analysis, I now ensure to place reinforcements to each face of the wall even when the compressive stress is less than 3MPa and check the N* < 0.5phiNu (as rscassar stated before) to comply with the new AS3600.

In case of block walls, as Retrograde suggested 'if the AS3700 allows it' then I should be able to specify it. I guess this will be the direction I will take or just refuse to change the concrete wall to block walls.

 

[nonplussed]

RayGon, here is a recent seminar that should have some answers for you. There is an explanation of the testing regime which resulted in the latest AS3700 release.

"Infrastructure Thought Leaders Series: Masonry use in High Rise Construction"

https://www.youtube.com/watch?v=DPew-D5RBX4

 

[RayGon]

nonplussed

Thank you for sharing. It explained and answered many of the questions I have.

There is one thing that got my attention was the brief discussion of the ductility factor for a centrally reinforced blockwalls and concrete wall.

In an event of earthquake, reversal of the loads causes the heavily loaded centrally reinforced concrete walls to crack and concrete to spall and fall off. When this happen the wall is deemed to have failed and this is the basis of the As3600 to specify that the walls be reinforced on each face. Would a centrally reinforced block wall not behave the same as centrally reinforced concrete wall?

The presentation by the lady professor from QUT was very interesting specially when she mentioned that the ductility factor for singly reinforced block walls is 2 while the singly reinforced concrete wall is 1.
Considering that both material is very similar in composition and with central reinforcement why would there be difference in ductility factor.

Anybody knows the reasons or any experimental study to prove that centrally reinforced blockwall have a ductility factor of 2 and the centrally reinforced concrete wall is less than 2.

 

[QSIIN]

Raygon,

I think you might be misunderstanding the function and meaning of the ductility factors.

The whole revision of 3600-2018 was to address this misunderstanding which is largely present in Australian engineering design.

That is, the ductility factor is a function of the behaviour of the building under the full elastic earthquake load. By using 2&0.77 you the engineer are acknowledging that the tension stress in the wall is going to exceed the tension reinforcement quantity by a factor of 2.6. If your wall - masonry or RC, doesn't go into tension, then this can't be true and isn't possible-it can't behave in a ductile manner (in the sense of the steel and the ductility factors). Hence why 3600 states that short stocky walls are to be designed as non-ductile, and the full elastic earthquake load, as the short stocky walls probably aren't in tension.

The same philosophy applies to masonry. It's not a 'code' responsibility to spell out. It's an understanding of seismic response that the engineer should understand, just like the concept of gravity, and F=MA.

The issue as I see it is 3700 states that reinforced masonry can use 2&0.77 and that's it. No other considerations. No accounting for flexural or shear overstrength. Singly reinforced masonry isn't great, especially if you're not considering the increase in compression from a ductile response (if you even have one). I've already seen engineers adopt masonry over RC to avoid section 14 of AS3600-2018, which is clearly a lack of knowledge that the concept behind the ductility factors applies to both.

 

[Retrograde]

Anybody knows the reasons or any experimental study to prove that centrally reinforced blockwall have a ductility factor of 2 and the centrally reinforced concrete wall is less than 2.

I'm not sure about the concrete walls, but for masonry walls the experimental studies used to justify a ductility factor of 2 were the studies undertaken by Dr Tatheer Zahra at QUT that she discussed in the presentation.

 

[RayGon]

Thanks Retrograde.

I did a little digging up and here is what I ended up with. I hope someone can give us a better understanding of the topic.

In the Video at around the 35minute mark, Dr Tatheer, mentioned that "... singly-reinforced masonry ... has ductility factor of 2..." and that "... a singly-reinforced concrete wall is a ductility factor of 1".
However, Checking the AS 1170.4, AS3700 and AS3600 i have the following clauses.

1. AS1170.4 Table 6.5A states that a concrete structure is designed with a minimum ductility factor of μ = 2.
2. Reinforced Masonry must be designed in accordance with AS3600, unless otherwise modified in AS3700.
AS3700 Clause 8.3 – “The structural design of reinforced masonry shall be in accordance with the general principles used for reinforced concrete design”
AS3700 Clause 8.4.4.1 – “The detailing of reinforcement in reinforced masonry shall be in accordance with AS3600, except as specifically modified by clauses C.4 to 8.11, or Clause 5.9.
3. AS3600:2018 states that limited ductile or ductile walls must be provided with two layers of reinforcement for earthquake requirements (see clause 14.6.1).

Therefore, wouldn’t reinforcement on both faces of the block walls be also required? Particularly for buildings that are designed with ductility factor of 2.

A reinforced masonry wall essentially has the same infill as a concrete wall. Then, shouldn’t both RC wall and Masonry Wall have the same ductility factor? What makes a reinforced masonry wall more ductile than a concrete wall with the same reinforcement?