AS3600 - 2 layers of reo in walls - Cl 10.7.3

[blihpandgeorge]

a question on AS3600 - 11.7.3 and need to provide 2 layers of reo to walls. The cases when this is required are clear and includes when slenderness is >20. My question relates to concrete cladding panels that are not part of any structural system and are purely for cladding, such that their own design actions are transmitting wind and their own seismic mass to supports. They will also be detailed to satisfy movements of the support structure. A typical example is a cladding panel to a steel braced industrial building.

If the cladding panel spans one way vertically, slenderness can easily approach 50 with central using previous design practices. On first impression it appears this is now not possible!?

The intro to section 11.1 refers to braced walls that are subject to in plane loads (11.1a) or simultaneous inplane and out of plane (11.1b), so if a walls doesnt have inplane loads then section 11 isnt required? I appreciate that walls are not defined in the standard and that unless detailed appropriately all walls will take inplane loadings - but want to explore this on the basis that they are detailed appropriately

in short - i feel a cladding panel (that doesnt have inplane forces) can have slenderness >20 and central reo. Any other opinions would be appreciated.

thanks in advance

 

[sdz]

I'll venture to say that if a concrete panel is cladding only then it wouldn't need to be designed to those requirements.
BUT, I would question whether this is the case for a "cladding panel to a steel braced industrial building.". Are you referring to tilt up construction? Are they really cladding and don't contribute to stability of the structure?
You also need to consider
11.7.3(b) Any part of a wall structure where tension exceeds the tensile capacity of the concrete under the design ultimate loads.
It's very likely that this will be the case for a thin panel and, even if it is cladding, consider the consequences of failure under wind load.

 

[Retrograde]

I have noticed that the requirements of 11.2.1 seem to contradict those of 11.7.3.

11.2.1 Two layers required if any part of the section is subject to tension for mu = 1 and Sp = 1

11.7.3 Two layers required if the tension exceeds the tensile capacity of the concrete for mu = 1 and Sp = 0.77

Maybe I'm missing something?

 

[QSIIN]

Retrograde,

It's not a contradiction, its a conservative look at two sides of the same coin.

11.2.1 says that if a wall is completely in compression, you can use one layer of reinforcement (if within the limits of CL11.5).But you have to check this using 100% of the earthquake load (not 38.5%).

11.7.3 says that if any part of the wall goes into tension, it needs two layers, but you have to check this using 77% of the earthquake load (not 100%).

If the wall goes into tension under 77% of the earthquake load, you must provide two layers, but can proportion the tension steel to 38.5% of the load, so long as you detail compression zone boundary elements as appropriate and required.

 

[tally25]

In regards to the OP's query. Is there any official advise or discussion regarding the requirement of panels to have 2 layers of reinforcement in cladding panels? As the OP pointed out, you will almost always exceed the slenderness limit of 11.7.3(d) for cladding panels in buildings such as warehouses with traditionally have high, thin panels with a central layer of reo. This has big cost implications, especially if two or more engineers are competing to win the job and one is specifying two-layers or reo, and the other is not.

In regards to 11.7.3(b), is this intended to be applicable to out-of-plane bending stresses or just in-plane? It seems to me that this clause is intended for in-plane loads (just a hunch), but the way it is written implies tension stress from in-plane or out-of-plane loads. Again tall, thin cladding panels could exceed the tensile capacity of concrete under out-of-plane bending due to wind load (precisely why we put reo in the concrete), however if you can get the panel to work in bending with a central layer of reo, I don't know why you need to add two layers - especially for panels with little axial load in them.

It would be helpful if the code (or a revised commentary) actually explained why we need two layers of reo so the engineer can make an informed decision.

There is a bit of discussion on walls with a single layer of reo in this thread: Link

Specifically, Agent666 has some interesting things to say about about it (from a NZ code perspective).

In NZS3101, it only allows the use of singly reinforced walls under certain conditions. There are quite restrictive axial load conditions, basically restricting their use to the panels self-weight or maybe a floor. They are most definitely not intended to be used in multi-storey construction by the code authors admissions. But they never expressly said so, so people use them. The provisions are written in a way that they say if the axial load is less than xxx, then you may use the following method...

What they don't say is when you are over this load, you are not supposed to use a single layer. Unfortunately, many engineers tend to think 'well I'm over this load, therefore cannot use that provision'. So they just design it normally like any other wall. They changed the strength reduction factor a few years ago to 0.7 (from 0.85), the intent was to make the use of singly reinforced walls less attractive (rather than explicitly banning them in certain applications) and also to effectively make the walls more or less elastic at the MCE level earthquake. But designers being designers just sucked up the change and carried on as they always had not going to double reinforcement.

There is a lot of unknowns, especially around biaxial seismic loading of singly reinforced which means they should be used with caution.

From discussions I've personally had with the code authors (NZS3101), their intent was that the provisions and hence use of singly reinforced walls should be limited to warehouse type industrial buildings or cladding elements only. They should not be used in multistorey construction, but we continue to see them being used. They were intending to have some seminars over here regarding the design of singly reinforced walls to help demystify when and where their use is appropriate.

Interestingly in ACI318-19 the use of a single layer of reinforcement is tied to shear, any shear higher than just the concrete component requires a double layer of reinforcement. This is a far more definitive cut-off, which would mean in any real wall you're going to require two layers.

Regarding the use of the ducts, etc. That advice has never been codified, it was interim advice put out by SESOC following the earthquakes in 2010/2011 to address what it thought were some pertinent observations they had made on building performance. Some people take this to mean the confinement specified is no longer required at all, given it was not included in our recent amendment to NZS3101. Some argue the code has always required the confinement of laps in certain applications, and this is where that equation comes from, the confinement of laps in areas of reversing stresses greater than 0.6f_y.

 

[QSIIN]

blihpandgeorge,

I think it's fair to say 11.1 (b)(i) gets you what you need.