Can you have two layers of reinforcement in 150mm precast wall?

[Gile_]

The client wants to change the 200mmx1000mm 'gravity' blade column to 150mm thick wall and don't mind increase the length to, say 2m, if needed. Due to the ductility requirement I still need to design these 150mm walls as columns.
So do you put two layers of reinforcement in OR,
if two layers of reinforcement is hard to arrangement due to the limited space, do you put one layer only and design as columns anyway and check the curvature?

 

[Agent666]

I'd say most people would want an idea of the loads and what is being supported here and the unsupported span to give you the best possible advice on detailing and slenderness, etc.

I'm guessing you're from Australia right? You chaps are always trying to bend the rules when it comes to calling a wall a column and a column a wall. If your codes saying design it as a column are there any minimum dimensional requirements for columns. When you refer to ductility requirement, is this member subject to seismic forces?

I wouldn't even consider anything 150 wide as a "column", something with a single layer of reinforcement I definitely wouldn't be calling it a column either. A 'column' by definition usually requires considerably more confinement than a 'wall' does, a single layer offers no confinement at all.

 

[skeletron]

You won't be able to fit 2 curtains in a 150mm wall.
Agree as noted above: it's less of a column and more of a wall, and so I would design it as such.

 

[QSIIN]

Haha Agent666

That's because in the Australian code, a singly reinforced wall without ties has better capacity than a doubly reinforced confined column! And it doesn't define the difference either, so you're free to choose the one that gives you the best (cheapest) result for the contractor.

The new 3600 has helped a little, but people are still trying to find ways to exploit the code to keep their rates as competitive as they used to be!

 

[Enhineyero]

You most likely cant place two layers of reo. If your client is really pushing for this I would analyse it as a non-ductile wall, put one layer of reo and check it for earthquake loads with Sp = 0.77 and Mu = 1.0

For warehousy type of buildings this is not an issue it, but if you are designing multi-storey building then...goodluck.

 

[Gile_]

‘Unfortunately’ I am. I used 200mm thick column as I mentioned but the builder and architect both ‘want’ them to be 150mm because they are SOOOO popular by other engineers here and I don’t know how to explain to them! I find it even hard to explain to my fellows here.
I don’t think AS3600 specify the minimum ‘thickness’ for columns. The issue I have here is I don’t think I can put two layers of reo in 150mm wall. I don’t care it is called a wall or a column, but it has to be designed as the way columns are designed instead checking the compression capacity only.
I think all vertical members will subject to seismic forces and there is no such ‘gravity only’ columns that engineers here tend to think they are. When I said ductility I mean limited ductility was assumed for the whole structure so I cannot have non-ductile singly reinforced walls.
So here is what I think, if the client insists 150mm wall and I have to design it that way, with limited ductility assumed, I am going to check the moment-curvature of these singly reinforced 150mm wall. Is that worth a try?

 

[Gile_]

But a singly reinforced wall will be non-ductile

 

[Gile_]

If you look into the old standard, actually singly reinforced was not allowed when the wall is not fully in compression! I don’t know why so many engineers ignore that.

 

[Gile_]

The new standard still give no guide to member ducility but rather for the whole structure. With no ductility is assumed, there is no chance that all walls remain fully in compression, which means 150mm walls should not be used if two layer reo is not possible. I am going to keep the limited ductility assumed for the whole structure and check these 150 walls curvature but I haven’t done this before!

 

[Enhineyero]

The new standard still give no guide to member ducility but rather for the whole structure. With no ductility is assumed, there is no chance that all walls remain fully in compression, which means 150mm walls should not be used if two layer reo is not possible. I am going to keep the limited ductility assumed for the whole structure and check these 150 walls curvature but I haven’t done this before!

It does get complicated when you mix non-ductile and limited ductile walls. If 99% of your other walls are limited ductile then I will retain the EQ loads as is and amplify the loads taken the non-ductile loads by a factor of 2.0

Having tension on the 150mm wall does not automatically mean you have to put two layers of reo. There is a couple of conditions you need to satisfy to avoid two layers of reo.
1. H/L < 2
this is possible for your case 2.7m high / 2m long = 1.35) Cl. 11.2.1(b)(i) states you use strut-tie to design this wall

2. Tensile stress is not greater than f'ctf, slenderness ratio < 20, ht <20m, and other parts of Cl 11.7.3. see image below

 

[hardbutmild]

I feel like three things (in addition to what everyone else mentioned already) might potentially be a problem
1) buckling of vertical bars
2) out of plane "following" of the structure (i.e. can the wall deform out of plane to approximately 1% of storey height without failure?)
3) horizontal sliding along the construction joints

slenderness ratio < 20

I don't think this is satisfied, right? It would require an inflection point distance of less than 85 cm.

 

[Gile_]

I believe that 'H' here is the total height of the continuous wall not just one story.

This is the tricky part. Unless the whole structure model is analysed with no ductility assumed, the tensile stress cannot be used to verify this clause. But then you have to adjust your model to non ductile, which will fail for sure so you end up going back to the ductile design, which means this clause doesn't meet the condition.

 

[Gile_]

Slenderness ratio looks okay to me as the unsupported height of wall is less than 3m.

 

[hardbutmild]

Slenderness ratio looks okay to me as the unsupported height of wall is less than 3m.

What am I missing here?
Clearly the slenderness ratio is the minimum of the two (it makes sense that two layers are required because of potential out of plane displacement).
I[sub]min[/sub] = 15[sup]3[/sup]*200/12 = 56250 cm[sup]4[/sup]
A = 15*200 = 3000 cm[sup]2[/sup]
i = (56250/3000)[sup]0,5[/sup] = 4,33 cm

if l is unsupported length, according to Enh's second picture to have one layer of reinforcement you need to satisfy:
l/i < 20
l < 20*i
l < 20*4,33
l > 86,6 cm

You need unsupported length smaller than 86,6 cm, right?

 

[MIStructE_IRE]

Will a 150mm wide column satisfy fire requirements?

 

[rowingengineer]

Are you sure that another engineer will accept a 150mm thickness? Most jobs I see are a minimum 200 thick for a column/wall under a slab between carparks, the only time I see 150 is if it a cold formed joist system.

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."

 

[r13]

On this front, I think ACI is more clear - the minimum number of bars for column is 3, one on each corner. That disqualifies 150mm thick element as column (with 4 layers of steel and clear spacing/cover requirements).

 

[Gile_]

Sorry you are right. slenderness ratio will easily be greater than 20. I messed up with thickness to effective height ratio as there is another 20 limit to it as well.
Looks like 2 layers cannot be avoided anyway!
So is there any chance to put 2 layers in and if not, what is the min. one for two layers?

 

[Gile_]

According to Australian standard, with aspect ratio not smaller than 4, the answer is yes for 90min. FRL. as long as Nf* is smaller than 0.35PhiNu

 

[Gile_]

I do see a lot of singly reinforced walls either they are 150mm or not. Same as blockwall.