Minimum Horizontal Reinforcement in RC Walls EC2 1

[ruchira2580]

According to the standard EC2, It states that the minimum horizontal reinforcement of a wall to be greater of 25% of vertical reinforcement or 0.1Ac% in each face. I have attached the detailing manual code for the reference for this clause published by Istruct. But it seems like it is really unrealistic in case of high rise buildings with lengthy shear walls.
For an example let's consider a RC wall having 450x2500 mm dimensions having H40@125 c/c (per 1m length) (As= 10053 mm2/m) . So according to this standards The minimum horizontal reinforcement comes as,

Max(450x2500x0.1/100 , 10053*0.25) = Max( 1125, 2513) = 2513 mm2/m which is a larger value per face, need at least T20 horizontal bars at 100 mm c/c(Asv = 3142 mm2/mm)per face. T20 links with 100 mm means a lot.

Is this minimum is correct or I have misinterpreted it...Please explain???thanks

 

[dik]

Depends on what you are looking for... absolute minimum I use is 0.2%, prescribed temperature steel. You can use 0.5 or 0.6% for crack control and making it resistant to water penetration.

Dik

 

[ruchira2580]

dik:
I'm troubling with the 25% of vertical reinforcement not 0.5 or 0.6% of Ac. 25% of vertical reinforcement seems much higher value which end up in dense horizontal links which is usually impracticable as per the code

 

[patswfc]

My understanding of this requirement is that the horizontal bars are to act as confinement steel, to prevent the vertical bars from bursting out, similar to links in a column (min dia. of column link is 25% of the main bars). Note, if you provide the hori. rebar on the inner face, then this wont contain the vertical bars and links are then required between the vertical rebar in the wall.

40s@125 is a lot of steel, is this provided along the full length of the wall or just at the ends with smaller bars in the central zone. If just at the ends,
you would only have to provide 20s@100 horizontally at the ends, with smaller hori. rebar in the middle.

Also, what are the 40s needed for, tension or compression capacity. If tension, then the bars arent going to want to burst out, thus you could relax the requirement on the horizontal steel. If they are also needed for compression but not to their full extent, then you could possibly work out what rebar is needed for compression capacity and then provide hori. rebar based on this smaller vert. rebar dia. needed for compression, even though you will be providing larger bars (due to tension capacity requirements).

 

[rapt]

That is a column, not a wall. it is about 5% reinforcement which is higher than many recommend for columns as maximum reinforcement. H40's at 125 need to be tied if they are actually needed for strength

 

[patswfc]

I missed the dimensions of the element, I thought we were talking about a lengthy shear wall!

Rapt, to eurocodes, this is considered a wall (if longer side is more than 4 x shorter side).

ruchira2580, for this size of element just provide the hori. rebar required by the code/IStructE manual.
Also note that when As provided > 0.02 Ac, that for a wall, you will also need to provide transverse
rebar (shear links) between the vertical bars. The horizontal rebar doesn't seem to be able to be used
for this purpose?
I'd be tempted to amend your wall dimensions to turn it into a column (per code speak) to overcome these
issues, if you can get this to work.

For a lengthy shear wall, would you need 40s@125. Probably not, thus the hori. rebar requirements wouldn't be as bad.

 

[ruchira2580]

Rapt:
This is a wall not a column as patswfc said according to euro code and you can go upto 6%.
Patswfc:
I have end up in this kind of heavy vertical reinforcement because of it's demand where this highrise structure has larger spans and restrictions in wall dimensions. This wall is not an core wall it is a load bearing wall of this shear wall structure. I have adopted uniform reinforcing in the wall. So nothing can do against it's vertical reinforcement. Seems like no release on this horizontal link issue

Dear patswfc can you further explain your quote
"I'd be tempted to amend your wall dimensions to turn it into a column (per code speak) to overcome these
issues, if you can get this to work"

 

[patswfc]

ruchira2580, if you can modify the size of your "wall" to that it can be classed as a "column", then you would just need links around the main steel in the "column".

Instead of having to provide 25% of the vertical steel as horizontal steel in the "wall" you would only have to provide links. The link size only needs to be 25% of the main bar dia., thus 10s at whatever centres. This will be much less than the requirement for walls that you have calc'd above.

Because your "wall" is just outside the limits of being able to be classed as a column, it does seem a bit daft that you would have to provide all of that horizontal rebar, hence my suggestion to amend the size to make it a column as per the code. Alternatively you could use you engineering judgement and consider this a column as is, and detail it accordingly, although technically you would not need the code requirements.

I'm not sure why the 1 to 4 limit was chosen as the cut off point between a column and a wall, and if there would be potentially detrimental consequences in detailing your wall as a column (i doubt it), so you should do a bit of research on this, before/if you decide to take this approach.

 

[rapt]

With all due respect to the eminent writers of EC2, it is a column and I would reinforce it as one.

Sometimes general rules like the 4:1 ratio and minimum horizontal reinforcement area related to vertical are sometimes too general. I doubt that the thought of a wall with 5% vertical reinforcement was ever considered by those who developed this rule resulting in minimum horizontal reinforcement of 1.25%. Unfortunately there does not appear to be any commentary on this section for Eurocode to give the reasoning.

There was once a 4:1 wall rule in old Australian codes, so clever designers used 800*190 walls rather than 800*200 columns to take advantage of some concessions in wall design and detailing relative to column design. It was removed many years ago because it distorted design intents illogically! Unfortunately it is creeping back in (for Fire) because it is in Eurocode now.

 

[hetgen]

@ Ruchira,

The recommended maximum vertical reinforcement for walls according to Eurocode is 4%. But your National Annex may override this figure.

Note also that for wall with vertical reinforcement > 2% the wall should be treated as a column.

(1) In any part of a wall where the total area of the vertical reinforcement in the two faces
exceeds 0,02 A c , transverse reinforcement in the form of links should be provided in
accordance with the requirements for columns (see 9.5.3).

 

[rapt]

Hetjun,

Not that I disagree with your final decision to design it as a column but,

It does say 4% unless you can prove you can fit more in. Which is normally the same for columns.

And it is unclear if the transverse reinforcement rule is removed > 2% . It does not say design as a column, just that it requires that vertical
bars be tied as for a column.

I will see if I can find out the reasoning for the 25% rule and get back.