Anchorage of rebar for punching shear. - eurocode 2 1

[BENG_MENG_CENG]

Hi Guys,

I was wondering what the general opinion on anchorage of additional top bars which are required for punching shear however are not required for bending.

In the past I have done a full tension anchorage however this feels conservative. So does this need to be a full tension anchorage or can this be reduced down to essentially the minimum anchorage?

Thanks,

 

[HTURKAK]

I read your post a few times .. Eurocode 2 proposes the use of shear links, bent -up bars , or as an alternative , proprietary shear stud rails . Refer to Eurocode 2 (Figure 6.22) to see how the punching rebar to be laid out.

In your case , you may use top rebars as bent -up bars to resist the punching..

 

[BENG_MENG_CENG]

Hi HTURKAK.

Sorry I was probably not that clear with my initial post.

Agreed, the punching can be controlled by links, studs or bent up rails if required, however this is only required if the shear at the control perimeter exceeds the resistance of the concrete which is defined by Vrd,c.

See below link for a full worked through example.

https://structville.com/2018/06/design-example-of-punching-shear-in-slabs-eurocode-2.html

One of the parameters of Vrd,c is the reinforcement ratio which additional bars can be provided over the support in order to increase this ratio and provide a higher Vrd,c and hence avoid the need for punching reinforcement. In most circumstances this would not be economical however in the example of a thin-ish piled slab with a high load such as a warehouse it becomes more economical to provide more rebar than providing punching shear strengthening to all the piles / pile heads on a 3.5m x 3.5m grid.

So the question is more to do with the anchorage of the additional top bars beyond the point at which the basic mat reinforcement can do provide a sufficient Vrd,c against punching.

Thanks.

 

[rapt]

It would need a minimum of full tension anchorage past the point where the shear plane intersects the reinforcement, B. Similar to the diagram in 6.2.2 for Beam Shear.

 

[HTURKAK]

Hi BENG_MENG_CENG (Structural),

I got your point , however , still my question is, why you do not use half of the top bars as BENT -UP bars ?

If you check the clause 6.4.5 Punching shear resistance of slabs and column bases with shear reinforcement , Expression (6.52)

vRd,cs = 0,75 vRd,c + 1,5 (d/sr) Asw fywd,ef (1/(u1d)) sinα , the second term for reinf. contribution will be zero for horizontal bars..

But the expression ( 6.47) Punching shear resistance of slabs and column bases without shear reinforcement includes the tension reinf. contribution in order to take into account the dowel action..

vRd,c = [CRd,c k{100* ρl* fck }**(1/ 3)

So , my opinion ;

- The top tension bars shall extent from BMD to tension anchorage length ( say the length from one side , 0.25 L + 40 Φ ),

- If you use additional top bars to increase the Punching shear resistance ( rather than punching reinf. for me absurd ) , providing additional say (10 Φ ) beyond the pt. where the shear plane intersects the reinforcement would be enough for dowel action.

I have attached the relevant section of EN 1992-1-1 . ( This is code in EU and law resource , one can get the full copy free of charge from internet ).


MY OPINION ONLY!!