Enhanced Shear capacity of concrete

[e104909]

Hi,

I have been checking concrete panels which resists both lateral earth and water pressure. It is a bridging panel of 1m width and 2m length.

Analyzing this as simple beam of 1m strip and 2m length of 800mm deep, it fails in shear. Ultimate shear is 850kN.

Now, I havent taken into account the enhanced shear stated in 3.4.5 of BS code.

Knowing that if shear failure is assumed acting at 30 degrees from the face of the support, shear will overlapped.

800mm/tan30 = 1300mm > 1000mm (half of beam length in which is know to have a zero shear)

Is this enhanced shear still applicable? can someone elaborate further?

Also, knowing that my beam falls under deep beam category and shear is located at less than 2d from the support.

Hoping for an expert point of view..

Thanks and Merry Christmas to all


Regards,
E104909

 

[asixth]

I would think the enhanced shear capacity is only applicable when a compression strut is developed within that zone (this is when the direction of the force is acting towards the support such as a beam supported by a column subject to gravity loads).

For your case, it sounds like horizontal links need to be provided for additional shear capacity. The V/A=1.0MPa so it is possible to use shear reinforcement to design for this.

 

[e104909]

Can you prove it mathematically? I actually have fighting for this in our meeting.

But my supervisor keeps on saying that it should be enhanced and with this beam that i have should not have any shear links requirement. This is because the original design that has been submitted already and our working drawings has been finalized. Yet having this kind of situation makes it too difficult to prove that it needs to have shear links. But he keeps on insisting that it should be enhanced.

I wanted to fully grasp the theory behind. Do you have any reference or journal that tackles about this? I remember 5 years ago ACI released a new design approach for shear design including deep beams. But i badly cannot find it

If this is considered as deep beam then i believe no shear links required and should detail the reinforcement carefully by placing horizontal and vertical bars in right location through strut and tie method. But i am not really experienced doing this deep beams. Please give me advise where to find answers?

But i wanted to know..does enhanced shear really applicable?

Regards,
E104909

 

[e104909]

Sir's,

Anyone who can explain further?

Comments and suggestions are greatly appreciated.

Thanks


Regards,
E104909

 

[Teguci]

CRSI covers this under their pile cap section. I think it tops out at 8 root f'c (shear at 45 degs is 2 root f'c).

If you're using ACI, the strut tie model is the place. Unfortunately, our BS is different than your BS.

 

[rapt]

It is a deep beam. They always have enhanced shear. Enhanced shear section in the code is for flexural members. This one is not!

 

[e104909]

Enhanced shear might have some limitation. This is not so clear in the code and unluckily my superior always tell me to multiply the shear capacity whenever it fails and he says that this could be enhanced. But he cannot explain and will just throw all the problem to me. In ACI a distance of 2d is where you have to enhance your shear..after that you can just put minimum. As per my superior, you can take advantage the enhancement by multiplying by 2. And this makes me confused

Regards,
E104909

 

[IDS]

Why not do it properly with a stut and tie analysis, rather than trying to apply the beam provisions where they do not apply?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[ishvaaag]

I agree with IDS. So you need compliant struts, ... since simple span as modular, tie rebar to get the equilibrium in the arcing action (and good transfer to the tie of the arch push) and on the limited design values of strength allowed by the code normally no mechanical crack should develop even at the factored level.

You can help to ascertain the same by making a 3D solid model (since this is a very simple one) and inspecting the stresses appearing, at least have a look for shear and tensile principal stresses in the concrete.

 

[Nowaday]

Mr e10..,

I think you can write the full description of the wall here, and let us see if we can help you, cause I know for sure that most of us has shear wall spreadsheets for design against wind. We can just adjust what we have to give you an estimate.

The previous comments are right, I dont support the method of designing wall as a beam, the old way sounds better and efficient.

Also know that it is wise to design the shear wall with the strenght of concrete only(like there was no reinforcement), then at the end you add minimum reinforcement on your final design.