Shear resistance of RC members 1

[mats12]

Im from Europe so I ll be refering to Eurocodes.

Lets say we are designing RC beam.

This beam has shear resistance without shear reinforcement. Lets call it VRd,c.
If design shear forces (lets say VEd) are greater than this value VRd,c then we have to design a shear reinforcement.
When we are designing a shear reinforcement (because VEd is greater than VRd,c) we are designing stirrups in assumption that they take over the whole shear force (whole VEd).

Does that mean that shear bearing capacity of a RC member is very conservative and is in reality much greater: CONCRETE SHEAR RESISTANCE (VRd,c) + STIRRUPS (VRd,s)?

Im confused because we either say that shear bearing capacity is equal to VRd,c (if VEd is smaller than VRd,c) or we say that shear bearing capacity is equal to VRd,s (stirrups) when in reality we have both?

Also I am wondering, are vertical stirrups consider to be in shear or in tension? I think they are in tension since EC2 is saying that the design of members with shear reinforcement is based on a truss model.

 

[KootK]

CONCRETE SHEAR RESISTANCE (VRd,c) + STIRRUPS (VRd,s)?

This is certainly how we do it in North America except in some high seismic situations. I'm surprised to hear that it's so different in Europe. Can you share the stirrup equation with us?

Also I am wondering, are vertical stirrups consider to be in shear or in tension? I think they are in tension since EC2 is saying that the design of members with shear reinforcement is based on a truss model.

That's right.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[atrizzy]

In Canada we certainly consider BOTH the shear resistance of the concrete and associated reinforcing acting together.
Sounds like from your description, the second you put in stirrups your total shear resistance is equal to that of the stirrups only? That doesn't make sense to me.

 

[KootK]

Well, the minute that shear cracks develop, you lose diagonal tension capacity over those cracks. You still have a concrete shear capacity but it's based on other mechanisms. I'm wondering if those are simply accounted for in other terms of the shear capacity equations in Europe.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mats12]

Sounds like from your description, the second you put in stirrups your total shear resistance is equal to that of the stirrups only?
thats excatly right!

someone from Europe should post here just for clarification...

KootK I will post a screenshot of equations.

 

[mats12]

 

[Guest262653]

The ENV version of EN1992 considered the concrete contribution, the EN version dropped it and Model Code 2010 changes these provisions again to VRd = VRd,c + VRd,s (see section 7.3.3). Probably the next generation of Eurocodes will address it again.

The background for the new MC2010 provisions is presented in the following link:

https://www.researchgate.net/public...010_shear_provisions_-_Part_I_Beams_and_slabs

Some additional info:

http://www.betonica.be/media/fib_2016/5_Mihaylov_fib_colloq_2016.pdf

 

[rapt]

Mats12,

Yes, in Eurocode, when Ved > Ved.c, Ved.c is set to zero and the full shear is taken by the stirrups in tension (less the vertical component of tendons in PT members).

But the minimum shear angle is about 21.7 degrees, increasing to about 45 degrees or more for very heavily loaded members. Because of this low angle, a lot more stirrups cross one shear plane than is generally assumed. The result generally is supposed to be similar to other approaches according to Walraven whose group developed the theory for the Eurocode committee.

The case where it is really wrong is pre-tensioned members, which have the same shear requirements as RC members once Ved > Ved.c as generally they have no vertical component of PT force as the strands are horizontal.