Shear Modulus of Concrete 1

[HardyParty]

What do you guys assume for the shear modulus of concrete?

I am looking at rigidity analysis in a shear wall. I am following guidance out of masonry textbook to determine the deflection of each wall and thus its relative rigidity and thus how much shear force it is likely to take.

In order to do this, you need the modulus of elasticity and shear modulus to determine deflection. For masonry, they advise using a shear modulus of 0.4 X modulus of elasticity. Is this comparable for concrete as well?

I know you can determine the shear modulus using Poissons ratio but doing testing to determine poissons seems a little excessive. Maybe I'm on the wrong track, let me know your thoughts.

 

[briancpotter]

Some conflicting equations show up. This post suggests using G = E/2(1-v), whereas this suggests using G = E/2(1+v). Possibly one of them is a typo?

Poisson's ratio for concrete typically is given as 0.18-0.2. I'd say measuring it is probably not necessary, as shear deflections tend to be a vanishingly small proportion of overall deflection.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[frv]

I believe the "+nu" is correct. However, I am quite certain that the relationship is only valid for isotropic materials, which concrete certainly is not.

 

[asixth]

I would be using the poisson's ratio rule as well and I think that is generally 0.2 for most codes. I also like to use the ACI provisions for calculations of E-modulus and a 0.7 stiffness factor to account for cracking. I'm not too sure on the derivations of the 0.7 stiffness factor.

 

[Ron]

I would use Poisson's ratio=0.15. The correct relationship is G=E/(2(1+mu)) or G=E/2.3

Using the common relationship of E=57000(f'c^0.5)......G= 24800 x (f'c^0.5); however, there are those who argue that the shear modulus doesn't change with strength...rubbish in my opinion.

As for comparison to masonry....in my opinion, the shear modulus of masonry would not be comparable to that of concrete...I would expect much lower for masonry.

 

[structSU10]

typically it follows G = E/2(1+v) in the elastic region, but once the concrete cracks there is a great reduction in the shear modulus. There is a paper "Post-cracking shear modulus of reinforced concrete membrane elements" on science direct that discusses this. It derived empirical formulas for under and over reinforced section.

 

[MJB315]

In the case of a shear wall, I'd be careful assuming outright that the shear deflection is a vanishingly small proportion of the overall deflection. That's typically true for a beam, but not for a wall.

If you're just trying to distribte lateral loads, and if you have squatty walls (very long and comparatively short), relative stiffnesses can be found using basically any reasonable shear modular assumption, as long as you're consistant overall.

"We shape our buildings, thereafter they shape us." -WSC

 

[HardyParty]

Awesome. Thanks for the help!