Modulus of Elasticity of Concrete Query

[otep_imported]

Good morning. I just want to know if the formula in aashto lrfd 2012 for elasticity of concrete which is Ec=0.043*wc^1.5*f'c^0.5 is for plain concrete only. The wc for normal weight concrete suggested by aashto is 2320kg/m^3 but for reinforced concrete we used 24kN/m^3 >> 2447.48kg/m^3. Is it right to substitute 2447.48kg/m^3 to wc and use the formula to calculate the elasticity of reinforced concrete. We will use the computed elasticity for our bridge global model in staad and my boss said that we should use 2447.48kg/m^3 to compute for the elasticity but I didn't think it is right. Please let me know if my boss' approach is right and kindly explain why. Thank you!

 

[dik]

You're splitting hairs... and design is almost never that precise.

Dik

 

[rapt]

It is the density of the concrete you are calculating. It has nothing to do with reinforcing and elasticity of reinforced concrete. And the actual value could be +- 20% from this value as Dik suggested.

The 24 you use is to calculate the self weight of the concrete and that has to allow for the higher density of the reinforcement in it. If it is very heavily reinforced, you should use a higher number than 24!

 

[otep_imported]

Thank you guys for the response! I just want to add this question. Why we are modeling reinforced concrete structures with its inertia based on gross concrete area and its elasticity based on concrete only? I think reinforcements has big effect on its stiffness and will yield to higher earthquake forces. Am I correct?

 

[dik]

Yes, until the first crack develops. It's an educated 'turkey shoot'.

Dik

 

[rapt]

For earthquake analysis, you never use even Ig unless the building is going to be uncracked under earthquake load(ie never). Much reduced inertias are used to account for the effects of cracking, normally dependant on how heavily reinforced the members is, but always less than Ig..