Vibrations:Reinforced Concrete

[cecil123]

Does vibaratory loading alter the strength of concrete and reinforced concrete compared to normal static loading?

 

[271828]

What kind of vibratory loading? Footsteps? Earthquake? Machine rotor out of balance?

 

[cecil123]

Thanks for the response.I want the effect of following vibratory loading:
1)Blast loading
2)Earthquake
3)Machine
4)and as you said, footsteps

 

[civilperson]

No.

 

[mitchelon]

The strength of the concrete is a function of the material properties.

 

[cecil123]

civilperson,

But, loading with increased strain rate does affect reinforced concrete strength?

 

[cecil123]

Am I right here:

Supposing in reinforced concrete, we have the vibrations produced due to blat loading.
Now,blast loading produces very high strain rate.And, the strength of concrete and steel bars increase significantly with increased strain rate.Right?
Am i right?
Can anyone point out the effects :
1Earthquake
2)Machine
3) footsteps
on concrete strength.

 

[271828]

Concrete is affected by loading rate. I'll leave the strength stuff to others.

For footsteps, resistance to unbalanced rotors, etc., the elastic modulus of concrete is typically taken as 35% higher than the ACI value. This is standard in the AISC DG11 and results in excellent comparisons to vibe test results for simple specimens.

 

[civilperson]

The change in strength due to increased strain rate is approximately the percentage effect of dynamic or impact loading compared to the same mass as static load. Thus, in calculations a net wash. How would you use the increase in strength for design purposes? Run a second set of calculations using a dynamic load and higher strength. AASHTO applies a impact factor to the static load for bridge design but uses the strength of reinforced concrete derived in the usual manner.

 

[NS4U]

For vibrations "technically" you should use the Icr for concrete since you theoretically have tension cracks that are expanding and contracting...

However, I have done tests that have shown using Ig is still valid and actually more accurate at predicting vibrations that Icr

 

[271828]

Mike, your second sentence is better than your first one, LOL!

I have a funny example, a 3 equal span lab specimen. It has large cracks over the supports and acts like 3 simple beams for deflection. For vibration, though, its first three natural frequencies are predicted within 2% if it's computed assuming a 3 span continuous beam. The mode shapes are almost exactly like the theoretical ones for a 3-span beam.

Vibe is a lot less like stiffness analysis than it would seem at first. It's a lot more like stability.

For example, nobody believes it at first, but an equal span continuous beam has the same natural frequency as the single span one. As one span flops up, the other flops down. Kinda reminds one of two span column buckling.

 

[cecil123]

A student in a seminar asked this question:

That, with increase in strain rate concrete strength increases as mentioned above.

However, he wanted to understand this intuitevely, that as the strain rate is fast the material is becoming stronger?Can anyone give an intuitive/physical understanding for this?

 

[civilperson]

Take a series of equal strength test cylinders. Load one set at the prescribed rate, 1000 psi increase per second. Load the second at 3000 psi increase per second and the third set at 9000 psi increase per second. The resultant strength of compression will measure higher for the faster loading rates. I think of it as a function of failure lagging the imposed load, so that the failure is occuring from an earlier stress which is lower than the gauge reading but takes some finite time to propagate across the cylinder dimensions.