Does prestressing affect the natural frequency of a slab. 1

[johnmcelwee]

I have been carrying out some research and believe that prestressing a concrete member may help to reduce deflection but it does not affect the dynamic modal properties of the member. The natural frequency (1st dynamic mode) of the structure remains the same. Has anyone any comments?

 

[JoshPlumSE]

I'm not an expert in PT design. But, here are my thoughts:

The slab would be looked as an uncracked slab and would, therefore, be somewhat stiffer.

Your transformed moment of inertia would change as well... But, I'd probably ignore this and just base the dynamic properties on the gross un-cracked properties of the slab.

 

[johnmcelwee]

Thank you for your post. I imagined the slab to be stiffer also (assuming the slab is uncracked due to prestress) therefore reducing deflections which would result in different natural frequencies but using a analysis software package the results showed the normal concrete section to have the same natural frequency as the section with prestress. Still not 100% as to why this happens, any ideas?

 

[Lion06]

Did the software assume some factor to apply to I to get the natural frequency that is automatically applied to the PT slab?

Did it assume Igross for both cases?

What exactly are you looking at? If it is just for the slab, then you should see a difference. If it's for the structure, as a whole, I'm not so sure. The lateral system would dictate that. The lateral system isn't PT is it?

 

[dik]

The internal damping would be diminished and this would have an impact also...

Dik

 

[rapt]

1 PT slabs are not necessarily uncracked. PT reduces cracking in many slabs, it does not eliminate it. Once significant restraints are introduced a PT slab will crack as much as or more than an RC slab, depending on how good the designer is.

2 What stiffness does your software assign to the RC slabs. Unless you specify otherwise it is probably using Igross!

 

[johnmcelwee]

thanks for the comments, Im using General Structural Analysis (GSA) 8.4 version, the software allows me to modify the stiffness parameters of the section (i.e damping, I) but in my case I am assuming an uncracked section for both the prestressed slab and the concrete slab with prestress removed. Therefore I use Igross for each section assuming the concrete has failed when cracked? I am considering only a section of the slab to analyse, not the structure as a whole. was wondering has anyone else came across this scenario before or is there still something I am missing!!

thanks!

 

[GregLocock]

Preloading in the linear range does not affect the natural frequency of a system, it is still controlled by the ratio of stiffness to mass distribution -there is no additional term in there for internal stresses.

The reason is that in the linear range there is no additional work done against the extra force, over one cycle. What you put in on the way up you get back on the way down.




Cheers

Greg Locock


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