Superimposed Beams

[Defecto Perfecto]

Hi All,

 

 

I would really be happy to get some reference or any standard specifications including ACI for the design and calculation of superimposed beams.

 

Hope you are all doing good.

 

Regards,

 

Waqas

[UmarMakhzumi]

Waqas,

 

Please describe what superimposed beams are. My guess is that you are talking about a new beam that will "bear" on an existing one? Is this understanding correct?

 

 

Thanks.

[Defecto Perfecto]

AOA Sir. Umar,

 

The condition is as follow,

 

We placed concrete for a circular Wind Turbine Foundation of Ø19m, but after concrete activity, client noticed a pour line at the edge of the foundation (horizontal direction), which according to their understanding is COLD JOINT. Now, our lender engineer want us to submit some calculation indicating that the COLD JOINT is not detrimental for structural integrity.

 

Our engineers, could simulate such situation through some Russian or Chinese standards, considering as a superimposed beam, but they are trying to figure this problem out using ACI.

 

Now the nut shell is, our engineers need to show off the lender engineers that if this is cold joint, will it still be able to take the load of wind turbine for future.

 

Would need your assistant in this regard.

 

Regards,

 

Waqas

[UmarMakhzumi]

Ws.

I can see the concern of the Lender's Engineer. For a turbine foundation, with an unintentional cold joint, he may be worried about vibrations and subsequent crack propagation. Well the irony is that you can't really have machine foundation poured in a single unit (unless its a simple small square pad). It has to has construction joints or sometimes unwanted cold joints.

Given the nature of problem, a super-imposed beam would not add any value to given problem. This approach shall make sense if you only think inside static design bubble. However, considering the limit states for turbine foundations, the addition of beam may not help. Here are two solutions:

1) Static Design:

Your engineers should demonstrate that enough reinforcement is present and crack control is not a problem. I have done huge compressor foundations and have allowed construction joints where required. Joints are inevitable and shouldn't be any problem. The impact of a cold joint on machine foundation is somewhat hard to assess as there are so many factors involved-quality of joint, location of joint, support framing . The above stated solution is an acceptable one if the dynamic loads being produced by turbine are very small- dynamics analysis wouldn't be of any significance.

2) For significant dynamic loads:

This solution is applicable if your turbine dynamic loads are significant enough to affect foundation response. Talk to your engineers to see what limit states governed foundation design.

If your foundation is table top, re-check the foundation using reduced section properties(since concrete is cracked). Normally, machine foundation design(other than table tops) is based on the assumption of rigid block and no one would consider reduced properties when checking response of foundation against dynamic loads. If your engineer re-checks the foundation based on reduced section properties(If foundation is table top) and amplitudes are still below the maximum limit, and there are no resonance issues, you shouldn't require any modification at all. However, You still need to add the crack control check to ensure that enough rebar has been provided and cracks won't propagate.

Don't float this idea to the Client unless someone who designed the foundation has actually done the check. Unless the foundation is excessively over-designed, this condition shall be very hard to meet. I will also discuss this with dynamic analysis subject matter expert and update you on what he thinks. #2 is a conservative way and is just one of many approaches out there.

So, adding a beam would do nothing to your wind turbine foundation. If the beam being added is a huge beam, it would add some/ little mass and damping to system but the response of the beam is not related to what your question is about. The question is also about what kind of dynamic loads are present. I have never seen any vendor data of wind turbines and can't say about kind of dynamic loads produced by them. If your dynamic loads are significant and foundation type is table top, see point 2 else point 1. I hope this helps.

Keep the updates coming.

Thanks.

Update: Regarding Point #2, for table top machines, reduced Section properties should be used. For rigid blocks, cold joint will not have any significant affect as foundation will be most likely in compression.

Edited April 15, 2015 by Umar Makhzumi

[Defecto Perfecto]

AOA

 

Thank you for you response, I have shared your suggestions with my engineering department.

 

I will update you after finding remarks from them.

 

Kind regards,

 

Waqas

[UmarMakhzumi]

W.s

So the SME says that if there is only compression, the cold joint shouldn't be an issue. But if there is moment in the foundation at cold joint location then reduced stiffness properties shall be considered. I don't know what the foundation framing is? Can you share that- probably a line diagram?

Thanks.

[Defecto Perfecto]

AOA,

 

I have shared all suggestions with my engineering department, and they have prepared a report and submitted to lender engineer for review purposes.

 

Will contact you as soon I get some remarks.

 

Regards,

 

Waqas