Redundancy for transfer beam supporting mid-rise tower column

[hetgen]

Hi good people,

How do you account for redundancy/robustness when designing a mega transfer beam supporting a multi-story building column?

I'm reviewing a 20-storey concrete frame building with a 50ft span transfer beam on the 2nd-floor level. The building structural geometry is very basic and the structural elements are conservatively designed.
The transfer beam is a critical element for the stability of the whole structure but it is a single span beam with no redundancy for alternative load path.

We could perhaps overdesign the transfer beam considering that it's a significant element, but I'm not sure if that count as adding redundancy and I'm also not sure by how much we have to overdesign.

N.B...The building is located in a non-seismic area.


Many thanks.

 

[DETstru]

A lot depends on the system, the connections, the confidence of the engineer, etc.

My 2 cents:
I often design transfers with a fair bit of conservatism. The big issue, in my book, is deflection/serviceability. On all the floors above you've assumed that the transferred column is just a normal column that does not deflect. When it lands on a transfer beam or transfer slab, that isn't necessarily true. So I try to design with as little deflection as is feasible, and that often results in a very conservative design for strength. This is even more important when you have transfers on transfers. The deflection quickly adds up, especially when you include long term effects.

 

[JAE]

For this building the stay cables were non-redundant and the design used added safety factors on the cables and cable connections to create a pseudo redundancy:

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[Deker]

Generally redundancy is achieved by providing continuity (indeterminacy) in our structures so that the forces can redistribute in the event of a local failure. In your case, you might consider extending the transfer beam so that it is continuous over multiple columns. This would also help with the deflection issues that DETstru brought up.

 

[ajos6183]

Hi hetgen,

This is what the Australian Building Code calls for. It sounds that you are not from Australia but this might be a good starting point.

 

[hetgen]

@ ajos6183 Thank you.

Does the Australian code give guidance on how to conduct a systematic risk assessment for critical components?

The transfer beam doesn't carry 25% of the total structure weight, but it is certainly a critical component.

@ JAE " The Guru!"

Thanks for the example....

Is there any guidance on the added factor of safety required to create a pseudo redundancy or is it normally a judgment-call by the designer?

 

[JAE]

We just used 2.0 and I think we even then kicked it up a bit.
No guidance other than general engineering judgement.

I recall also that after the Kansas City Hyatt Regency walkway collapse there was another project in a city where I saw the engineer interviewed on TV and he was designing a similar building with a hung walkway and he mentioned a safety factor of 10.0. Not sure if he was pandering to the public to comfort them that his walkway was going to be OK.

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[ajos6183]

To the best of my knowledge, the Australian code does not give any guidance on how to conduct a systematic risk assessment.

 

[KootK]

A couple of strategies not yet mentioned explicitly.

1) Ideally, each floor would be viable in a temporary, ULS sense with the transferred columns no longer in play. This is a strong form of redundancy.

2) Use staged, harped, bonded post-tensioning in the transfer girder. Then add reinforcing capable of holding up the transferred column all by itself. In a sense, it's like having two beams. You also get some serious benefits with respect to deflection control. This is a weaker form of redundancy, of course, since an explosion taking out the beam may well neuter both load paths. Additionally, the reliability of the beam is no better than the reliability of the columns supporting it.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[patswfc]

Agree with KootK, that each floor above should try to be designed and detailed to "stand up" by itself using catenary action (large deflections), to avoid disproportionate collapse.

 

[MrHershey]

What you're describing isn't a code requirement for most buildings. Structural engineers are of course free to include as many alternate load paths as they'd like in their designs and perhaps may be prudent or even best practice to do so in some cases. I know we've done it for critical elements before.

That said, for Risk Category III/IV high rise buildings (>75' tall per IBC 2015), extra ties to prevent the sort of progressive/disproportionate collapse you're concerned about are actually required. In IBC 2015, this is Section 1615. Also typically required for US Military buildings 3 floors and taller, though they have their own requirements outside of the IBC. This section may be worth exploring if you'd like to include some extra redundancy, even if it may not technically be required.