A question about redundancy factor r

[pinestone3]

Dear experts,

I have an important question to ask about the redundancy factor r.
We are designing steel structures only in elastic range.

According to ASCE7-05 table 15.4-1 we are considering a response factor R=1 for nonbuilding moment frames similar to buildings.

As for the redundancy factor, we are considering a value r=1 (instead of r=1.3) because if we use a value r=1.3 would result in an increase of the seismic load, higher than the elastic one. (R/r =0.77). We think it's impossible.

Our client does not contest such a choice but he wants us to justify such an selection

What do you think about this and if you think it is positive, can you provide us some evidence to support our selection?

An early reply would be highly appreciated.

 

[KootK]

I vote for keeping the redundancy factor in the mix.

1) Even at R=1, you're still not designing for the MCE earthquake.

2) The trouble with the designing for the elastic earthquake is that it's damn hard to estimate that earthquake's intensity with any accuracy. And, if you're wrong about the intensity, you won't have much residual ductility lying around to cover the difference.

3) The R value and the redundancy factor are two different concepts. Apples and oranges. Dumping one because you're messing with the other seems irrational to me.

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.

 

[pinestone3]

Thank you for your quick reply.

So you think we should follow ASCE7-05 section 12.3.4 to check the value of redundancy factor?

 

[KootK]

Can you tell us a bit more about the kind of non-building structure that you're designing? What kind of a thing is it? From where does it derive it's lateral resistance?

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.

 

[pinestone3]

The structure is like a pipe rack. What do you think about the R value and redundancy factor of the structure? Thanks.

 

[KootK]

First off, cool pipe rack!

I really don't see this structure being classified as redundant. For starters, it's almost certainly extremely torsionally irregular.

Regarding the R factor:

1) I would think that you'd be table 15.4-2 rather than 15.4-1. I wouldn't consider this similar to a building

2) At worst, you could take R=1.25 for "other" structures.

3) You could probably make an argument for either an inverted pendulum (R=2) or a trussed tower (R=3).



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.

 

[pinestone3]

Thank you for your reply.

If I use table 15.4-2, the structure will be non-building structures not similar to buildings. Then according to 12.3.4.1, I can use rho=1.0, is it right?

But for pipe rack, in 15.5.2, pipe rack is taken as non-building structures similar to buildings, also in the note b, c, d of Table 15.4-1, it talks about the height limit of pipe racks. So what category pipe rack belongs to?

If we think the structure is not redundant, we should use rho=1.3, right?

 

[KootK]

If I use table 15.4-2, the structure will be non-building structures not similar to buildings. Then according to 12.3.4.1, I can use rho=1.0, is it right?

That's my interpretation

So what category pipe rack belongs to?

I think you're right, the pipe rack category sounds good to me. That will get you R=3.25 then, right?

If we think the structure is not redundant, we should use rho=1.3, right?

Yes.[pre][/pre]

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.

 

[pinestone3]

"I think you're right, the pipe rack category sounds good to me. That will get you R=3.25 then, right?"

Because the height of pipe rack is 51.5m, I think I can only use R=1.5 for my case.

For Detailing Requirements, what is the difference between AISC 360 and AISC 341? If I am designing the structure only in elastic range, I have to refer to AISC 360 code because to assume R=1.5 is equivalent to apply to the structure the “elastic” response spectra considering a minimum intrinsic internal dissipation of energy, is it correct?

 

[KootK]

I didn't realize your rack was that tall.

For Detailing Requirements, what is the difference between AISC 360 and AISC 341?

341 is ductile stuff; 360 is ordinary stuff.

If I am designing the structure only in elastic range, I have to refer to AISC 360 code because to assume R=1.5 is equivalent to apply to the structure the “elastic” response spectra considering a minimum intrinsic internal dissipation of energy, is it correct?

Yes, that is how I interpret things.

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.