short building & short period, R=1 only?

[ottles]

https://www.ideals.illinois.edu/bitstream/handle/2142/17027/1_Zafar_Adeel.pdf?sequence=4

"They concluded that for a structure of a natural period less than 0.2 second (short period structures), the ductility does not help in reducing the response of the structure. Hence, for such structures, no ductility reduction factor should be used."

Does it mean for short building like 1 to 2 storey. Making it R=8 special moment frame would be useless because the period would be too short for the ductility to be engaged? Are they saying that to design short period buildings.. you need R=1 even for seismic location (meaning making the column, beam, foundation bigger size sections to resist seismic forces at service elastic limit?) But this would increase the cost enough to build higher storeys.

For short period building. If ductility can't be deployed because of the short period, then how do you strengthen short period building in seismic location?

 

[MrHershey]

Would keep in mind that the 'they' he's talking about is a couple of papers from the '70s and '80s. That's not current practice. Also annoying that he didn't actually include those papers in the references.

Digging a bit further into other references it appears this is primarily a computational concern right now. It's a possibility based on mathematical/computer models but there are very, very few real world examples of buildings with periods this low behaving poorly during an earthquake specifically because their period is low (and not because they were detailed and/or constructed poorly).

 

[ottles]

Would keep in mind that the 'they' he's talking about is a couple of papers from the '70s and '80s. That's not current practice. Also annoying that he didn't actually include those papers in the references.

Digging a bit further into other references it appears this is primarily a computational concern right now. It's a possibility based on mathematical/computer models but there are very, very few real world examples of buildings with periods this low behaving poorly during an earthquake specifically because their period is low (and not because they were detailed and/or constructed poorly).

Do you mean short period building is excellent for earthquake? Hope you can address directly my questions in the original message because I don't understand what you mean.

I'm asking whether you need to design short period building directly for the earthquake forces (R=1) rather than focus on the building ductility as we do in special moment frame (R=8). Thank you.

 

[MrHershey]

I'm not saying they're good or bad. I'm saying that there's close to zero real world evidence that extreme short period structures perform poorly during earthquakes specifically because they have extremely short periods. Computer models show that extreme short period structures may not be able to behave as ductilely as intended and thus would have a higher probability of collapse, but real world evidence that this is actually a problem does not exist, at least as far as I can find. You would think we'd have some more evidence by now given the number of low period structures is much higher worldwide than the number of longer period structures.

For this reason current codes (edit for clarity: US codes) do not require a reduction in the R factor for extreme short periods.

Another reference if you're interested: [URL unfurl="true"]http://www.iitk.ac.in/nicee/wcee/article/WCEE2012_5015.pdf[/url]

And a quote from that reference:
Given the lack of clear evidence that short-period systems are problematic outside of the computational/theoretical area, it seems unwise to proceed with a recommendation to make significant adjustments to R for short period systems. However, adjustments to the computed deflection of short period systems might be warranted. Thus, the principal preliminary recommendation is to make no modification to R but to further develop a period dependent relationship for Cds.

 

[ottles]

Something confuses me. Ductile building means the beam can form plastic hinges.. but in short period building.. the beams don't even rotate because the period is short and there is not much storey shear.. the whole building moves.. so I don't understand what the paper means when it mentions "For these very short period systems, ductility demands become very large and are impossible to accommodate with traditional detailing".

To building strong building that respond elastically. You need big member sizes and sufficient rebars. Ductile means forming plastic hinges.. and these don't occur if the building can respond elastically.. so why is ductility demands very large for short period building. I'm kinda confused.

 

[ottles]

to continue the above.. remember that

R=8 means one focus on moment capacity of the members (enough to form plastic hinges) or ductile

R=1 means one focus on strength or elastic

Now the paper says "For these very short period systems, ductility demands become very large and are impossible to accommodate with traditional detailing"

why is ductility or moment capacity demands very large for short period building when they have lessened joint rotations.. are you saying short period buildings can rotate into plastic hinges even more than long storey building??

 

[wannabeSE]

ottles,
I didn't read the article. I just skimmed portions of it. But, I thought I would mention a couple things.
1. It is written by a student in a master's program (PhD or post-doctorate would hold more weight).
2. I think the "traditional detailing" is the traditional concrete detailing used in Pakistan. I didn't see a description of the detailing in paper. This "traditional detailing" probably doesn't come close to special moment frames requirements in ACI 318 or other codes with specific seismic detailing requirements. Does it even meet ACI requirements for non-seismic beams and columns?

 

[ottles]

wannabeSE, I'm referring to MrHeshey message where he said that "Computer models show that extreme short period structures may not be able to behave as ductilely as intended"... this means they need to behave at service load elastic range. But the paper mentions that "For these very short period systems, ductility demands become very large and are impossible to accommodate with traditional detailing". Can you see the conflict in the two statements?

Someone, please show whether short period structures have more elastic or ductile demands...

ductile means the rebars are beyond yield point
elastic means the rebars are below yield point

what particular structural member is the paper describing about much larger ductility demands for short period system? I've thought for this for over 3 hours and still confused.

 

[ottles]

to add to the above comment.. short period structure are very stiff.. so there is minimal drift and not much storey shear to make the joint rotates and creating plastic hinges.. so why did MrHershey paper says there is much larger ductile demands for short period structure when it is very stiff and there should be more elastic demand than ductile.. can anyone explain this.. thank you.

 

[ottles]

I read this...

http://peer.berkeley.edu/tbi/wp-con...t_Requirements_For_Long-Period_Structures.pdf

"By increasing the stiffness of low-rise structures, the strength of these structures is generally
increased, and ductility demands on these structures as they struggle to escape the constant acceleration
region of the response spectrum are generally decreased."

It's really true that if you increase the elastic demand (or stiffness?) of the short period building.. the ductility demands decreased. Maybe the reason short period structures have more ductility demands is because it's assumed they assumed it's not very stiff?