Seismic Load Diaphragm Deflections

[IngeIvan]

Hi,

I'm calculating the lateral deflections of my flexible (metal deck) diaphragm. My question is:

1. Do we have to use the Amplification factor and calculate inelastic deflections like it is done with the main seismic force resisting system?

2. If we don't have to apply Cd, is the diaphragm going to be okay being designed for a force that is dependent on the magnitude of the Response Modification Factor of the system? In other words, for big R's, the diaphragm is designed for smaller forces, and viceversa.

3. I guess the main question is, do we want the diaphragm to behave elastically, if so, do we want to use overstrength factors for the unit shears checks???

Thanks! Any help or reference will help!

 

[DaveAtkins]

1. I believe the answer is no. The large deformations required to reduce seismic force are in the frames, not the diaphragm. In other words, the seismic force is relieved before it reaches the diaphragm.

2. I believe the answer is yes--which is the reason for overstrength factors, dependent on SDC. If this were not true, you would need to design for R = 1, which is unreasonable.

3. You must design using the overstrength factors required by code--depending on the SDC, they apply to various parts of the diaphragm, as I recall.

DaveAtkins

 

[wannabeSE]

1. No - see Dave's response
2. The same R is used as what is used for the base shear. However, the diaphragm forces are calculated different than the story shear. If using ASCE 7, see section 12.10.
3. Overstrength is not typically used for the deck, but is often required for collectors and their connections. Again, for ASCE 7, see section 12.10 for when the overstrength factor is required.

 

[IngeIvan]

Thank you for your comments, I have a few other questions based on your responses:

The seismic force that is generated in the building is assumed to be distributed to the lateral resisting systems by the Diaphragm, so the seismic force will go from the diaphragm and into the MSFRS isn't it? So in my mind, I would think that the diaphragm needs to withstand and not yield/fail until the forces have made it into the MSFRS so that the system performs as desired and dissipate all the energy (i.e. yielding of braces in special braced frames).

Any comments about this?

 

[KootK]

Any comments about this?

Yeah, your statement is backwards. Remember, there's no such thing as an externally applied seismic force. There're only externally applied seismic displacements which, in concert with inertial effects, create internal member forces. So the seismic effect starts at the earth and works it's way up to the diaphragms, not the other way around.

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.

 

[KootK]

It is implied in the codes provisions, although not explicitly stated, that diaphragms should remain essentially elastic. After all, if diaphragms yielded early in the seismic load history, VLFRS elements would start responding independently and all hell would break loose. Rationally, it would seem to make sense for all diaphragm components to be designed with over strength in order to keep them reliably elastic.

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.

 

[IngeIvan]

Yeah, I would agree it needs to stay elastic, although it is not well specified in the code.

And seismic forces are inertial forces generated where there is a mass in the structure due to the ground acceleration. So the forces are generated at the diaphragm elevations and then work their way down into the lateral resisting elements, right? or am I really that wrong about this?

 

[KootK]

Yeah, I would agree it needs to stay elastic, although it is not well specified in the code.

I'm pretty sure that the lack of OS diaphragm design has something to do with the difference between Fx and Fpx like WannabeSE implied. Peak single floor Fpx forces happen at different instants in time than peak whole building Fx forces which don't actually even represent a real expected condition. The part that baffles me is how R can be relied upon to reduce diaphragm forces that occur due to higher mode effects when plastic hinging may not actually accompany those higher mode effects. I know... at some point it devolves into a Schrodinger's cat kind of conversation.

or am I really that wrong about this?

I'm afraid so. Google D'Alembert's principle of pseudo acceleration force. Something like that. You probably learned it in dynamics but filed it under voodoo nonsense long ago. Pseudo being science speak for utterly imaginary of course. Don't beat yourself up over this. I was 3-4 years into practice before I clued in. In retrospect, you'll likely find the notion of an externally applied seismic force quite ridiculous. I mean, what could the source of that external lateral load possibly be? It's not as though there are little seismo-fairies up there flapping their tiny wings and pushing your buildings around.

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.

 

[wannabeSE]

I have a somewhat intuitive understanding of why the entire diaphragm is not designed with Ω[sub]0[/sub]. So, I am not prepared to explain myself. But, if you have questions on how to apply ASCE 7 requirements, NERHP has several technical briefs on diaphragm design

http://www.nehrp.gov/library/techbriefs.htm.

Unfortunately, they don't have a tech brief specifically for bare metal decks. But, the other briefs may answer many questions that are not material specific. I just noticed they recently added one for wood diaphragms.