Collector Design Force 2

[ChiEngr]

Hello,

I am working on a project in which I am trying to transfer about 50 kips of seismic load into a concrete shear wall. My question is with regards to the collector design forces. I have calculated the maximum force of the (3) possible forces as per Section 12.10.2.1. However, the force associated with exception #1 states that the forces calculated above need not exceed those calculated using the load combinations of Section 12.4.2.3 with seismic forces determined by Equation 12.10-3. My seismic design category is C, and thus rho = 1. Therefore, this exception ends up governing. But I am skeptical to use the exception to my benefit since it results in a force that is lower than what you would calculate using the Equivalent Lateral Force Procedure. Why would the code allow you to design the collector for forces less than those of the ELF?

As a separate question, would the amplified forces also apply to the diaphragm to collector connection? Or just to the collector design itself and its connection to the lateral force resisting element?

Thanks for your input!

 

[sticksandtriangles]

ChiEngr,

What is your Fx (ELF) vs your Fpx (diaphragm design force) at your level of interest? I only have ASCE 7-16 in front of me so some of the code references seem a bit wonky, but I would be surprised if you have a condition where Fpx is less than Fx.

Also, if you have a transfer type diaphragm, taking load from wall above out to a concrete wall, I would recommend designing for the worst case of the overall ELF response or the Fpx response. I tried to explain this in this blog post.

As a separate question, would the amplified forces also apply to the diaphragm to collector connection? Or just to the collector design itself and its connection to the lateral force resisting element?

See this post:

https://www.eng-tips.com/viewthread.cfm?qid=489226

I do not design collector to diaphragm connections for omegafied forces, but there are others that do.






S&T -

http://www.re-tug.com

 

[Deker]

The upper bound diaphragm force was initially thought to result in diaphragms that behaved elastically or with limited inelasticity, but newer research suggests that the upper bound equation falls short of ensuring elastic response. You'll notice that the alternative diaphragm provisions of ASCE 7-16 12.10.3 no longer contain an upper bound diaphragm force. The upper bound limit is non-mandatory, so you're free to design for a larger force if you deem it appropriate. That may take the form of an "in-between" solution where you design for a force that's larger than 0.4S[sub]DS[/sub]IW[sub]px[/sub] but still results in a practical connection, as opposed to designing for Ω[sub]0[/sub]F[sub]px[/sub] with an impractical connection.

I second reading through the thread that S&T linked. In general I recommend designing diaphragm to collector connections for overstrength, but at the very least you should consider the available ductility of the connections to determine whether omitting the overstrength factor is appropriate.

To add to S&T's comment about transfer diaphragms, keep in mind that transfer forces need to be considered concurrently with diaphragm intertial forces, so you can't choose to design for the worst-case of one vs the other. You may have two different analytical models to parse out transfer and intertial forces, but the results of those analyses need to be superimposed with the appropriate load factors for diaphragm and collector design.

 

[sticksandtriangles]

keep in mind that transfer forces need to be considered concurrently with diaphragm intertial forces

This might be the concept that has been missing from my design ideology for transfer diaphragms and what e95kwon was referencing with this image

Do you have a reference for transfer forces and diaphragm forces being considered concurrently?
My mind assumed that the max ELF xfer forces would not be concurrent with max inertial diaphragm forces.

 

[Deker]

A reference stating that it's required, or a reference showing how to perform the analysis? It's true that the ELF force distribution doesn't necessarily occur at the same time as the peak diaphragm response at a given level, but there's no way to account for that in an ELF based design. When a transfer diaphragm experiences its max inertial response it also clearly experiences some level of transfer force from the VLFRS above. See the code snip below that requires the two load cases to be considered concurrently. If you felt that was too conservative, you could perform a nonlinear time history analysis (which is obviously way beyond what you would do for a standard building).

When combined, the load combinations for collectors take the following form:

1. Ω[sub]0[/sub]F[sub]x[/sub] + MIN( Ω[sub]0[/sub]F[sub]px[/sub], 0.4S[sub]DS[/sub]IW[sub]px[/sub])
2. ρF[sub]x[/sub] + 0.2S[sub]DS[/sub]IW[sub]px[/sub]

For design of the diaphragm itself, you would drop the overstrength factor from F[sub]px[/sub]. Some would also drop it from F[sub]x[/sub], and the code does seem to support this for structures that do not have a Type 4 Horizontal Irregularity, but I choose to keep it in. See this thread for more discussion on that: Link.

 

[sticksandtriangles]

Thanks Deker, I glazed over the "shall be increased by overstrength before being added to the diaphragm inertial forces". That's very clear.

S&T -

http://www.re-tug.com

 

[ChiEngr]

S&T,

This is a one story building. It is around 100 years old and the owner wants to build an addition to the east. Therefore, the proposal is to demolish the existing east wall and replace with steel framing and a new concrete shear wall.

I do not have substantial experience with seismic design, but my Fpx is less than my Fx (123.1 kips vs 106 kips). The existing building has a very low R and its roof consists of hollowcore plank, so it is much heavier than conventional roof structures. For one story buildings, would you expect Fpx to exceed Fx?

 

[ChiEngr]

Also, for what it's worth, the building does not have any irregularities. I believe that would mean that I would not have to worry about amplifying the transfer forces by the overstrength factor in the case that I had a transfer diaphragm.