Shear on an interior shearwall with a collector will include Fpx on top of Fx?

[LockeBT]

Question is based on the text "Design of Wood Structures" by Donald Bryer.

In one example of a box-shaped 2-story building with stacked interior shearwalls, the text mentioned that the shearwall below will take the lateral force from the shearwall directly above it IN ADDITION to the shear force from the diaphragm (via a collector).

Usually I never use the diaphragm force, Fpx, to design shearwalls, only Fx. However it seems when you have a collector tying into a shearwall, that shearwall will now also take the force transferred by said collector (ie a force derived from Fpx) ON TOP OF the Fx force based on its tributary.

Appreciate any clarifications.

Edit: Picture for clarity

 

[JoshPlumSE]

Look at it this way:
1) You calculate the values for the main lateral force resisting system based on ASCE 7-2016 section 12.8. Right? Part of this is diaphragm forces that get into the shear wall and such.

2) Per ASCE section 12.10 (Diaphragms, chords and collectors) you have to make sure your diaphragm design forces are at least equal to the Fpx forces. This is to ensure that any localized inertial effects that are greater then what you did from the main lateral force resisting system do not inhibit the desire ductility of the overall structure. However, this only applies to the diaphragm itself and the chords and collectors.

3) The shear wall is NOT considered to be a chord or collector. However, the CONNECTION between the shear wall and the diaphragm or chord would (in my opinion) have to be designed to this elevated (or potentially elevated) Fpx force.

Does that make it any more clear? I think of this kind of like in steel where we design connections for an elevated force to ensure ductility of the oveall system.

 

[KootK]

I agree with JP's response. Summarizing:

1) In my opinion, you do not have to design the low wall for Fpx. Fx is the appropriate value.

2) Yes, intellectually, it feels a bit weird to not design the wall for the load being delivered by it by collector. That whole "load path" thing.

3) Some of the reasons for the disconnect are:

a) We are generally designing the diaphragm and it's connections to remain elastic. That is not true of the walls.

b) Our estimation of seismic forces is very much a statistical/probabilistic thing. In this case, the salient things to consider in that regard include:

i) Tigher mode effects mean that the force experienced by any one diaphragm may exceed that calculated using ELF which assumes first mode behavior with some modification for higher mode effects.

ii) To apply the higher diaphragm loads from to all level of a building will be punitive in some instances. This tends to be more the case the more floor levels are involved.

 

[LockeBT]

Very much appreciated for the response guys.

KootK, on top of your 2) point it also feels weird when the force delivered by the collector into your shearwall (a force based on Fpx) is HIGHER than the force distributed in that shearwall based on Fx. This is often the case for me whenever the redundancy factor, rho, is 1. Also, as you mentioned, the Fpx force is typically higher than the Fx story shear for a given story.

To reiterate both your points to make sure I understand:
1) Design your shearwalls for based on the distribution of Fx
2) Design your diaphragm, collector and its connection using Fpx
3) When constructing the collector loading diagram (shown below), all the shear loading (even the shear in the shearwall) is based on Fpx. That shear is NOT used to to design your shearwall, but rather to establish the loading diagram to determine the maximum collector force.

 

[KootK]

Sounds good to me LockeBT.