Collector Element Design 2

[dgengineering]

Hi,

I have a question about collector design. I use ENERCALC to generate seismic base shear and force distribution, story shears. But ENERCALC has 2 level and story forces: Fx and Fpx. Should I use Fpx, diaphragm force to design collector elements? I think if I use it the collector diagram won't balance out to 0 right? also what is the use of Fpx? I read ASCE but it's not clearly explained.

Thanks

 

[driftLimiter]

Asce 7 provides loading for collectors in ch 12. Fpx or sometimes Omega Fx. It's almost always more than Fx.

So when constructing the collector force diagram what I do is apply the amplified load and then I scale the shear wall forces by the same amount which act as reactions on the collector. This way the diagram does balance out to zero where needed.

 

[dgengineering]

My Fpx design value is lower than story shear. Should I scale the story shear down to Fpx design value? Thanks

 

[driftLimiter]

I dont think that should be possible.

For example in a single story building, equation 12.10-1 of ASCE 7-16 ends up being Fx * Wr / Wr = Fx.

Definitely don't go less than Fx for collector design. And don't forget to check Fpx min = 0.2SdsIeWpx.

 

[dgengineering]

I agree, Fx is lower then design Fpx but the story shear is higher than Fpx at the 2nd level. It's a 2 story house. Fx is the level force but I'm talking about story shear

 

[driftLimiter]

Okay my bad I failed to absorb your statement regarding story shear.

But even so the story shear isn't really relevant to diaphragm design unless you have transfer forces in the diaphrams due to discontinuities. Think about the load path, the diaphragm doesn't experience cumulative forces like the shear walls do (sans discontinuities).

Without transfer forces you need only compare Fx with Fpx. Furthermore the reaction on your collectors per floor is only based on the forces generated at that floor level.

The reason we need Fpx is because the vertical distribution of forces from the ELF procedure is based on a first mode displacement. In reality the building will have mass participation in higher modes as well. These higher modes would make less story shear and overturning in the MLFRS but higher diaphragm inertial forces. Hence the need to amplify the diaphragm force as you go down the building (Fpx >= Fx)

 

[dgengineering]

ok got it. One more question. For collector diagram we use shear wall load and diaphragm load (plf). I use story shear for my shear wall line loads but you're saying I should use Fx level force for shear wall load? Thanks

 

[driftLimiter]

No you should use Fpx level loads otherwise the collector force diagram won't be in equilibrium (won't zero out).

What I usually do is calculate Fx, then Fpx for each level. Then find the ratio Fpx/Fx for each level. Now when you go to a collector line you multiply the Fx level forces in that line by the ratio, the result are the reactions on the collector.

Clear as mud ?

 

[dgengineering]

Very clear. Thank you

 

[alt0]

Here's a similar post from a while back, may help to visualize things.