Two-Stage Analysis & Upper Structure Base Anchorage

[renzasaurus]

7-16 §12.2.3.2 permits the use of two-stage analysis, allowing the upper structures to be analyzed separately using equivalent lateral force method. Normally (non-two-stage analysis) mass at the base level would be assumed to dump right into the foundation (h = 0, thus Cvx = 0). My scenario is modular over concrete podium. The weight of the modular unit floor is non-insignificant and I want to anchor to the podium properly. How is this mass accounted for with regard to base anchorage in two-stage analysis then? Does my anchorage to the podium also need to be amplified by (R_upper / R_lower)? Would my upper structure "base" actually be h = height of floor framing = ~1', giving me a Cvx value (however small)?

 

[HTURKAK]

The requirements of Two-Stage Analysis Procedure are defined in ( 12.2.3.2 ) and I think pretty straight forward. However, could not see your problem despite I read twice.

Will you provide a sketch ? Is the modular unit storey is a single timber structure?

 

[Boof19]

Interesting question. The inertial forces generated by the podium level modular floor will absolutely contribute to the overall shear the anchors need to resist, to not include them in the anchorage design would be unconservative. On the other hand I would think long and hard about including that bottom floor mass for any diaphragm design on my upper stories, if that unit floor mass is large enough it's going to significantly increase your diaphragm forces for those upper levels which isn't realistic in my mind.

Would my upper structure "base" actually be h = height of floor framing = ~1'

The base of the upper structure is still and always will be the podium. The modular floor could be looked at as an additional story ~1' above base.

Does my anchorage to the podium also need to be amplified by (R_upper / R_lower)

Yes

 

[sandman21]

Anchorage should be increased by overstregnth as well.