Modeling to capture backstay effect

[bvass]

Hi All,

I'm currently working on a 19 story core wall building with 2 levels of below grade parking. I've been researching the backstay effect for some time now and am trying to capture its effects in design. I'm using ETABS as my structural modeling program. In order to design for the shear reversal that will happen at level 1, I've modeled in my basement walls around the perimeter. Right now, I've got my diaphragm at this level modeled as a rigid membrane. I've read that a semi-rigid diaphragm should be used in lieu of rigid, which I plan to change in the future, however right now I'm only trying to capture the backstay effect.

What I am seeing in the story shear response plot is no reversal whatsoever at level 1. This is counter-intuitive because the rigid diaphragm stiffness in conjunction with the in-plane stiffness of the basement shear walls (10" thick) should provide a restraint on the core walls to reverse the shear load. I don't have any compression only lateral springs modeled to account for the passive resistance provided by the soil on the walls perpendicular to the shear force, however I've read in the PEER-ATC-72-1_report that these springs can mostly be neglected as their stiffness isn't comparable to that of the in-plane stiffness provided by the basement walls. The walls extend down to level P2 and span between piers/pier caps that I have modeled as pins.

Has anyone modeled to capture this effect before that can provide me with some insight as to why I'm not seeing the reversal? I've attached the story response plot so you can see what I'm talking about.

Thanks in advance for your help.

 

[Agent666]

The storey force plot is the total seismic or wind Lateral force envelope, so you won't see the reversal of shear on the global plots. You need to look at the individual pier shear force for this, it's a local effect if you like in the individual wall(s).

The reason why you won't see the reversal in the global forces is the shear in the internal wall(s) is in an opposite direction to those on the perimeter walls. They cancel and the difference is the force applied at the level above (I.e. The force being applied at that diaphragm).

Do change to a semi-rigid diaphragm and ensure you do a sensitivity check on diaphragm stiffness. Rigid is just that, it's not realistic at all when considering this effect.