2009 NYC Building Code 1620.2.5

[ship4885]

Section 1620.2.5 states we must design the diaphragm for the force Fp = 0.2I*Sds*Wp + Vpx, where Vpx is defined as:

"The portion of the seismic shear force at the level of the diaphragm, required to be transferred to the components of the vertical seismic-force-resisting system because of the offsets or changes in stiffness of the vertical components above or below the diaphragm."

Can anyone enlighten me to exactly what Vpx is because I do not understand their definition.

What I originally was going to design the diaphragm and collector elements for is my Story Shear output from ETABS divided by the length of my diaphragm to create the line load(times my over strength factor for collector elements) until I looked further into the NYC code.

 

[SteelPE]

This is the same Vpx that is located in the 2003 IBC building code (1620.2.5).

IBC 2006 defaulted to ASCE7-05 section 12.10.1.1. Although Vpx is not in the equation, the description is still in the description below the equation.

I always though this provision was to be use for bracing systems that are offset as you drive your forces down your frame and into the ground. You might have a brace bay that has to move from one gridline to another due to architectural reasons. The force that is being transferred must be accounted for in the diaphragm.

 

[ship4885]

Thank you, now I understand. If for example, on level 3 my brace frame is 20 ft away from the frame on the rest of the levels, the forces from the rbaced frame must transfer through the slab above and below the offset braces in order to continue down.

We do not have this condition of vertical offsets therefore my Vpx is zero.

Thanks!

 

[SteelPE]

You can also get a transfer of force if you have changes in the stiffness of the lateral force resisting system

Say you have a moment frame on levels 1-10 and you have a shear walls on level 1-5. Since the stiffness of a shear wall is much greater than a moment frame you will end up with a large Vpx on level 5 (Force comming out of the moment frame and into the shear walls).

 

[ship4885]

Great point! We have brace frames on the interior of the building with exterior moment frames.