P Delta Analysis - On/Off

[PEinVA]

I'm working on a structural steel building 5 stories tall. I am in SDC = B, Site Class = D, R = 5, using Ordinary Concentrically Braced Frame system. I am in IBC 2003, AISC Seismic 2002, AISC 1989-ASD Method. I have my model built in RAM Frame and I cannot determine if I should be using P-Delta effects or not? How do I determine this? Please let me know ASAP. Thanks.

RC

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

http://www.tbastructures.com

 

[csd72]

There are several options, check out the following thread:

javascriptpenindex(450,450,'

http://www.tipmaster.com/includes/refinfo.cfm?w=450&h=450')

 

[csd72]

sorry that should have been

thread507-201140

 

[PEinVA]

I'm in IBC 2003 and using AISC 89-ASD. Those methods are for AISC 2005 aren't they? I'm just wondering if I have to take into account P-Delta effects or not. Thanks.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[JAE]

Under the IBC 2003 you have ASCE 7-02.

See section 9.5.2.6.2.1 and 9.5.5.7.2 of ASCE 7-02 for Pdelta effect requirements under seismic for SDC B.

The question would be what exactly does RAM do to develop their Pdelta effects within their model. Technically, there are two Pdelta effects (P[Δ] and P[δ]) the first being the overall frame Pdelta from nodal drift and the second being the Pdelta effects from column bending along its length.

AISC ASD 9th edition dosn't say anything about Pdelta except some discussion in the commentary on Chapter C.

 

[PEinVA]

JAE,
Thanks for your response. I found those sections in the code already.

The equation to check if P-Delta effects should be accounted for is:

Theta = (Px) * Delta
----------------
Vx * hsx * Cd

Px = Total Vertical Design Load at and above level x
Delta = Story drift occurring simultaneously with Vx
Vx = Seismic Shear at level x
hsx = story height
Cd = deflection amplification factor


It states Px = total vertical design load at and above level x. So as I move down the building I take a cumulative vertical load? For my building this becomes very large and since it is in the numerator of the equation, this makes my building becomes " potentially unstable" according to this.

However my building meets all drift limits by code and strength design. The design story drift listed in the equation, does that include the deflection amplification factor and it is only in the equation to bring deflection back to analyzed deflection?

This whole section doesn't seem to make much sense. Can someone help make some sense of it? Thanks.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[PEinVA]

Wait...
I figured out that the Vx is also cumulative. But inverse from the distribution of the seismic forces at each particular level. It all is clear now. Thanks guys.

RC

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[JAE]

RCraine - great. Sounds like a "doh" moment!