P-Delta Effects 1

[JrStructuralEng]

Anyone have any documentation on simplified approaches to P-Delta effects, particularly with respect to 3 or 4 story wood framed buildings. (I am hoping for a simplified approach or a document that explains the pseudo load approach better)

 

[Lion06]

I don't know of a simplified document, but P-big delta effects are typically not significant for shear walls or braced frames. I suppose there could be some slight increase in the chord members of the shearwall, but I don't see it as an issue.

P-big delta effects are typically more of a concern in moment frames. P-little delta effects should always be investigated, but this isn't dependent on number of stories or lateral system.

I'll be interested to hear others' opinions on this.

 

[paddingtongreen]

As far as I know, it used to be that we assumed a floor drift in each direction, multiplied them by the sum of the loads in the columns of each row, at the top of that section, divided by the height of that section and applied the answer as a dummy horizontal force at the top of that section, on that row. Do this for each story. After running the analysis, including these dummy forces, see if the assumed drift was realistic, if not, do it again with the newly calculated drifts. I used to put it in the analysis as a separate load case to run with the vertical cases.

I don't think it can be simpler than that.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[JoshPlumSE]

There are a number of simplified methods out there that are codified:

1) The seismic chapter in ASCE-7 has a simplified method. Section 12.8.7.

2) There is also the B1-B2 method that AISC gives. That's given in section C2.1b (of the 13th edition - Black book). The B1 is intended for moment frames and would probably be taken as insignificant for a shear wall system. Therefore, you'd just use the B2 method.


In addition, there is a good chapter in the 2nd edition of the Seismic design handbook (Farzad Naiem)

 

[westheimer1234]

oh well i just run it "pdelta analysis" using staad.. i believe any FEM software can do this..

the way i understand it you can do pdelta 2 ways.. one using moment magnification B1-B2 method.. two using rigorous pdelta analysis done by FEM softwares..

 

[Lion06]

I don't believe he's using software to model a wood framed building, but I could be wrong.

Also, you have to be careful with the P-delta button. Most only do P-big delta, not P-little delta, so in order to capture the member second order effects you need to break the members you're concerned about into 4 (or so) parts.

 

[ToadJones]

STAAD now does both.

 

[JrStructuralEng]

Correct StructEIT, I am not using software to model a wood framed building. I am more concerned with determining the approximate bending moments in the stud members

 

[Lion06]

If you are looking for second order effects in the studs, that is a P-small delta analysis.

If you are designing with the NDS, this is already accounted for in the Combined Bending and Axial Compression equation(Eq 3.9-3).

The terms account for this are the [1-(fc/Fce1)] and 1-[1-(fc/Fce2)-(fb1/Fbe)^2]. That takes care of the member second order effects. It's on page 20 in the '05 NDS.

 

[JoshPlumSE]

For what it's worth, the NDS equations really only account for the individual member buckling effects (i.e. P-little delta) and do not account for the overall story drift effect (i.e. P- Big Delta).

Sort of an academic point, though. When you're talking about a shear wall structure, the story drift effect should be small.

 

[Lion06]

Josh-

That's correct regarding P-little delta. I noted that in the post.

I would also hold that the story drift effects are not relevant for an individual stud. I say that for this reason. A single stud, even in a shearwall, is pin-pin member. Even if the stud is differentially displaced by some amount at the top and bottom, it still is only seeing axial load from the floor above. That axial load might be somewhat skewed now in order to follow the longitudinal axis of the stud, but it's still an axial force only. The P-small delta effects are a result of the out-of-plane wind load against the wall. The P-big delta effects are on the shearwall as a whole, not on an individual stud.

 

[Lion06]

I guess I'm comparing this to an exterior,pin-pin building column with wind girts framing to it. The column would be designed for the bending from the girts, the axial load from gravity, and the member second order effects. You would not consider global second order effects for design of this column would you?