P-Delta Analysis

[DCBII]

Hi,

Is it necessary to consider P-Delta effects when designing trusses with significant bending in the chord members, or are P-Delta effects for the main lateral force resisting system only.

If it is necessary, does anyone have any tips on how to approach it. Is it necessary for me to multiply my ASD loads by 1.6 and then divide the results by 1.6 if I'm not using the Direct Analysis Method of Appendix 7? Section C2.2a(b) of the Specification has me a little confused. Are P-Delta analyses ALWAYS carried out under factored loads, no matter what?

 

[ToadJones]

Sometimes I really wonder about all these types of analysis....no computers and no one would dream of trying it.

 

[Lion06]

Toad-
I agree 100%!

 

[ToadJones]

I guess it gets frustrating. I spent half my days and sometimes all evening doing research to learn how to do things THAT I HAVE DONE A 100 TIMES!!!!!!!!!!!!!

 

[JoshPlumSE]

By the code, I don't know that you are "required" to use the DA Method for trusses. My last post probably implies that it's required. But, really meant to say only that the DAM method is a very general procedure that can be used for any analysis where you want to account for stability effects.

FWIW: I just took a look and the 2010 version of the code removes the term "lateral" from that quote... "all components that contribute to the displacements of the structure." So, the applicability is more generalized that was implied by the language from 2005.

 

[abusementpark]

Has a gravity steel truss ever failed because it wasn't designed for second-order effects?

These codes are getting a little out of hand.

 

[ToadJones]

The codes have officially made it impossible to analyze a structure without a computer.
This scary.
I have worked with lots of folks who could hardly model a single column properly in a FEM program.
I have found it more difficult over the years to grasp the finer points of properly modeling a structure with software than actually grasping the design philosophy.
At times I get genuinely PO'd when checking another's work that turns out to be a botched FEM model.

I'd hate like hell to think that there are guys coming out of school having learned some FEM in school thinking they are design engineers

 

[Lion06]

Removing "lateral" from that sentence seems to include every single piece of the building, right down to the bent plates at the slab edge.

I still don't think it's the intent to include gravity members in the reduced stiffness requirements. It just doesn't make logical sense to me. I guess I'll see how it reads, but it's worded now to be for stability of the structure. To me, that means if it doesn't contribute to the stability of the structure that it's not intended to be required to meet the requirements of the DAM. That makes sense to me , including gravity members doesn't.

I could be way off, though.

 

[RFreund]

Thanks for the correction. I should have put more thought into that because as soon as I read your post, it was one of those "oh, thats right" moments.

How about this statement:

So really the second order analysis (or B1, B2 amplified 1st order analysis, I usually use this as or projects don't normally get to out of hand) is to account for the p-delta and P-DELTA effects. The DAM method is really to account for: 1. Notional loads due to initial imperfections and 2.Inelastic effect of column buckling (I believe this was previously stated.

I think part of my confusion was/is coming from the fact that the results of the DAM effect my second order analysis (amplified 1st order). In that by increasing the horizontal load and decreasing stiffness (or increasing deflection) gives a higher B2 value. So I guess I was just looking at it as an extension of the second order analysis, which I guess it sort of is?

As far as FEM goes, I like learning about it but not sure I'll get to put it to use. Currently our office is more about making some safe assumptions and going back to some sort of strength of materials equations.



EIT

 

[JoshPlumSE]

AbusementPark

To some extent, we have been designing for 2nd order effects since the early sixties. That's why we always used the Cm / (1-fa/Fe) multipler on our bending stresses.

Look at it in terms of that old equation. Is it possible for the compression chord to fail if you considered only fa/Fa + fb/Fb <1.0 without any consideration of momen amplification? Of course it is....

The new codes are a bit more "genearal" than they used to be. They tell us to account for 2nd order effects, they just don't tell us how. We could certainly use the old type of Cm/(1-fa/Fe) amplifier, or we can include the effect directly using a P-Delta analysis. to me that is a rational change reflecting the fact that so many engineers are using analysis programs these days.

My point is only that the 2nd order analysis requirements in the code are not a whole heck of a lot different from old codes..... Rather, it's all the other portions of the new analysis requirements (stiffnesss reductions, and notional loads) that represent changes to the code philosophy.

 

[WillisV]

I use the DM to design trusses all the time when doing erection engineering. Lets say for instance you are setting a truss that has a bolted splice in the middle of it and you want to know the out of plane moment that splice needs to take under construction dead + wind loading prior to the truss being locked in to any diaphragm. The DM method can accurately capture the increase in out of plane moment due to the dead load in the top chord plus the out of plane wind loading as well as let you know if the truss is even stable without additional bracing or guying (the P-delta analysis will not merger). As JoshPlum was alluding to this is actually one of the major benefits of the DM method - using it for stability problems and to determine bracing loads in lieu of Appendix 6.

 

[WillisV]

BTW the notional loads concept is definitely not calibrated to the design of trusses etc, for that I use direct out of plane modeled initial geometry (sine wave with L/500 out of plane sweep).

 

[DCBII]

WillisV,

Instead of using a factor for on the actual gravity loads to get notional loads, couldn't you use a factor on the chord forces caused by gravity loads instead?

I'm just not sure how to model the initial geometry. How would you model a curved beam in a finite element program? It seems like a lot of effort.

 

[WillisV]

You model it as piecewise linear using the panel points. It is not too difficult. Again DO NOT use notional loads to do this, as they were calibrated for building column design, and the distance that the notional load pushes the column over will in all likelihood not equal the L/1000 (where L is the entire length of the truss) value for maximum initial curvature that you are shooting for in this type of design. Plus, out of plane notional loads will add fake out of plane moments to push it over into the deformed shape that are not really there.

 

[DCBII]

I guess that's not too bad. It's just a matter of knowing which buckling mode to shape the initial geometry to.

 

[WillisV]

DCBII - true, that is indeed the hard part. You can look at Eigen buckling modes to get a feel for it, but if it is braced in multiple locations etc. you might have to brute force through several different modes to get the controlling forces.