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RE: P-Delta

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>I am using the RAM software to model ... a simple
>rectangular, 3-bay, 2-story building with a rigid
>frame on each end in each direction. When looking
>at individual frames in the small model, with only
>the wind loads applied in one direction (also the
>innate mass load, which would be the "P" of P-delta),
>the horizontal reactions at the base are greater
>than the applied story forces for each direction.
>I modeled one of the end frames using Visual Analysis
>4.0 and did not get the same results as I did with
>RAM despite using the P-delta analysis.  The reactions
>added up to the applied loads in VA.
>I am hoping someone can explain to me why (or if) the
>reactions are greater than the applied loads when using
>a second-order analysis such as P-delta because it would
>seem based on pure statics/dynamics, that the building
>is no longer standing still.

In the frame you modeled in RAM (a 3-by-3 bay, 2-story building), the middle
two lines of framing lean on the outer two. Essentially, all the reactions
required to stabilize the two interior frames against stability effects (or
instability effects) are adding to the exterior two. I think that is why you
are getting larger shear on the outer two.

In the singular frame you modeled in Visual Analysis, you probably are not
getting the additional effects of the interior frames. Does this explain the

One important side note: when doing P-Delta (second order) analysis, you
have to have the gravity loads on the frames in addition to the lateral
loads. The second-order effects are generated by the effects of the gravity
loads on the deflected frame. If the gravity loads are omitted in the
analysis, you're not going to get second-order effects.

Hope this helps.


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