Disconnected Node Mass

[scanf1027]

I have a model with multiple diaphragms at different levels, but also at different locations in plan; it is an "L" shaped structure. With the various stories, I have columns that RAM treats as segmented at the different story locations. The problem is the little segmented column masses are not associated with any diaphragm. In some cases, I don't want to disregard that mass, so I associated the disconnected node masses with the closest diaphragm above so it is included for seismic. The problem I'm having is in one of my models, I get a building story shear for the DL case. How can this be?

Also, because of the multiple diaphragms and different roof heights, my wind load exposure ranges are not correct. Is it easier to manually input the respective diaphragm wind load or try to edit the exposure boundaries?

I know this is a lot of info, thanks.

 

[SethGuthrie]

Interesting question. First off, your discussion of mass distribution is all correct, but it should not have any effect on the results of the Dead Load case at all. Masses are used in calculated seismic loads, dynamic analysis results (eigenvalue and response spectrum), and in some case in P-Delta calculations only.

If you are getting some net shear in the structure (as shown in the building story shear report) due to dead load, here are some things to look for:

1. P-Delta effects. Try running the model without P-Delta under criteria - General to determine if the shear is an effect of our P-Delta amplification or not.

2. Stepping foundations. If some of the support nodes are on one level and others are at the base, the total shear in one story might not be zero, though the total reactions would be.

3. Multi-level braces. Due to the way we do our story shear calculation, if braces skip a level, going from say the base to story 2 without touching story 1, our accounting of the total shear at story 1 won't include the horizontal component of those braces. I've seen similar issues with models imported from Revit where the braces are like knee-braces, but the vertical offset is nearly equal to the story height.

4. Sloped framing. Loads on the sloped framing can cause a net shear.

 

[scanf1027]

Seth,
Thanks for your response, but your response leaves me with a couple of questions:
My case is the one with braces that by-pass a level, because there are no diaphragms at those locations (I have multiple diaphragms, at different levels). My question is, based on how RAM is determining the story shear, how can I be sure the member forces are correct and the frame shears are correct? I have a particular frame with 7 kips of DL shear at a level. This doesn't seem correct to me. Maybe I'm missing something.

 

[SethGuthrie]

For those cases, it's best to hand-calculate frame shear, or building shear, based on member forces which should be in equilibrium and reliable. Here is a small attachment that might clarify further.

A brace often does resist significant force under dead loads because the columns shorten, and the brace goes into compression.

Only tension-only braces are except from the gravity loads. Our forum has more on that here:

http://communities.bentley.com/prod..._and_design__wiki/ramss-tension-only-faq.aspx