Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Disconnected Node Mass

Disconnected Node Mass

Disconnected Node Mass

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.

RE: Disconnected Node Mass

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.

RE: Disconnected Node Mass

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.

RE: Disconnected Node Mass

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:



Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close