How to include equipment loads into modal analysis and in dead load. 2

[petb]

Hi!
I don't use SAP2000 very frequently, and this always puzzles me.
I would like to add some equipment masses/weights in a separate load pattern and would like to have the option of letting those masses be included both in the modal analysis and in the Dead load case (or at least be treated as a Dead load or auperimposed dead for subsequent load combinations as required by the design code).

The equipment is not modeled per se but known masses at known locations within the structure.

I have tried adding them as e.g. line or point masses, but then they're only "active" in the modal case and not included in the dead load.
If i use a super load pattern, they are included in the dead load but not in the modal analysis due to being a force only, not a mass.

Is there a reasonable work flow for what I'm trying to achieve? Or am I doing things in a way it's not supposed to be done? Any tips will be greatly appreciated.

Pete

 

[STpipe]

Are you dealing with a model with only a frame? Maybe you could slide the lines into the areas where the equipment is resting and manually change the properties of the members at those locations (by changing the density) to account for the increased mass?

 

[petb]

Thanks, that is what I normally do. But the caveat here is that I want to be able to exhange the profile without the hazzle of using property modifiers to account for the extra masses/weights. We have some structures that are similar but per project fabricated near the actual erection sites, so setting up the models to be able to quickly change profiles according to local availability and to local design codes is my main goal here.

 

[STpipe]

Ok, in that case, could you perhaps create "dummy" members that have little to no stiffness but have the correct density to account for the added mass. You would create that "dummy" member at a small offset away from your actual frame member, and then tie it back to your frame members using rigid links. The only downside to this approach would be you will pickup low frequency or 0 Hz. modes because of the flexibility of your "dummy" members.

Another suggestion would be, are you able to use shells in your model? Same idea/principle as having a "dummy" member to include your added mass.

 

[petb]

Thanks, I'll give that a try. I was really hoping for a neat way of adding mass in separate load pattern or similar to avoid work around. How do you structural guys address this situation? Can't be that uncommon to change a profile for architectural or other reasons. Setting for the property modification route seems error prone to me.

In the past I have also set the self weight multiplier, used line masses to simulate equipment and set self weight multiplier to zero and used a gravity load in z-dir. This also works but is also a convoluted way in my opinion.

 

[Guest262653]

Add additional weights as a separate load pattern. Then define the mass source including this additional load pattern.
In version 21.2.0, the procedure is as follows:
- Define -> Mass source -> Modify/Show Mass Source... -> Check both checkboxes for mass source (including Specified Load Patterns) -> Add the new Load Pattern to the list of Mass Multipliers for Load Patterns
- Use this Mass Source for your modal analysis

 

[petb]

So, in the later versions you're allowed to add weights in a specific load pattern? Great, I'll try to upgrade as soon as possible, running 19.1 now.

 

[Guest262653]

Not only in latest versions. I have been using this method since I started using SAP2000 and that was more than 10 years ago. Maybe the dialog boxes and menus are a bit different but the method is essentially the same.

 

[petb]

Thanks, works like a charm. Is there a possibility to set up multiple modal load cases with different mass sources then? I defined a couple of new mass sources and would like to run one modal case for each mass source. When I set up a modal load case the Mass Source Box is greyed out and always shows the mass source that I set as default in Define -> Mass Source -> Default Mass Source.

 

[Guest262653]

I couldn't find a way to do it either. I guess that the program always considers the default mass source for modal analysis. Other defined mass sources are used only for other analysis cases (nonlinear?).

Check the last point in the following link from CSIAmerica:

Link

 

[petb]

That would be a nice feature though. Thank you for your fast responses.

 

[Guest262653]

Update: I found a way to create multiple modal analyses cases from differente mass sources.

The trick is to create a dummy nonlinear case for each additional mass source, with no load applied and with the appropriate mass source selected. This forces SAP2000 to create a new mass matrix for this mass source. Then, in the definition of the new modal case, in the stiffness to use box, select the dummy nonlinear case and the mass source updates accordingly.

Hope it helps.

 

[petb]

Thanks! Very clever solution indeed.