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Modal analysis of LSF building
2

Modal analysis of LSF building

Modal analysis of LSF building

(OP)
Hello

This past few weeks, it was required of me to model and design an LSF structure for work (nothing fancy, it was a one-story house.
I had previously never worked with LSF structures and this has been a decent learning process for me, though I'm still a long way from understanding all the nuances of it.

I decided to use SAP2000 to model the structure because it's the software I'm most familiar with and the one I have the easiest access to. There are a couple of things that make me believe other software would be better for designing LSF structures, but this is what I have. By now, I believe I've also developed a decent library of cross-sections to make my work in the future a lot easier.

I decided to open this thread because I have some question about the modal analysis of my structure. Here are a few pictures of my model:

3D view


Roof view


Base floor view


Foundations


So, at the foundation level, I have a concrete frame (which represents the footing of the building) which, as you can see, is continuous. It is connected to the base floor by a concrete shell. The base floor is also a concrete shell.
All the other elements are typical of LSF buildings. I've decided to model the walls with shell elements as well, with the characteristics of the OSB. The columns are cold-formed sections. I've modelled all the internal walls. In the roof, there is also has an OSB shell connecting all the beams.

My question is regarding the Modal analysis though. Most vibration modes are local, but the first "global" vibration mode is the 3rd, and is mostly a translational mode in the y-direction, with a little bit of rotation (torsion). It happens for a frequency of 11.46 Hz, which I've read from reliable sources to be typical for these buildings.

MY PROBLEM: I've analyzed up to 300 vibration modes and the accumulated mass % never goes beyond 45% in any of the relevant vibration modes.
The structure has many low mass elements and most of the vibration modes are local but this still sounds very odd to me. I have not set any kind of constraints to the wall or roof elements (plate/diaphragm), as I do not believe that gives an accurate representation of the structure at all.

Can anyone tell me whether something is up with my model or if this is normal in these structures?

As if this post isn't long enough, I have another question:

I have access to some older projects of my company, where other people (more experienced engineers) made the same kind of analysis to different but similar houses built with the same system. In their modal analyses, in more than one project, they often got close to 100% mass participation within the first 10 vibration modes. They also achieved natural frequencies of 1.14 Hz, which is not, at all, in line with what I've read to be normal for this type of buildings.

Best regards
Filipe

RE: Modal analysis of LSF building

(OP)
eh, well, I think I figured it out on my own. Much of my mass comes from the modeled infrastructure (the foundations) and the bottom floor slab. These elements do not participate in the vibration modes. This is why the accumulated percentage of mass will never come close to 100%.

RE: Modal analysis of LSF building

be careful with the modelling if you just did the OSB properties and those shells are continuously attached to the CFMF studs/post and floor framing you are likely over estimating the stiffness of the panels, in the real construction there will be slip of the fasteners and slip of any hold down devices.

We'd generally back into equivalent wall panel stiffness and use a single shell for the lateral analysis if even using software for that analysis at all.

Open Source Structural Applications: https://github.com/buddyd16/Structural-Engineering

RE: Modal analysis of LSF building

Lateral resisting elements for these types of structures, in my experience, usually consists of Cold Form Strapping which would be modeled more like a steel braced frame.

Open Source Structural Applications: https://github.com/buddyd16/Structural-Engineering

RE: Modal analysis of LSF building

That does make sense - if most/a lot of your mass is at locations where the mode shapes have almost zero disp. it will not add much to the mass P.F.

RE: Modal analysis of LSF building

(OP)
Thanks for the replies.

Quote (Celt83)

Lateral resisting elements for these types of structures, in my experience, usually consists of Cold Form Strapping which would be modeled more like a steel braced frame.
The structure I'm modeling isn't exactly your typical LSF structure. It's a modular type of construction developed by the company I'm working on. The walls are produced in the factory and are then placed and connected onsite. Neighboring wall panels are connected from the sides but also from the top and bottom. There are no steel bracing frames, all the lateral stiffness is provided by the OSB panels. All the wall modules (interior and exterior) serve both a structural and insulation purpose.

Quote (Celt83)

be careful with the modelling if you just did the OSB properties and those shells are continuously attached to the CFMF studs/post and floor framing you are likely over estimating the stiffness of the panels, in the real construction there will be slip of the fasteners and slip of any hold down devices.
There may be an overestimation of the stiffness of the walls, but right now I don't exactly have a better way to model these structures, because I don't really know any better. Unfortunately, I also do not have anyone else to discuss these issues with.

RE: Modal analysis of LSF building

NCSEA just hosted a webinar on the topic of cfs shear walls a couple days ago. You can access ($$$) the recorded version on their website.

Out of curiosity, have you figured out what was different with the previous analyses of your colleagues that resulted in the high mass participation and low frequency?

Also curious what you decided to do as a workaround with the mass participation issue in your model.

RE: Modal analysis of LSF building

(OP)
Now that I know the reason for the low mass participation, I don't think there's an issue at all. It's just a matter of fact that most of my model's mass does not participate in the vibration modes, because I decided to actually model the foundation and bottom floor elements (I could have just inserted restraints to the columns and not model the bottom floor slab).

My colleagues, when they modeled their structures, did not model the foundation or bottom floor elements. That explains why they got a high mass participation in their modal analyses. Nevertheless, the low frequencies of the structures they modeled are not expected in this type of structures (the best book available in Portugal regarding LSF structures says it's expected for the natural frequency of these structures to be above 10 Hz).
So their results denote that the innate stiffness of these structures is absent from their models. It seems to me that they did not model the walls or any equivalent bracing frames.

RE: Modal analysis of LSF building

Makes sense. I guess you could temporarily assign a zero-density concrete material to the foundation elements, just for the modal analysis.

RE: Modal analysis of LSF building

I think you already have it figured, but if you consider a simple cantilever. It will not require a huge number of modes to reach 90%. But say that you add a large mass at the base, the fixed base. Then it is impossible to reach 90%.

I have experienced similar issues with box-like concrete buildings for primarily the vertical seismic response. You mention 400 modes, check the forces you have with 400 modes. Then increase to the number of modes to 2000 och 4000 and check the forces again. If there is no signifacant change than 400 is enough include the full loading.

Thomas

RE: Modal analysis of LSF building

A number of comments:
1) You will need to "soften" the G value for your shell elements modeling OSB. This is to account for nail slip and such. Wait until you know what your nailing pattern is going to be. You can use the NDS which has the concept of "apparent shear stiffness" to account for nail slip.
2) In my experience wood shear wall can be overly stiff if they're not meshed enough.
3) I might have two separate models. One for gravity only system where the vertical studs and beams are all modeled. And, another for only the lateral system. The problem is you might be getting a lot of lateral stiffness out of your studs. Where each of them acts like a hold down point.

4) You might manually "discretize" your mass. Meaning that instead of relying on member self weight, try lumping the mass at the floor locations only. That will get rid of local modes and allow you to get closer to 100% mass participation.
5) Alternatively, you could go with Ritz Vectors. Ritz vectors are very similar to true modes, but they are biased towards a certain deflected shape. This gives them a few advantages:
a) They inherently get rid of lot of local modes that aren't consistent with the deflected shape.
b) They already account for the "residual" mass associated with the base of the structure. Though they acceleration used for this mass can be overly conservative at times.

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