×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• 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.

# Modal analysis of LSF building2

## 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.

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:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!