Choosing between braced frames and moment frames
Choosing between braced frames and moment frames
(OP)
When you have a typical, multi-story (say 3-5 stories), steel-framed, commercial office building, what do you generally find to be the most efficient/least expensive lateral force resisting system?
Do you always try to make braced frames work if the architecture allows for it? Or are there some scenarios where moment frames might be a better option, even if braced frames are possible? I know that each job is unique, but what is your general process for making this decision?
Do you always try to make braced frames work if the architecture allows for it? Or are there some scenarios where moment frames might be a better option, even if braced frames are possible? I know that each job is unique, but what is your general process for making this decision?






RE: Choosing between braced frames and moment frames
RE: Choosing between braced frames and moment frames
If you are limited to moment frames vs braced framed, I agree with Steellion
One of the main disadvantages with moment frames is deflection- you will most likely need to fix the based to keep this in check
RE: Choosing between braced frames and moment frames
We do mid-rise buildings, 10 to 15 story high, and typically fall in SDC B or C. Since R for the above mentioned lateral force resisting systems is comparable, we have used concentrically braced steel frames not detailed for seismic resistance. For OSCB and OSMF, seismic detailing per AISC 341 is required.
RE: Choosing between braced frames and moment frames
I've always envisioned that the foundation system would be a lot more efficient if you used moment frames. But, I guess that doesn't come close to outweighing the costs associated with upping all the member sizes to keep the drift in check.
RE: Choosing between braced frames and moment frames
I just ran through this exercise on a building. It was a small building (3 stories) but on a site class E. The preliminary foundations are ridiculously large for such a small building. The architect barely allowed any place for bracing. I quickly ran through the numbers for the system using moment frames and found that the steel sizes went through the roof. In the end I decided to keep the brace frames with the ridiculous foundations.
If the owner or GC have a problem with what I came up with well I have plenty of suggestions to make things more reasonable..... they just need to live with some of my changes.
RE: Choosing between braced frames and moment frames
RE: Choosing between braced frames and moment frames
They suggest fixing the base of the columns to control drift in the first story.... everyone on this forum always says to never "fix" the bases because you never get a truly fixed base at that increases the cost of the system to much.
RE: Choosing between braced frames and moment frames
Another item to note is that you can gain a good bit of siffness in the frame by taking into account composite action at the beams if you have it. Most people don't because it is easier to ignore it and most commercial programs (Ram for instance) ignores it, but it is there if you want to put forth the effort and properly account for negative moment regions etc.
RE: Choosing between braced frames and moment frames
Did you do a one-for-one substitution of the moment frames for the braced frames? Or did you make all the connections moment-resisting?
RE: Choosing between braced frames and moment frames
Part of what was causing problems was frame drift. This particular structure was clad in brick so that didn't help at all. I stayed away from fixing the bases because of what others have warned me about and because the building slab was 4' above grade (to pull it out of a flood plain) making the footings very deep and pilasters very tall.
RE: Choosing between braced frames and moment frames
RE: Choosing between braced frames and moment frames
RE: Choosing between braced frames and moment frames
What about the costs of the extended end plate bolted moment connection, instead of the traditional connection with field welding of the flanges? I've wondered how much more expensive that connection is compared to a typical shear connection, since you eliminate the field welding.
RE: Choosing between braced frames and moment frames
It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN
RE: Choosing between braced frames and moment frames
RE: Choosing between braced frames and moment frames
RE: Choosing between braced frames and moment frames