Drift for Moment Frames
Drift for Moment Frames
(OP)
I haven't done that much steel design, mostly concrete. I have been doing more steel design lately. I have read on here that you cannot control drift in moment frames like you can with braced frames. I understand this. However, what is the maximum drift limit you use for desing using moment frames? I use 0.0025 x story height. This is very restrictive and difficult to achieve. I have done a few one story strip malls and they generally require moment frames. The roof heights are usually 16' or 18' from finished floor. The architects like to use 8" columns and a max of 10" columns. These columns along with the story height make achieving the drift limit difficult. What would you do in this situation? What is the drift limit for moment frames? Do you fix the bases of your columns in moment frames?






RE: Drift for Moment Frames
I try to keep a drift limit of H/400 as a general rule, depending on the type of elements attached (masonry, drywall, metal siding, etc.) Obviously, a warehouse with metal siding doesn't require such tight limits. AISC Design Guide 3 recommends H/400 to H/600 with an absolute limit of 3/8"
The way I stiffen moment frames is to: fix the base of columns (not preferable), or often I will make the frame a 3-beam, 2-column frame... that is, to have 4 beam-to-column moment connections (making it look like the pi symbol in elevation). But that is not alot of help if your columns are very flexible.
RE: Drift for Moment Frames
RE: Drift for Moment Frames
Wind drift limits vary with who is designing the building and what it is made of. I look at service level wind (0.7W) and keep the floor movement at H/400 or something like this due to occupant discomfort. If it is a single story building, I will limit wind drift to H/300 or H/360 for brick. Flexible facade buildings-like a steel wall panel prefab building-often have a lot higher wind drift limit.
It sounds like you are trying to get seismic drift to work on a single story building. What makes seismic worse is that the drift is calculated under ultimate loads and has to be amplified by Cd. It is common to have problems getting the drift to work with steel moment frames in moderate and high seismic locations. How many frames do you have on the entire building? I have seen people make every frame a moment frame to get the drift to work and maintain a smaller column size. You can also re-orient WF columns to use some strong axis bending in each direction instead of having almost all weak axis column orientations in a single direction. What R value are you using?
RE: Drift for Moment Frames
RE: Drift for Moment Frames
RE: Drift for Moment Frames
RE: Drift for Moment Frames
RE: Drift for Moment Frames
Don't you think it is funny how the IBC addresses member deflections but does not address story or building drift at all?
RE: Drift for Moment Frames
RE: Drift for Moment Frames
RE: Drift for Moment Frames
Typically I do not fix the base unless I have deflection problems and the members cannot be upsized or additional frames added.
Mike McCann
McCann Engineering
RE: Drift for Moment Frames
RE: Drift for Moment Frames
Mike McCann
McCann Engineering
RE: Drift for Moment Frames