Drift for Moment Frames 6

Don't you hate the answer "it depends"?

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.

Oh, and remembering to use .7W if your governing lateral drift loading is wind (from AISC Design Guide 3) This effectively converts the ASCE 7 wind loading to a "ten year recurrence" level.

The drift limits for seismic are in the IBC. I also put in an angle at every floor level to support the brick or stone facade. This will allow for floor movement without tearing up the facade during an earthquake and lessen the chance of bricks falling off the building.

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?

Fixing the bases can substantilly reduce drift. If I do this I try to use thicker base plates with larger anchor bolts and no gussets to keep the costs down. Don't forget to minimize footing rotation.

I am trying to get wind to work. With pinned bases, it is impossible to achieve H/400. The building has a continuous glass storefront and EIFS w/drivit above the windows. I haven't read anything in the IBC about 0.7W. Where do you see it in the IBC? Where is it in AISC DG 3? I haven't found it there either. I am also in a situation where I have to use helical piers under the footing for bearing capacity.

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I have found a discussion in the commentary of AISC '05 specification. It punts and states some designers design for drift using the 10 year return and some the 50 year return. I also found the discussion in AISC Design Guide No. 3 which states to use 0.7W (or the 10 year return) because components and cladding are not life threating issues typically. The author suggests to design for drift based on perception and not to use the same design wind force as is used to design the members for strength. The IBC suggests to design for deflections of members using 0.7W but does not mention frame drift at all. I think I will use 0.7W because the arguement makes sense to me and it appears to be common practice.

Thanks to all those that have answered my question. I appreciate your time and effort.

Don't you think it is funny how the IBC addresses member deflections but does not address story or building drift at all?

Story drift is dependent upon the cladding material and a servicability issue not safety. As others have said, use AISC Design Guide No. 3.

To fix the column bases, Have you thought about embedding he column base into a thicker footing? Just an idea. You could maybe have some rebar or studs welded onto the column to provide resistance.

I typically use the limits mentioned above, but add my own story drift limit of 1/2" in order to limit damage to windows asmuch as possible.

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

In high seismic zone, like Los Angeles area, it will be really hard to make drift under limit. Strength level, 6x6x1/4 column can work while 6x6x1/2 may still not work by considering drift. Anyone has better ideas to deal with this issue???

Place the entire Los Angeles basin on Seismic Isolators?

Mike McCann
McCann Engineering

How would you fix a column in practice without gussets? Just by providing a thick base plate and longer stronger anchors? Is the moment from the column base transmitted directly to the footing? Just doing a quick calc here with a large moment, the footing area gets pretty large because of the eccentric loading for a given allowable bearing pressure.