4 Story Moment Frame and Drift

[mzaitz]

All

Working on a 4 story moment framed building and having issues with drift for wind. Building is 61'-4" tall (15'-4" floor to floor) not counting the mansard roof. I have worked on taller buildings but they ended up with braced frames or concrete shear walls. Building is L shaped with dimensions approximately 270 ft x 230 ft for the outside bend. The long side on the outside of the corner has a 5 degree slope in plan so all of my moment frames on the outside of the building are 5 degrees from orthogonal. After lots of reading, I decided to use H/400 for the building. Building is located in a 120 mph wind speed and SDC B. Using a 25 year return for the wind (factor of 0.49 for checking drift), I cannot seem to get the drift at the second floor to meet H/400 or even H/300. I am limited to W14 columns and they are getting large (W14x132 and larger) and increasing the weight has a small affect on the drift.

Having not done moment frames this tall I am working to eliminate the doubler plates (web panel reinf). Also, looking at using bolted flange plates instead of direct welding beam flanges to column. Anything else that is a major concern cost or design wise?

Thoughts for stiffening the building:

Use W18's for the columns. This will be a hard sell to the architect.

Fix the bottom of the columns to the footings. Not something I normally like to do.

Are there any other insightful ways to stiffen the building that I am missing?

 

[Guastavino]

Have you talked to the folks at sideplate? They do proprietary moment frames. They help with design and the "fee" is on the fabricator. All bolted connections in the field, and very stiff, or so I hear

 

[mzaitz]

I did chat with the people from Sideplate, but in conversation with the contractor (and his preferred fabricator) they did not want to go that route. I plan to recommend it again but we will see what they say.

 

[MotorCity]

Have you looked at using stair or elevator shafts for lateral resistance? If constructed of CMU or concrete, they can offer considerable lateral resistance if designed as shear walls.

 

[KootK]

If stiffening the columns is yielding diminishing returns, perhaps it's time to focus on the beams. You could stiffen the beams or tighten up the column spacing to achieve that. Lastly, are there more bays available to use as frames?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Boiler106]

in my experience, i have never been able to get multistory moment frames to work without fixing the base. Most of my buildings are in 115mph (ASCE 7-10). I use 10 year wind for drift check, as recommended by the aisc design guide, and i also do not apply importance factors in my drift check. perhaps, you have a reason for 25 years and i wouldnt mind hearing it.

i would also make sure that if you have multiple frames in one line that they are all sharing columns such that they dont have a gravity beam between two frames in series.

I would assume that you are using W27/W30 beams for the frames at the base. W14x132's dont sound unreasonable.

 

[mike20793]

Are you saying you have the bases of your moment frames column pinned? You'll want those fixed for a brick clad building that tall. I just did a 4 story moment frame building in hurricane winds and got drift to work with W14 columns and W24 beams, so it's definitely doable. I'd stick with heavier W14s and keep increasing them to avoid using the doubler plates and then look at what that size will do for you. What size beams are you using?

Also, what is this 0.49 factor for drift? I haven't ever heard of that before. Usually drift is taken at a 10 year event rather than the design 50 year event. I reduce my drift requirements based on the conversion (multiply the 400 in H/400 by 0.84^2 or use an additional 0.7 factor for your drift combinations). For brick it reduces down to H/282 for that wind speed and 0.7 factor is listed in the IBC, but is really 0.84^2. Try using that drift limit and see if it helps.

 

[JoshPlumSE]

I worked on a very similar problem not long ago, though it was for seismic and it was 10 stories. But, we went with the same basic logic you've suggested.

We used W18's for the columns. This was more challenging for seismic because of AISC 358 limitations on members we could use. Then we fixed the base. We met our drift ratios. Though it wasn't easy.

I don't know if it will ever get built though. It was more of a conceptual project for the client. I wanted to use an elevator core or stairwell core to stiffen up the building, but the client said no. They had some kind of bias against field welding and concrete walls. Or, that was their cost comparison seeing if the increase in steel cost could sufficiently offset the reduction in certain types of field labor.

 

[mzaitz]

@Motorcity - Cannot use the elevator shaft as there is only 1 and it of course is located adjacent to the large chase that runs full height of the building. Trying to figure out how to isolate it from the LFRS and still maybe using it for gravity. The stairwells are not CMU.

@KootK - do not see adding any more frames. I feel I have too many already but we will see.

@Boiler106 - Beams are in the W27 and W30 range. a couple at the extreme ends are larger but not many.

@mike20793 - Currently have the base plates pinned. Already avoided the doubler plates so that is not an issue.

I came up with the 0.49 factor by taking a 25 year return, using the nominal wind speed in the commentary, changing it to ultimate, ratio it with my design wind speed, squaring it and then multiplying it by 0.6 for actual. The V nom is 84 mph which is 108.4 mph ultimate so my factor is 0.6*((84/0.775)/120)^2 = 0.49. Lots of brick and a fair amount of curtainwall is on the building. The problem with fixing the base plates is there are a lot of piers due to a partial basement under the 220 ft leg. All of the columns on the exterior wall at the basement are on piers and I cannot get the room to fix the base plates in the direction perpendicular to the wall. My column is 16" in from face of brick and 8" from edge of slab/face of stud. I had to bump out the pier 1 1/2" just to get the W14 base plate to fit on it with any sort of construction tolerance. I can fix some of the base plates so I may look at that. Using the 0.42 (10 year) reduced the drift at the 2nd floor to H/329 which seems more reasonable. I meet the H/400 at the penthouse level.

 

[KootK]

Cannot use the elevator shaft as there is only 1 and it of course is located adjacent to the large chase that runs full height of the building.

For most layouts, this isn't a deal breaker. Hit the shaft with drag struts, even if they have to pass through the shaft and be fire rated. I like this much better than trying to isolate the shaft laterally.

The stairwells are not CMU.

Even if they're not CMU, They are probably convenient places for some steel chevron bracing.

All of the columns on the exterior wall at the basement are on piers and I cannot get the room to fix the base plates in the direction perpendicular to the wall.

You gotta work a little harder for it but there are ways. I've shown one below. Now that I think of it, erection might be easier if the embedded steel column extended to the footing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SteelPE]

According to an AISC seminar you attended years ago, you can assume a partial fixity at the base of the columns of 10% of the full fixity and still have pinned bases. This is based upon the old column k charts. A majority of you drift is probably happening at the first floor do this should help.