Story Drift

[ars001]

I'm designing a 1 story retail building approximately 75'x230' in size. I'm providing moment frames on the ends to accomodate my wind loads (no seismic in this area). Is it too conservative to say that both the deflection of the roof diamphragm and the story drift from my frames should be accumulative when looking at the overall drift of the building? Or should they be assumed as seperate entities?

 

[DaveAtkins]

Definitely add in the diaphragm deflection. I think you may have trouble getting this diaphragm to work--it is very long and not very deep.

I had a similar project where I felt the diaphragm was deflecting too much, so I designed the interior columns as "flagpoles" to help resist lateral loads.

DaveAtkins

 

[lkjh345]

Agree with DaveAtkins. For this shape of building, I would supperimpose the diaphragm deflection on thop of the moment frame displacment to find my overall story drift at the building centerline. Decide if this total movement is excessive for building cladding, windows, etc.

At 75'x230', assuming a reasonable roof height, you probably can still get 1 1/2" roof deck to work. May need to beef up you fasteners towards the ends to get the diaphragm shear strength rrequired. But nothing to to far out of the ordinary.

 

[ars001]

That's not the answer I wanted to here from you guys/gals!!! Unfortunetly that was the answer I was expecting. I can get the shear resistant needed with an 1.5-22ga. deck. My deck stiffness is lacking. I can get my stiffness to a resonable value but have to go to a 36/7 weld pattern. With my frame drift and diaphragm deflection I'm about at a drift of 1/400 or 1/2". Which I find acceptable for the cladding. Obviously I'm backing on other factors such as a full length block wall across the back of my building (Perp. to my frames)that will greatly stiffen my building.

 

[JAE]

OK, here's another 2 cents worth.

You say that you have no seismic in your area, but under the IBC, everywhere has seismic, just to greater or lesser degrees.

With seismic design with SDC B or higher, you check lateral drift (with limits) for a seismic load on your building (which may be less than the wind), but this is the "design" seismic. To check drift, you also have to include the Deflection Amplification Factor, Cd in your diaphragm and frame drifts which can get large with a long diaphragm like you have.

So be sure to check that as well unless you are an SDC A.

 

[tngolfer]

Two additional options:

1. Is it possible to put another frame near the center?

2. A rep from Loadmaster came to give our company a lunch seminar. The system uses a composite roofing system to gain much higher shear loads and span distances. Here's their webiste:

http://www.loadmaster.net/

Might be something you can look into.

 

[lkjh345]

Agree with tngolfer. Adding a moment frame (or portal frame) or two near the centerline may make most of these proplems go away.

Or, if your contractor is on board already, ask them how much it would cost to use a 36/7 pattern fastening for the whole building. It may surprise you how relatively inexpensive this is. Particularly if you allow them to use fasteners instead of welding. I've had to do 36/7 fastening before, and have been surprised how little the extra attachments cost.

 

[ars001]

Thank you everyone for your help. I've done a little more investigating and found using moment frames and 1 1/2" deck wasn't giving me the stiffness I'm needing. So, I've convinced the architect (for now) that we need to provide chevron frames w/ drag struts on each end. Obviously, this reduces my drift substantially.

 

[haynewp]

There is no seismic drift limit for a 1 story building as long as the exterior walls, interior walls, and ceiling can accommodate the drift. I think H/400 is more for occupancy comfort for multi-story under 10 year wind. I would check that the stability of the structural system is alright for the combined diaphragm and frame drift and look for around H/240 under 10 year wind. Check general compatibility of architectural elements for this drift allowable. Is there brick on the building?

 

[ars001]

JAE is correct in saying that seismic design is everywhere, but to a larger or lesser degree depending on the location. But, MN ammendments eliminate seismic and earthquake provisions. So we do not design for any seismic loads in MN.

haynewp,
Take a look at the ASCE 7-02 commentary section CB.1.2 for Drift of Walls and Frames from the effects of wind. They suggest limits between 1/600 and 1/400. I've generally used 1/400 and have had pretty good luck. I would think H/240 could me a litte excessive. But thats just my 2 cents.

 

[Galambos]

Perhaps you already know this, but I would make sure you are checking your drift based on a ten year storm. For most wind speed ranges, you can multiply the wind speed by 0.84. Check with the AISC design guide and ASCE commentary.

 

[haynewp]

Check the AISC design guide on serviceability. It has H/200 for drift of reinforced masonry walls on steel frames under 10 year wind. It is your choice how strict you want to keep the wind drift, and depending on the finishes etc. I have heard of some engineers using H/600 for frame drift when there is brick.

 

[SteveGregory]

One more thought. Do you need an expansion joint in a 230' long building?

If so, you need a separate set of columns at the center. Now you have 2 separate roof diaphragms and 4 moment frames.

 

[civilperson]

If the building is heated/cooled, not much temperature change so no expansion joint required. If a parking structure or unheated warehouse, then 300 feet is the longest dimension without an expansion joint using free standing details, (no restraints or ties to other buildings).

 

[vmirat]

ASCE 7 Commentary also mentions an absolute maximum interstory drift of 3/8". Anybody use that?

 

[jike]

3/8" seems reasonable for floor to floor of 14 feet or less.

It is approx. H/450