Metal Bldg Drift w/ Brick Veneer 1

[STR04]

http://www.eng-tips.com/viewthread.cfm?qid=111916

http://www.eng-tips.com/viewthread.cfm?qid=86767

http://www.eng-tips.com/viewthread.cfm?qid=92878

We engineers truly cannot agree on a drift criteria. I’m looking at a metal building with brick veneer/metal stud back-up and would like to give the metal building mfg'r a drift requirement but cannot decide where to go on this one. Does anyone have a code reference that addresses this issue and not judgment, rule-of-thumb or past job experience?

TIA

 

[ashleyassociates]

I doubt seriously that you will find a code provision alluding to this that will suit what you are needing. Unfortunately, you are the one that probably has to make the determination. The Metal Building Code Manufacturers Association calls out a building drift of H/60 for buildings with non brittle finishes and H/100 for building drift of buildings with brittle finishes. H being the mean eave height of the building (basically the average between eave and peak). I am a metal building engineer of 10 years experience that also owns my own structural engineering firm, and if you wanted to really make sure, call out for H/240 with H being the mean eave height. I hope this helps you.

 

[structuralaggie]

I never thought I would hear a metal building designer say that. I guess that is because you are a metal building engineer, not a designer or salesman. H/240 is what the IBC says (table 1604.3), so I don't know how the metal building industy says H/100 is ok. The problem is the metal building guys will tell the owner that it will cost too much to go with H/240 and that his engineer is being too conservative. The savings in cost is often too tempting to turn down.

 

[mrengineer]

IBC Table 1604.3 has requirements for deflection, but not drift. (Notice that first sentence of 1604.3 refers to both "deflections and lateral drift").

There was an article in December 2004 by the MBNA in "The Construction Specifier" magazine "Specification and Design of A Metal Building System" that discussed both drift and deflection. For "non-reinforced masonry" they refer you to the AISC "Serviceability Design Considerations" design guide.

A few years age I did a metal building with non-load bearing cmu at the exterior walls. I reviewed the NCMA publication "Concrete Masonry Walls for Metal Buildings", which had suggested details for creating a hinge at the base of the walls. I decided that the details were not terribly practical, and ended up specifying max drift = L/150, with careful detailing.

 

[grandcasita]

For non-strutural veneer applications, in out-of-plane bending, the architects always tell us L/600, because a crack is a crack, is a crack.

If the issue is drift, and the backup studs will stay "straight", simply tilting with the drift, assuming a hinge at the bottom may be just fine. Then you get into the new eccentricity of the brick weight, bearing footprint, and mortar strength.

 

[SteveGregory]

I just spec'd a mtl building with masonry veneer for a drift of H/360. For serviceability, the wind load was 0.7 x the design wind load.

 

[haynewp]

That sounds like what I would use, H/360 or 0.3" max gap at the bottom of the brick using 0.7 x design wind.

 

[JKStruct]

Our specs call for an H/600 limit with a brick fascia. So far, no owner complaints.

 

[SteveGregory]

We use L/600 for deflection of masonry or masonry veneer. This is out-of-plane bending deflection which is not the same as building drift.

 

[STR04]

SteveGregory,

How do you address building drift then?

 

[SteveGregory]

STR04,
My post yesterday (see above) addresses drift. H/360 with 0.7 x Wind

 

[structuralaggie]

H/300 should be equivalent drift to L/600, so why H/360? Sure, its better, but the logic?

 

[SteveGregory]

structuralaggie,
360 is 50% higher (better) than the code minimum of l/240 for "deflection/drift" in IBC Table 1604.3.

However, it is only a number! I have not gotten any response back from the metal bldg. mfr yet.

 

[miecz]

AISI publishes the Steel Stud Brick Veneer Design Guide.

 

[haynewp]

I should have said 1/16" gap previously.

UFC guideline is L/600.

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_310_02a.pdf

 

[ashleyassociates]

I would say that you can spec h/500, L/750, drift/1000....hide behind anything that you want....just remember that this linear term has exponential cost. Know what you have spec'd before just giving it a blank term off of another set of drawings.

H/240 as I have said before is an extremely stringent set of criteria to make a low rise building fall into. This is not stringent from the fact of design, but from cost....h/240 is a very serviceable number....h/300 is still possibly serviceable, but h/600 has huge cost implications all the way around...is it really worth this???

I am pointing these facts out not from the fact of a Pre-
Engineered Metal Building Manufacturer keeping things cheap....but from the point of a structural engineer trying to keep deflections reasonable, costs down, and variability between deflection calculations within tolerance....as all are!!!

 

[grandcasita]

While not constructive on their own, I'll just echo and summarize AA's comments:

Tax dollars at work (the UFC),

and

Anyone can build a brick...outhouse.