Mezzanine Lateral loads 1

[sponton]

Hello,


I am modelling a mezzanine with 5x1 25 ft bays. I am wondering whether this should in fact be or not braced. I was debating on the lateral load that should be placed per code and my coworkers argue that it should be .02*(TOTAL DEADLOAD) but I cannot find anything in the code regarding this [I was going to add an accidental excentricity to the columns], this in consequence arises other questions such as: "should it be placed in the center of mass of the diaphragm?" should I have besides said lateral load a given eccentricity as in the case of earthquake loads.

I would check the lateral displacements which for a 1 story mezzanine must be ridiculously small. I am in fact more concerned about the frame that's going to hold a monorail on top of the beam, the whole point of the lateral analysis is to see how it behaves, then again it's only a 1 kip load but still I'd like to see how other people approach these sorts of problems.

I've seen plenty of people modeling them in STAAD or etabs and they model the connections as rigid when in fact they are simply connected, which should increase deflections. Any ideas?

 

[TehMightyEngineer]

The 0.02 dead they mention is likely coming from the requirement for stability from the AISC steel specification part 2 or appendix 7 (if you're in the US). For a mezzanine designed by first-order analysis all load combinations must include an additional lateral load at each level of the structure equal to 0.0042*(LRFD gravity load of the load combination or 1.6*ASD gravity load of the load combination). For a mezzanine this ends up being REALLY small. I typically increase this to at least a few hundred pounds. This way the mezzanine can take a minor impact of a fork truck or other misuse. In addition, your handrails have minimum lateral loads on them, these have to be supported by your structure. Then, as you said, you'll have some accidental eccentricity but this is likely negligible for a single story.

As for lateral stability, model the structure as it's idealized. If the connections are simple then model them as such. Depending on the post bases you may be able to call them fixed but you'll want to verify the anchor capacity, baseplate, and welds. On mezzanines I've designed typically this isn't hard to do.

I've seen lots of issues with improperly designed mezzanines that had little to no lateral stiffness. In addition, make sure that if you're considering the beams fully laterally braced for LTB that floor plate and not floor grating is being used. Floor plates welded to the compression flange can brace beams, floor grating with metal clips probably can as well but good luck quantifying it (to say nothing about them removing the grating).

The monorail is also something to watch out for. ASCE 7 prescribes some minimum lateral loads that I always use on a monorail, even if it's not a powered hoist. Typically 10-20% of the vertical load. This will help when they inevitably lift something off center. Pay very close attention to that cantilever, if you've never designed a cantilever monorail I have a few papers for you to look at.

Lastly, make sure your deflections are good. As you said, simple span beams will deflect more. A bouncy floor really makes people uncomfortable with loading up a mezzanine to it's maximum rated load.



Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

 

[ajc2014]

Not sure whats codes you're using, but ASCE 7-10 in section 1.4.3 that the minimum design lateral force shall be 0.01*DL. I'm sure it's also stated in most building codes.
It's to cover lateral loads that will be imposed by people walking, moving, and etc (or at least that's how I've thought of it)

 

[sponton]

@TehMighty..


Great, if you don't mind can you share those papers on monorails, I am struggling a bit with them

I forgot to mention that on top of the mezzanine, we have 26 gauge type b metal deck with a 3" concrete top. So i'm taking the beams as fully braced. I ended up putting a 10K load [.02*TL at the center of mass of the diaphragm DL=50psf LL=125psf) it gave me no problems with respect to drift or anything, so i decided not to brace it. In fact the existing mezzanine which is basically a copy of the one I am making [except it was designed in 1988] has no bracing and no apparent issues until this day. Apparently it was very conservatively designed, even the point load from the column that's going to be built midspan for the monorail frame didn't even tickle the existing beam.


@ajc

I am using ASCE

 

[BadgerPE]

Out of curiosity, what are your beam/column connections? Are they simple shear tabs, or are the beams directly welded to the columns? I'm trying to understand where your lateral load resistance is coming from. Unless you have a cantilever column/diaphragm type design.

 

[sponton]

@BagerPE


Simple shear tabs.

 

[TehMightyEngineer]

Badger: I expect that he's getting fixity at the base of the columns and the 3" deck is acting as a diaphragm. I've seem similar mezzanines work like this (though how many people actually check it during the design is another story).

Sponton:

http://sydney.edu.au/engineering/civil/publications/2009/r906.pdf

I've also attached a great paper for your cantilevered monorail.

A very good spreadsheet, though it's set for AISC 9th edition, makes a good double check and the flange bending calculations are highly useful:

http://www.steeltools.org/resources/viewdocument/?DocumentKey=98ce81af-1b6b-4c4e-ba2a-2692bbb337f3

I'd also pick up a copy of ASME B30.11 - Safety Standards for Monorails and Underhung cranes.

https://www.asme.org/products/codes-standards/monorails-and-underhung-cranes

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

 

[TehMightyEngineer]

Also, are you in a seismically active area Sponton?

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

 

[sponton]

Not exactly, I'm in Center Kentucky.

 

[TehMightyEngineer]

Hmmm, given the size of your mezzanine you may want to look into the seismic design requirements for non-building structures in ASCE 7, I suspect that will end up being your governing lateral load.

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

 

[Triangled]

Are you adding 25% of your 125 LL for seismic? Maybe not an issue in KY.

 

[sponton]

I'm more concerned about this rigidity that god knows where it comes from. I added 1000K lateral load and I am only getting a story drift of 0.00005 which doesn't make sense, I don't have shearwalls or anything that would justify the rigidity in the building. I am really hating etabs lately.

 

[TehMightyEngineer]

Never used etabs but your gut-feeling check is definitely correct. You have something giving stiffness, likely a support or end condition that shouldn't be there.

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

 

[BadgerPE]

TME,

That is what I was thinking, but wasn't sure about the base situation. For low loads on an industrial slab, a small amount of base fixity could be designed for.

I also agree that most likely there is are some incorrect end releases in the beam/column members. Another thing I have run into, while modeling the diaphragm using plates, I have had too much stiffness when the plates were fixed to the beams. I have not used eTabs so I don't know how your diaphragm is modeled, but that could be a source of additional stiffness.

 

[Brad805]

I would look at your error log and use the standard solver to see what is going on during the analysis. To me it seems the system has an overall lack of restraint. Unless you have very large HSS columns, numerous fixed end connections, or concrete shearwalls, I do not see how you would deal with biaxial bending.

 

[sponton]

I have TS10x10x5/16 columns, "simply connected" with W21x44 Beams so M33/M22 are released. Columns were both pinned and fixed but no change has been shown. I would say the opposite, the system has a great amount of restrain, I just don't know where it is coming from. I mean numbers match, the base forces match the W of the bdg, the base shear, overturning moments, everything, but the lateral deflection isn't right. This is also part of an existing building so, mezzanine is to be connected to some existing 'long' columns.

 

[BadgerPE]

Did you only model the steel or did you model the diaphragm somehow? Also, do you have this mezz attached to the existing building in your model?

 

[sponton]

well the diaphragm is a steel deck with 4" of concrete, set as a rigid diaphragm for the analysis. I only modeled the mezzanine itself. Figured if much isn't happening in the mezzanine it won't affect the overall structure, otherwise it just becomes cumbersome modeling the whole thing.

 

[sponton]

I give up, I have to turn this crap soon and i keep running out of time. I just fixed the columns, removed the diaphragm load and put two point loads on the joint at the edge of the mezzanine. That gave me a max of 0.75 in deflection in the x-direction and 1.3in on the y direction, both below limits. Now, the question is why the hell my diaphragm isn't working when I assign loads directly. Well, at least there's movement.

 

[TehMightyEngineer]

You could check your deflection in etabs, 99% of all my modeling errors were caught by looking at the deflected shape.

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)