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?

 

[MacGruber22]

Why wouldn't you use Sds for the site and the appropriate response modification for the mezzanine system? Because of lower seismic area and given that steel columns must have 4 anchor bolts minimum, my mezzanines are usually cantilevered columns with a lateral point load of 4.0 to 4.5% of the tributary dead load for the mezz.

 

[sponton]

@MacGruber22

Actually that's how I checked the first time but the lack of response of the system was what was worrying some. Maybe I'm overreacting, which I do quite often, but since the mezzanine is attached to existing columns, I don't want to allow too much bending to occur. (The old construction plans aren't very enlightening and the building seems to be grossly conservative.

 

[MacGruber22]

"lack of response of the system was what was worrying some"

What do you mean by response? Drift?

What is your Sds?

 

[sponton]

yes, the frame wasn't really stiff.

0.241, but the thing that I think boosts up the loads is the R-value, we have non-braced non-moment frames. which aren't even in a category. [Inverted pendulum, cantilevered column has special steel moment frames or ordinary steel moment frames..]

 

[TehMightyEngineer]

My understanding is you use the ordinary steel moment frame R values, this only means that you've detailed it towards the requirements of ordinary steel moment frames. Ordinary steel design essentially means you're not using any provisions of the AISC Seismic Design spec to increase your R factor. Watch out for the axial load requirements of ASCE 7 12.2.5.2

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

 

[MacGruber22]

"we have non-braced non-moment frames. which aren't even in a category"

I don't know what that is.

That said, tension-only braced frames are the most efficient lateral frames - is there any chance of using cables or rod, or will walls and doors/windows get in the way?

How about just kickers to brace?

 

[sponton]

I was just checking the "new" ASCE-10, and it seems that they have made some modifications ( I was using ASCE-7-02) It'd seem that you can either go for a cantilevered column system and choose a steel special or ordinary cantilever column system. Which seem to be the same but as somebody suggested above there are some requirements for the columns such as not exceeding 15% of the capacity of the column including slenderness effects. I guess this sorts of settles down the problem. To answer your question, it'd seem that bracing is to be avoided at all costs, they have plenty of transit inside this plant. and they want to keep open spaces.