Seismic Loads for Intermediate Mezzanine

[JAE]

I have a large building - 3 floors each with about 300,000 sf. Between the second floor and the roof there is a small (8000 sf) mezzanine attached to the columns.

Under IBC 2000, how do you determine the lateral seismic force on this small mezzanine? I can get the story forces for each level based upon the overall building - just not sure if the mezzanine counts as a "level" or if its mass gets added to the main floors and the seismic load is calculated separately (to check the columns) as an Architectural component?

 

[Bagman2524]

Verify that the mezzanine is "rigid" f > 16.67 Hz. If it is then you should use the accelerations from the floor level that it is attached to.

 

[haynewp]

Will it also have its own LFRS or are you just relying on the main building columns?

 

[JAE]

Well, technically it is dependent for lateral bracing on the building's concrete columns. But there is also a concrete shaft that could be connected to. In either case I just don'e see counting the mezzanine as a "level" since it's so small a part of the whole area (8000 sf vs. 300,000).

 

[haynewp]

That was my first response since it is less than 3% of the floor area. I would probably just add the additional weight to the second floor and design the overall lateral system without considering it as a level. Then come back and look at the local effects from the mezzanine inertia applied to the building columns.

 

[youngstructural]

I would tend to neglect this floor in the calculation of the seismic moment and base shear for the overall building, and simply not allow the resulting provisions and design to be within 2.7% (~8/300) of the required factored applied loads calculated. As for designing and detailing of the mezzanine floor and the supporting structure, I would check the columns for the imposed loads (really no choice, since they will be present) and design these components accordingly.

I am actually a Canadian engineer, however I'm working in New Zealand. Some of the world's most ground breaking seismic work has been done, and continues to be done, here in New Zealand. Tom Paulay, one of the team of Professors at Canterbury University here in Christchurch who created Capacity Design has been quoted as saying that something to the effect of, and I am paraphrasing here, "Forget the last 10% of an earthquake. An earthquake doesn't know about the last 10%; It's much more important that the structure knows how to behave when in an earthquake than the structure has the capacity to resist forces who's calculation is in the end arbitrary." Basically if you make sure that your plastic hinges can dissipate energy, there is no soft storey mechanism and (if in Reinforced Concrete) the detailing allows for the capacity design assumed behaviour to occur, you're good even if you acheive a capacity 10%.

Good luck,
Let us know what you choose to do!

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[youngstructural]

Darn; heynewp beat me by seconds!

But it is nice to see we're pretty close to agreement on how such a scenario should be handled.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[JAE]

Thanks for the replies.

So do I use Arch component forces from the IBC or what? How does one take the mezzanine mass and get a demand from it?

The IBC takes the total base shear and distributes it to the various levels. Since the mezzanine isn't a "level" then I can't use that approach.

 

[haynewp]

Yes, use equation 16-67 with the weight of the mezzanine to get the component force. Look at treating it as an access floor, you may can justify using Rp=2.5.

 

[Zhuge]

I don't know How advance your software is. You can generate a level 2 spectra. from that spectra you can design the mezzanine level separately by assuming the mezzanine sitting on the ground with seismic spectra = level 2 spectra.

 

[whyun]

I think using the component seismic force is overly conservative. Why not apply the roof coefficient on the mass of the mezzanine floor?

Though the area of the mezzanine is small relative to the whole building area, 8000 sf isn't that small. It is too large as a "component" and too small to be considered a "level".

 

[haynewp]

There is probably around 20 kips of difference in treating it as a component.