When does a mezzanine need to be considered a story for seismic design ?
Mezzanine is defined in 2012 IBC Section 202 as “An intermediate level or levels between the floor and ceiling of any story and in accordance with Section 505.” 2012 IBC Section 505.2 requires: “The aggregate area of a mezzanine or mezzanines within a room shall be not greater than one-third of the floor area of that room or space in which they are located. …”
The topic of when a mezzanine needs to be considered a story has recently come up. We consulted Igor Marinovic, Senior Research Engineer at BlueScope Buildings North America, Inc. for an answer. Here is his interesting and informative reply:
“Good question; unfortunately, there is no simple answer. As you know, mezzanines are not defined by ASCE 7-10 as structural elements, but they exist. 2012 IBC Chapter 5 has a definition related to occupancy and fire; however, that has nothing to do with seismic design. Using the 33% area-based limit would be applicable if the unit weight (psf) of the mezzanine floor is the same as the unit weight of the roof, but in low-rise buildings, the mezzanine can be much heavier, and in fact, often drive the overall building response.
The better answer to your question would be to follow the intention of ASCE 7-10 Chapters 13 and 15, where the weight of the components/nonstructural elements is limited to 25% of the overall building (frame) weight. It can easily be shown that this is effectively the same limit as the 33% limit mentioned above. At least this weight-based limit looks rational when talking about seismic design. But, in real world design, this option is usable for very small mezzanines only.
When these limits are exceeded, it is common to explore other options. Mezzanine separation is the first choice where the mezzanine columns are kept away from the building’s seismic force-resisting system. For a one-story building with a mezzanine, the ‘outer shell’ remains a one-story system. Another option may be used for cases where the roof diaphragm is flexible. Consider a common metal building system layout with 5 equal bays (Figure 1), where the moment frames are perpendicular to the ridge, and a heavy mezzanine is located in two bays at one end of the building. For the transverse direction of loading (perpendicular to ridge), the roof diaphragm is flexible, so there is little interaction between adjacent moment frames. Therefore, each frame line acts as an independent line of resistance, with the exception of frames connected to the mezzanine. This can lead to a practical solution where three frames around the mezzanine are considered as a functional unit – the two-story system, while the remaining three bays would not be affected by the mezzanine, i.e., are ‘isolated’ from the mezzanine-related effects.
Note that for the orthogonal direction of loading, the longitudinal seismic force-resisting systems (SFRSs) will need to be designed as either one- or two-story, depending on whether the mezzanine is connected to a particular SFRS or not.
The Metal Building Manufacturer’s Association (MBMA) also worked on this problem, and there is a recent UCSD study by Uang and Smith, who provide worthwhile recommendations based on structural dynamics.
In 2011, three full-scale metal building system moment frames were tested on the shake table at UCSD. The third specimen (https://nees.org/warehouse/experiment/3052/project/986) included a mezzanine that was approximately 50% of the whole building weight, and the specimen performed well, and the initiation of failure was at somewhere between 200% and 250% of the design basis earthquake (DBE). But, none of the three specimens could be collapsed, since the system has inherent multiple redundancies. This video is a summary, with the mezzanine case shown in the last 5-6 minutes.”
We are very grateful to Mr. Marinovic for sharing his experience and knowledge on this subject.
Via S.K.Ghosh Assoc.