Adding in a mezzanine
Adding in a mezzanine
(OP)
I have a SDC C all-steel 3-story building. The building is an OMF building for the top two stories and OCBF building at the bottom story.
The owner wants to add a mezzanine between the foundation and the first level for part of the building plan (not full). He's very adamant against adding in additional columns or braced frames to make this mezzanine a self-supporting separate structure. We did anticipate that the columns and footings needed to be designed for some extra load for a mezzanine situation so we have some extra capacity, but we'd been expecting to add in braces where needed.
Due to this situation, my boss has asked me to investigate a lateral system where the seismic loads from the mezzanine are applied as point loads at the mezzanine elevation, and the columns span this load between the foundation and the second floor. The loads would then be distributed through the second floor diaphragm to the overall existing building braced frames. He wants us to do this to the extent that he wants us to investigate if we need to reinforce the existing columns. This is of course an iterative nightmare because if I repair a column, the stiffness will change and we'll have to redistribute seismic forces. And of course, the building plan is irregular, so I have different column shapes, sizes, and even orientations to worry about.
I have some serious questions/doubts:
1. Is this system really allowed by code? I don't know that it's specifically prohibited, but I'm having trouble classifying the
"system".
2. What kind of system is it? Do I need to adjust my seismic "R" factor?
3. Any other ideas/questions/advice I need to consider? Anyone done this before?
The owner wants to add a mezzanine between the foundation and the first level for part of the building plan (not full). He's very adamant against adding in additional columns or braced frames to make this mezzanine a self-supporting separate structure. We did anticipate that the columns and footings needed to be designed for some extra load for a mezzanine situation so we have some extra capacity, but we'd been expecting to add in braces where needed.
Due to this situation, my boss has asked me to investigate a lateral system where the seismic loads from the mezzanine are applied as point loads at the mezzanine elevation, and the columns span this load between the foundation and the second floor. The loads would then be distributed through the second floor diaphragm to the overall existing building braced frames. He wants us to do this to the extent that he wants us to investigate if we need to reinforce the existing columns. This is of course an iterative nightmare because if I repair a column, the stiffness will change and we'll have to redistribute seismic forces. And of course, the building plan is irregular, so I have different column shapes, sizes, and even orientations to worry about.
I have some serious questions/doubts:
1. Is this system really allowed by code? I don't know that it's specifically prohibited, but I'm having trouble classifying the
"system".
2. What kind of system is it? Do I need to adjust my seismic "R" factor?
3. Any other ideas/questions/advice I need to consider? Anyone done this before?






RE: Adding in a mezzanine
ASCE 7-05 Section 12.1.3 gives some information on "smaller portions of the structure tied to the remainder...".
Furthermore Section 12.2.3.1 lists a few exceptions for the rule about using the smaller R factor, those are:
1. Rooftop structures..., [and]
2. Other supported structural systems with a weight equal to or less than 10 percent of the weight of the structure.
3. Detached ...
RE: Adding in a mezzanine
If I add this mezzanine in as a another floor and set the columns I want to use as "lateral" columns with the tops pinned at level 2 and bottoms pinned at foundation and all the Level 1A mezzanine beams as "gravity" beams, will RAM distribute this correctly based on the rigidities of the columns? Or will it just give an error since there's no frame of any kind? I'm just not sure if RAM will properly redistribute part of this load back up into the level 2 diaphragm?
RE: Adding in a mezzanine
Note, these types of structures tend to be very sensitive to P-Delta amplification and I usually recommend no P-Delta analysis, at least in the preliminary design.