QUESTION
I am designing a masonry addition with a small mezzanine inside. My walls are designed to span between the slab and the roof, the mezzanine diaphragm is not supporting the masonry wall. However, my masonry wall is supporting the mezzanine floor by a ledger. ASCE 7-05 12.11 gives the out of plane anchorage forces for structural walls. This section specifies the out of plane attachment forces required for diaphragms that support the walls. Do these forces still apply in my situation? In the past I have used the same anchorage forces because I couldn’t find anywhere else that specifies differently. In this case the mezzanine is extremely small, so the strapping and bolting to resist the out of plane bending on the ledger seems excessive. Am I overkilling this?
This mezzanine is attached to an exterior CMU wall that is part of the LFRS. The mezzanine is supported by a ledger that transmits gravity and seismic lateral loads to the CMU shear wall. This is why I have always used the structural wall anchorage forces to determine the anchorage of the mezzanine. For a mezzanine this small (8’-0" x 12’-0") the minimum 280 plf force is controlling, which is the same force that controls at the top of the masonry wall at the roof connection (wall is only 17’-0" tall. Just wanted to see if there was some supporting information out there to reduce the loads for something like this where the mezzanine is so small.
REPLIES
Michel60
I’d say your current approach is conservative but reasonable. However, you could adjust for some of the actual conditions if they warrant. For example, if the mezzanine is small and has no connection to vertical elements of the LFRS and is otherwise supported for gravity loads by light framed walls, hangers or columns of small cross-section, then I’d treat the mezzanine as the load “generating” element and calculate an Fp force to design the attachment to the masonry wall which would then be the load “resiting” element. However if you have frames or shear walls that are part of the LFRS connecting to the mezzanine or even substantial gravity columns (ones with significant flexural stiffness) then in my opinion your current approach is conservative and necessary if only to account for the load path uncertainty.
I do think it is possible that the design forces may be less and not be subject to the 280plf minimum of Section 12.11.2. If the only connection to the LFRS is the connection at the CMU wall being considered then the load path is the mezzanine loading applied the wall and not the mezzanine supporting the wall load. In the case of the mezzanine loading the wall, the mass considered for seismic would be only that of the mezzanine and any tributary elements not including the mass of the CMU wall itself (although the CMU mass would be considered in the wall design but not in the diaphragm connection to the wall). Section 12.11 references “…walls to supporting construction…” your mezzanine is “supported” and the lateral forces resulting are only those of it’s own inertial mass (possibly with minimum limits according to 12.10.1.1.) and the limits of Section 12.11.2 do not apply.
In terms of detailing you likely end up doing all the same things you’ve suggested with ledger bolts and tension ties but possibly being slightly smaller and/or more widely spaced.
SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=327448
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