Assessment of the Rigid Or Flexible Diaphragm Condition for Floor Systems Used in Buildings With Large Plan Aspect Ratios

Abstract eng:
A study devoted to define the diaphragm condition (rigid, semi-rigid, semi-flexible or flexible) for buildings with the floor systems most widely used in Mexico is presented. The selected floor systems are two-way ribbed RC slabs, precast RC beam and block, steel decks and RC waffle flat slabs. Subject buildings were design as moment frames according to current Mexico´s Federal District Code (MFDC). Selected dimensions for the floor systems were those typically used in office and apartment buildings currently constructed in Mexico City. Each floor system under study was designed according to current design practices of Mexican engineering firms. Different plan aspect ratios (L1/L2) for the buildings were considered in the study. For apartment buildings, the following floor systems were studied: two-way ribbed RC slabs, precast RC beam and block and RC waffle flat slabs. The typical separation among columns was 6 m in both orthogonal directions. Considered plan aspect ratios for the subject buildings were L1/L2=1, 1.5, 2, 2.5, 3, 3.5 and 4. For office buildings, the studied floor systems were two-way ribbed RC slabs, steel decks and RC waffle flat slabs. The typical separation among columns was 15 m in the longitudinal direction and 10 m in the transverse direction. Considered plan aspect ratios were L1/L2=1, 1.5, 2.25, 3, 3.75 and 4.5. All models were analyzed under uniformly distributed lateral loading with Ansys finite element software using refined meshes. Isotropic material modeling was considered for two-way ribbed RC slabs and for waffle RC flat slabs. To account for the difference in lateral stiffnesses in the main two orthogonal directions, orthotropic material modeling was considered for precast RC beam and block and steel deck floor systems. In order to assess the behavior of the different floor systems as a diaphragm (rigid, semi-rigid, semi-flexible or flexible), force and displacement criteria previously proposed in the literature were used (for example, Ju and Lin 1999, ASCE-7 2010). Based upon the results of the described parametric study, it was found that criteria based exclusively on the relative difference of lateral displacements between the supporting resisting lateral members (frames and/or walls) and the midspan of the floor system, such as the one available in ASCE-7, it is not a good criterion to classify most diaphragms, unless they are indeed very flexible. Mixed displacement and force-based criteria are much more adequate to classify diaphragms for most floor systems used in building construction; therefore, the index proposed by Ju and Lin (1999) is much more adequate on this regard. Using the index proposed by Ju and Lin, it was found that for the typical spans considered for apartment buildings, all the studied floor systems (two-way ribbed RC slabs, precast RC beam and block and RC waffle flat slabs) reasonably behave as rigid diaphragms for the considered plan aspect ratios. However, for the typical spans considered for office buildings, most of the studied floor systems behave as rigid diaphragms when 1≤ L1/L2<3, and as semi-rigid diaphragms when L1/L2≥3, but RC waffle flat slabs currently being built in Mexico City behave as semi-rigid diaphragms when 1≤ L1/L2<3.75, and as semi-flexible diaphragms when L1/L2≥3.75.

• Export as: BibTeX | MARC | MARCXML | DC | EndNote | NLM | RefWorks
• Add to your basket