GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.2 467 asymmetrical in the transverse direction (Walls P1 and P2). The log-walls are made of glulam timber derived from Scots Pine trees (Pinus sylvestris L.) and belong to the C24 class of resistance. The dimensions of the cross sections of the logs used in the outer walls and inner walls are 160x160mm and 80x160mm. In the corner joints the presence of construction gaps of 1mm was accounted in order to simulate the real construction behaviour. The building has been analyzed indipendently in the two plan directions (X,Y). The Modal- DBD steps defined in Figure 1 are developed in both the X and Y direction analyses. The first step of Modal-DBD method requires to define the displacement limits for different hazard levels.For each displacement level a Non-Exceedance (NE) probability has been assigned. Currently, for timber structures, there aren’t specific drift limits, in particular interstory drift level that reach a building up to collapse has not yet been universally agreed on. Based on the evidences of experimental test find in licterature, the limits presented in Table 3 for Blockhaus structures are too conservative. For the Immediate Occupancy level has been chosen the limitation of interstorey drift in suggested in [§ 4.4.3.2(b)] of EC8 [4] for buildings having ductile non-structural elements, corresponding to 0.75%. For the Life Safety level, has been adopted the limit suggested in §12, Table 12.12.1 of ASCE 7-16 (The American Society of Civil Engineers, 2016) of 2.5% for structures other than masonry with four of less stories and risk category I or II. Based on licterature values of collapse displacements for Blockhouse structures, the drift of 3% has been incremented to 4% as the collapse prevention drift level (Tab. 1). Table 1 - Adopted drift limits for Blockhaus structures. Performance Expectations Level Exceedance probability Drift limit Level IO 50%/50 yr 0.75% Level LS 10%/50 yr 2.5% Level CP 2%/50 yr 4% The building is designed for the site (Lat. 40.79, Long. -124.16) located in Eureka, California, assuming Site Class C (very dense soil and soft rock) (ASCE 7-16 soil classification). The design response spectrum curves have been evaluated based on the prescription of ASCE 7-16 (2016) §11. The base of the Modal-DBD is the development of a modal analysis to introduce the Equivalent Structure. The procedure consists in an equivalent linearization of a non-linear multigrade freedom system (MDOF) in which the story stiffness of the linear elastic MDOF system is estimated with a rigidity equivalent at the target inter-story drift. The X-direction and Y-direction were analyzed indipendently. The natural frequencies, ω n and mode shape φ n , are determined by solving the eigenvalue problem. After, the modal participation factor Γ n for the n -th mode was calculated In order to measure the contribution of each mode to Γ n , has been introduced the the inter-story drift factor γ jn , independent from the normalization. To generate the inter-story drift spectra the SRSS modal combination rule has been employed. At last, the weakest floor and the equivalent stiffness has been evaluated. In order to define the backbone parameters, the entire log-walls named P1, P2, P3 and P4 have been respectiverly divided into three parts, named GF (Ground Floor), FF (First Floor) and SF (Second Floor). Each part has been modelled as full scale log-wall via FE, using the softwere Abaqus/Explicit (Dassault Systèmes, 2015). The model has been realized by assembling different parts. The general longitudinal log, consists on 3D continuous deformable regular element.