Date of Award

5-2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Civil Engineering

Advisor

Pang, Weichiang

Committee Member

Schiff , Scott

Committee Member

Ross , Brandon

Abstract

The use of flexible roof diaphragms is very common in the United States, both for multi-family residential buildings and large-scale commercial buildings. Due to its simplicity, the traditional diaphragm design method is commonly used in diaphragm design, in particular for the design of diaphragms with relatively small dimensions. The traditional diaphragm design method assumes the axial forces developed in framing members under in-plane loading carried only by the perimeter chords. The traditional diaphragm design method has always been thought to be a conservative design method, especially when applied to large diaphragms. In recent years, the engineering community began to question the applicability of the traditional diaphragm design method. A new design approach known as the collective chord design method was proposed by Lawson (2007) to analyze the chord forces for very large flexible roof diaphragms. This method utilizes strain compatibility of a simple beam to estimate the axial forces in chord members. According to this method, the axial force carried by each continuity chord is proportional to its distance from the neutral axis. Since the collective chord method distributes the axial forces to intermediate chord members, the axial forces in the perimeter chords or end chords usually are much smaller than that estimated by the traditional diaphragm design method. While, the collective chord method yields more economical design than the traditional method (due to smaller forces in the end chords), the design assumptions have not been fully verified via full-scale experiment or rigorous analytical models (e.g. finite element models).
The main objectives of this thesis study were (1) to perform numerical analysis of large panelized all-wood roof diaphragms to observe the chord force and shear force distributions under in-plane loading, and (2) to use the analysis results to determine the applicability of the traditional and collective chord design methods.
The roof diaphragms utilized in this study were numerically modeled using a program called M-CASHEW (Matlab - Cyclic Analysis of Shear Walls), which was initially developed to analyze wood shear walls. The M-CASHEW program was modified to include new features for modeling large panelized diaphragms. The M-CASHEW diaphragm model was validated by comparing the data from actual diaphragm tests to the model results. Various sensitivity analyses were performed to calibrate the most computationally efficient modeling parameters, in terms of both the computational demand (speed) and accuracy, for use in large diaphragm analysis.
The calibrated diaphragm modeling parameters were then used to model the behavior of a case study all-wood panelized roof diaphragm. The dimensions of the case study diaphragm were 192 ft x 96 ft. Analyses were performed for both loads applied in the longitudinal and transverse directions (i.e. aspect ratios of 2:1 and 1:2). Twelve large diaphragm models were created to investigate the influences of various modeling and construction parameters (e.g. uniform nail schedule versus multiple nail zones) on the overall performance of the large diaphragm. The results obtained from numerical analyses were compared to the calculations of the traditional as well as the collective chord design methods.
For the case study diaphragm with an aspect ratio of 2:1 (length to width), i.e. with uniform load applied perpendicular to the longitudinal direction, the analysis results showed that the tension force in the longitudinal chord member at one-quarter of the width from the end chord in tension was about 10% of that in the end chord (Fo). According to the collective chord method, this chord should carry a tension force equal to 50% of Fo. This suggests that the collective chord method has the potential of overestimating the forces in the intermediate chords. The influence of diaphragm aspect ratio on chord force distribution was studied using three diaphragms of different aspect ratios (1:2, 2:1 and 4:1). The results showed that the chord force distributions for the range of aspect ratio considered did not follow the collective chord model. The analysis results also showed that the traditional diaphragm design method is not overly conservative and may be used for large diaphragm design. The behaviors of diaphragms with different sheathing nail schedules were also investigated. The results confirmed that the use of multiple nail zones, which is a common practice in panelized roof construction, is not only more efficient and economical but also structurally sound.

Share

COinS