Date of Award

5-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Civil Engineering

Advisor

Nielson, Bryant G

Committee Member

Schiff , Scott D

Committee Member

Juang , Hsein

Committee Member

Kaye , Nigel B

Abstract

Post hurricane damage investigations of light frame wood residential structures reveal that roof envelope failure induces considerable damage to the structure and its contents. Roof - to - wall (RTW) connection failures though not as common as roof sheathing failure also cause significant structural and material damage. Considerable changes have been made in the ASCE structural loads standard and International Building code (IBC) after hurricane Andrew in order to prevent RTW connection and sheathing failures. That includes not only a substantial increase in the design wind load in the past two decades but also a strict enforcement of tighter nailing schedules and stronger RTW connections (metal straps and hurricane ties). However a significant number of older buildings constructed with toenailed RTW connections exist and their safety and reliability needs to be investigated. Hence there is an apparent need to statistically understand the behavior of toenailed RTWconnections in existing buildings. Fragility analysis of roofs of older buildings will provide an insight on the prevailing level of safety and help to identify the shortcomings and the associated ramifications. Experimental statistics and analytical models of the toenail behavior and sheathing fasteners will help to formulate accurate roof fragility estimations. Estimation of the effect of wind load spatial correlation on the fragility estimation and the sensitivity of fragility curves to various modeling assumptions will further enhance the credibility of roof system fragility analysis methodologies. Since hurricane ties have replaced toenailed RTW connections in modern residential construction and are used as a retrofit measure to complement existing toenail connection capacities, understanding their behavior under high loads is essential. Experimental tests on hurricane ties subjected to uplift and combined (uplift and lateral) loads will not only provide an insight on the advantage of their usage in hurricane prone areas but also help in identifying the available design space when subjected to multi-axial loads. This information is crucial while developing statistical and analytical models for hurricane ties.
This research study evaluated the in-situ capacity of roof-to-wall connections and sheathing to rafter fasteners in light-framed wood construction. The outcome of this study was an analytical model designed to approximate the uplift behavior of toenail connections and to facilitate modeling of roof systems. In addition, the study experimentally examined three very common hurricane ties under uni-axial, bi-axial and tri-axial loads. After testing over 350 connections and performing detailed analyses, the currently used design equation for combined loads was found to be inefficient (least usable design space) and overly conservative. A new design space taking a 25% reduction on all allowable loads for hurricane ties when subjected to multi-axis load is proposed.
A finite element model of a light frame gable roof system was created using the developed analytical model of the RTW toenail connections and sheathing fasteners. Assessment of the overall impact of RTW and sheathing connector behavior on the wind-uplift fragility curves for the roof system was achieved using a Latin-hypercube based simulation strategy. It was found that the treatment of post ultimate connection behavior had a significant influence on the fragility assessment of the roof system. However assigning variable and uniform stiffness for roof-to-wall connectors and sheathing fasteners had little to no effect on the distribution pattern of wind uplift load among connectors. Additionally, the effects of gable end supports, sheathing thickness, nailing schedule and wind pressure spatial correlation on the fragility estimation were explored. The results indicated that the fragility estimations of both roof to wall connections and sheathing panel systems are not sensitive to the spatial correlation of wind pressure for wind perpendicular to the ridge.

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