UVM Theses and Dissertations
Format:
Print
Author:
Ernst, Matthew R.
Title:
Dept./Program:
Civil Engineering
Year:
2009
Degree:
MS
Abstract:
The motivation for this research was to apply methods of vibrations testing in order to determine axial loads in the pin-ended truss members of the trusses supporting the roof lantern as well as the adjacent x-bracing of the Breeding Barn located in Shelburne, Vermont. The method of vibrations testing was necessary in order to detennine the in-situ axial loads of the members in the barn. Other common methods, such as strain gauges, were not useful for this application because strain gauges can only detect changes in strain and load from a reference state. The large size and weight of the roof at the Breeding Barn produce significant axial loads in the members. The reference state of stress in the members is therefore the desired in-situ measurement, making strain gauge experiments insensitive.
In order to determine the axial loads in the truss members, acceleration time histories of the individual truss members were collected using wireless accelerometers provided by MicroStrain of Williston, Vermont. The vibrations testing technique was improved from previous tests at the bam by using two accelerometers on each member and also by using a higher sampling rate to observe higher frequencies. Using the Fourier transform on the acceleration data, power spectral density plots were produced, from which the resonant frequencies of each member were determined. Knowing the resonant frequencies for a member and the beam vibration equation developed for pin-ended members, the axial loads of the truss members were calculated.
A major concern for the Breeding Bam is how the roof structure will handle a full snow load if and when it occurs. Two and three dimensional computer models of the roof structure were developed to predict the axial loads in the truss members over a range of loading conditions. Due to member pretensions and loose connections that are assumed to be present throughout the barn, "superposition" models were developed to predict the axial loads in the structure due to a full snow event. The superposition models added the in-situ tensions detennined experimentally to the tensions predicted by the computer models for the snow load case. By neglecting the dead load case from the models and instead including the experimental results in the superposition models, the pretensions present in the members were incorporated. Based on the results of the 3-D superposition model, the lowest factor of safety in the tested members is 1.34, and thus the members are safe for snow loading.
In order to determine the axial loads in the truss members, acceleration time histories of the individual truss members were collected using wireless accelerometers provided by MicroStrain of Williston, Vermont. The vibrations testing technique was improved from previous tests at the bam by using two accelerometers on each member and also by using a higher sampling rate to observe higher frequencies. Using the Fourier transform on the acceleration data, power spectral density plots were produced, from which the resonant frequencies of each member were determined. Knowing the resonant frequencies for a member and the beam vibration equation developed for pin-ended members, the axial loads of the truss members were calculated.
A major concern for the Breeding Bam is how the roof structure will handle a full snow load if and when it occurs. Two and three dimensional computer models of the roof structure were developed to predict the axial loads in the truss members over a range of loading conditions. Due to member pretensions and loose connections that are assumed to be present throughout the barn, "superposition" models were developed to predict the axial loads in the structure due to a full snow event. The superposition models added the in-situ tensions detennined experimentally to the tensions predicted by the computer models for the snow load case. By neglecting the dead load case from the models and instead including the experimental results in the superposition models, the pretensions present in the members were incorporated. Based on the results of the 3-D superposition model, the lowest factor of safety in the tested members is 1.34, and thus the members are safe for snow loading.