Department of Sustainable Construction

Management and Engineering

 

Faculty


Non-DestructiveTimber Research

Home/About Validation With Frame Testing

Beam Construction For Testing

Case Study 1 Agricultural Barn Case Study 2 - Visitors Center
ESF barn - Lafayette Rd
Case Study 3 - Museum 1 Case Study 4 - Museum 2 Case Study 5 Academic Building Case Study 6 - Residential Timber House Testing Video
Crouse College - from Flickr Platt barn compressed

For information about trainng or questions about the procedure, please contact Paul Crovella at plcrovella@esf.edu

About Non Destructive Testing

There is a growing recognition of the need for a more sustainable approach to construction. With this awareness comes an interest in studying existing buildings as a resource for redeveloping the built environment. Existing buildings can be deconstructed and the individual structural members reused, however “the greenest building is the one that is already built” (Carl Elefante). Successful reuse of entire buildings will depend on the ability to determine their current structural condition. In structural rehabilitation of timber-framed bridges, churches, mill buildings, homes, and barns, an engineering analysis is often necessary to determine the load bearing capacity. To perform this analysis the properties of the wood and the properties of the connections must be determined (Brungraber, 1985). There are a number of accepted non-destructive methods for determining in-situ mechanical properties of wood (Ross et al., 2004, Kasal et al., 2009). However there are no commonly accepted methods for non-destructively determining the strength and stiffness of timber connections (Anthony,2008). This paper presents results on the use of a frequency-based method to non-destructively determine the initial rotational stiffness of timber joints in two timber structures.

here is a growing recognition of the need for a more sustainable approach to construction. With this awareness comes an interest in studying existing buildings as a resource for redeveloping the built environment. Existing buildings can be deconstructed and the individual structural members reused, however “the greenest building is the one that is already built” (Carl Elefante). Successful reuse of entire buildings will depend on the ability to determine their current structural condition. In structural rehabilitation of timber-framed bridges, churches, mill buildings, homes, and barns, an engineering analysis is often necessary to determine the load bearing capacity. To perform this analysis the properties of the wood and the properties of the connections must be determined (Brungraber, 1985). There are a number of accepted non-destructive methods for determining in-situ mechanical properties of wood (Ross et al., 2004, Kasal et al., 2009). However there are no commonly accepted methods for non-destructively determining the strength and stiffness of timber connections (Anthony,2008). This paper presents results on the use of a frequency-based method to non-destructively determine the initial rotational stiffness of timber joints in two timber structures

Theory On Non Destructive Testing

The fundamental frequency for a uniform beam is governed by the beam’s geometry, material properties, and the joint stiffness (Eq. 1). If the geometry, material properties, and natural frequency can be determined non-destructively, the stiffness of the joints can be determined. This equation is derived by combining expressions for the displacement of beam with the equation of motion of a harmonic oscillator. The solution of this combined equation results in the general form sine, cosine, and hyperbolic sine and cosine. The exact shape is found by applying boundary conditions to the  solution. This solution can then be expressed in terms of the frequency that matches the boundary conditions. For a uniform beam, the resulting equation is: 

conditions and mode, w= uniform mass per unit length (including beam mass), E = modulus of elasticity, I = second moment of area, l= length of beam 

The value of Kn for the first mode of vibration is 9.87 for a simply supported beam, and 22.4 for a beam fixed at each end. (Figure 1)  A useful expression for describing the joint stiffness between the two extreme values is the “normalized frequency” which represents a linear variation between 0 for a simply supported beam, and 1.0 for a fixed-fixed beam.

Figure 1:Fundamental Frequency of a beam depending on boundary conditions (adapted from McGuire, 1995)
A useful expression for describing the joint stiffness between the two extreme values is the “normalized frequency” which represents a linear variation between 0 for a simply supported beam, and 1.0 for a fixed-fixed beam.

The value of Kn is not the value of the rotational stiffness of the joint, but rather a value that can be correlated to the rotational stiffness of the joint. An important point arises here: When considering a

Beam with semi-rigid joints, whether the joint stiffness will be significant in the analysis depends on the ratio of the joint stiffness to the bending stiffness of the beam. As shown in Figure 2, when the beam flexural or the joint rotational stiffness is large in relation to the other, the effect of changing joint stiffness has a minimal effect on frequency. When the joint stiffness is between 1 and 100 times that of the flexural stiffness, a change in joint stiffness produces a relatively large change in frequency.

 

Figure 2-Effect on natural frequency of varying rotational joint stiffness relative to beam stiffness

Methodology

 

Experimental Apparatus

Hardware and software The frequency was determined using an accelerometer placed on the beam to produce an electrical signal due to the free vibration of the member. The accelerometer was a quartz shear type single-axis accelerometer (500 mV/g). The accelerometer was attached through a 9 m (30 ft) foot cable to a USB powered signal conditioner. This signal conditioner is powered through the USB port of the computer, and provides a constant current input to the accelerometer. The voltage at which this current flows is measured by the signal conditioner, and then amplified and output to the computer via a stereo jack. The sound card of a personal computer accepted the stereo jack signal as an input. A digital audio editor “Audacity 1.2.6" was used to capture, edit, and save the signal as a .wav format file. The signal was then processed using a Fast Fourier Transform (FFT), with code based on an applet from Evan Mroz. The resulting frequency response spectrum output file was imported to Excel® to identify the peak of the fundamental frequency. This frequency was then correlated to a joint stiffness, based on either a closed form solution (see references) or using a finite element model.