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By Mark Gilberg

Historic buildings are being abandoned and demolished at an alarming rate across America. One of the principle structural reasons for this is our inability to accurately assess the load bearing capacity of the wood floor systems in these buildings.

With the help of NCPTT, wood scientists at Purdue University and the USDA Forest Products Laboratory are developing a rapid, cost effective method for nondestructively evaluating the structural integrity of wood floor systems in older buildings. Researchers are studying the use of vibration frequency and stiffness as a means to assess the structural integrity of wood floor systems. The wooden floor is placed under a transverse load and the vibrational frequency and stiffness are measured. Researchers believe that this information will provide a new methodology for determining the soundness of wooden floors. Currently, there are no standard methods for examining and assessing the integrity of wood floors. Consequently, any methods used tend to lead to results that are extremely Preservation Technology from the Ground Floorsubjective and arbitrary.Under these circumstances building inspectors and engineers often assign overly conservative load bearing capacities for existing wood floors.

The assignment of low allowable floor loads often means that the continued use or adaptive reuse of older buildings becomes unfeasible, which can lead to their continued disuse or even demolition.

Researchers hypothesize that wood decay reduces the strength and stiffness of the floor, which should affect the dynamic behavior of the wood floor when subjected to loads. The relationship between the natural frequency and stiffness of a structural system is based on mathematical equations governing the motion of an idealized beam subjected to transverse loads.

This relationship yields the curve seen in Figure 1. Although this is an idealized system, it was hoped that this relationship could be used to assess the condition of an in-place floor system.

To test their hypothesis, researchers measured the fundamental frequency and stiffness of a floor and then compared these data to the curve generated by the theoretical model. First, researchers demonstrated that it is possible to accurately measure the fundamental frequency of vibration of an in-place floor. Next, researchers compared the vibrational response of individual joists with that of laboratory built floor sections as well as similarly constructed in-place wood floor systems in older buildings. Floors were vibrated at various frequencies and at different locations using an electric vibrator attached to the plank sheathing. Then researchers compared the natural frequency of each joist as an individual and as part of the floor system. They found that joist response is similar in both cases irrespective of the location of the electric vibrator. Finally, they determined the floor stiffness of wood floor systems by measuring the deflection of floors when they were weighed with bags of rock salt.

Having determined experimentally both natural frequency and floor stiffness, the researchers plotted these values against the theoretical curve. As can be seen in Figure 2, there is a strong correlation between the model and the experimental results.

These results demonstrate that the fundamental frequency of a wood floor can be used to estimate floor stiffness by simply referring to the graph in Figure 2. The derived value for stiffness can then be compared with building code design requirements to determine load bearing capacity. This is a major advance and with continuing trials it is hoped the compiled data will form the basis of computer software that will help structural engineers look beyond water and insect damage as well as age to accurately determine the load bearing capacity of wood floors in older buildings. For further information contact Dr. Michael Hunt, Purdue University, at (765) 494-3636 or e-mail mhunt@fnr.purdue.edu.


Mark Gilberg was NCPTT’s Director of Applied Research and Technology Transfer.
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Updated: Thursday, April 19, 2007
Published: Sunday, January 11, 2009


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