Inspeccion de puentes de madera

5
Detecting decay in wood components
R.J. ROSS,
USDA Forest Products Laboratory, USA and

X. WANG and B.K. BRASHAW
Natural Resources Research Institute, USA

5.1

Introduction

Wood deterioration is one of the most common damage mechanisms in timber bridges and other wood structures and often inflicts damage internally. This may occur without visible signs appearing on the surfaceuntil a member’s load-bearing capacity has been largely destroyed. Determining an appropriate load rating for an existing timber structure and establishing rational rehabilitation. repair, or replacement decisions can be achieved only when an accurate assessment of its existing condition is made. Knowledge of the condition of the structure can lead to savings in repair and replacement costs byminimizing labor and materials and extending its life. In wood structures, the degradation of a load-bearing (in-service) member may be caused by any one of several organisms that derive their nourish­ ment or shelter from the wood substrate in which they live. For example, several types of fungi attack wood. The hyphae of these fungi secrete enzymes that depolymerize the chemical components of wood,thereby lowering the density, strength, and hardness of a member. This results in a significant reduction in load-carrying capacity, which in turn may result in the member’s failure. Accurate detection of decay in wood structures is therefore critical to ensuring the safety of the public and extending the service life of the structures. Recently, we prepared a comprehensive manual on the inspectionof wood structural elements titled Wood and Timber Condition Assessment Manual (Ross et al. 2004). It was prepared at the request of the American Forest and Paper Association to assist field engineers and other inspection professionals. It is published by the Forest Products-Society and includes chapters on visual inspection techniques, ultrasound or stress wave based inspection tools, and probingtype techniques. A chapter on post-fire inspec­ tion and assessment is included in addition to a sample inspection report and summaries from numerous inspections. Detailed descriptions of the 100

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various available tools, guidelines on their use, and interpretation of data obtained from them are included. This chapter presents a summary of the Woodand Timber Condition Assessment Manual. It focuses on current inspection techniques for decay detection and provides guidelines on the use of various non-destructive evaluation (NDE) methods in locating and defining areas of deterioration in timber bridge components and other civil structures.

5.2

Conventional methods

Conventional methods for detecting deterioration in bridges are dividedinto two categories: those for exterior deterioration and those for interior deterioration. In both cases, specific techniques or tools are appropriate for certain types of damage, and their usefulness varies depending on the type of structure. Although a variety of inspection methods may be employed, in practice the inspector uses only a few tools. The methods or tools are often dictated bybudget, previous experience, and the types of problem that are encountered.

5.2.1 Methods for determining exterior deterioration
Exterior deterioration is the easiest to detect because it is often readily accessible to the inspector. The ease of detection depends on the severity of damage and the method of inspection. Commonly used methods include visual inspection and probing. When areas ofexterior deterioration are located by these methods, further investigation by other methods is required in order to confirm and define the extent of damage.

Visual inspection
Visual inspection is the simplest method for locating deterioration in timber structures. The inspector observes the structure for signs Of actual or poten­ tial deterioration, noting areas for further investigation. Visual...