1 trail bridge inspection timber deterioration & decay


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    OBJECTIVES: Recognize the Types and Causes of Timber DeteriorationRecognize Natural Defects In Timber ElementsUnderstand What Causes DecayIdentify Where Decay Is Likely To OccurIdentify Methods Used to Detect Deterioration

  • Types and Causes of Timber DeteriorationNatural Defects In Timber ElementsFungi (Brown and White Rot)InsectsTermitesCarpenter AntsPowder-post BeetlesCaddis FliesMarine Borers

  • Types and Causes of Timber DeteriorationDelamination of Glulam BeamsFireImpact or CollisionsAbrasion or WearOverstressWeathering or WarpingAnimals


  • Fungi

  • Mold and Stain

  • Brown and White RotFungi types that weaken the wood include:

    Brown rot - degrades the cellulose and hemi-cellulose leaving the lignin as a framework which makes the wood dark brown and crumbly

    White rot - feeds upon the cellulose, hemi-cellulose, and the lignin and makes the wood white and stringy

  • Brown and White Rot

  • InsectsInsects tunnel in and hollow out the insides of timber members for food and shelter. Some common types of insects include:TermitesCarpenter antsPowder-post beetles or lyctus beetlesCaddis flies

  • Termites

  • Carpenter Ants

  • Powder-post Beetles or Lyctus Beetles

  • Caddis flies

  • Marine Borers

  • Marine Borers

  • Marine Borers

  • Types and Sources of DeteriorationDelaminations Delaminations occur in glued-laminated members when the layers separate due to failure within the adhesive or at the bond between the adhesive and the laminate. They provide openings for decay to begin and may cause a reduction in strength.

  • Other Types and Sources of DeteriorationDelaminations

  • Types and Sources of Deterioration Loose connectionsLoose connections may be due to shrinkage of the wood, crushing of the wood around the fastener, or from repetitive impact loading (working) of the connection. Loose connections can reduce the bridges load-carrying capacity.

  • Types and Sources of Deterioration Loose connections

  • Types and Sources of Deterioration Surface depressionsSurface depressions indicate internal collapse, which could be caused by decay.

  • Types and Sources of DeteriorationFire

  • Types and Sources of Deterioration Impact or CollisionsSevere damage can occur to truss members, railings, and columns from floating debris or ice.

  • Types and Sources of Deterioration Abrasion or Wear

  • Types and Sources of DeteriorationExcessive Wear Usually indicated by knots that are above the surrounding wood. Nails protruding above the surface and bent over.Rounding on the edges of the planks at the bridge end.Dished and polished Planks.

  • Types and Sources of Deterioration Overstress

  • Types and Sources of DeteriorationOverstress

  • Types and Sources of Deterioration Weathering or Warping

  • Other Types and Sources of DamageWood swellAnimal


  • NATURAL DEFECT DEFINITIONSCheck A separation of the wood normally occurring across or through the rings of annual growth and usually as a result of seasoning.Split A separation of the wood through the piece to the opposite surface or to an adjoining surface due to the tearing apart of wood cells.Shake A lengthwise separation of the wood which occurs between or through the rings of annual growth.

  • Defect - Check

  • Defects - Split

  • Defect - Shake

  • Review of Natural Defects

  • WHAT CAUSES DECAY?Decay detection is a significant part of the timber bridge inspection.Basic understanding of the decay is essential.The source of the decay is a plant a fungus that has tiny microscopic, thread-like roots called hyphae that penetrate throughout the wood in search of food. The hyphae penetrate the wood secreting powerful enzymes that reduce the wood into food for the plant. This chemical alteration of the wood cell is what we call decay.

  • Life Cycle of Fungus

  • Fungus

  • DECAY MECHANICSThe wood is initially infected when the fungus spores land on the surface or in the checks and splits of wood and geminate. The following four conditions must exist for germination:A sufficient supply of oxygen.A favorable temperature range. (32 - 90 F)An adequate food supply.Available water. (Wood must be above the fiber saturation point of the wood. Moisture content of approximately 25%).

  • Controlling DecayDepriving the spores of any one of these requirements will effectively slow, stop or prevent decay.

    For example:(1) If wood is underwater, there is no usable oxygen present and the wood will not decay; HOWEVER DECAY WILL FLOURISH AT THE WATERLINE(2) If the wood is treated, there is no palatable food for the fungi; HOWEVER, IF THE TREATMENT ENVELOPE IS BREACHED AS IN A CHECK - -DECAY MAY BE PRESENT.(3) If the wood is always dry there is no moisture for germination.

  • Where do I look for decay?

  • Locations to look for decay:Around ChecksAround SplitsAround ShakesAround CracksAround FastenersAreas in contact with soilAreas where debris and water collectCommon Areas Where Decay Occur

  • Where do I look for decay?Conditions and locations conducive to decay can be readily identified:Members that are in contact with soil such as posts, piling, abutments, wing walls, etc. Sawn timber members that have large checks open to the weather or stream flow (water entry).Wood penetrating fasteners and wood to wood contact.End grained surfaces.

  • Decay Detection MethodsMethod for Detecting Exterior DeteriorationVisual InspectionProbingMethod for Detecting Interior DeteriorationSoundingMoisture MeterDrilling and Coring

  • Method for Detecting Exterior Deterioration -Visual InspectionThe simplest and most common method for locating deterioration and decay The inspector observes the structure for signs of actual or potential deterioration and decay Visual inspection requires a strong light and is useful for detecting intermediate or advanced surface decay Visual inspection cannot detect decay in the early stages, cannot detect internal decay, and should never be the sole method used.

  • Method for Detecting Exterior Deterioration -Visual InspectionWhat to Look for:Fruiting bodies almost always indicate a severe decay problem. Staining and discoloration indicate areas of potential decay. Sunken faces and localized surface depressions can indicate decay.Horizontal checks that can entrap water.Interfaces between different wood members, wood and concrete, etc.Where dirt and debris has been placed against the wood.At fastener locationsInsect activity Most insects prefer the softer decayed wood to the hard dry wood.

  • Visual Signs of External DecayFungi or fruiting bodies (conks)

    Under the bridge, on girders and sills, anywhere the wood is subject to wet/dry cycles.

  • Visual Signs of External DecaySpongy and Rotted areas.

    Look at wood/soil interfaces and anywhere water collects. Stains may be rot indicators.

  • Visual Signs of External Decay - Discoloration

  • Visual Signs of External Decay - Brown Rot

  • Method for Detecting Interior Deterioration -ProbingProbing with a pointed tool such as an awl will locate decay near the wood surface. Decay will be evidenced by excessive softness or lack of resistance to the probe penetration and the breakage pattern of the splinters. A brash break (straight line break directly above and parallel with the axis of the awl) indicates decayed wood, whereas a crisp splintered break with the splinter hinging from one end indicates sound wood. However, care must be taken to differentiate between decay and water-softened wood.

  • Method for Detecting Interior Deterioration - Pick TestDecayed wood breaks abruptly across grain without splintering.Sound wood pries out as long splinters.

  • External DecayWhat are the Rot Indicators?

  • External DecayHow Deep is that Rot?

  • External DecayWhat is Underneath the Surface?

  • Method for Detecting Interior Deterioration - SoundingSounding the surface by striking it with a hammer is one of the oldest and most commonly used inspection methods. Although sounding is widely used, interpretation is VERY subjective.Soundings are based on the tonal quality of the ensuing sounds and the rebound of the hammer. Practical experience has shown that sounding is only useful for members less than 4 inches thick.

  • Method for Detecting Interior Deterioration - SoundingInterpreting Soundings:Sound timber gives a crisp sound. Defective timber gives a dull sound.Loose hardware will vibrate.

    Note: A 2 inch thick shell of competent wood is sufficient to mask any interior rot.

  • Method for Detecting Interior Deterioration -Moisture MetersAs wood decays, electrolytes are released and the electrical properties are altered. Based on this phenomenon, a resistance-type moisture meter can be used to detect these changes. Although the moisture meter does not detect decay, it does identify wood that has a moisture content high enough for decay growth. Moisture contents of greater than 20% to 30% would indicate conditions suitable for decay.

  • Method for Detecting Interior Deterioration - Drilling and CoringDrilling and coring are probably the most common methods used to detect internal decay in wood members. Drilling is usually done with a battery powered


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