6.1-i

Table of Contents

Section 6 ..................................................................................................................................................

Inspection and 

Evaluation of 

Common Timber 

Superstructures 

 6.1 Solid Sawn Timber Bridges ................................................................... 6.1.1 

6.1.1 Introduction............................................................................... 6.1.1 

 6.1.2 Design Characteristics............................................................... 6.1.2 

 Multi-beam Bridges ........................................................... 6.1.2 

 Covered Bridges ................................................................. 6.1.3 

 Trusses ......................................................................... 6.1.4 

 Arches .......................................................................... 6.1.6 

 Primary and Secondary Members ...................................... 6.1.7 

 6.1.3 Overview of Common Defects ................................................. 6.1.8 

 6.1.4 Inspection Procedures and Locations........................................ 6.1.8 

 Procedures .......................................................................... 6.1.8 

 Visual........................................................................... 6.1.8 

 Physical........................................................................ 6.1.8 

 Advanced Inspection Techniques ................................ 6.1.8 

 Locations ............................................................................ 6.1.9 

 Bearing Areas............................................................... 6.1.9 

 Shear Zones.................................................................. 6.1.9 

 Tension Zones............................................................ 6.1.10 

 Areas Exposed to Drainage........................................ 6.1.10 

 Areas of Insect Infestation ......................................... 6.1.11 

 Areas Exposed to Traffic ........................................... 6.1.11 

 Areas Previously Repaired......................................... 6.1.11 

 Secondary Members................................................... 6.1.12 

 Fasteners and Connectors .......................................... 6.1.12 

 

 6.1.5 Evaluation ............................................................................... 6.1.13 

 NBI Rating Guidelines ..................................................... 6.1.13 

Element Level Condition State Assessment..................... 6.1.13 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1-ii

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6.1.1 

Section 6 

Inspection and Evaluation of 

Common Timber Superstructures 

Topic 6.1 Solid Sawn Timber Bridges 

6.1.1 

Introduction Timber bridges are gaining a resurgence in popularity throughout the United 

States. There are two basic classifications in timber construction: solid sawn and 

glued-laminated (glulam). A solid sawn beam is a section of tree cut to the desired 

size at a saw mill. Solid sawn multi-beam bridges are the simplest type of timber 

bridge (see Figure 6.1.1). 

Figure 6.1.1 Elevation View of a Solid Sawn Multi-Beam Bridge 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.2 

6.1.2 

Design 

Characteristics 

Multi-beam Bridges Solid sawn multi-beam bridges consist of multiple solid sawn beams spanning 

between substructure units (see Figure 6.1.2). The deck is typically comprised of 

transversely laid timber planks, and longitudinally laid planks called runners which 

are supported by the beams. Sometimes a bituminous wearing surface is placed on 

the deck planks to provide a skid resistant riding surface for vehicles, as well as a 

protective surface for the planks. Beam sizes typically range from about 150 mm 

by 300 mm (6 inches by 12 inches) to 200 mm by 400 mm (8 inches by 16 inches), 

and the beams are usually spaced about 600 mm (24 inches) on center. 

Figure 6.1.2 Underside View of a Solid Sawn Multi-Beam Bridge 

This bridge type is generally used in older, shorter span bridges, spanning up to 8 

m (25 feet). Shorter spans are sometimes combined to form longer multiple span 

bridges and trestles. Many older timber trestles were built for railroads and trolley 

lines. Solid sawn timbers have become obsolete for most modern bridge members 

due to the development of high quality glulam members (see Topic 6.2). 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.3 

Covered Bridges Covered bridges are generally found along rural roads and get their name from the 

walls and roof which protect the bridge superstructure (see Figures 6.1.3 and 

6.1.4). Covered bridges are usually owned by local municipalities, although some 

are owned by states or private individuals. Some still carry highway traffic, but 

many are only open to pedestrians or light weight vehicles. While most covered 

bridges were built during the 1800's and early 1900's, there are a number of 

covered bridges being built today as historic reconstruction projects. 

Figure 6.1.3 Elevation View of Covered Bridge
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.4 

Figure 6.1.4 Inside View of Covered Bridge Showing King Post Truss Design

 Trusses 

The majority of covered bridges are essentially truss bridges (see Figure 6.1.5). 

Solid sawn timber members make up the trusses of these historic structures. The 

covers on the bridges prevent decay of the truss and are responsible for their 

longevity. Typical truss types for covered bridges include the king post, queen 

post, Town, Warren, and Howe (see Figure 6.1.6). The floor system consists of 

timber deck planks, stringers, and floorbeams. The span lengths of covered 

bridges generally range from 15 to 30 m (50 to 100 feet), although many are well 

over 30 m (100 feet) and some span over 61 m (200 feet). 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.5 

Figure 6.1.5 Town Truss Covered Bridge

King Post

Double Warren

Queen post

Town

Howe

Figure 6.1.6 Common Covered Bridge Trusses 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.6 

 Arches 

Timber arches were first used in covered bridges by Theodore Burr to strengthen 

the series of truss configurations normally used in covered bridges. These became 

known as Burr arch-trusses (see Figures 6.1.7, 6.1.8 and 6.1.9). The arch served as 

the main supporting element, and the king posts simply strengthened the arch. The 

span lengths for Burr-arch truss bridges generally range from 15 to 53 m (50 to 

175 feet). Because of their greater strength, many of these structures still exist 

today. 

Figure 6.1.7 Schematic of Burr Arch-truss Covered Bridge

Figure 6.1.8 Burr Arch-truss Covered Bridge
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.7 

Figure 6.1.9 Inside View of Covered Bridge with Burr Arch-truss Design

Primary and Secondary 

Members 

The primary members of solid sawn multi-beam bridges are the beams, and the 

secondary members are the diaphragms or cross bracing if present (see Figure 

6.1.2). These bridges usually have timber diaphragms or cross bracing between 

beams at several locations along the span. 

The primary members in truss and arch structures are the truss members (chords, 

diagonals, and verticals), arch ribs, stringers, and floorbeams (see Figures 6.1.9 

and 6.1.10). The secondary members are the diaphragms and cross bracing 

between stringers, the upper and lower lateral bracing, sway bracing, and the 

covers on the roof and sides when present. 

Figure 6.1.10 Town Truss Design 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.8 

6.1.3 

Overview of 

Common Defects 

Common defects that occur on solid sawn timber beams include: 

3⁄4 Checks, splits, shakes, and knots 

3⁄4 Decay by fungi 

3⁄4 Damage by insects and borers 

3⁄4 Damage from impact/collisions 

3⁄4 Damage from wear, abrasion, and mechanical wear 

3⁄4 Damage from weathering/warping 

3⁄4 Damage from overstress 

3⁄4 Damage from fire 

3⁄4 Loose connections 

3⁄4 Failure of protective system 

A less common defect that may be encountered by the inspector includes damage 

from chemical attack. Refer to Topic 2.1 for a more detailed presentation of the 

properties of timber, types and causes of timber deterioration, and the examination 

of timber. 

6.1.4 

Inspection 

Procedures and 

Locations 

Inspection procedures to determine other causes of timber deterioration are 

discussed in detail in Topic 2.1.7. 

Procedures Visual 

The inspection of timber splits, cracks, shakes, fungus decay, deflections, 

crushing, delaminations, and loose connections is primarily a visual activity. 

 Physical 

The physical examination of a timber member can be conducted with a hammer or 

pick. The hammer is used to sound the members to detect hollow areas or internal 

decay. Picks are used to determine the condition of the surface. 

 Advanced Inspection Techniques 

Several advanced techniques are available for timber inspection. Nondestructive 

methods, described in Topic 13.1.2, include: 

3⁄4 Pol-Tek 

3⁄4 Spectral analysis 

3⁄4 Ultrasonic testing 

3⁄4 Vibration 

Other methods, described in Topic 13.1.3, include: 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.9 

3⁄4 Boring or drilling 

3⁄4 Moisture content 

3⁄4 Probing 

3⁄4 Shigometer

Locations Bearing Areas 

Check the bearing areas for crushing of the beams near the bearing seat (see Figure 

6.1.11). Investigate for decay and insect damage by visual inspection and 

sounding and/or probing at the ends of the beams where dirt, debris, and moisture 

tend to accumulate. Also verify the condition and operation of the bearing 

devices, if they are present (see Topic 9.1). 

Figure 6.1.11 Bearing Area of Typical Solid Sawn Beam 

 Shear Zones 

As discussed in Topic P.2, maximum shear occurs near supports. A horizontal 

shear force of equal magnitude accompanies the vertical shear component of this 

force. Because of timber’s orthotropic cell structure, it has excellent resistance 

against vertical shear but low resistance against horizontal shear. The failure of a 

solid sawn timber member due to load is generally preceded by horizontal shear 

cracking along the grain. A horizontal shear “crack” is effectively a longitudinal 

split. 

Investigate the area near the supports for the presence of horizontal shear cracking. 

The presence of transverse cracks on the underside of the girders or horizontal 

cracks on the sides of the girders indicate the onset of shear failure. These cracks 

can propagate quickly toward midspan and represent lost moment capacity of up to 

75% (see Figure 6.1.12). Measure these cracks carefully for length, width, and if 

possible, the depth. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.10

Figure 6.1.12 Horizontal Shear Crack in a Timber Beam 

 Tension Zones 

Examine the zones of maximum tension for signs of structural distress. The 

maximum tension generally occurs at the bottom half of the middle third of the 

beam span. Investigate for section loss due to decay or fire, especially near midspan. Examine beams for excessive deflection or sagging. Tension cracks in 

timber break the cell structure perpendicular to the grain and are typically 

preceded by the appearance of horizontal shear cracks. 

Solid sawn beams with sloping grain that intersects the surface in the tension zone 

are particularly susceptible to flexure cracking because the tensile stress and 

horizontal shear stress combine to split the grain apart. 

 Areas Exposed to Drainage 

Timber bridges with plank decks are exposed to drainage throughout the length of

the span. Plank decks with asphalt overlays in good condition offer some 

protection. In these cases, deck joint areas at span ends are candidates for drainage 

exposure. 

Investigate for signs of decay along the full length of the beam but especially 

where the beam is subjected to continual wetness and areas that trap moisture. 

These include member interfaces between deck planks and stringers, deck planks 

and beams, beams and bearing seats, stringers and floorbeams, floorbeams and 

trusses, truss member connections, arch connections, and all fastener locations. 

(see Figure 6.1.13). 

Decay and chemical attack may be evidenced by discolored wood, brown and 

white rot, the formation of fruiting bodies (the result of fungal attacks, which 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.11

produce disc-shaped bodies that distribute reproductive spores), “sunken” faces in 

the wood, or soft “punky” texture of the wood. When surface probing for expected 

decay is inconclusive, the next step is to drill the suspect area. If this has been 

done in a previous inspection, the drill hole area should be examined carefully for 

proper preservation treatment and dowel plug installations. 

Figure 6.1.13 Decay in a Timber Beam 

 Areas of Insect Infestation 

Insect infestation can be detected in various ways. Carpenter ants generally leave 

piles of sawdust; powder-post beetles leave small holes in the surface of the wood; 

and termites can often be readily seen. Another indication of insect infestation is 

hollow sounding wood. Further probing or drilling should be performed in suspect 

areas. 

 Areas Exposed to Traffic 

For overhead and through structures, check for collision damage from vehicles 

passing below or adjacent to structural members. 

 Areas Previously Repaired 

Thoroughly examine any repairs that have been previously made. Determine if 

repaired areas are sound and functioning properly. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.12

 Secondary Members 

Inspect bracing members for decay and fire damage. Examine connections of 

bracing to beams for tightness, cracked or split members, and corroded, loose, or 

missing fasteners (see Figure 6.1.14). Deteriorated secondary members may 

indicate problems in the primary members. 

Figure 6.1.14 Typical Timber End Diaphragm 

 Fasteners and Connectors 

Check all fasteners (e.g., nails, screws, bolts, and deck clips) for corrosion. Also 

inspect for loose or missing fasteners. Check for moisture and decay around the 

holes. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.13

6.1.5 

Evaluation State and federal rating guideline systems have been developed to aid in the 

inspection of timber bridges. The two major rating guideline systems currently in 

use are the FHWA's Recording and Coding Guide for the Structural Inventory and 

Appraisal of the Nation's Bridges used for the National Bridge Inventory (NBI) 

component rating method and the AASHTO Guide for Commonly Recognized 

(CoRe) Structural Elements used for element level condition state assessment. 

NBI Rating Guidelines Using NBI rating guidelines, a 1-digit code on the Federal Structure Inventory and 

Appraisal (SI&A) sheet indicates the condition of the superstructure. Rating codes 

range from 9 to 0, where 9 is the best rating possible. See Topic 4.2 (Item 59) for 

additional details about the NBI rating guidelines. 

The previous inspection data should be used along with current inspection findings 

to determine the correct rating. 

Element Level Condition 

State Assessment 

In an element level condition state assessment of a solid sawn timber bridge, the 

AASHTO CoRe elements are: 

Element No. Description

111 Open Girder/Beam 

117 Stringer 

135 Truss/Arch 

156 Floorbeam 

The unit quantity for the timber superstructures is in meters or feet, and the total 

length must be distributed among the four available condition states depending on 

the extent and severity of deterioration. The sum of all condition states must equal 

the total quantity of the CoRe element. Condition state 1 is the best possible 

rating. See the AASHTO Guide for Commonly Recognized (CoRe) Structural 

Elements for condition state descriptions. 

A Smart Flag is used when a specific condition exists, which is not described in 

the CoRe element condition state. The severity of the damage is captured by 

coding the appropriate Smart Flag condition state. The Smart Flag quantities are 

measured as each, with only one each of any given Smart Flag per bridge. 

For damage due to traffic impact, the “Traffic Impact” Smart Flag, Element No. 

362, can be used. There are three condition states for the “Traffic Impact” Smart 

Flag, with condition state 1 being the best. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.1: Solid Sawn Timber Bridges 

6.1.14

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6.2-i

Table of Contents

Section 6 ..................................................................................................................................................

Inspection and 

Evaluation of 

Common Timber 

Superstructures 

 6.2 Glulam Timber Bridges ......................................................................... 6.2.1 

 6.2.1 Introduction............................................................................... 6.2.1 

 

 6.2.2 Design Characteristics............................................................... 6.2.2 

 Multi-beam Bridges ........................................................... 6.2.2 

 Truss Bridges ..................................................................... 6.2.3 

 Arch Bridges....................................................................... 6.2.4 

 Primary and Secondary Members ...................................... 6.2.5 

 

 6.2.3 Overview of Common Defects ................................................. 6.2.6 

 

6.2.4 Inspection Procedures and Locations........................................ 6.2.7 

 Procedures .......................................................................... 6.2.7 

 Visual........................................................................... 6.2.7 

 Physical........................................................................ 6.2.7 

 Advanced Inspection Techniques ................................ 6.2.7 

 Locations ............................................................................ 6.2.7 

 Bearing Areas............................................................... 6.2.7 

 Shear Zones.................................................................. 6.2.8 

 Tension Zones.............................................................. 6.2.9 

 Areas Exposed to Drainage........................................ 6.2.10 

 Areas of Insect Infestation ......................................... 6.2.10 

 Areas Exposed to Traffic ........................................... 6.2.10 

 Areas Previously Repaired......................................... 6.2.10 

 Secondary Members................................................... 6.2.10 

 Fasteners and Connectors .......................................... 6.2.11 

 

 6.2.5 Evaluation ............................................................................... 6.2.12 

 NBI Rating Guidelines ..................................................... 6.2.12 

Element Level Condition State Assessment..................... 6.2.12 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2-ii

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6.2.1 

Topic 6.2 Glulam Timber Bridges 

6.2.1 

Introduction A glued-laminated (glulam) member is made by gluing strips of wood together to 

form a structural member of the desired size. An advantage of glulam members is 

that they allow for a higher utilization of the wood, since a lower grade of material 

can be used in of lower stress. Many strength reducing characteristics of wood, 

such as knots and checks, are minimized due to relatively small laminate 

dimensions. Also, the size and length of a glulam member is not limited by the 

size or length of a tree. Strips of wood used in glulam members are generally 20 

to 40 mm (3/4 to 1-1/2 inches) thick (see Figures 6.2.1 and 6.2.2). 

Figure 6.2.1 Elevation View of a Glulam Multi-beam Bridge 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2.2 

Figure 6.2.2 Underside View of a Glulam Multi-beam Bridge

6.2.2 

Design 

Characteristics 

Multi-beam Bridges Glulam multi-beam bridges are very similar to solid sawn multi-beam bridges, but 

they generally use larger members to span greater distances. Glulam multi-beam 

bridges are typically simple span designs (see Figure 6.2.1). They usually support 

a deck consisting of glulam panels with a bituminous wearing surface. Beam sizes 

typically range from 150 mm by 610 mm (6 inches by 24 inches) to 310 mm by 

1525 mm (12-1/4 inches by 60 inches), and the beams are usually spaced 1.7 m to 

2.0 m (5'-6" to 6'-6") on center (see Figure 6.2.2). 

These more modern multi-beam bridges can typically be used in spans of up to 24 

m (80 feet), although some span as long as 46 m (150 feet). These too can be used 

to form longer multiple span structures. They are generally found on local and 

secondary roads, as well as in park settings. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2.3 

Truss Bridges Trusses may be of the through-type or of the deck-type. Usually the floor system 

consists of a timber deck supported by timber stringers and floorbeams, all of 

which are supported by the trusses (see Figures 6.2.3 and 6.2.4). Timber trusses 

are generally used for spans that are not economically feasible for timber multibeam bridges. Timber trusses are practical for spans that range from 46 to 76 m 

(150 to 250 feet) (see Figure 6.2.5). 

Truss

Deck Stringer

Floorbeam

Timber Through Truss

Typical Section

Figure 6.2.3 Timber Through Truss Typical Section
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2.4 

Figure 6.2.4 Bowstring Truss Pedestrian Bridge 

Figure 6.2.5 Parallel Chord Truss Pedestrian Bridge (Eagle River, Alaska)

Arch Bridges Glulam arch bridges usually consist of two- or three-hinged deck arches, which 

support a glulam deck and floor system (see Figures 6.2.6 and 6.2.7). Glulam 

arches are practical for spans of up to about 91 m (300 feet). Although they are 

not widely used for highway bridges, they are frequently used for pedestrian 

overpasses and in locations such as parks where aesthetics is important. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2.5 

Figure 6.2.6 Glulam Arch Bridge over Glulam Multi-beam Bridge (Keystone 

Wye interchange, South Dakota)

Figure 6.2.7 Glulam Arch Bridge (West Virginia)

Primary and Secondary 

Members 

The primary members of glulam multi-beam bridges are the beams, and the 

secondary members are the diaphragms or cross bracing (see Figure 6.2.8). Due to 

the larger depth of the glulam beams, diaphragms or cross bracing should always 

be present. Diaphragms are usually constructed of short glulam members, and 

cross bracing is usually constructed of steel angles. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2.6 

The primary members of glulam arch and truss structures are the arch, truss, 

stringers, and floorbeams, spandrel bents and hangers. The secondary members 

are the diaphragms and cross bracing between the stringers and the lateral bracing 

between the arch or truss. 

Figure 6.2.8 Typical Glulam Diaphragm

Recent technology has also produced glulam timber materials which are reinforced 

with fibers such as aramids, carbon, and fiberglass. These fiber reinforced glulam 

beams help increase the strength and mechanical properties of timber bridges. 

6.2.3 

Overview of 

Common Defects 

Common defects that occur on glulam timber beams include: 

3⁄4 Checks, splits, shakes 

3⁄4 Decay by fungi 

3⁄4 Damage by insects and borers 

3⁄4 Damage from impact/collisions 

3⁄4 Damage from wear, abrasion, and mechanical wear 

3⁄4 Damage from weathering/warping 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2.7 

3⁄4 Damage from overstress 

3⁄4 Damage from fire 

3⁄4 Loose connections 

3⁄4 Failure of protective system 

A less common defect that may be encountered by the inspector includes damage 

from chemical attack. Refer to Topic 2.1 for a more detailed presentation of the 

properties of timber, types and causes of timber deterioration, and the examination 

of timber. 

6.2.4 

Inspection 

Procedures and 

Locations 

Inspection procedures to determine other causes of timber deterioration are 

discussed in detail in Topic 2.1.7. The inspection locations and procedures for 

glulam bridges are similar to those for solid sawn bridges. 

Procedures Visual 

The inspection of timber splits, cracks, shakes, fungus decay, deflections, 

crushing, delaminations, and loose connections is primarily a visual activity. 

 Physical 

The physical examination of a timber member can be conducted with a hammer or 

pick. The hammer is used to sound the members to detect hollow areas or internal 

decay. Picks are used to determine the condition of the surface. 

 Advanced Inspection Techniques

Several advanced techniques are available for timber inspection. Nondestructive 

methods, described in Topic 13.1.2, include: 

3⁄4 Pol-Tek 

3⁄4 Spectral analysis 

3⁄4 Ultrasonic testing 

3⁄4 Vibration 

Other methods, described in Topic 13.1.3, include: 

3⁄4 Boring or drilling 

3⁄4 Moisture content 

3⁄4 Probing 

3⁄4 Shigometer 

Locations Bearing Areas 

Inspect the bearing areas for crushing of the beams (see Figure 6.2.9). Investigate 

for decay and insect damage by visual inspection, sounding, and/or probing at the 

ends of the beams. Also check the condition and operation of the bearing devices 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2.8 

if they are present (see Topic 9.1). 

Figure 6.2.9 Bearing Area of Typical Glulam Beam 

 Shear Zones 

Examine for horizontal shear cracks and delaminations near the ends of the beam. 

Delaminations (i.e., separations in the laminations) can occur due to either failure 

of the glue or failure at the bond between the glue and the lamination (see Figure 

6.2.10). Delaminations that extend completely through the cross section of the 

member are the most serious since this makes the member act as two smaller 

members. Delaminations that are located near the center of the cross section are 

more serious than those near the top or bottom of the beam. Delaminations 

directly through a connector are also undesirable. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2.9 

Figure 6.2.10 Close-up View of Glulam Bridge Showing Laminations 

 Tension Zones 

Examine the zone of maximum tension for signs of structural distress (see Figure 

6.2.11). The maximum tension generally occurs at the bottom half of the middle 

third of the beam span. Investigate for section loss due to decay or fire, especially 

near mid-span. Inspect for excessive deflection or sagging in the beams. 

Figure 6.2.11 Elevation View of Beam of Glulam Multi-beam Bridge 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2.10

 Areas Exposed to Drainage

Investigate for signs of decay along the full length of the member but especially 

where the beam is subjected to continual wetness or prolonged exposure to 

moisture (see Figure 6.2.12). Decay and chemical attack may be evidenced by 

discolored wood, brown and white rot, the formation of fruiting bodies (the result 

of fungal attacks, which produce disc-shaped bodies that distribute reproductive 

spores), "sunken" faces in the wood, or the soft “punky” texture of the wood. 

Figure 6.2.12 Decay on Glulam Beam 

 Areas of Insect Infestation 

Insect infestation can be detected in various ways. Carpenter ants generally leave 

piles of sawdust; powder-post beetles leave small holes in the surface of the wood; 

and termites can often be readily seen. Another indication of insect infestation is 

hollow sounding wood. Further probing or drilling should be performed in suspect 

areas. 

 Areas Exposed to Traffic 

For overhead and through structures, check for collision damage from vehicles 

passing below or adjacent to structural members. 

 Areas Previously Repaired 

Thoroughly examine any repairs that have been previously made. Determine if 

repaired areas are sound and functioning properly. 

 Secondary Members 

Examine diaphragms for decay, fire damage, and insect damage (see Figure 

6.2.13). Check steel cross bracing for corrosion, bowing, or buckling (see Figure 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2.11

6.2.14). Examine connections for tightness, cracks and splits, and corroded, loose, 

or missing fasteners. Deteriorated secondary members may indicate problems in 

the primary members. 

Figure 6.2.13 Typical Diaphragm for a Glulam Multi-beam Bridge 

 Fasteners and connectors 

Inspect all fasteners for corrosion, tightness, and missing parts (see Figure 6.2.13). 

Figure 6.2.14 Glulam Beams with Numerous Fastener Locations 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Glulam Timber Bridges 

6.2.12

6.2.5 

Evaluation State and federal rating guideline systems have been developed to aid in the 

inspection of timber bridges. The two major rating guideline systems currently in 

use are the FHWA's Recording and Coding Guide for the Structural Inventory and 

Appraisal of the Nation's Bridges used for the National Bridge Inventory (NBI) 

component rating method and the AASHTO Guide for Commonly Recognized 

(CoRe) Structural Elements used for element level condition state assessment. 

NBI Rating Guidelines Using NBI rating guidelines, a 1-digit code on the Federal Structure Inventory and 

Appraisal (SI&A) sheet indicates the condition of the superstructure. Rating codes 

range from 9 to 0, where 9 is the best rating possible. See Topic 4.2 (Item 59) for 

additional details about NBI rating guidelines. 

The previous inspection data should be used along with current inspection findings 

to determine the correct rating. 

Element Level Condition 

State Assessment 

In an element level condition state assessment of a glulam timber bridge, the 

AASHTO CoRe elements are:

Element No. Description

111 Open Girder/Beam 

117 Stringer 

135 Truss/Arch 

156 Floorbeam 

The unit quantity for the timber superstructures is in meters or feet, and the total 

length must be distributed among the four available condition states depending on 

the extent and severity of deterioration. The sum of all condition states must equal 

the total quantity of the CoRe element. Condition state 1 is the best possible 

rating. See the AASHTO Guide for Commonly Recognized (CoRe) Structural 

Elements for condition state descriptions. 

A Smart Flag is used when a specific condition exists, which is not described in 

the CoRe element condition state. The severity of the damage is captured by 

coding the appropriate Smart Flag condition state. The Smart Flag quantities are 

measured as each, with only one each of any given Smart Flag per bridge. 

For damage due to traffic impact, the “Traffic Impact” Smart Flag, Element No. 

362, can be used. There are three condition states for the “Traffic Impact” Smart 

Flag, with condition state 1 being the best. 
6.3-i

Table of Contents

Section 6 ..................................................................................................................................................

Inspection and 

Evaluation of 

Common Timber 

Superstructures 

 6.3 Stress-Laminated Timber Bridges.......................................................... 6.3.1 

 6.3.1 Introduction............................................................................... 6.3.1 

 

 6.3.2 Design Characteristics............................................................... 6.3.1 

 Stress-Laminated Timber Slab Bridges.............................. 6.3.1 

 Stress-Laminated Timber Tee Beam Bridges..................... 6.3.3 

 Stress-Laminated Timber Box Beam Bridges.................... 6.3.4 

 Stress-Laminated Timber K-frame Bridges ....................... 6.3.5 

 Primary and Secondary Members ...................................... 6.3.5 

 

 6.3.3 Overview of Common Defects ................................................. 6.3.6 

 

6.3.4 Inspection Procedures and Locations........................................ 6.3.6 

 Procedures .......................................................................... 6.3.6 

 Visual........................................................................... 6.3.6 

 Physical........................................................................ 6.3.6 

 Advanced Inspection Techniques ................................ 6.3.6 

 Locations ............................................................................ 6.3.7 

 Stressing Rods.............................................................. 6.3.7 

 Bearing Areas............................................................... 6.3.7 

 Shear Zones.................................................................. 6.3.7 

 Tension Zones.............................................................. 6.3.8 

 Areas Exposed to Drainage.......................................... 6.3.8 

 Areas of Insect Infestation ........................................... 6.3.8 

 Areas Exposed to Traffic ............................................. 6.3.8 

 Areas Previously Repaired........................................... 6.3.9 

 Secondary Members..................................................... 6.3.9 

 Fasteners and Connectors ............................................ 6.3.9 

 

 6.3.5 Evaluation ............................................................................... 6.3.10 

 NBI Rating Guidelines ..................................................... 6.3.10 

Element Level Condition State Assessment..................... 6.3.10 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.2: Stress-Laminated Timber Bridges 

6.3-ii

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6.3.1 

Topic 6.3 Stress-Laminated Timber Bridges 

6.3.1 

Introduction Stress-laminated timber bridges were first developed in Canada, in 1976, by the 

Ontario Ministry of Transportation and Communications. These bridges consist of 

multiple planks mechanically clamped together using metal rods to perform as one 

unit (see Figure 6.3.1). The compression induced frictional resistance within the 

timber laminations is the mechanism that makes this structural system effective. 

Figure 6.3.1 Stress-Laminated Timber Slab Bridge Carrying a 90,000-Pound 

Logging Truck (Source: Barry Dickson, West Virginia University)

6.3.2 

Design 

Characteristics 

Stress-Laminated 

Timber Slab Bridges 

Stress-laminated timber slab bridges can be used for simple spans of up to 15 m 

(50 feet) and are capable of carrying modern highway loadings (see Figures 6.3.1 

and 6.3.3). Stressed deck bridges have also been constructed using glulam 

members. Combining glulam technology with stress-lamination increases 

practical span lengths to 19 m (63 feet) (see Figure 6.3.4).
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.3: Stress-Laminated Timber Bridges 

6.3.2 

Figure 6.3.2 Typical Section of a Stress-Laminated Timber Slab Bridge

Figure 6.3.3 Stress-Laminated Timber Slab Bridge
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.3: Stress-Laminated Timber Bridges 

6.3.3 

Figure 6.3.4 Glulam Stress-Laminated Timber Slab Bridge

Stress-Laminated 

Timber Tee Beam 

Bridges 

The idea for stress-laminated timber tee beam bridges was developed at West 

Virginia University. These bridges consist of a stress-laminated deck and glulam 

beams (see Figure 6.3.5). High strength steel rods are used to join the stresslaminated deck and glulam beams together to form stress-laminated timber tee 

beams. The first structure of this type was built in 1988, near Charleston, West 

Virginia. It is 23 m (75 feet) long and has stressing rods spaced at two feet. It has 

performed well, and stressed tee beams will likely be used in the future to achieve 

even longer span lengths (see Figure 6.3.6). 

Figure 6.3.5 Typical Section of a Stress-Laminated Timber Tee Beam Bridge 

(Source: Barry Dickson, West Virginia University)
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.3: Stress-Laminated Timber Bridges 

6.3.4 

Figure 6.3.6 Elevation View of Stress-Laminated Timber Tee Beam Bridge 

(West Virginia)

Stress-Laminated 

Timber Box Beam 

Bridges 

Stress-laminated timber box beam bridges represent further development of timber 

bridges by West Virginia University. These bridges consist of adjacent box beam 

panels individually comprised of stress-laminated flanges and glulam beam webs 

(see Figure 6.3.7). This bridge type is also known as a cellular stressed deck. 

Span lengths of up to 18 m (60 feet) have been designed, and there is a potential 

for longer spans (see Figure 6.3.8). 

Figure 6.3.7 Typical Section of a Stress-Laminated Timber Box Beam (Source: 

Barry Dickson, West Virginia University)
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.3: Stress-Laminated Timber Bridges 

6.3.5 

Figure 6.3.8 Stress-Laminated Timber Box Beam Bridge Being Erected

Stress-Laminated 

Timber K-frame 

Bridges 

Stressed K-frame bridges represent further development of the stressed deck 

bridge by the Ontario Ministry of Transportation and Communications. These 

bridges consist of three spans in which the stressed deck is supported at two 

intermediate points by stressed laminated timber struts (see Figure 6.3.9). This 

bridge type has been used for a bridge with a total length of 13 m (43 feet), and it 

has a potential for span lengths over 15 m (50 feet). 

Elevation

Prestress Rod

Prestress Rods

Timber Laminations

Typical Deck Section

Typical Leg Section

Wearing Surface

Figure 6.3.9 Stress-Laminated Timber K-frame Bridge

Primary and Secondary 

Members 

The primary members are the decks, slabs, tee beams, box beams, and frame legs. 

The secondary members are the diaphragms and cross bracing between beams. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.3: Stress-Laminated Timber Bridges 

6.3.6 

6.3.3 

Overview of 

Common Defects 

Common defects that occur on stressed timber bridges include: 

3⁄4 Checks, splits, shakes 

3⁄4 Decay by fungi 

3⁄4 Damage by insects and borers 

3⁄4 Damage from impact/collisions 

3⁄4 Damage from wear, abrasion, and mechanical wear 

3⁄4 Damage from weathering/warping 

3⁄4 Damage from overstress 

3⁄4 Damage from fire 

3⁄4 Loose connections 

3⁄4 Failure of protective system 

A less common defect that may be encountered by the inspector includes damage 

from chemical attack. Refer to Topic 2.1 for a more detailed presentation of the 

properties of timber, types and causes of timber deterioration, and the examination 

of timber. 

6.3.4 

Inspection 

Procedures and 

Locations 

Inspection procedures to determine other causes of timber deterioration are 

discussed in detail in Topic 2.1.7. The inspection of stress-laminated timber 

bridges is similar to those for glulam bridges. 

Procedures Visual 

The inspection of timber splits, cracks, shakes, fungus decay, deflections, 

crushing, delaminations, and loose connections is primarily a visual activity. 

Physical 

The physical examination of a timber member can be conducted with a hammer or 

pick. The hammer is used to sound the members to detect hollow areas or internal 

decay. Picks are used to determine the condition of the surface. 

 Advanced Inspection Techniques

Several advanced techniques are available for timber inspection. Nondestructive 

methods, described in Topic 13.1.2, include: 

3⁄4 Pol-Tek 

3⁄4 Spectral analysis 

3⁄4 Ultrasonic testing 

3⁄4 Vibration 

Other methods, described in Topic 13.1.3, include: 

3⁄4 Boring or drilling 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.3: Stress-Laminated Timber Bridges 

6.3.7 

3⁄4 Moisture content 

3⁄4 Probing 

3⁄4 Shigometer

Locations Stressing Rods

Examine the condition of the steel stressing rods, and inspect for crush and splits 

in the fascia members. Check for loss of prestress in the rods, which would be 

indicated by shifted planks in the stress-laminated timber element and excessive 

deflection or loose rods. This may be observed when the bridge is subject to a 

moving live load. 

Figure 6.3.10 Broken Stressing Rods

 Bearing Areas

Inspect the bearing areas for crushing of the beams. Investigate for decay and 

insect damage by visual inspection, sounding, and/or probing at the ends of the 

beams. Also check the condition and operation of the bearing devices if they are 

present (see Topic 9.1, Bearings). 

 Shear Zones

Examine for horizontal shear cracks and delaminations near the ends of the beam. 

Delaminations (i.e., separations in the laminations) can occur due to either failure 

of the glue or failure at the bond between the glue and the lamination (see Figure 

6.3.11). Delaminations that extend completely through the cross section of the 

member are the most serious since this makes the member act as two smaller 

members. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.3: Stress-Laminated Timber Bridges 

6.3.8 

Figure 6.3.11 Close-up View of End of a Stress-Laminated Timber Bridge 

Showing Laminations 

 Tension Zones

Examine the zone of maximum tension for signs of structural distress. The 

maximum tension generally occurs at the bottom half of the middle third of the 

beam span. Investigate for section loss due to decay or fire, especially near midspan. Inspect for excessive deflection or sagging in the beams. 

 Areas Exposed to Drainage

Investigate for signs of decay along the full length of the member but especially 

where the beam is subjected to continual wetness or prolonged exposure to 

moisture. Decay and chemical attack may be evidenced by discolored wood, 

brown and white rot, the formation of fruiting bodies (the result of fungal attacks, 

which produce disc-shaped bodies that distribute reproductive spores), "sunken" 

faces in the wood, or the soft “punky” texture of the wood. Examine the curb line 

areas. Standing water soaks into the beam and corrodes the stressing rods. 

 Areas of Insect Infestation

Insect infestation can be detected in various ways. Carpenter ants generally leave 

piles of sawdust; powder-post beetles leave small holes in the surface of the wood; 

and termites can often be readily seen. Another indication of insect infestation is 

hollow sounding wood. Further probing or drilling should be performed in suspect 

areas. 

 Areas Exposed to Traffic

Check stress-laminated timber members for collision damage from vehicles 

passing below. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.3: Stress-Laminated Timber Bridges 

6.3.9 

 Areas Previously Repaired

Thoroughly examine any repairs that have been previously made. Determine if 

repaired areas are sound and functioning properly. 

 Secondary Members

Examine solid sawn or glulam diaphragms for decay, fire damage, and insect

damage. Check steel cross bracing for corrosion, bowing, or buckling. Examine 

connections for tightness, cracks and splits, and corroded, loose, or missing 

fasteners. Deteriorated secondary members may indicate problems in the primary 

members. 

 Fasteners and Connectors 

Inspect all fasteners for corrosion, tightness, and missing parts. Stressing rod 

hardware is the most important fastener system on a stress-laminated timber 

bridge. 
SECTION 6: Inspection and Evaluation of Common Timber Superstructures 

TOPIC 6.3: Stress-Laminated Timber Bridges 

6.3.10

6.3.5 

Evaluation State and federal rating guideline systems have been developed to aid in the 

inspection of timber bridges. The two major rating guideline systems currently in 

use are the FHWA's Recording and Coding Guide for the Structural Inventory and 

Appraisal of the Nation's Bridges used for the National Bridge Inventory (NBI) 

component rating method and the AASHTO element level condition state 

assessment method. 

NBI Rating Guidelines Using NBI rating guidelines, a 1-digit code on the Federal Structure Inventory and 

Appraisal (SI&A) sheet indicates the condition of the superstructure. Rating codes 

range from 9 to 0, where 9 is the best rating possible. See Topic 4.2 (Item 59) for 

additional details about NBI rating guidelines. 

The previous inspection data should be used along with current inspection findings

to determine the correct rating. 

Element Level Condition 

State Assessment 

In an element level condition state assessment of timber bridges, the AASHTO 

CoRe elements are: 

Element No. Description

11 Timber Deck - Bare 

12 Timber Deck with A/C Overlay 

54 Timber Slab - Bare 

55 Timber Slab with A/C Overlay 

111 Open Girder/Beam 

The unit quantity for the timber superstructures is in meters or feet, and the total 

length must be distributed among the four available condition states depending on 

the extent and severity of deterioration. The sum of all condition states must equal 

the total quantity of the CoRe element. The unit quantity for decks and slabs is 

“each”, and the entire element must be placed in one of the five available condition 

states based on the condition of the deck. Some states have elected to use the total 

area (m2 or ft2). When a total area is used, the total area must be distributed among 

the five available condition states depending on the extent and severity of 

deterioration. The sum of all condition states must equal the total quantity of the 

CoRe element. Condition state 1 is the best possible rating. See the AASHTO 

Guide for Commonly Recognized (CoRe) Structural Elements for condition state 

descriptions. 

A Smart Flag is used when a specific condition exists, which is not described in 

the CoRe element condition state. The severity of the damage is captured by 

coding the appropriate Smart Flag condition state. The Smart Flag quantities are 

measured as each, with only one each of any given Smart Flag per bridge. 

For damage due to traffic impact, the “Traffic Impact” Smart Flag, Element No. 

362, can be used. There are three condition states for the “Traffic Impact” Smart 

Flag, with condition state 1 being the best.