Laser Scanning Technology 

as Part of a Comprehensive 

Condition Assessment for 

Covered Bridges

Brian K. Brashaw

Samuel Anderson

Robert J. Ross

Research Note

FPL–RN–0335

United States Department of Agriculture

Forest 

Service

Forest Products

Laboratory

June

2015
June 2015

Brashaw, Brian K.; Anderson, Samuel; Ross, Robert J. 2015. Laser scanning technology as part of a comprehensive condition assessment for covered bridges. Research Note FPL-RN-0335. Madison, WI: U.S. Department 

of Agriculture, Forest Service, Forest Products Laboratory. 10 p.

A limited number of free copies of this publication are available to the 

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Contents

Introduction...................................................................... 1

Initial Work on Laser Scanning 

with Covered Bridges ...................................................... 1

Literature Review............................................................. 2

Demonstration Study ....................................................... 2

Amnicon Falls Covered Bridge—An Overview......... 2

Field Testing with 3D Laser Scanning Equipment...... 2

Processing of Data Obtained from Laser Scans............... 3

Role of Laser Scanning Technologies in Condition 

Assessment of Timber Bridges and Structures................ 8

3D Laser Scanning........................................................... 8

Visual Inspection.............................................................. 8

Hammer Sounding and Probing....................................... 8

Moisture Content Determination ..................................... 9

Stress-Wave Timing Techniques...................................... 9

Resistance Microdrilling.................................................. 9

Summary.......................................................................... 9

Literature Cited .............................................................. 10

Abstract

New noncontact technologies have been developed and 

implemented for determining as-built condition and current 

dimensions for a wide variety of objects and buildings. In 

this study, a three-dimensional laser scanner was used to 

determine the dimensions and visual condition of a historic 

bridge in the Amnicon Falls State Park in northern Wisconsin. 3D scanning provides millions of data points of information about the bridge being inspected. The point clouds of 

data collected depict all visible aspects and actual dimensions of the bridge. This information can be used to determine areas of the bridge that show excessive deflection, 

rotation or skew of the structure, damaged members, and 

other visual indicators that a human eye cannot easily identify or quantify. This technique could be incorporated into 

a comprehensive inspection protocol for historic covered 

bridges that includes 3D laser scanning, visual inspection, 

hammer sounding and probing, moisture content determination, stress-wave timing, and resistance microdrilling.

Keywords: Condition assessment, 3D laser scanning, timber 

bridge historic, covered bridges, laser scanning, condition 

assessment
Laser Scanning Technology as Part of a 

Comprehensive Condition Assessment for 

Covered Bridges

Brian K. Brashaw, Program Manager, Forest Products Laboratory

Samuel Anderson, Engineering Technician, University of Minnesota Duluth 

Natural Resources Research Institute

Robert J. Ross, Supervisory Research General Engineer, Forest Products Laboratory

Introduction

Covered bridges have long been a part of the fabric of 

American life. Today several hundred historic covered 

bridges remain in the United States. Although much effort 

has been taken to preserve these structures, the high cost of 

restoration, neglect, and vandalism has taken its toll, and 

many are lost forever. The U.S. National Park Service’s 

Historic American Engineering Record (HAER) has efforts 

under way to document historic structures. Their Level I 

documentation consists of measured and interpretive drawings, large-format photographs, and written historical reports. To assist in this effort, newer technologies need to be 

explored that can provide as-built records at a faster rate and 

with more accuracy. The USDA Forest Products Laboratory (FPL), in cooperation with the University of Minnesota 

Duluth (UMD) and the Federal Highway Administration 

(FHWA), have explored the use of advanced laser scanning technologies for providing as-built records of covered 

bridges (Brashaw 2010). This report presents our recent 

work with laser scanning technologies and covered bridges. 

It includes a summary of our initial work in this area, results 

of a literature review, and summarizes results of a study we 

conducted to demonstrate laser scanning technologies with 

covered bridges. It also discusses the important role they 

may play as part of a detailed condition assessment procedure for timber bridges and other timber structures.

Initial Work on Laser Scanning with 

Covered Bridges

Our initial work was a component of the Research, Technology, and Education portion of the National Historic Covered 

Bridge Preservation (NHCBP) Program, administered by 

the Federal Highway Administration. The NHCBP program includes preservation, rehabilitation, and restoration 

of covered bridges that are listed or are eligible for listing on the National Register of Historic Places; research 

for better means of restoring and protecting these bridges; 

development of educational aids; and technology transfer 

to disseminate information on covered bridges in order to 

preserve the Nation’s cultural heritage. We conducted an assessment and demonstration of the use of three-dimensional 

(3D) high-definition laser scanning technology to scan existing bridges for purposes of obtaining as-built records. Six 

covered bridges in Wisconsin, Minnesota, and Iowa were 

scanned using portable scanning equipment and the resulting scans were processed using computer software to create 

3D point cloud images accurate within 5 mm. These images 

were then imported into AutoCAD® for further processing 

into as-built documentation. In addition, the Zumbrota, Minnesota, Bridge was recreated as a 1/100th scale model using 

rapid prototyping technology as a demonstration of the technology to create as-built replicas. A publication that summarized our work was prepared and distributed widely (Ross 

and others 2012). It presents a review of laser scanning 

technology, data processing techniques, and the potential 

cost savings that may accrue when using the technology and 

briefly discusses the potential for using this technology as 

part of an inspection protocol for historic covered bridges.

Our work successfully demonstrated the potential for using 

3D laser scanning to accurately develop as-built documentation of historic covered bridges. Based on our work, we 

concluded the following: 

• Three-dimensional laser scanning can be used to rapidly 

scan historic covered bridge structures and a number of 

technologies and commercial vendors are available. 

• Post-processing of the scan data requires experience and 

skill to cost-effectively create as-built documentation. 

• A 3D scanner can be used to create a range of outputs that 

include point cloud scans, parametric images, and 2D and 

3D AutoCAD® drawings. For historic covered bridges, 

this information can be used for a variety of purposes 

including as-built documentation and structural assessment, while also providing detail on the land topography 

adjacent to the bridge.

• One of the objectives of our work was to assess the 

accuracy of laser scanning to determine accurate dimensions of the bridges. Each bridge was then scanned using 
Research Note FPL–RN–0335

2

the laser scanner at 20+ points. The raw scan data were 

processed into point cloud data. The locations measured 

using traditional measuring equipment on the photographs were then located in the digital files and the digital 

dimensions noted. Based on these data, we compared the 

actual and digital measurements. The comparison data 

are reported for each of the four Madison County, Iowa, 

bridges in the following sections. Fourteen to sixteen 

points were sampled from each bridge with a very small 

error noted. This mean error of the 14–16 measurements 

ranged from 0.04 to 0.08 in. (1.01 to 2.02 mm) within the 

rated scanner accuracy of 1/4 in. at 150 ft. 

Literature Review

Three-dimensional (3D) laser scanners are instruments that 

record precise and accurate surface data of objects in a nondestructive manner. These instruments use an infrared beam 

of light to calculate and record the distance to an object, 

typically as data points with spatial coordinates. These data 

are then analyzed using various types of computer software 

to generate a final detailed image of coordinates and dimensions. 3D laser scanners have successfully been used to digitize objects of various sizes ranging from small diagnostic 

artifacts to large, complex sites of monumental architecture 

(FARO 2011b). 

A wide number of companies manufacture various types 

of 3D laser scanners. Generally, these units use LiDAR 

(light detection and ranging) technology, where laser pulses 

determine the distance to an object or surface. The distance 

to an object is determined by using time-of-flight between 

transmission of a pulse and detection of the reflected signal. 

A point cloud of data is then collected and processed into 

the true shape of the object. The FARO scanner we used 

in this project used phase shift technology rather than time 

of flight. This means that instead of a single pulse being 

reflected and the time of flight measured, constant waves of 

varying length are projected. Upon contact with an object, 

they are reflected back to the scanner. The distance from 

the scanner to the object is accurately measured by measuring the phase shifts in the waves of infrared light (FARO 

2011b).

Several publications have documented the use of 3D laser 

scanning on bridges. The Pennsylvania Department of 

Transportation (PennDOT) completed an initial study in 

1999. In their study, their stated goals were to evaluate the 

technology for creating as-built drawings (Foltz 2000). 

They compared traditional and 3D scanning methods and 

estimated an overall time savings of 100+ man hours, which 

was achieved through the use of 3D scanning. Based on 

this assessment, PennDOT purchased two laser scanners in 

2000. A second assessment of the technology was completed by Jaselskis and others (2003). They showed that 

laser scanning could be used cost effectively for preliminary 

surveys to develop TIN meshes of roadway surfaces and to 

measure bridge beam camber in a safer and quicker fashion 

compared to conventional approaches. Other potential applications noted in their report included developing as-built 

drawings for historic structures.

The use of laser scanners to inspect bridges is well documented. Jaselskis and others (2005), for example, noted the 

potential for using laser scanners in the bridge inspection 

process. Their use has been reported on, specifically on the 

inspection of concrete decks (Tang and others 2011) and 

concrete columns (Zhu and others 2010). Atasoy and others 

(2011) completed an assessment of a variety of visualization techniques to improve a bridge inspector’s assessment 

through the use of point cloud data.

Demonstration Study

The study described in the following sections was designed 

to demonstrate use of laser scanning technologies in the 

assessment of covered bridges. The bridge selected for scanning and assessment was located near Superior, Wisconsin, 

in the Amnicon Falls State Park. Built in 1900, this bowstring pony truss bridge has a span of 55 ft.

Amnicon Falls Covered Bridge—An Overview

This bridge was moved to its current location from a nearby 

site over the Amnicon River in 1930. A roof was added to it 

around 1939. The bridge is a bowstring pony truss design. It 

is considered to be historically significant as a consequence 

of its age and unusual construction details. Designed and 

constructed by Mr. Charles Horton, the bowstrings were 

unique in that no holes were drilled in the truss members 

and they were held together with a unique system of clips. 

His design allowed quick assembly without expensive 

machinery or tools and minimal labor. The construction 

method called for using arched beams secured with hooks 

and clips rather than rivets and bolts (Wisconsin Department 

of Natural Resources 2013). The roof was replaced in 1941 

after collapsing from heavy snow and again in 1971 when 

destroyed by a fire set by vandals. Figures 1–3 are photographs of the Amnicon River Bridge in October 2011.

Field Testing with 3D Laser Scanning 

Equipment 

Laser scanning was completed by staff from UMD using a 

FARO 3D Focus 120 scanner. Approximately 18 scans were 

completed of the bridge from a variety of angles. Figure 4 

shows a FARO laser scanner in use.

The scanning process consisted of steps, listed below: 

1. Visible reference points were placed at several locations 

on the bridge for use in linking the individual laser scans 

together. Since the scanner is a line of sight device, it will 

only capture reflection from objects. Therefore, all visible 

references must be in the line of site for the scanner. Typically, a combination of paper targets and globes are used 
Laser Scanning Technology as Part of a Comprehensive Condition Assessment for Covered Bridges

3

as visible references. Each scan requires a minimum of 

three visible references, however, four or more references 

are recommended. These targets are used to register or 

“connect” multiple scans, each on their own coordinate 

system into a single, aligned coordinate system. Figures 5 

and 6 show the use of paper targets and spherical globes 

we used in this study. 

2. A 3D laser scanner (FARO Focus 3D 120, FARO 2013) 

was used to conduct the scan. The scanner was placed at 

18 unique vantage points inside and outside the bridge, 

such that all visible surfaces of the bridge would be 

documented. This included upstream, under the bridge 

and downstream locations. Individual scans were completed in approximately 10 minutes. When a single scan 

was completed, it was saved to a flash memory card on 

the unit as an .ls file. Because Figures 7–9 show example 

locations that were scanned for the Amnicon Bridge. 

Processing of Data Obtained from 

Laser Scans

After all visible portions of the bridge were scanned, the 

software files were linked together using FARO Scene software. This process is referred to as “registration.” Because 

of the large number of data points collected and resulting 

file size, it is recommended that a computer with significant 

processing power and high levels of RAM memory be used. 

The scanner saves scan measurements in its own coordinate system, with the origin being the center of the scanner 

mirror. This software allowed an individual to identify the 

targets placed prior to the scanning process and use them 

Figure 1—Amnicon River Bridge photograph from upstream.

Figure 2—Amnicon River Bridge profile showing bowstring 

trusses and timber components.

Figure 3—Entrance to the Amnicon River Bridge.

Figure 4—A FARO Focus 3D 120 laser scanner 

was used to scan the Amnicon River Bridge.
Research Note FPL–RN–0335

4

to link or attach one scan to another. The process of linking 

two individual scans was repeated several times until all 

scans were compiled into one large scan depicting the entire 

bridge. Figures 10–12 are point cloud images of the multiple 

scans that were completed for the Amnicon River Bridge. 

Once the Amnicon Bridge was completely assembled using all of the individual scans, it was exported into AutoCAD using an add-in provided by Kubit USA. This add-in 

enabled us to import point cloud files in addition to the 

ones inherently recognized by AutoCAD and has additional 

modeling tools for working directly with point cloud data 

in AutoCAD. Once a point cloud has been exported into 

AutoCAD, it can be divided into multiple cross-sections. 

This is done so that specific components of the bridge can 

be seen more clearly. From this point, two and three-dimensional (2D and 3D) models of the bridge can be generated. 

Figures 13–16 show AutoCAD images of the Amnicon 

Bridge. The steel floor beams and bowstring members were 

omitted from the AutoCAD images. 

Figure 5—Paper reference targets used during the inspection process.

Figure 6—Spherical globes are often used in combination 

with paper targets as references for laser scanning. 

Figure 7—Laser scanner test setup, inside bridge.

Figure 8—Scanner test setup, bridge entrance.

Figure 9—Scanner test setup, under bridge.
Laser Scanning Technology as Part of a Comprehensive Condition Assessment for Covered Bridges

5

Figure 10—All scans have been connected to create a point cloud by using a process 

called registration. Point clouds depicting all visible aspects and actual dimensions 

of the bridge.

Figure 11—A point cloud image can be rotated to look at other views, such as this 

landscape image of the Amnicon Bridge.

Figure 12—Point cloud images collect a wide range of data points from reflective 

surfaces such as vegetation and land. This image shows the white pine located near 

the bridge and the cross section of the river valley. 
Research Note FPL–RN–0335

6

14'-1115

16"

57'-315

16"

6'-07

8"

2'-13

8"

3'-1 1

16"

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

Figure 13—An isometric view of the Amnicon River Bridge.

Figure 14—AutoCAD cross-section of the Amnicon River Bridge.
Laser Scanning Technology as Part of a Comprehensive Condition Assessment for Covered Bridges

7

13'-81

4"

8'-1 7

16"

102°

6 3

16"

8'-01

8"

16'-01

8"

3'-13

8"

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

15'-35

8"

20'-21

4"

54'-57

8"

15

8" 11

2" 1'-103

4"

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

PRODUCED BY AN AUTODESK EDUCATIONAL PRODUCT

Figure 15—End view of the Amnicon River Bridge.

Figure 16—View of the underside of the Amnicon River Bridge roof system.
Research Note FPL–RN–0335

8

Role of Laser Scanning 

Technologies in Condition 

Assessment of Timber Bridges 

and Structures

Assessment of the condition of wood in a historic bridge 

can be conducted for a variety of reasons. Federal inspection requirements and historic preservation would be the 

most likely reasons for conducting a condition assessment. 

Historically, a structural condition assessment consists of 

the following: 1) a systematic collection and analysis of 

data pertaining to the physical and mechanical properties 

of materials in use; 2) evaluation of the data collected; and 

3) providing recommendations, based on evaluation of the 

collected data, regarding portions of an existing structure 

that affect its current or proposed use (Ross and others 

2006). Such an assessment relies upon an in-depth inspection of the wood members in the structure. A wide variety of 

techniques are used to assess the condition of wood in structures. Visual, resistance drilling (probing), and stress-wave 

or ultrasound-based techniques are all used either individually or in combination to inspect in-service wood. 

The use of new laser scanning technology should also be 

incorporated into a comprehensive inspection protocol. 3D 

scanning provides millions of data points of information 

about the bridge being inspected. The point clouds of data 

collected depict all visible aspects and actual dimensions 

of the bridge. This information can be used to determine 

areas of the bridge that show excessive deflection, rotation or skew of the structure, damaged members, and other 

visual indicators that a human eye cannot easily identify or 

quantify.

A comprehensive inspection protocol for historic covered 

bridges should include the following:

• 3D laser scanning

• Visual inspection

• Hammer sounding and probing

• Moisture content determination

• Stress-wave timing

• Resistance microdrilling

Each of these techniques is covered generally in the following sections. Comprehensive information on visual inspection, hammer sounding and probing, stress-wave inspection 

and resistance microdrilling can be found in Wood and Timber Condition Assessment Manual, Second Edition (White 

and Ross 2014).

3D Laser Scanning

As noted earlier in this report, 3D laser scanning consists of 

the use of an appropriate commercial laser scanning system. 

There are a number of commercial companies that supply 

equipment that would be appropriate. The resolution of the 

unit should be approximately ±5 mm (0.2 in.) at a distance 

of 120 m (393.7 ft). Ideally, the laser scanning should be 

completed in advance of the other inspection techniques. 

Depending on the size of the structure, 15–30 laser scans 

should be completed and registered to form one comprehensive point cloud image. This information can be used to 

identify areas of the bridge that show excessive deflection 

while not under load, rotation, or skew of the structure, 

damaged members, and other visual indicators that a human eye cannot easily identify or quantify. It can provide 

information that can be used by the inspection team as they 

conduct the other inspection techniques.

Visual Inspection

Visual inspection is the simplest method for locating deterioration in timber structures. The inspector observes the 

structure for signs of actual or potential deterioration, noting 

areas for further investigation. Visual inspection requires 

strong light and is suitable for detecting intermediate or 

advanced surface decay, water damage, mechanical damage, 

or failed members.

Visual inspection cannot detect early stage decay when 

remedial treatment is most effective. During an investigation, the following signs of deterioration should be carefully 

investigated. Fruiting bodies provide positive indication of 

fungal attack but do not indicate the amount or extent of decay. Some fungi produce fruiting bodies after small amounts 

of decay have occurred; whereas others develop only after 

decay is extensive. When fruiting bodies are present, they 

almost certainly indicate serious decay problems.

Sunken faces or localized surface depressions can indicate 

underlying decay. Decay voids or pockets may develop 

close to the surface of the member, leaving a thin, depressed 

layer of intact or partially intact wood at the surface. Staining or discoloration indicates that members have been 

subjected to water and potentially high moisture contents 

required for decay activity. Rust stains from connection 

hardware are also a good indication of wetting. Insect activity is visually characterized by holes, frass, and powder 

posting. The presence of insect activity may also indicate 

the presence of decay. Plant or moss growth in splits, cracks, 

or soil accumulations on the structure indicates that adjacent 

wood has been at relatively high moisture content suitable 

for decay for a sustained period of time.

Hammer Sounding and Probing

Simple mechanical tests are frequently used for in-service 

inspection of wood members in timber bridge structures. 

Hammer sounding can allow inspectors to identify areas that 

sound hollow and require further inspection. Loose hardware or the presence of carpenter ants can also be detected 

using hammer sounding. Further, the use of probes can 
Laser Scanning Technology as Part of a Comprehensive Condition Assessment for Covered Bridges

9

(parallel to fiber) and longest across the grain (perpendicular 

to fiber). For common timber bridge species, Douglas-fir 

and Southern Pine, at dry conditions, stress-wave transmission time is approximately 200 μs/m (60 μs/ft) parallel to 

grain, but ranges from 850 to 1000 μs/m (259 to 305 μs/ft) 

in the perpendicular or cross-grain direction. 

The presence of deterioration from decay can greatly affect 

stress-wave transmission time in wood. Transmission times 

for decayed wood are much greater than that for nondecayed 

wood. For example, transmission time for nondegraded 

Douglas-fir is approximately 850 μs/m (244 μs/ft), whereas 

severely degraded members exhibit values as high as 3,200 

μs/m (975 μs/ft) or greater. White and Ross (2014) demonstrated that a 30% increase in stress-wave transmission time 

implies a 50% loss in strength. A 50% increase indicates 

severely decayed wood.

Resistance Microdrilling

Resistance drilling, an automated form of mechanical probing, is being increasingly used in the field to characterize 

wood properties and to detect abnormal physical conditions 

in structural timber members. This system was originally 

developed for use by arborists and tree care professionals to 

evaluate the condition of urban trees and locate voids and 

decay. It is now being used to identify and quantify decay, 

voids, and termite galleries in wood beams, columns, poles, 

and piles. This technique is now the preferred drilling and 

coring technique for timber bridge elements (Brashaw and 

others 2005). 

The resistance drill system measures the resistance of wood 

members to a 1.5-mm drill bit with a 3.0-mm head that 

passes through them. The drill bit is fed at a fixed movement 

rate allowing the inspector to determine the exact location 

and extent of the damaged area. This system produces a 

chart showing the relative resistance over its travel path. 

This chart can be produced either as a direct printout or 

electronic data that can be downloaded to a computer. Areas 

of sound wood have varying levels of resistance depending 

on the density of the species and voids show no resistance. 

The inspector can determine areas of low, mild, and high 

levels of decay. Further, termite galleries can be identified by the characteristic of high resistance (solid wood) 

followed immediately by no resistance (gallery). Larger 

diameter holes can be drilled and inspected with a magnified 

inspection scope to verify the presence of live termites.

Summary

• Three-dimensional laser scanning can be used to rapidly 

scan historic bridge structures. There are a number of 

technologies and commercial vendors available.

• Post-processing of the scan data requires experience and 

skill to cost-effectively create as-built documentation. 

• A 3D scanner can be used to create a range of outputs that 

allow the inspector to determine the depth of any checks or 

splits. The underlying premise for such tests is that degraded 

wood is relatively soft and will have low resistance to probe 

penetration or withdrawal.

Moisture Content Determination

Moisture meters can effectively be used in conducting 

inspections of timber bridge elements. It is well documented 

that the presence of moisture is required for decay to occur 

in timber. Typically, moisture conditions in timber of less 

than 20% will not allow decay to occur. However, as the 

moisture increases above 20%, the potential for decay to occur increases. Serious decay occurs only when the moisture 

content of the wood is above 28%–30%. This occurs when 

dry wood is exposed to direct wetting through rain, moisture 

infiltration, or contact with ground water or bodies of water. 

Wood that is fully saturated with water and without enough 

oxygen will not be affected by wood decay fungi. Timber 

piles should be carefully inspected near the water line since 

rivers and streams have varying water levels throughout the 

year.

Stress-Wave Timing Techniques

Stress-wave timing is an effective method for locating and 

defining areas of decay in timber bridges (Brashaw and 

others 2005). Stress-wave propagation in wood is a dynamic 

process that is directly related to the physical and mechanical properties of wood. In general, stress waves travel 

faster in sound and high quality wood than in deteriorated 

and low quality wood. By measuring wave transmission 

time through a timber bridge beam, pile cap, or piling in 

the radial direction, the internal condition of the structural 

element can be fairly accurately evaluated. A stress wave 

is induced by striking the timber member with an impact 

device instrumented with an accelerometer that emits a start 

signal to a timer. A second accelerometer, held in contact 

with the other side of the timber, senses the leading edge 

of the propagating stress wave and sends a stop signal to 

the timer. The elapsed time for the stress wave to propagate 

between the accelerometers is displayed on the timer. This 

measured time, when converted to a transmission time on a 

per length basis (or wave propagation speed), can be used 

as a predictor of the physical conditions inside the timber 

bridge member. 

Stress-wave timing is especially useful on thick timbers or 

glued-laminated timbers (≥89 mm (3.5 in.)) where hammer sounding is not effective. Access to both sides of the 

member is required. Because wood is an organic substance, 

the speed of wave propagation varies with grain direction. 

Hammering the side of a timber member will cause a sound 

wave across or transverse to the wood cells (perpendicular 

to grain). The speed of sound across the grain is about onefifth to one-third of the longitudinal value. 

Stress-wave transmission times are shortest along the grain 
Research Note FPL–RN–0335

10

include point cloud scans, parametric images, and 2D and 

3D AutoCAD® drawings. For historic bridges, this information can be used for a variety of purposes including 

as-built documentation and structural assessment, while 

also providing detail on the land topography adjacent to 

the bridge.

• The use of new laser scanning technology should be 

incorporated into a comprehensive inspection protocol. 

Three-dimensional scanning provides millions of data 

points of information about the bridge being inspected. 

The point clouds of data collected depict all visible 

aspects and actual dimensions of the bridge. This information can be used to determine areas of the bridge that 

show excessive deflection, rotation, or skew of the structure, damaged members, and other visual indicators that a 

human eye cannot easily identify or quantify.

• With new access to lower cost laser scanning technology, 

a comprehensive inspection protocol for historic covered 

bridges should include (1) 3D laser scanning, (2) visual 

inspection, (3) hammer sounding and probing, (4) moisture content determination, (5) stress-wave timing, and 

(6) resistance microdrilling.

Literature Cited

Atasoy, G.; Tang, P.; Zhang, J.; Akinci, B. 2010. Visualizing 

laser scanner data for bridge inspection. 2010 Proceedings 

of the 27th International Symposium on Automation and 

Robotics in Construction, Bratislava, Slovakia: 390–400.

Brashaw, B.K.; Vatalaro, R.J.; Wacker, J.P.; Ross, R.J. 2005. 

Condition assessment of timber bridges. 1. Evaluation of 

a micro-drilling resistance tool. General Technical Report 

FPL–GTR–159. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 8 p. 

Brashaw, B.K.; Vatalaro, R.J.; Wang, X.; Ross, R.J.; Wacker, 

J.P. 2005. Condition assessment of timber bridges. 2. Evaluation of several stress wave tools. General Technical Report 

FPL–GTR–160. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 11 p.

Brashaw, B.K. 2010. Use of laser scanning technology to 

obtain as-built records of the Zumbrota covered bridge. 

NRRI Technical Report NRRI/TR-2011/04. University of 

Minnesota Duluth.

FARO. 2013. FARO laser scanner technical specifications. 

FARO laser scanner focus 3D - the leader in 3D documentation.

FARO. 2011a. Measuring arms, laser tracker, laser scanner. 

FARO Laser Scanner Focus 3D - APPLICATIONS. 

FARO. 2011b. FARO laser scanner training. Training 

presentation. 

Foltz, L. 2000. 3D laser scanner provides benefits for Penn 

DOT Bridge and Rockface surveys. Professional Surveyor. (May). http://www.profsurv.com/magazine/article.

aspx?i=590

Jaselskis, E.; Cackler, E.; Andrle, S; Gao, Z. 2003. Pilot 

study on improving the efficiency of transportation project 

using laser scanning. Final Report CTRE Project 02-109. 

Center for Transportation Research and Education, Iowa 

State University.

Jaselskis, E.; Gao, Z.; Walters, R. 2005. Improving transportation projects using laser scanning. Journal of Construction 

Engineering and Management. 131(3): 377–384.

Ross, R.J.; Brashaw, B.K.; Anderson, S.J. 2012. Use of laser 

scanning technology to obtain as-built records of historic 

covered bridges. Research Paper FPL–RP–669. Madison, 

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