Nondestructive Assessment 

of Wood Members from a 

Historic Viewing Tower

Robert J. Ross

Xiping Wang

C. Adam Senalik

R. Bruce Allison

Lujing Zhou

United States Department of Agriculture

Forest

Service

Forest Products

Laboratory

Research Note

FPL–RN–0349

August

2017
Abstract

The state of Wisconsin’s Department of Natural Resources 

is responsible for operating one of the largest state park 

systems in the United States. Peninsula State Park, one of 

the state’s most visited parks, attracts more than one million 

visitors a year. This park contained one of the most iconic 

structures in the Wisconsin State Park system, a tall, 

wooden observation tower that was constructed in 1932.

Because of concerns about the structural integrity of several 

of the large timbers in the tower, it was closed in the spring 

of 2015 and dismantled in fall 2016 at the request of the 

Department of Natural Resources. The USDA Forest 

Service, Forest Products Laboratory, was contacted and 

asked to conduct an assessment of the timbers after the 

structure was dismantled. This report summarizes results 

obtained from our inspection and assessment of the timbers. 

A brief summary of the nondestructive testing techniques 

used, observations, and data from tests conducted on the 

timbers are included.

Keywords: nondestructive assessment, decay, moisture, 

timbers

August 2017

Ross, Robert J.; Wang, Xiping; Senalik, C. Adam; Allison, R. Bruce; Zhou, 

Lujing. 2017. Nondestructive assessment of wood members from a historic 

viewing tower. Research Note FPL–RN–0349. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products Laboratory. 8 p.

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Introduction

The USDA Forest Service, Forest Products Laboratory 

(FPL), was contacted in late 2015 to assist engineers from 

the Wisconsin Department of Natural Resources (DNR) in 

evaluating the condition of timbers in a historic viewing 

tower located in Wisconsin’s Peninsula State Park. Located 

in Door County, the Eagle Tower (Fig. 1) was a 75-ft (1 ft = 

0.3048 m) observation tower that sat atop a 180-ft limestone 

bluff. From its top platform, one could see Peninsula State 

Park, surrounding islands in Lake Michigan, and 

Michigan’s Upper Peninsula.

The tower structure was built in 1932. It replaced the 

original tower built on Eagle Bluff, which was constructed

in 1914. This iconic tower stood as a well-known Wisconsin 

landmark. Peninsula State Park is visited by more than a 

million people every year.

The DNR periodically examined the structure using visual 

inspection techniques. Engineers looked for signs of 

distress, such as failed members and those showing 

evidence of attack by decay fungi or carpenter ants or 

nesting activity for local bird populations. After an 

extensive inspection, the DNR concluded that the main 

structural members were severely deteriorated and, 

consequently, closed the structure. The tower was 

dismantled September 19, 2016.

The objective of the work summarized in this report was to 

nondestructively evaluate the quality of timbers that had 

comprised the tower after it was dismantled. Several 

nondestructive techniques were used to assess the condition 

of the large timber poles from the tower.

History of the Tower in Peninsula 

State Park

The original tower on Eagle Bluff was constructed in 1914 

at a cost of $1,061.92. The tower was 76 ft tall, and it stood 

225 ft above Lake Michigan. It was constructed during the 

summer of 1914 to serve as a fire tower with the expectation 

it would become a tourist attraction. The ledger of 

Peninsula’s first manager, A.E. Doolittle, lists payment to 

men for fire watch duty.

A construction crew cut logs and boards from timber in the 

park using tools and saws available at the time. To erect the 

tower, they first raised the center pole. Then they used the 

center pole to raise other support poles. Three trees made up

each corner pole, one tree for each level, with platforms 

between each level. Horizontal landing support beams were 

added, followed by planks for decking at the three levels.

A telephone line connected the tower to the manager’s 

residence and the local exchange so that fires could be 

quickly reported. Historical records indicate there was a 

large buildup of fire-prone material in the park. Peninsula’s 

last significant fire was in 1921.

The original tower was dismantled. Then, Eagle Tower was 

built in 1932. Foreman Sam Erickson and crew used horses, 

Nondestructive Assessment of Wood 

Members from a Historic Viewing Tower

Robert J. Ross, Supervisory Research General Engineer and Research Professor

USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin, and Michigan Technological 

University, Houghton, Michigan

Xiping Wang, Research Forest Products Technologist

C. Adam Senalik, Research General Engineer

USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin

R. Bruce Allison, Adjunct Professor and Arborist

University of Wisconsin-Madison, Madison, Wisconsin, and Allison Tree, LLC, Verona, Wisconsin

Lujing Zhou, Ph.D. Candidate

Beijing Forestry University, Beijing, China

Figure 1. Eagle Tower, Peninsula State Park, Door 

County, Wisconsin.
Research Note FPL–RN–0349

2

tractors, trucks, and other machinery. They wrapped cable 

around nearby trees to raise poles. A stump wrapped with 

rusty cable can still be seen a short distance from the tower

site, along the road leading toward Eagle Terrace (GPS 

N45.16275 W 87.19730). The poles (western redcedar) 

came from Washington State.

Although funded with state relief, this was not a Civilian 

Conservation Corps project.

Eagle Tower was 75-ft tall and stood 250 ft above water 

level. Safety improvements were made in 1972, including 

slanting the top deck railings. Hardware and decking were 

replaced and stained in subsequent years.

Inspection of Eagle Tower

An in-depth inspection of Eagle Tower was conducted in the 

spring of 2015 because of its age. The inspection, contracted

by the DNR, included core sampling to determine the 

general internal condition of the structural components and 

overall load-bearing capacity of the structure. The tower 

inspection report indicated that the structure was in poor 

condition and should be razed. Subsequent reviews of the 

tower inspection report supported this recommendation. On 

May 20, 2015, the tower was closed for public use. Eagle 

Tower was dismantled on September 19, 2016.

Assessment of Dismantled 

Members from Eagle Tower

Accepted inspection techniques were used to assess the 

condition of the timbers from the tower. These techniques 

are discussed in detail in Wood and Timber Condition 

Assessment Manual, Second Edition (White and Ross 2014). 

A brief description of the techniques we used follows.

Visual Inspection

The simplest method for locating deterioration in wood 

members is visual inspection. An inspector observes the 

structure for signs of actual or potential deterioration, noting 

areas that require further investigation. Visual inspection is 

useful for detecting intermediate or advanced surface decay, 

water damage, mechanical damage, or failed members. 

Visual inspection cannot detect early stage decay or 

deterioration.

Several key indicators are looked for in visual inspections: 

fruiting bodies (evidence of advanced decay), staining or 

discoloration of members (indicators of water damage), 

evidence of insect activity (holes, frass, and powder 

posting), plant or moss growth in a member, deep checks or 

splits, and failed or missing members.

In our visual inspection of the tower members, we 

specifically looked for the following: Evidence of water 

intrusion and subsequent damage, especially near where the 

legs of the tower were in contact with the concrete pads they 

rested on and other areas where the exterior of the wood 

member was compromised, and evidence of structural 

failure of the timbers.

Sound Transmission

A significant volume of research has been devoted to the 

use of sound waves for locating areas of deterioration in 

timber structures, and a practical set of guidelines for their 

use has been prepared by FPL (Ross and others 1999). The 

transmission of sound in wood is affected significantly by 

the presence of deterioration. Consequently, ultrasound and 

stress-wave-based technologies have been developed and 

are widely used to inspect wood structures (Allison and 

others 2008; Brashaw and others 2005b; Clausen and others 

2001; Emerson and others 2002; França and others 2015; 

Ross and others 1999, 2006) and have been used for the 

assessment of culturally significant historic ships and 

artifacts (Ross and others 1998, Wang and others 2008,

Dundar and Ross 2012).

In this assessment, we used several simple, commercially 

available stress wave timers in our inspection. Sensors were 

placed on opposite sides of a timber. The timber was then 

impacted, generating a stress wave. The time it took for the 

wave to travel between the sensors was measured by the 

timer and recorded. Transmission times for wood from 

several species are known and were used as a baseline. 

Transmission times significantly longer than baseline values 

indicated the presence of deteriorated wood in the Eagle 

Tower legs.

Microdrilling Resistance

Simple mechanical tests are frequently used for in-service 

inspection of wood members in structures. Drilling and 

coring are the most common simple tests used to detect 

internal deterioration. Both are used to detect the presence 

of voids and to determine the thickness of the residual shell 

when voids are present.

Microdrilling resistance is a commercially developed 

technique 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 (Brashaw and others 

2005a). The underlying premise for this technique is that 

degraded wood is relatively soft and will have low 

resistance to drill penetration.

Microdrilling resistance tests were conducted in areas of the 

Eagle Tower timbers that were believed to contain

deteriorated wood based on results from our visual 

assessments and stress wave testing.
Nondestructive Assessment of Wood Members from a Historic Viewing Tower

3

Results

Intensive testing of the four large timbers that served as the 

legs for the tower was conducted. Figures 2 through 5 show

the dismantled timbers. Each timber was sawn into three 

sections prior to testing. Figures 6 through 11 show each of 

the sections and corresponding test results. One of the legs 

was severely deteriorated with significant decay throughout 

its entire length. Another leg had significant deterioration.

The other two contained limited areas of deterioration. A 

visual inspection of many of the horizontal cross members 

from the tower revealed significant deterioration. One of 

these members was severely split and was deteriorated 

along its entire length (Fig. 12).

Conclusions

Based on our visual inspection and testing of the timbers 

from the tower, the following were concluded:

1. Critical support members of the tower were severely 

deteriorated. One of the legs from the tower was 

severely deteriorated along its entire length.

2. Many of the members were significantly deteriorated; 

they should not be reused in any load-bearing 

application.

Recommendations

The following are recommended:

1. All of the large timbers should be stored under cover 

and off the ground. This will minimize further 

deterioration.

2. None of the members should be reused in any loadbearing application. Reuse of any recovered materials 

should be limited to non-load-bearing applications, for 

example, as interior paneling in offices, signage, or 

craft-type products.

Literature Cited

Allison, R.B.; Wang, X.; Ross, R.J. 2008. Visual and 

nondestructive evaluation of red pines supporting a ropes 

course in the USFS Nesbit Lake Camp, Sidnaw, MI. In: 

Proceedings, 15th International Nondestructive Testing of 

Wood Symposium, September 10–12, 2007, Duluth, MN. 

Peachtree Corners, GA: Forest Products Society: 43-48.

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

2005a. Condition assessment of timber bridges. 1. 

Evaluation of a micro-drilling resistance tool. Gen Tech. 

Rep. FPL–GTR–159. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

8 p.

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

2005b. Condition assessment of timber bridges. 2. 

Evaluation of several stress-wave tools. Gen. Tech. Rep.

FPL–GTR–160. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

11 p.

Clausen, C.A.; Ross, R.J.; Forsman, J.W.; Balachowski, 

J.D. 2001. Condition assessment of roof trusses of Quincy 

mine blacksmith shop in Keweenaw National Historical 

Park. Research Note FPL–RN–0281. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 4 p.

Dundar, T.; Ross, R.J. 2012. Condition assessment of 2500 

year old wood coffin. Res. Note FPL–RN–0327. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 3 p.

Emerson, R.; Pollock, D.; McLean, D.; Fridley, K.; Pellerin, 

R.; Ross, R.J. 2002. Ultrasonic inspection of large timber 

bridge members. Forest Products Journal. 52(9): 88-95.

França, Frederico J.N.; França, Tâmara S.F. Amorim; 

Yeary, L.A.; Hohnholt, C.; Forsman, J.W.; Ross, R.J. 2015. 

Condition assessment of a historic trout rearing station in 

upper Michigan. Res. Note FPL–RN–0334. Madison, WI: 

U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 5 p.

Ross, R.J.; Brashaw, B.K.; Wang, X. 2006. Structural 

condition assessment of in-service wood. Forest Products 

Journal. 56(6): 4-8.

Ross, R.J.; Soltis, L.A.; Otton, P. 1998. Assessing wood 

members in the USS Constitution using nondestructive 

evaluation methods. APT Bulletin. 29(2): 21-25.

Ross, R.J.; Volny, N.; Pellerin, R.F.; Salsig, W.W.; Falk, 

R.H. 1999. Inspection of timber bridges with stress wave 

nondestructive evaluation tools. A guide for use and 

interpretation. Gen. Tech. Rep. FPL–GTR–114. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory.

Wang, X.; Wacker, J.P.; Ross, R.J.; Brashaw, B.K. 2008. 

Condition assessment of main structural members of steam 

schooner WAPAMA. Gen. Tech. Rep. FPL–GTR–117. 

Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory. 29 p.

White, R.H.; Ross, R.J. 2014. Wood and timber condition 

assessment manual. Second edition. Gen. Tech. Rep. FPL–

GTR–324. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory. 102 p.
Research Note FPL–RN–0349

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Figure 2. Materials from the dismantled tower.

Figure 3. Sections from the four timbers that served as 

legs for the tower. Sections identified as TL#1 through 

TL#4 were the leg sections that rested on concrete 

support pads.

Figure 4. Timber sections TL#5 through TL#8 were from 

the uppermost portion of the tower’s legs.

Figure 5. Timber sections TL#9 through TL#12 were the 

middle section of the tower’s legs.
Nondestructive Assessment of Wood Members from a Historic Viewing Tower

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Figure 6. Timber sections TL#1 through TL#4, with corresponding stress wave transmission test results. Test results 

indicated severe deterioration in sections TL#1 and TL#2.

Figure 7. Microdrilling resistance results indicated deterioration in the vicinity of a bird nest in the timber.
Research Note FPL–RN–0349

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Figure 8. Timber section TL#3 and corresponding stress wave test results. No deterioration was observed.

Figure 9. Deterioration was noted for timber section TL#4.
Nondestructive Assessment of Wood Members from a Historic Viewing Tower

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Figure 10. Results obtained from stress wave tests of the uppermost sections of the tower’s legs.

Figure 11. Test results from the middle sections of the tower’s legs.
Research Note FPL–RN–0349

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Figure 12. Horizontal support member that contained 

severe deterioration.