Nondestructive Assessment of 

Wood Members in a Viewing 

Tower in Potawatomi State Park, 

Door County, Wisconsin, USA

Robert J. Ross

Xiping Wang

C. Adam Senalik

United States Department of Agriculture

Forest

Service

Forest Products

Laboratory

Research Note

FPL–RN–0366

December

2018
December 2018

Ross, Robert J.; Wang, Xiping; Senalik, C.Adam. 2018. Nondestructive 

assessment of wood members in a viewing tower in Potawatomi State Park, 

Door County, Wisconsin, USA. Research Note FPL-RN-0366. Madison, 

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

Laboratory. 12 p.

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

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53726-2398. This publication is also available online at 

www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries 

in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin. 

The use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the United States Department of 

Agriculture (USDA) of any product or service.

In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil rights regulations and policies, the USDA, its Agencies, 

offices, and employees, and institutions participating in or administering USDA programs are prohibited from discriminating based on race, color, national 

origin, religion, sex, gender identity (including gender expression), sexual orientation, disability, age, marital status, family/parental status, income derived 

from a public assistance program, political beliefs, or reprisal or retaliation for prior civil rights activity, in any program or activity conducted or funded by 

USDA (not all bases apply to all programs). Remedies and complaint filing deadlines vary by program or incident. 

Persons with disabilities who require alternative means of communication for program information (e.g., Braille, large print, audiotape, American Sign 

Language, etc.) should contact the responsible Agency or USDA’s TARGET Center at (202) 720–2600 (voice and TTY) or contact USDA through the 

Federal Relay Service at (800) 877–8339. Additionally, program information may be made available in languages other than English. 

To file a program discrimination complaint, complete the USDA Program Discrimination Complaint Form, AD-3027, found online at http://www.ascr.usda.

gov/complaint_filing_cust.html and at any USDA office or write a letter addressed to USDA and provide in the letter all of the information requested in 

the form. To request a copy of the complaint form, call (866) 632–9992. Submit your completed form or letter to USDA by: (1) mail: U.S. Department of 

Agriculture, Office of the Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250–9410; (2) fax: (202) 690–7442; or 

(3) email: program.intake@usda.gov. 

USDA is an equal opportunity provider, employer, and lender.

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. Potawatomi State Park, 

located on the Door County peninsula, consists of about 

1,200 acres of flat to gently rolling upland terrain bordered 

by steep slopes and rugged limestone cliffs along Lake 

Michigan’s shoreline. A 75-ft observation tower sits atop 

a 150-ft bluff overlooking Lake Michigan. The USDA 

Forest Service, Forest Products Laboratory, was asked to 

conduct an assessment of the main support timbers of the 

tower. This report summarizes the results obtained from 

the inspection and assessment. It includes a brief summary 

of the nondestructive testing techniques that were used, 

observations, data from tests conducted on the timbers, and 

recommendations.

Keywords: Nondestructive assessment, decay, moisture, 

timbers, observation tower

Contents

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

History of the Tower in Potawatomi State Park...................1

Inspections of the Tower in Potawatomi State Park ............2

Results..................................................................................2

Conclusions..........................................................................3

Implications..........................................................................3

Recommendations................................................................4

Literature Cited ....................................................................5
Introduction

The USDA Forest Service, Forest Products Laboratory 

(FPL), was contacted in early 2018 by the Wisconsin 

Department of Natural Resources (DNR) to assist DNR 

engineers in assessing the condition of timbers in a historic 

viewing tower located in the Potawatomi State Park. 

Located in Door County, the 75-ft observation tower (Fig. 1) 

sits atop a 150-ft limestone bluff. From its top platform, one 

can see 16 miles across Lake Michigan’s Green Bay.

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, carpenter ants, or nesting 

activity of local bird populations. 

Nondestructive Assessment of Wood 

Members in a Viewing Tower in Potawatomi 

State Park, Door County, Wisconsin, USA

Robert J. Ross, Ph.D., Project Leader, and Research Professor

USDA Forest Products Laboratory, Madison, Wisconsin, USA, and

Michigan Technological University, Houghton, Michigan, USA

Xiping Wang, Ph.D., Research Forest Products Technologist

C. Adam Senalik, Ph.D., P.E., Research General Engineer

USDA Forest Products Laboratory, Madison, Wisconsin, USA

The objective of the work summarized in this research note 

was to nondestructively evaluate the quality of timbers 

that comprise the tower. Several nondestructive techniques 

were utilized to assess the condition of the large-diameter 

timber poles and rectangular columns and beams used in the 

construction of the tower. 

History of the Tower in 

Potawatomi State Park

In 1834, a federal quarry was established on the shores 

of Green Bay to supply limestone for the building of 

breakwaters and harbors. This quarry eventually became 

known as Government Bluff. The land that was to become 

Potawatomi State Park was purchased from the federal 

government by the State of Wisconsin in 1928.

Figure 1. Potawatomi Observation Tower: (a) southwest view; (b) southeast view.

(a) (b)
Research Note FPL–RN–0366

2

In April 1929, the Sawyer Commercial Club donated $500 

for the purchase of materials to build a 75-ft observation 

tower at the peak of Government Bluff, overlooking Sawyer 

Harbor. Construction of the tower began in late summer of 

1931 and was completed by the Wisconsin Conservation 

Commission. The tower consisted of three platforms 

supported by four wood poles, each 75 ft in length.

Inspections of the Tower in 

Potawatomi State Park

In summer 2012, a large crack was discovered on a main 

support timber and the tower was closed for an extended 

period because of concerns for public safety. In 2017, 

inspections of the tower found evidence of decay and 

movement of the structural wood members. Based upon 

the 2017 inspections, the Wisconsin DNR contacted FPL 

and requested a nondestructive inspection similar to the 

inspection performed by FPL personnel on Peninsula State 

Park’s Eagle Tower in 2016 (Ross and others 2017).

An in-depth, nondestructive inspection of the Potawatomi 

State Park Tower was conducted on February 6, 2018. 

Accepted inspection techniques used to assess the condition 

of the timbers in the tower are discussed in detail by White 

and Ross (2014). Following are brief descriptions of the 

techniques.

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 inspection of the tower members, we specifically 

looked for

1. evidence of water intrusion and subsequent damage, 

especially where the legs of the tower were in contact 

with concrete pads they rested on and other areas where 

the exterior of the wood member was compromised, and

2. 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). In summary, 

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 2005; Clausen and 

others 2001; Emerson and others 2002; França and others 

2015; Ross and others 1999; Ross and Wang 2005; Ross 

and others 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).

A simple, commercially available sound transmission 

nondestructive testing device was used in the inspection. 

With this device, 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 device and recorded. 

Transmission times for wood in good condition from 

several species are known and were used as a baseline. 

Transmission times significantly longer than baseline values 

indicate the likely presence of deteriorated wood.

Micro-Drilling Resistance

Simple mechanical tests are frequently used for in-service 

inspection of wood members in structures. Drilling and 

coring are the most common 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.

Micro-drilling resistance is a commercially available 

inspection technique originally developed for use by 

arborists and tree care professionals to evaluate the 

condition of urban trees and to locate voids and decay. It is 

now commonly used to identify and quantify decay, voids, 

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

piles (Brashaw and others 2005). The underlying premise 

for this technique is that degraded wood is relatively soft 

and will have low resistance to drill penetration and that 

voids will have no resistance to drill penetration.

Micro-drilling resistance tests were conducted in areas of 

the timbers that were believed to contain deteriorated wood, 

based on results from the visual assessment and sound 

transmission testing.

Results

The condition assessment of the Potawatomi Observation 

Tower was performed on February 6, 2018. Figures 1a 

and 1b show the tower on the day of the inspection. The 

assessment began with a visual inspection. Deterioration 

of structural and nonstructural wood members was evident 

in the tower. Numerous deep splits and cracks within many 

of the members, and several deep splits on the main load 

bearing support columns, were observed. Holes, probably 
Nondestructive Assessment of Wood Members in a Viewing Tower in Potawatomi State Park, Door County, Wisconsin, USA

3

resulting from nesting activities of birds, were observed. 

Of concern were splits and deterioration in the vicinity of 

connections and the evidence of lateral movement of the 

upper sections of the tower.

After the visual inspection was completed, sound 

transmission and micro-resistance drilling tests were 

performed. Figure 2a shows the southwest tower support 

pole near ground level. Note the large patched area sealing a 

previous woodpecker hole. Similar patches were present on 

all four support legs of the tower. The pink attached notes 

indicate sites where micro-resistance drilling inspections 

were performed. Figure 2b shows the resistance drilling tool 

positioned for use, and Figure 2c shows the results of the 

inspection. The diameter of the pole at the inspection site 

was 17 in. The measured resistance noticeably decreases 

between approximately 7 and 10 in. from the edge. The 

decrease indicates the presence of interior decay or a void, 

or both.

Figure 3a shows the steel collar securing the first platform to 

the southwest support. The steel collar had displaced relative 

to the location of the support approximately 3-1/4 in., 

causing noticeable damage (Fig. 3b). The displacement 

was sufficiently deep to completely penetrate through the 

surface preservative treatment. Figure 3a also shows surface 

deterioration on the south side of the support. The wood at 

that location was easily penetrated by a probe with minimal 

effort, indicating the presence of advanced decay. Figure 3c 

shows the results of the micro-resistance drilling at location 

no. 5, 10 in. above the steel collar (as shown in Fig. 3b). 

The resistance profile showed sign of decay for the outer 

1-1/2 in. in this location.

Figure 4a shows the southeast support of the tower. The 

diameter of the support pole at the location shown is 17 in. 

Micro-resistance drilling at location no. 3 is 37 in. above the 

footing. The resistance profile shown in Figure 4b reveals 

center decay of 1 in. diameter and multiple internal cracks.

Figure 5a shows the southeast support pole at the first 

platform. Micro-resistance drilling was performed at 

location no. 7, 58 in. above the platform, where the diameter 

of the support pole was 15 in. Large patched cracks of 

unknown depth are observed on this timber. The resistance 

profile results shown in Figure 5b reveal an internally sound 

support with some surface deterioration (marked red in the 

resistance profile).

Figure 6a shows the northwest support pole at the second 

platform. Micro-resistance drilling was performed at 

location no. 10, where the support pole diameter was 

12 in. Figure 6b shows visible surface deterioration on the 

east side of the support. The resistance profile shown in 

Figure 6c reveals an internally sound support.

Figure 7 shows the central rectangular timber supporting 

the staircase at the second platform. The stress wave timing 

tool is shown in Figure 7a. Results of the stress wave timing 

measurement were significantly high. Any measurement of 

greater than 300 μs/ft is indicative of internal decay. The 

measurement across the support was 853 μs/ft, indicating 

severe decay. In addition, Figure 7b shows visible decay. 

The extent of the decay on the support was such that the 

wood could be removed by hand with little to no effort.

Conclusions 

The following conclusions are based on the visual 

inspection and nondestructive testing of the tower 

components: 

1. The tower contained support members that were 

deteriorated. Surface decay is present on all four 

support poles. The southwest support contains internal 

and surface decay over a significant length. The central 

stair support is decayed.

2. The inspections performed revealed several areas of 

decay in the tower. Given the current state-of-the-art 

in timber inspection, it would be difficult to accurately 

determine, in-place, the extent to which the deterioration 

has spread. Although it can be stated with certainty 

that the load-carrying capacity of the tower has been 

compromised, the residual load-carrying capacity is 

unknown.

3. It is difficult to estimate the quantity or quality of 

recoverable material from the timbers. Many of the 

members were deteriorated, and they should not be 

reused in any load-bearing application.

4. Several of the same deficiencies that were observed with 

the Potawatomi Observation Tower were observed during 

the 2015 assessment of the Eagle Tower. Both structures 

were of similar construction and age, so this is to be 

expected.

Implications

The presence of deterioration caused by decay has a 

significant effect on various properties of structural timber. 

The following sections summarize the effect of decay on 

various strength properties of wood. Table 1 summarizes 

early scientific literature on the topic.

Toughness (Impact Bending)

Toughness (the ability of a wood member to withstand 

shock loading) is generally considered to be the strength 

property most affected in early stages of decay. Research 

dating to 1954 indicates a loss of toughness of 50% with a 

corresponding loss of only 1% loss in weight.

Static Bending

Research dating to the 1930s has been conducted on 

modulus of elasticity, modulus of rupture, and work-tomaximum load. Reported results indicate a significant loss 

in modulus of rupture (strength) with decayed wood.
Research Note FPL–RN–0366

4

Other Strength Properties

Research has shown that compression perpendicular to the 

grain, compression parallel to the grain, tension parallel to 

the grain, shear parallel to the grain, and tangential hardness 

are also impacted by deterioration caused by decay.

Recommendations

1. If the tower is dismantled and reuse of the materials is 

desired, then each member should be evaluated using 

advanced nondestructive assessment procedures.

2. The FPL has no data or published reports on, and can 

therefore make no recommendation related to, bonding 

of deteriorated wood.

3. None of the members should be reused in any loadbearing application.

4. 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).

Table 1—Estimated values for strength losses in softwoods and hardwoods at early stages of decay (indicated by 

weight loss) by brown-rot and white-rot fungi as a percentage of the value for non-decayed samplesa,b

Strength property loss (%)

Approx. 

weight 

loss

(%)

Static bending Compression 

perpendicular 

(radial)

ToughnessImpact 

bending

General 

bending 

strength

Work 

to max. 

load MORc MOEc

Compression 

parallel

Tension 

parallel

Shear 

parallel Hardness

Brown rot

Softwoods

1 57 20–38 — — — — — — — 2 —

2 — 20–50 5 27 13–50 4–55 18–24 10 23–40 — —

4 75 25–55 — — — — 25–35 — — 6 7

6 — 62–72 16 — 61 66 48 25 60 — —

8 — 78 — — — — 48–60 — 50 15 21

10 — 85 36 — 70 — 66 45 — 20 —

Hardwoods

1 — 6–27 — — — — — — — — —

2 36 31–50 — 54 32 — 6–10 — 56 — —

4 — 60–70 — 69 49 — — — — — —

6 — 80 — 75 61 — 16–25 — — — —

8 — 9–89 13–34 — — — 19 — 82 — —

10 60 70–92 — — — — — — — — —

White rot

Softwoods

1 55 — — — — — — — — — —

2 — — — — — — 10–20 — 4–38 — —

4 — — — — — — — — 8–43 — —

6 75 — — — — — 32–61 — 10–49 — —

8 — — — — — — — — 14–58 — —

10 85 — — — — — — — 20–63 — —

Hardwoods

1 — 21 — — — — 4 — — — —

2 — 26 — 28–35 13–14 4 5 — 22–42 — —

4 70 44 — 38 20 — — — 17–44 — —

6 75 50 — 45–53 20–27 10 12–27 14 12–58 — 18

8 — — — — — — — — 14–49 —

10 85 60 58 24 14 35 20 20–50 25

a

From Wilcox (1978). Values obtained from published experimental results and adjusted to equivalent weight loss levels.

b

Sources: Brown (1963), Cartwright and others (1931), Gillwald and Michalak (1963), Hartley (1958), Henningsson (1967), Kennedy (1958), Kennedy 

and Ifju (1962), Kubiak and Kerner (1963), Mizumoto (1966), Mulholland (1954), Pechmann and Schaile (1950), Richards (1954), Scheffer (1936), Toole 

(1971), and Wilcox (1968).

c

MOR, modulus of rupture; MOE, modulus of elasticity.
Nondestructive Assessment of Wood Members in a Viewing Tower in Potawatomi State Park, Door County, Wisconsin, USA

5

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. Proceedings, 15th Nondestructive Testing of Wood 

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

Products Society: 43–48.

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.

Brown, F.L. 1963. A tensile strength test for comparative 

evaluation of wood preservatives. Forest Products Journal. 

13(9): 405–412.

Cartwright, K.St G.; Findlay, C.J.; Campbell, W.G. 1931. 

The effect of progressive decay by Trametes serialis Fr. 

on the mechanical strength of the wood of Sitka spruce. 

Great Britain: Dep. Sci.Ind. Res. Forest Products Research 

Bulletin No. 11.

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 a 

2,500-year-old mummy coffin. Research 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, Lon A.; Hohnholt, Christopher; Forsman, John 

W.; Ross, Robert J. 2015. Condition assessment of a 

historic trout rearing station in upper Michigan. Research 

Note FPL–RN–0334. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

5 p.

Gillwald, W.; Michalak, J. 1963. Vergleichende 

Untersuchungen uber die Einwirkung von verschiedenen 

holzzerstoreden Pilzen anf einige physikalische 

Eigenschaften und die Festigkeit des Pappelholzes 

(Populus marilandica Bosc.). Wiss.Z. Humboldt-Univ. 

Berlin, Math. Naturwiss. Reihe. 12(2): 265–273.

Hartley, C. 1958. Evaluation of wood decay in experimental 

work. USDA Forest Products Laboratory Mimeo No. 2119.

Henningsson, B. 1967. Changes in impact bending strength, 

weight, and alkali solubility following fungal attack on birch 

wood. Stud. For. Suec. (Stockholm). No. 41.

Kennedy, R.W. 1958. Strength retention in wood decayed 

to small weight losses. Forest Products Journal. 8(10): 

308–314.

Kennedy, R.W.; Ifju, G. 1962. Applications of microtensile 

testing of thin wood sections. Tappi. 45(9): 725–733.

Kubiak, M.; Kerner, G. 1963. Die Veranderungen einiger 

physikalischer Eigenschaften, der Druckfestigkeit und der 

chemischen Zusammensetzung des Holzabbanes durch 

Conophora cerebella Pers. Und Steveum hirsutum Wiild. 

Drev. Vysk. 1963(4): 181–193.

Mizumoto, S. 1966. The effect of decay caused by 

Gloeophyllum trabeum on the strength properties of 

Japanese red pine sapwood. Journal of Japan Forestry 

Society. 48(1): 711.

Mulholland, J.R. 1954. Changes on weight and strength of 

Sitka spruce associated with decay by a brown-rot fungus, 

Poria monticola. Journal of the Forest Products Research 

Society. 4(6): 410–416.

Pechmann, H. von; Schaile, O. 1950. Uber die Andeerung 

der dynamischen Festigkeit und der chemischen 

Zusammensetzung des Holzes durch den Angriff 

holzzerstorenender Pilze. Forstwiss Centralbl. 69(8): 

411–466.

Richards, D.B. 1954. Physical changes in decaying wood. 

Journal of Forestry. 52: 260–265.

Ross, R.J.; Wang, X. 2005. Quincy mine blacksmith shop—

Condition assessment of timbers. Structure. September 

2005: 32–34.

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. General Technical Report FPL–GTR–114. 

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

Service, Forest Products Laboratory.

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.; Wang, X.; Senalik, C.A.; Allison, R.B.; Zhou, L. 

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. 
Research Note FPL–RN–0366

6

Scheffer, T.C. 1936. Progressive effects of Polyporous 

versicular on the physical and chemical properties of red 

gum sapwood. USDA Tech. Bull. No. 527.

Toole, E.R. 1971. Reduction in crushing strength and weight 

associated with decay by brown rot fungi. Wood Science. 

3(3): 172–178.

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

Condition assessment of the main structural members of 

historic steam schooner Wapama. Research Paper 

FPL–RP–649. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory.

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

assessment manual. Second Edition. General Technical 

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

Agriculture, Forest Service, Forest Products Laboratory. 

102 p.

Wilcox, W.W. 1968. Changes in wood microstructure 

through progressive stages of decay. Research Paper 

FPL–RP–70. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory.

Wilcox, W.W. 1978 Review of literature on the effects of 

early stages of decay on wood strength. Wood and Fiber 

Science. 9(4): 252–257.
Nondestructive Assessment of Wood Members in a Viewing Tower in Potawatomi State Park, Door County, Wisconsin, USA

7

Figure 2. Southwest support pole at ground level. (a) View of southern side of support. The attached pink notes indicate 

locations of micro-resistance drilling test. The black patch is the location of a sealed woodpecker hole. (b) Micro-resistance 

drilling test at location no. 1, 32 in. above the footing. (c) Resistance profile for location no. 1 reveals a 3- to 4-in. center 

deterioration, which was likely caused by water entering through the woodpecker hole 1 ft above.

(c)

[%]

[inch]

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

55

50

45

40

35

30

25

20

15

10

5

0

(a) (b)

Location no.  2

Location no.  1

Location no.  1
Research Note FPL–RN–0366

8

Figure 3. Southwest support pole at platform 1. (a) Steel collar securing the first platform to the southwest support. 

Displacement of the collar has damaged the support and penetrated the surface layer of preservative. Deterioration is 

apparent where the clamp has damaged the support. (b) Steel clamp has displaced 3-1/4 in. The attached pink notes show 

micro-resistance drilling locations no. 4 and no. 5, 10 in. above the collar. (c) Resistance profile for location no. 5 shows 

1-1/2 in. of surface decay.

Location no.  4 Location no.  5

(c)

(a) (b)

[%]

[inch]

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

40

35

30

25

20

15

10

5

0
Nondestructive Assessment of Wood Members in a Viewing Tower in Potawatomi State Park, Door County, Wisconsin, USA

9

Figure 4. Southeast support pole at ground level. (a) Micro-resistance drilling test at location no. 3, 37 in. above the footing. 

The support diameter is 17 in. at this location. (b) The resistance profile reveals center deterioration (marked red) of 1 in. 

diameter and multiple internal cracks (marked purple).

Location no.  3

(b)

(a)

[%]

[inch]

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

35

30

25

20

15

10

5

0
Research Note FPL–RN–0366

10

Figure 5. Southeast support pole at platform 1. (a) Micro-resistance drilling test at location no. 7, 58 in. above the platform. 

The support diameter is 15 in. Large patched cracks of unknown depth can be observed on this timber. (b) The resistance 

profile reveals an internally sound support with surface deterioration.

Location no.  7

(b)

(a)

[%]

[inch]

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

35

30

25

20

15

10

5

0
Nondestructive Assessment of Wood Members in a Viewing Tower in Potawatomi State Park, Door County, Wisconsin, USA

11

Figure 6. Northwest support pole at platform 2. (a) Micro-resistance drilling test at location no. 10. (b) Visible surface 

deterioration. (c) Resistance profile reveals an internally sound support.

Location no.  10

(c)

(a) (b)

[%]

[inch]

0 1 2 3 4 5 6 7 8 9 10 11 12

50

45

40

35

30

25

20

15

10

5

0
Research Note FPL–RN–0366

12

Figure 7. Central stairway support at platform 2. (a) Stress wave timing measurement showed significantly high readings 

(426 μs, equivalent to 853 μs/ft), indicating severe decay. (b) Visible deteriorated wood.

(a) (b)