BENDING STRENGTH AND STIFFNESS OF BRIDGE PILES AFTER 85 YEARS IN THE MILWAUKEE RIVER

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1 BENDING STRENGTH AND STIFFNESS OF BRIDGE PILES AFTER 85 YEARS IN THE MILWAUKEE RIVER USDA Forest Service Research Note FPL U.S. Department of Agriculture Forest Service Forest Products Laboratory Madison, Wisconsin

2 ABSTRACT Static bending tests were conducted on small clear specimens from untreated piles salvaged from the Milwaukee River. Strength and stiffness of red pine pile material was substantially lower than published values for the species; white pine and tamarack results showed little or no apparent effects due to the river exposure. ACKNOWLEDGMENT The author wishes to acknowledge Dr. Joe Clark (FPL) who collected the study piles. A more comprehensive study of the same piles has been undertaken by Clark, in which he hopes to identify agents of deterioration (bacteria, fungi, or chemicals) and relate them to strength degradation.

3 ~ ~~ ~~ BENDING STRENGTH AND STIFFNESS OF BRIDGE PILES AFTER 85 YEARS IN THE MILWAUKEE RIVER By B.A. BENDTSEN, Technologist Forest Products Laboratory, 1 Forest Service U.S. Department of Agriculture INTRODUCTION It is commonly believed that wood can be submerged in water or buried underground indefinitely without undergoing major changes in physical or mechanical properties. A lack of oxygen apparently provides an effective deterrent to attack by most micro-organisms. However, Scheffer, Duncan, and Wilkinson, 2 recently studied piles removed from the Potomac River, Washington, D.C. after 62 years of service. They reported substantiated reductions in the crushing strength of sapwood due to bacteria; some reduction was also observed in the crushing strength of heartwood. The Forest Products Laboratory frequently receives inquiries concerning the suitability for structural application of wood that has been submerged in water or used underground for many years. The report by Scheffer et al. is the only basis we have for responding to these inquiries. When it was learned that the Pleasant Street Bridge over the Milwaukee River in downtown Milwaukee was being dismantled, we took this opportunity to add to our limited knowledge of the long term effects of these environments on wood properties. This report presents the results of an evaluation of the static bending properties of small, clear specimens removed from piles supporting the bridge. BRIDGE TEST MATERIAL The bridge, built in 1887, had been a swinging drawbridge. One hundred and ninety-seven untreated piles driven in concentric circles, and supporting the swinging portion of the bridge, were the source of material for this study. The piles, averaging 20 feet in length and 10 to 12 inches in diameter, were driven to 8 feet below the river bottom, and stood about 7 feet below the water s surface. The river bottom had apparently been dredged and then backfilled with riprap after the piles were driven. Fender piles upstream protected the foundation piles from physical damage. Although the fender piles were periodically inspected and replaced as necessary, none of the foundation piles had ever been replaced. The sample material for this study consisted of twenty-one 2-foot sections taken from 12 different piles. Piles had been obtained with the belief that they were all tamarack, as called for by the original bridge specifications, but examination of the sections indicated a mixture of red pine, white pine (presumed eastern), and tamarack. 1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 2 Scheffer, Theo. C., Duncan, C. G., and Wilkinson, Thomas. Condition of Pine Piling Submerged 62 Years in River Water. Wood Preserv. 47(1):

4 The location of the piles in the foundation layout is unknown because the piles had been removed before we learned of the salvage operation. The 2-foot sections came from three different vertical locations in the piles: Nine came from near the bottom ends of the piles, well below mudline; eight were obtained at the mudline (the river bottom); and four came from near the tops of the piles. The number of piles and the number of sections by species and location in the pile are described in table 1. Because of damage to some of the piles during the salvage operation, it was not possible to get a section from each of the three vertical positions in each pile. A 2-foot section was obtained from each vertical position in two red pine piles, in one white pine, and in one tamarack pile. The pile sections were stored frozen in plastic bags before processing into static bending specimens. Three 1- by 1- by 16-inch specimens were prepared from each section-one from sapwood, one from outer heartwood, and one from inner heartwood (as near the pith as possible while still retaining reasonably flat grain orientation). The specimens were taken along a radius where the sapwood ring was broadest, and tested in the saturated moisture condition according to standard procedures for testing small, clear specimens of timber. 3 RESIDUAL PROPERTIES OF PILES The results of tests are summarized in table 2. The original properties of the pile material are, of course, not known. However, published average values 4 of small, clear specimens of the species involved provide a reasonably good basis for comparison and are included in the table. The average modulus of rupture of the red pine test material was 32 percent lower than the published average. Modulus of elasticity was 27 percent lower and specific gravity 12 percent lower. White pine specimens proved to be 1 percent lower in modulus of rupture, 10 percent higher in modulus of elasticity, and 3 percent higher in specific gravity. Tamarack test values were 8 percent, 2 percent, and 6 percent lower than published values in modulus of rupture, modulus of elasticity, and specific gravity, respectively. Because no meaningful statistical analysis can be performed, it would be difficult to es- tablish whether these differences are significant. Results are less conclusive than they would have been had the piles all been tamarack as believed when the sample material was collected. This is particularly true for white pine and tamarack as material from only three white pine piles and one tamarack pile was tested. Each average test result was within the normal variability range for tests of small clear specimens. The average coefficient of variation for the modulus of rupture of small clear specimens is 16 percent; for modulus of elasticity, 22 percent; and for specific gravity, 10 percent. 4 However, the large difference between the average test results for red pine suggests that a significant reduction in properties occurred. It is very improbable that average test results from eight piles (actually eight different trees) would exhibit deviations from average properties of the species of the magnitude observed unless a degradation in properties had taken place. The results for white pine and tamarack are not as conclusive because the differences between the test results and published averages are small and because few piles of these species were tested. However, there is some evidence that degradation has occurred. Modulus of elasticity is commonly not affected to the same degree as most strength properties when wood is deteriorated by heat, chemicals, decay, or other agencies. If some degradation has taken place, the modulus of elasticity of the test material should compare more favorably than modulus of rupture to published average species property values. Table 2 indicates that is true for all three species. Table 3 shows average results for three radial positions in the piles; inner heart, outer heart, and sapwood. With one exception (modulus of elasticity of tamarack), outer heart material is stronger and stiffer than either inner heart or sapwood. This is perhaps due, at least in part, to growth conditions of the trees from which the piles were cut. Of greater importance, sapwood modulus of rupture values are consistently lower when compared to heartwood values than are modulus of elasticity values. This suggests that some degradation has occurred in the bending strength of sapwood of all three species. 3 American Society for Testing and Materials. Standard Methods for Testing Small Clear Specimens of Timber. ASTM D American Society for Testing and Materials. Standard Methods for Establishing Clear Wood Strength Values. ASTM D

5 Although average differences in properties between heartwood and sapwood are generally small, individual differences in some instances are unusually large, particularly in red pine. In two pile sections of this species, the modulus of rupture of sapwood specimens was about one-half or less that of either the corresponding inner or outer heartwood specimens. Apparently, heartwood offered some natural resistance to degradation in some piles. Table 4 summarizes the results by vertical position in the piles. The table includes only data from piles for which material from each of three vertical positions was tested: Two red pine piles and one each of white pine and tamarack. The data offer no evidence that conditions for deterioration were more severe at one vertical location than another. Were there a difference in severity of exposure, its effect should be most pronounced in red pine, yet the mechanical properties for this species are essentially the same at the bottom and top of the piles and at the mudline. In summary, a significant reduction in bending strength, stiffness, and specific gravity apparently occurred in untreated red pine piles during 85 years exposure in the Milwaukee River. No definite conclusions are drawn for white pine and tamarack as differences measured were generally small and few piles were tested for these species. Changes measured in modulus of elasticity were substantially less than those in modulus of rupture and on the average, sapwood bending strength was slightly lower than heartwood. In a few instances, however, the bending strength of the sapwood specimen was only about one-half that of the heartwood specimens from the same pile section. Although lumber cut from the piles would be suitable for many applications, lumber from the red pine piles could not be recommended for structural use, using published allowable design stresses for the species. White pine and tamarack test material compared favorably to published values but too few piles were tested to make general recommendations. A more extensive study is still underway at the Laboratory to study the extent of degradation of the compression strength of the piles. That study will also attempt to identify agencies of degradation (bacteria, fungi, or chemicals) and to establish their relative importance at various locations in the pile. Table 1.--Numbers of piles and 2-foot sections, by species and vertical position in piles 3

6 Table 2.--Average specific gravity, bending strength, and stiffness of pile specimens compared to published average values for the same species 1 Based on ovendry weight and green volume. 2 ASTM D 2555 (see text footnote 4). Table 3.--Summary of test results by radial position 1 Based on ovendry weight and green volume. 4

7 1 Table 4.--Summary of,tests by vertical position 1 Includes only piles for which sections from each vertical position were tested. 2 Based on ovendry weight and green volume. U.S. GOVERNMENT PRINTING OFFICE /

SOME STRENGTH AND RELATED PROPERTIES OF YAGRUMO HEMBRA

U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON, WIS. U.S. FOREST SERVICE RESEARCH NOTE JUNE 1964 SOME STRENGTH AND RELATED PROPERTIES OF YAGRUMO HEMBRA ( CECROPIA PELTATA