Havens, Jas H.

Rahal, Assaf

English

University of Kentucky

EVALUATION OF TIMBER CAISSON US 2.5 BRIDGE OVER OHIO RIVER Jas. H. Havens, Director of Research and Assaf Rahal, Research Engineer Kentucky Department of Highways INTRODUCTION Timber foundations under masonry structures have been employed tluoughout recorded history. Wood buried below the oxygen-diffusion zone is preserved tluough oxygen starvation of fungi and bacteria. Decay may proceed for a short time. Anaerobic bacteria have an implied capability of regenerating and utilizing oxygen from host organic matter; however, the digestive cycle is likely to become unbalanced, poisoned , or at least arrested. Obviously , buried woods do not persist forever. Some commentaries indicate that wood pilings suffice for 80 to JOO years; others suggest hundreds of years. However, the tenure of structures in tl1is country rarely exceeds a hundred or more years. American engineers do not usua lly build to withstand the ages. Obsolescence limits tenure. European engineers strive for greater tenure . Pier No. 2 of the former C&O Bridge at Covington is off-shore from the Kentucky side of the Ohio River. It was built in 1887. ln 1927, this pier was extended downstream to support a new railroad bridge. The other three piers remained independent. The original structure was then converted to highway use and was purchased by the Commonwealth of Kentucky in 1937. ln 1968, an engineering analysis of the superstructure indicated critical deficiencies in terms of "safety factors" , and the bridge was closed to all traffic. Subsequently, various plans for reconstruction came under consideration. Of grea test significance he,e. is the consideration toward re-use I I ,;(9'®1.:>.0l\'v j-OtL. /\~\111,( vttt11tcx,t~ ;)'3,t,'~e;. PLATE XIII. ~ :: tdx&&fufi TRANS. AM .SOC .CI V. ENG.RS VOLXXIII N ~ 446 BURR ON C I NCINNATI BRIDGE . ,_ --~ I 11111111 II of Pier No. 2 -- jointly with a new highway bridge and the existing railroad bridge. Cost estimates appeared persuasive; the structural feasibility remained dependent upon the integrity of the pier -- more specifical ly, the worthiness of the masonry, concrete, and the underlying timber caisson. Prior to removal of the steel superstructure (fall of 1970), vertical cores were extracted from Pier No. 2 for evaluation. This report concerns the evaluation of specimens of wood from the timber caisson. Consideration of use of Pier No. 2 in the new structure afforded a rare opportunity for historical but purposeful inspection of the existing condition of the wood. Discovery of decay or rot wou ld probably have led to immediate rejection of the alternative. Upon recovery, the cores appeared frayed and severely damaged. Later, when sawed along their diameters, bright wood appeared. A slightly acrid odor was detectable; but there was also a distinctive scent of newly-cul wood. Specimens of pine were distinctly odorous. A specimen of poplar was recovered but was examined only by the Forest Products Laboratory. In the main, the cores consisted of while oak. Not all of the specimens were bright (yellowish); some were dark ·· approaching gun-metal blue or dark gray. Miniature specimens were cut from the cores and subjected to compressive loading. Stress-strain curves are presented here. Comparative stress-st rain curves obtained from new white oak wood are al so provided. The subst ructure construction was described by Wm. H. Burr (I). Plate XIII , therein, is most pertinent and is reproduced herein as Figure I. The I ~ -- +J - , __ 1111111 1 11 1 Figure 1. Caisson for Cincinnati Bridge (] ). 15 Figure 3. r > 15r 15 4 (' NEW Figure 2. Composite Photo of Cores Received for Evaluation. Recovery of wrought iron pin in NW core was incidental. Specimen of Bright White Oak in Compression. Note weeping from pores near base of specimen. 16 Figure 4. Dark White Oak Specimen Showing Residual Distortion Following Severe Compression. main piers of the Roebling suspension bridge immed iately upstream from the C&O site, was begun in 1857; the piers are founded on hewed log mats set on gravel 10-12 feet above bedrock. The bridge was opened to traffic January I , 1867 (2). Fig,;re 2 is a composite photograph of the three cores received for eva luation. Attention is directed to the incidental recovery of a wrought iron pin in the upper portion of the NW core. COMPRESSION TESTS Small cubical spec imens, ranging from 0.7 inch to 1.2 inches in size, were cut from the cylindrical cores. Except for slight surface drying during fine shaping, the specimens were maintained in a wet cond ition until the compression tests were completed. A specimen of bright wood in test is shown in Figure 3. AJI compression tests were made perpendicular to the grain. There was noticeable weeping of water from the pores. Figure 4 shows a specimen of dark wood distorted (residual) by compression. None of the specimens ruptured, split, or tore ; all specimens, including new wood, exJ1 ibited a yield point followed by strain hardening. The resu lting stress-strain relationsMps are shown in Figure 5. The minimum yield point occurred at about 350 psi; the yield stress of new wood was in the order of 2 to 2.5 times that of old wood . The handbook yield point for white oak, at 70 percent moisture (green and unseasoned), is 850 psi. New wood specimens were soaked in water for five days before testing. Differences (between new and old wood) in the number of annual rings per inch are also shown in Figure S. Because of these differences, the woods are not directly comparab le in terms of strength. The new wood specimens contain a significantly greater proportion of late wood growth and are thereby adjudged to be superio r in strength. Due to an assumed improbab ility of finding new wood comparab le in anatomical attributes to the old wood, it was decided to air dry specimens of old wood and to make strength tests in that condition and lo compare those strengths with handtook values for new wood. The average handbook value for white oak in this condition is 1410 psi. The strengths of the two specimens selected (one bright wood and one dark wood) are shown in Figure 6. By this comparison, the old wood would necessarily be adjudged equal to average new wood. This interpretation minimizes any effects otherwise attributable to decay. It does not suffice to explain the low, wet strength of the old wood .(3). It is suggested that moisture contents approach ing a state of supersaturation --possibly somewhat greater than that of green wood and of osmotic origin -- affects the wet strength. The effect of swe lling, attending unloading (coring), on moisture content was not determined. However, a small but significan t swe ll pressure was measured upon rein1mersion (discussed subsequently). SWELL AND RELAXATION TESTS (TRIAXIAL) A cylindrica l specimen of dark wood, 2.72 inches x 1.30 inches, was placed in a triaxia l test chamber and surrounded by water under 20 psi pressure; the specimen was restrained in the axial direction by a nominal preload of I pound and a load cell. After 24 hours, the swell pressure was 6:1 pounds. The load was increased to SO pounds; and under constant strain, the specime n was allowed to relax for I to 6 hours; the residual load then was recorded. Then the load was removed and the residual strain recorded (usua lly 20 to 30 minutes after unloading) . This procedure was repeated with five add itional , SO-pound increments of loads; the maximum load was 300 pounds. The resulting stra in-hysteresis data were plotted as shown in Figure 7. The relaxation loads after a time ( I to 6 hours) are shown plotted against applied load, in Figure 8. There, the deviations from the line of equality indicate the relative creep-relaxation of load with respect to the applied load and time. From these data, the relaxation modulus, G, was calcu lated from the equation : G P/3Ae 1, in which : p Applied Load A Area of Spec imen (after st ra ining) e 1 Strain (from original height of specimen) The respective moduli for o ld and new wood are: 17 1~00 1400 1300 1200 1100 IOOO 900 . 800 ,:. u c 700 ~ § ~ 600 "' "' w a: I-"' ~ z ::, 100 Figure 5. 1800 1600 1400 .~ " 1200 ~ .,; ~ en en 1000 w Q'. I-"' f- 800 z ::, 600 F igure 6. 9 rings per in. 9 roys per in. 9 rings per in . 23 rings 9 rays 2 2 rings per 8 rays pe r 0 .1 0.2 0 .3 UNIT ST RAIN ( inch I inch ) per per in . in . 20 rings per in. 8 rays per in . in . in , 0.4 Stress-Strain Relationships Obtained from Old Wood and New Wood (Saturated). NW Dork wood --- - - - ~:l.0-152.5 --\ /\~ /\ I Light wood 2 3 4 5 6 7 STRAIN (percent) Stress-Strain Relationship Obtained from Air-Dry Specimens of Old Wood. 240 220 200 180 160 140 "'c ~ 120 v, 100 v, LIJ a: I-<J) 80 60 40 20 0 0 220 200 180 .. 160 c: "":' 1/) 140 ~ Cl) Cl) 120 w a:: I-Cl) 100 8 0 60 40 20 Figure 7. O I_ D WHITE OAK .3 . 4 . 5 .6 STRAIN(%) POINTS ( RES·fOUAL STRAIN) 0 . 1 NEW WHITE OAK 0 -2 0.3 STRAIN(%) Strain-Hysteresis Tests: Old and New Wood. Arrows show cycling of loads; points show total strains with respect to original height" of specimen. Residual strain points show the unrecovered strains from each load-rebound cycle. 18 LOADED POINTS .7 . 8 II 0 .4 0 -5 LOAD (lbs) 50 100 150 200 250 300 58 100 150 200 250 300 MODULUS (psi) Old White Oak (From Cores) 2,398 2,856 3,834 4,668 5,596 6,348 New White Oak 7,145 7,903 9,371 10,501 11,572 13,096 TIME (min) 60 164 110 75 360 70 62 60 60 61 68 65 Finally, after a period of rest, the specimen was loaded at a constant rate of strain (I/1000 inch per minute) while monitoring the load. The resulting stress-strain graph is shown in Figure 9 . The effects of the previous strain history of the specimen are evident in the lower portion of the curve; some collapse of internal fibers undoubtedly occurred during previous loadings ; thereafter, near-linearity is resumed. Also shown there is the stress-strain curve obtained from successive, quick applications of 50 pound increments of load (from Figure 7). Comparative compression graphs obtained from similar tests on a specimen of new white oak are provided for each of the test situations. Significant differences between U1e old and new wood are: 320 OLD WOOD 300 70 min . 280 260 24 220 ~ .0 200 ,_ 180 z w :. w a:: 160 u ::: 110 min. 140 0 " 0 .J 120 100 164 min . 80 Line of Equality ( Hookeon Elastlclty) 60 60mln . 40 20 0 0 20 40 60 80 100 120 140 160 180 200 220 RELAXED LOAD lbs . ) 1. The old wood was kept wet except for sl ight drying while shaping the specimens; the new wood specimens were shaped and then soaked for five days. 2. The specimen of old wood contained 19 rings and 13 rays per inch; the new wood specimen contained 9 rings and 9 rays per inch. ANATOMY OF WOOD The inner structure of white oak wood is similar to that of red oak, but white oak has perceptibly more abundant tyloses filling large spring-growth tracheid. Figure 10 is a cross-sectional view of new wood (magnified 13.5 times). The parts are labeled. It is reportedly possible to blow air through the pores of a short length of red oak whereas the pores in white oak are plugged with tyloses. Tyloses are an intrusive growth of parenchyma cells into tracheid cells after sap flow subsides . Tracheids comprise the principal vertical (axial) piping system; they feed smaller, horizontal (radial) tracheids. These occur principally in the rays. The rays are discontinuous in the vertical direction and are an inch or more in height in white oak (4). The porous springtime growth and the rays weaken the wood structurally. Strength is somewhat proportional to the amount of dense-wood (late growth) between the porous rings. Generally, the wider the rings the stronger the wood (hardwoods only). In the dense-wood areas, the cells are smaller and have much thicker walls and are richer in intercellular resins (glues). Figure 11 is a companion to Figure IO and shows a side view of the same specimen. Figures 12 and 13 illustrate dark wood core specimens. Figure 14 is a bright wood from Pier No. 2 and is comparative to Figure 13. Figure 15 illustrates the pine in cross-section. 320 280 240 ~ 200 ,_ z w :. ~ 160 u ~ 0 " g 120 80 40 0 40 NEW l'ltlITE OAK 80 61mln. Line of Equality {Hookeon Elasticity) 120 160 RELAXED LOAD (1bs . ) 65mln. 200 240 Figure 8. Relaxation or Subsidence of Load Due to Creep. 19 260 240 220 200 180 QUICK STRAIN, / LOAD APPLIED i INCREMENTALLY,/ ( FIG. 6) / ;__. I ti f I I I 240 OLD 220 200 180 160 14 WO OD STRESS v, STRAIN 'i' QUICK STRAIN, I LOAD APPLIED I INCREMENTALLY, ~ ( Fl G 6) v, I I f' I IN PER MIN. , CONT INU OUS 1000 en 140 "' I / ,r,J"i"'o"to 1N PER MIN. , CONTINUOUS I w "' I t; 120 I / - " . t, I 100 BO 60 40 ,, .. " / / / L / / ," / Figure 9. DISCUSSION ~I I I I "' 80 "' w "' ,_ "' 60 40 2 ~ .. .5 .6 .7 STRAIN (%) Stress-Strain Curves Obtained in Triaxial Test Apparatus (20 psi Lateral Pressure) Following Creep-Relaxation Tests. .8 .9 1.0 I. I 1.2 1.3 1.4 1.5 1.6 The wood specimens tested were necessarily selected from portions within the cores wh ich were recovered intact - that is, showing the least internal dama ge (fray ing, etc.) from the cutting bit. It should be recognized that there was not complete recovery; tl1e possibility remains that the recovered portion of the cores, and thereby the specimens tested , represent only the best wood . This situation seems unrecognizable unl ess, through insight or conjecture , the imperfect recovery is attributed altoge th er to the coring equipment. The only specimen of pine availab le in cores as received (specimens submitted to Forest Products Laboratory were selected fro m cores beforehand} contained a large knot and was not suitable for physica l tests. No specimens of yellow poplar were ava ilab le. The tests were, therefo re, limited to the white oak wood. The simple compression tests and the relaxation modu li indicated an apparent loss of strength in comparison lo new wood . If it were assumed that the old wood was originally as strong as the new wood , the differences in strength might be attr ibuted directly to age , deterioration, decay, etc. However, there are significant reasons, based on anatomical or structural Late< Wood .... Ty loses (White Plugging Material) Figure IO. Radial Rays (9 per inch) Cross-Sectional View of New White Oak Magnified 13.SX. I. Annual rings are large vertical pores (tracheid} produced by early spring growth; as growing season progresses, additiona l pores form but become successfully smaller and farther apart. The late wood growth is more dense and is stronger. 2. Radial rays are horizontal cells and lateral conductors of sap; they act as a lateral (radial) piping system. Rays are discontinuous in the vertical direction; each bundle is about one inch in height. 20 Annual Ring (9 per inch) ~ Tracheid (Discon-tinuous)-.............. Ray -Figure 11. Figure 12. Figure 13. Axial Direction ---------Side View of New White Oak, Magnified 13.SX. Cross Sectional View of Dark Wood from Pier No. 2, Magnified 17.SX. Note greater abundance of large pores; specimen contains 17.5 rings per inch and 10 rays per inch. Side View of Dark Wood from Pier No. 2, Magnified 17.SX. Greater abundance of large pores are evident here also. 21 Ray (Discon-.....____ tinuous) ~ Tracheids (Pores) ....._ Ray Tracheid (filled) '1 comparisons, to suspect that the new wood is superior to the original quality of the old wood and that interpretation of strength differences as a loss in strength of the old wood is not altogether justifiable. The old wood contains about twice as many porous rings per lineal inch as the new wood. Strength varies in some inverse proportion to the number of rings per inch -· probably more d iscretely with the percentage of the area occupied by large pores. Visual comparison of Figure l O with Figure 12 suffices to show that the new wood and old wood are qot identical in these dimensional attributes. A cursory ratio of 2: 1 would, indeed, minimize the strength loss attributable to deterioration of the old wood - that is, if wet-strength loss is used as an estimate or measure of deterioration. Air-dry strengths furt her minimize the extent of deterioration. NOTE J. In weighing these observations, attention should be directed also to Forest Products Laboratory 's report and the discussions therein concerning losses in acety l content and the implied relationship between these chemical changes and strength (5) (6). On the basis of these observations, the loss in wet strength with time might be in the order of 25 percent -- part of which may be accountable Early Wood ---in terms of saturation (cf. acetyl loss, FPL repo rts) and an undefined portion to bacterial decay (7 ). NOTE 2: The design bearing pressure was less than JOO psi (cf. 1). NOTE 3: 17iere seems to be a noticeable degree of uncertainty impl(ed in the FPL reports in regard to strength loss. "Inadvisable" was the word used in the FPL report of September 1970 to summarize all uncertainties _bearing on re-use of the pier. In the earlier report, the term "not be depended on" was used. These sam~ uncertainties appear in the FPL evaluation of pine piles under the 14th Street bridge in Washington, D. C. The judgements rendered there were doubtlessly precedential. The source of the white oak timbers is now unknown. Obviously, the new wood grew in an environment distinctly different from that where the old wood grew. It would be interesting to know if the old wood came from virgin forests in more northern climates or if the new wood specimen is merely typical of second-growth timbers. ---Annual Ring (Late Wood) Figure 14. Side View of Bright Wood from Pier No. 2, Magnified 17 .5X. Figure 15. Axial Direction Cross-Sectional View of Pine from Pier No. 2, Magnified 17 .SX. In contrast to hardwoods, strength of conifer woods increases as the number of rings per inch increases. 22 ----Ray ? Tracheid (with Ty loses) --Ray REFERENCES 1. The River Spans of the Cincinnati and Covington Elevated Railway, Transfer and Bridge Company, Wm. H. Burr; Transactions, ASCE, Vol. XXIII, August 1890. 2. Report of John A. Rockling, Civil Engineer, to the President and Board of Directors of the Covington and Cincinnati Bridge Company; April 1, 1867. 3. The Superswol/en State of Wood, Harold Tarkow and Wm. C. Feist ; The Journal of the Technical Association of the Pulp and Paper Industry; Vol. 51, No. 2, February 1968. 4. Plant Anatomy, Katherine Esau; John Wiley and Sons, 1965. 23 5. Condition of the Pier-Supporting Wood Grillage of the U. S. 25 Highway-Railroad Bridge over the Ohio River between Covington and Cincinnati, Report to Department of Highways, by Joe W. Clark, Pathologist, Forest Products Laboratory, Forest Service, U. S. Department of Agriculture, at the University of Wisconsin, Madison; May 1970, and supplement, September 19, 1970. 6. Condition of Pine Piling Submerged 62 years in River Water ( 14th Street Bridge Over Potomac River, Washington, D. C.), Theo. C. Scheffer, C. G. Duncan, and Thos. Wilkinson; Wood Preserving, January 1969. 7. Evaluation of Cored Specimens from Timber Caisson beneath Pier No. 2 of the U. S. 25 Bridge over the Ohio River between Covington and Cincinnati, F 141 (1), Research Report 301; Jas. H. Havens and Assaf Rahal ; Depa1tment of Highways, December 1970. 

Evaluation of Timber Caisson US 25 Bridge Over Ohio River

https://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1170&amp;context=ktc_proceedings

Evaluation of Timber Caisson US 25 Bridge Over Ohio River

Abstract

Similar works

Full text

Available Versions

University of Kentucky