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tte a ee

PUBLIC ROADS

Pa Su

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UNITED STATES DEPARTMENT OF AGRICULTURE

BUREAU OF PUBLIC ROADS

Vib See Nels We) Vv DECEMBER, 1928

1 eet

THE YADKIN RIVER BRIDGE

U.S. GOVERNMENT PRINTING OFFICE: 1928

epovbiks

iad _—> ne

ROADS

A JOURNAL OF HIGHWAY RESEARCH

U. 8S. DEPARTMENT OF AGRICULTURE BUREAU OF PUBLIC ROADS

CERTIFICATE: By direction of the Secretary of Agriculture, the matter contained herein is published as administrative information and is required for the proper transaction of the public business

obtained.

The reports of research published in this magazine are necessarily qualified by the conditions of the tests from which the data are Whenever it is deemed possible to do so, generalizations are drawn from the results of the tests; and, unless this is done

the conclusions formulated must be considered as specifically pertinent only to the,described conditions

VOL. 9, NO. 10

TABLE OF

Loading Tests on a Reinforced Concrete Arch

THE U. S. BUREAU

DECEMBER, 1928

R. E. ROYALL, Editor

CONTENTS

Page

185

OF PUBLIC ROADS

Willard Building, Washington, D. C.

REGIONAL HEADQUARTERS Mark Sheldon Building, San Francisco, Calif.

DISTRICT

DISTRICT No. I, Oregon, Washington, and Montana. Box 3900, Portland, Oreg.

DISTRICT No. 2, California, Arizona, and Nevada. Mark Sheldon Building, San Francisco, Calif.

DISTRICT No. 3, Colorado, New Mexico, and Wyoming. 301 Customhouse Building, Denver, Colo. DISTRICT No.4, Minnesota, North Dakota, South Dakota, and Wasconsmn, 410 Hamm Building, St. Paul, Minn. DISTRICT No. 5, Iowa, Kansas, Missouri, and Nebraska. 8th Floor, Saunders-Kennedy Bldg., Omaha, Nebr.

DISTRICT No. 6, Arkansas, Oklahoma, and Texas.

1912 Fort Worth National Bank Building, Fort Worth, Tex.

OFFICES

DISTRICT No. 7, Illinois, Indiana, Kentucky, and Michigan. South Chicago Post Office Building, Chicago, III. DISTRICT No. 8, Louisiana, Alabama, Georgia, Florida, Mississippi,

South Carolina, and Tennessee. Box J, Montgomery, Ala. DISTRICT No. 9, Connecticut, Maine, Massachusetts, New Hamp- shire, New Jersey, New York, Rhode Island, and Vermont. Federal Building, Troy, N. Y. DISTRICT No. 10, Delaware, Maryland, North Carolina, Ohio, Penn- sylvania, Virginia, and West Virginia. _ Willard Building, Washington, D. C. DISTRICT No. 11, Alaska. . Goldstein Building, Juneau, Alaska. DISTRICT No. 12, Idaho and Utah. Fred J. Kiesel Building, Ogden, Utah.

Owing to the necessarily limited edition of this publication it will be impossible to distribute it free to any persons or institutions other than State and county officials actually engaged in planning or constructing public highways, instructors

in highway engineering, periodicals upon an exchange basis, and Members of both Houses of Congress. At the present

time names can be added to the free list only as vacancies occur.

Others desiring to obtain Pustic Roaps can do so

by sending 10 cents for a single number or $1 per year to the Superintendent of Documents, U.S. Government Printing

Office, Washington, D. C.

' :

LOADING TESTS ON A REINFORCED CONCRETE ARCH

REPORT ON TESTS MADE ON YADKIN RIVER BRIDGE IN NORTH CAROLINA

Reported by ALBIN L. GEMENY, Senior Structural Engineer, Bureau of Public Roads, and W. F. HUNTER, Designing Bridge Engineer, North Carolina Highway Commission

tests it 1s desired to discuss briefly some of the assumptions which are made in arch-bridge design. The hingeless, reinforced-concrete arch rib is a statically indeterminate structure which can be ana-

one describing the details of these particular

by expansion joints at one or more points. It may be rigidly attached to the tops of the spandrel columns or it may have movable bearings on the columns. In prac- tically all cases the columns are integral with the rib.

In designing, the effect of the superstructure on the deformation of the rib is generally neglected even

lyzed only by considera- tion of the elastic prop- erties of the concrete and steel of which it is constructed. In apply- ing the theory of elastic structures to the anal- ysis of an arch rib, it is assumed that the modulus of elasticity of the concrete is con- stant for all parts of the arch and at all intensities of stress up to the working stress for which the arch is designed. It is further assumed that a plane section of the rib re- mains plane after the rib has been deformed.

In open-spandrel arch construction, the floor system is sup- ported by the spandrel columns through which the loads are transmit- ted to the ribs. In cur- rent design practice it is assumed that the loads are distributed only to adjacent panel points and are applied as vertical forces at the points of the rib at which the columns are attached, although it is apparent that a con-

STATEMENT BY THE ADVISORY COMMITTEE!

HE NORTH CAROLINA STATE HIGHWAY COMMIS- SION built in 1922, asa Federal-aid project, a 3-span concrete arch bridge over the Yadkin River, also known as the Pee

Dee River, between Albemarle and Mount Gilead. In 1926 the Carolina Power & Light Co. began the construction of a dam on a site about 9 miles downstream from the bridge. The water of the river, upon the closing of the dam, was to be backed up to such a height as to submerge the bridge and necessitate its replacement by a new bridge at a higher elevation. Between the time of completion of the new bridge and the closing of the dam, a period of several months, the old bridge was to be demolished so as to offer no ob- struction to the flow of water in the river.

These circumstances presented a unique opportunity to test a modern, full-size, reinforced concrete arch bridge with moderately long spans. In recent years the popularity of the arch bridge has increased greatly because of its superior esthetic value and, in this country, millions of dollars are spent annually on this type of bridge alone. Consequently, there is a widespread tendency on the part of bridge engineers to embrace any idea which may lead to more economical or more satisfactory arch design without sacrificing safety. In departing from current practice, the judgment of the engineer is based more and more on data developed by the various research agencies of the world.

The North Carolina State Highway Commission, recognizing the opportunity to make this test, and desiring to make it as complete as the available time and money permitted, requested the coopera- tion of the Bureau of Public Roads.

The bureau acceded to the State’s request, and the two agencies then jointly issued to various technical and scientific societies and colleges invitations to participate in the experiment by appointing one or more of their members to serve on an advisory committee. ! The purpose of this committee was to formulate general plans for the test, and, by meeting from time to time, assist those in active charge in the solution of problems which would certainly be en- countered during the period of the test, and assist in interpreting the results. The advisory committee first met in April, 1927, and formulated general plans; several meetings were held during the course of the test, and at a final meeting on November 8, 1928, this report was approved by the committee. Acknowledgments by the committee are given below. ?

though it isobvious that this effect may be of con- siderable importance. The degree to which the rib deformation is mod- ified by the superstruc- ture depends upon the number of breaks in the continuity of the floor system, the method of connecting it to the col- umns, and upon the stiffness of the columns. In the case of an arch with the floor system continuous over the whole span and rigidly attached to the tops of the columns, we have, in fact, a fixed, spandrel braced arch in which the diagonals are omit- ted and their functions performed by the rigid joints at the ends of the columns. In the case of an arch with expan- sion joints at each panel point and with the floor system supported onex- pansion bearings, the condition would ap- proach those assumed in designing. Usually the conditions lie some- where between these two extreme _ cases. The free rib is three

tinuous floor system distributes the loads to panels be- yond those in which they are applied, thus rendering indeterminate the distribution of the load to the rib. The floor system may be continuous or it may be broken

1 Membership of the advisory committee formed as a result of invitations issued by the Bureau of Public Roads and the North Carolina Highway Commission was as follows: University of North Carolina represented by Dean G. M. Braune; North Carolina State College represented by Dean W. C. Riddick; American Association of State Highway Officials represented by Searcy B. Slack, bridge engineer of the Georgia State Highway Board; American Society of Civil Engineers represented by Prof. Clyde T. Morris of Ohio State University; American Railway Engineering Association represented by J. B. Hunley, engineer of structures of Cleveland, Cin- cinnati, Chicago & St. Louis Ry. Co.; American Concrete Institute represented by A. B. Cohen, consulting engineer, New York, N. Y.; Highway Research Board rep- resented by A. T. Goldbeck, director of the bureau of engineering, National Crushed Stone Association; U. S. Bureau of Standards represented by D. E. Parsons, associate engineer; American Society for Testing Materials represented by F. E. Schmitt, editor, Engineering News-Record; U. 8S. Bureau of Public Roads repre- sented by E. F. Kelley (chairman), chief, division of tests; O. L. Grover, principal bridge engineer; H. M. Westergaard, professor of theoretical and applied mechanics, University of Illinois; and L. W. Teller, senior engineer of tests; North Carolina State Highway Commission represented by L. R. Ames, State highway engineer; Wm. L. Craven, bridge engineer; M. M. Trumbull, assistant bridge engineer; and E. H. Kivett, engineer of tests.

24135—28——1

times indeterminate and the complete arch, in the present case, thirty-nine times indeterminate.

2 The instruments and scientific apparatus used in this test were furnished by the following organizations: The American Society of Civil Engineers and the Bureau of Standards furnished the electric telemeters. The Bureau of Standards furnished the Berry strain gauges and temperature coils. The Bureau of Public Roads fur- nished the radiusmeter, weighing cells, thermometers and deflection wires. The committee on concrete and reinforced concrete of the American Society of Civil Engineers furnished the clinometers.

The installation of instruments and making of field observations were under the direction of G. W. Davis of the Bureau of Public Roads, assisted by W. F. Hunter and W. M. Price of the North Carolina Highway Commission and Albin L. Gemeny and E. C. Sutherland of the Bureau of Public Roads. The electric telemeters were calibrated and installed by O. 8. Peters of the Bureau of Standards. All computations were made by W. F. Hunter and Albin L. Gemeny. The preliminary model analysis was made by D. H. Overman of the Ohio State Highway Department under the direc- tion of Prof. Clyde T. Morris of Ohio State University. The brass wire model analysis was made by G. W. Davisand E. C. Sutherland. The final model analysis was made by Prof. J.T. Thompson of Johns Hopkins University and Albin L. Gemeny, using a model constructed by the Bureau of Public Roads. The bridge maintenance depart- ment of the North Carolina Highway Commission made available one of its forces during the entire period of the test to do all construction work and operate the ferry. The foreman of this force was J. P. Beach under the general direction of C. B. Taylor. Success in the prosecution of the test was due in large measure to the enthusiastic cooperation of the North Carolina Highway Commission through Messrs. Craven, Trumbull, and Hunter of the bridge department.

185

PUBLIC ROADS

Vol.9 No 10

pe 125

A method of analyzing indeterminate structures, developed comparatively recently by Professor Beggs, of Princeton University, consists in studying the elastic action of a model, each member of which is of the same relative stiffness as the corresponding member of the structure. By the application of the Maxwell theorem of reciprocal deflections, and Miller Breslau’s principle that any influence line is a deflection diagram, the moment, thrust, and shear at any section may be found and the stresses computed.

OBJECTS OF THE TESTS OUTLINED

The North Carolina bridge tests were conducted for the following specific purposes:

(1) To compare the measured deformations of a full-size, reinforced concrete arch rib with the deformations as determined by the theory of elastic structures, when the rib carried loads producing stresses of moderate intensities, and was as free as practicable from the restraining action of the superstructure.

(2) To make the same comparisons when the rib carried loads which produced stresses of high intensities.

(3) To determine the effect of the superstructure on rib de- formations by comparing deformations measured when the superstructure was intact and the measured and computed deformations of the rib free from restraint by the superstructure.’

3 Further references will be made simply to the “free rib.”

“ RIB REINFORCEMENT 6-% ROUND BARS TOP AND BOTTOM

EXPANSION JOINT

CENTER LINE

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BEARING PLATES

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PLAN Fic. 1.—Detaits or Test SPAN

(4) ‘To compare the measured deformations of the rib, both with and without the restraining action of the superstructure, and the deformations as determined from an analysis made by the use of an elastic model.

TEST BRIDGE DESCRIBED

The test bridge consisted of three 2-rib open-spandrel arch spans of 146 feet 3 inches clear span and 28 feet 3 inches rise with seven 42-foot 6-inch deck-girder ap- proach spans at each end. The floor system of the arches rested on sliding bearing plates at each panel point. These plates were found to be badly corroded and probably had ceased to function freely as sliding bearings. The intermediate arch piers below the springing line were of hollow construction, the hollow space being filled with field stones. The end arch piers had buttresses on the shore side to increase their resist- ance to overturning under the unbalanced thrust. The piers were founded on solid rock. Details of the test span are shown in Figure 1.

The bridge was built by contract, using cement and reinforcing steel furnished by the State highway commission. ‘The coarse aggregate consisted of crushed field stones found in the vicinity of the bridge site. Inspection of the aggregate in cores taken from the

December, 1928 PUBLIC

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bridge disclosed several varieties of stone, all of which were apparently hard, sound, and durable. The fine aggregate consisted of a mixture of 5 parts of sand to 1 of stone screenings. <A well-known brand of Portland cement was used.

The concrete for the piers below the springing line was mixed in the proportions 1:2'4:5 and, for the remainder of the bridge, in the proportions 1:2:4. Tests of 6 by 12 inch cylinders of the 1:2:4 concrete made at 28 days showed strengths of 2,140, 1,900, 1,655, and 1,258 pounds per square inch. Each of these strengths is for a single cylinder representing about 50 cubic yards of rib concrete and are arranged in the order of location of the batch from springing line to crown. Inspection records do not show clearly which of the arch spans is represented by these cylinders.

The reinforcing steel consisted principally of round, deformed bars of intermediate grade steel. Some of the minor reinforcing consisted of square, deformed bars. Tension tests on the steel showed an average yield point of about 48,000 pounds per square inch and an average ultimate strength of 75,300 pounds per square inch.

Cuts made in the concrete for installing instruments, taking test specimens and destroying the continuity of the superstructure showed dense, hard concrete ap- parently of good quality. The steel, where the cover- ing of concrete was stripped off, showed clean surfaces, free from all signs of corrosion.

PRELIMINARY EXPERIMENTS MADE

Various possible methods of measuring vertical de- flections of the rib and horizontal movements of piers were considered and it was decided to use suspended wires. ‘The wires for measurement of pier movements were to be fixed at the far piers of the adjacent spans on the assumption that temperature movements of any point on the wire would be vertical, the position of such point horizontally remaining fixed. In order to test this assumption, a wire was stretched between two firmly planted posts at the Arlington Experiment Farm and observations made of the movements of a number of points fixed on the wire over a period of time during which there was a considerable change in temperature. It was observed that no appreciable horizontal move- ments of the points took place.

The deflection wires (described in detail on p. 192) were installed in June, 1927. At the same time ther- mometers were placed in holes drilled in the ribs at different distances from the surfaces of the concrete. The holes were filled with cup grease and closed with corks through which the stems of the thermometers passed. Temperature movements at the crown of both ribs of the center arch span were observed daily over a period of several months. The observations showed an average movement of one-fortieth of an inch for a change of 1° C. in average rib temperature.

TANKS FILLED WITH WATER USED FOR TEST LOADS

The test span was loaded with tanks of water, filled by pumping from the river. The tanks were 12 feet 6 inches wide by 20 feet long and 18 feet high, inside dimensions, and were built of timber with structural steel underframes. The length was such as to permit supporting the load at two adjacent panel points. Rollers were provided so that the empty tanks could be easily moved into any desired position on the bridge. After being placed in position, the tanks were jacked

up and allowed to rest symmetrically on four bearing blocks located over the center of the columns at which the loads were to be applied, as shown in Figure 2, page 190. The tanks were leveled by the use of plumb bobs suspended at each end and then filled with water. The tanks were moved while empty to avoid overstraining the floor system.

Force to move the tanks was applied by a truck through a block and tackle arrangement anchored to the solid handrails on each side of the bridge. At ordinary temperatures the tank could be rolled over the rock as- phalt surface of the bridge floor but at high tempera- tures it was necessary to use plank runways to prevent the rollers from sinking into the asphalt.

It was not possible to weigh the tanks by ordinary methods and a special weighing cell was used for the purpose. This device makes use of a small copper cylinder, specially heat-treated and of fixed size, which when compressed under load, is permanently deformed according to a fixed law.

The complete weighing cell consists of a hollow steel cylinder into which a steel piston fits closely. On the inside of the cylinder head is a hardened steel face or anvil with a plane, smooth surface. On the entering end of the piston is a corresponding hardened steel face. The copper cylinder, one-half inch in diameter by one-half inch high, is placed on end in the steel cylinder on the smooth surface and the piston is allowed to rest on it. The load whose magnitude is desired is then applied to the piston and its entire weight is

LOADING TANK

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PuMPING PLANT FoR FILLING TANKS

transmitted to the copper cylinder. The length of the copper cylinder is measured with a micrometer caliper before and after the load is applied. The weight corre- sponding to the deformation of the cylinder is taken from a calibration curve which has been previously determined in the laboratory for the particular size and quality of copper cylinder used.

The empty tank was weighed by placing a cell under each corner of one end and two cells with an equalizer under the center of the opposite end. The total weight of the tank was the sum of the four weights indicated by the cells. The weight of the tank was also calculated from the unit weights of the timber determined by

188

weighing specimens of the material used in constructing the tanks. The weights determined by the two methods checked within 1 per cent. The weight of each empty tank was found to be approximately 47,000 pounds and this figure was used in the computa- tions. The water capacity of each tank was 4,500 cubic feet, 33,750 gallons, or 281,250 pounds. The increments of water load at each panel point were 22,750, 45,500, and 68,250 pounds.

Two WEIGHING CELLS IN PLACE AT ONE END or TANK TESTS DIVIDED INTO THREE PHASES

The test was divided into three phases which, for convenience, are designated as series 1, 2, and 3.

In series 1 a single tank was placed so as to apply its load at two adjacent panel points and deformations observed over the entire rib from springing line to springing line with the superstructure intact. The series of loadings began at a pier and continued suc- cessively to the crown. The tank was also placed on one of the adjacent spans in a panel next to the crown, and deformation readings were taken on the span under observation. .

In series 2, the same procedure was followed except that the deck and railings were cut at each panel point and supported on new, greased bearing plates so as to destroy, as far as practicable, the continuity of the floor system and its restraining action at the tops of the columns, and the curtain wall near the crown was broken out. In this series, the load at panel points 1 and 2 (fig. 2) was omitted because of the small deformations caused by the load in this position .

In series 3, two tanks were placed in the position to produce the maximum stress in the rib and the de- formations were measured as in series 1 and 2, with the superstructure in the same condition as in series 2.

PUBLIC ROADS

Vol. 9, No. 10

Four increments of load were applied at each posi- tion of loading: The empty tank, the tank filled with 91,000, 182,000, and 273,000 pounds of water.

In destroying the continuity of the superstructure for series 2 and 3 the slab, girders, and handrail over each cross beam were cut through with air drills and the steel severed with an oxyacetylene flame. The ends of the oirders were then jacked up and new, well-greased bear- ing plates inserted at each girder bearing. The girder ends on the entire east half of the span were shattered by the cutting operation to such an extent as to make the bearings on the cross beam unsafe. To relieve these bearings of the tank loads, holes were cut through the deck at the panel points and timber bearing blocks placed directly on the cross beams. In two panels it was thought necessary to support the dead weight of the floor system on timbering built up from the ribs. This was done in such a way that the arch was not stiffened.

DEFORMATIONS MEASURED

In order to measure completely the deformation of the rib under live load, the following six measurements were made:

(1) Deformations of the concrete on the extrados and the intrados at nine sections of the rib spaced 18 feet 6 inches apart along the axis.

(2) Deformations of the reinforcing steel near the springing lines and at the crown.

(3) Rotation of the arch axis at nine points spaced 18 feet 6 inches along the axis.

(4) Deflections of the rib at nine columns, no meas- urements being taken at the column next to each pier.

(5) The change in length of mid-ordinates of each of the 31 consecutive 5-foot arcs of the axis.

ScAFFOLD FrRoM WHICH OBSERVATIONS WERE MADE

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