\JU) CORNELL UNIVERSITY LIBRARY GIFT OF Prof. Ludlow Brown FINE ARTS UNIVERSITY OF ILLINOIS BULLETIN Vol. XV May 27, 1918 No. 39 ' tE^Dtered^U BecondniUM matter Dm. 'li»- 1912, ^t the Foet Office at Urbana, ilL. onder the Act ol AuK* 24 ,1912 TEST OF A FLAT SLAB FLOOR OF THE WESTERN NEWSPAPER UNION BUILDING BY :; ARTHUR N.TALBOT AND HARRISON F. GONNERMAN ;■ BULLETIN No. 106 ENGINEERING EXPERIMENT STATION FCBLISHBD BT THB UnITVBSITT OF IlLINOZS, UbBANA Pbicb: Twhnti Cbnts EUBOPSAH AqEHT Chapuan 4c Bali., Ltd., London THE Engineering Experiment Station was established by act of the Board of Trustees, December 8, 1903. It is the purpose of the Station to carry on investigatioins along various- lines of engineering and to study problems of importance to professional engi- neers and to the maijiufacturing, railway, mining, constructional, and industrial interests of the State. The control pf the Engineering" Experiment Station is vested in the heads of the several departments of the College of Engineering. These constitute the Station Staff, and, with the Director, determine the character of the investigations to be undertaken. The work is carried on under the supervision of the Staff, sometimes by research fellows as graduate work, sometimes by members of the instructional staff of the College of. Engineering, but more frequently by investigators belonging to the Station corps. The results of 'fhese investigations are pubUshed in the form of bulletins, which record mostly the experiments of the Station's own staff of investigators. There will also be issued from time to time, in the form of circulars, compilations giving the results of the experi- ments of engineers, industrial works, technical institutions, and gov- ernmental testing departnients. The volume and number at the top of , the front cover page are merely arbitrary numbers and refer to the general publications of the University of Illinois: eith&- above the title or below the seal is given the number of the Engineering Experiment -Station bulletin or cir- cular which should be used in referring to these pv^lications.^ For copies of bulletins, circulars, or other information address thp Engineehino Expebiment Station, Uhbana, Illinois. UNIVERSITY OF ILLINOIS ENGINEERING EXPERIMENT STATION Bulletin No. 106 May, 1918 TEST OF A FLAT SLAB FLOOR OF THE WESTERN NEWSPAPER UNION BUILDING BY ARTHUR N. TALBOT Professor of Municipal and Sanitary Engineering AND IN Charge of Theoretical and Applied Mechanics AND HARRISON F. GONNERMAN Research Associate in Theoretical and Applied Mechanics ENGINEERING EXPERIMENT STATION Published by the Univebsitt o» Iluhois, Ubbana Cornell University Library The original of tiiis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924015389566 CONTENTS PAQE 1. Preliminarj' 7 2. Acknowledgment 7 3. The Building 8 4. The Test 13 5. Deflection of Slab 19 6. Cracks in Slab .... 19 7. Load-strain Diagrams 21 8. Stresses in Reinforcing Bars in Upper Side of Slab . . 25 9. Stresses in Reinforcing Bars in Lower Side of Slab ... 27 10. Strains in Concrete at Upper Surface of Slab 27 11. Strains in Concrete at Lower Surface of Slab 27 12. Influence of Position of Bar ... 29 13. Resisting Moment Accounted for by Stresses in Reinforcing Bars 31 14. Bending of Columns 39 15. Time Effect of Load and Recovery upon Removal of Load 40 16. General Comments . . . . ... . . 41 17. Wrecking of the Floor .43 18. Summary 43 LIST OF FIGURES HO. PAGE 1. View of Western Newspaper Union Building at Time of Test ... 9 2. View Showing Full Load on Floor .... 9 3. Thickness of Floor in Test Area .... 11 4. Arrangement of Reinforcement in Test Area 12 5. Location of Test Panels 15 6. Location of Gage Lines on Upper Side of Slab 16 7. Location of Gage Lines on Lower Side of Slab 17 8. Load-deflection Diagrams and Location of Deflection Points .... 18 9. Location of Main Cracks in Upper Side of Slab . .... 20 10. Location of Main Cracks in Lower Side of Slab . . 21 11. Load-strain Diagrams for Gage Lines on Reinforcing Bars on Upper Side of Slab 22 12. Load-strain Diagrams for Gage Lines on Reinforcing Bars on Upper and Lower Sides of Slab 23 13. Load-strain Diagrams for Gage Lines in Concrete on Upper and Lower Sides of Slab 24 14. Stresses in Reinforcing Bars in Pounds per Square Inch at Load of 913 Pounds per Square Foot . 26 15. Unit-strains in Concrete at Load of 913 Pounds per Square Foot ... 28 16. Position of Neutral Axis for the Several Layers of Bars at Column 22 . 30 17. Sections of Positive Moment and Negative Moment Considered in the Calculations . . .... 32 18. Sections of Positive and Negative Moment Considered in the Calculation of the Resisting Moments Accounted for by Stresses in the Rein- forcing Bars .... 35 19. Load-strain Diagrams for Gage Lines Located on Columns . . .39 20. View Showing Arrangement of Reinforcement at Column 22 . . .45 21. View of Slab after Reinforcing Bars were Exposed . . . . 45 LIST OF TABLES NO. PAGB 1. Compression Tests of Concrete Prisms .... 13 2. Tension Tests of Eeinforcing Bars ... 14 3. Resisting Moments Accounted for by Stresses Observed in Eeinforcing Bars 36 TEST OP A PLAT SLAB FLOOR OP THE WESTERN NEWS- PAPER UNION BUILDING 1. Preliminary. — This bulletin gives the results of the test made on a four-way reinforced concrete flat slab floor of the Western News- paper Union Building in Chicago in August and September, 1917. A load of 913 lb. per sq. ft. was applied over fouj;* panels. The build- ing, which was nine years old at the time of the test, was to be torn down to clear the site for the new Union Passenger Station; the op- portunity was utilized to apply a test load much greater in propor- tion to the design load than had been used in previous tests of build- ings. The test was carried far enough to give stresses in the rein- forcing bars and concrete markedly higher than have been obtained in other building tests. The information on the action of the slab in its various parts given by the strain measurements has an important bearing on the design of the flat slab structure. 2. Acknowledgment. — The test was made as investigative work of the Engineering Experiment Station The testing work was done under the direct supervision of Mr. Gonnerman. He and Mr. N. B. Ensign, Associate in Theoretical and Applied Mechanics, acted as observers. The results have been reduced and prepared for publication as a bulletin of the Engineering Experiment Station. Acknowledgment of valuable aid received in carrying out the test is made. The Portland Cement Association furnished the labor for preparing for the test and for hauling the loading material and loading and unloading the floor. The Universal Portland Cement Company assisted in making arrangements and gave assistance on the test. The pig iron used for loading material was lent by the Illinois Steel Company. The freight on the pig iron was borne jointly by the Pennsylvania Railroad and the Portland Cement Association. Opportunity to use the building for the purpose of the test was given by the Chicago Union Station Company; the test was made at the suggestion of A. J. Hammond, Principal Assistant Engineer. The Condron Company provided an assistant for tracing and checking the transfer of the loading material. 8 ILLINOIS ENGINEERING EXPERIMENT STATION 3. The Building. — The "Western Newspaper Union Building was an eight-story reinforced concrete structure located at Clinton and Adams Streets, Chicago. The building was erected in the spring of 1909 by the George Hinchcliff Company, contractors, according to plans furnished by S. N. Crowen, architect, and Ritter and Mott, en- gineers. It had been in use by a printing company until 1916. The floor tested had been occupied by printing presses. Pig. 1 is a view of the building at the time of the test ; the wrecking of the building had begun. Two types of floor construction were used in the building. The first five floors wera slab and girder type; the sixth, seventh, and eighth floors were Turner mushroom flat slab type (four-way rein- forcement). The floors of the building were divided into panels 17 ft. 5y2 in. by 19 ft. 4i/^ in. The test was made on the sixth floor. This floor was designed for a live load of 250 lb. per sq. ft. and was nominal- ly 8% in. thick. A considerable variation in thickness was found, the measured thickness over the test area ranging from 7.5 to 9.8 in. Fig. 3 gives the thickness of the floor at a number of places as determined by readings with an engineer's level. In general, the thickness was greater away from the columns than in the vicinity of the columns. The interior columns were octagonal in form, 24 in. in short diameter below the floor tested and 21 in. in short diameter above it. The inside diameters of the hooping of the columns on the fifth and sixth floors are given on the plans as 21 in. and 18 in., respectively. The column capitals were pyramidal ; the short diameter at the top of the capital was 54 in. The building plans called for 15 %-in. round bars in each of the four bands of reinforcement in the floor slab and indicated that over most of the columns in the test area there were laps in certain bands. After the test was made, the floor was broken into and the location and extent of all laps and the position of reinforcing bars with respect to the surfaces of the slab were found. Fig. 4 shows the arrangement of the reinforcement found over the test area, including the iDosition of the laps. In several places the arrangement of rein- forcement differs from that given in the building plans. In three places in rectangular bands the reinforcement for positive moment was double that given on the plans (30 bars instead of 15). The lapping of bars at columns was generally greater than that indicated on the plans. At column 15 three bands were lapped; at columns 14, 16, 21, 26, 27, and 28, two bands; and at columns 22 and 28, one band. In most cases, the length of lap and its position were such that the extra Fig. 1. View or Western Newspaper Union Building at Time of Test Fig. 2. View Showing Full Load on Floor TEST OP THE WESTERN NEWSPAPER UNION BUILDING 11 metal was effective in regions of greatest moment. In some cases the laps were poorly arranged, as at columns 15, 16, and 27. No reason is apparent for the lapping of bars between columns. There was no reinforcement for negative moment in the region midway between columns. The eight 1%-in. column rods were bent out into the slab, and two circumferential ring rods (circles of 5 ft. 6 in. and 8 ft. 6 in. diameters) rested on these and supported the lower layers of rein- forcing bars. The measurement of position of bars with respect to r/ Secfions of f/oor fro/n hvhich fesf /3Hsf?7S "7 3,00 J _8.ee tvffrfi cv^ shon^n fhas \s.ae S.SS ^oa I a^-s' „a>/ &i^ I o o o o "I 1 /=ans/ e I Pane/ ^ | \s.7-4 3./4 \s.33 ^.ez S/e\ I 8.34 / \33 \3.4/ T.3S 7sa \zSS ° \ °' ? ° I ■^,^£C'\8i4S -si^J^'J-'!' ^^■SV^^' \Be3 s.os J.oe \e.3z 7.3/ J.ee \ o ° ? 1 S.3Z l«?/ , J? II \j? <0 Pone/ D Pone/ C ^ K. •X^^" Pone/ /? loz A Pane/ B i-| Concre/s W Sfee/ /T. ^ Fig. 6. Location of Gage Lines on Upper Side of Slab readings into form for analysis renders it necessary to restrict the number of gage lines and so their distribution over the test area be- comes a matter of importance. The gage lines in this test were placed with a view of getting some information on (1) the amount and dis- tribution of the stresses in the reinforcement along sections through TEST OF THE WESTERN NEWSPAPER UNION BUILDING 17 Fig. 7. Location of Gage Lines on Lower Side of Slab the panel centers and through the panel edges, and at other points, (2) the strain in the concrete at the more important points in the slab, (3) the moment of resistance accounted for by the stresses in the rein- forcing bars which cross sections through the panel centers and through the panel edges, and (4) something on the stresses in columns at the edges of the loaded area due to bending under the applied load. 18 ILLINOIS ENGINEERING EXPERIMENT STATION o/ Def/ect/on In/nches f Fig. 8. Load-deflection Diagrams and Location of Deflection Points TEST OF THE WESTERN NEWSPAPER UNION BUILDING 19 The gage lines in panels A and C with few exceptions were laid out in duplicate in order that readings obtained on the gage lines in the one panel might serve as a check on readings obtained at corresponding gage lines in the other, and also to find out whether there was similarity of action in the two panels. 5. Deflection of Slab. — The deflections of the slab at the several deflection points are plotted in Fig. 8, the diagrams for p'oints similar- ly located being grouped together. The recovery of deflection one day after the load was removed is indicated by the points plotted at the bottom of the diagram. The second point plotted for the load of 637 lb. per sq. ft. is the deflection 16 hours after the last of the load was applied ; the second point for the load of 855 lb. per sq. ft., 37 hours after ; the second point for the load of 913 lb. per sq. ft., 66 hours after. Where no second point is plotted, the change in deflection was negligible. The deflections at the centers of panels A, B, C, and D under the load of 913 lb. per sq. ft. were 1.06, 1.12, 1.04, and 0.87 in., respectively. It may be noted in this connection that panels C and D had a greater amount of reinforcement than A and B and that panel D was thicker. The concrete of panels A and D was of unusually good quality. Of the deflections at the middle of the inner edges of the loaded panels, that at point 8 (Fig. 8) was considerably greater than that at point 16, and that at point 12 was more than at point 20. The dif- ferences are explainable by differences in quality of concrete and in amount and arrangement of reinforcement. At the outer edges of the loaded area, the deflections at points 5 and 9 were considerably greater than at points 15 and 18 ; and the de- flections at points 2 and 11 were greater than at points 14 and 19 — the same circumstances explain these differences in deflections. It may be noted that points 1 and 6 (Fig. 8) distant one-quarter of the panel lengths from the panel edges gave a measurable deflec- tion. Point 7 (center of adjacent panel) remained stationarj^ and point 4 showed an upward movement. 6. Cracks in 8lab. — It was noted before the load was applied that there were numerous checks in the upper surface of the slab in the regions around the column and along the panel edges. Most of these were evidently surface checks ; others were tension cracks formed under previous loads. The latter opened upon the application of load. 20 ILLINOIS ENQINEEEINS EXPERIMENT STATION Fig. 9 gives the location of the more important cracks on the upper side of the slab which either opened or formed under the test load. These cracks were all open cracks — much more marked than hair cracks. Under the load of 913 lb. per sq. ft. they ranged in width from 0.02 to 0.06 in. These cracks show the regions of high tensile stress in the top of the slab. The main cracks at the columns were generally at or near the edges of the column capital ; they branched out to join the cracks extending along the panel edges between the columns. The cracks at the capitals of the columns bordering the loaded area were Fig. 9. Location of Main Cracks in Upper Side of Slab fully as wide as those at the capital of the central column. The cracks along the panel edges were generally as pronounced as those around the capitals. Fig. 10 shows the location of the more important cracks on the lower side of the slab in panels A and C, the panels in which the principal tension gage lines on the lower side of the floor were placed. In panels B and D, in addition to those noted in the figure, there were cracks over the panels in positions similar to those noted in panels A TEST OP THE WESTERN NEWSPAPER UNION BUILDING 21 and C. In panel D the cracks were not so wide nor so numerous as in the other panels, even at the maximum load. Panel B had larger and more numerous cracks than the other panels. Most of the cracks on the lower side of the slab were found in bands extending in rec- tangular directions. In a small area at the panel centers cracks ex- tended in diagonal directions. No cracks or checks had been noted on the lower side of the slab before loading was begun. The first hair cracks here were observed at a load of 234 lb. per sq. ft. At a load of 425 lb. per sq. ft., the cracks were fairly well defined and for higher /9-4i Fig. 10. Location of Main Cracks in Lower Side of Slab loads they gradually opened up and extended. The main cracks on the lower side of the slab did not open so much as the main cracks on the upper side of the slab. The construction joint shown in Fig. 9 and 10 opened appreciablj'^ under the application of load. Upon the removal of the load the cracks closed, giving the slab an appearance similar to that which it had before the load was applied. 7. Load-strain Diagrams. — In Pigs. 11 to 13 the load-strain dia- grams are given for gage lines on the upper and lower sides of the 22 ILLINOIS ENGINEERING EXPERIMENT STATION Unit Sfra/n - mper in. Fig. 11. Load-strain Diagrams FOR Gage Lines ON Reinforcing Bars ON Uppee Side of Slab TEST OP THE WESTERN NEWSPAPER UNION BUILDING 23 Unit Strain - in. per t'n. Fig. 12. Load-strain Diagrams for Gage Lines on Reinforcing Bars on Upper AND Lower Sides op Slab 24 ILLINOIS ENGINEEBINQ EXPERIMENT STATION , 00/>4 '^• y ■/} \ ^ f -^ y y / ^ -" r- -^ r ^> { ^ ■^ I 1 / b ' 1 / / / 7 / ^ / , / r \ { / / / \ I/as \/OI U7 A NO l^^ 123 124 lOS 07 lie) L _^ ,0004 ^ SM- 400 - Unit Strain - in. per in. Fig. 13. Load-strain Diagrams for Gage Lines in Concrete on Upper and Lower Sides of Slab TEST OF THE WESTERN NEWSPAPER UNION BUILDING 25 slab. In these diagrams tensile strains are plotted to the right and compressive strains to the left of the axis. Gage lines having similar positions on the floor have been grouped. Averages of the several read- ings were used in computing the strains. It will be noted that for gage lines on bars in the upper side of the slab the diagrams show de- formations of considerable amount at the load of 234 lb. per sq. ft ; the diagrams for gage lines on bars on lower side show markedly smaller deformations. For loads greater than 234 lb. per sq. ft. the deforma- tions in bars on the lower side of the slab increased fairly uniformly with increase of load ; it has already been noted that the first cracks on the lower side appeared at this load. The diagrams for gage lines on the concrete on both upper and lower sides of the slab show deforma- tions of considerable amount at the first load and a fairly uniform in- crease in deformation for higher loads. 8. Stresses in Reinforcing Bars in Upper Side of Slab. — The stresses in the reinforcing bars at the several loads may be computed from the strains given in Fig. 11 and 12. For convenience of compari- son the stresses corresponding to the strains for the maximum load (913 lb. per sq. ft.) are given in Fig. 14. The values shown at points around the columns are in bars at the upper side of the slab ; the others are in bars at the lower side. For bars on the upper side of the slab the greatest stresses were found at gage lines located on diagonal bars at the edge of the column capitals. At column 22 (the central column of the loaded area) the stresses in diagonal bars over the edge of the column capital ranged from 49 200 to 57 300 lb. per sq. in. The stresses in diagonal bars at column 22 at gage lines located some distance from the column capital ranged from 15 000 to 34 500 lb. per sq. in. The stresses in diagonal bars at the columns bordering the loaded area ranged from 36 600 to 54 900 lb. per sq. in. In general, the stresses in the diagonal bars at the columns bordering the loaded area were nearly as great as those observed at corresponding gage lines at the central coliunn. The stresses given do not include the stress due to the load of the slab itself ; allowing for this, it is apparent that the yield point of the steel was not reached in even the most highly stressed bars. The stresses in the east and west rectangular band at column 22 averaged about 41 400 lb. per sq. in. The stresses in the north and south rectangular band at column 22 ranged from 23 700 lb. per sq. in. 26 ILLINOIS ENGINEERING EXPERIMENT STATION at the edge of the column capital to 40 500 lb. per sq. in. at gage lines near the edge of the band on either side of the column. The bars in this band were lapped at column 22, the bars ending about 80 inches north and south, of the column center. It will be noted that at gage lines near the edges of the rectangular bands the stresses were greater ■S400 /r/oo( ^,,-, Z3I00 I ^'■-^, 'iJlOO — T-('^! r I V ?! ^> V J^'^ §1 ' ■^ Jl" IS :^ ■^^ .00 l^" egoo_ s/oo ■ S| »0° J) {/■Th • Uppar 5'n/p Fig. 14. Stresses in Ebinporcing Bars in Pounds per Square Inch at Load op 913 Pounds per Square Foot than in bars at the middle of the band. In bars outside the loaded area and near the edge of a rectangular band stresses were found as great as 6000 lb. per sq. in. It is evident that portions of the slab out- side the loaded area contributed measurably to the resistance developed in the slab. TEST OF THE WESTERN NEWSPAPER UNION BUILDINQ 27 9. Stresses in Reinforcing Bars in Lower Side of Slab. — On the lower side of the slab the greatest stresses were found in the bars of the rectangular bands (Fig. 14). At the maximum load stresses from 24 300 to 30 000 lb. per sq. in. were observed in rectangular bands with- in the loaded area: In the one which had 30 bars instead of the usual 15, stresses from 18 000 to 21 000 lb. per sq. in. were found. The stresses in the rectangular bands at the edge of the loaded area varied from 5100 lb. per sq. in. in a bar outside the loaded area to 22 500 lb. per sq. in. in a bar inside the loaded area. At one gage line outside the loaded area and near the edge of the band a stress of 15 600 lb. per sq. in. was found. The stresses in the diagonal bands in the region of the center of the panels were smaller than those in bars of the rectangular bands in the region between columns. The stresses in diagonal bars at gage lines away from rectangular sections which pass through panel centers ranged from 6300 to 18 300 lb. per sq. in. The effect of position of reinforcing bars with respect to surface of slab on the stress developed is discussed in another place. 10. Strains in Concrete at Upper Surface of Slab. — The unit- strains in the concrete at the several loads are plotted in Fig. 13. For convenience of comparison, the unit-strains at the maximum load are recorded in Fig. 15. The values for points around the columns are for gage lines on the lower side of the slab; a few gage lines which cross the panel edges between columns are also on the lower side. The remaining gage lines are on the upper side of the slab. On the upper surface of the slab the greatest compressive strains were found at gage lines along the inner panel edges midway between columns. Strains from 0.00089 to 0.00097 in. per in. were observed at these gage lines. Assuming a modulus of elasticity of concrete of 4: 000 000 lb. per sq. in. and a straight-line stress-strain relation the stresses in the concrete corresponding to these deformations would be 3560 and 3880 lb. per sq. in. Strains as great as 0.00054 in. per in. (corresponding stress in the concrete on the assumption just given, 2160 lb. per sq. in.) were found at gage lines at the panel centers. 11. Strains in Concrete at Lower Surface of Slab. — The greatest strains in concrete on the lower surface of the slab were found close to the edge of the capital of column 22. The strains at this column at 28 ILLINOIS ENGIJSTEEEING EXPERIMENT STATION the diagonal gage lines (see Fig. 15) ranged from 0.0012 to 0.0016 in. per in. at the maximum load. These strains are as great as the strains which were found at failure in the tests of the concrete prisms cut from the slab ; they represent the range in deformation at the ultimate load usually found in compression tests of concrete. Spalling or chipping — -t. oooo^ •^+.00007 ■/3-4i- -13-4^ fCol:._ ' +.0007S -.00006 ^f If Pane/ D /'ane/ /4 -ooo^ /^ne/C I %;^±0047 ''■^-.OOJZS ^-.ocns \9 /■ -.OOOS€ ,^^(j£- -. 00/31 Jr.ooose -.OOOS4'\' I -.00087 - -.0007.9 •» ,>-^-:00053 fane/ 3 - Ctunpressire Strain + Tensile 5^r£'/n *-• Upfief" Si^e k Fig. 15. Unit-strains in Concrete at Load of 913 Pounds per Square Foot of the concrete surface was plainly visible near the edge of the capital of column 22 in panel A. At rectangular gage lines near the capital of this column a strain in the concrete of 0.0014 in. per in. was observed. These high deformations indicate that the concrete near the capital of column 22 was highly stressed and that at certain gage lines it was stressed to its ultimate strength, the action of the surrounding con- crete preventing its complete failure. TEST OP THE WESTERN NEWSPAPER UNION BUILDING 29 Near the edges of the capitals of colmnns bordering the loaded area strains from 0.00054 to 0.0011 in. per in. were observed at di- agonal gage lines, and strains from 0.00044 to 0.00081 in. per in. at rectangular gage lines. With a modulus of elasticity of concrete of 4 000 000 lb. per sq. in. the stresses corresponding to these strains would range from 1760 to 4400 lb. per sq. in. At the gage lines crossing the inner panel edges at a section of negative moment between columns 15 and 22, compressive strains about one-half those found near the column capital were observed at the lower loads, even though there was no tension reinforcement in the upper side of the slab in this region. For the highest load when the concrete at the capital had begun to crush there was a relatively great increase in the strains in this middle region, a value as great as 0.0012 in. per in. being found. It should be noted that there were compressive strains of some amount in regions of negative moment outside the loaded area. 12. Influence of Position of Bar. — The stresses in Fig. 14 are given without reference to the position of the bar with respect to the surface of the slab and without reference to its position in the band. It may be expected that the stresses in the several layers of bars will vary. The average depths of the layers of bars at the edge of the capital of column 22 ranged from 0.90 in. below the upper surface of the slab for the upper layer to 3.65 inches for the lower layer. At this column the layers of bars in the order of their position with respect to the upper surface of the slab were as follows: (1) north and south rectangular bars (one lap), (2) diagonal bars running to northwest, (3) east and west rectangular bars, (4) diagonal bars running north- east, (5) north and south rectangular bars (second lap), (6) circum- ferential ring bars, (7) radial column bars. (See Fig. 20.) The strains in steel and concrete at gage lines near the capital of column 22 for the load of 913 lb. per sq. ft. have been plotted in Fig. 16 to show the position of the neutral axis for the various layers of bars on which readings of deformation were taken. The strains found at gage lines 5 and 17 are markedly smaller than those found at gage lines 8, 9, 20, and 22A which were placed on bars of the same layer. Gage line 22A, like gage lines 5 and 17, was placed over the edge of 30 ILLINOIS ENGINEEEING EXPERIMENT STATION the capital and it will be noted that the deformation found at this gage line is more in harmony with the deformations found at other gage lines on bars of the same laver than are the deformations at 5 and 17. L/n/Y 3fra/n m 3 tee/ -/h per //7. {/n/V Sfra/'n /n Concrete -/h.per in. Fig. 16. Position of Netjteal Axis for the Sevekal Layers of Bars at Column 22 No reason for the markedly smaller value in gage lines 5 and 17 is apparent. For the gage lines other than 5 and 17 the position of the neutral axis with respect to the under side of slab ranged from 0.43 to 0.46 of the effective depth of the several layers. The value of j (which represents the ratio of the distance between the bar and the eentroid of compressive stresses to the distance between the bar and the face of the slab) for these bars is, therefore, about 0.85. The value of j found in a similar way for gage lines located at columns border- ing the loaded area was 0.87. It will be noted in Pig. 16 that the strain in the lower layer of diagonal bars is nearly as great as that in the upper layer of diagonal bars. The strain in the layer of rectangular TEST OF THE WESTERN NEWSPAPER UNION BUILDING 31 bars between the two layers of diagonal bars was less than that found in the diagonal bars. No measurements of strain were made on the rings nor on the column bars bent out radially into the slab. These bars were placed low in the slab and near where the neutral axis may be expected to be. It is probable that they were stressed somewhat. As has been stated elsewhere these bars were not taken into account in the calculations of resisting moment. For the diagonal bars in the central area of the panels, the stresses in the bars of the lower layers were generally considerably greater than those in the upper layer. In the one case where the lower bar shows considerably less stress, the gage lines were near the end of lapped bars. The bars in the rectangular bands at sections of positive moment were farther from the under surface of the slab than were the bars of the diagonal bands, but the stresses in the former were greater than in the latter except where laps occurred. It should be noted that the bars at the middle of the rectangular bands were farther from the un- der surface of the slab than were the outer bars. Differences in the magnitude of stresses in bars in similar places at sections of positive moment are partly accounted for by differences in the position of the bars ; for example, the bar at gage line 236 was 2.7 in. from the lower surface and the one at 237 was 1.15 in. from th'C lower surface, while the stresses were 12 600 and 21 600 lb. per sq. in. respectively. Sim- ilarly, the bars for gage lines 219, 221, and 239 are higher in the slab than the corresponding bars at the edges of the band. Gage lines 2, 7 and 10 were placed on the same diagonal bar. Similarly, gage lines 11, 14, and 18 were on another diagonal bar. The bar having gage lines 2, 7, and 10 was about 1.95 in. below the upper surface of the slab and the other bar about 3.15 in. below the upper surface. The stresses at gage lines 2, 7, and 10 were 27 000, 34 500 and 19 800 lb. per sq. in., respectively ; the stresses at gage lines 11, 14, and 18 were 16 200, 23 700, 15 000 lb. per sq. in., respectively. It will be seen that the stresses at the gage lines opposite the column capital (7 and 14) were greater than those observed at the other gage lines. 13. Resisting Moment Accounted for iy Stresses in Reinforcing Bars. — It will be of interest to find the magnitude of the resisting moments developed in sections of the slab and to compare the values 32 ILLINOIS ENGINEERING EXPERIMENT STATION with the bending moment due to the external forces. As the part played by the tensile stresses in the concrete is unknown and uncertain, only that part of the resisting moment found by using the measured stress in the reinforcing bars can be considered. The resisting moment based on the tensile stresses in these bars may not be expected to ac- count fully for the bending moment. The two sections considered will be (1) a section across the panels midway between columns (AB, Fig. Fig. 17. Sections of Positive Moment and Negative Moment Considered in THE Calculations 17) and (2) one along an edge of the panels parallel to the first section but skirting the part of the periphery of the column capitals at the comers of the panels (CDB, Fig. 17). It will be noted that the edges of the area here considered are along lines of zero shear, except around the column capitals. The external forces acting on the row of half panels are the load on the two half panels and the reaction or external shear at the three column capitals. The moment of the cou- ple formed by these two external forces will be resisted by the numeri- TEST OP THE WESTERN NEWSPAPER UNION BUILDING 33 cal suin^of the resisting moments developed in the two sections AB, and CDB. The moment of the internal stresses at the section of the panel midway between columns is referred to as the positive re- sisting moment and that at the edge of the panel as the negative re- sisting moment. The actual distribution of these resisting moments along the sections need not be considered in making the desired com- parison. The point of application of the resultant of the load on the two half panels is shown at F ; that of the resultant of the reaction of the three supports at G. The moment of the couple formed by these two resultants is the bending moment due to the external forces and is the moment to be considered. Analysis shows that, for a uniformly distributed load, and round columns, the value of this bending mo- ment for a load two panels wide is given quite closely by the equation for sections at right angles to the long way of the panel, and for sections at right angles to the short way of the panel where • M = bending moment for a width of two panels W = load on one panel Ij = long side of an oblong panel measured from center to center of column h = short side of an oblong panel measured from center to center of column c = diameter of column capital. With the load of 913 lb. per sq. ft. over four panels, the bending moment for a width of two panels, as obtained by equations (1) and (2), is 12 820 000 Ib.-in. for the long way of the panels (resisted at north and south sections) and 11 060 000 Ib.-in. for the short way (re- sisted at east and west sections). The positive moment (the resisting moment at the section across the panels midway between columns) and the negative moment (the resisting moment at the section at the edge of the panels) together 34 ILLINOIS ENGINEERING EXPERIMENT STATION must resist this bending moment. "With a condition of ends of slab for which the tangent to the curve of flexure at the edges of the panels remains horizontal when the load is applied (usually termed fixed ends), the condition assumed in the analysis, the positive moment will be found by analysis to be one-third of the total resisting mo- ment and the negative moment two-thirds, provided the slab is uni- formly stiff throughout. If the tangents at the panel edges deflect somewhat, the positive moment will be greater than one-third and the negative moment less than two-thirds. In the comparisons to be made it will be assumed that the proportions given by analysis are one-third for the positive moment and two-thirds for the negative moment. As stated in "14, Bending of Columns," calculations of the resisting moment developed at sections of the slab at the boundaries of the loaded area as accounted for by the observed stresses in the reinforc- ing bars, the results of analytical determinations of the moments at sections at the edges and at the middle of the loaded area, and the amount of flexure in the columns all go to show that there is little in- accuracy in. this assumption in the case under consideration. The analytical value of the positive resisting moment will be termed the analytical positive moment and that of the negative resisting moment the analytical negative moment. For the north and south sections the magnitude of these analytical moments becomes 4 270 000 and 8 550 000 Ib.-in. respectively, and for the east and west sections 3 700- 000 and 7 400 000 Ib.-in. respectively. The sections used in obtaining the resisting moment accounted for by the stresses observed in the reinforcing bars are shown in Fig. 18. QKC-BJT is the east and west section of positive moment used, and MNOP the east and west section of negative moment; similarly, IJO-NKL is the north and south section of positive moment, and ABCD and EFGH the north and south sections of negative moment. Sections of negative moment in the north and south direction are taken on two sides of the column capitals, because the available rein- forcement differed in the two sections. The sections of positive mo- ment were taken as shown because they cross the greatest number of gage lines. In the calculation of the resisting moments developed, lapped bars were considered as contributing to the resisting moment where- ever the bars extended beyond the section a sufficient distance to in- sure adequate anchorage with respect to the magnitude of the stress TEST OF THE WESTERN NEWSPAPER UNION BUILDING 35 developed in the bar. Many of the laps were made in such a way that the additional section may not be expected to contribute much to the resisting moment. As measurable stresses were found in bars near the edge of the band outside the loaded area, they were taken into account in the calculations. The ring rods around the columns and the column bars which were bent out into the slab were not in- cluded in the reinforcement, for no measurements of strain were made on these bars. For the diagonal bars the component of the stress was taken in a direction at right angles to the direction of the panel edge. The average of the stresses at the principal critical gage lines was generally used. For the bands at the edges of the loaded area the stresses were considered to vary over the band from gage line to gage (^ (§> Fig. 18. Sections of Positive and Negative Moment Considered in the Cal- culation OF the Resisting Moments Accounted for by Stresses IN the Reinforcing Bars line. Some judgment had to be used in determining the stress varia- tion" as well as the availability of lapped bars, but it is believed that the assumptions made give results well below the actual resistance de- veloped in the slab. It should be noted that the maximum stresses observed in bars of negative moment were 15 per cent greater than the average of the stresses used in computing the resisting moment accounted for by these stresses, not counting bars in bands near the 36 ILLINOIS ENGINEERING EXPERIMENT STATION edges of the loaded area, and similarly the maximum stresses in bars at sections of positive moment were 25 per cent greater than the aver- age of the stresses used in computing the positive moment. The measured position of the bars with reference to the face of the slab and the measured thicknesses of the slab were used. The position of the neutral axis was determined from the strains measured in the re- inforcing bars and in the concrete at the face of the slab; knowing this distance, the values of id, the distance from the bar to the center of gravity of the compressive stresses, were computed in the usual way. The value of jd was generally about 0.87 of the distance from the center of gravity of the bars to the face of the slab. Table 3 Resisting Moments Accounted fob by Stresses Obseevbd in Eeinfoecing Bars (Moments are given in pound inches) North and South Sections Negative Moment Positive Moment Width AB Fig. 18 Width BC Width CD 2 760 000 2 720 000 1 690 000 Width IJ Width JO-NK Width KL 560 000 1 570 000 750 000 Total 7 170 000 Total 2 880 000 Width EF 2 950 000 2 890 000 2 030 000 Width FG Width GH Total 7 870 000 Average total 7 520 000 East and V^ ^est Sections Negative Moment Positive Moment Width MN Width NO Width OP 2 190 000 2 950 000 1 680 000 Width QK Width KG-FJ Width JT 600 000 1 340 000 730 000 Total 6 820 000 Total 2 670 000 Table 3 gives the resisting moments accounted for by the stresses observed in the reinforcing bars as calculated by the method described. A comparison of these moments with the values of the analytical moments already given indicates that about 88 per cent of the analyti- TEST OP THE WESTERN NEWSPAPER UNION BUILDING 37 cal negative moment is accounted for by tlie stresses in the rein- forcing bars in the case of the north and south section and 92 per cent in the case of the east and west section. For positive moments 68 per cent of the analytical positive moment is accounted for in the north and south section and 72 per cent in the east and west section. The average percentages for the sections in the two directions are 90 for the negative moment and 70 for the positive moment. The reinforcing bars are not the only source of tensile resistance in the slab ; tensile strength of concrete assists. It is evident that at sections of positive moment tension in the concrete may make up a considerable part of the resistance offered by the slab, and even at sections of negative moment it may have an effect. This tension will be especially influential in regions away from cracks. At points of the sections which are outside the loaded area and where the stresses in the bars are small it may be expected to form a not inconsiderable part of the total resisting moment developed. Of course, if all panels were loaded, the effect of tension in the concrete would be much less, since its greatest proportional effect is outside the loaded area. In making comparisons, it must be kept in mind that the unit- strain computed from the measurements gives the average strain over the gage length and that at a crack the stress in the bar will be more than the average stress over the gage length. Measurements made in beam tests in various laboratories gener- ally show that up to loads near the ultimate load the measured stress in the reinforcing bars is less than that necessary to account for the full bending moment due to the applied load. At the lower stresses the deficiency is considerable; at stresses near the yield point of the steel it may not be much, and the measured stresses may even be larger than necessary to account for the bending moment. The ef- fect is particularly noticeable in concrete of high quality. On the whole the analytical Values of the moments are closely ap- proached, as closely as may be expected in tests of this kind. The negative moment of course is most fully accounted for. The positive moment is not wholly accounted for; but as the section of positive moment involves relatively low stresses the effect of the tensile re- sistance of the concrete on the measured stresses may be considerable, especially in the part of the slab outside the loaded area. It must not be overlooked, also, that the stress in the bars at the cracks will be larger than the average stress over the gage length. 38 ILLINOIS ENGINEERING EXPERIMENT STATION It should be noted that in the case of negative moments the magnitudes of the moments found for the part of the sections ad- jacent to the central column are smaller than those for the corres- ponding parts of the sections adjacent to the outer columns. In the case of column 15, part of this difference is due to the additional amount of reinforcement furnished by the extra laps at this place, and it should be noted too that the larger number of laps at the outer columns makes this portion of the slab stiffer than the portion around the central column. In general, however, most of it appears to be due to the stresses developed in slab and reinforcing bars outside the loaded area, a part of the slab which has not usually been considered to contribute to the resisting moment. In the sections of positive mo- ment, the magnitudes of the resisting moments found are less propor- tionally at the ends of the sections than at the middle, a condition which may be due to the greater proportional effect of the tensile re- sistance developed in the concrete in these regions. The foregoing comparisons have been made on the basis of the full analytical value of the bending moment and by considering one- third of it as positive moment and two-thirds as negative moment. The Joint Committee on Concrete and Reinforced Concrete recom- mended for the sura of the positive and negative moments a value which is about 85 per cent of the analytical value heretofore used and recommended that the distribution be three-eighths positive moment and five-eighths negative moment. It may be of interest to note that the sum of the positive and negative moments accounted for by the measured stresses in the reinforcing bars has nearly the same value as the sum of the moments recommended by the Joint Committee. The negative moments so accounted for are 13 per cent higher than the value recommended by the Joint Committee, and the positive mo- ments are about 73 per cent of the committee's values. In judging of the results, it should be remembered that at the section of positive moments tensile stresses in the concrete have a considerable influence at the stage of the loading indicated by the stresses in the bars in the loaded area and outside of it. The reference to the tensile resistance of the concrete as contributing to the resisting moment of the slab in the test should not be taken to mean that it will be effective as a re- sistance when the ultimate load is reached. It is apparent, also, that requirements less than those of the Joint Committee will not provide for all the moment developed by the bars of the slab. TEST OF THE WESTERN NEWSPAPER UNION BUILDING 39 In making a comparison with methods used in design it should be borne in mind that the principal observed maximum stresses were from 15 to 25 per cent greater than the average of the observed stresses which were used iu computing the resisting moments accounted for by the stresses in the bars ; in designing, a uniform stress over the section is assumed. 14. Bending of Columns. — A few gage lines were placed on columns bordering the loaded area. In Fig. 19 the location of these is shown, together with the load-strain diagrams. Although for some y— ^ Concrete I I /ooo soo aoo 700 600 soo 400 soo too /oo 406 \ 407 Unit .Sfra/n ■ /a per /n. Fig. 19. Load-strain Diagrams for Gage Lines Located on Columns of the gage lines the strains show as tensile strains, it is probable that for the gage lines given as in tension the compressive strain in the column due to the weight of the floors above was sufficient to over- come the tensile strain measured by the instrument. 40 ILLINOIS ENGINEERING EXPERIMENT STATION Tlie direction of the bending was in the direction which would be expected from the condition of loading. For gage line 406 on a column reinforcing bar the deformation at the load of 913 lb. per sq. ft. corresponded to a flexural tensile stress of 10 500 lb. per sq. in. and for gage line 407 on the concrete on the opposite face of the column the strain was 0.00054 in. per in. corresponding to a stress of 2160 lb. per sq. in. with a modulus assumed as 4 000 000 lb. per sq. in. Similarly at gage line 401 on a column reinforcing bar at the east face of column 14 a stress of 8100 lb. per sq. in. was found, and for gage line 403 on the concrete on the opposite face of the column the compressive stress was 1200 lb. per sq. in. based on the same modulus of elasticity of concrete. The highest compressive strain on column 14 was found at gage line 404 on the southwest face of the column. Gage line 402 was placed across an opening which had been cut into the concrete of the column in an unsuccessful effort to ex- pose column bars at this point. A fine crack formed across this gage line. It is seen that the deformation at this gage line is large. Fine cracks formed on the tension side of the columns at or near the juncture of the column shaft and capital. At column 15 a crack was found about half way up the column capital. All the cracks noted were fine cracks — much smaller than those observed on the lower surface of the slab. The amount of flexure in the columns was apparently not suf- ficient to give more than a slight reduction in the proportion of re- sisting moment carried by the slab at the sections at the edges of the loaded area, nor more than a slight increase in the moment carried at the sections through the middle of the loaded area. This view is borne out by calculations of the resisting moment developed at sections of the slab at the boundaries of the loaded area, as accounted for by the observed stresses in the reinforcing bars, and also by the results of analytical determinations of the moments at sections at the edges and the middle of the loaded area. 15. Time Effect of Load and Recovery upon Removal of Load. — As has been stated, two or more sets of strain gage and deflection readings were generally taken after an increment of load had been applied, — the first set immediately after the last of the load had been placed and another set 12 hours thereafter. A third set was taken 66 hours after the load of 913 lb. per sq. ft. had been applied. TEST OF THE WESTERN NEWSPAPER UNION BUILDING 41 The effect of time on tlie strains in the steel and concrete was very small; the reading in a few of the gage lines increased slightly after 12 hours, but generally not enough to allow two points to be plotted on Pig. 11 to 13 and not more than may be considered to be within the error of observation. This was true for both steel and con- crete even in the most highly stressed places at the maximum load after a period of 66 hours had elapsed. The deflection readings were affected but little through the 12- hour period until a load of 637 lb. per sq. ft. was reached; Pig. 8 shows that at this load a marked change in deflection occurred at the centers of panels B, C, and D in the 12 hours' time. At the maxi- mum load the increase in deflection through the 66-hour period was small. Four days were consumed in removing the load. Pour hours after the last of the load was removed, readings were taken on the gage lines and deflection points, and twenty hours later another set was taken. The cracks in both upper and lower surfaces, of the slab had closed so that they were not easily traced. The recovery in de- flection is given by the plotted points at the bottom of the diagrams in Pig. 8. The recovery at the centers of the panels was about 75 per cent ; at other points it was generally greater. The recovery in strains in steel and concrete was also large. Even where high stresses and open cracks had been observed the residual strain in the steel was not more than that to be expected from the lack of interlocking of parti- cles at the cracks. In the region of high compressive stress in the con- crete the recovery at the principal gage lines was 75 per cent or more ; in the reinforcing bars the recovery averaged about 80 per cent. In general the action of the slab was that of concrete of high quality. 16. General Comments. — Although there were differences in the stresses and deflections found at corresponding points in the four loaded panels, these differences can generally be accounted for by (1) difference in the arrangement and number of the reinforcing bars, (2) differences in position of the bars with respect to the surfaces of the slab, and (3) differences in strength and stiffness of concrete in the four panels. The large amount of steel in the region of the column capitals bordering the loaded area (where there were two or three bands lapped) added greatly to the stiffness of the slab in this 42 ILLINOIS ENGINEERING EXPEEIMENT STATION region. The effect of this added stiffness was to cause higher relative stresses to be developed at the columns bordering the loaded area, and relatively large negative resisting moments were developed at col- umns where lapped bars were numerous. Another effect of the lapping of bars at columns and in certain of the rectangular bands was to de- crease the deflection of the slab in the panels affected by the laps. The influence of the position of the reinforcing bars has already been dis- cussed. Higher stresses in reinforcing bars and greater deflections in the slab were found in the panels for which the compression tests showed weaker and less stiff concrete. Cracks, also, were wider and more numerous in these panels. It is apparent that the tensile resist- ance of the concrete at sections of positive moment, particularly near the outer edges of the loaded panels, contributed to the resistance of the slab even at the maximum load. With reference to the design of the slab, it may be noted that the slab was strongly reinforced, though the reinforcing bars were not distributed to the best advantage and the laps were not placed so as to be fully effective. Taking the total reinforcement found over the loaded area and immediately outside, and including such lapped bars as had sufficient anchorage beyond critical sections to be effective (there were many lapped bars which were not counted as effective), and considering the thinness of the slab, the amount of reinforcement for negative moment was on the average as much as that required for the negative moments recommended by the Joint Committee on Con- crete and Reinforced Concrete. The amount of reinforcement avail- able for positive moment was on the average more than 50 per cent greater than that required for the positive moments recommended by the same committee. The distribution of the reinforcing bars was, however, quite different from that recommended by this committee. It may be noted also that the amount of reinforcement was greater than that required by most building regulations. Although the nominal thickness of the slab (8% in.) was less than that required by building regulations, the slab fulfilled the common requirements for compressive and shearing stresses in con- crete of the high quality shown in the tests of prisms taken from the slab. The provisions of the Joint Committee on Concrete and Rein- forced Concrete for bending moments and working stresses in con- crete of 3000 lb. per sq. in. strength give a thickness about the same as the designed thickness of the slab. TEST OF THE WESTERN NEWSPAPER UNION BUILDING 43^ In view of the large number of lapped bars not shown on the building plans, the use of high-carbon steel instead of the mild steel specified, and the unexpectedly high strength of the concrete, it is not strange that the floor carried a much higher load than was antici- pated when the test was begun. 17. Wrecking of the Floor. — As soon as the load was removed, portions of the floor in the four panels of the loaded area were broken into to uncover the reinforcing bars at important sections. Measure- ments of the position and location of the reinforcing bars were made for use in calculation. The thickness of the floor was measured at a number of points to check the values obtained from level readings. Photographs were taken to give a record of the position and location of reinforcing bars and their laps ; Fig. 21 is a sample. As the wrecking of the floor by the contractor progressed, further measurements of the position of the reinforcing bars and their laps, including those in the bands outside of the loaded area, were taken. The wrecking of the building by the contractor was an interest- ing operation. Fires were first built around the columns on the fioor below the one to be wrecked; the effect on the concrete at the base of the column after several hours application of heat was to crack and loosen the concrete shell and expose the reinforcement. To assist in cracking the concrete and separating it from the steel, in many cases water was thrown over the columns after they were well heated. A heavy iron pear-shaped weight (about 1600 pounds) was dropped on the floor immediately over the column capital close to where the column of the story above had been. After the column capital had been sheared and shattered by this operation, the portion of the floor surrounding the column and that directly between columns was broken up with the weight and with sledge hammers. After the con- crete of sections of the floor had been removed in this way, the re- inforcing bars were cut with the oxyacetylene flame. Many of the bars were taken out in good condition. The process was continued until the entire floor was wrecked. The longitudinal reinforcing bars in the column were then cut near the floor below and the columns were pulled down on the floor and broken up with the heavy weight and hammers. 18. Summary. — The following deductions have been drawn in the foregoing presentation of the results of the test : 44 ILLINOIS ENGINEERING EXPERIMENT STATION (1) The tests of samples of the concrete from the slab, as well as the hardness and toughness of the concrete observed in breaking Tip the slab, indicate that the concrete was of unusually good quality and that it had high strength and stiffness. The action of the slab under load was that to be expected with high grade, well-seasoned concrete. The effect of time on the stresses in steel and concrete and on the deflection of the slab under a sustained load was slight, even over a period of 66 hours under the maximum load of 913 lb. per sq. ft., — conditions which would not exist at an early age of concrete. Upon removal of load, the recovery in deflection at the centers of the panels was about 75 per cent of that under load, and at other points generally more ; the recovery in strains in steel and concrete was as large. (2) The position of the important cracks on both upper and lower sides of the slab may be expected to indicate the region of high tensile stresses in the reinforcing bars; it is also an indica- tion of the general action of the slab iu flexure. The cracks on the upper side at the load of 913 lb. per sq. ft. opened to a width of 0.02 to 0.06 in. ; those on the lower side were not so wide. Up- on removal of the load the cracks closed, leaving the surfaces of the slab with the appearance which they had before the load was applied. (3) For reinforcing bars in the upper side of the slab in the regions of negative moment, the stresses in bars of diagonal bands were greater than those in bars of rectangular bands, a stress of 57 300 lb. per sq. in. being observed in a diagonal bar and one of 42 000 lb. per sq. in. in a rectangular bar at the maxi- mum load. Stresses were found in both rectangular and diagonal bars at the columns bordering the loaded area nearly as great as those at corresponding points at the central column. Stresses of some magnitude were found in bars outside the loaded area. The stresses given do not include the stress due to the load of the slab itself. (4) For reinforcing bars in the lower side of the slab in the regions of positive moment, the stresses in bars of rectangular bands were greater than those in bars of diagonal bands even though the former were farther above the lower surface of the slab than the latter; in the one apparent exception, the presence Col ■ -^^^^ ^^i "^A ii^-C Fig. 20. View Showing Arrangement op Eeinporcement at Column 22 Pig. 21. View of Slab after Eeinpoecing Bars were Exposed TEST OF THE WESTERN NEWSPAPEB UNION BUILDINQ 47 of laps doubled the usual number of bars. At the maximum load stresses of 24 000 to 30 000 lb. per sq. in. were observed in bars of rectangular bands and of 20 000 to 24 000 lb. per sq. in. in bars of diagonal bands. A stress of 15 600 lb. per sq. in. was observed in a bar outside the loaded area at the edge of a rec- tangular band. (5) On the upper surface of the slab the greatest com- pressive strains were found at gage lines along the inner panel edges midway between columns, ranging from 0.0009 to nearly 0.001 in. per in. at the maximum load; at the centers of the panels values about half as great were found. On the lower sur- face the greatest strains were found at the middle column; these ranged from 0.0012 to 0.0016 in. per in., values which are as great as the strains found at failure in the tests of the concrete prisms cut from the slab and as great as are ordinarily found in com- pression tests of concrete at the ultimate load. In some places there was chippiag and spalling of the concrete. It is evident that the action of the surrounding concrete assisted in prevent- ing failure. At gage lines crossing the inner panel edges high compressive strains also were found, even though there was no tension reinforcement in the upper side of the slab in this region. It may be noted that the maximum compressive strains on the upper surface of the slab were one-half to three-fifths those found on the lower surface. The intensity of the strains at various points along sections of positive moment and negative moment will give some measure of the distribution of intensity of moments along those sections. (6) The observed stresses in the reinforcing bars accounted for about 90 per cent of the analytical negative moment and about 70 per cent of the analytical positive moment, as given by the methods used. It should be noted that the observed stresses used are average stresses over the gage length, and the stress at a crack may be expected to be greater than the average over the gage length. It seems probable that tensile resistance of the con- crete contributed to the resistance of the slab, particularly in the sections of positive moment and in regions near the edges of the loaded area. A similar influence of the tensile resistance of con- crete, when the stresses in the steel are well below its yield point, has been observed in numerous beam tests. That the tensile re- 48 ILLINOIS ENGINEERING EXPERIMENT STATION sistance of the concrete contributed to the resisting moment of the slab in the test should not be taken to mean that it will be effective in resisting moment when the ultimate load is reached. It may be noted also that the sum of the positive and negative mo- ments accounted for by the measured stresses in the reinforcing bars has almost the same value as the sum of the positive and negative moments recommended by the Joint Committee on Con- crete and Reinforced Concrete, and that the negative moments so accounted for are about 113 per cent and the positive moments about 73 per cent of the moments recommended by this commit- tee. In making a comparison with methods used in designing it should be borne in mind that the principal maximum stresses were from 15 to 25 per cent greater than the average stresses which were used in computing the resisting moments accounted for by the stresses in the bars; in designing, a uniform stress over the section is assumed. Although the arrangement of bars was not as recommended, the amount of reinforcement for negative moment, considering all available bars over the area used, was as much as that required for the negative moments recommended by the Joint Committee on Concrete and Reinforced Concrete, even though the slab was thinner than recommended for ordinary concrete. The amount of reinforcement for positive moment was more than 50 per cent greater than that required for the positive moments recommended by the committee. Although the nominal thickness of the slab was less than that required by building regulations, it fulfilled the provisions of the committee for bending moments and working stresses for concrete of a test strength of 3000 pounds per square inch. (7) The action of the floor slab under test should give added confidence in the suitability and reliability of the flat slab as a load-carrying structure. LIST OF PUBLICATIONS OF THE ENGINEERING EXPERIMENT STATION Bulletin No. /. Teata of Reinforced Concrete Beams, by Arthur N. Talbot. 1904. None azailable. Circular No. 1. High-Speed Tool Steels, by L. P. Breckenridge, 1905. None available. Bulletin No. S. Tests of High-Speed Tool Steels on Cast Iron, by L. P. Breckenridge and Henry B. Dirks. 1905. None available. Circular No. S. Drainage of Earth Roads, by Ira O. Baker. 1906. None available. Circular No, S. Fuel Testa with Illinois Coal (Compiled from tests made by the Technological Branch of the U. S. G. S., at the St. Louis, Mo., Fuel Testing Plant, 1904-1907), by L. P. Breckenridge and Paul Diserens. 1908. Thirty cents. Bulletin No. S. The Engineering Experiment Station of the University of Illinois, by L. P. Breckenridge. 1906. None available. Bulletin No. 4- Tests of Reinforced Concrete Beams, Series of 1905, by Arthur N. Talbot 1906. Forty-five cents. Bulletin No. 5. Resistance of Tubes to Collapse, by Albert P. Carman and M. L. Carr. 1906. None available. . Bulletin No. 6. Holding Power of Railroad Spikes, by Roy I. Webber. 1906. None available. Bulletin No. 7. Fuel Tests with Illinois Coals, by L. P. Breckenridge, S. W. Parr, and Henry B. Dirks. 1906. None available. Bulletin No. 8. Tests of Concrete: I, Shear; II. Bond, by Arthur N. Talbot. 1906. None available. Bulletin No. 9. An Extension of the Dewey Decimal System of Classification Applied to the Engineering Industries, by L. P. Breckenridge and G. A. Goodenough. 1906. Revised Edition 1912. Fifty cents. Bulletin No. 10. 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The Strength of Chain Links, by G. A. Goodenough and L. E. Moore. 1908. Forty cents. Bulletin No. 19. Comparative Testa of Carbon, Metallized Carbon and Tantalum Filament Lamps, by T. H. Amrine. 1908. None available. Bulletin No. SO. Tests of Concrete and Reinforced Concrete Columns, Series of 1907, by Arthur N. Talbot. 1908. None available. Bulletin No. Bl. Tests of a Liquid Air Plant, by C. S. Hudson and C. M. Garland. 1908. Fifteen cents. Bulletin No. 82. Tests of Cast-iron and Reinforced Concrete Cidvert Pipe, by Arthur N. Talbot. 1908. None available. Bulletin No. 23. Voids, Settlement and Weight of Crushed Stone, by Ira O. Baker. 1908. Fifteen cents. *Bulletin No. S4' The Modification of Illinois Coal by Low Temperature Distillation, by S. W. Parr and C. K. Francis. 1908. Thirty cerUs. Bulletin No. 25, Lighting Country Homes by Private Electric Plants, by T. H. Amrine. 1908 Twenty cents. *A limited number of copies of bulletins starred is available for free distribution. 49 50 PUBLICATIONS OF THE ENGINEERING EXPERIMENT STATION Bulletin No. 26. High Steam-Pressures in Locomotive Service. A Review of a Report to the Carnegie Institution of Washington, by W. F. M. Goas. 1908. Twenty-five cents. Bulletin No. B7. Tests of Brick Columns and Terra Cotta Block Columns, by Arthur N. Talbot and Duff A. Abrams. 1909. Twenty-five cents. Bulletin No. 28. A Teat of Three Large Reinforced Concrete Beams, by Arthur N. Talbot. 1909. Fifteen cents. Bulletin No. 29. Tests of Reinforced Concrete Beams: Resistance to Web Stresses, Series of 1907 and 1908, by Arthur N. Talbot. 1909. Forty-five cents. *Bulletin No. 30. On the Rate of Formation of Carbon Monoxide in Gas Producers, by J. K. Cle- ment, L. H. Adams, and C. N. Haskins. 1909. Twenty-five cents. ^Bulletin No. SI. Tests with House-Heating Boilers, by J. M. Snodgrass. 1909. Fifty-five cents. 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Bulletin No, 41. Tests of Timber Beams, by Arthur N. Talbot. 1910. Thirty-five cents. ^Bulletin No. 4^. The Effect of Keyways on the Strength of Shafts, by Herbert F. Moore. 1910. Ten cents. Bulletin No. 43. Freight Train Resistance, by Edward C. Schmidt. 1910. Seventy-five cents. Bulletin No. 44. An Investigation of Built-up Columns Under Load, by Arthur N. Talbot and Herbert F. Moore, 1911. Thirty- five cents. ^Bulletin No. 45. The Strength of Oxyacetylene Welds in Steel, by Herbert L. Whittemore. 1911. Thirty-five cents. =^Bulletin No. 46. The Spontaneous Combustion of Coal, by S. W. Parr and F. W. Kressman. 1911. Forty-five cents. ^Bulletin No. 47. Magnetic Properties of Heusler Alloys, by Edward B. Stephenson. 1911. Tw^i~ ty-five cents. ^Bulletin No. 48. Resistance to Flow Through Locomotive Water Columns, by Arthur N. Talbot and Melvin L. Enger. 1911. Forty cents. *Bulletin No. 49. Testa of Nickel-Steel Riveted Joints, by Arthur N. Talbot and Herbert F. Moore. 1911. Thirty cents. *Bulletin No. 60. Tests of a Suction Gas Producer, by C. M. Garland and A. P. Kratz. 1912. Fifty cents. Bulletin No. 51. Street Lighting, by J. M. Bryant and H. G. Hake. 1912. Thirty-five cents. ^Bulletin No. 62. An Investigation of the Strength of Rolled Zinc, by Herbert F. Moore, 1912, Fifteen cents. ^Bulletin No. 53. Inductance of Coils, by Morgan Brooks and H. M. Turner. 1912. Forty cents. ^Bulletin No. 54' Mechanicfl.1 Stresses in Transmission Lines, by A. Guell. 1912. Twenty cents. ^Bulletin No. 56. Starting Currents of Transformers, with Special Reference to Transformers with Silicon Steel Cores, by Trygve D. Yensen. 1912. Twenty cents. ^Bulletin No. 56. Tests of Columns: An Investigation of the Value of Concrete as Reinforcement for Structural Steel Columns, by Arthur N. Talbot and Arthur R. Lord. 1912. Twenty-five cents. ^Bulletin No. 67. Superheated Steam in Locomotive Service. A Review of Publication No. 127 of the Carnegie Institution of Washington, by W. F. M. Goss. 1912. Forty cents. *A limited number of copies of bulletins starred is available for free distribution. PUBLICATIONS OF THE ENGINEERING EXPERIMENT STATION 51 *Bullelin No. 68. A New Analysis of the Cylinder Performance of Reciprocating Engines, by J. Paul Clayton. 1912. Sixty cents. ^Bulletin No. 69. The Effect of Cold Weather Upon Train Resistance and Tonnage Rating, by Edward C. Schmidt and F. W. Marquis. 1912. Twenty cents. *BuUetin No. 60, The Coking of Coal at Low Temperatures, with a Preliminary Study of the By-Products, by S. W. Parr and H. L, Olin. 1912. Twenty-five cents. * Bulletin No. 61. Characteristics and Limitation of the Series Transformer, by A. R. Anderson and H. R. Woodrow. 1913. Twenty-five cents. Bulletin No. 6S. The Electron Theory of Magnetism, by Elmer H. Williams. 1913. Thirty-five cents. Bulletin No. 63. Entropy-Temperature and Transmission Diagrams for Air, by C. R. Richards. 1913. 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Wind Stresses in the Steel Frames of Office Buildings, by W. M. Wilson and G. A. Maney. 1915. Fifty cents. *Bulletin No. 81. Influence of Temperature on the Strength of Concrete, by A. B. McDaniel. 1915. Fifteen cents. Bulletin No. 88. Laboratory Tests of a Consolidation Locomotive, by E. C. Schmidt, J. M. Snod- grass, and R. B. Keller. 1915. Sixty-five cents. *Bulletin No. 83. Magnetic and Other Properties of Iron-Silicon Alloys. Melted in Vacuo, by Trygve D. Yensen. 1915. Thirty-five cents. Bulletin No. 84. Testa of Reinforced Concrete Flat Slab Structure, by Arthur N. Talbot and W. A. Slater. 1916. Sixty-five cents. ^Bulletin No. 86. The Strength and Stiffness of Steel Under Biaxial Loading, by A. T. Becker 1916. Thirty-five cents. ^Bulletin No. 86. The Strength of I-Beams and Girders, by Herbert F. Moore and W. M. Wilson. 1916. Thirty cents. *Bulletin No. 87. Correction of Echoes in the Auditorium, University of Illinois, by F. R. Watson and J. M, White. 1916. Fifteen cents. 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Magnetic and Other Properties of Iron-Aluminum Alloys Melted in Vacuo, by T. D. Yensen and W. A. Gatward. 1917. Twenty-five cents. ^Bulletin No. 96. The Effect of Mouthpieces on the Flow of Water Through a Submerged Short Pipe, by Fred B. Seely. 1917. Twenty-five cents. ^Bulletin No. 97. Effects of Storage Upon the Properties of Coal, by S. W. Parr. 1917. Twenty cents. "^Bulletin No. 98. Tests of Oxyacetylene Welded Joints in Steel Plates, by Herbert F. Moore. 1917. Ten cents. Circular No. 4- The Economical Purchase and Use of Coal for Heating Homes, with Special Reference to Conditions in Illinois. 1917. Ten cents. *Bulletin No. 99. The Collapse of Short Thin Tubes, by A. P. Carman. 1917. Twenty cents. *CiTcular No. 5. The Utilization of Pyrite Occurring in Illinois Bituminous Coal, by E. A. Holbrook. 1917. Twenty cents. ^Bulletin No. 100. Percentage of Extraction of Bituminous Coal with Special Reference to Illinois Conditions, by C. M. Young. 1917. '^Bulletin No. 101. Comparative Tests of Six Sizes of Illinois Coal on a Mikado Locomotive, by E. C. Schmidt, J. M. Snodgrass, and O. S. Beyer, Jr. 1917. Fifty cents. * Bulletin No. 102, A Study of' the Heat Transmission of Building Materials, by A. C. Willard and L. C. Lichty. 1917. Twenty-five cents. '^Bulletin No. 103. An Investigation of Twist Drills, by Bruce W. Benedict and W. P. Lukens. 1917. Sixty cents. *Bulletin No. 104- Tests to Determine the Rigidity of Riveted Joints of Steel Structures, by W. M. Wilson and H. F. Moore. 1917. Twenty-five cents. Circular No. 6. The Storage of Bituminous Coal, by H. H. Stoek. 1918. Forty cents. Circular No. 7. Fuel Economy in the Operation of Hand Fired Power Plants. 1918. Twenty cents. '^Bulletin No. 106. Hydraulic Experiments with Valves, Orifices, Hoze, Nozzles, and Orifice Buckets, by Arthur N. Talbot, F. B. Seely, V. R. Fleming and M. L. Enger. 1918. Thirty-five cents. *Bulletin No. 106. Test of a Flat Slab Floor of the Western Newspaper Union Building, by Arthur N. Talbot and Harrison F. Gonnerman. 1918. Twenty cents. *A limited number of copies of bulletins starred is available for free distribution. THE -UNIVEESITiY OF ILLINOIS;: THE* STATE UNR^RSITY "'" Urbana EbMTJND J. JAJIBS, Ph.D., LL.D., President T^ UNIVERSITY INCLUDES THE FOLLOWING DEPARTMENTS The Graduate ^cttoi^; The College of LiBeral Arts and' Sciences (Ancient and 'Modem Languages and rByuuuiugjr,.iJuui;ai/iuii, JHJ.»wJciiiixi.niD, .miMuuuiijjf , vjrcuuvi^, jriiya' try; Botany, Zqology, Entomology; Physiology; ^ArtvandJSBsign) The College of Commerce and Business Administration (General Business, Bank- ■ ig, fiwurance, Accountancy, Railway,, Administration, Foi^K.' 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The Courses in Chemistry and Chemical Engineering The Scl^l of Railway Engineering and Administration IThe School of Music (four years' course) The School of Library Science (two years' course); The College of Medicine (in Chicago) The College of Dentistry (in Chicago) - iliiSchQpl of Pharmacy (in Chicago; Ph. G. and Ph. C. courses) .--■- The Summer Session (eight weeks) fc^S^^fe^* Stations ^^d^Scientific Bureaus: ^U. S. *p^eult\iral Experiment !^?!^ Station; Engineering Experiment Station; State Laboratory of Natural His- ■' tory; State TEiJiBSffiS B lJI jf s Office; Biologieai Experiment Station -(># Illinois River; State water SSTCy; State Geological Snrv^; U. S. Bureau' of Mines Experirnfent Station. , :' 'f ■., X The bbrary collections contain .(December 1, 1^17) 411,737 volumes and 104,524 pamphlets. , .; t^ , For catalogs and information iiddress r.^ '•■' ^,- 'fc'^^^i ' ■ " ■ ' '•'^'t'.. '^™ "tU&GISTRAR ^^ ' ■' ■ 'v-S^'V ,;„ UBskNA, Illinok TA 445 Tj5™"""'™""y Library JmmmlfJlV *'''' "°°'' <" ^>^^ Western 3 1924 015 389 566