1、SP-86-2ACI 435.8R-85(Reapproved 1991) (Reapproved 1997)Observed Deflections of Reinforced ConcreteSlab Systems, and Causes of Large DeflectionsBy ACI Committee 435D. R. Buettner*ChairmanS. K. Ghosh*Chairman, Subcommittee on FieldMeasurementsD. E. Branson S. V. Kulkarni A. Scanlon*R. G. Drysdale* M.
2、S. Mirza* J. M. SpangA. Farah E. G. Nawy* M. K. TadrosA. B. Gogate M. V. Pregnoff A. F. ShaikhJ. Grossman G. M. Sabnis* S. ZundelevichC-T. T. Hsu C. G. Salmon*Members of the Subcommittee on Field Measurements which preparedthis report.*Please see Preface for the entire 435 membership.Synopsis: This
3、report is in two distinct parts.Part I is a summary of published studies on slab deflections(3 from Australia, 1 from Scotland, 1 from Sweden, 2 from U.S.).The summary focuses on construction practices and materialsquality. Comparison of deflections calculated by various methodswith actual long-term
4、 deflections is made in some cases.Part II summarizes several construction problems and mate-rial deficiencies which can contribute to large long-term deflec-tions. Focusing on large construction loads, the authors showCopyright 1985, American Concret e Institute.12 ACI Committee 435that constructio
5、n loads may be considerably higher than designloads and that high construction loads cause high initial deflec-tions because concrete has a lower modulus of elasticity whenloaded at an early age. Furthermore, concrete creeps more whenit is loaded at an early age, thereby causing additional highlong-
6、term deflections, even when construction loads are sus-tained only for a few days.The authors then suggest a method of form removal andreshoring that has proved successful in the New York City areain preventing large slab deflections. Essentially, no more thanan 8-foot slab span is left unsupported
7、until a slab is mature.A reader interested only in the Committees findings andrecommendations may proceed straight to Part II of the report.; concrete slabs creepproperties; deflection; flat concrete plates; form removal;loads (forces); modulus of elasticity; reinforced concrete;shorinq; shrinkage;
8、two-way slabs.Contents:Part IFIELD DEFLECTION MEASUREMENTS OF REINFORCED CONCRETE FLAT PLATES,FLAT SLABS AND BEAMS: A REVIEW OF LITERATUREInvestigation A (Commonwealth Scientific and Industrial ResearchOrganization, Melbourne, Australia - experimentalflat plate structures)Investigation B (Jenkins, P
9、lowman and Haseltine - Scottishapartment building)Investigation C (Army Engineer Waterways Experiment Station,Vicksburg, Mississippi - Army barracks flatplate structure)Investigation D (Taylor, Heiman - five Sydney area buildings)Investigation E (Chalmers University, Goteborg Sweden - twoapartment h
10、ouses)Investigation F (Jenkins - Australian flat plate building)Investigation G (Sbarounis -multistory flat plate building)-IL-Slab Systems and Large DeflectionsPart IIFACTORS CONTRIBUTING TO DEFLECTION PROBLEMS IN TWO-WAYREINFORCED CONCRETE SLABSFactors Contributing to Slab Deflection ProblemsLoads
11、 During ConstructionProperties of Concrete at Early AgesCreep of Concrete Loaded at Early AgesControl of Slab DeflectionsSummary and Conclusions3ACI Committee 435Part IFIELD DEFLECTION MEASUREMENTS OF REINFORCED CONCRETEFLAT PLATES, FLAT SLABS AND BEAMS: A REVIEW OF LITERATUREThis part of the report
12、 reviews and summarizes the existingliterature on field deflection measurements of reinforced con-crete flat plates,flat slabs and beams.INVESTIGATION ASummaryThree experimental flat plat structures were erected at theDivision of Building Research, Commonwealth Scientific andIndustrial Research Orga
13、nization, Melbourne, Australia. Theinvestigations were carried out under field conditions, thestructures being completely exposed to the weather.Structure Mark I consisted of an expanded shale concreteslab, 3-l/2 in. thick, spanning three bays of 9 ft in one direc-tion and three bays of 12 ft in the
14、 other, with cantilevers 4 ft6 in. long in this direction. The reinforcement was conventionalindividual plain round bars and was designed by the empiricaldesign method given in ACI 318-56. The slab was carried on 16steel columns of box section with a grillage type shear connec-tion. The significant
15、features of this structure were (1) span/depth ratios of 41 in one direction and 31 in the other; (2) theratio 4:3 of the sides of the panels; and (3) the steel columns.of lightweight aggregate concrete was also an importantFurther, no edge beams or torsion reinforcement nearthe edge columns was use
16、d.The long-term deflections reached “annoying“ proportions.The slab was allowed to stand under its own weight for 8 months,during which time the deflection at the center of the middlepanel increased by O.62 in. This was 12 times the initial elas-tic deflection of O.O5 in. In a study of the long-term
17、 deforma-tion of this structure it was suggested that about 2O% of theincrease at the center of the middle panel was due to differen-tial settlement of inner and outer columns, about 4O% was due tofurther cracking causing a reduction in stiffness, and to localbond slip, and about 4O% to creep. This
18、analysis also suggestedthat the increment of deflection due to creep was about 85% ofthe immediate deflection of a completely cracked slabIn connection with the large long-term deformations, threefeatures were pointed out. First, the structure was constructedof expanded shale concrete. Available evi
19、dence suggests that inSlab Systems and Large Deflections5concrete made with well-coated, expanded shale aggregate, thecreep may in certain cases be 20% greater than for natural rockconcrete at the same stress, which would be an insignificantcontribution in this case. Secondly, the experimental struc
20、turewas built in the summer, and during its early history was exposedregularly to high ambient temperatures and direct sunlight. Ithas been shown that creep is directly proportional to tempera-ture, for set cement pastes. Finally, since the structure wasoutdoors, completely exposed, it was under wid
21、ely fluctuatingconditions of temperature and relative humidity. Creep andshrinkage under fluctuating conditions have been shown to begreater than under constant average conditions of storage.Structure Mark II had 9-ft spans over two bays in one direc-tion and three bays in the other. Cantilevers 3 f
22、t long extendedin the two-span direction. The slab was of expanded shale con-crete and was intended to be 4-in. thick, but because of distor-tion of the formwork it was much thicker in some places. Theconcrete, supplied by an outside contractor, contained in errorsome dense basalt in addition to the
23、 expanded shale aggregate.These two factors combined to make the slab much stiffer than wasintended and useless for studies of deformation. No attempt,therefore, was made to examine its deflection under imposed load-ing, and it was tested directly to destruction.StructureMark III, probably the first
24、 prestressed, post-tensioned flatplate in Australia, was allowed to stand underits own weightto obtain data on loss of prestress.ReferencesA.la. “Experimental Flat Plate Structure of Expanded Shale Con-crete,“Constructional Review (Sydney), Vol. 33, No. 2,Feb. 196O, pp. 22-29.A.lb.“Experimental Ligh
25、tweight Flat Plate Structure, Part I -Measurements and Observations During Construction,“ Con-structional Review (Sydney), Vol. 34, No. 1, Jan. 1961,pp. 21-32.A.lc.“Experimental Lightweight Flat Plate Structure, Part II -Deformations due to Self Weight,“ Constructional Review(Sydney), Vol. 34, No. 3
26、, Mar. 1961, pp. 25-33.A.ld.“Experimental Lightweight Flat Plate Structure, Part III -Long-Term Deformations,“Constructional Review (Sydney),Vol. 34, No. 4, Apr. 1961, pp. 21-26.6 ACI Committee 435A.le.“Experimental Lightweight flat Plate Structures, Part IV -Design and Erection of Structures with C
27、oncrete Columns,“Constructional Review (Sydney), Vol. 35, No. 1, Jan. 1962,pp. 29-33.A.lf. Beresford, F.D.,“Experimental Lightweight Flat PlateStructure, Part V- Deformations under Lateral Load,“Constructional Review (Sydney), Vol. 35, No. 2, Dec. 1962,pp. 17-23.A.lg. Lewis, R.K.,“Experimental Light
28、weight Flat PlateStructure, Part VI- Design and Erection of aPost-tensioned Flat Plate,“ Constructional Review(Sydney), Vol. 36, No. 3, Mar. 1963, pp. 21-24.A.lh. Beresford, F.D., and Blakey, F.A., “ExperimentalLightweight Flat Plate Structure, Part VII - A Test toDestruction,“Constructional Review
29、(Sydney), Vol. 36, No.6, June 1963, pp. 18-26.A.2. Blakey, F.A.,“Deformations of an Experimental LightweightFlat Plate Structure,“Civil Engineering Transactions(Sydney), Institution of Engineers Australia, Vol. CE3,No. 1, Mar. 1961, pp. 18-22.A.3. Blakey, F.A.,“Australian Experiments with Flat Plate
30、s,“ACI Journal Proceedings Vol. 6O, No. 4, Apr. 1963, pp.- .A.4. Blakey, F.A. “The Deflection of Flat Plate Structures,“Civil Engineering and Public Works Review, Vol. 58, Sept.1963, pp. 1133-1136.INVESTIGATION BSummaryThe paper (Ref. B.l) describes large deflections found inelectrically heated rein
31、forced concrete floor slabs in a con-siderable number of Scottish apartment buildings and gives thebasic reasons why such deflections occurred. The slabs weresupported on three sides on load-bearing walls, and were freealong the fourth edge. The slabs had noticeable deflectionsalong their free edges
32、. Some of the deflections were up to1.25 in. in a clear span of 12 ft 5-l/2 in.The paper describes a number of laboratory tests to ascer-tain the shrinkage of the aggregates used for constructing thefloors and the influence on concrete made with such aggregatesand similar sands. The results of modul
33、us of elasticity testsare given. An investigation into the temperature and deflectionSlab Systems and Large Deflectionscharacteristics with or without live load of a typical apartmentfloor is described and the results are given, together withexamples of deflection readings from other apartment floor
34、s andcore crushing results.Full-scale laboratory tests were set up using pairs of slabscast with shrinkable Scottish and unshrinkable flint aggregateconcretes operating at a range of temperatures.The conclusions drawn were as follows:(1) Shrinkage of Aggregates- Almost all the aggregates testedfrom
35、the lowlands of Scotland gave rise to higher concreteshrinkage than are expected from good aggregate, e.g. flint.Depending on the degree of this shrinkage, the deflection ofmembers constructed with such aggregates will be greater thanwhen unshrinkable ones are used.(2) Effect of Floor Heating- Under
36、-floor heating caused partialdrying out of the slabs and was responsible for a small part ofthe deflections that had occurred.(3) Modulus of Elasticity - It is essential that designers checkthe deflection likely to occur in a beam or slab by consideringthe instantaneous E and the anticipated shrinka
37、ge and creepvalues of the concrete being used, and assessing a value for theeffective E which can be used in the normal formulae. The long-term deflection of the test apartment floor was approximately 12times the estimated instantaneous deflection under dead and liveload. In the full-scale laborator
38、y tests the movement at thetime of writing was up to approximately 8 times the instantaneousvalues.(4) Use of Top Reinforcement -The full-scale tests showed thata considerable reduction can be made in the deflections of slabsby use of a suitable quantity of top reinforcement. In continu-ous slabs a
39、large amount of top steel is used for continuity andthis is, no doubt, partly the reason such slabs suffer fewerproblems. Cantilevers are usually unsatisfactory.(5) Span/Depth Ratios - The results obtained point very stronglyto the need for a close reexamination of the recommended valuesof span/dept
40、h ratios given in many codes of practice.Underconditions where the full design live load is likely to besupported for long periods,especially when combined with simplysupported spans,a significant reduction in such permitted ratiosmay be necessary.8ACI Committee 435ReferenceB.l Jenkins, R.A.S.,Plowm
41、an, J.M. and Haseltine, B.A.,“Investigation into the Causes of the Deflection of HeatedConcrete Floors, Including Shrinkage,“ The StructuralEngineer (London), Vol. 43, No. 4., April 1965, pp. 1O5-117.INVESTIGATION CSummaryThe report (Ref. C.1)investigation to determineand concrete strains in anFord
42、Hood, Killeen, Texas.summarizes the results of a fieldthe short- and long-time deflectionsArmy barracks flat-plate structure atDue to the rather great slab thickness of 9 in., correspond-ing to a span-to-depth ratio of approximately 28, all observeddeflections were small and in no instance exceeded
43、O.O22 ft, orabout l/8OO of the shorter span,during the 45-month observationperiod, in spite of an early temporary construction load esti-mated to have been almost 3O percent in excess of the totaldesign load.The measured short-time deflections under various loadingconditions compared reasonably well
44、 with deflections predictedby use of the Ersatz frame analysis method (as proposed byVanderbilt, Sozen and Siess).ReferenceC.l Geymayer, H.G., and McDonald, J.E., “Short-and Long-timeDeflections of Reinforced Concrete Flab Slabs,“ TechnicalReport C-7O-1, U.S. Army Engineer Waterways ExperimentStatio
45、n, Vicksburg, Mississippi, February 1970, 9 pp. plus4 tables with 17 figs.C.2 Vanderbilt, M.D., Soren, M.A., and Siess, C.P., “Deflectionsof Reinforced Concrete Floor Slabs,“ Structural ResearchSeries No. 263, Civil Engineering Department, University ofIllinois, Urbana, Illinois, April 1963.Slab Sys
46、tems and Large Deflections9INVESTIGATION DSummaryField deflection measurements were taken on five buildingsin and around Sydney, Australia for periods of up to 9 years.The buildings and some of the results of these investigationsare described below. Some recurring design and constructionproblems, po
47、inted up by these investigations, are enumerated.Structure I (Refs. D.l, D.2, D.3, Building No. 1 ofRef. D.8 “Taylors Flat Plate“ of Ref. D-9, Structure 1 ofRef. D.1O Building 1 of Ref. D.11) - The dimensions are shownin Fig. la. The typical interior panel of 2O ft 1O in. x 16 ft8 in. with the 8 in.
48、 thick plate gives a longer span to depthratio of 31. The slab was designed for a dead load of 110 psfand superimposed load of 75 psf. The concrete design strengthwas f(= 3 ksi.bars fy minTypical reinforcement was of hard grade deformed= 5O ksi); each mid-panel had a layer of top rein-forcement of w
49、elded wire fabric (fy subscript = 7O ksi). The deflec-tion behavior of the slab was expressed by the equation:Long term deflection=A+B+C+D+E+Fwhere A =B=C=D=E =F =initial elastic deflection (13% of total) caused byslab dead load on removal of props.long-term elastic deflection (l-1/2%) caused bysuperimposed loads and finishes, without producingareas of cracked section in the slab.initial cracking deflection (l/2%) due to productionof cracked sections in the concrete slab at the timeof prop removal.long-term cracking deflection (19%) due to trans-formation of slab from untracked to partia
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