《ACI-435.8R-1985-R1997.pdf》由会员分享,可在线阅读,更多相关《ACI-435.8R-1985-R1997.pdf(47页珍藏版)》请在三一文库上搜索。
1、SP-86-2ACI 435.8R-85 (Reapproved 1991) (Reapproved 1997) Observed Deflections of Reinforced Concrete Slab Systems, and Causes of Large Deflections By ACI Committee 435 D. R. Buettner* Chairman S. K. Ghosh* Chairman, Subcommittee on Field Measurements D. E. Branson S. V. Kulkarni A. Scanlon* R. G. Dr
2、ysdale* M. S. Mirza* J. M. Spang A. Farah E. G. Nawy* M. K. Tadros A. B. Gogate M. V. Pregnoff A. F. Shaikh J. Grossman G. M. Sabnis* S. Zundelevich C-T. T. Hsu C. G. Salmon *Members of the Subcommittee on Field Measurements which prepared this report. *Please see Preface for the entire 435 membersh
3、ip. Synopsis: This 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 materials quality. Comparison of deflections calculated by various metho
4、ds with actual long-term 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 show Copyright 1985, American Concrete Institute. 2 ACI Co
5、mmittee 435 that construction loads may be considerably higher than design loads and that high construction loads cause high initial deflec- tions because concrete has a lower modulus of elasticity when loaded at an early age. Furthermore, concrete creeps more when it is loaded at an early age, ther
6、eby causing additional high long-term deflections, even when construction loads are sus- tained only for a few days. The authors then suggest a method of form removal and reshoring that has proved successful in the New York City area in preventing large slab deflections. Essentially, no more than an
7、 8-foot slab span is left unsupported until a slab is mature. A reader interested only in the Committees findings and recommendations may proceed straight to Part II of the report. ; concrete slabs;creep properties; deflection; flat concrete plates; form removal; loads (forces); modulus of elasticit
8、y; reinforced concrete; shorinq; shrinkage; two-way slabs. Contents: Part I FIELD DEFLECTION MEASUREMENTS OF REINFORCED CONCRETE FLAT PLATES, FLAT SLABS AND BEAMS: A REVIEW OF LITERATURE Investigation A (Commonwealth Scientific and Industrial Research Organization, Melbourne, Australia - experimenta
9、l flat plate structures) Investigation B (Jenkins, Plowman and Haseltine - Scottish apartment building) Investigation C (Army Engineer Waterways Experiment Station, Vicksburg, Mississippi - Army barracks flat plate structure) Investigation D (Taylor, Heiman - five Sydney area buildings) Investigatio
10、n E (Chalmers University, Goteborg Sweden - two apartment houses) Investigation F (Jenkins - Australian flat plate building) Investigation G (Sbarounis -multistory flat plate building) -IL- Slab Systems and Large Deflections Part II FACTORS CONTRIBUTING TO DEFLECTION PROBLEMS IN TWO-WAY REINFORCED C
11、ONCRETE SLABS Factors Contributing to Slab Deflection Problems Loads During Construction Properties of Concrete at Early Ages Creep of Concrete Loaded at Early Ages Control of Slab Deflections Summary and Conclusions 3 ACI Committee 435 Part I FIELD DEFLECTION MEASUREMENTS OF REINFORCED CONCRETE FLA
12、T PLATES, FLAT SLABS AND BEAMS: A REVIEW OF LITERATURE This part of the report reviews and summarizes the existing literature on field deflection measurements of reinforced con- crete flat plates, flat slabs and beams. INVESTIGATION A Summary Three experimental flat plat structures were erected at t
13、he Division of Building Research, Commonwealth Scientific and Industrial Research Organization, Melbourne, Australia. The investigations were carried out under field conditions, the structures being completely exposed to the weather. Structure Mark I consisted of an expanded shale concrete slab, 3-l
14、/2 in. thick, spanning three bays of 9 ft in one direc- tion and three bays of 12 ft in the other, with cantilevers 4 ft 6 in. long in this direction. The reinforcement was conventional individual plain round bars and was designed by the empirical design method given in ACI 318-56. The slab was carr
15、ied on 16 steel columns of box section with a grillage type shear connec- tion. The significant features of this structure were (1) span/ depth ratios of 41 in one direction and 31 in the other; (2) the ratio 4:3 of the sides of the panels; and (3) the steel columns. of lightweight aggregate concret
16、e was also an important Further, no edge beams or torsion reinforcement near the edge columns was used. 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 middle panel increa
17、sed by O.62 in. This was 12 times the initial elas- tic deflection of O.O5 in. In a study of the long-term deforma- tion of this structure it was suggested that about 2O% of the increase at the center of the middle panel was due to differen- tial settlement of inner and outer columns, about 4O% was
18、due to further cracking causing a reduction in stiffness, and to local bond slip, and about 4O% to creep. This analysis also suggested that the increment of deflection due to creep was about 85% of the immediate deflection of a completely cracked slab In connection with the large long-term deformati
19、ons, three features were pointed out. First, the structure was constructed of expanded shale concrete. Available evidence suggests that in Slab Systems and Large Deflections5 concrete made with well-coated, expanded shale aggregate, the creep may in certain cases be 20% greater than for natural rock
20、 concrete at the same stress, which would be an insignificant contribution in this case. Secondly, the experimental structure was built in the summer, and during its early history was exposed regularly to high ambient temperatures and direct sunlight. It has been shown that creep is directly proport
21、ional to tempera- ture, for set cement pastes. Finally, since the structure was outdoors, completely exposed, it was under widely fluctuating conditions of temperature and relative humidity. Creep and shrinkage under fluctuating conditions have been shown to be greater than under constant average co
22、nditions of storage. Structure Mark II had 9-ft spans over two bays in one direc- tion and three bays in the other. Cantilevers 3 ft long extended in 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 w
23、as much thicker in some places. The concrete, supplied by an outside contractor, contained in error some dense basalt in addition to the expanded shale aggregate. These two factors combined to make the slab much stiffer than was intended and useless for studies of deformation. No attempt, therefore,
24、 was made to examine its deflection under imposed load- ing, and it was tested directly to destruction. StructureMark III, probably the first prestressed, post- tensioned flat plate in Australia, was allowed to stand under its own weight to obtain data on loss of prestress. References A.la. “Experim
25、ental Flat Plate Structure of Expanded Shale Con- crete,“ Constructional Review (Sydney), Vol. 33, No. 2, Feb. 196O, pp. 22-29. A.lb.“Experimental Lightweight Flat Plate Structure, Part I - Measurements and Observations During Construction,“ Con- structional Review (Sydney), Vol. 34, No. 1, Jan. 196
26、1, pp. 21-32. A.lc.“Experimental Lightweight Flat Plate Structure, Part II - Deformations due to Self Weight,“ Constructional Review (Sydney), Vol. 34, No. 3, Mar. 1961, pp. 25-33. A.ld.“Experimental Lightweight Flat Plate Structure, Part III - Long-Term Deformations,“ Constructional Review (Sydney)
27、, Vol. 34, No. 4, Apr. 1961, pp. 21-26. 6 ACI Committee 435 A.le.“Experimental Lightweight flat Plate Structures, Part IV - Design and Erection of Structures with Concrete Columns,“ Constructional Review (Sydney), Vol. 35, No. 1, Jan. 1962, pp. 29-33. A.lf. Beresford, F.D., “Experimental Lightweight
28、 Flat Plate Structure, Part V- Deformations under Lateral Load,“ Constructional Review (Sydney), Vol. 35, No. 2, Dec. 1962, pp. 17-23. A.lg. Lewis, R.K.,“Experimental Lightweight Flat Plate Structure, Part VI- Design and Erection of a Post-tensioned Flat Plate,“ Constructional Review (Sydney), Vol.
29、36, No. 3, Mar. 1963, pp. 21-24. A.lh. Beresford, F.D., and Blakey, F.A., “Experimental Lightweight Flat Plate Structure, Part VII - A Test to Destruction,“Constructional Review (Sydney), Vol. 36, No. 6, June 1963, pp. 18-26. A.2. Blakey, F.A.,“Deformations of an Experimental Lightweight Flat Plate
30、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 Plates,“ ACI Journal Proceedings Vol. 6O, No. 4, Apr. 1963, pp. - . A.4. Blakey, F.A. “The Deflection of Flat Plate Str
31、uctures,“ Civil Engineering and Public Works Review, Vol. 58, Sept. 1963, pp. 1133-1136. INVESTIGATION B Summary The paper (Ref. B.l) describes large deflections found in electrically heated reinforced concrete floor slabs in a con- siderable number of Scottish apartment buildings and gives the basi
32、c reasons why such deflections occurred. The slabs were supported on three sides on load-bearing walls, and were free along the fourth edge. The slabs had noticeable deflections along their free edges. Some of the deflections were up to 1.25 in. in a clear span of 12 ft 5-l/2 in. The paper describes
33、 a number of laboratory tests to ascer- tain the shrinkage of the aggregates used for constructing the floors and the influence on concrete made with such aggregates and similar sands. The results of modulus of elasticity tests are given. An investigation into the temperature and deflection Slab Sys
34、tems and Large Deflections characteristics with or without live load of a typical apartment floor is described and the results are given, together with examples of deflection readings from other apartment floors and core crushing results. Full-scale laboratory tests were set up using pairs of slabs
35、cast with shrinkable Scottish and unshrinkable flint aggregate concretes operating at a range of temperatures. The conclusions drawn were as follows: (1) Shrinkage of Aggregates- Almost all the aggregates tested from the lowlands of Scotland gave rise to higher concrete shrinkage than are expected f
36、rom good aggregate, e.g. flint. Depending on the degree of this shrinkage, the deflection of members constructed with such aggregates will be greater than when unshrinkable ones are used. (2) Effect of Floor Heating- Under-floor heating caused partial drying out of the slabs and was responsible for
37、a small part of the deflections that had occurred. (3) Modulus of Elasticity - It is essential that designers check the deflection likely to occur in a beam or slab by considering the instantaneous E and the anticipated shrinkage and creep values of the concrete being used, and assessing a value for
38、 the effective E which can be used in the normal formulae. The long- term deflection of the test apartment floor was approximately 12 times the estimated instantaneous deflection under dead and live load. In the full-scale laboratory tests the movement at the time of writing was up to approximately
39、8 times the instantaneous values. (4) Use of Top Reinforcement -The full-scale tests showed that a considerable reduction can be made in the deflections of slabs by use of a suitable quantity of top reinforcement. In continu- ous slabs a large amount of top steel is used for continuity and this is,
40、no doubt, partly the reason such slabs suffer fewer problems. Cantilevers are usually unsatisfactory. (5) Span/Depth Ratios - The results obtained point very strongly to the need for a close reexamination of the recommended values of span/depth ratios given in many codes of practice. Under condition
41、s where the full design live load is likely to be supported for long periods, especially when combined with simply supported spans,a significant reduction in such permitted ratios may be necessary. 8ACI Committee 435 Reference B.l Jenkins, R.A.S.,Plowman, J.M. and Haseltine, B.A., “Investigation int
42、o the Causes of the Deflection of Heated Concrete Floors, Including Shrinkage,“ The Structural Engineer (London), Vol. 43, No. 4., April 1965, pp. 1O5-117. INVESTIGATION C Summary The report (Ref. C.1) investigation to determine and concrete strains in an Ford Hood, Killeen, Texas. summarizes the re
43、sults of a field the short- and long-time deflections Army barracks flat-plate structure at Due to the rather great slab thickness of 9 in., correspond- ing to a span-to-depth ratio of approximately 28, all observed deflections were small and in no instance exceeded O.O22 ft, or about l/8OO of the s
44、horter span, during the 45-month observation period, in spite of an early temporary construction load esti- mated to have been almost 3O percent in excess of the total design load. The measured short-time deflections under various loading conditions compared reasonably well with deflections predicte
45、d by use of the Ersatz frame analysis method (as proposed by Vanderbilt, Sozen and Siess). Reference C.l Geymayer, H.G., and McDonald, J.E., “Short-and Long-time Deflections of Reinforced Concrete Flab Slabs,“ Technical Report C-7O-1, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Missi
46、ssippi, February 1970, 9 pp. plus 4 tables with 17 figs. C.2 Vanderbilt, M.D., Soren, M.A., and Siess, C.P., “Deflections of Reinforced Concrete Floor Slabs,“ Structural Research Series No. 263, Civil Engineering Department, University of Illinois, Urbana, Illinois, April 1963. Slab Systems and Larg
47、e Deflections 9 INVESTIGATION D Summary Field deflection measurements were taken on five buildings in and around Sydney, Australia for periods of up to 9 years. The buildings and some of the results of these investigations are described below. Some recurring design and construction problems, pointed
48、 up by these investigations, are enumerated. Structure I (Refs. D.l, D.2, D.3, Building No. 1 of Ref. D.8 “Taylors Flat Plate“ of Ref. D-9, Structure 1 of Ref. D.1O Building 1 of Ref. D.11) - The dimensions are shown in Fig. la. The typical interior panel of 2O ft 1O in. x 16 ft 8 in. with the 8 in.
49、 thick plate gives a longer span to depth ratio of 31. The slab was designed for a dead load of 110 psf and superimposed load of 75 psf. The concrete design strength was f ( = 3 ksi. bars fy min Typical reinforcement was of hard grade deformed = 5O ksi); each mid-panel had a layer of top rein- forcement of welded 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+F where A = B = C = D = E = F = initial elastic deflection (13% of total) caused by slab dead load on removal of props. long-term elastic defl
链接地址:https://www.31doc.com/p-3728211.html