ENVIRONMENTAL ENGINEERING CONCRETE STRUCTURES

1、ENVIRONMENTAL ENGINEERING CONCRETE STRUCTURESCE 498 Design ProjectSeptember 26, 2006OUTLINEINTRODUCTIONPERFORMANCE CRITERIADESIGN LOADS AND CONDITIONSSTRUCTURAL DESIGNCONCRETE MIX DESIGNADDITIONAL CRITERIAINTRODUCTIONWhy concrete?Concrete is particularly suited for this application because it will n

2、ot warp or undergo change in dimensionsWhen properly designed and placed it is nearly impermeable and extremely resistant to corrosionHas good resistance to natural and processing chemicalsEconomical but requires significant quality controlWhat type of structure?Our focus will be conventionally rein

3、forced cast-in-place or precast concrete structures Basically rectangular and/or circular tanksNo prestressed tanksINTRODUCTIONHow should we calculate loads?Design loads determined from the depth and unit weight of retained material (liquid or solid), the external soil pressure, and the equipment to

4、 be installedCompared to these loads, the actual live loads are smallImpact and dynamical loads from some equipmentsWhat type of analysis should be done? The analysis must be accurate to obtain a reasonable picture of the stress distribution in the structure, particularly the tension stressesComplic

5、ated 3D FEM analysis are not required. Simple analysis using tabulated results in handbooks etc. PERFORMANCE CRITERIAWhat are the objective of the design?The structure must be designed such that it is watertight, with minimum leakage or loss of contained volume. The structure must be durable it must

6、 last for several years without undergoing deteriorationHow do you get a watertight structure?Concrete mix design is well-proportioned and it is well consolidated without segregationCrack width is minimizedAdequate reinforcing steel is usedImpervious protective coating or barriers can also be usedTh

7、is is not as economical and dependable as the approach of mix design, stress & crack control, and adequate reinforcem.PERFORMANCE CRITERIAHow to design the concrete mix?The concrete mix can be designed to have low permeability by using low water-cement ratio and extended periods of moist curingUse w

8、ater reducing agents and pozzolans to reduce permeability.How to reduce cracking? Cracking can be minimized by proper design, distribution of reinforcement, and joint spacing.Shrinkage cracking can be minimized by using joint design and shrinkage reinforcement distributed uniformlyPERFORMANCE CRITER

9、IAHow to increase durability?Concrete should be resistant to the actions of chemicals, alternate wetting and drying, and freeze-thaw cyclesAir-entrainment in the concrete mix helps improve durability. Add air-entrainment agentsReinforcement must have adequate cover to prevent corrosionAdd good quali

10、ty fly-ash or pozzolansUse moderately sulphate-resistant cement DESIGN LOADS AND CONDITIONSAll the loads for the structure design can be obtained from ASCE 7 (2006), which is the standard for minimum design loads for building structures endorsed by IBCContent loadsRaw Sewage 63 lb/ft3Grit from grit

11、chamber . 110 lb/ft3Digested sludge aerobic. 65 lb/ft3Digested sludge anerobic 70 lb/ft3For other numbers see ACI 350. Live loadsCatwalks etc 100 lb/ft2Heavy equipment room 300 lb/ft2 DESIGN LOADS AND CONDITIONSWhen using the LRFD (strength or limit states design approach), the load factors and comb

12、inations from ACI 318 can be used directly with one major adjustmentThe load factors for both the lateral earth pressure H and the lateral liquid pressure F should be taken as 1.7The factored load combination U as prescribed in ACI 318 must be increased by durability coefficients developed from crac

13、k width calculation methods: In calculations for reinforcement in flexure, the required strength should be 1.3 UIn calculations for reinforcement in direct tension, including hoop tension, the required strength should be 1.65 UThe required design strength for reinforcement in shear should be calcula

14、ted as fVs 1.3 (Vu-fVc)For compression use 1.0 USTRUCTURAL DESIGNLarge reinforced concrete reservoirs on compressible soil may be considered as beams on elastic foundations. Sidewalls of rectangular tanks and reservoirs can be designed as either: (a) cantilever walls fixed at the bottom, or (b) wall

15、s supported at two or more edges.Circular tanks normally resist the pressure from contents by ring tensionWalls supporting both interior water loads and exterior soil pressure must be designed to support the full effects of each load individuallyCannot use one load to minimize the other, because som

16、etimes the tank is empty.STRUCTURAL DESIGNLarge diameter tanks expand and contract appreciably as they are filled and drained. The connection between wall and footing should either permit these movements or be strong enough to resist them without crackingThe analysis of rectangular wall panels suppo

17、rted at three or four sides is explained in detail in the PCA publication that is available in the library and on hold for the courseIt contains tabulated coefficients for calculating stress distributions etc. for different boundary conditions and can be used directly for designIt also includes some

18、 calculation and design examplesSTRUCTURAL DESIGNReinforced concrete walls at least 10 ft. high that are in contact with liquids should have a minimum thickness of 12 in. The minimum thickness of any minor member is 6 in., and when 2 in. cover is required then it is at least 8 in. For crack control,

19、 it is preferable to use a large number of small diameter bars for main reinforcement rather than an equal are of larger barsMaximum bar spacing should not exceed 12 in. The amount of shrinkage and temperature reinforcement is a function of the distance between joints in the directionShrinkage and t

20、emperature reinforcement should not be less thank the ratios given in Figure 2.5 or ACI 350The reinforcement should not be spaced more than 12 in. and should be divided equally between the two surfacesSTRUCTURAL DESIGNFigure showing minimum shrinkage reinforcement and table showing minimum cover for

21、 reinforcement requiredSTRUCTURAL DESIGNIn order to prevent leakage, the strain in the tension reinforcement has to be limitedThe strain in the reinforcing bars is transferred to the surrounding concrete, which cracks. Hence, minimizing the stress and strain in the reinforcing bar will minimize crac

22、king in the concrete.Additionally, distributing the tension reinforcement will engage a greater area of the concrete in carrying the strain, which will reduce cracking even more. The strength design requires the use of loads, load combinations and durability coefficients presented earlierSTRUCTURAL

23、DESIGNServiceability for normal exposuresFor flexural reinforcement located in one layer, the quantity Z (crack control factor of ACI) should not exceed 115 kips/in.The designer can use the basic Gergley-Lutz equation for crack width for one way flexural members.The reinforcement for two-way flexura

24、l member may be proportioned in each direction using the above recommendation too. Alternate design by the working stress method with allowable stress values given and tabulated in ACI 350. Do not recommend this method for us. STRUCTURAL DESIGNImpact, vibration, and torque issuesWhen heavy machines

25、are involved, an appropriate impact factor of 1.25 can be used in the designMost of the mechanical equipment such as scrapers, clarifiers, flocculators, etc. are slow moving and will not cause structural vibrationsMachines that cause vibration problems are forced-draft fans and centrifuges for dewat

26、ering clarifier sludge or digester sludgeThe key to successful dynamic design is to make sure that the natural frequency of the support structure is significantly different from frequency of disturbing forceSTRUCTURAL DESIGNTo minimize resonant vibrations, ratio of the natural frequency of the struc

27、ture to the frequency of the disturbing force must not be in the range of 0.5 to 1.5. It should preferably be greater than 1.5Methods for computing the structure frequency are presented in ACI 350 (please review if needed)Torque is produced in most clarifiers where the entire mechanism is supported

28、on a central columnThis column must be designed to resist the torque shear without undergoing failureMATERIAL DESIGNThe cement should conform to:Portland cement ASTM C150, Types I, IA, II, IIA, .Blended hydraulic cement ASTM C595Expansive hydraulic cement ASTM C845They cannot be used interchangeably

29、 in the same structureSulfate-resistant cement must have C3A content not exceeding 8%. This is required for concrete exposed to moderate sulfate acctak (150 to 1000 ppm)Portland blast furnace slab cement (C595 may be used)Portland pozzolan cement (C595 IP) can also be usedBut, pozzolan content not e

30、xceed 25% by weight of cementitous materialsMATERIAL DESIGNThe air entraining admixture should conform to ASTM C260Improves resistant to freeze-thaw cyclesImproves workability and less shrinkageIf chemical admixtures are used, they should meet ASTM C494. The use of water reducing admixtures is recom

31、mendedThe maximum water-soluble chloride ion content, expressed as a % of cement, contributed by all ingredients of the concrete mix should not exceed 0.10%MATERIAL DESIGNMix proportioning all material should be proportioned to produce a well-graded mix of high density and workability28 day compress

32、ive strength of 3500 psi where the concrete is not exposed to severe weather and freeze-thaw28 day compressive strength of 4000 psi where the concrete is exposed to severe weather and freeze-thawType of cement as mentioned earlierMaximum water-cement ratio = 0.45If pozzolan is used, the maximum wate

33、r-cement + pozzolan ratio should be 0.45Minimum cementitious material content1.5 in. aggregate max 517 lb/yd31 in. aggregate max 536 lb/yd30.75 in. aggregate max 564 lb/yd3MATERIAL DESIGNAir entrainment requirements 5.5 1 % for 1.5 in. aggregate6.0 1 % for 1.0 or 0.75 in. aggregateSlump requirements

34、1 in. minimum and 4 in. maximumConcrete placement according to ACI 350 (read when you get a chance)Curing using sprinkling, ponding, using moisture retaining covers, or applying a liquid membrane-forming compound seal coatMoist or membrane curing should commence immediately after form removalADDITIO

35、NAL CRITERIAConcrete made with proper material design will be dense, watertight, and resistant to most chemical attack. Under ordinary service conditions, it does not require additional protection against chemical deterioration or corrosionReinforcement embedded in quality concrete is well protected

36、 against corrosive chemicalsThere are only special cases where additional protective coatings or barriers are requiredThe steel bars must be epoxy coated (ASTM A775)In special cases, where H2S evolves in a stagnant unventilated environment that is difficult or uneconomical to correct or clean regula

37、rly, a coating may be requiredREFERENCESACI 350 (1989)Books on reserve in the libraryEmails from Jeffrey Ballard, structural engineer, HNTB. He will visit to talk with us soon.ENVIRONMENTAL ENGINEERING CONCRETE STRUCTURESCE 498 Design ProjectNovember 16, 21, 2006OUTLINEINTRODUCTIONLOADING CONDITIONS

38、DESIGN METHODWALL THICKNESSREINFORCEMENTCRACK CONTROLINTRODUCTIONConventionally reinforced circular concrete tanks have been used extensively. They will be the focus of our lecture todayStructural design must focus on both the strength and serviceability. The tank must withstand applied loads withou

39、t cracks that would permit leakage.This is achieved by:Providing proper reinforcement and distributionProper spacing and detailing of construction jointsUse of quality concrete placed using proper construction proceduresA thorough review of the latest report by ACI 350 is important for understanding

40、 the design of tanks. LOADING CONDITIONSThe tank must be designed to withstand the loads that it will be subjected to during many years of use. Additionally, the loads during construction must also be considered.Loading conditions for partially buried tank. The tank must be designed and detailed to

41、withstand the forces from each of these loading conditionsLOADING CONDITIONSThe tank may also be subjected to uplift forces from hydrostatic pressure at the bottom when empty.It is important to consider all possible loading conditions on the structure. Full effects of the soil loads and water pressu

42、re must be designed for without using them to minimize the effects of each other. The effects of water table must be considered for the design loading conditions. DESIGN METHODSTwo approaches exist for the design of RC membersStrength design, and allowable stress design.Strength design is the most c

43、ommonly adopted procedure for conventional buildingsThe use of strength design was considered inappropriate due to the lack of reliable assessment of crack widths at service loads. Advances in this area of knowledge in the last two decades has led to the acceptance of strength design methods The rec

44、ommendations for strength design suggest inflated load factors to control service load crack widths in the range of 0.004 0.008 in. Design MethodsService state analyses of RC structures should include computations of crack widths and their long term effects on the structure durability and functional

45、 performance.The current approach for RC design include computations done by a modified form of elastic analysis for composite reinforced steel/concrete systems.The effects of creep, shrinkage, volume changes, and temperature are well known at service levelThe computed stresses serve as the indices

46、of performance of the structure. DESIGN METHODSThe load combinations to determine the required strength (U) are given in ACI 318. ACI 350 requires two modificationsModification 1 the load factor for lateral liquid pressure is taken as 1.7 rather than 1.4. This may be over conservative due to the fac

47、t that tanks are filled to the top only during leak testing or accidental overflowModification 2 The members must be designed to meet the required strength. The ACI required strength U must be increased by multiplying with a sanitary coefficient The increased design loads provide more conservative d

48、esign with less cracking.Required strength = Sanitary coefficient X UWhere, sanitary coefficient = 1.3 for flexure, 1.65 for direct tension, and 1.3 for shear beyond the capacity provided by the concrete. WALL THICKNESSThe walls of circular tanks are subjected to ring or hoop tension due to the inte

49、rnal pressure and restraint to concrete shrinkage. Any significant cracking in the tank is unacceptable.The tensile stress in the concrete (due to ring tension from pressure and shrinkage) has to kept at a minimum to prevent excessive cracking. The concrete tension strength will be assumed 10% fc in

50、 this document.RC walls 10 ft. or higher shall have a minimum thickness of 12 in. The concrete wall thickness will be calculated as follows:WALL THICKNESSEffects of shrinkageFigure 2(a) shows a block of concrete with a re-bar. The block height is 1 ft, t corresponds to the wall thickness, the steel

51、area is As, and the steel percentage is r. Figure 2(b) shows the behavior of the block assuming that the re-bar is absent. The block will shorten due to shrinkage. C is the shrinkage per unit length.Figure 2(c) shows the behavior of the block when the re-bar is present. The re-bar restrains some sho

52、rtening. The difference in length between Fig.2(b) and 2(c) is xC, an unknown quantity. WALL THICKNESSThe re-bar restrains shrinkage of the concrete. As a result, the concrete is subjected to tension, the re-bar to compression, but the section is in force equilibriumConcrete tensile stress is fcs =

53、xCEcSteel compressive stress is fss= (1-x)CEsSection force equilibrium. So, rfss=fcsSolve for x from above equation for force equilibriumThe resulting stresses are:fss=CEs1/(1+nr)and fcs=CEsr/(1+nr)The concrete stress due to an applied ring or hoop tension of T will be equal to:T * Ec/(EcAc+EsAs) =

54、T * 1/Ac+nAs = T/Ac(1+nr)The total concrete tension stress = CEsAs + T/Ac+nAsWALL THICKNESSThe usual procedure in tank design is to provide horizontal steel As for all the ring tension at an allowable stress fs as though designing for a cracked section. Assume As=T/fs and realize Ac=12tSubstitute in

55、 equation on previous slide to calculate tension stress in the concrete. Limit the max. concrete tension stress to fc = 0.1 fcThen, the wall thickness can be calculated as t = CEs+fsnfc/12fcfs* TThis formula can be used to estimate the wall thicknessThe values of C, coefficient of shrinkage for RC i

56、s in the range of 0.0002 to 0.0004. Use the value of C=0.0003Assume fs= allowable steel tension =18000 psiTherefore, wall thickness t=0.0003 TWALL THICKNESSThe allowable steel stress fs should not be made too small. Low fs will actually tend to increase the concrete stress and potential cracking. Fo

57、r example, the concrete stress = fc = CEs+fs/Acfs+nT*TFor the case of T=24,000 lb, n=8, Es=29*106 psi, C=0.0003 and Ac=12 x 10 = 120 in3If the allowable steel stress is reduced from 20,000 psi to 10,000 psi, the resulting concrete stress is increased from 266 psi to 322 psi. Desirable to use a highe

58、r allowable steel stress. REINFORCEMENTThe amount size and spacing of reinforcement has a great effect on the extent of cracking. The amount must be sufficient for strength and serviceability including temperature and shrinkage effectsThe amount of temperature and shrinkage reinforcement is dependen

59、t on the length between construction jointsREINFORCEMENTThe size of re-bars should be chosen recognizing that cracking can be better controlled by using larger number of small diameter bars rather than fewer large diameter barsThe size of reinforcing bars should not exceed #11. Spacing of re-bars sh

60、ould be limited to a maximum of 12 in. Concrete cover should be at least 2 in. In circular tanks the locations of horizontal splices should be staggered by not less than one lap length or 3 ft. Reinforcement splices should confirm to ACI 318Chapter 12 of ACI 318 for determining splice lengths. The l