1、ENVIRONMENTAL ENGINEERING CONCRETE STRUCTURES,CE 498 Design Project September 26, 2006,OUTLINE,INTRODUCTIONPERFORMANCE CRITERIADESIGN LOADS AND CONDITIONSSTRUCTURAL DESIGNCONCRETE MIX DESIGNADDITIONAL CRITERIA,INTRODUCTION,Why concrete? Concrete is particularly suited for this application because it
2、 will not warp or undergo change in dimensions When properly designed and placed it is nearly impermeable and extremely resistant to corrosion Has good resistance to natural and processing chemicals Economical but requires significant quality controlWhat type of structure? Our focus will be conventi
3、onally reinforced cast-in-place or precast concrete structures Basically rectangular and/or circular tanks No prestressed tanks,INTRODUCTION,How should we calculate loads? Design loads determined from the depth and unit weight of retained material (liquid or solid), the external soil pressure, and t
4、he equipment to be installed Compared to these loads, the actual live loads are small Impact 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 tensio
5、n stresses Complicated 3D FEM analysis are not required. Simple analysis using tabulated results in handbooks etc.,PERFORMANCE CRITERIA,What 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 mus
6、t be durable it must 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 segregation Crack width is minimized Adequate reinforcing steel is used Impervious protective coating or bar
7、riers can also be used This is not as economical and dependable as the approach of mix design, stress & crack control, and adequate reinforcem.,PERFORMANCE CRITERIA,How to design the concrete mix? The concrete mix can be designed to have low permeability by using low water-cement ratio and extended
8、periods of moist curing Use water 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 distribut
9、ed uniformly,PERFORMANCE CRITERIA,How to increase durability? Concrete should be resistant to the actions of chemicals, alternate wetting and drying, and freeze-thaw cycles Air-entrainment in the concrete mix helps improve durability. Add air-entrainment agents Reinforcement must have adequate cover
10、 to prevent corrosion Add good quality fly-ash or pozzolans Use moderately sulphate-resistant cement,DESIGN LOADS AND CONDITIONS,All 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 IBC Content l
11、oads Raw Sewage 63 lb/ft3 Grit from grit chamber 110 lb/ft3 Digested sludge aerobic. 65 lb/ft3 Digested sludge anerobic 70 lb/ft3 For other numbers see ACI 350. Live loads Catwalks etc 100 lb/ft2 Heavy equipment room 300 lb/ft2,DESIGN LOADS AND CONDITIONS,When using the LRFD (strength or limit state
12、s design approach), the load factors and combinations from ACI 318 can be used directly with one major adjustment The load factors for both the lateral earth pressure H and the lateral liquid pressure F should be taken as 1.7 The factored load combination U as prescribed in ACI 318 must be increased
13、 by durability coefficients developed from crack width calculation methods: In calculations for reinforcement in flexure, the required strength should be 1.3 U In calculations for reinforcement in direct tension, including hoop tension, the required strength should be 1.65 U The required design stre
14、ngth for reinforcement in shear should be calculated as fVs 1.3 (Vu-fVc) For compression use 1.0 U,STRUCTURAL DESIGN,Large 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: (
15、a) cantilever walls fixed at the bottom, or (b) walls supported at two or more edges. Circular tanks normally resist the pressure from contents by ring tension Walls supporting both interior water loads and exterior soil pressure must be designed to support the full effects of each load individually
16、 Cannot use one load to minimize the other, because sometimes the tank is empty.,STRUCTURAL DESIGN,Large 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 with
17、out cracking The analysis of rectangular wall panels supported at three or four sides is explained in detail in the PCA publication that is available in the library and on hold for the course It contains tabulated coefficients for calculating stress distributions etc. for different boundary conditio
18、ns and can be used directly for design It also includes some calculation and design examples,STRUCTURAL DESIGN,Reinforced 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.
19、cover is required then it is at least 8 in. For crack control, it is preferable to use a large number of small diameter bars for main reinforcement rather than an equal are of larger bars Maximum bar spacing should not exceed 12 in. The amount of shrinkage and temperature reinforcement is a function
20、 of the distance between joints in the direction Shrinkage and temperature reinforcement should not be less thank the ratios given in Figure 2.5 or ACI 350 The reinforcement should not be spaced more than 12 in. and should be divided equally between the two surfaces,STRUCTURAL DESIGN,Figure showing
21、minimum shrinkage reinforcement and table showing minimum cover for reinforcement required,STRUCTURAL DESIGN,In order to prevent leakage, the strain in the tension reinforcement has to be limited The strain in the reinforcing bars is transferred to the surrounding concrete, which cracks. Hence, mini
22、mizing the stress and strain in the reinforcing bar will minimize cracking 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, loa
23、d combinations and durability coefficients presented earlier,STRUCTURAL DESIGN,Serviceability for normal exposures For 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 cra
24、ck width for one way flexural members. The reinforcement for two-way flexural 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 u
25、s.,STRUCTURAL DESIGN,Impact, vibration, and torque issues When heavy machines are involved, an appropriate impact factor of 1.25 can be used in the design Most of the mechanical equipment such as scrapers, clarifiers, flocculators, etc. are slow moving and will not cause structural vibrations Machin
26、es that cause vibration problems are forced-draft fans and centrifuges for dewatering clarifier sludge or digester sludge The key to successful dynamic design is to make sure that the natural frequency of the support structure is significantly different from frequency of disturbing force,STRUCTURAL
27、DESIGN,To minimize resonant vibrations, ratio of the natural frequency of the structure 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 1.5 Methods for computing the structure frequency are presented in ACI 350 (please review if n
28、eeded)Torque is produced in most clarifiers where the entire mechanism is supported on a central column This column must be designed to resist the torque shear without undergoing failure,MATERIAL DESIGN,The cement should conform to: Portland cement ASTM C150, Types I, IA, II, IIA, . Blended hydrauli
29、c cement ASTM C595 Expansive hydraulic cement ASTM C845 They cannot be used interchangeably 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 (C59
30、5 may be used) Portland pozzolan cement (C595 IP) can also be used But, pozzolan content not exceed 25% by weight of cementitous materials,MATERIAL DESIGN,The air entraining admixture should conform to ASTM C260 Improves resistant to freeze-thaw cycles Improves workability and less shrinkage If chem
31、ical admixtures are used, they should meet ASTM C494. The use of water reducing admixtures is recommended The 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 DESIGN,Mix proportioning all mater
32、ial should be proportioned to produce a well-graded mix of high density and workability 28 day compressive strength of 3500 psi where the concrete is not exposed to severe weather and freeze-thaw 28 day compressive strength of 4000 psi where the concrete is exposed to severe weather and freeze-thaw
33、Type of cement as mentioned earlier Maximum water-cement ratio = 0.45 If pozzolan is used, the maximum water-cement + pozzolan ratio should be 0.45 Minimum cementitious material content 1.5 in. aggregate max 517 lb/yd3 1 in. aggregate max 536 lb/yd3 0.75 in. aggregate max 564 lb/yd3,MATERIAL DESIGN,
34、Air entrainment requirements 5.5 1 % for 1.5 in. aggregate 6.0 1 % for 1.0 or 0.75 in. aggregate Slump requirements 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 liqu
35、id membrane-forming compound seal coat Moist or membrane curing should commence immediately after form removal,ADDITIONAL CRITERIA,Concrete made with proper material design will be dense, watertight, and resistant to most chemical attack. Under ordinary service conditions, it does not require additi
36、onal protection against chemical deterioration or corrosion Reinforcement embedded in quality concrete is well protected against corrosive chemicals There are only special cases where additional protective coatings or barriers are required The steel bars must be epoxy coated (ASTM A775) In special c
37、ases, where H2S evolves in a stagnant unventilated environment that is difficult or uneconomical to correct or clean regularly, a coating may be required,REFERENCES,ACI 350 (1989) Books on reserve in the library Emails from Jeffrey Ballard, structural engineer, HNTB. He will visit to talk with us so
38、on.,ENVIRONMENTAL ENGINEERING CONCRETE STRUCTURES,CE 498 Design Project November 16, 21, 2006,OUTLINE,INTRODUCTIONLOADING CONDITIONSDESIGN METHODWALL THICKNESSREINFORCEMENTCRACK CONTROL,INTRODUCTION,Conventionally reinforced circular concrete tanks have been used extensively. They will be the focus
39、of our lecture today Structural design must focus on both the strength and serviceability. The tank must withstand applied loads without cracks that would permit leakage. This is achieved by: Providing proper reinforcement and distribution Proper spacing and detailing of construction joints Use of q
40、uality concrete placed using proper construction procedures A thorough review of the latest report by ACI 350 is important for understanding the design of tanks.,LOADING CONDITIONS,The tank must be designed to withstand the loads that it will be subjected to during many years of use. Additionally, t
41、he loads during construction must also be considered. Loading conditions for partially buried tank. The tank must be designed and detailed to withstand the forces from each of these loading conditions,LOADING CONDITIONS,The tank may also be subjected to uplift forces from hydrostatic pressure at the
42、 bottom when empty. It is important to consider all possible loading conditions on the structure. Full effects of the soil loads and water pressure 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 cond
43、itions.,DESIGN METHODS,Two approaches exist for the design of RC members Strength design, and allowable stress design. Strength design is the most commonly adopted procedure for conventional buildings The use of strength design was considered inappropriate due to the lack of reliable assessment of c
44、rack 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 recommendations for strength design suggest inflated load factors to control service load crack widths in the range of 0.004 0.008 in.,Design Methods,Serv
45、ice state analyses of RC structures should include computations of crack widths and their long term effects on the structure durability and functional performance. The current approach for RC design include computations done by a modified form of elastic analysis for composite reinforced steel/concr
46、ete systems. The effects of creep, shrinkage, volume changes, and temperature are well known at service level The computed stresses serve as the indices of performance of the structure.,DESIGN METHODS,The load combinations to determine the required strength (U) are given in ACI 318. ACI 350 requires
47、 two modifications Modification 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 fact that tanks are filled to the top only during leak testing or accidental overflow Modification 2 The members must be designed to meet the requir
48、ed strength. The ACI required strength U must be increased by multiplying with a sanitary coefficient The increased design loads provide more conservative design with less cracking.Required strength = Sanitary coefficient X UWhere, sanitary coefficient = 1.3 for flexure, 1.65 for direct tension, and
49、 1.3 for shear beyond the capacity provided by the concrete.,WALL THICKNESS,The walls of circular tanks are subjected to ring or hoop tension due to the internal pressure and restraint to concrete shrinkage. Any significant cracking in the tank is unacceptable. The tensile stress in the concrete (du
50、e 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 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:,
