Chapter 5 Bridge Deck Slabs. Bridge Engineering 1

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1 Chapter 5 Bridge Deck Slabs Bridge Engineering 1

2 Basic types of bridge decks In-situ reinforced concrete deck- (most common type) Pre-cast concrete deck (minimize the use of local labor) Open steel grid deck Orthotropic steel deck Timber deck Bridge Engineering 2

3 In-situ reinforced concrete deck Bridge Engineering 3

4 Pre-cast concrete deck Bridge Engineering 4

5 Open steel grid deck Bridge Engineering 5

6 Orthotropic-steel deck Bridge Engineering 6

7 Timber deck Bridge Engineering 7

8 Bridge Deck Slab Background A) AASHTO , Distribution of Loads and Design of Concrete Slabs Main reinforcement perpendicular to traffic S + 2 M = P20;2 S 24, ft 32 S M = P18;0.61 S 7.315, m 9.74 ft-lb/ft or kn-m/m P 20 Where is 16,000 lb for H20 or HS20 loading, is 72 kn for M18 or MS18 loading and S is the effective span length P 18 Bridge Engineering 8

9 Distribution reinforcement Bridge Engineering 9

10 Bridge Deck Slab Main reinforcement parallel to traffic Distribution of wheel loads E=4+0.06S or ( S) maximum 7.0 ft (2.134m) HS20 (MS18) loading Span up to and including 50 ft (15.24 m) LLM = 900 S ft-lb (13.14 S kn-m) Span 50 ft to 100 ft (15.24 to m) LLM= 1000(1.30S-20.0) ft-lb 14.6(1.3 S-6.1) kn-m Bridge Engineering 10

11 Bridge Deck Slab. The above design method developed based on Westergaard theory, (Ref. below) Computation of Stresses in Bridge Slabs Due to Wheel Loads, Public Roads, March, 1930 Bridge Engineering 11

12 Bridge Deck Slab B) CSA-CAN3-S6-M78 CAN/CSA-S6-88 Similar to AASHTO Above methods very conservative Substantial strength enhancement Arching action developed in system C) CHBDC Limit States Design 1) Serviceability limit states i) deformations (8.13.1) ii) vibration (3.4.4) iii) control of cracking (8.12.3) Bridge Engineering 12

13 Bridge Deck Slab 1) Serviceability limit states i) deformations (8.13.1) Deflections and rotations occurring immediately upon the application of loads shall be determined by elastic methods using the value of E c at the time of loading and considering the effects of cracking and reinforcement. Bridge Engineering 13

14 Bridge Deck Slab 1) Serviceability limit states ii) vibration (3.4.4) Superstructures, other than for long span bridges, shall be proportioned so that the maximum deflection due to the factored traffic load, including the dynamic load allowance, does not exceed the limit given in Figure (A) for the anticipated degree of pedestrian use. The deflection limit state shall apply at the centre of the sidewalk or, if there is no sidewalk, at the inside face of the barrier. Bridge Engineering 14

15 Bridge Deck Slab An approved method shall be used to ensure that vibration likely to occur in normal use will not cause discomfort or concern to users of a pedestrian bridge. Bridge Engineering 15

16 Figure (A) Bridge Engineering 16

17 Bridge Deck Slab 1) Serviceability limit states iii) control of cracking (8.12.3). Bridge Engineering 17

18 Bridge Deck Slab 2) Fatigue limit state, reinforcing bars ( ) Stress range in straight bars shall not exceed 125MPa Stress range at anchorages, connections and bends shall not exceed 65 MPa Tack welding of primary reinforcement shall not permitted Stress range in the vicinity of welds shall not exceed 100 MPa For other types of welded splices, the stress range shall not exceed 65 MPa Bridge Engineering 18

19 The ultimate limit state strength (or stability) All sections of the slab shall be proportioned to have factored resistance that are at least equal to the sum of the force effects of the factored loads. Besides satisfying equilibrium and compatibility, the following assumptions are considered i) strains in bars and concrete proportional to the distance from neutral axis ii) maximum usable strain at the extreme compression fiber is iii) Stress in steel, 1) E s times steel strain if stress is less than f y 2) f y if strain is more than yield strain Bridge Engineering 19

20 The ultimate limit state strength (or stability) iv) Concrete has negligible tensile strength in calculation of flexural and axial tensile resistance v) stress-strain distribution pattern is as follows Bridge Engineering 20

21 Bridge Engineering 21 The ultimate limit state strength (or stability) For reinforced concrete slabs, the factored resistance may be calculated by: Where, = Φ 2 a d f A M y s u b f f A a c y s ' α 1 = b f f A a f A ba f T C c y s y s c ' 1 ' 1 α α = = = s c Φ Φ =0.75 =0.90 Material resistance factors }

22 Methods of analysis Yield line method Westergaard theory Influence line Grillage analogy Orthotropic plate theory Folded plate method Finite element and finite strip method Bridge Engineering 22

23 Yield Line Method The principal is similar to that of the plastic design theory of steel frames Reflects the true behavior at ultimate limit state Especially for existing bridges It is a crack in a reinforced bridge, along which the reinforcement has yielded The section must be under-reinforced (as required by bridge design codes) Helps find Moments at the plane of failure Load at which the slab fails Gives an upper bound solution Bridge Engineering 23

24 Yield Line Method Characteristics: Yield lines are straight. Axes of rotation pass along lines of support Axes of rotation pass over columns. A yield line dividing two slab parts must pass through the point of intersection of the axes of rotation of the two parts. Yield lines must end at slab boundary Simple supports attract positive or sagging yield lines while continuous supports do the opposite. Equilibrium or virtual work method Bridge Engineering 24

25 Yield Line Method Virtual work method Upper bound Study many failure patterns Choose the pattern with highest moment or least load Energy dissipation at yield line External work by loads D( l, θ, M ) E( p, V ) θ Where, l is the length of yield line, is the rotation of the yield line and M is the moment of resistance per unit length. P represents the external load and V is the volume between the deflected surfaces and the original plane of the slab. Bridge Engineering 25

26 Yield Line Method Bridge Engineering 26

27 Yield Line Method W e ( P ) = pδdxdy = e i = ( mblθ ) ( P ) ( m lθ ) W = c b The virtual work gives an upper bound to the failure load P or lower bound to resistance moment M. So, try many patterns and select the lowest P or highest M. Reference: Wood, R.H. Plastic and Elastic Analysis of Slabs and Plates, Thames and Hudson, London, Bridge Engineering 27

28 Behavior of a restrained slab When subjected to a concentrated load, a restrained slab goes through these stages: 1) Development of fixed boundary action 2) Cracking 3) Development of compressive membrane action, if the slab is unreinforced, or superposition of the latter action and fixed action if the slab is reinforced 4) Failure Bridge Engineering 28

29 Behavior of a restrained slab Bridge Engineering 29

30 Behavior of a restrained slab Bridge Engineering 30

31 Empirical method - background Conventional design of deck slabs based on flexure and shear can be quite conservative And significant increase in strength is possible from internal arching action developed within the slab and the supporting beam system Consequently, it is possible to reduce the amount of reinforcement in such slabs quite considerably, without undermining the level of safety. Restraints at the edges of simply supported slabs increase their load bearing capacity Bridge Engineering 31

32 Empirical method - background Development of fixed boundary action and compressive membrane action are grouped and named as arching action. Arching action leads to increase in slab strength. This fact is reflected in CHBDC by suggesting minimal reinforcement in the deck slab, provided certain conditions are met. Bridge Engineering 32

33 Empirical method Conditions (CHBDC) Slab of uniform thickness, bounded by exterior supporting beams and: (a) Slab is composite with parallel supporting beams, for which the lines of support are also parallel (b) The ratio of the supporting beams spacing and thickness is less than The spacing of the supporting beams in calculating this ratio is taken parallel to the direction of the transverse reinforcement. Bridge Engineering 33

34 Empirical method - Criteria (c) Spacing of the supporting beams not to exceed 4.0 m. The slab extends beyond external beams wide enough for the development length of bottom transverse bars (d) Provide longitudinal rebars in the deck slab in the negative moment regions of continuous composite beams. Bridge Engineering 34

35 General and specific Criteria for empirical method General criteria Bridge Engineering 35

36 Minimum concrete cover and tolerances Bridge Engineering 36

37 Minimum concrete cover and tolerances Bridge Engineering 37

38 Minimum concrete cover and tolerances Bridge Engineering 38

39 Minimum concrete cover and tolerances Bridge Engineering 39

40 Minimum concrete cover and tolerances Bridge Engineering 40

41 Minimum concrete cover and tolerances Bridge Engineering 41

42 General and specific Criteria for empirical method General criteria, continued Bridge Engineering 42

43 Empirical method - Criteria Negative reinforcement on supports is provided accordingly If the general criteria plus the specific ones are fulfilled, then empirical method is applicable Decks normally need 4 layers of re-bars Main top and bottom re-bars to transfer live load to supporting girders Distribution bars on the top of lower main bars and bottom of upper main bars to aid distribution of wheel loads and act as temp. shrinkage re-bars Bridge Engineering 43

44 Empirical method - Criteria Bridge Engineering 44

45 Empirical method For skew angle of more than 20 Bridge Engineering 45

46 Empirical method Transverse reinforcing bars are placed on a skew, the reinforcement ratio for these bars is not less than θ ρ 2 cos θ where is the skew angle The spacing of reinforcement in each direction does not exceed 300 mm Bridge Engineering 46

47 Deck reinforcement 4 layers Bridge Engineering 47

48 Example Bridge Engineering 48

49 Example Bridge Engineering 49

50 Example Bridge Engineering 50

51 Example Bridge Engineering 51

52 Example Bridge Engineering 52

53 Example Bridge Engineering 53

54 Example Bridge Engineering 54

55 Example Bridge Engineering 55

56 Bridge deck deterioration Chloride-containing deicing salt causes corrosion of rebars and later damage to concrete In US, over 200 million/year on highway bridge deck repair In Canada, Ontario, over 20 million/year on bridge repairs Bridge Engineering 56

57 Spalling Basic spalling mechansim Bridge Engineering 57

58 Deck protection methods Protection systems Bituminous waterproofing Pre-fabricated sheeting Thin adhesive films Galvanized Rebars Epoxy coating of rebars Stainless steel Cathodic protection Bridge Engineering 58

59 Cathodic protection Developed by California Department of Transportation Bridge Engineering 59

60 Thicker Cover Use thicker cover and denser concrete IOWA method Slump 12.5 ~ 25 mm Air content 6% Bridge Engineering 60

61 Composites CFRP, GFRP (bars, sheets) Fiber Matrix FRP vs. steel Lighter, more durable, stronger, lower E, brittle, more initial cost, less life-cycle cost? Bridge Engineering 61

62 Composites, Matrix Thermoset Polyester Vinyl Resin Epoxy Phenoic Polyurethane Thermoplastic Bridge Engineering 62

63 Composites, Fibers Aramid Boron Carbon/graphite Glass Nylon Polyester Polyethylene Polypropylene Bridge Engineering 63

64 Composite Carbon fiber bars Bridge Engineering 64

65 Composites Glass fiber bars Bridge Engineering 65

66 Composites, Surface roughness Bridge Engineering 66

67 Composites Domain of application Construction of new structures Renovation, repair of existing bridges Retrofit of existing bridges Embedded or externally applied rods Bridge Engineering 67

68 Composites Important issues: Design to be consistent with limit states design principles Rigorous material testing procedures Design provisions for reinforced and prestressed components Site preparation and construction procedure Fire resistance Long term durability Ultraviolet rays, temp., humidity Bridge Engineering 68

69 Composites Testing FRP internal reinforcement Cross sectional area Anchor for testing FRP specimens Tensile properties Development length Bond strength Surface bonded FRP reinforcement Direct tension pull-out Tension of flat specimens Overlap splice tension test Bridge Engineering 69

70 Composites Design Flexure Deformability condition to ensure concrete crushes first Crack limitations less severe than for steel bars Deflection limitation similar to conventional members Shear Stirrups fail in corners due to premature fracture at the bends Few tests show shear resistance is less than predicted Bridge Engineering 70

71 Composites Design Thermal stress Expansion of FRP very different than concrete Large thermal stresses in harsh climates Must consider thermal stress in the design Fire resistance depends on Critical temperature of FRP varies for various types Thickness of concrete cover, aggregates Ultraviolet rays Not concern in embedded bars Use protective coatings, additives to the resin Bridge Engineering 71