The content for this class has been provided by the following PB employees: With assistance from: Martine Klein, P.E. Narration by Greg Metzger, PB

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2 The content for this class has been provided by the following PB employees: With assistance from: Martine Klein, P.E. Narration by Greg Metzger, PB University If you have any questions about the content of this course please contact Julio Valvezan or If you experience any technical difficulties, please contact your IT Help Desk. 2

3 As with all of our LRFD courses, you may download a PDF version of this course for your future reference. Click on the ATTACHMENTS link located in the upper right corner of this course window to access the document and save the file to your desktop. 3

4 After completing the content within each class you will be asked to take a test to ensure that you mastered the key training objectives. You will need to make a minimum score of 80% to receive credit for passing the class. Successful completion of the class will earn point one IACET CEU Please refer to your state s specific continuing education requirements regarding applicability 4

5 This class is the third class in the Structures TRC curriculum for LRFD Design, developed internally at PB. It focuses on the following ten areas of major change introduced by the LRFD Bridge Design Specifications: 5

6 After successfully completing this course you should be able to identify: The common types of joints and bearings. The Load Factor equation for the various limit states that generally apply to the design of joints and bearings. The differences between the load factors to accommodate factored loads and those to accommodate factored displacements and rotations. The class will take you approximately one hour to complete. 6

7 This course contains 3 lessons. We will start with an overview of joints and bearings and some specific items in the LRFD code that affect their design. Lesson 2 covers the details of the design of joints and bearings applying LRFD. Finally, Lesson 3 provides some design examples. 7

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9 The purpose of this course is to become familiar with the LRFD Code requirements for joints and bearings. Joints and Bearings are covered in Section 14 of the LRFD Code. The new code increases design displacements and rotations so that the design movements are greater than the actual movements thereby ensuring that the structure can displace and rotate without restrictions. The new code also accounts for construction tolerances in determining the design displacements and rotations. 9

10 The selection and layout of the joints and bearings must allow for: deformations due to temperature construction tolerances elastic shortening due to prestressing effects time-dependent causes, such as creep and shrinkage and must be consistent with the proper functioning of the bridge. 10

11 A joint is a structural discontinuity between two elements. There are many types of deck joints including the following: Modular Bridge Joint System (MBJS) Compression Seals Finger Joints Poured Seals Strip Seals Bridge Plate Joint The picture shows and example of a modular joint which can accommodate large movements. The joint is designed so that the spaces between the bars are equal. 11

12 There are other types of joints that are not necessarily located on a bridge deck such as: Construction Joints- contractor needs to stop a pour at controlled point Contraction Joints- which allow the joint to move and is used for crack control. 12

13 Finger joints are used for long span structures such as suspension bridges and cable stayed bridges. Strip seal joints are used for small joint movements. Strip seals can be replaced without replacing the joint armor. 13

14 Bridge plate joints are for moderate span lengths and can be noisy. Asphalt plug joints are for very small movements. 14

15 A bearing is a structural device that transmits loads, usually between superstructure and substructure. Bearings can be designed to allow translation, rotation or they can be designed to restrain movement. The type of bearing selected depends on the load requirements, the rotations and the displacements. 15

16 There are many types of bearings including the following: Plain Elastomeric Bearing Pads (PEP) Disk Bearings Bronze Bearings Cylindrical Bearings Seismic Isolation Bearings Roller Bearings 16

17 The reinforced elastomeric bearing consists of steel plates bonded to relatively thin layers of elastomer. On steel structures, elastomeric bearings are usually attached to sole plates and masonry plates. On concrete structures, elastomeric bearings are usually bonded to the concrete. 17

18 Spherical bearings have curved sliding surfaces and they may also have sliding flat surfaces Rocker bearings allow rotation without translation. 18

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20 We have now completed Lesson 1. We will continue with lesson 2 which covers LRFD design concepts. 20

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22 The left side of the equation is the sum of the factored loads. The right side is the load capacity or the rotation or displacement capacity. 22

23 The load modifier Eta is related to ductility, redundancy and operational importance. The maximum value of gamma (eye) is listed, refer to the code when the minimum value is used For deformations use eta I equal to one 23

24 Each component and connection shall be designed for the following limit states: 24

25 Thermal loads, displacements and rotations are considered under TU (Uniform Temperature). Creep and Shrinkage effects, when applicable, are also considered. For the Strength Limit States the 0.5 factor is used to factor loads and the 1.2 factor is used to factor displacements and rotations. For the Service limit states the factors are 1.00 and 1.20 respectively. 25

26 In general, the permanent loads that apply to joints and bearings consist of only the dead load and superimposed dead load. 26

27 This slide displays a list of both permanent and transient loads and their corresponding abbreviations. 27

28 The code also provides coefficients of friction for use in the various types of bearing designs. Frictional forces must be overcome before rotation or translation can take place. The table shows the various types of PTFE materials commonly used. Bearing manufacturers should be contacted to determine the best type of PTFE material that can accommodate the required loads and service life of the bearing. Access for inspection, maintenance and future bearing replacement should also be considered. Please Note that for Teflon, as the pressure increases the coefficient of friction decreases. 28

29 The maximum joint gaps based on factored joint movements can also be found in the code. Special consideration should be given to gaps that meet the code limit at the time of installation but over time, widen due to time dependent effects such as creep and shrinkage. 29

30 Bearings and joints must be designed to accommodate movements and to carry loads. The following items must be considered: Factored displacements and rotations are used Factored loads are used Joints and their supports shall be designed to withstand factored force effects over their factored range of movement The Dynamic load Allowance IM, for Deck Joints for all Limit States is equal to 75% 30

31 Bearings loaded to less than 20 percent of their vertical capacity require special design. Lightly loaded bearings can shift over time so the condition of the minimum vertical load with the maximum horizontal loads need to be checked. Bearings are not recommended for horizontal to vertical load ratios of over 40 percent. Bearings with large horizontal loads and small vertical loads can become unstable. Skewed bridges with over 30 degrees skew, have a tendency to rotate under seismic loading and bearings should be designed and detailed to accommodate this effect. 31

32 Article provides Design Requirements for uniform temperature TU The thermal movements shall be computed from the extreme temperatures specified in Article and the estimated setting temperatures. The design thermal movement associated with uniform temperature change may be calculated using Procedure A from the table below or by using Procedure B and the contour maps in the code. The minimum and maximum temperatures are specified in table for Procedure A Procedure A is the same method as in the previous codes. 32

33 If you choose to use procedure B, there are contour maps for minimum and maximum temperatures for steel and concrete bridges. This slide displays the minimum values. 33

34 Only the contours for steel girder bridges with concrete deck are shown here. The maximum values are displayed. 34

35 The actual thermal movement range can be determined by using the following equation. Please take a moment to review the terms. 35

36 The size of the deck joint gaps depends on the setting temperature. The setting temperature of a bridge or any component thereof shall be taken as the actual air temperature averaged over the 24-hour period immediately preceding the setting event. Movements are measured relative to the setting temperatures to determine the size of the joint. For long spans, offset charts for installation of joints are recommended. The charts are used to account for the uncertainty in the setting temperature at the time of design. C

37 Similarly, the position of the bearing depends on the setting temperature. The setting temperature of a bridge or any component thereof shall be taken as the actual air temperature averaged over the 24-hour period immediately preceding the setting event. (Section ) It is recommended that offset charts be defined in appropriate increments and included in the design drawings so that the position of the bearing can be adjusted to account for differences between setting temperature and an assumed design installation temperature (C ) For elastomeric bearings, the maximum horizontal displacement of the bridge superstructure shall be taken as 65% of the design thermal movement range. (Section ) 37

38 The total design rotation is the sum of the rotations due to dead + live load + the allowances for profile grade effects and construction tolerances. Section specifies radians as the construction tolerance for elastomeric bearings, pot bearings, curved sliding surface bearings, and disc bearings 38

39 Metal or concrete components are susceptible to damage under a single rotation that causes contact. Metal to metal contact is not allowed. To prevent contact, factored rotations must be used for design. 39

40 Article requires that tapered plates be used to provide a level surface for the bearing if under the factored permanent loads the inclination of the underside of the girder exceeds 0.01 radians. 40

41 Steel reinforced elastomeric bearings can be designed using either of two methods, Method A or Method B. Method A allowable stresses are lower than the allowable stresses for bearings designed using Method B because Method B requires bearings to be tested and requires greater quality control. Steel reinforced bearings have greater strength and superior performance in practice C

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43 The code provides the Ultimate Stress for use in determining the Ultimate Capacity for the various types of bearings. 43

44 Method A shows the allowable stress without testing which should be less than 1.0 KSI 44

45 Method B allows 1.6 KSI since the material is actually tested. Note that the shear modulus is required to determine the allowable design stress. 45

46 The force required to displace the bearing in shear depends on the shear modulus. The Hardness or Durometer is a measure of the bearing s stiffness. 46

47 The minimum elastomeric bearing height is twice the factored shear deformation. The load factor for deformation is the larger of the two factors provided in table shown earlier. This ensures that the bearing is flexible enough to deform without damaging the elastomer. 47

48 Note that since the bearing height is in the denominator, the shorter bearing is harder to deform. As the bearing height decreases, H increases. 48

49 The shape factor is a function of the elastomer proportions. The shape factor is calculated for each layer using the equation below. 49

50 Using the compressive stress and the shape factor, the strain can be determined. Using the strain, the change in bearing height due to the application of vertical loads can be calculated. Note that the maximum strain in the chart is 7%. The compressive deflections are considered separately for: Dead Load + Long Term Creep Instantaneous Live Load 50

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52 We have now completed Lesson 2 which covers the fundamental LRFD concepts related to joints and bearings. We will now address specific design examples. 52

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54 The examples used throughout this training module are greatly simplified to illustrate the use of the code. Elastomeric bearings are used in these examples because they are usually designed by the engineer. Many bearings are purchased from fabricators fully assembled but are not designed by the engineer. When purchasing a bearing, the required loads and movement range are provided to the bearing fabricator. The fabricator supplies a bearing that meets those requirements. Even if the bearing is designed by a fabricator, the engineer still needs to verify that the bearing provided conforms to the code by performing their own check of the bearing capacity. 54

55 This slide highlights the load combinations and factors for joints and bearings force effects. In determining deformations, creep and shrinkage effects should also be considered. 55

56 This slide outlines factors when calculating the factored joint load under Strength I conditions. The joint impact of 75% is used regardless of the span length 56

57 This slide and the next provide an example for the change in length of the span due to temperature variation. Note that this slide shows the actual change in length not factored. 57

58 Assume that we have an 80 foot long bridge with one fixed and one expansion bearing. The full unfactored movement range due to the temperature variation from the maximum to the minimum design temperature is calculated below. The factored temperature range is the actual temperature range multiplied by 1.2 or 20% greater than the actual. 58

59 This slide outlines the factored lengthening of the bridge due to a temperature rise. The thermal movement due to a temperature drop measured from the setting temperature should also be checked and in many cases controls the design temperature range. 59

60 This is an example of how to apply the factors to loads. Assume that you are designing the expansion bearing for a bridge with normal vehicular use. Assume that lateral loads are resisted by other elements such as a diaphragm keyed into the pier cap beam. Assume a 200 kip Dead Load, a 25 kip wearing surface Dead Load, a 50 kip Live Load, a 10 kip Dynamic Allowance. All loads are vertical. 60

61 This slide shows the maximum factored design vertical load. The key factors are highlighted in red. 61

62 This slide shows the calculation of the horizontal design load. The Key factors are highlighted in red 62

63 In order to deform this 10 inch x 20 inch x 2 inch thick elastomer one inch, a shear force of 13,000 pounds is required. Note that the plan area is used in determining the shear force. Please also note that Hrt is the sum of the individual elastomer layers excluding the steel reinforcement layers. 63

64 Using the compressive stress and shape factor you can determine the compressive strain. 64

65 This marks the conclusion of the course. We have now covered all of the objectives and you should now be able to identify: The common types of joints and bearings. The Load Factor equation for the various limit states that generally apply to the design of joints and bearings. The differences between the load factors to accommodate factored loads and those to accommodate factored displacements and rotations. 65

66 You are now ready to begin the final assessment. The assessment consists of 10 multiple choice questions. You will need to achieve a minimum score of 80% to receive credit for passing the course. If you score below 80%, please go back and review the content of this course, and then retake the assessment to achieve a passing score. When ready, click the Right arrow below to advance to the assessment. 66

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68 Thank you for completing this course. If you received a passing score on the assessment, simply close this window to exit the course. Your score will be recorded on your transcript. If you did not achieve a passing score, please review the content of this course and then retake the assessment to achieve a passing score. If you have any questions or problems with the functionality of this course, please us at pbu@pbworld.com and specify the name of the course and the issue you are experiencing so that we may assist you. You may print a certificate from the My Transcript area of PB University by clicking the cert. icon. If you need a certificate that specifically states the IACET certification and credit hours, please a request to us at pbu@pbworld.com. 68