The discussion of continuity creation and its consideration in Pre-cast concrete bridges

1. Pre-cast beam / continuity
Senior Structural Engineer
Structural Engineer
1
kamel Farid;
Ashraf Alsayed;

2. Objective
 The discussion of continuity creation and its consideration in pre-cast concrete
bridges
2

3. Continuous Concrete Bridges Advantages
 Reduction of joint installation and maintenance costs
 Protection of beam ends and pier caps
 Improved ride quality
3

4. Continuity design concepts
 Fully continuous:
 Slab and beams are continuous
 Possible to design for continuous behavior for superimposed dead and live loads
 Partially continuous:
 Only the deck is continuous
 Spans behave as a series of simple spans
 Link slab
 Discontinuous
Spans behave as a simple spans
 Tied slab
 Messenger slab
4

5. Fully continuous
DESIGN AND CONSTRUCTION CONSIDERATIONS
5

6. Fully continuous
 Requires girder ends to be embedded in a
common diaphragm.
 Requires connection for positive and
negative moments to be established.
6

7. Phases of construction stages
 Placement of pre-cast girders
 Forms and rebar are installed for
deck slab
 Cast slab on pre-cast girders in
assembly areas
 Leave slab block out for eventual
closure pour and pier diaphragm
 Form pier diaphragm and closure
slab
 Place diaphragm and slab
reinforcing
 Pour and cure the final closure
 Complete railing closures
7

8. General considerations
 Subsequent applied loads (Barrier, wearing surface, live load) applied to a
continuous system
 Remaining creep and shrinkage potential of the system must be resisted by the
pier joints
 Need to check joint effectiveness
 Might still have to design as simple spans due to construction stages
8

9. General considerations
9

10. Modeling
10

11. Modeling
 Slab shells are continuous for all spans
 Pre-cast beams frames are continuous for
all spans
 Construction stages load case is defined
to account for stages of construction,
creep and shrinkage
11

12. Modeling
Precast beam Continuity Model
12

13. Modeling
Rigid link Continuity Model
13

14. Modeling
Bearing Continuity Model
14

15. Modeling
Rigid link Continuity Model
15

16. Modeling
Cap beam Continuity Model
16

17. Modeling
Column Continuity Model
17

18. Results
18
Construction stages: before continuity Construction stages: after continuity

19. Results
19
The effect of moving load

20. Creep and shrinkage effects
It is though that creep
and shrinkage will
redistribute dead load, so
simple spans may be
used for dead load and
assuming a continuous
bridge for live load and
superimposed dead load.
20
Shrinkage strainCreep coefficient

21. Creep and shrinkage effects
21
Moments are in ton.m on the entire
bridge section
-1200
-1000
-800
-600
-400
-200
0
200
400
600
800
1000
0 10 20 30 40 50 60 70 80
0 days 60 days 365 days 36500
0.0
200.0
400.0
600.0
800.0
1000.0
sec 1 (Mid span) sec 2 (Support span) sec 3 (Mid span)
0 days 60 days 365 days 36500
Sec-1 Sec-2
Sec-3
Sec-2 Sec-1
0 days 60 days 365 days 36500
sec 1 (Mid span) 696.5 771.6 727.3 710.7 10%
sec 2 (Support span) 949.8 764.6 849.0 883.3 -9%
sec 3 (Mid span) 175.5 375.1 304.8 277.3 53%

22. Creep and shrinkage effects
 After final placement, the beams will
continue to creep and shrink; cambering
up
 Temperature will also cause camber
 Positive moments will form causing
cracking
22

23. Partially continuous
DESIGN AND CONSTRUCTION CONSIDERATIONS
23

24. Partially continuous
 Only the deck is to be made continuous
for practical reasons
 Reduced exposure of beam ends,
 Better ride quality
 In some cases, it may be more
advantageous than the fully continuous
due to simpler forming and reduced field
pour volume
24

25. Modeling
25

26. Modeling
 Slab shells are continuous for all spans
 Pre-cast beams frames are NOT
continuous at supports
 Construction stages load case is defined
to account for stages of construction,
creep and shrinkage
26

27. Modeling (slab)
27

28. Modeling (Pre-cast)
28

29. Modeling (Diaphragm)
29

30. Modeling (Rigid links)
30

31. Modeling (Bearing)
31

32. Modeling (Columns)
32

33. Results
33
The effect of dead load before continuity The effect of dead load after continuity

34. Results Creep and shrinkage effects
34
-500
0
500
1000
1500
0 10 20 30 40 50 60 70 80
Start End

35. 35
The effect of moving load on the entire deck

36. Results
36
The effect of moving load on the Slab The effect of moving load on the Pre-cast

37. Link slab
 Slab provides minimal continuity over
center supports
 Applied loads produce end rotations
 Slab is forced to bend or comply with the
induced curvature
37

38. Modeling
 Typically, SCI bridge models the
composed action between slab and pre-
cast by creating a body constrain between
each joint of the frames and each
corresponding joint of the slab.
 The linked slab will be modeled same as
the partially continuous model. Only at the
slab at the link slab part, will not have any
constrains with the frame.
38

39. General Considerations
 Link slabs are used to eliminate deck joints
at piers where each span is supported on
elastomeric deck with a length that extends
approximately 5% of each adjacent span.
 Shear stud connectors shall be omitted
within the limits of the link slab and a bond
breaker is applied between the top flange
and the kink slab to prevent composite
action.
 only spray applied membrane waterproofing
shall be used on decks with link slabs.
39

40. Discontinuous Slab
TIED SLAB
MESSENGER HINGE SLAB
40

41. Discontinuous Slab
 Beams behave as simply supported spans
 Separate bearings and end diaphragms
are provided for each span
 Tied slab
 Messenger slab (Created by ACE)
41
Messenger slab
Additional rebar for
messenger slab detail
T T/3
Tied slab

42. General considerations
 The tie reinforcement at mid depth of the slab deboned for a short length either
side of the joint to permit deck rotation.
 No moment of continuity between spans
 Slabs between spans are separated using compressible joint fillers but deck
waterproofing and dick surfacing are continuous and special seals are provided
over the joint for double protection
 In case of settlement. Simply supported span are most favorable
42

43. Modeling
 As shown in the deformed shape figure,
neither the shells nor the frames are
continuous.
 All joints between the spans are separated
43

44. Results (Dead)
44
The effect of dead load on the entire deck
Of messenger slab
The effect of dead load on the entire deck
Of tied slab

45. Results (live)
45
The effect of moving load on the entire deck
Of messenger slab
The effect of moving load on the entire deck
Of tied slab

46. Results of tied slab
46
The effect of moving load on the Slab The effect of moving load on the Pre-cast

47. Results of messenger slab
47
The effect of moving load on the Slab The effect of moving load on the Pre-cast

48. Conclusion
Reinforcement (Ton)
Beams Slab Diaphragms Total
simple 4.0 23.5 1.1 28.6
Patially continuous 3.9 24.0 1.1 29.0
Continuous 4.4 23.5 0.7 28.6
48
0
5
10
15
20
25
30
35
Beams Slab Diaphragms Total
simple Patially continuous Continuous

49. Conclusion
 Provision of continuous spans in place of single span causes considerable
reduction in moment due to dead load, live load. However, in case of construction
stages, the continuity has a slight effect on reinforcement especially in dead load.
49

50. Conclusion
 For fully continuous, Shrinkage produces considerable additional moments.
 For partially continuous, Shrinkage produces slight additional moments.
50
-500
0
500
1000
1500
0 20 40 60 80
Start End
-1500
-1000
-500
0
500
1000
0 10 20 30 40 50 60 70 80
0 days 60 days 365 days 36500

51. Conclusion
 Fully continuous girders are more durable than partially continuous girders
because main reinforcement covers the negative tension moment which controls
the crack width. This effect is partially archived in the partially contiguous case.
 Generally, partial continuity is more preferred than full continuity for the following
reasons:
 Elimination of the cost of expansion joints
 Better riding quality
51

52. Moving load comparison
52
-1000
-500
0
500
1000
1500
Sec.1 Sec.2 Sec.3
Moving load
Cont Part. Cont tied slab Messenger slab
-1000
-500
0
500
1000
1500
0 10 20 30 40 50 60 70 80
Moving load comparison
Cont min Cont max Part. Cont Max Part. Cont min
tied slab max tied slab min Messenger slab max Messenger slab min

53. SIDL comparison
53
-300
-200
-100
0
100
200
300
400
0 10 20 30 40 50 60 70 80
SIDL
Partially continuous Continuous Tied slab Messenger slab
-300
-200
-100
0
100
200
300
400
Sec.1 Sec.2 Sec.3
SIDL
Continuous Partially continuous
Tied slab Messenger slab

54. Important notes
 The continuity of moment in girder is not affected by the bearing pad stiffness.
54
-1500
-1000
-500
0
500
1000
1500
0 10 20 30 40 50 60 70 80
k=400000
k=200000
k=100000
Pad bearing
stiffness