Masonry Reinforcement for Arch Bridges

Transcription

1 Bersche-Rolt. Masonry Reinforcement for Arch Bridges

2 Crack Tensile forces induce cracking in masonry which can be resisted using rebar

3 Thermal cracking Tension induced by thermal effects

4 Moisture cracking Tension induced by moisture effects

5 Failed beam No tension resisting element in the bottom of the beam

6 Reinforced Masonry Beam Nominal Beam Depth Reinforcement Hangers

7 Section through beam Grout Reinforcement Hanger

8 Masonry Beam Model Compression Nominal beam depth Tension Cross Section Stress Distribution

9 Design of masonry beams Design standard BS 5628 Part 2 Formula for singly reinforced rectangular members Md = As x fy x Z/γms (maximum of 0.4 x fk x b x d²/ γmm) Material partial safety factors can be adjusted to suit the condition of the structure

10 Materials Reinforcement : Stainless steel deformed bar and spacers Grade 304 or 316 Characteristic strength 460 Mpa Mortar: Cementitious, shrinkage compensated, thixotropic Characteristic strength 50MPa Mortar strength can be varied but the key property of bond strength has to be retained to ensure composite action

11 Simple beam

12 Beam for Temporary Works

13 Spey Bridge Masonry parapets failed assessment for bending capacity

14 Parapet reinforcement Vertical reinforcement Horizontal reinforcement Concrete slab

15 Parapet reinforcement Through ties Vertical reinforcement Horizontal reinforcement Through ties Vertical reinforcement Horizontal reinforcement Arch ring Reinforced masonry Tie bar Fill Arch ring Contained masonry Fill

16 Parapet reinforcement Core drilled holes for grouting in rebar

17 Drilling for vertical bars Portable drilling rig

18 Completed parapet

19 Retaining wall Upper courses displaced by traffic action

20 Retaining wall Vertical and horizontal rebar installed to unify wall then repointed in hydraulic lime

21 Requirements for Arch Strengthening Increase load carrying capacity of structure Avoid changing beneficial characteristics of masonry arch form by over-stiffening locally Avoid significant increase in self-weight Minimum disruption for users and buried services Maintain arch profile Use materials that are tolerant of real site conditions particularly water penetration Be cost effective

22 Arch reinforcement Transverse bar Radial bar Longitudinal bar

23 Delph Bridge

24 Arch discontinuity Transverse drilled bars form dowels across step in arch

25 Hambleden Bridge Transverse bars cross cracks allowing a higher condition factor to be used in the assessment

26 Transverse Reinforcement Detail Drilled hole filled with grout Stainless steel Reinforcing bar Stainless steel spacer

27 Transverse bar

28 Drilling transverse holes

29 Drilling transverse holes Drilling water is collected and solids settled out before disposal

30 Drill cores

31 Obstructions to drilling Temporary holes were drilled in the web to allow drilling of the arch

32 Hinge mechanism Load 4-HINGE MECHANISM Hinges occur at the springings, under the load and between the crown and springing

33 Reinforced hinge Load 4-HINGE MECHANISM The reinforcement prevents the crack under the load from opening

34 Hinge positions at failure Note multiple small cracks under load position where reinforced compared to single large crack in extrados

35 Delph sandstone Sandstone at the springing provides rebar anchorage resulting in a second reinforced hinge

36 Analysis Load P Fill M H V Hy 1 -Vx 1 +F 1 a 1 +M=0 Hy 2 -Vx 2 +F 2 a 2 +P(x 2 -x 1 )=0 Taking moments about hinge positions gives collapse load. Hinge positions are moved until minimum value of load found.

37 Hinge detail Compression Tension Hinge model Cross section Stress distribution

38 Longitudinal reinforcement Sawn groove filled with grout Stainless steel spacer Stainless steel Reinforcing bar

39 Cutting grooves

40 Completed intrados Transverse bars are not seen but longitudinal bars in slots are

41 Hungerford Bridge

42 Hungerford Bridge Establishing arch ring thickness and height of backing by trial drilling

43 Hungerford Working hours and noise levels restricted by closeness to housing.

44 Working platform Floating platform as canal kept open during the works

45 Longitudinal reinforcement Drilled hole filled with grout Stainless steel Reinforcing bar Stainless steel spacer

46 Longitudinal drilling Three bricks removed to gain access to correct depth for drill head

47 Longitudinal drilling

48 Longitudinal drilling

49 Drill bits & Flexible drive

50 Drilling rig

51 Drilling rig extended

52 Grouting Low pressure grouting to prevent damage to arch

53 Cold weather grouting B-R grout can be used down to 3deg C

54 Ring separation Load

55 Radial ties Load 4-HINGE MECHANISM Pins drilled across bed joints prevent premature ring separation

56 Radial ties Various types of pin were tested with no significant difference in performance therefore the simple straight pin was adopted

57 Complete reinforcement Arch extrados Radial bar drilled through rings Bed joint between rings Transverse bar in drilled hole Longitudinal bars in drilled hole Arch intrados

58 Completed arch Both transverse and longitudinal bars are sub-surface and not visible

59 Completed bridge

60 Willow Bank 3 Bars in slots to increase axle load were installed

61 Willow Bank 1 Bars in slots to increase axle load were installed 10m upstream of the access point

62 Alternative access

63 Low headroom

64 Vacuum cutting

65 Washing down

66 Pollution control

67 Stone masonry Similar techniques to those used for brickwork can be applied to stone

68 Advantages Improves robustness, ductility and load distribution capability of existing structure Increases load carrying capacity of structure Does not change beneficial characteristics of masonry arch construction form Does not increase self weight of structure Minimum disruption to users of structure and buried services Arch profile is maintained Accommodates variations in the existing masonry Uses well tried durable materials

69 Hydraulic lime Testing to establish the levels of creep using hydraulic lime mortars to bond reinforcement

70 Bersche-Rolt Award Winner for Design and Execution The I.C.E.Historic Bridge and Infrastructure Awards 2004