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Bridge Design Manual - 2002 Bridge Details

Ethiopian Roads Authority Page 8-1

8 BRIDGE DETAILS

8.1 SCOPE

This section contains requirements for the design and selection of structural bearings. It also

contains requirements for railings for new bridges and for rehabilitated bridges to the extentthat railing replacement is determined to be appropriate. This section provides bridge railingtest levels and associated crash test requirements. All bridge traffic barrier systems will

referred to as railings herein.

The section also contains the requirements for the design of deck expansion joints. It also

mentions the reference for drainage of bridge decks, and mentions the means of dealing with

utilities.

8.2 NOTATIONS

A1 = area under bearing deviceA2 = notional area (see Figure 8-3)

B = length of pad if rotation is about its transverse axis or width of pad if rotation is

about its longitudinal axle (mm)

d = the diameter of the hole or holes in the bearing (mm)D1 = diameter of curved surface of rocker or roller unit (mm)

D2 = diameter of curved surface of mating unit (D2 = for a flat plate) (mm)Es = Youngs modulus for steel (MPa)

Fc = compressive strength of concreteFy = specified minimum yield strength of the weakest steel at the contact surface (MPa)

G = shear modulus of the elastomer (MPa)

hrmax = thickness of the thickest elastomeric layer in elastomeric bearing (mm)hri = thickness of i

th elastomeric layer in elastomeric bearing (mm)

hrt = total elastomer depth in an elastomeric bearing (mm)

hs = thickness of steel laminate in steel-laminated elastomeric bearing (mm)L = length of a rectangular elastomeric bearing (parallel to longitudinal bridge axis)

(mm)

m = modification factor

n = number of layers of elastomerPn = nominal bearing resistance

Pr = factored resistance of pot wall (N)

Ps = service compressive load due to total load (N)

S = shape factor of thickest layer of an elastomeric bearingSi = shape factor of ith layer of an elastomeric bearing

W = width of the bearing in the transverse direction (mm)

FTH = constant amplitude fatigue threshold for Category A (MPa)o = maximum horizontal displacement of the bridge deck at the service limit state

(mm)

s = maximum shear deformation of the elastomer at the service limit state (mm) = instantaneous compressive deflection of bearing (mm)

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Bridge Details Bridge Design Manual - 2002

Page 8-2 Ethiopian Roads Authority

I = instantaneous compressive strain in ith elastomer layer of a laminated bearing

s = maximum service rotation due to total load (RAD)L = service average compressive stress due to live load (MPa)s = service average compressive stress due to total load (MPa) = resistance factor for the strength limit state specified in Chapters 5,6,7, and 8 or

for the extreme event limit state specified in Chapter 2: General Requirements

8.3 BEARINGS

8.3.1 GENERAL

Bearings shall be fixed or movable as required for the bridge design. Movable bearings mayinclude guides to control the direction of translation. Fixed and guided bearings shall be

designed to resist all loads and restrain unwanted translation. Bearings support relatively

large loads while accommodating large translation or rotations.

Bearings can be named according to their function as fixed or expansion bearings, after thematerial they are made of such as steel, cast steel, alloy, bronze, elastomeric or PTFE

bearings. Movements in both directions sometimes justify the use of spherical bearings. Thegeneral rule for steel roller bearings is that the higher the steel quality, the less radius of theroller needed (see Figure 8-1 below). If the load is too large, multiple rollers are sometimes

used.

Figure 8-1 Steel Roller Bearing

Steel reinforced elastomeric bearings and steel plate/PTFE sliding bearings are relatively

cheap and require a minimal construction height (see Figure 8-2). Steel reinforcedelastomeric bearings can take movements in all directions but only to a certain limit. Theyare therefore suitable for small or medium sized bridges. The bearing has to be changed after

some 30 - 50 years when the rubber is worn out.

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Figure 8-2 Reinforced Elastomeric Bearing

From a maintenance point of view it is wise to promote uniformity through the use of as few

types of bearings as possible. Other types of bearings are however shown in Figure 12-7.

8.3.2 DESIGN

Contact Stresses

Unless otherwise noted, the resistance factor for bearings, , shall be taken as 1.0.

Friction for bearings: Steel roller bearings and steel plate bearings with PTFE layer in-

between shall be designed with a friction factor of 5 % of the actual vertical load, or 0 %

which is most unfavorable. The friction shall belong to the actual load giving the friction.

At the service limit state, the contact load, Ps, shall satisfy:

For cylindrical surfaces:

(8.1)

For spherical surfaces:

(8.2)

where: D1 = the diameter of the roller surface (mm), andD2 = the diameter of the mating surface (mm) taken as:

Positive if the curvatures have the same sign, and Infinite if the mating surface is flat.

Fy = specified minimum yield strength of the weakest steel at the contact surface

(MPa)Es = Young's modulus for steel (MPa)

sy

2

1

1

s E

F

D

D1

WD8

P

2

2

3

s

y

2

2

1

1s

E

F*

D

D1

D40P

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W = Width of the bearing (mm)

The service limit state loads are limited so that the contact causes calculated shear stressesno higher than Fy/3 or surface compression stresses no higher than 1.25 Fy. The maximum

compressive stress is at the surface, and the maximum shear stress occurs just below it.

The formulas are derived from the theoretical value for contact stress between elastic bodies(Ref. 1). They are based on the assumption that the width of the contact area is much less

than the diameter of the curved surface.

The two diameters have the same sign if the centers of the two curved surfaces in contact are

on the same side of the contact surface, such as is the case when a circular shaft fits in a

circular hole.

Concrete Supporting the Bearing

In the absence of confinement reinforcement in the concrete supporting the bearing device,the factored bearing resistance, Pr, shall be taken as:

Pr = Pn for which: (8.3)

Pn = 0.85 f'c A1 m (8.4)

where: Pn = nominal bearing resistance (N)

Al = area under bearing device (mm2)

m = modification factor (see formulae below)A2 = a notional area defined in Figure 8-3 (mm

2)

The modification factor (m) shall be determined as follows:

where the supporting surface is wider on all sides than the loaded area:

m = 0.75A2/A1< 2.0

where the loaded area have non-uniformly distributed bearing stresses:

m=0.75A2/A1 1.50

Where the supporting surface is sloped or stepped, A2 shall be taken as the area of the lowerbase of the largest frustum of a right pyramid, cone, or tapered wedge contained wholly

within the support and having for its upper base the loaded area, and having side slopes of

1.0 vertical to 2.0 horizontal as shown in Figure 8-3 below.

Where the factored applied load exceeds the factored resistance, as specified herein,

provisions shall be made to resist the bursting and spalling forces.

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Figure 8-3 Determination of A2 for a Stepped Support

8.3.3 LOAD PLATES ANDANCHORBOLTS

Load Plates

The bearing together with any additional plates shall be designed so that:

The combined system is stiff enough to prevent distortions of the bearing that wouldimpair its proper functioning.

The bearing can be replaced within a jacking height of 400 mm without damage to thebearing, distribution plates or supporting structure

In lieu of a more refined analysis, the load from a bearing fully supported by a grout bed

shall be assumed to be distributed at a slope of 1: 1.5, vertical to horizontal, from the edge of

the smallest element of the bearing that resist the compressive load.

Sole plate and base plate connections shall be adequate to resist lateral loads, including

seismic loads. Sole plate shall be extended to allow for anchor bolts inserts, when required.

Anchorages and Anchor Bolts

All girders shall be positively secured to support bearings by a connection that can resist the

horizontal forces that shall be imposed on it. Separation of bearing components shall not be

permitted. Connections shall resist the least favorable combination of loads at the Strength

Limit State and shall be installed wherever deemed necessary to prevent separation.

The factored resistance of the anchor bolts shall be greater than the factored force effects dueto Strength I or II load combinations and to all applicable extreme event load combinations.

The tensile resistance of anchor bolts shall be determined.

The shear resistance of anchor bolts and dowels shall be determined.

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8.3.4 PTFE (Polytetrafluorethylene also known as Teflon)

PTFE Sliding Surfaces

PTFE may be used in sliding surfaces of bridge bearings to accommodate translation or

rotation. All PTFE surfaces other than guides shall satisfy the requirements specified herein.

PTFE is also known as TFE and is commonly used in bridge bearings. This article does not

cover guides. The friction requirements for guides are less stringent, and a wider variety of

materials and fabrication methods can be used for them

The PTFE surface shall be made from pure virgin PTFE resin satisfying the requirements of

ASTM D1457. It shall be fabricated as unfilled sheet, filled sheet, or fabric woven from

PTFE and other fibers.

PTFE may be provided in sheets or in mats woven from fibers. The sheets may be filled with

reinforcing fibers to reduce creep, i.e. cold flow and wear or they may be made from pureresin.

The friction coefficient depends on many factors, such as sliding speed, contact pressure,lubrication, temperature, and properties such as the finish of the mating surface (Ref. 2).

The material properties that influence the friction coefficient are not well understood, but the

crystalline structure of the PTFE is known to be important, and it is strongly affected by the

quality control exercised during the manufacturing process.

Unfilled dimples can act as reservoirs for contaminants (dust, etc.) which can help to keep

these contaminants from the contract surface.

Mating Surface

The PTFE shall be used in conjunction with the mating surface. Flat mating surfaces shall

be steel.

Stainless steel is the most commonly used mating surface for PTFE sliding surfaces. Frictiontesting is required for the PTFE and its mating surface because of the many variables

involved. The finish of this mating surface is extremely important because it affects the

coefficient of friction. ASTM A 240M, Type 304, stainless steel, with a surface finish of4.0x10-4mm (0.40 m) RMS or better, is appropriate, but the surface measurements are

inherently inexact, and hence it is not a specified alternative. Friction testing is required for

the PTFE and its mating surface because of the many variables involved.

Minimum Thicknesses

A minimum thickness is specified to ensure uniform bearing and to allow for wear. Duringthe first few cycles of movement, small amounts of PTFE transfer to the mating surface and

contribute to the very low friction achieved subsequently. This wear is acceptable and

desirable.

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PTFE: For all applications, the thickness of the PTFE shall be at least 1.5 mm after

compression. Recessed PTFE-sheet shall be at least 4.5 mm. thick when the maximum

dimension of the PTFE is less than or equal to 600mm, and 6.0 mm when the maximumdimension of the PTFE is greater than 600 mm. Woven fabric PTFE, which is mechanically

interlocked over a metallic substrate, shall have a minimum thickness of 1.5 mm and a

maximum thickness of 3.0 mm over the highest point of the substrate.

PTFE continues to wear with time (Ref. 2), and movement; wear is exacerbated by

deteriorated or rough surfaces. This wear is undesirable because it usually causes higherfriction and reduces the thickness of the remaining PTFE. Unlubricated, flat PTFE wears

more severely than the lubricated material. The evidence on the rate of wear is tentative.

High travel speeds, such as those associated with traffic movements, appear to be more

damaging than the slow ones due to thermal movements. However, they shall be avoided byplacing the sliding surface on an elastomeric bearing that will absorb small longitudinal

movements. No further allowance for wear is made in these specifications due to the limited

research available to quantify or estimate the wear as a function of time and travel.

However, wear may ultimately cause the need for replacement of the PTFE, so it is wise toallow for future replacement of the PTFE, in the original design.

Steel Mating Surfaces: The thickness of the stainless steel mating surface shall be at least

1.5 mm when the maximum dimension of the surface is less than or equal to 300 mm and at

least 3.0 mm when the maximum dimension is larger than 300 mm.

The minimum thickness requirements for the mating surface are intended to prevent it from

wrinkling or buckling. This surface material is usually quite thin to minimize cost of the

highly finished mating surface. Some mating surfaces, particularly those with curvedsurfaces, are made of carbon steel on which a stainless steel weld is deposited. This welded

surface is then finished and polished to achieve the desired finish.

Contract Pressure

The contract stress between the PTFE and the mating surface shall be determined at thestrength limit state using the nominal area.

The average contact stress shall be computed by dividing the load by the projection of the

contract area on a plane perpendicular to the direction of the load. The contract stress at theedge shall be determined by taking into account the maximum moment transferred by the

bearing assuming a linear distribution of stress across the PTFE. The contact stress at the

edge shall be determined based on the factored load and the extreme factored momenttransferred by the bearing.

Stresses shall not exceed those given in Table 8-1. Permissible stresses for intermediate filler

contents shall be obtained by linear interpolation within Table 8-1.

The contact pressure must be limited to prevent excessive creep or plastic flow of the PTFE,

which causes the PTFE disc to expand laterally under compressive stress and may contribute

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to separation or bond failure. The lateral expansion is controlled by recessing the PTFE into

a steel plate or by reinforcing the PTFE, but there are adverse consequences associated with

both methods. Edge loading shall be particularly detrimental because it causes large stressand potential flow in a local area near the edge. Values of the average and edge contact

pressure in Table 8-1 are in appropriate proportions to one another relative to the currently

available research.

Average Contact Stress Edge Contact StressMaterial Permanent

Loads

All Loads Permanent

Loads

All

Loads

Filled Sheets with Maximum FillerContent

28 40 35 55

Table 8-1 Permissible Stresses (MPa) for Filled PTFE Bearings

Coefficient of Friction

Where friction is required to resist non-seismic loads, the design coefficient of friction under

dynamic loading shall be taken as not more than 10 percent of the values listed in Table 8-1

for the bearing stress and PTFE type indicated.

The coefficients of friction in Table 8-2 are based on a 0.20 m finish mating surface.Coefficients of friction for rougher surface finishes must be established by test results.

The friction factor decreases with lubrication and increasing contact stress but increases with

sliding velocity (Ref. 2). The coefficient of friction also tends to increase at low

temperatures. Static friction is larger than dynamic friction, and the dynamic coefficient of

friction is larger for the first cycle of movement than it is for later cycles.

8.3.5 ELASTOMERICBEARINGSMETHODB

General

Steel-reinforced elastomeric bearings are treated separately from other elastomeric bearingsbecause of their greater strength and superior performance in practice (Ref. 4 and 5). The

design method described in this section allows higher compressive stresses and more slender

bearings than are permitted for other types of elastomeric bearings, both of which can lead tosmaller horizontal forces on the substructure.

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Coefficient of FrictionPressure

MPa 3.5 7 14 >20

Type PTFE TemperatureoC

Dimpled Lubricated 20

-25

0.04

0.08

0.03

0.045

0.025

0.04

0.02

0.03Unfilled or Dimpled

Unlubricated

20

-25

0.08

0.20

0.07

0.18

0.05

0.13

0.03

0.10

Filled 20

-25

0.24

0.44

0.17

0.32

0.09

0.25

0.06

0.20

Woven 20-25

0.080.20

0.070.18

0.060.13

0.0450.10

Table 8-2 Design Coefficients of Friction Service Limit State

Steel-reinforced elastomeric bearings are generally designed using either of two methods,

commonly referred to as Method A and Method B. In this Specification Method B is used(if, however, Method A is approved or circular bearings are used, refer to the provisions of

Ref. 3).

The stress limits associated with Method A usually result in a bearing with a lower capacity

than a bearing designed using Method B. This increased capacity resulting from the use of

Method B requires additional testing and quality control.

Steel-reinforced elastomeric bearings shall consist of alternate layers of steel reinforcement

and elastomer bonded together. In addition to any internal reinforcement, bearings may haveexternal steel load plates bonded to either or both of the upper or lower elastomer layers.

Tapered elastomer layers shall not be used. Tapered layers cause larger shear strains and

bearings made with them fail prematurely due to delamination or rupture of thereinforcement. All internal layers should be the same thickness because the strength and

stiffness of the bearing in resisting compressive load are controlled by the thickest layer.

The top and bottom cover layers shall be no thicker than 70 percent of the internal layers.

The shape factor of a layer of an elastomeric bearing, Si, shall be taken as the plan area of thelayer divided by the area of perimeter free to bulge. For rectangular bearings without holes,

the shape factor of a layer shall be taken as:

Si = LW (8.5)2hri(L + W)

where: L = length of a rectangular elastomeric bearing (parallel to longitudinal bridgeaxis) (mm)

W = width of the bearing in the transverse direction (mm)

hri = thickness of ith

elastomeric layer in elastomeric bearing (mm)

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The shape factor, Si, is defined in terms of the gross plan dimensions of layer 1. Refinements

to account for the difference between gross dimensions and the dimensions of the

reinforcement are not warranted because quality control on elastomer thickness has a moredominant influence on bearing behavior.

Holes are strongly discouraged in steel-reinforced bearings. However, if holes are used, theireffect should be accounted for when calculating the shape factor because they reduce theloaded area and increase the area free to bulge. The suitable shape factor formula for

rectangular bearings is:

L W- d2

Si = ________ 4_________

hri(2 L+2 W + . d ) (8.6)

where: d = the diameter of the hole or holes in the bearing (mm)

Material Properties

The material requirements shall be as specified in the below specifications, including shearmodulus G, and nominal hardness.

The elastomer shall have a shear modulus between 0.6 and 1.3 MPa and a nominal hardnessbetween 50 and 60 on the Shore A scale.

The shear modulus of the elastomer at 230C shall be used as the basis for design. If the

elastomer is specified explicitly by its shear modulus, that value shall be used in design, and

the other properties shall be obtained from Table 8-5. If the material is specified by its

hardness, the shear modulus shall be taken as the least favorable value from the range forthat hardness given in Table 8-5. Intermediate values may be obtained by interpolation.

Hardness (Shore A)

50 60 70

Shear Modulus, G, @ 23oC (MPa) 0.66-0.90 0.90-1.38 1.38-2.07

Creep deflection @ 25 years divided by

instantaneous deflection

0.25 0.35 0.45

Table 8-3 Shear Modulus, G

Materials with a nominal hardness greater than 60 are prohibited because they generallyhave a smaller elongation at break and greater stiffness and greater creep than their softer

counterparts. This inferior performance is generally attributed to the larger amounts of filler

present. Their fatigue behavior does not differ in a clearly discernible way from that of softer

materials. Table 8-5 goes up to 70 hardness because the table also refers to plain elastomericpads.

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Creep varies from one compound to another and is generally more prevalent in harder

elastomers but is seldom a problem if high-quality materials are used. This is particularly

true because the deflection limits are based on serviceability and are likely to be controlledby live load, rather than total load. The creep values given in Table 8-5 are representative of

neoprene and are conservative for natural rubber.

Table 8-4 below gives the minimum elastomer grade to be used.

50-Year Low Temperature (oC) -10

Maximum number of consecutive days when the temperature does not riseabove 00C

3

Minimum low-temperature elastomer grade 0

Table 8-4 Low-Temperature and Minimum Grades of Elastomer

Shear modulus, G, is the most important material property for design, and it is, therefore, thepreferred means of specifying the elastomer. Hardness has been widely used in the pastbecause the test for it is quick and simple.

Design Requirements

Steel-reinforced bearings are designed to resist relatively high stresses. Their integrity

depends on good quality control during manufacture, which can only be ensured by rigorous

testing.

Bearings designed by the provisions herein shall be tested in accordance with the

requirements in the Technical Specifications or in Ref. 3, or similar method approved by theEngineer.

Compressive Stress: In any elastomeric bearing layer, the average compressive stress at the

service limit state shall satisfy:

For bearings subject to shear deformation:

s1.66 G S 11.0 MPa (8.7)

L 0.66 G S (8.8)

For bearings fixed against shear deformation:

s 2.0 G S 12.0 MPa (8.9)

L 1.0 G S (8.10)

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where: s = service average compressive stress due to the total load (MPa)L = service average compressive stress due to live load (MPa)G = shear modulus of elastomer (MPa)

S = shape factor of the thickest layer of the bearing

These provisions limit the shear stress and strain in the elastomer. The relationship betweenthe shear stress and the applied compressive load depends directly on the shape factor, withhigher shape factors leading to higher capacities

The compressive limits, in terms of G and S, were derived from tests correlated with theory.

The specified stress limits provide a safety factor of approximately 1.5 against initialdelamination.

The compressive stress limits, in terms of GS, were derived from tests and are based on theobservation that fatigue cracking remained acceptably low if the maximum shear strain due

to total dead and live load was kept below 3.0, and the maximum shear strain range for

cyclic loading was kept below 1.5.

Compressive Deflection of Elastomeric Bearings

Deflections of elastomeric bearings due to total load and to live load alone shall beconsidered separately. Instantaneous deflection shall be taken as:

= .=i hri (8.11)

where: =i = instantaneous compressive strain in ith elastomer layer of a laminated bearing

hri = thickness of ith

elastomeric layer in a laminated bearing (mm)

Values for =i shall be determined from test results or by analysis when considering long-term

deflections. The effects of creep of the elastomer shall be added to the instantaneous

deflection. Creep effects should be determined from information relevant to the elastomeric

compound used, or from the above specifications.

Limiting instantaneous deflections is important to ensure that deck joints and seals are not

damaged. Furthermore, bearings that are too flexible in compression could cause a smallstep in the road surface at a deck joint when traffic passes from one girder to the other,

giving rise to impact loading. A maximum relative deflection across a joint of 3 mm is

suggested. Joints and seals that are sensitive to relative deflections may require limits that

are tighter than this.

Long-term deflections should be considered where joints and seals between sections of thebridge rest on bearings of different design and when estimating redistribution of forces in

continuous bridges caused by settlement. Provided high-quality materials are used, the

effects of creep are unlikely to cause problems.

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Laminated elastomeric bearings have a nonlinear load deflection curve in compression. In

the absence of information specific to the particular elastomer to be used, Figure 8-4 shall be

used as a guide.

Reliable test data on total deflections are rare because of the difficulties in defining the true

0.0 for deflection. However, the change in deflection due to live load can be reliablypredicted either by design aids based on test results or by using theoretically based equations

(Ref. 6). In the latter case, it is important to include the effects of bulk compressibility of the

elastomer, especially for high shape factor bearings.

Figure 8-4: Stress-Strain Curves

Shear Deformation of the Bearing

The horizontal movement of the bridge superstructure, o, shall be taken as the extremedisplacement caused by creep, shrinkage, and post-tensioning, combined with thermal

effects computed in accordance with section 3.21: Uniform Temperature.

The maximum shear deformation of the bearing, at the service limit state, s, shall be takenas o, modified to account for the substructure stiffness and construction procedures. If alow friction sliding surface is installed, s need not be taken to be larger than thedeformation corresponding to first slip.

The bearing shall satisfy: hrt 2s (8.12)

where: hrt = total elastomer thickness (mm)

s = maximum shear deformation of the elastomer at the service limit state (mm)

The shear deformation shall be limited in order to avoid rollover at the edges and

delamination due to fatigue.

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Fatigue tests that formed the basis for this provision were conducted to 20,000 cycles, which

represents one expansion/contraction cycle per day for approximately 55 years (Ref. 7). Theprovisions will, therefore, be unconservative if the shear deformation is caused by high-cycle

loading due to braking forces or vibration. The maximum shear deformation due to these

high-cycle loadings should be restricted to no more than 0.10 hrt, unless better informationis available. At this strain amplitude, the experiments showed that the bearing has anessentially infinite fatigue life.

If the bridge girders are lifted to allow the bearings to realign after some of the girdershortening has occurred, that shall be accounted for in design.

Pier deflections sometimes accommodate a significant portion of the bridge movement, andthis may reduce the movement that must be accommodated by the bearing. Construction

methods may increase the bearing movement because of poor installation tolerances or poor

timing of the bearing installation.

Combined Compression and Rotation of Bearings

The provisions of this section shall apply at the service limit state. Rotations shall be takenas the maximum sum of the effects of initial lack of parallelism and subsequent girder end

rotation due to imposed loads and movements.

Bearings shall be designed so that uplift does not occur under any combination of loads and

corresponding rotations.

Rectangular bearings shall be taken to satisfy uplift requirements if they satisfy:

2

ri

ss

h

B

nGS0.1

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B = length of pad if rotation is about its transverse axis, or width of pad if rotation

is about its longitudinal axis (mm)

s = maximum service rotation due to the total load (RAD)

These provisions address two conditions. Equation 8.13 ensures that no point in the bearing

undergoes net uplift, and Equations 8.14 and 8.15 prevent excessive compressive stress onan edge. When the thickness of an outer layer of elastomer is more than one-half thethickness of an interior layer, the parameter, n, shall be increased by one-half for each such

exterior layer. Uplift must be prevented because strain reversal in the elastomer significantly

decreases its fatigue life.

A rectangular bearing should normally be oriented so its long side is parallel to the axis

about which the largest rotation occurs. The critical location in the bearing for bothcompression and rotation is then at the midpoint of the long side. If rotation occurs about

both axes, uplift and excessive compression should be investigated in both directions.

The interaction between compressive and rotation capacity in a bearing is illustrated inFigure 8-5. It is analogous to the interaction diagram for a reinforced concrete column.

Because a high shape factor is best for resisting compression, but a low one accommodates

rotation most readily, the best choice represents a compromise between the two. The"balanced design" point in Figure 8-5, where uplift and compressive stress are

simultaneously critical, will in many cases provide the most economical solution for a given

plan geometry. Table 8-5 gives coordinates for the balance point for different bearingshapes.

Figure 8-5 Elastomeric BearingInteraction between Compressive Stress and Rotation

Angle

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Bearing TypeKs_

GS

2

ri

s

h

B*

n

Fixed Rectangular 1.636 1.636

Movable Rectangular 1.364 1.364

Table 8-5 Steel-Reinforced Elastomeric Bearings - Balanced Design

Stability of Elastomeric Bearings

Bearings shall be investigated for instability at the service limit state load combinationsspecified in Table 3-2.

Bearings where 2A B (A and B as per below) shall be considered stable, and no furtherinvestigation of stability is required.

for which:

W

L0.21S

L/h92.1A

ri

+

= (8.16)

++=

W4

L1)20S(S

67.2B (8.17)

where: G = shear modulus of the elastomer (MPa)

L =length of a rectangular bearing (parallel to longitudinal bridge axis) (mm)

W =width of the bearing in the transverse direction (mm)

where:

For a rectangular bearing where L is greater than W, stability shall be investigated by

interchanging L and W in Equations 8.16 and 8.17.

For rectangular bearings, the service average compressive stress due to the total load, s,shall satisfy:

If the bridge deck is free to translate horizontally:Ks < G (8.18)

2A - B

If the bridge deck is fixed against horizontal translation:

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BA

Gs

(8.19)

Equation 8.18 corresponds to buckling in a sideways mode and is relevant for bridges in

which the deck is not rigidly fixed against horizontal translation at any point. This shall be

the case in many bridges for transverse translation perpendicular to the longitudinal axis. Ifone point on the bridge is fixed against horizontal movement, the sideways buckling mode is

not possible, and Equation 8.19 should be used. This freedom to move horizontally should

be distinguished from the question of whether the bearing is subject to shear deformationsrelevant to the previous sub-subchapters of this subchapter, entitled Compressive Deflection

of Elastomeric Bearings and Shear Deformation of the Bearing. In a bridge that is fixed at

one end, the bearings at the other end will be subject to imposed shear deformation but willnot be free to translate in the sense relevant to buckling due to the restraint at the opposite

end of the bridge.

Reinforcement of Bearings

The thickness of the steel reinforcement, hs, shall satisfy the following:

At the service limit state:

y

smaxrs

F

h3h

(8.20)

At the fatigue limit state:

TH

smaxrs

F

h0.2h

(8.21)

where: FTH = Constant amplitude fatigue threshold of 165 MPa

hr max = thickness of thickest elastomeric layer in elastomeric bearing (mm)

KL = service average compressive stress due to live load (MPa)

Ks = service average compressive stress due to total load (MPa)

Fy = yield strength of steel reinforcement (MPa)

If holes exist in the reinforcement, the minimum thickness shall be increased by a factor

equal to twice the gross width divided by the net width.

Seismic Provisions for Bearings

Elastomeric expansion bearings shall be provided with adequate seismic resistant anchorageto resist the horizontal forces in excess of those accommodated by shear in the pad. The sole

plate and the base plate shall be made wider to accommodate the anchor bolts. Inserts

through the elastomer should not be allowed, unless approved by the Engineer. The anchorbolts shall be designed for the combined effect of bending and shear for seismic loads.

Elastomeric fixed bearings shall be provided with horizontal restraint adequate for the full

horizontal load.

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The seismic demands on elastomeric bearings exceed their design limits. Therefore, a

positive connection between the girder and the substructure concrete is needed. Holes in

elastomer cause stress concentrations that can lead to tearing of the elastomer duringearthquakes.

8.3.6 ROLLERBEARINGS

The rotation axis of the bearing shall be aligned with the axis about which the largest

rotations of the supported member occur. Provision shall be made to ensure that the bearing

alignment does not change during the life of the bridge. Gearing to ensure that individualrollers remain parallel to each other and at their original spacing, shall connect multiple

roller bearings.

Roller bearings shall be detailed so that they can be easily inspected and maintained.

Cylindrical bearings contain no deformable parts and are susceptible to damage if the

superstructure rotates about an axis perpendicular to the axis of the bearing. Thus, they areunsuitable for bridges in which the axis of rotation may vary significantly under different

situations, such as bridges with a large skew. They are also unsuitable for use in seismic

regions because the transverse shear caused by earthquake loading can cause substantialoverturning moment.

Good maintenance is essential if mechanical bearings are to perform properly. Dirt attractsand holds moisture, which, combined with high local contact stresses, can promote stress

corrosion. Metal bearings, in particular, must be designed for easy maintenance.

Material for roller bearings shall conform to the requirements of the below specifications:

Roller bearings shall be made of hard structural steel conforming to AASHTO M 169

(ASTM A 108), M 102 (ASTM A 668M), or M 270M (ASTM A 709M), Grades 250, 345,or 345W, or similar European Steel according to Table 8-1.

Carbon steel has been the traditional steel used in mechanical bearings because of its goodmechanical properties. Surface hardening shall be considered. Corrosion resistance is also

important. The use of stainless steel for the contact surfaces may prove economical when

life-cycle costs are considered. Weathering steels should be used with caution as theirresistance to corrosion is often significantly reduced by mechanical wear at the surface.

8.4 EXPANSION

JOINTS

If possible, expansion joints should be avoided because of the complexities and the

maintenance needed. Short bridges with a total length less than 15 m and bridges less than

80 m with end-walls need no expansion joints. Medium sized bridges can preferably use asimple type of expansion joint as shown on the Standard Detail Drawings-2002, Chapter 7:

Bridge Drainage, Drawing B-33.

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At long bridges it is wise to use as few large capacity expansion joints as possible (see Figure

8-6). The normal requirements for a factory-made expansion joint is that it should be:

durable and resistible easy to install and inspect

easy to maintain, clean and repair without closing the whole bridge deck and withoutspecialists

designed to create only minor horizontal forces free from disturbing noise when in use easy to receive spare parts without delay

Similar to bearings only a limited number of expansion joint types approved by ERA should

be used as a first choice.

Expansion joints shall be provided where structurally needed. The coefficient of thermal

expansion is given in Chapter 9: Reinforced Concrete, and the temperature range in section

3.21: Temperature Ranges. In addition to the calculated expansion, a displacement toleranceof 10 20 mm shall be added, the lower value for 6 m high support and the higher for 12 m

high support. Linear interpolation shall be used in-between these values. Support heightabove 15 m or in seismic zone 4 shall be investigated separately.

Figure 8-6 Principle for Multi-Panel Expansion Joint

If possible, Type J1 or J2 from the Standard Detail Drawings-2002, Chapter 7: BridgeDrainage, Drawing B-33 shall be used for 10 - 40 mm expansion joints. Greater openings

than 40 mm shall be designed with factory made expansion joints approved by the Engineer.

8.5 RAILINGS

For safety reasons all bridges should be provided with railings. All railings should be

designed and tested, unless the railing from the Standard Detail Drawings-2002, Chapter 2:Guardrail Drawings or Chapter 7: Bridge Drainage, Drawing B-35 is selected. Preferably

the ERA Standard Railing (from Standard Detail Drawings-2002) should be used. Spacing

between posts should be between 1.5 - 2.0 m. Since steel railings have to be imported andfor other reasons, only concrete railings will be considered by ERA especially in rural areas,

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unless otherwise stated in the Contract documents. These are easy to repair and generally

require less maintenance.

The bridge railing performance need not be identical over the whole highway network. New

railing designs should match site needs leading to a multiple test level concept (see Ref. 8).

Previously crash tested railing should retain its test level approval and should not have to bere-tested.

With the finite resources available, it is not reasonable to expect all existing rails to be

updated. Many existing bridge rails have proven functional and need only be replaced whenremoved for bridge widenings.

All railings for traffic lanes or pedestrian lanes shall resist the loads given in Chapter 3:

Load Requirements, Section 3.16: Vehicular Collision Force. Railing material other than

reinforced concrete shall be approved by ERA. Traffic lane guardrail shall not be lower than

900 mm. Railing for pedestrian bridges shall not be lower than 1000 mm. Exterior railing for

bicycle lanes shall not be lower than 1200 mm.

If possible ERA Standard Detail Drawing-2002 No. R-01 shall be used. If this Standard

Railing is not used, the detail design of the railing shall be approved together with the bridgedesign.

8.6 DRAINAGE OF BRIDGE DECKS

Usually a bridge deck over water will be made without curbs, edge-beams, or raised

pedestrian walkways hindering the flow of surface water. Provision for drainage shall beomitted if the deck is designed without a curb and with at least a 2% crossfall

(superelevation).

Where curbs are specified, outlets shall normally be spaced every 5 meters in high rainfallareas, 10 meters in normal rainfall areas, and 20 meters in dry areas, unless otherwise proved

with detail calculations according to the ERA Drainage Design Manual-2002, Chapter 10:

Storm Drainage Facilities

For underpasses especially in urban areas, curbs shall be provided and drainage outlets shall

be provided for at least every 10m unless detail design according to the ERA Drainage

Design Manual-2002, Chapter 10: Storm Drainage Facilities or made according to the ERA

Standard Detail Drawings-2002, Chapter 7: Bridge Drainage, Drawing B-32.

8.7 UTILITIES (SERVICE DUCTS, CABLES, ETC.)

Smaller plastic pipes for cables should be cast in the concrete deck as a first option.

Otherwise they should be placed in the upper part of the girders or, in the case of slabs, inthe center of the slab. Signal cables (Tele-, opto-cables, etc) and electric power cables should

always be laid in separate pipes. In urban areas at least 3 pipes 70 mm are recommendedfor future use.

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Water and/or Sewage pipes should always be regarded as filled when calculated as

supported by the bridge. Arrangements for pipe expansion should preferably be applied at

the same place as the expansion joints for the bridge. The pipes cannot interfere with thewaterway area. Pipes under pressure should be protected by insertion in a protection pipe.

Electric Light posts should be avoided at bridges. Where this is not possible, they shall beplaced for safety reasons on brackets 0.5 m outside the guardrail. Here they will not interfere

in a collision with the railing.

REFERENCES

1. Roark, R. J., and W. C. Young. Formulas for Stress and Strain. 5th

Ed. McGraw Hill:New York, 1976.

2. Campbell, T. I., and W. L. Kong. TFE Sliding Surfaces in Bridge Bearings. Report ME-

87-06. Ontario Ministry of Transportation and Communications, Downsview, Ontario,

1987.3. AASHTO LRFD Bridge Design Specifications 2nd Edition, 1998.

4. Roeder, C. W., J. F. Stanton, and A. W. Taylor. Performance of Elastomeric Bearings.

NCHRP Report 298. TRB, National Research Council, Washington DC, October 1987.5. Roeder, C. W., and J. F. Stanton. State of the Art Elastomeric Bridge Bearing Design.

ACI Structural Journal, Vol. 88, No. 1, 1991.

6. Stanton, J. F., and C. W. Roeder. Elastomeric Bearings Design, Construction, andMaterials. NCHRP Report 248. TRB, National Research Council, Washington DC,August 1982.

7. Roeder, C. W., J. F. Stanton, and A. W. Taylor. Fatigue of Steel-Reinforced

Elastomeric Bearings. Journal of Structural Division, ASCE, Vol. 116, No. 2, February1990.

8. Ross. H. E., D. L. Sicking, R. A. Zimmer, and J. D. Michie. Recommended Procedures

for the Safety Performance Evaluation of Highway Features. NCHRP Report 350. TRB,National Research Washington, D. C., 1993.

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