751.11 Bearings

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751.11.1 General

751.11.1.1 Bearing Types

Elastomeric Bearings

Elastomeric bearings are economical bearings. Typically the footprint of elastomeric bearings is rectangular or square; however, circular elastomeric bearings may be considered for skewed bridges or short span curved steel bridges with lower vertical loading, translational and rotational demands where multi-rotational (pot) bearings could be unnecessary overdesign. See Structural Project Manager or Structural Liaison Engineer before using circular elastomeric bearings. Elastomeric bearings may be either plain or laminated (steel reinforced) pads. There are three types of elastomeric bearings utilized by MoDOT based on pad type and function:

  1. Plain fixed – integral bents only (Use laminated fixed if slope or grade dictates.)
  2. Laminated fixed (Use girder chairs if minimum dead load, slope or grade dictates.)
  3. Laminated expansion


PTFE Bearings

PTFE type bearings shall be used when laminated elastomeric expansion bearings are not capable of handling the required expansion length or the required total thickness of elastomer exceeds 5”.


Multi-Rotational High Load Bearings

Pot bearings can be considered for use on bridges when bearing design service reaction exceeds 300 kips or bearing area exceeds 15” x 24” and effects of curvature are considered. Due to high cost, limited use of these types of bearings are recommended. Consult Structural Project Manager before using Pot Bearings.


Girder/Beam Chairs

Chairs shall be used at integral bents for prestressed and steel girders and beams if slope requirements cannot be met by elastomeric laminated fixed bearings.

Chairs shall also be used for prestressed girders and beams at fixed intermediate bents where minimum dead load or slope requirements cannot be met by elastomeric laminated fixed bearings and under the smaller girder where girder sizes change.


Bearing Type Location

The following figure shows an example of what type of bearings should be used at the given locations.

751.11 Bearing Type Location.gif
  1. Integral construction (end bent typ.) – Use plain fixed elastomeric bearing or laminated fixed if necessary.
  2. Flexible intermediate bent (pile cap) – Use laminated fixed bearings for flexible intermediate bents.
  3. Rigid intermediate bent w/out thermal expansion – Use laminated fixed bearings at bents with no temperature movement.
  4. Rigid intermediate bent w/ thermal expansion – Use laminated expansion bearings or PTFE/Pot bearings if necessary.
  5. Rigid end bent w/ thermal expansion - Use laminated expansion bearings or PTFE/Pot bearings if necessary.

751.11.1.2 Limit States and Load Factors

Limit States and Load Factors

In general, each component shall satisfy the following equation:

\,Q = \sum{n_i}\ {\gamma_i} \ {Q_i} \le \phi  {R_n}= R_r


Where:

\,Q = Total factored force effect
\,Q_i = Force effect
\,n_i = Load modifier
\,\gamma_i = Load factor
\,\phi = Resistance factor
\,R_n = Nominal resistance
\,R_r = Factored resistance

Limit States

The following limit states shall be considered for bearing design:
STRENGTH – I
SERVICE – I
See LRFD Table 3.4.1-1 and LRFD 3.42 for Loads and Load Factors applied to each given limit state.
Service loads without including dynamic load allowances shall be used for bearing design.

Resistance Factors

STRENGTH limit states, see LRFD Article 6.5.4.2 and 5.5.4.2
For all other limit states, \,\phi= 1.00

Load Modifiers

751.11.2 Design

751.11.2.1 Elastomeric Bearings

General

Elastomeric Bearing design shall follow AASHTO LRFD “Method A”.

The elastomeric expansion bearings and fixed bearings for steel girders consist of a sole plate and elastomeric bearing pad. Elastomeric bearings at integral end bents and fixed bearings for prestressed girders at intermediate bents consist of elastomeric bearing pad without a sole plate.

The sole plates are flat or beveled to match the profile grade of the roadway surface along the centerline of the girder. If the profile grade is equal to or less than 0.01 or the total drop across the bearing is equal to or less than 1/8 inch, then a flat sole plate may be used and if necessary, elastomer thickness increased to address the bearing rotation due to the profile grade. Sole plates are used to anchor girders, ensure uniform distribution of the compressive stress and strain over bearing area and reduce dead load bearing rotations.

At fixed bearings, girder chairs may be considered as an alternate if roadway slope, rotation or bearing pressure is requiring tall or large bearing pads.

The elastomer bearing pad shall be 60 durometer hardness and reinforced with 1/8 inch steel shim plates when laminated pads are required by design.

When rectangular bearings are used, increased rotation bearing capacity can be achieved by orientating the pad with the shorter dimension of the pad parallel with the span of the girder.

Plain elastomeric bearing pads should be utilized where vertical loads, translations and rotations are relatively small. For integral end bents, use ½” fixed plain pads when taper due to girder slope or grade is less than 1/8”, or use a laminated bearing pad when taper equals or exceeds 1/8” due to girder slope or grade. For integral intermediate bents, though rarely used, consideration can be given to utilizing ½” plain pads under similar conditions of slope.

Laminated elastomeric bearing pads should be utilized where there is greater need for vertical loading, translational and rotational capacity. For non-integral end bents, non-integral intermediate bents and for integral end bents when taper exceeds 1/8” due to girder slope or grade use laminated elastomeric bearing pads where the pad thickness and number of laminates is based on design that should account for larger vertical loads, translation, rotation and meet slope of girder and grade requirements.


Size Limitations

Use the values in the standard bearing pad tables if possible.

Bearing pad length shall be 8” minimum for MoDOT Standard Prestressed (P/S) I-Girders, Adjacent P/S Box Beams and Steel I-Girders.

Bearing pad length shall be 5” minimum for P/S NU Girders and P/S Spread Box Beams. Not preferred but for consideration of some lower bound limits as used by the Nebraska Department of Roads (NDOR) that developed the NU I-Girder, for P/S NU Girders only, and based on NDOR guidelines, a 4” minimum bearing pad length and/or 2 ft. minimum bearing pad width can be used with Structural Project Manager or Structural Liaison Engineer approval.

Plain Fixed for P/S I-Girder:
W, width of bearing ≤ Bottom flange width – 1.5”


Plain Fixed for steel girders:
Bottom flange width – 2” ≤ W ≤ Bottom flange width


Laminated Fixed for P/S I-Girder:
W = Bottom Flange width – 1.5”


Laminated Expansion Bearing Pads for PS I-Girders and Laminated Fixed and Expansion Pads for Steel Girders:
9” ≤ W ≤ Bottom flange width + 7”


Stability

The following requirement shall be met for ensuring stability of the bearing pad:

Rectangular Pads

\,h_{rt} \le MIN \big\{L/3, W/3\big\}

Circular Pads

\,h_{rt} \le {D/4}


Temperature Movement

Determining temperature movements for bearings requires the calculation of the thermal origin of the bridge. To accomplish this, the stiffness of each bent must be calculated. Once the thermal origin is established, the total temperature movement for each bearing location can be found by the following equations:

\,\triangle_s = \gamma \alpha (temperature range)x(distance from thermal origin)(0.65)

Where:

\,\triangle_s = maximum shear deformation of the pad
\,\gamma = 1.2 for laminated pads
  = 1.0 for PTFE bearings
\,\alpha = coefficient of thermal expansion
  = 0.0000065 IN/IN/ºF (steel superstructure)
  = 0.000006 IN/IN/ºF (concrete superstructure)
(0.65) = 65% reduction due to forgiving nature of elastomer (LRFD 14.7.6.3.4)
temperature range = 150ºF (steel superstructure)
  = 120ºF (concrete superstructure)


Shear Deformation

Both plain elastomeric and laminated elastomeric shall meet the following criteria for shear deformation.

\, h_{rt} \ge\ 2 \triangle_s

Where:

\, h_{rt} = total elastomer thickness, in.

The following table represents the available heights of laminated expansion pads that are required due to the shear deformation criteria. PTFE type bearings shall be used if \,\triangle_s >2.5\ in.

Laminated Expansion Bearing Pad Heights
Laminated Expansion Pads
\,\triangle_s, in. Interior layer thickness, in. Exterior layer thickness, in. \,h_{rt}, in. n C, in.
1/2 1/2 1/4 1 2 1 1/4
3/4 1/2 1/4 1 1/2 3 1 7/8
1 1/2 1/4 2 4 2 1/2
1 1/4 1/2 1/4 2 1/2 5 3 1/8
1 1/2 1/2 1/4 3 6 3 3/4
1 3/4 1/2 1/4 3 1/2 7 4 3/8
2 1/2 1/4 4 8 5
2 1/4 1/2 1/4 4 1/2 9 5 5/8
2 1/2 1/2 1/4 5 10 6 1/4

Where:

C = total thickness of bearing including steel shims, in.
n = total number of interior layers of elastomer + 1*

* The additional “1” is accounting for the two exterior layers as per LRFD 14.7.5.3.5


Compressive Stress

Service loads without including dynamic load allowance shall be used for design checks.

At intermediate bents with 2 bearing pads per girder line (i.e. PS I-girders), use ½ of the live load reaction for each pad.


Total Load

Plain Elastomeric Pad

\, \sigma_{TL} \le \ 0.800 ksi


Laminated Elastomeric Pad

\, \sigma_{TL} \le 1.00 GS

and

\, \sigma_{TL} \le 1.0 ksi


Minimum Dead Load

\, \sigma_{DLmin} \ge 0.200 ksi


The 200 psi minimum requirement is intended for preventing the horizontal crawling of the bearing when it is not attached to the top surface. This requirement has been applied to the bearing designs detailed in EPG 751 even though these bearings are detailed with positive attachment to the flange of the girder. Compliance with the requirement is desirable but is not mandatory if it results in a special bearing design or special superstructure treatments.


Where:

\,\sigma_{TL} = compressive stress due to total load = \, \frac{DC + DW + LL} {L \times W}
\,\sigma_{DLmax} =compressive stress due to maximum dead load = \, \frac{DC + DW } {L \times W}
\,\sigma_{DLmin} =compressive stress due to minimum dead load = \, \frac{DC} {L \times W}
G = shear modulus = 0.130 ksi for compressive stress calculations
S = shape factor for thickest layer of elastomer = \, \frac{LW}{2h_{ri}(L + W)}
S = shape factor for circular pads = \, \frac{D}{4h_{ri}}
\,h_{ri} = thickness of the ith elastomer layer, in.


Rotation

Rotations shall be taken as the maximum possible change in slope between the top and bottom surfaces of the bearing caused by the initial lack of parallelism between the bottom of girder flange and top of bearing and the girder end rotation due to imposed loads and movements. Rotations may be calculated by a straight-line approximation of dead and live load deflections taken at 1/10 or 1/4 points. The following equation must be satisfied to ensure that uplift does not occur under any combination of loads and corresponding rotation:

Rectangular Laminated Elastomeric Pad

\,\sigma_{s} \ge \ 0.5GS \left (\frac{L}{h_{ri}} \right)^2 \frac{\theta_s}{n}

Where:

L = length of bearing pad in the direction of traffic
W = width of bearing pad in the direction perpendicular to traffic
G = shear modulus = 0.200 ksi for rotation calculations
\,\theta_s = maximum rotation about the transverse axis due to initial lack of parallelism and total service load, rad
n = total number of interior layers of elastomer + 1*

* The additional “1” is accounting for the two exterior layers as per LRFD 14.7.5.3.5


Plain elastomeric pads contained within integral concrete diaphragms are not subject to this rotation requirement.

This criteria is intended as an uplift check. If uplift is not possible at the bearing due to an integral diaphragm/beam at the abutment or an integral diaphragm/beam at an intermediate bent, then this criteria would not be applicable.

Circular Laminated Elastomeric Pad

\,\sigma_{s} \ge \ 0.375GS \left (\frac{D}{h_{ri}} \right)^2 \frac{\theta_s}{n}

Where:

D = diameter of pad


Compressive Deflection

Deflections of elastomeric bearings should be limited to ensure that deck joints and seals are not damaged. Also, bearings that are too flexible can produce a small step across a deck joint giving rise to a high impact loading when traffic passes from one girder to the other. The maximum relative deflection across a joint is suggested to be less than 1/8”.

Initial compressive deflection of plain elastomeric or in any layer of steel reinforced elastomeric bearing at the service limit state without impact shall not exceed \, 0.07 h_{ri}.

Values for compressive strain can be calculated by using LRFD Figure C14.7.6.3.3-1 for 60 durometer reinforced bearings.

There is a code check for compressive deflection of a single layer but no code check for total compressive deflection.


Taper

When the difference between the slope of the girder and the slope of a bearing pad exceeds 1/8” taper shall be considered. For laminated expansion pads for both PS I-Girders and steel girders and for laminated fixed pads for steel girders where sole plates are required, sole plates shall be tapered to account for the girder slope. Sole plates shall have a minimum thickness of 1 1/2” at the centerline of bearing, and a minimum thickness of 1” at the edge. Plain fixed pads shall not be tapered. At integral end bents where girder slope is greater than 1/8”, use laminated fixed pads. Laminated bearing taper is provided by tapering the top shim to match the slope of the girder to the nearest 1/8” total difference in thickness. Thickness of shims shall be a minimum of 1/8” and a maximum of 1/2”. For excessive girder slopes it may be necessary to taper the top two shims. Tapered layers of elastomer are not permitted.

751.11 Elastomer Bearing Tapered Single.gif 751.11 Elastomer Bearing Tapered Multi.gif


Anchor Bolts

Check with Structural Project Manager or Liaison before using anchor bolts other than ASTM F1554 Grade 55. When anchor bolts are used for laminated fixed for steel girders or laminated expansion for steel and P/S I-girders, they should be designed for a minimum horizontal force equal to 20% of the maximum dead load applied to the bearing.

ASTM F1554 Grade 105 anchor bolts may be used when Grade 55 is not adequate. (Revise anchor bolt notes in EPG 751.50 Standard Detailing Notes for plans with different grade and nuts, e.g. “ASTM F1554 Grade 55” to “ASTM F1554 Grade 105” and “ASTM A563 Grade A Hex or Heavy Hex nuts” to “ASTM A563 Grade DH Heavy Hex nuts”.)


Maximum Dead Load Reactions for given Anchor Bolts and Height of Pad


Maximum Dead Load Reaction, kips (Non-Seismic)
\, \triangle_s 1/4 1/2 3/4 1 1 1/4 1 1/2 1 3/4 2 2 1/4
\, h_{rt} 1/2 1 1 1/2 2 2 1/2 3 3 1/2 4 4 1/2
Anchor Bolt Diameter, in.
1 78 53 41 33 27 23 21 18 16
1 1/4 153 105 80 64 54 46 41 36 33
1 1/2 265 182 138 112 94 80 71 63 57
2 628 431 329 265 222 191 168 150 135
2 1/2 1227 843 642 519 435 374 329 293 264
3* 2120 1457 1110 897 752 647 568 507 457
3 1/4* 2696 1853 1412 1140 956 823 723 644 581

Note: Calculations were based on 1-1/2" thick sole plate at centerline of bearing and a 2 anchor bolt (ASTM F1554 Grade 55) pattern.

* Consult Structural Project Manager before using bolt diameters larger than 2 1/2".


Use shear blocks if necessary to limit to two anchor bolts.

751.11.2.2 Standard Pad Tables for P/S I-Girders

Standard Laminated Expansion Bearing Pads for Prestress I-Girders
Bottom Flange Width in. L in. W in. S Int. Layer S Ext.Layer DL min kips DLmax + LL kips \, \sigma_{TL} < ksi
17 9 12 5.14 10.29 21.6 72.2 0.669
17 10 14 5.83 11.67 28.0 106.2 0.758
17 11 15.5 6.43 12.87 34.1 142.6 0.836
17 12 15.5 6.76 13.53 37.2 163.5 0.879
17 13 15.5 7.07 14.14 40.3 185.2 0.919
17 14 15.5 7.36 14.71 43.4 207.5 0.956
18 9 12 5.14 10.29 21.6 72.2 0.669
18 10 14 5.83 11.67 28.0 106.2 0.758
18 11 16 6.52 13.04 35.2 149.1 0.847
18 12 16.5 6.95 13.89 39.6 178.8 0.903
18 13 16.5 7.27 14.54 42.9 202.8 0.945
18 14 16.5 7.57 15.15 46.2 227.4 0.985
19 9 13 5.32 10.64 23.4 80.9 0.691
19 10 14 5.83 11.67 28.0 106.2 0.758
19 11 16 6.52 13.04 35.2 149.1 0.847
19 12 17.5 7.12 14.24 42.0 194.3 0.925
19 13 17.5 7.46 14.92 45.5 220.6 0.970
19 14 17.5 7.78 15.56 49.0 245.0 1.000
24 12 18 7.20 14.40 43.2 202.2 0.936
24 13 20 7.88 15.76 52.0 260.0 1.000
24 14 22 8.56 17.11 61.6 308.0 1.000
24 15 22.5 9.00 18.00 67.5 337.5 1.000
24 16 22.5 9.35 18.70 72.0 360.0 1.000
24 17 22.5 9.68 19.37 76.5 382.5 1.000
24 18 22.5 10.00 20.00 81.0 405.0 1.000

Note:

Pads shown in table must be checked for:

Shear deformation, Stability, Rotation, and Compressive deflection.


Standard Laminated Fixed Bearing Pads for Prestress I-Girders
Bottom Flange Width in. L in. W in. S Ext.Layer DL min kips DLmax + LL kips \, \sigma_{TL} < ksi
17 8 15.5 10.55 24.8 124.0 1.000
17 9 15.5 11.39 27.9 139.5 1.000
17 10 15.5 12.16 31.0 155.0 1.000
17 11 15.5 12.87 34.1 170.5 1.000
17 12 15.5 13.53 37.2 186.0 1.000
17 13 15.5 14.14 40.3 201.5 1.000
17 14 15.5 14.71 43.4 217.0 1.000
18 8 16.5 10.78 26.4 132.0 1.000
18 9 16.5 11.65 29.7 148.5 1.000
18 10 16.5 12.45 33.0 165.0 1.000
18 11 16.5 13.20 36.3 181.5 1.000
18 12 16.5 13.89 39.6 198.0 1.000
18 13 16.5 14.54 42.9 214.5 1.000
18 14 16.5 15.15 46.2 231.0 1.000
19 8 17.5 10.98 28.0 140.0 1.000
19 9 17.5 11.89 31.5 157.5 1.000
19 10 17.5 12.73 35.0 175.0 1.000
19 11 17.5 13.51 38.5 192.5 1.000
19 12 17.5 14.24 42.0 210.0 1.000
19 13 17.5 14.92 45.5 227.5 1.000
19 14 17.5 15.56 49.0 245.0 1.000
24 8 22.5 11.80 36.0 180.0 1.000
24 9 22.5 12.86 40.5 202.5 1.000
24 10 22.5 13.85 45.0 225.0 1.000
24 11 22.5 14.78 49.5 247.5 1.000
24 12 22.5 15.65 54.0 270.0 1.000
24 13 22.5 16.48 58.5 292.5 1.000
24 14 22.5 17.26 63.0 315.0 1.000
24 15 22.5 18.00 67.5 337.5 1.000

Note:

Pads sizes in above tables (not at integral bents) must be checked for:
Shear deformation, Stability, Rotation, and Compressive deflection.

For pads at integral bents:
Shear deformation and Rotation need not be checked.

For fixed bearings requiring more than two layers of 1/4” elastomer separated by 1/8” shim, use shape factor calculated for the interior layer of elastomer in the laminated expansion bearing pad table.


Standard Plain Fixed Bearing Pads for Prestress I-Girders
Bottom Flange Width in. L in. W in. S Single Layer DL min kips DLmax + LL kips \, \sigma_{TL} < ksi
17 9 12 5.14 21.6 86.4 0.800
17 9 14 5.48 25.2 100.8 0.800
17 9 15.5 5.69 27.9 111.6 0.800
17 10 14 5.83 28.0 112.0 0.800
17 10 15.5 6.08 31.0 124.0 0.800
17 11 15.5 6.43 34.1 136.4 0.800
17 12 15.5 6.76 37.2 148.8 0.800
17 13 15.5 7.07 40.3 161.2 0.800
17 14 15.5 7.36 43.4 173.6 0.800
18 9 12 5.14 21.6 86.4 0.800
18 9 14 5.48 25.2 100.8 0.800
18 9 16.5 5.82 29.7 118.8 0.800
18 10 14 5.83 28.0 112.0 0.800
18 10 16.5 6.23 33.0 132.0 0.800
18 11 16 6.52 35.2 140.8 0.800
18 11 16.5 6.60 36.3 145.2 0.800
18 12 16.5 6.95 39.6 158.4 0.800
18 13 16.5 7.27 42.9 171.6 0.800
18 14 16.5 7.57 46.2 184.8 0.800
19 9 13 5.32 23.4 93.6 0.800
19 9 15 5.63 27.0 108.0 0.800
19 9 17.5 5.94 31.5 126.0 0.800
19 10 14 5.83 28.0 112.0 0.800
19 10 16 6.15 32.0 128.0 0.800
19 10 17.5 6.36 35.0 140.0 0.800
19 11 17.5 6.75 38.5 154.0 0.800
19 12 17.5 7.12 42.0 168.0 0.800
19 13 17.5 7.46 45.5 182.0 0.800
19 14 17.5 7.78 49.0 196.0 0.800
24 12 18 7.20 43.2 172.8 0.800
24 12 20 7.50 48.0 192.0 0.800
24 12 22.5 7.83 54.0 216.0 0.800
24 13 20 7.88 52.0 208.0 0.800
24 13 22.5 8.24 58.5 234.0 0.800
24 14 22.5 8.63 63.0 252.0 0.800
24 15 22.5 9.00 67.5 270.0 0.800
24 16 22.5 9.35 72.0 288.0 0.800
24 17 22.5 9.68 76.5 306.0 0.800
24 18 22.5 10.00 81.0 324.0 0.800


Note:

For integral bents, use a laminated bearing when taper over 1/8” is required.

751.11.2.3 Standard Pad Tables for Steel Girders

Standard Plain Fixed Bearing Pads for Steel Girders of Beams
L in. W in. S Single Layer DL min kips DLmax + LL kips \, \sigma_{TL} < ksi
10 10 5.00 20.0 80.0 0.800
12 10 5.45 24.0 96.0 0.800
14 10 5.83 28.0 112.0 0.800
10 11 5.24 22.0 88.0 0.800
12 11 5.74 26.4 105.6 0.800
14 11 6.16 30.8 123.2 0.800
12 12 6.00 28.8 115.2 0.800
14 12 6.46 33.6 134.4 0.800
16 12 6.86 38.4 153.6 0.800
12 13 6.24 31.2 124.8 0.800
14 13 6.74 36.4 145.6 0.800
16 13 7.17 41.6 166.4 0.800
12 14 6.46 33.6 134.4 0.800
14 14 7.00 39.2 156.8 0.800
16 14 7.47 44.8 179.2 0.800
14 15 7.24 42.0 168.0 0.800
16 15 7.74 48.0 192.0 0.800
18 15 8.18 54.0 216.0 0.800
14 16 7.47 44.8 179.2 0.800
16 16 8.00 51.2 204.8 0.800
18 16 8.47 57.6 230.4 0.800
14 17 7.68 47.6 190.4 0.800
16 17 8.24 54.4 217.6 0.800
18 17 8.74 61.2 244.8 0.800
16 18 8.47 57.6 230.4 0.800
18 18 9.00 64.8 259.2 0.800
20 18 9.47 72.0 288.0 0.800
16 19 8.69 60.8 243.2 0.800
18 19 9.24 68.4 273.6 0.800
20 19 9.74 76.0 304.0 0.800
18 20 9.47 72.0 288.0 0.800
20 20 10.00 80.0 320.0 0.800
22 20 10.48 88.0 352.0 0.800


Use only plain pads at integral bents. Do not use sole plate.
Use girder chairs instead of widening beam.


Standard Laminated Bearing Pads for Steel Girders and Beams
B min. L in. W in. S Int. Layer S Ext. Layer \, h_{rt} max in. \, \triangle_s max in. DL min kips DLmax + LL kips \, \sigma_{TL} < ksi
9 9 12 5.14 10.29 3.0 1.50 21.6 72.2 0.669
9 10 14 5.83 11.67 3.0 1.50 28.0 106.2 0.758
9 11 16 6.52 13.04 3.5 1.75 35.2 149.1 0.847
11 12 18 7.20 14.40 4.0 2.00 43.2 202.2 0.936
13 13 20 7.88 15.76 4.0 2.00 52.0 260.0 1.000
15 14 22 8.56 17.11 4.5 2.25 61.6 308.0 1.000
17 15 24 9.23 18.46 5.0 2.50 72.0 360.0 1.000
19 16 26 9.90 19.81 5.0 2.50 83.2 416.0 1.000
21 17 28 10.58 21.16 5.0 2.50 95.2 476.0 1.000
23 18 30 11.25 22.50 5.0 2.50 108.0 540.0 1.000
27 20 34 12.59 25.19 5.0 2.50 136.0 680.0 1.000
9 12 9 5.14 10.29 3.0 1.50 21.6 72.2 0.669
9 14 10 5.83 11.67 3.0 1.50 28.0 106.2 0.758
9 16 11 6.52 13.04 3.5 1.75 35.2 149.1 0.847
9 18 12 7.20 14.40 4.0 2.00 43.2 202.2 0.936
9 20 13 7.88 15.76 4.0 2.00 52.0 260.0 1.000
9 22 14 8.56 17.11 4.5 2.25 61.6 308.0 1.000
9 24 15 9.23 18.46 5.0 2.50 72.0 360.0 1.000
9 26 16 9.90 19.81 5.0 2.50 83.2 416.0 1.000
10 28 17 10.58 21.16 5.0 2.50 95.2 476.0 1.000
11 30 18 11.25 22.50 5.0 2.50 108.0 540.0 1.000
13 34 20 12.59 25.19 5.0 2.50 136.0 680.0 1.000


Where:

DL+LL max||= DC + DW + LL w/out dynamic load allowance
B min = minimum allowable bottom flange width
\, h_{rt} max = maximum allowable total thickness of elastomer
\, \triangle_s max = maximum allowable shear deformation
DL min = DC
DL max = DC + DW

751.11.2.4 PTFE (Polytetraflouroethylene) Bearings

General

PTFE sliding bearing shall consist of a lower unit and an upper unit as follows:

The lower unit shall be PTFE bonded to a 1/8” stainless steel plate which is vulcanized to a neoprene pad.

The upper unit shall be a 1/8” highly polished stainless steel plate welded to a steel sole plate.

70 Durometer hardness is to be used for sliding bearings.


Design:

Neoprene Elastomeric Pad

Maximum (DL +LL) Compressive Stress = 1000 psi
Minimum (DL) Compressive Stress = 200 psi

751.11.2.5 Multi-Rotational High Load Bearings

General

Pot and Disc bearings are the most commonly used multi-rotational bearings. Pot bearings carry vertical loads by compression of an elastomeric disc confined in a steel cylinder and accommodates rotation by deformation of the disc. Disc bearings accommodate vertical loads and rotation through the deformation of an unconfined or partially confined single elastomeric disc that is molded from a urethane compound.

Pot and disc bearings are commonly available in three different types:

1)Fixed – movement is not allowed and horizontal forces can be resisted from any direction.

2)Guided Expansion – allows for free movement in one direction. Horizontal forces can be resisted perpendicular to the free movement direction. For curved bridges, assume the free movement direction to be along a chord connecting the ends of the beam.

3)Expansion – unguided movement is allowed in all horizontal directions. Horizontal forces cannot be resisted.


The following design information shall be shown on the bearing sheets for each bearing location.

  • Type (fixed, guided-expansion, expansion)
  • Maximum Vertical Design Load
  • Minimum Vertical Design Load
  • Lateral Design Load (only when exceeding 10% of the Vertical Design Load)
  • Total Required Movement
  • Minimum Required Rotation
  • Slope of girder bottom
  • Bottom of girder elevations at centerline of bearing
  • Maximum Estimated Bearing Assembly Height
  • Location, Size, and Number of anchor bolts
  • Length and Width of masonry plate

Include the following note on the appropriate substructure sheets when multi-rotational bearings are used:

“Any modifications required to meet the height of bearings shown on bridge plans will be the responsibility of the contractor. Cost of any modification required shall be borne by the contractor.”

Inverted pot bearing configurations are not permitted.


Design

Pot or disc bearing design for structures shall follow LRFD 14.7.4 or 14.7.8 respectively.

Maximum and Minimum vertical loads shall be determined at the Service-I limit state. The minimum vertical load should not be less then 20 percent of the maximum vertical design load.

The lateral load provided by the manufacturer may be assumed 10 percent of the total vertical load. If applicable strength or extreme event load combinations result in a load greater than 10 percent, the required load shall be shown on the plans.

The total temperature movement required at a bent may be calculated using the procedure given in this section. Transverse movements should also be considered for large roadway widths. On wide roadways (greater than 40’) an increase in the standard clearance between the guide bar and bearing may be required.

The rotation due to factored loads, \, \phi_u, shall be determined from the controlling strength limit state. The minimum rotation required shall include an additional 0.01 radians for fabrication and installation tolerances and uncertainties.

Due to a variety of preferences among multi-rotational bearing fabricators, explicit details are not provided in the plans. Instead, the fabricator determines the sizes of all the bearing components, from the masonry plate to the sole plate. As a guide, the following equation may be used to estimate the height of the assembly for design or the table below.

H = 6.5 + P/ 400
Where:
P = Total load calculated at the service-I limit state (excluding impact), k
H = Height of the bearing assembly measured from the bottom of the girder to the beam seat elevation, in

When determining the longitudinal load distributed to the substructure for expansion bearings, a friction coefficient of 0.05 shall be used.


Details

Bevel sole plate to match the slope of girder to the nearest 1/8” total difference in thickness across the plate.

When large movements are anticipated at a bearing location, three pairs of bearing stiffeners should be considered to help distribute the load.

Minimum of four anchor bolts are required for the connection of the masonry plate to the substructure.


Estimated Pot Bearing Dimensions

The values shown are from one manufacturer and are based on assumed horizontal load requirements (Seismic Performance Category A). The designer should contact manufacturers with specific requirements for their products to get more detailed information on bearing dimensions.

These tables are to be used as a reference to assist the designer in estimating the size of the sole plate and the masonry plate.

Dimensions for Guided Expansion Bearing   Dimensions for Fixed
Total Vertical Load kips Total Horiz. Load kips A Inches B Inches O.D. Inches D Inches   Total Vertical Load kips Total Horiz. Load kips A Inches B Inches O.D. Inches D Inches
50 10 10 10.25 6.375 4.125   50 10 6.5 6.5 6.375 3
75 14.9 11 11.25 7.25 4.25   75 14.9 7.5 7.5 7.25 3
100 20 11.75 12.25 8.125 4.5   100 20 8.25 8.25 8.125 3.25
150 30 14.25 13.5 10 4.875   150 30 9.75 9.75 10 3.375
200 40 15.75 14.5 11.75 5   200 40 10.75 10.75 11.75 3.5
250 50 17.75 16 12.875 5.5   250 50 11.75 11.75 12.875 3.625
300 60 19.25 16.25 14.25 6   300 60 12.75 12.75 14.25 3.625
350 70 20 18.25 15.875 6.375   350 70 13.5 13.5 15.875 4.125
400 80 22.25 19 17.125 6.875   400 80 14.5 14.5 17.125 4.125
450 90 23 19 18.375 7   450 90 15 15 18.375 4.125
500 100 23.75 22.75 19.375 7   500 100 15.75 15.75 19.375 4.25
550 110 24.5 22.75 19.75 7.375   550 110 16.5 16.5 19.75 4.375
600 120 26.5 22.75 21.375 8.25   600 120 17 17 21.375 4.625
650 130 27.25 22.75 22.625 8.25   650 130 17.75 17.75 22.625 4.75
700 140 27.75 27.25 23.625 8.375   700 140 18.25 18.25 23.625 4.75
750 150 28.25 27.25 24.125 8.5   750 150 18.75 18.75 24.125 4.875
800 160 28.25 27.25 25 8.875   800 160 19.5 19.5 25 5
850 170 30.75 26.5 25.875 9.375   850 170 20 20 25.875 5
900 180 31.5 26.5 26.75 9.375   900 180 20.5 20.5 26.75 5
950 190 32 31.75 26.875 9.625   950 190 21 21 26.875 5.375
1000 200 32.25 31.75 27.5 9.75   1000 200 21.5 21.5 27.5 5.375
1100 220 33.5 31.75 28.875 9.875   1100 220 22.25 22.25 28.875 5.5
1200 240 26.75 26.5 30.75 10.375   1200 240 23.25 23.25 30.75 5.625
1300 260 27.875 27.5 32.25 10.625   1300 260 24 24 32.25 5.75
1400 280 28.75 28.25 32.625 11.125   1400 280 25 25 32.625 6.125
1500 300 29.625 29 33.5 11.5   1500 300 25.75 25.75 33.5 6.125
1600 320 30.5 29.75 34.75 11.75   1600 320 26.5 26.5 34.75 6.25
1700 340 31.5 30.5 35.75 12   1700 340 27.25 27.25 35.75 6.375
1800 360 32.5 31.5 37 12.25   1800 360 28 28 37 6.5
1900 380 33 33.125 37.375 12.5   1900 380 28.5 28.5 37.375 6.75
2000 400 33.75 35.125 38.125 12.5   2000 400 29.25 29.25 38.125 6.875
2100 420 34.25 38.125 39.125 12.375   2100 420 30 30 39.125 6.875
2200 440 35 38 40.125 12.625   2200 440 30.5 30.5 40.125 6.875
2300 460 35.875 38.75 41.625 13.125   2300 460 31.25 31.25 41.625 7.25
2400 480 36.25 41.25 41.875 13.125   2400 480 31.75 31.75 41.875 7.5
2500 500 37.125 42 42.625 13.375   2500 500 32.5 32.5 42.625 7.625
2600 520 37.875 42.75 43.5 13.625   2600 520 33 33 43.5 7.625
2700 540 38.5 43.25 44.375 13.75   2700 540 33.75 33.75 44.375 7.75
2800 560 39.125 43.75 45.25 14   2800 560 34.25 34.25 45.25 7.75
2900 580 40 44.5 46.25 14.125   2900 580 34.75 34.75 46.25 7.875
3000 600 40.5 45 46.625 14.375   3000 600 35.25 35.25 46.625 7.875
3100 620 41.125 45.5 45.625 14.75   3100 620 36 36 45.625 8.25
3200 640 41.75 46 46.875 15.125   3200 640 36.5 36.5 46.875 8.375
3300 660 42.375 46.5 47.625 15.375   3300 660 37 37 47.625 8.5
3400 680 43 47 48.5 15.5   3400 680 37.5 37.5 48.5 8.5
3500 700 43.75 47.75 49.375 15.625   3500 700 38 38 49.375 8.625


751.11 Fixed Bearing Plan.gif   751.11 Expansion Bearing Plan.gif
Fixed Bearing   Expansion Bearing
751.11 Fixed Bearing Elev.gif   751.11 Expansion Bearing Elev.gif
Section A-A   Section B-B
Fixed Bearing   Expansion Bearing


See table above for dimension A, B, O.D. and D.

751.11.3 Details

751.11.3.1 Elastomeric Bearings for P/S I-Girders

Expansion Bearings

Note: The location of anchor bolts in relation to the slotted holes in the sole plate shall correspond with the temperature at the time of erection. At 60°F the slotted holes should center on the anchor bolts.

751.11.3.1.1.jpg
SECTION A-A SIDE VIEW
751.11.3.1.2.jpg
PART PLAN VIEW  


Notes:

For use of smaller or larger diameter anchor bolts than shown in the table “Anchor Bolt Diameter” and with approval of SPM or SLE, see Fabrication Operations Engineer.

Bevel sole plate to match the slope of girder to the nearest 1/8" total difference in thickness across the plate. Minimum thickness shall be 1 1/2" at centerline bearing and 1" at the edge of bearing. (By design)

For \, \triangle > 2 1/2", use PTFE bearings.

* Slot width may be increased on wide roadways to accommodate lateral expansion. See Structural Project Manager.

** Designer should verify that minimum dimensions are met or neoprene will melt and flow into hole during manufacturing of bearings.

751.11.3.2 Elastomeric Bearings for Steel Girders

Laminated Expansion Bearings

Note: The location of anchor bolts in relation to the slotted holes in the sole plate shall correspond with the temperature at the time of erection. At 60°F the slotted holes should center on the anchor bolts.

751.11.3.2.1 Laminated Expansion.jpg
SECTION A-A SIDE VIEW
751.11.3.2.2 Laminated Expansion.jpg
PART PLAN VIEW  


Notes:

For use of smaller or larger diameter anchor bolts than shown in the table “Anchor Bolt Diameter” and with approval of SPM or SLE, see Fabrication Operations Engineer.

Bevel sole plate to match the slope of girder to the nearest 1/8" total difference in thickness across the plate. Minimum thickness shall be 1 1/2" at centerline bearing and 1" at the edge of bearing. (by design)

For \, \triangle > 2 1/2", use PTFE bearings.

* Slot width may be increased on wide roadways to accommodate lateral expansion. See Structural Project Manager.

** Designer should verify that minimum dimensions are met or neoprene will melt and flow into hole during manufacturing of bearings.


Laminated Fixed Bearing

751.11.3.2.1 Laminated Fixed.jpg
SECTION A-A SIDE VIEW
751.11.3.2.2 Laminated Fixed.jpg
PART PLAN VIEW  


Notes:

For use of smaller or larger diameter anchor bolts than shown in the table “Anchor Bolt Diameter” and with approval of SPM or SLE, see Fabrication Operations Engineer.

Bevel sole plate to match the slope of girder to the nearest 1/8" total difference in thickness across the plate. Minimum thickness shall be 1 1/2" at centerline bearing and 1" at the edge of bearing. (by design)

* Slot width may be increased on wide roadways to accommodate lateral expansion. See Structural Project Manager.

** Design should verify that minimum dimensions are met or neoprene will melt and flow into hole during manufacturing of bearings.



Sole Plate Clipping Details
(for Steel Structures)

751.11 SolePlateClippingDetails Steel.gif SECTION THRU BEARING

Minimum edge distance shall be maintained on all holes in sole plate. Slotted holes shown are for expansion bearings.

751.11.3.3 PTFE Bearings

Note: The location of anchor bolts in relation to the slotted holes in the sole plate shall correspond with the temperature at the time of erection. At 60°F the slotted holes should center on the anchor bolts.

751.11.3.3 Sec AA.jpg
SECTION A-A     SIDE VIEW
751.11.3.3 Part Plan.jpg
PART PLAN VIEW      

751.11 PTFE Bearings Notes.gif

Notes:

For use of smaller or larger diameter anchor bolts than shown in the table “Anchor Bolt Diameter” and with approval of SPM or SLE, see Fabrication Operations Engineer.

* Slot width may be increased on wide roadways to accommodate lateral expansion. See Structural Project Manager.

**** Designer should verify that minimum dimensions are met or neoprene will melt and flow into the hole during manufacturing of bearing.


Details of the sole plate connection to the prestress girder.

751.11.3.3 Section thru Girder Detail.jpg
751.11.3.3 Notes Sec thru Girder.jpg

SECTION THRU GIRDER


751.11.3.3 Sec End of Girder Detail.jpg

SECTION AT END OF GIRDER

* See EPG 751.22.3.6 Girder Reinforcement

751.11.3.4 Bearing Edge Distance

Steel or Prestress Girders

Beam Ledges

Heavy loads placed near the edge of a concrete surface can produce spalling. Two precautions should be taken to avoid the problem. Additional reinforcement should be placed under the load and around the corner. The bearing area of the load should not project beyond the straight portion of the stirrups, nor beyond the interior face of a transverse anchor bar (if one is provided). The recommended distance to a step, or the end of the bent cap in the transverse direction is 9 inches.


Bearing Clearance

When beams are placed to end at an intermediate bent, the clearance is governed by the bent up strands, diaphragm reinforcement, skew, and any vertical or horizontal curvature in the profile grade. A 9 inch minimum between bearing pads for prestress is a general rule. For simply supported steel or constant depth girders the recommended minimum clearance is 2 inches between bearing pads. See Dimension "C". Also consider transverse girder offsets when using prestress girders on horizontally curved alignments.


751.11 Bearing Edge Distance.gif


Check that all anchor bolts are located inside stirrup reinforcement.


Beam Steps

DIFFERENCE IN BEARING ELEV. METHOD OF OBTAINING ELEVATION
1/8" or less Retain in vertical dimension but use no fill plates

Use the lower 1/8" for bent height.

1/4" thru 1/2" Increase thickness of top plate of bearing or haunch.
5/8" thru 1" and up* Monolithic bearing pad or step beam.


* Use same method of obtaining elevaton throughout a structure where practical. Do not mix monolithic bearing pads and steps.

751.11 Beam Steps- Step Beam.gif

STEP BEAM

751.11 Beam Steps- Monolithic Bearing Pads.gif

MONOLITHIC BEARING PADS

751.11.3.5 Anchor Bolts

Show anchor bolt well locations and details on the plans.

Corrugated steel pipe is allowed for use as a permanently placed form unless located under an open expansion device due to concerns about accelerated corrosion. Use only removable forms and note on plans. In lieu of anchor bolts wells at the contractors option anchor bolts holes may be drilled.

Other methods of forming are allowed and determined by contractor in accordance with Sec 712 but shall be removable. This could include PVC, styrofoam, cardboard, etc.

Drilling anchor bolt holes should not be permitted if reinforcement is congested unless reinforcement is detailed on plans to clear anchor bolt wells.

If uplift is a concern, determine capacity of anchorage based upon strength developed utilizing either well or drilled hole. Use engineering judgment in determining capacity of either system, i.e. friction capacity, grout capacity, anchor capacity. Corrugations may be considered for developing strength resistance. Identify specific method of anchoring required if there is a concern about differences in resistance capacity.


Clearance for reinforcement:

751.11.3.5 Part Plan.jpg
SQUARE   SKEWED  

PART PLAN OF BEAM TOPS

(INT. BENTS SHOWN)


* Use even number of bars in beam tops for square bridges when possible.


Note: Details shown above are for information only. Place the following note on the plans, preferably with "General Notes."

"All reinforcing bars in the tops of substructure beams or caps shall be spaced to clear anchor bolt wells for bearings by at least 1/2"."


751.11.3.5 Detail Wells and Reinforcement.jpg
TABLE OF ANCHOR BOLT WELLS
Anchor Bolt Diameter "A" Well Diameter "B" Well Depth "C" Spiral Depth "D" Spiral Diameter Spiral Bar Length Bolt Extension into Concrete
1" 3" 12" 9" 8 1/8" 15'-1" 9"
1 1/4" 4" 15" 12" 9 1/8" 19’-9" 12"
1 1/2" 4" 18" 15" 9 1/8" 23'-0" 15"
2" 4" 21" 18" 9 1/8" 26'-1" 18"
2 1/2" 4" 28" 25" 9 1/8" 33'-2" 25"

Omit spiral on 21" WF if anchor bolt spacing is less than 6"


TABLE OF BOLT WEIGHTS
Description 1" 1 1/4" 1 1/2" 2" 2 1/2"
Unthreaded (#/in.) ---- ---- 0.5 0.89 1.39
Threaded (#/in.) ---- ---- 0.425 0.755 1.20
Nut (lbs.) ---- ---- 1.31 2.99 5.64
Weight used for entire bearing (#/ft.) 2.673 4.175 ---- ---- ----

Anchor bolt sizes were based on 10% dead load reactions. Check the anchor bolts for any centrifugal forces.

Epoxy coat anchor bolt wells at bents with expansion devices.

751.11 Swedge Anchor Bolt-A.gif 751.11 Swedge Anchor Bolt-B.gif
DETAIL "A" DETAIL "B"
  (Optional detail for 1 3/8" Ø thru 2 1/2" Ø Anchor Bolts)

SWEDGE ANCHOR BOLT DETAILS

751.11.3.5 Swedge Notes.jpg

Above detail shall be placed on the bearing sheet of all structures using swedge anchor bolts.

751.11.3.6 Girder/Beam Chairs

Chairs shall be used at integral bents for prestressed and steel girders and beams if slope requirements cannot be met by elastomeric laminated fixed bearings.

Chairs shall also be used for prestressed girders and beams at fixed intermediate bents where minimum dead load or slope requirements cannot be met by elastomeric laminated fixed bearings and under the smaller girder where girder sizes change.

P/S I-girder shown, steel structures are similar.

751.11.3.6 front elevation 2017.jpg (a) 3 in. minimum when using beam step.
(b) 1 in. minimum and 5 in. maximum. Use beam step if necessary.
Maximum Service
Limit State Load Reaction
90 kips
Based on details shown, ASTM A709 Grade 36 steel and bolt threads in the shear plane. Larger service load reactions will require modified details by design.


751.11.3.6 beam chair.jpg


751.11.3.6 side May 17 2017.jpg
Instructions:
When determining service limit state load reaction use DC Loading (excluding Safety Barrier Curb) and an additional 50 lb/sf construction load.
See EPG 751.50 (Notes A4a3.1 and G1.16 thru G1.19) for appropriate notes to be placed with details.