ITEM 412 – ELASTOMERIC BEARING PADS

412.1 Description

412.1.1 Scope

This Item shall consist of the requirement for elastomeric bearing pads whose main function is to transfer loads or accommodate relative movement between a bridge superstructure and its supporting structure, or both while avoiding damaging strain and additional tension. Elastomeric bearings shall include unreinforced pads (consisting of elastomer only) and reinforced bearings with steel laminates.

 

412.1.2 General Requirements

All bearings shall be designed in accordance with specifications contained in the latest edition of the AASHTO Standard Specification for Highway Bridges, 15^th Edition, 1992.

412.1.3 Classification and Use

The following are the types of elastomeric bearings:

a. Plain Elastomeric Bearing Pad

b. Plain Elastomeric Sandwich Bearing

c. Steel-Laminated Elastomeric Bearing without External Load Plates

d. Steel-Laminated Elastomeric Bearing with External Load Plate(s)

A laminated bearing pad is required when the thickness of the plain pad is more than 31.75 mm and the compressive strain is more than 15%.

412.2 Material Requirements

The elastomer for the manufacture of the bearing is furnished in two types as follows:

1. Type CR-Chloroprene Rubber

2. Type NR-Natural Rubber

The elastomer compound used in the construction of a bearing shall contain only either natural rubber or a chloroprene rubber as the raw polymer. No reclaimed rubber shall be used.

Steel laminates used for reinforcement shall be made from rolled mild steel conforming to ASTM A 36, A 570 or equivalent, unless otherwise specified by the Engineer. The laminates shall have a minimum nominal thickness of 20 gage.

412.2.1 Physical Requirements

A. The elastomer compound shall meet the minimum requirement of Table 1.

B. Dimensions and Permissible Variations

All elastomeric layers, for example, plain bearing pads, laminates, and covers, shall be of uniform thickness unless otherwise specified in the contract or purchase order.

All internal steel laminates shall be of uniform thickness. When specified in the contract or purchase order, the thickness of the outer steel laminates may differ if not adjacent to an external load plate.

The minimum thickness of internal steel laminates shall be 1.5 mm or (16 gage) when the greater of the length or width of a rectangular bearing or diameter of a circular bearing is less than 450 mm. In all other cases, the minimum thickness shall be 2 mm or (14 gage).

Bearing dimensions and elastomer layer thicknesses shall satisfy the tolerances in Table 2, in which D is the length, width or diameter as appropriate, and T is the total elastomer thickness.

Variation from a plane parallel to a design surface shall not exceed an average slope of 0.005 for the upper surface and 0.006 for a side surface.

Table 1 – Quality Control Properties of Elastomer

ASTM Requirements
Properties	Natural Rubber(NR)		Chloroprene Rubber (CR)
Grade (Durometer) Physical Properties: A. Before Aging Tensile Strength, MPa (D412) Elongation, % (D412) Durometer Hardness, Shore Pts.(D2240) Tear Resistance, N/mm (D624) Compression Set , % (D395) Method B, Temperature, 0C B. After Aging Temperature of the Test, 0C Aging Time, Hours Tensile Strength, % (D573) Elongation, % Durometer Hardness, Shore Pts. Other Properties: Ozone Resistance (D1149) Partial Pressure, MPa Duration, Hours Tested at 20% strain 37.7 ±10C Mounting procedure D518 Procedure A Brittleness D2137, low temp. Brittleness at -400C Shear Modulus Nominal Hardness Shear Modulus at 230C, MPa	60 15.5(min.) 400(min.) 60±5 31.5(min.) 25(max.) 70 70 168 -25 (max.) -25 (max.) +10(max.) 25 48 - Pass 0.85-1.1	70 15.5(min.) 300(min.) 70±5 31.5(min.) 25(max.) 70 70 168 -25 (max.) -25 (max.) +10(max.) 25 48 - Pass 1.13-1.84	60 15.5(min.) 350(min.) 60±5 31.5(min.) 35(max.) 100 100 70 -15(max.) -40(max.) +15(max.) 100 100 No cracks Pass 0.85-1.1	70 15.5(min.) 300(min.) 70±5 31.5(min.) 35(max.) 100 100 70 -15(max.) -40(max.) +15(max.) 100 100 No cracks Pass 1.13-1.84

Plain pads and laminated bearings shall be manufactured to the design dimensions and these specifications with the tolerances listed in Table 2, unless other tolerances are shown on the design drawings.

Table 2 – Tolerances

Design Dimension/Parts

Tolerances (Mm)

Overall vertical dimensions Design thickness 32 mm or less Design thickness over 32 mm Overall horizontal dimensions 914 mm and less Over 914 mm Thickness of individual layers of elastomer (laminated bearing only) at any point within the bearing Variation from a plane parallel to the theoretical surface: (as determined by measurements at the edge of the bearings) Top Sides Position of exposed connection members Edge cover of embedded laminated connection members Size of holes, slots or inserts Position of holes, slots, or inserts

-0, +3 -0, +6 -0, +6 -0, +12 ±20 percent of design value but no more than ±3 mm Slope relative to the bottom of no more than 0.005 radian 6  3 -0, +3 ±3 ±3

412.3 Construction Requirements

412.3.1 Handling, Transport, Storage and Installation

During handling, transport, storage, and installation, bearings shall be kept clean and protected from mechanical damage, heat, contaminants and other deleterious effects.

Bearings shall be placed on surfaces that are plane to within 1.6 mm and unless the bearings are placed in opposing pairs, horizontal to within 0.01 radians.

Any lack of parallelism between the top of bearing and the underside of the girder that exceeds 0.01 radians shall be corrected either grinding of the surface, grout pack bearing seats or modification of the bearing such that the intended bearing placement is as originally designed with the least amount of bearing modification, or as otherwise directed by the Engineer.

Exterior plates of the bearing shall not be welded unless at least 38.1 mm of steel exists between the weld and the elastomer.

Each completed bearing shall have its components clearly identified, be securely bolted, strapped or otherwise fastened to prevent any relative movement and marked on its top as to location and orientation in each structure in the projects conformity with the plans.

Dismantling at the site shall not be done unless absolutely necessary for inspecting or installation.

412.3.2 Sampling

Sampling, testing and acceptance consideration will be made on a lot basis. A lot shall be further defined as the smallest number of bearings as determined by the following criteria:

a. A lot shall not exceed a single contract or project quantity.

b. A lot shall not exceed 50 bearings.

c. A lot shall consist of those bearings of the same type regardless of load capacity.

For acceptance purposes, bearing from within the lot shall be selected at random as samples for inspection and testing.

A minimum of three bearings shall be taken from the lot for testing. If the number of bearings in the lot exceeds 50 then for each additional 50 or part thereof, one additional bearing shall be taken for testing.

412.3.3 Acceptance Requirements

1. If lack of elastomer to steel bond is indicated, the bearing shall be rejected.

2. If laminate placement faults are observed which results in elastomer layer thickness that exceed the tolerances in Table 2, the bearing shall be rejected.

3. If there are at least three separate surface cracks which are each at least 2 mm wide and 2 mm deep. The bearing shall be rejected.

4. Record the median compressive stiffness (K) of the bearing of median stiffness. The compressive stiffness of each bearing tested shall not differ form (K) by more than 10%.

5. For each bearing that fails to meet the requirements in (1), two additional bearings maybe sampled and shall meet the requirements in (1) or the lot shall be rejected.

6. If the lot is not rejected, the bearing of median stiffness (K) shall be subjected to the elastomeric material tests in (2).

412.4 Method of Measurement

The quantity of elastomeric bearing pad to be paid for will be the final quantity placed and accepted in the completed structure. The dimensions of elastomeric bearing pads will be the quantity placed in accordance with the Plans or as otherwise directed by the Engineer.

412.5 Basis of Payment

The accepted quantity, measured as prescribed in Section 412.4, shall be paid for at the contract unit price for Elastomeric Bearing Pads which price and payment shall be full compensation for furnishing and placing all materials, including all labor, equipment, tools and incidentals necessary to complete the work prescribed in this Item.

Payment will be made under:

Pay Item Number	Description	Unit of Measurement
412	Elastomeric Bearing Pads	Each