Bridge Bearing Pad Calculation: A Worked Elastomeric Bearing Case Study
A bridge bearing has a demanding job. It must transfer vertical reaction while accommodating the movements and rotations that occur in a bridge superstructure. For a steel-reinforced elastomeric bearing, the engineering problem is a balance of compressive, shear and rotational stiffness; it is not a simple choice of pad length, width and thickness.3
This article is an educational adaptation of a published Federal Highway Administration example for an interior-girder bearing at an intermediate pier. It illustrates the sequence of checks used in that example. It is not a design for a live project and must not be substituted for the governing code, the current project documents or the responsible engineer’s calculations.3
The illustrative inputs
The FHWA example considers a rectangular bearing with a 24 in transverse width and a 7.5 in longitudinal length. It assumes a 60 Shore A elastomer with a shear modulus, G, of 0.150 ksi. The cited maximum Service I reaction is 290.5 kips and the cited maximum live-load reaction is 129.9 kips.3
Input from the published example | Symbol | Value |
Bearing length | L | 7.5 in |
Bearing width | W | 24 in |
Service reaction | P | 290.5 kips |
Live-load reaction | PLL | 129.9 kips |
Assumed shear modulus | G | 0.150 ksi |
Chosen internal elastomer layer | hri | 0.5 in |
Step 1: Establish a preliminary plan area
The published example uses a 1.75 ksi limit to establish a preliminary minimum plan area:
Arequired = 290.5 / 1.75 = 166.0 in²
A 24 in × 7.5 in plan gives:
Aprovided = 24 × 7.5 = 180.0 in²
The selected preliminary plan area exceeds the cited minimum. This does not complete the bearing design; it simply establishes a reasonable starting geometry for the further checks.3
Step 2: Check average compressive stress
Using the service reaction and the 180 in² plan area:
σ<sub>service</sub> = 290.5 / 180 = 1.614 ksi
For the live-load reaction:
σ<sub>live</sub> = 129.9 / 180 = 0.722 ksi
The values above reproduce the rounded results presented in the published example. They become inputs to the shape-factor check, which relates the geometry of the elastomer layer to the permitted stress behaviour.3
Step 3: Calculate the shape factor
For a rectangular elastomeric layer without holes, the FHWA example defines the shape factor as:
S = LW / [2hri(L + W)]
The example derives minimum shape-factor demands of 5.38 under total load and 4.81 under live load. Using the 0.5 in internal layer selected in the example:
S = 7.5 × 24 / [2 × 0.5 × (7.5 + 24)] = 5.71
That value exceeds the governing 5.38 value for this worked example. Rearranging the same equation gives limiting internal-layer thicknesses of approximately 0.531 in for the total-load check and 0.594 in for the live-load check; the selected 0.5 in layer therefore meets both stated conditions.3
What the preliminary calculation does—and does not—say
The calculation demonstrates why the internal elastomer-layer thickness cannot be selected by appearance alone. It affects the shape factor, and the shape factor in turn influences compressive and rotational stiffness. But a bearing whose area and shape factor look acceptable has not yet completed the necessary review.
The same FHWA design path goes on to address the following items:3
Further check | Why it matters |
Compressive deflection | Helps assess vertical movement and joint-related implications where relevant. |
Shear deformation | For movable bearings, total elastomer thickness must accommodate the design translation. |
Combined compression and rotation | Helps prevent uplift and excessive edge stress under applicable service combinations. |
Stability | Assesses whether geometry and load lead to an instability concern. |
Reinforcement and fatigue | Confirms that steel shims/reinforcement are adequate for induced tensile stresses and fatigue considerations. |
Temperature and compound grade | Elastomer selection must suit the service temperature conditions specified for the bridge. |
The FHWA example notes that elastomeric materials are stiff in compression but flexible in shear, enabling a bearing to support gravity load while allowing controlled deformation. It also explains that the selected elastomer compound should suit the climatic conditions because elastomers stiffen at low temperatures.3
A disciplined way to start a project enquiry
A project discussion should include the governing standard, dead/live/seismic/wind or other actions as applicable, movement at each support, design rotations, bearing layout, available support dimensions, temperature range, corrosion environment, fixed or guided/free intent, installation geometry and required test or inspection documentation.
Berzelius Performance Materials can discuss neoprene and elastomeric bearing-pad product requirements. The final calculation, detailing, specification, acceptance criteria and project approval must remain with the responsible engineer and the governing design standard.
For a technical discussion, visit www.bpmaterials.uk or email sales@berzelius.uk.