Bridge Bearing Seized Expansion Joint Thermal Restraint Case Study

Bridge expansion joints and bearings are small compared with the bridge deck, but they control a critical boundary condition. If a joint fills with debris or a bearing seizes, thermal movement that should be harmless can become horizontal restraint. The deck may still look serviceable while abutments, pier caps, bearing seats, shear keys, and diaphragms receive loads that were not intended to be permanent.

This case study follows a highway overbridge where the expansion joint appears open, but measured movement is much smaller than the movement expected from temperature change. The purpose is to show how a civil engineer can combine field inspection, free thermal movement, restraint stiffness, bearing shear strain, crack mapping, and return-to-service criteria into a defensible decision.

The case is a screening calculation, not a substitute for a bridge-code rating or a detailed finite-element assessment. Its value is that it turns a vague maintenance defect into a quantified structural and asset-management problem.

Case Context

A two-span composite bridge has one nominally fixed pier line and an expansion joint at the east abutment. The east bearing line is intended to allow longitudinal movement. During a summer inspection, the joint seal is packed with grit and vegetation, two bearings show rust staining at the sliding interface, and diagonal cracks are visible on one bearing seat.

Item	Field value or assumption
Effective expansion length from fixed line	L=60\ \text{m}
Installation reference temperature	15^\circ\text{C}
Hot inspection deck temperature	50^\circ\text{C}
Temperature rise from reference	\Delta T=35\ \text{K}
Effective thermal expansion coefficient	\alpha=12\times10^{-6}\ /\text{K}
Measured joint movement from cold reference	8\ \text{mm}
Number of restrained bearing paths used in screen	n=8
Equivalent horizontal stiffness per seized path	k_b=18\ \text{kN/mm}
Elastomeric bearing pad thickness	t_b=50\ \text{mm}
Bearing-seat and shear-key screening capacity	R_h=1600\ \text{kN}
Inspection trigger	new crack growth or movement less than 70% of expected

The inspection team does not assume that the bridge is unsafe simply because the joint is dirty. It also does not accept the joint as functional just because some gap remains visible. The engineering question is whether the bridge is actually moving by the amount implied by the measured temperature change.

Field Evidence

The site evidence is consistent with restrained thermal movement:

Evidence	Engineering interpretation
joint trough filled with compacted debris	the joint may have a hard stop before full thermal closure
torn seal and standing water near the abutment	drainage and debris control are not functioning
bearing sole plates have rust lines but no fresh sliding witness marks	the bearing line may not be moving freely
measured movement is much smaller than expected	the deck expansion is being restrained or diverted
diagonal cracks appear near one bearing seat	horizontal load may be entering the pier cap locally
crack width increases on hot afternoons and closes slightly overnight	temperature, not truck loading alone, is driving the symptom

This pattern separates the problem from a simple deck-joint maintenance issue. The joint, bearings, drainage, and bearing-seat concrete form one movement system. The structural assessment must therefore review the boundary condition, not only the deck.

Free Thermal Movement

The free longitudinal thermal movement is:

Using millimetres for bridge length:

so:

The movement per degree is:

For a 35\ \text{K} temperature rise, the expansion joint should therefore show about 25\ \text{mm} of closure or equivalent bearing movement, subject to measurement tolerances and actual temperature distribution.

The measured movement is only:

The movement ratio is:

Only about 32\% of the expected movement is appearing at the intended location. That is below the inspection trigger of 70\%, so the bridge should not remain in unrestricted service without further assessment.

Displacement Deficit

The unaccommodated movement is:

This does not prove that exactly 17.2\ \text{mm} is stored elastically in one member. Some movement may be taken by local crushing, bearing deformation, abutment backwall contact, pier flexibility, deck curvature, or measurement error. It does prove that the intended movement mechanism is not behaving as assumed.

For a bridge inspection decision, the deficit is large enough to justify lane restriction during hot weather, debris removal, bearing-release planning, and detailed structural review before normal operation is restored.

Equivalent Restraint Force

A first-pass force screen can be made by combining the restrained movement with the equivalent horizontal stiffness of the seized bearing paths.

The bearing-line stiffness used for the screen is:

The equivalent horizontal restraint force is:

So:

Compare this with the screened bearing-seat and shear-key horizontal resistance:

The utilization is greater than one. This is not a final rating result, because the real stiffness distribution and load path require a detailed model. It is a strong diagnostic result: the restraint is large enough to explain concrete cracking and to require immediate action.

Bearing Shear Strain Check

The visible bearing deformation provides a second check. If the bearing pad thickness is:

and the observed relative movement is:

then the observed bearing shear strain is:

That observed value alone may look acceptable for a short-term inspection screen. The problem is that the bridge should have accommodated about:

If the bearing system were forced to absorb that movement without sliding or joint relief, the implied shear strain would be:

An inspection action threshold of \gamma=0.35 would be exceeded. The bearing has not merely moved a little; it has failed to provide the intended movement path, and the remaining movement is entering other structural details.

Why the Joint Gap Can Mislead

A remaining visible joint gap does not prove that the joint is functioning. The useful question is not only “is there a gap?” but:

1. Did the gap change by the amount predicted from temperature?
2. Is debris creating local hard contact before the nominal gap closes?
3. Are both edges of the deck moving consistently, or is one bearing line locked?
4. Do bearing witness marks agree with the joint measurement?
5. Are crack widths or abutment spalls correlated with temperature?

In this case, the joint still has visible open space, but debris and seized bearings prevent most of the expected displacement from occurring. A static photograph would understate the problem. Temperature-indexed displacement records reveal it.

Immediate Engineering Decision

The bridge is not automatically closed, but it is not accepted as normal. The engineering team selects controlled service with restrictions while the movement system is released and assessed.

The immediate actions are:

1. restrict heavy permit vehicles and avoid unnecessary lane loading near the affected abutment during hot afternoons;
2. mark cracks, bearing positions, and joint edges so movement can be compared with temperature;
3. remove loose debris from the joint only under an approved maintenance procedure, because sudden release can change load distribution;
4. inspect bearing seats, anchor bolts, sole plates, sliding surfaces, diaphragms, shear keys, and abutment backwalls;
5. check drainage details so the cleaned joint does not refill with grit and water;
6. plan bearing reset or replacement where sliding surfaces are corroded or deformed;
7. review whether temporary jacking is required and define jacking-force limits before lifting any deck line.

The sequence matters. Cleaning the joint without checking bearings may leave a seized bearing path in service. Jacking without a structural lift plan can damage diaphragms, utilities, expansion devices, or adjacent bearings.

Corrective Release and Repair

The repair package includes:

• removal of compacted debris and failed seal material from the expansion joint;
• replacement of the torn gland and restoration of joint drainage;
• cleaning or replacement of seized bearing sliding plates;
• reset of bearing alignment where the pad has walked or rotated;
• repair of damaged bearing-seat concrete after movement restraint is removed;
• installation of durable debris shields or improved drainage where the joint is exposed to grit;
• survey baselines for joint gap, bearing position, bearing rotation, and crack width.

Concrete repair is deliberately late in the sequence. If cracks are patched before the thermal restraint is removed, the repair hides an active load path rather than fixing it.

Validation During a Temperature Cycle

The repair is not accepted when the joint looks clean. It is accepted when the bridge moves as a bridge with free thermal movement should move.

For a monitored temperature swing of:

the expected movement is:

A practical acceptance band for this test may be:

provided that the deck temperature measurement is representative and both sides of the joint move consistently.

Additional release criteria are:

Criterion	Acceptance intent
joint movement follows about 0.72\ \text{mm/K}	restored thermal movement path
bearing shear strain remains below the project action threshold	no hidden bearing overdistortion
crack-width change is less than 0.05\ \text{mm} over the monitored cycle	no active crack propagation from restraint
no new spalling or bearing-seat crushing appears	local concrete demand is controlled
drainage discharges after washdown or storm event	debris mechanism has been addressed
jacking records, torque records, and bearing positions are retained	future inspections have a baseline

If movement remains below the acceptance band after cleaning, the residual restraint is still active. The bridge should stay restricted until the bearing line, abutment contact, or hidden obstruction is corrected.

Measurement Uncertainty

Assume the representative deck temperature has uncertainty:

and the joint displacement measurement has uncertainty:

The movement uncertainty from temperature is:

Combining temperature and displacement uncertainty gives:

The observed deficit is:

which is nearly ten times the combined measurement uncertainty. The conclusion that the bridge is restrained is therefore robust. The exact force still needs a refined structural model, but the maintenance and restriction decision does not depend on a marginal measurement.

Risk Screen

A simple risk-priority-number screen helps communicate why this is more than routine joint cleaning:

Before action:

Severity is high because the restraint can damage bearing seats, abutments, diaphragms, and the deck load path. Occurrence is moderate because the event recurs with temperature cycles. Detection is weak because a visual gap can hide a nonfunctional movement system.

After joint cleaning, bearing release, movement monitoring, and drainage correction:

The consequence of recurrence remains serious, but recurrence and detection improve when the movement path is restored and measured.

Lessons for Bridge Asset Management

The transferable lessons are:

1. Expansion joints and bearings must be inspected as a movement system, not as independent components.
2. Temperature-indexed displacement is stronger evidence than a single visual gap measurement.
3. A seized bearing can create structural demand even when the deck, girders, and pavement look normal.
4. Joint cleaning is not a complete repair unless drainage, bearing movement, crack response, and future access are addressed.
5. Return to service should be based on measured movement, crack stability, bearing condition, and retained records.

The engineering decision is to treat the defect as an active boundary-condition failure. The repair is complete only when the bridge demonstrates the expected thermal movement over a measured temperature cycle.