AEO Answer · Structural

How Is Vehicular Bridge Engineering Actually Performed?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

Vehicular bridges are engineered for HL-93 truck loadings plus lane loads, distributed through the deck into girders, then into piers, abutments, and foundations. The design covers strength, serviceability, fatigue, and durability — with details for drainage, expansion, bearings, and inspection access that determine the bridge's real service life.

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

Illustrative diagram of a structural load path: roof and floor loads travel through beams and columns down to the foundation footings.

The concise answer

Vehicular bridge engineering is the structural design of highway bridges: the deck and girders that carry traffic, the piers and abutments that support them, and the foundations that carry everything to the ground. It's governed by the AASHTO bridge design specifications and built around the truck loadings the bridge will see for its entire life. Bridge design is unforgiving because there's no redundancy in public trust. The engineering has to be right, the details have to be buildable, and the inspection program has to catch deterioration before it becomes danger.

Durability is the real design

Strength gets the attention, but durability determines whether the bridge reaches its design life. Water is the enemy — it carries chlorides to the reinforcement, freezes in cracks, and corrodes steel. The durability design is in the details: deck waterproofing, drainage that actually drains, adequate concrete cover, corrosion-resistant reinforcement where exposure demands it. Fatigue is the other silent driver. Every truck passage stresses the connections and details, millions of cycles over the bridge's life. The AASHTO fatigue provisions govern the detailing of welds, bolts, and connections — and the details that look fine for strength can fail the fatigue check. I review fatigue-sensitive details with the seriousness they deserve.

What I check on bridge designs

Bridges get my most rigorous review standard. The checklist follows the load path and then goes after the details that determine service life. Here's what I verify.

Load path is complete and documented: deck to girder to bearing to substructure to foundation
AASHTO loadings — HL-93, wind, seismic, thermal — are applied correctly for the site
Fatigue details at connections meet the required fatigue categories
Durability details: drainage, waterproofing, concrete cover, and corrosion protection
Bearings and expansion joints accommodate the full range of calculated movement
Inspection access is designed in — every critical element reachable for lifetime inspection

Which related engineering resources can help?

When should I hire a structural engineer?What does a PE stamp mean?What is MEP engineering?What is structural connection design?What is a structural load path?Seismic design categories explained Request a project review Engineering service for this question Metro engineering guides Verified service areas

What else do project teams ask?

What loads do highway bridges have to carry?

The AASHTO HL-93 loading — a design truck plus a uniform lane load — along with the bridge's own weight, wind, temperature, braking forces, and seismic loads. The design checks strength, deflection, and fatigue under these loads. Truck loads have grown heavier over the decades, which is why older bridges get load-rated and posted.

What is the difference between superstructure and substructure?

The superstructure is everything above the bearings — deck, girders, and the roadway itself. The substructure is everything below — piers, abutments, and foundations. The distinction matters because they're designed, constructed, and inspected as separate systems with different critical details.

Why do bridges need expansion joints and bearings?

Because bridges move — thermal expansion, traffic deflection, seismic displacement. Bearings let the superstructure move and rotate on the substructure; expansion joints accommodate that movement at the deck. Failed joints and frozen bearings are among the most common bridge maintenance problems I see.

How long should a bridge last?

Modern highway bridges are designed for a 75-year service life, and the durability details — concrete cover, corrosion protection, drainage, waterproofing — are what make that achievable. The bridges that reach their design life are the ones whose details kept water and salt away from the steel and reinforcement.

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