Day 4 of 11 — Class 4

Bridge Portal Frame
& AASHTO Loads

Shift from buildings to bridges. Model a single-span portal frame and apply AASHTO HL-93 truck and lane loads to find critical load positions.

AASHTO ETABS HL-93 Truck Lane Load Portal Frame
01 — Why

Why do bridges need their own loading code?

Buildings are loaded by people and furniture. Bridges are loaded by trucks — moving, dynamic, and far heavier per axle than anything in a building. AASHTO quantifies this.

Code Reference — AASHTO LRFD

HL-93 design vehicle: tandem (2 × 110 kN axles at 1.2 m spacing) or design truck (3-axle: 35 + 145 + 145 kN), combined with lane load of 9.3 kN/m. Take the worst case.

Content Coming Soon Why moving loads matter: influence lines explained visually. How the critical position of the truck changes the maximum moment location.
02 — Concept

Portal frames and horizontal thrust

A portal frame is a rigid frame with two columns and a beam. Unlike a simply-supported beam, it develops horizontal thrust at the base — which is why the foundations must be designed for both vertical and horizontal forces.

Content Coming Soon Portal frame free body diagram. How rigidity at beam-column joints transfers moment. Fixed vs. pinned base and effect on thrust.
03 — Walkthrough

Step-by-step: Bridge portal in ETABS

Build a 20 m span portal frame (columns 4 m tall, 400×600 mm sections) and apply HL-93 loads.

01

Set up 1-bay portal geometry

Grid: 1 bay × 20 m (X), 1 story × 4 m (Z). Two columns and one bridge girder.

02

Assign fixed base to both columns

Select bottom nodes → Assign → Joint → Restraints → Fixed (all 6 DOF). This creates moment at the base — a key portal frame behaviour.

03

Apply lane load (9.3 kN/m, full span)

Define load pattern LANE. Apply distributed 9.3 kN/m along the full girder length.

04

Apply HL-93 truck as point loads

Define load pattern TRUCK. Apply 3 point loads: 35 kN at 0 m, 145 kN at 4.3 m, 145 kN at 8.6 m from one end. Reposition to maximise midspan moment.

05

Define AASHTO load combination

AASHTO LRFD Strength I: 1.25×DC + 1.75×(LL+IM). Dynamic load allowance IM = 33% for truck. Run and view results.

04 — Check Yourself

Bridge load quiz

Test your understanding of AASHTO HL-93 loading.

Interactive Quiz Coming Soon Questions: What is IM and why is it applied? What is the difference between tandem and design truck? Where does the truck sit to maximise midspan moment? Why does the portal frame develop horizontal thrust at the base?