Concrete Bridges Design - Fundamentals

Classify bridges by span length, defined as the distance between piers. Culverts under 6.1 m; short 6.1–30 m; medium 30–100 m; long over 100 m guide superstructure choices.

Identify bridge type by interspan relation: simply supported, continuous, or cantilever, and understand when each is preferred, including construction ease, soil settlement effects, and moment behavior.

Classify five bridge types and identify the static kill system of the mean girders for each bridge, using concepts from the previous lecture.

Explore cantilever, continuous, and simply supported bridge systems through analyzed bay configurations, pier caps, and depth changes, including double cantilevers and intermediate bays, to identify structural arrangements in everyday bridges.

This lecture outlines the six structural forms of bridges—slab, beam, truss, arch, suspension, and other forms—and explains how classification guides design methods, checks, and overall bridge behavior.

Slab bridges offer the easiest and fastest construction and design, using a slab between piers, with solid or voided slabs and an inverted beam to prevent edge cracks.

Explore girder bridges as the most common and economical bridge type, with T and box sections; box sections provide higher bending rigidity and better eccentric load distribution for longer spans.

Explore voided slabs for precast prestressed bridges using FHWA design tables for spans 7.6–15.2 m, detailing void configurations and a fixed 48-inch mold width.

Explore precast prestressed I-girders and their cross sections, showing why types two to six with wider upper flanges accommodate prestressing strands on the upper and lower flanges for practical spans.

Explore box girders as precast prestressed structures for spans from 15.2 to 24.4 meters, assembled to form a bridge platform; learn about connections between segments and their challenges.

Examine shear keys for box girders, contrasting partial-depth and full-depth connections, and compare traditional concrete with ultra high performance concrete for economy and cross-section efficiency.

Analyze box girders, cast-in-place prestressed, for spans from 30 to 180 meters. Compare cost in place versus precast options and note post-tensioned girders for longer spans.

Explore the ASD versus LFD design methods for bridges, showing how dead load and live load factors are used to address variation in loading and material strength.

Explore load resistance design using the reliability index and normal distribution to assess probability of failure for bridge design, linking mean load, mean resistance, and standard deviation.

Explains the LRFD design equation by combining strength reduction with load factors, detailing eta components for ductility, redundancy, and importance, and applying dead and live load modifiers.

Explore how bridge diaphragms transfer lateral and live loads from the deck to main girders using cross girders, and compare with building diaphragms, highlighting spacing and cost implications.

Place crossbeams at the start and end of each span; add intermediates for spans over 12.2 meters, and for box bridges, add intermediates when curvature is under 244 meters.

Explore the five design limit states for bridges, including dead and live loads, wind effects, and state-specific requirements, contrasting with traditional structures and guiding safe, durable bridge design.

Explore the four serviceability limit states for concrete bridges, including deflection, crack control, and tension in concrete girders and columns, with steel yield and connection slip considerations.

Identify fatigue and extreme event limit states as essential design checks for bridges, addressing infinite and finite load-induced fatigue, seismic events, collisions, wind, and other extreme loads.

Identify permanent and transient bridge loads, including dead loads, creep and shrinkage, wind, seismic, and vehicle interactions, plus settlement and temperature effects shaping load combinations.

Explore how concrete bridge design uses load combinations and load factors across limit states, extreme events, dead and live loads, and special loads like wind, water pressure, and settlement.

Set maximum load factors for permanent loads, including the debt load at 1.25 and the surface factor at 1.5, guiding bridge components and maintenance planning.

Assess dead loads on bridges: the concrete deck (20–30 cm) and asphalt wearing surface (125–170 kg/m²), plus construction loads for steel or precast girders and shoring considerations.

Calculate dead loads on a 26 m concrete bridge: slab, girders, parapets, and wearing surface, distributed to intermediate girders with spacing 2.4 m.

calculate design lanes by dividing the roadway by 3.6 m and rounding down. set the design lane at three meters, even if traffic lanes are wider.

Explore concrete bridge design for loads including truck, tandem, and uniform loads, with factors like weight, vehicle dimensions, position along the bridge, lane count, and dynamic effects.

The lecture traces the evolution of truck load design for bridges, from uniform distribution to 15- and 20-ton trucks, with front axle 20 percent and rear axle 80 percent.

Explain the HL-93 live-load system for concrete bridges, including design truck HS20 plus design lane load, or design tandem plus design lane, with span-driven governing cases and impact factors.

Covers four load combinations for bridges: truck plus lane, tandem plus lane, two trucks spaced 15 meters for continuous girders, and fatigue with truck load only at 9 meters.

Explore how influence lines track the effect of moving loads on beams, revealing maximum reactions, bending moments, and the distinction between concentrated and distributed loads.

Explore using influence lines to obtain maximum internal forces in simple span bridges, starting from a simple beam and incorporating truck, tandem, and lane loads for maximum bending moments.

Explain the multiple presence factor for bridge live loads across one to four lanes, and when to apply it in three-dimensional models, lever-rule calculations, or alongside distribution factors.

Explain dynamic load allowance in concrete bridge design, using a typical 1.33 factor, with 1.75 for joints and 1.15 for fatigue, and assess sidewalk and bicycle loads.

Compare concentrated versus distributed tire loads on a girder, showing how distributed contact area reduces moments; cover standard tire contact dimensions and a CWD empirical equation for tire weight.

Apply the lever rule to calculate maximum bending moments and shear forces on exterior bridge girders using the Sigmon moment about the second girder, truck positions, and edge distances.