For a typical multi-girder highway overpass, which load combination generally would be the only one requiring design calculations unless Strength II and IV are needed?

• A. Service I
• B. Fatigue and Fracture limit states
• C. Strength I Load Combination
• D. Extreme-Event I

Answer: (C)

In bridge design, different load combinations are checked for different limit states, and one combination is used to size members for their ultimate capacity. For a typical multi-girder highway overpass, the standard set of factors that governs the member’s strength is the Strength I load combination. This is the primary design calculation because it represents the worst credible combination of dead and live loads (and other permanent loads) that the members must resist.

Serviceability checks, which ensure crack widths, deflections, and overall usability under regular service conditions, are handled by the Service I combination and do not drive the member sizing in the same way. Fatigue and fracture limit states address the effects of repeated traffic loading on connections and details, which may be critical for certain details but are separate checks from the main sizing. Extreme-Event I covers rare, unusual loads and is only used if such events are within the design scope.

So, under typical conditions, the Strength I load combination is the one that requires the primary design calculations, unless Strength II or IV are needed for specific design considerations.