A structural frame where rigid beam-to-column connections resist wind and earthquake forces through bending.
A lateral force-resisting structural system in which beams and columns are connected with moment connections that resist lateral loads through bending of the members. Moment frames allow open floor plans without diagonal bracing or solid shear walls. They are classified as ordinary, intermediate, or special depending on their ductility and seismic detailing requirements.
The lateral system choice largely determines a steel package's cost, and a moment frame concentrates expense into many costly moment connections rather than diagonal braces. Estimators must catch the frame's classification—ordinary, intermediate, or special—because special seismic detailing multiplies fabrication, welding, and inspection requirements. Knowing the system also helps when proposing value-engineering alternates that could meet the same lateral demand for less.
On a hospital project in a high-seismic zone, the estimator prices a special moment frame, accounting for the demand-critical welds, protected zones, and continuity plates that a braced-frame alternate would have avoided.
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