In Plain English
Quick Answer
A structural frame where rigid beam-to-column connections resist wind and earthquake forces through bending.
Definition
Definition
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.
Context
Why It Matters in Bidding
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.
Example
Example
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.
See Also
Related Terms
FAQ
Questions Contractors Ask
Why does a moment frame cost more to bid than a braced frame?
Moment frames resist lateral loads through many rigid beam-to-column connections, each requiring extra welding, plates, and inspection. Braced frames use simpler connections plus diagonal members. The trade-off is open floor plans without bracing, which owners value, but estimators should expect higher steel connection costs when a moment frame is specified.
What is the difference between ordinary, intermediate, and special moment frames?
The categories reflect increasing ductility and seismic detailing. Ordinary frames suit low-seismic regions with minimal special detailing, while special moment frames require demand-critical welds, protected zones, and rigorous quality control for high-seismic areas. Each step up adds fabrication and inspection cost, so estimators must price to the classification shown in the structural notes.
Can value engineering replace a moment frame to lower a bid?
Sometimes. If the architecture permits, substituting braced frames or shear walls can cut connection cost, but the swap must satisfy the engineer's lateral and seismic requirements. Estimators usually offer this as a priced alternate during bidding rather than assuming it, since the structural design intent and code demands govern what is acceptable.
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