Moment-resisting frame design

The connection is where a moment frame is won or lost. To transfer bending between a beam and a column the joint must be genuinely stiff and strong, so bolted extended end plates, often with a haunch below the beam, or fully welded connections are used. We design each connection to carry the full design moment, shear and axial force together, and we check the column web at the connection for the concentrated forces the beam flanges deliver, adding stiffeners or web plates where the web would otherwise yield or buckle. The stiffness of the connection is not a detail; it is an input to the frame analysis, because a semi-rigid joint changes how the whole frame shares load.

Because the connections are heavier and the members are often larger to control drift, a moment frame usually costs more steel and more fabrication than a braced frame doing the same job. The design pays close attention to buildability, keeping connection types repeatable and access for bolting or welding realistic, so the saving in open floor space is not lost to slow erection. Where only part of a building needs to be open, we often brace the rest and use moment frames only where the architecture demands them, which keeps the overall frame economical.

Moment frames suit buildings where bracing or cores cannot be tolerated: fully glazed facades, showrooms, open retail floors, buildings that need every bay clear for future flexibility, and some low to medium-rise commercial work. They are also used in seismic regions across parts of Europe because a well-detailed moment frame is ductile and can absorb energy by yielding in a controlled way, though ductile detailing is a specialist addition to the standard wind design. Single-bay portal frames are the simplest moment frames of all, using the same principle at the eaves.

The limits are stiffness, cost and height. As buildings get taller the flexibility of a pure moment frame makes drift hard and expensive to control, and at that point a braced core or an outrigger arrangement usually takes over, sometimes combined with perimeter moment frames. For the low and medium-rise buildings where they are most at home, the trade is clear: you accept heavier connections and a little more steel in exchange for elevations and floors that stay completely open. We set out that trade-off honestly and design the frame to the Eurocodes for whichever route suits the building.