Unlike a shear-wall assembly, where the anchor points between the wall base and the foundation are assumed to pick up an overturning load on the shear wall, moment frames do their work in the upper corners of the frame. There they resist the “moment”—the force of bending or rotation—with a rigid connection at the joint between the column and beam, rather than relying on the joint between the base of the column and the foundation it rests on.
In terms of overturning, a moment frame is analyzed like a single large shape. Its two narrow columns aren’t considered to be overturning independently; instead, the entire frame around a garage door or large window may be trying to overturn as a 9- by 20-foot unit. If the weight of an upper story is resting on the header beam, the uplift on either column from the overturning motion could be completely outweighed by the gravity load on the whole structure. This means that, in total, the uplift stress on the bolted connections between each column and its concrete base is less problematic than is the case with narrow shear walls, even though each of the frame’s two columns may be quite narrow.
This difference is reflected in the connection details of the various systems. The Simpson steel Strong-Wall at right, for example, has heavy bolts tying it to the foundation at its base, where the overturning and uplift forces could be extreme (1). At the top of the panel (2), by contrast, the connection consists only of Simpson SDS screws—lighter-duty fasteners. But in the upper corner of a Simpson Ordinary Moment Frame (3), heavy-duty bolting is required to hold the joint stiff.