Introduction

'Moment frame' is used in this book as a shortened version of 'moment-resisting frame'. While terminology varies between countries and some structural engineers simply prefer 'frame' , the term ' moment frame' is intended to remind readers of the primary method moment frames transfer force - through bending. The three essential characteristics of a moment frame are: firstly, columns deep enough to resist significant bending moments; secondly, beams and columns with similar depths; and finally, rigid connections between columns and beams. Beams and columns of a moment frame are subject to relatively large bending moments as it drifts or deflects sideways while resisting seismic forces (Fig. 5.32).

Wherever bending occurs in a frame it is accompanied by shear force. The two are inseparable. Unless rigorously designed for seismic forces according to the Capacity Design approach columns, beams and beam-column joints fail in shear before bending (Fig. 5.33). Columns

▲ 5.33 Column shear failure in a moment frame. 1994 Northridge earthquake, California.

(Reproduced with permission from A.B. King)

▲ 5.33 Column shear failure in a moment frame. 1994 Northridge earthquake, California.

(Reproduced with permission from A.B. King)

Correct layout: Columns strong in direction of frame. Frame effective for loading in this direction

Poor layout: Columns weak and flexible when bending about their minor axis Reinforced Concrete Steel

Poor layout: Columns weak and flexible when bending about their minor axis Reinforced Concrete Steel

Beam-column joint

Column depth

Elevation of typical reinforced concrete frame

Beam depth

Beam

Beam-column joint

Column depth

Elevation of typical reinforced concrete frame tt]

Beam depth

▲ 5.34 Column orientation in moment frames and frame elevation.

are especially vulnerable. They are subject to very high shear forces compared to columns of gravity resisting frames (Fig. 5.32 (c) and (d)). The end columns of seismic resisting frames can also experience significant tension and compression forces. This is how most of the overturning moment on a frame is resisted. The narrower and higher a frame the greater these axial forces become.

While Fig. 5.32 shows the effects of seismic and gravity forces separately, moment frames resist them simultaneously. Even where moment frames by virtue of their orientation with respect to gravity-resisting structure support no floor loads they still support their self-weight. So moment frames are always designed for the combined effects of seismic and gravity forces.

Like a shear wall a moment frame is effective in the direction of its length only. A one-way frame cannot resist forces at right angles to its length since there are no beams framing into the columns in that direction. Columns need to be orientated correctly with respect to the frame length to utilize their maximum strength and stiffness (Fig. 5.34). Where beams frame into columns from two usually orthogonal directions the columns become members of two frames (see Fig. 5.2(c)) and require additional strength.

Section A-A

▲ 5.35 Possible single-storey moment frame forms. Almost any shape is feasible structurally.

Three single-storey frames

Moment frames can take a myriad of forms. So far one and two-bay rectilinear frames have been presented. Multi-bay frames are common and so are frames whose geometry breaks free from orthogonality to include pitched rafters or even embrace the curve. But provided certain structural requirements are met almost any scale or shape of moment frame is possible (Fig. 5.35). As illustrated in Fig. 5.36 three single-storey single-bay frames can be stacked vertically to form a three-storey one-bay frame, or joined sideways to form a single-storey three-bay frame. Architects in collaboration with structural engineers choose the layout of frames, numbers of bays, and bay width or distance between columns to suit architectural planning requirements. The chosen structural configuration must ensure sufficient overall strength and stiffness against seismic forces and hopefully enhance the realization of the architectural design concept. An architect should consider a moment frame, a shear wall, or even a braced frame for that matter, not only as a device to resist seismic forces but also as an opportunity to enhance the clarity with which an architectural idea or concept is realized.

It should come as no surprise that moment frames are such a popular method of providing seismic resistance. Moment frames meet the aspirations of contemporary building owners and inhabitants for minimum disruption to spatial planning, minimum structural foot prints and maximum opportunities for natural light and views. However, after having informed readers of the geometrical and configurational freedom associated with moment frames some necessary structural requirements must be mentioned.

One bay three-storey frame

Single storey three-bay frame

▲ 5.36 Single-bay frames as modules of multi-storey and multi-bay frames.

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