Structural requirements

Relatively deep columns are the first prerequisite for a seismic moment frame. The column depth measured in the direction that the frame acts, and to a lesser extent the column width if it is rectangular in cross-section, must provide sufficient stiffness, bending and shear strength. To ensure ductile strong column - weak beam frames (to be discussed shortly) column depths are usually equal to or greater than those of the beams. Since reinforced concrete moment frames require special reinforcement detailing any column cross-section should be larger than 230 mm wide by 400 mm deep and even then such a small member might prove structurally inadequate for a building more than one-storey high.

Where columns are part of a two-way framing system they are normally square or circular in cross-section in order to possess sufficient strength in both directions. For a one-way frame cost-effective

▲ 5.37 Two suspended waffle slabs have collapsed. Retail store. 1994 Northridge earthquake, California.

(Bertero, V.V., Courtesy of the National Information Service for Earthquake Engineering, EERC, University of California, Berkeley).

▲ 5.37 Two suspended waffle slabs have collapsed. Retail store. 1994 Northridge earthquake, California.

(Bertero, V.V., Courtesy of the National Information Service for Earthquake Engineering, EERC, University of California, Berkeley).

columns are usually rectangular, contributing strength in one direction only. Columns need to be continuous from the foundations to the top of the moment frame at roof level.

A moment frame beam is approximately the same depth and width as the column it frames into to facilitate the direct transfer of bending moments between the two elements. The beam is also considerably deeper than a typical floor slab. The depths of moment frame beams are rarely less than their span/12 whereas slab depths typically range between span/25 to span/30. The beams of a moment frame can not take the form of slabs. Even waffle slabs have exhibited very poor seismic performance. During numerous earthquakes they have demonstrated their unsuitability to resist cyclic seismic forces sometimes by pancaking (Fig. 5.37). Slabs without beams are too flexible and weak to provide adequate lateral resistance even though they are perfectly adequate for gravity forces.

Shear wall resists y direction forces

Moment frame resists x direction forces

Column of a flat plate structure

▲ 5.38 Plan of a flat plate gravity-resisting structure provided with seismic resistance by a pair of shear walls and moment frames.

Where either a flat plate or flat slab flooring system carries gravity forces a separate and recognized lateral force resisting system needs to be provided in each orthogonal direction (Fig. 5.38).

Ideally, moment frame beams should be continuous and form a straight line in plan. Horizontal off-sets along a beam line are to be avoided either along a beam or at columns. Beams, slightly curved in plan, may be possible structurally but straight beams avoid undesirable secondary effects. Beam centerlines should coincide with those of columns although codes do allow small offsets but not nearly enough for a beam to be attached to a column face.

Another defining feature of a moment frame is its rigid joints. As shown in Fig. 5.39 , a moment frame requires at least one rigid joint. The more rigid joints the more evenly bending moments and shear forces are distributed around structural members and the greater the horizontal rigidity. Member sizes are also kept to a minimum. In most moment frames beam-column joints are designed and constructed to be rigid. One significant exception occurs in the case of single-storey frames. Columns are commonly pinned at their bases to facilitate y x

Plan

Inertia forces

Three-pinned frames n

Two-pinned frames

Single-pinned frames construction and reduce the need for enlarged foundation pads to withstand column bending moments. The penalty for this approach is that the frame is more flexible than if it were fully rigid so larger members are required.

Frames that are regular in elevation and plan display the best seismic performance. Regular frames comprise those with approximately equal bay widths and where all columns are oriented in the correct direction. Bay widths of multi-bay frames may be varied but the best configuration for seismic resistance is where beam spans are identical. Experience suggests an optimum distance between column centrelines of between 5 and 8 m. Once a span exceeds 8 m deeper beams can require increased inter-storey heights.

Materials and heights of frames

Rigid frame

Moment frames are fabricated from wood, reinforced concrete and ▲ 5.39 Moment frame forms with steel. Glue-laminated and laminated veneer lumber (LVL) wood frames different numbers of pins. Members are detailed to partially express their bending are reasonably popular in countries well endowed with forests. The moment diagrams under seismic forces. main challenge facing designers is how to achieve rigid beam-column joints. Some jointing techniques are shown in Fig. 5.40.7 Their complexity explains why the most practical rigid joint in sawn wood construction is formed with a diagonal brace. Wood moment frames are normally restricted to low-rise light-weight buildings (Fig. 5.41).

▲ 5.40 Methods of forming rigid joints in

(glue-laminated) wood moment frames. ▲ 5.41 An elegant wood moment frame. Commercial building, Austria.

▲ 5.40 Methods of forming rigid joints in

(glue-laminated) wood moment frames. ▲ 5.41 An elegant wood moment frame. Commercial building, Austria.

Steel frames are suitable for light-industrial buildings through to high-rise construction (Fig. 5.42). Where reinforced concrete is the dominant structural material concrete moment frames can provide seismic resistance for buildings many storeys high (Fig. 5.43). Once over a

height of around 20 storeys they may require supplementing with shear walls or transformed into a different structural system such as a bundled-tube structure to restrict wind-induced drift.

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