Moment frames

At the most basic level, structural actions such as the axial forces, bending moments and shear forces arising from seismic forces can be expressed in the detailing or shaping of structural members. Usually the choice of just one action, such as bending, provides enough architectural potential (Fig. 17.3). If this were the chosen action, then one consequence for beam and column detailing is that members are haunched or tapered from beam-column junctions to achieve minimum cross-sectional depths at their mid-spans (Fig. 17.3(a)). Shear walls shaped to reflect their seismic actions also offer interesting possibilities (Fig. 17.3(b)).

A further level of detailing sophistication can express the concept of Capacity Design (Chapter 3). The fundamental requirement for moment frames is for seismic damage to columns to be suppressed and damage confined only to beams. The concept of strong columns-weak beams can be expressed easily. Architectural attention can also be paid to the expression of potential plastic hinges at ends of beams. Engineers anticipate and design for concentrated damage in these areas. In reinforced concrete construction, beam cores at the ends of beams are confined by closely-spaced ties wrapped around the horizontal reinforcing. Provision of this confinement conjures up images of binding, strapping or bandaging. The retrofitting of some existing earthquake-prone structures involves wrapping column plastic hinge zones with high-strength materials (Fig. 17.4). The utilitarian nature of the solution illustrated here does not preclude more elegant alternatives for new construction.

For glue-laminated wood frames architects can use ductile steel beam-column joints to articulate the structural goal of preventing damage to the brittle wooden members (see Fig. 5.40).

There are other opportunities to integrate specific reinforcement detailing requirements with architectural expression. Take the beam stubs projecting from corner columns that were popular in New Zealand multi-storey frame buildings in the 1970s. Stubs solved the problem of adequately anchoring top and bottom beam longitudinal bars, and lessened reinforcement congestion in the beam-column core,

▲ 17.5 Typical beam studs. Educational building, Wellington.

easing concrete placement (Fig. 17.5). Recent research has led to other satisfactory anchorage methods.

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