Earthquakes inflict powerful and unpredictable forces on buildings. Some of the most difficult design problems structural engineers face involve anticipating and accommodating seismic forces. Making the situation even more difficult is the fact that, almost universally, design considerations such as building efficiencies, aesthetics, and code compliance come first, then the engineers are called on to fix any discrepancies that present a weakness (or stiffness, for that matter) that might precipitate a structural failure during an earthquake. Close cooperation between architect and engineer from the very outset of design is critical to seismic safety. To illustrate this, I provide just a few examples of how design decisions can affect a building’s performance during an earthquake.

Occupancy-related configuration
In many large public buildings, such as hospitals or office buildings, the lobby level can be two times the height of the building’s other stories and have few or no walls. While this may create a grand, welcoming entrance, it also means a “soft” or weak structural plan at the bottom level, where seismic loads are greatest. If the stories above incorporate a shear-wall system with cellular layout, you may be creating a lobby level weaker than the story above and shear walls that are discontinued at the lobby level. Even with frame structures, these tall lobby levels can be much more flexible (i.e., softer) than other levels.

Fire protection
In Type I construction, reinforced masonry or concrete walls or columns of reinforced concrete or well-fire-protected steel will be required, and the diaphragm will be reinforced concrete. From a seismic-design standpoint, the locations of structural materials and their mass are not necessarily problematic. But note that exposed steel or timber walls or floors may be eliminated as options and that buildings with high fire-protection levels tend to be high in mass. Fire-sprinkler water tanks also present a significant concentration of mass in high-rise buildings.

Acoustics
Concrete floors and concrete or masonry walls commonly provide sound-transmission dampening. Likewise, concrete topping on wood floors increases their rigidity and mass. In a wood-frame building, this introduction of localized areas of stiffness can be advantageous or disadvantageous, depending on location and orientation. Placed correctly, these elements can stiffen the structure to withstand shear forces more effectively. Walls that cause eccentricity of center of mass from the center of rigidity, on the other hand, lead to torsion.

Construction cost
Low constructions budgets are a fact of life. When they lead to a low budget for structural design, you have less time to spend on design analysis and construction inspection. In such cases, even something as seemingly simple as introducing a steel moment frame at strategic places in a wood-frame building (e.g., garage openings or “soft front walls” of retail buildings) can be neglected because of budget considerations.

Appearance
Beauty is in the eye of the beholder, or as the late architect George Simonds once said, “form follows fashion.” Again, the seismic result of pursuing an aesthetic goal can be unfavorable or opportune. Unfavorable examples are pilotis, the “stilts” once popular in Modern architecture that can cause a soft story. Among the favorable examples are buildings with expressed structural elements, such as Norman Foster’s Hong Kong and Shanghai Bank Building. With the structure becoming the aesthetic, there is more justification for the added expense of increased attention to detailing, especially connections.

Energy conservation, HVAC system, environmental concerns
Structure is affected by systems and material selection for energy efficiency as well. For example, a system that requires large ducts may necessitate a truss in a suspended ceiling space, rather than beams, although beams would more easily allow the development of a moment-resistant frame. Introducing high-mass concrete or masonry walls into wood residences as a “thermal flywheel” also introduces high stiffness at those walls.

Gravity load resistance
With the exception of the eccentrically braced frame, all of the structural elements used in seismic design were first developed to resist gravity loads or wind and were adapted to seismic resistance in the 20th century. So, typically, column or wall spacing is set by span limits for resisting gravity and to correspond to floor plan requirements. Only then are seismic considerations considered. In the decades prior to the 1994 Northridge (Calif.) Earthquake, engineers found ways to span longer distances, with fewer columns placed farther apart and supporting beams with increased depths. This trend worked well enough for gravity loads, but, as it turned out, the ductility of steel moment-resisting frames suffered, and the impact on seismic resistance was negative.

Copyright 2002 The American Institute of Architects. All rights reserved.